Recent developments in thermoelectric materials and applications

Resume : We present here a complete study of the structure and thermoelectric properties of colusite Cu26T2(Ge,Sn)6S32 (T = V, Cr, Mo, W). In the first part of this presentation, we pinpoint the key role of the densification process on the formation of short-to-medium range structural defects in Cu26V2Sn6S32 [1]. A simple and powerful way to adjust carrier concentration combined with enhanced phonon scattering through point defects and disordered regions is described. By combining experiments with band structure and phonons calculations, we elucidate, for the first time, the underlying mechanism at the origin of intrinsically low thermal conductivity in colusite samples as well as the effect of S vacancies and antisite defects on the carrier concentration. In the second part, we will show the spectacular role of the substitution of V5+ by hexavalent T6+ cations (Cr, Mo and W) on the electronic properties, leading to high power factors [2]. In particular, Cu26Cr2Ge6S32 sample shows a PF value of 1.53 mW m-1 K-2 at RT that reaches a maximum value of 1.94 mW m-1 K-2 at 700 K. The rationale is based on the concept of conductive “Cu-S” network, which in colusites corresponds to the more symmetric parent sphalerite structure. The interactions within the mixed octahedral-tetrahedral [TS4]Cu6 complexes are shown to be responsible for the outstanding electronic transport properties. [1] C. Bourgès et al., J. Amer. Chem. Soc. 140 (2018) 2186 [2] V. Pavan Kumar et al., Adv. Energy Mater. (Published online)

Resume : Chalcopyrite CuFeS2, known as an antiferromagnetic semiconductor, has been confirmed as a low-cost and environmentally benign n-type thermoelectric material owing to its high power factor exceeding 1 mW / m K^2 at near room temperature achieved through tuning the carrier concentration [1]. However, the dimensionless figure of merit ZT is still quite low because of the high lattice thermal conductivity. Here we performed a phonon transport analysis based on ab initio calculations in order to obtain insights from microscopic view which lets us reduce lattice thermal conductivity of CuFeS2 via an adequate control of microstructure. We calculated the phonon transport properties by using AlmaBTE, a solver of the phonon Boltzmann transport equation [2]. The cumulative thermal conductivity profile revealed that phonons with mean free path ranged from 100 nm to 1 µm have dominant contributions to thermal conductivity. Moreover, we will discuss the effects of microstructure control on the lattice thermal conductivity for experimental bulk and thin film samples together with more detail of computational results. [1] N. Tsujii and T. Mori, Appl. Phys. Express 6, 043001 (2013), R. Ang et al., Angew. Chem. Int. Ed. 54, 12909 (2015), H.Takaki et al., Mater. Today Phys. 3, 85 (2017), [2] J. Carrete et al., Comput. Phys. Commun. 220, 351 (2017).

Resume : Copper-based chalcogenides have received increasing attention as promising thermoelectric materials due to their high efficiency, tunable transport properties, high elemental abundance and low toxicity. Among these materials, copper antimony selenide (CuSbSe2) is a good host material for further modification due to its high S (535 μV/K at 625 K) and intrinsically low k (0.38 W/(m K) at 625 K). Therefore, we design a series of new CuSbSe2 based compounds, namely AeCuSbSe3 and SrOCuSbSe2, by introducing ionic motifs (Ae2 = Alkaline earth metal ions, and Ae2 /O2- building blocks) to the structure. AeCuSbSe3 crystallize in the orthorhombic space group Pbam, and feature 3D [CuSbSe3]2- frameworks. SrOCuSbSe2 crystallizes in the monoclinic space group P21/m, and features 2D [CuSbSe2] layers. These new compounds are typical semiconductors with relatively large band gaps (~1.5 eV) and high S (~600 μV/K at 550-800 K). Ultralow thermal conductivity was also observed for these compounds (~0.38 W/(m K) at 850 K). Density functional theory (DFT) calculation results indicate that the lone pair electrons of Sb3 are responsible for the soft phonon modes and strong vibrational anharmonicity in the lattice, which can lead to ultralow thermal conductivity.

Resume : Zinc oxide has been one of the most promising n-type thermoelectric oxides with relatively large power factor values competitive to major non-oxide candidates such as skutterudites and clathrate compounds. However, a substantially high lattice thermal conductivity of ZnO has limited its ZT values below 0.7, despite a number of attempts to suppress the thermal conduction in the oxide. Here we report an anomalously large reduction of the lattice thermal conductivity, ph, of ZnO doped with Al and Cu simultaneously. The ZnO samples doped with equimolar amounts (x at. % to Zn) of Al and Cu showed a significant peak shift in their XRD patterns, which has never been observed on single doping of Al or Cu. Moreover, the ph values of the samples at x = 8 was as low as 5 and 1.5 W/Km at room temperature and 800 °C, respectively. By comparing with those of Al-doped ZnO at x = 2, which are 40 and 8 W/Km at the corresponding temperatures, a 80% reduction of ph is overwhelming. A remarkable increase in the occupancy of Al in the 4-fold coordination was clearly confirmed by 27Al MAS solid state NMR, evidencing an extended solubility limit of Al into the Zn site in the presence of Cu as a co-dopant. It should also be noted that the ph value at 800 °C, 1.5 W/Km, is very close to the theoretical lower limit of 1.2 W/Km predicted for ZnO above room temperature.

Resume : Thermoelectric (TE) Generators allow the transformation of thermal into electric energy, presenting an interesting mechanism of energy harvesting. The efficiency of a TE material can be measured by its Figure of Merit (ZT), which depends on the Seebeck Coefficient, the Electrical and Thermal Conductivities. The thermoelectric ZT can be enhanced via nanostructuring due to the phonon scattering at nanodomains. In this work, TE materials were developed and studied to enhance their properties, through the manipulation at the nanoscale. Despite its high thermal conductivity, ZnO was used due to its characteristics as an oxide TE material, such as high stability, non-toxicity, reduced costs and very low environmental impact, although, its major limitation remains with the high thermal conductivity. The ZnO nanoparticles were synthesized by a chemical co-precipitation route based on the addition of two solutions: zinc salt and the sodium hydroxide. It was possible to manipulate the preferential direction and the morphology of ZnO (passing from hexagonal prism-shaped particles to ‘desert-rose stones’), by only varying the concentration of the NaOH in the co-precipitation. During the morphologic change, the phononic phenomena predominate on the thermal conductivity, where the structures are smaller (~50nm), compared with the clusters formed afterwards (~4μm). It was also possible to reduce the thermal conductivity by 30% (0.9 Wm−1K−1) due to the occurrence of quantum confinement.

Resume : Because of chemical stability, flexibility, tunable electrical conductivity, and non-toxicity, perovskite oxides (ABO3) are great alternative to the main thermoelectric (TE) materials based on scarce toxic tellurides. Indeed, n-type TE ABO3 such as La-doped SrTiO3 can exhibit large TE power factor[1]. However, efficient p-type TE ABO3 still remains challenging. P-type transparent conducting Sr-doped LaCrO3 is appealing in this context[2]. We will present high-quality n-type La-doped SrTiO3 and p-type Sr-doped LaCrO3 TE epitaxial films grown by molecular beam epitaxy (MBE)[3-4]. All the films are flat and fully strained with low mosaicity (<0.1°). In particular, we will show that La-doped SrTiO3 epitaxial films can reach large electrical conductivity (up to 10E4 S/cm) and power factor (up to 40 µW/cmK2). Optimized Sr-doped LaCrO3 films can present relatively large Seebeck coefficient of about +180 μV/K with electrical resistivity around 1 Ωcm at room temperature. The impact of doping level, cationic stoichiometry (A/B) and epitaxial strain (ranging from -2.1% to +1.7%) on structural and physical properties will be shown and discussed. Finally, the integration of these films on Si has been done using a SrTiO3 epitaxial buffer layer with a view to building an integrated micro-thermoelectric module. [1] G. Bouzerar et al., EPL 118, 67004 (2017) [2] K.H.L. Zhang et al., Adv. Mat. 27, 35 (2015) [3] M. Apreutesei et al., Sci. Tech. & Adv. Mat. 18, 430 (2017) [4] D. Han et al., submitted

Resume : Dopants play an important role in engineering the electronic properties of semiconductor materials. At the same time they can strongly influence the phonon scattering processes and thereby the thermal conductivity. We have recently shown how Boron point defects in 3C-SiC act as “superscatterers” and exhibit resonant phonon scattering which is one to two orders of magnitude higher than other defects.[1] The increased scattering leads to a thermal conductivity that is suppressed by one to two orders of magnitude. In order to understand the physics behind and the factors causing resonance scattering, we explain the results with the help of a simple 1D mono-atomic linear chain.[2] We show that small lattice distortions emanating from two or more close energy minima in potential energy surface lead to very large perturbations of the interatomic force constants. Such a behavior is characterized by a peak in the trace of imaginary part of the T matrix (which is closely related to the scattering rates) and a reflection coefficient approaching unity. The strong influence of the potential energy surface surrounding the defect atom on the thermal conductivity opens a new path to tailor thermal conductivities where required values range from very low in thermoelectric materials to very high in power electronics applications. Using doping of GaAs as an example, we show how the provided insights can be used to identify potential superscatterers. A. Katre, J. Carrete, B. Dongre, G. K. H. Madsen, and N. Mingo, Phys. Rev. Lett. 119 075902 (2017). B. Dongre, J. Carrete, A. Katre, N. Mingo, and G. K. H. Madsen, J. Mater. Chem. C 6, 4691 (2018).

Resume : Based on the general framework of the linear response theory to thermal perturbation by Luttinger together with the method developed by Jonson and Mahan, range of validity of Sommerfeld-Bethe relation (and Mott formula) between two correlation functions L_11 and L_12 associated with Seebeck coefficient S = L_12/T L_11 with T being temperature is clarified for a model with both periodic and random potentials, electron-phonon interaction, and electron-electron interaction. The contributions which do not satisfy the Sommerfeld-Bethe relation are identified, including the well-known phonon drag. The general expression of the contribution of phonon drag to L_12 is derived which is valid even under the strong disorder, i.e., beyond the Boltzmann transport regime. Based on this method, we study phonon drag effects on Seebeck coefficient by focusing on the roles of impurity bands with the aim to explore the possible microscopic understanding of huge Seebeck coefficient observed in FeSb_2 at low temperatures. We find that the phonon drag in the presence of impurity bands with a narrow band gap below the conduction band with large effective mass will be the key factors behind the remarkable phenomemon.

Resume : Nanostructuring is a promising approach for the next generation thermoelectric materials yielding ultra-low thermal conductivities. More specifically, some of the lower thermal conductivities in materials have been achieved by the inclusion of hierarchically sized structures, at the atomic size, the nanoscale, and mesoscale, which can scatter phonons of various wavelengths and reduce phonon transport throughout the spectrum. Less attention has been paid, however, to the power factor (PF), which in most cases is reduced. In this work, we present our recent simulation results on the thermoelectric PF of hierarchically nanostructured materials than include nanocrystalline potential barrier boundaries, nanoinlusions and voids. We employ a fully quantum mechanical simulator based on the Non-Equilibrium Green’s function method, including electron-phonon interactions, which allows robust treatment of nanoscale transport physics, and includes all geometrical and scattering details without the need of approximations. We discuss the conditions under which the PF is less affected due to the presence of nanostructuring, and conditions under which the PF is even enhanced. We further couple our results with Molecular Dynamics simulations to compute the reduction in thermal conductivity in our structures and compute ZT. Our findings can complement the large volume of work on reducing thermal conductivity, and point to design directions that further increase the ZT figure of merit.

Resume : Printable thermoelectric(TE) materials have significant potential to be the key to unlock technologies ranging from flexible thermal energy-harvesting devices for wearable electronics up to large-scale waste heat recovery to increase the power output of power plants . However, with the advantage of good processability for mass production from roll-to-roll, these materials lag behind conventional TE materials in terms of efficiency. To increase the Figure of Merit ZT of printed amorphous TE materials morphological effects that lead to percolative charge transport can be exploited. We present a theoretical model for thermoelectric thin films showing an increase in the ZT-value in TE material systems where two locally distinct charge transport mechanisms coexist compared to materials where only a one type of charge transport predominates. The two transport mechanisms are spatially distributed in a binary random resistor network with a probability ratio near the percolation threshold. Macroscopic thermoelectric properties of the thin film are then calculated by an expanded nodal analysis method, which enables an analysis of the spatial dependencies and path formation of the current flow. Emerging Anomalies due to fractal cluster formation lead to a macroscopic collapse of the anti-correlation of the Seebeck coefficient and the electrical conductivity, resulting in a simultaneous enhancement of both and an increase in the TE performance by almost one order of magnitude.

Resume : Owing to the possibility of fairly easy tuning of their properties through the suitable structural modifications, 2D layered materials (e.g., graphene, silicene, germanene, transition metal dichalcogenides) have emerged recently as very promising candidates for novel devices in the fields of opto- and electronics, spintronics, quantum computing, and recently also for thermoelectric applications. The figure of merit for thermoelectric devices favors systems characterized by (i) large Seebeck coefficient and (ii) by as large as possible the ratio of electric to the thermal conductivity. Recently, the transition metal carbides and nitrides (MXenes) have joined the family of the 2D materials and are considered as promising candidates for novel thermoelectric devices. Here, we present results of ab initio calculations based on nonequilibrium Green’s function and density functional theories for thermoelectric properties of titanium carbides and nitrides, either pristine or with structural defects and/or suitably functionalized. Our studies reveal that MXenes have much better thermoelectric characteristics than graphene, silicene, and germacene based systems, and comparable to structures based on hexagonal SiGe alloys. The thermoelectric figure of merit for 2D MXenes exceeds considerably the value of two that is considered to be a prerequisite for reasonable thermoelectric systems. Acknowledgements: Support of NCN through the grant OPUS (UMO-2016/23/B/ST3/03567) is acknowledged.

Resume : It has provoked a new thermoelectric research heat that the n-type SnSe ZT has reached 2.8±0.5 at 773K, published on Science magazine lately. However the cheap, fast and high-efficient SnSe synthesis methods are researchers’ continuous pursue and ultimate target. Electroless plating Ag in polycrystalline SnSe offers a rapid and effective way to prepare high ZT materials, a peak ZT=1.0 was achieved in 1.0at% Ag doping compared with the original none doping ZT=0.4 at 873K, and the thermal conductivity reduced 40% to 0.5 Wm-1K-1, while keeping the electrical conductivity and Seebeck coefficient unchanged at the same temperature. What’s more, Ag doping more than 1.0at% will cause arise of thermal conductivity, owing to the balance between charge carrier transportation and phonon scattering caused by Ag doping defects. In a word, this work could offer a reference for the high performance TE material research.

Resume : The topological crystalline insulator tin telluride (SnTe) belongs to a recently discovered class of materials in which a crystalline symmetry ensures the existence of topologically protected surface states [1]. As an emerging thermoelectric material, SnTe has also received extensive attention because of its low toxicity and eco-friendly nature [2]. Band engineering and nanostructuring can enhance the thermoelectric performance of SnTe at intermediate temperatures (400-800 K) [3]. In this work, we report a fully ab-initio calculation of the temperature dependence of the electronic structure of SnTe in the high temperature phase, using density functional perturbation theory [4] and electron-phonon Wannier [5] approach. We obtain the temperature variation of the direct band gap in SnTe in very good agreement with experiments. Also, we analyze the temperature dependence of the electron-phonon coupling and its possible consequences on thermoelectric transport. Our results indicate that thermoelectric transport properties crucially depend on the electronic band structure of SnTe. [1] T. H. Hsieh, H. Lin, J. Liu, et al., Nat. Comm. 3, 982 (2012). [2] W. Li, Y. Wu, S. Lin, et al., ACS Energy Lett. 2, 2349 (2017). [3] R. Moshwan, et al, Adv. Funct. Mater. 27, 1703278 (2017). [4] S. Baroni et al., Rev. Mod. Phys. 73, 515 (2001). [5] F. Giustino et al., Phys. Rev. B 76, 165108 (2007); F. Giustino, Rev. Mod. Phys. 89, 015003 (2017).

Resume : For thermoelectric applications, ab initio methods generally fail to predict the transport properties of materials because of their inability to predict properly the carrier concentrations that control the electronic properties. In this presentation, a methodology to fill in this gap is shown and applied in important materials for thermoelectric applications: NiTisn, ZnSb and Mg2Si. For that, we show that the main intrinsic defects act as donors/acceptors of electrons and are responsible of the measured electronic properties of the materials. Moreover, combining the density of states of the solid solutions with the determination of the energy of formation of charged defects, we show how to obtain numerically the measured carrier concentrations in experimentally supposed “pure” compounds. Subsequently the thermoelectric properties of the compounds can be calculated using a fully ab initio description and, depending on the system, an overall correct agreement with experiments is obtained.

Resume : Nowadays, the research to find semiconducting materials for energy conversion applications has raised a huge interest in the scientific community. Particularly, a high number of investigations are focused to find suitable compounds for thermoelectric applications[1]. In this work, we report a detailed investigation about the deposition of 4 different Cu-Se chalcogenides films (orthorhombic CuSe, hexagonal CuSe, orthorhombic Cu2Se, cubic Cu2Se) on flexible polycarbonate substrates by using PHRMS, a novel semiconductor thin film deposition technique developed in our lab that allows the growth at room temperature (RT) and in a few minutes of many metal selenides (Ag-Se, Cu-Se, Sn-Se, …) with different compositions[2,3]. The obtained films were characterized by X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDX), micro-Raman spectrometry and scanning electron microscopy (SEM-FEG). The transport properties (Seebeck coefficient, resistivity and mobility carriers) were measured from room temperature up to 100ºC. All the four compounds investigated show p-type conductivity with different values of the Seebeck coefficient (ranging from 10V/K to 350V/K), resistivities measured at RT from 10-4 to 100 cm and a maximum power factor (PF) of the order of 0.16mW m-1 K-2. [1] M. Martín-González, O. Caballero-Calero, P. Díaz-Chao, Renew. Sustain. Energy Rev. 2013, 24, 288. [2] J. A. Perez-Taborda, L. Vera, O. Caballero-Calero, E. O. Lopez, J. J. Romero, D. G. Stroppa, F. Briones, M. Martin-Gonzalez, Adv. Mater. Technol. 2017, 2, 1700012. [3] J. A. Perez-Taborda, O. Caballero-Calero, L. Vera-Londono, F. Briones, M. Martin-Gonzalez, Adv. Energy Mater. 2018, 8, 1702024.

Resume : Harvesting energy from temperature gradient between human body and its environment is an attracting and challenging goal. Ergonomics and health-related issues however impose some restrictions. Alternative to Bi2Te3 alloys, the room temperature best performing but toxic thermoelectric (TE) materials, have to be considered while a good skin contact necessitate a flexible TE material. We propose to use health compatible perovskite oxide TE films deposited on a flexible porous silicon (PSi) membrane to fulfill these requirements. The porosification of a Si wafer top surface is obtained by electrochemical etching. The 30m thick PSi layer is then peeled-off to get a self-supporting membrane. The idea is to peel-off the membrane after TE generator micro-processing completion, thereby ensuring high temperature mechanical stability for perovskite crystallization and cleanroom handling easiness. We report here on the synthesis and characterization of TE thin film libraries grown by combinatorial pulsed laser deposition. Polycristalline or epitaxial films of (Nb,La):SrTiO3 and (Sr,Ba):LaCoO3 semiconducting compounds (n and p type respectively) are deposited on PSi/Si or single crystal perovskite substrate. Local measurements of the Seebeck coefficient, the electrical conductivity and the thermal conductivity (on PSi) allows for the estimation of the figure of merit ZT. The evolution of the TE properties versus composition and microstructure will be discussed.

Resume : Transition metal nitrides in their thin film forms were and are still intensively studied for their mechanical properties. they also exhibit interesting electric properties which makes them suitable for other applications [1]. More recently, CrN and ScN well known as a degenerated n-type semiconductor have shown promising thermoelectric properties with high Seebeck, low electrical properties and moderate thermal conductivity[2]. In this work, CrNx films were deposited by magnetron sputtering at 650 C on different substrates with different amount of reactive gas in the plasma. Under the present deposition conditions, over- and under- stoichiometry in nitrogen (CrN1?? x) is obtained in the epitaxial rock-salt structure. Structural (XRD), morphology (SEM) composition (RBS, ERDA, XPS) and electrical characterization were performed on the different CrN films. The thermoelectric properties of the films depend on the nitrogen content and on the substrate nature. The over-stoichiometry in nitrogen and/or Cr deficiency lead to a p-type semiconductor behaviour of CrN, with promising thermoelectric properties. The under-stoichiometry in nitrogen leads to a n-type semiconductor of CrN up to the metallic behaviour of the film when Cr2N is formed. Parallel DFT calculation on the effect of the different vacancies or interstitials confirmed the stability and the effect observed experimentally. Both, Cr vacancies and nitrogen interstitials are favourable and lead to a shift of the fermi level into the valence band, thus confirming the p-type character of the semiconductor material. The control of the semiconductor behaviour of CrN films can be tailored by controlling of the stoichiometry. These results are a starting point for designing p-type and n-type thermoelectric materials based on chromium nitride thin film, a material cheap and routinely grown at industrial scale. 1. Gao, B., et al., Recent progress in nanostructured transition metal nitrides for advanced electrochemical energy storage. Journal of Materials Chemistry A, 2019. 7(1): p. 14-37. 2. Eklund, P., S. Kerdsongpanya, and B. Alling, Transition-metal-nitride-based thin films as novel energy harvesting materials. Journal of Materials Chemistry C, 2016. 4(18): p. 3905-3914.

Resume : Co-sputtering method with Cu and Bi2Te3 target were used to make thermoelectric Cu-doped Bi2Te3 thin films. To keep the low Cu concentration in the thin film under control, different RF power used on Cu target, and DC power on Bi2Te3 target was fixed. We investigated the effect of different low-level Cu doping concentration on Bi2Te3 thin films from the perspective of structural and thermoelectric properties. In these, we compared two types of the films fabricated by different process. At first, Cu doped Bi2Te3 thin films deposited at room temperature were annealed at high temperature. The other films were deposited at high temperature directly. As Cu contents increase, CuxTe and Te-deficient BixTey phases were formed in both cases. But the prominent difference can be detected in the amount of the second phases from the different process. Thermoelectric properties of the films were measured at room temperature under an Ar gas atmosphere.

Resume : GeTe-based thermoelectric materials have been used for the medium temperature application. To improve a ZT value, it is necessary to increase the power factor (S2σ) and reduce the thermal conductivity. GeTe-based thermoelectric materials have high carrier concentrations because of Ge vacancy, which results in low Seebeck coefficient and high thermal conductivity. To improve the ZT value of GeTe-based materials, carrier concentration control is essential. GeTe-based thermoelectric materials have herringbone structure which shows bright and dark contrast. Herringbone structure of GeTe-based thermoelectric materials can be observed by using transmission electron microscopy. If the width and length of the herringbone structure can be controlled, the improvement of thermoelectric properties of the GeTe-based thermoelectric materials is expected. In this study, we selected Cu and Sb as dopants, and we will discuss on doping effects of Cu and Sb on thermoelectric properties and microstructure of GeTe.

Resume : Half-Heusler thermoelectric compounds have attracted extensive attention because of their promising thermoelectric properties. The properties of thermoelectric materials are determined by dimensionless figure-of-merit, ZT=S^2σT/κ, where S is the Seebeck coefficient, σ, κ and T represent the electrical conductivity, thermal conductivity and absolute temperature, respectively. To increase the value of ZT, decent Seebeck coefficient, electrical conductivity, and low thermal conductivity are desired simultaneously. The half-Heusler materials have the general formula XYZ, where X and Y can be a transition metals and Z can be a main group element. (Hf,Zr,Ti)NiSn and (Hf,Zr,Ti)CoSb are mainly investigated but Hf has expensive cost. Thus, we chose NbFeSb-based materials among half-Heusler compounds. It was reported that NbFeSb-based materials have high power factor(S^2σ) and high thermal conductivity. In our presentation, we fabricated metal-doped NbFeSb thermoelectric materials for inducing point defect scattering and higher ZT. We analyzed thermoelectric properties and microstructure of NbFeSb-based materials using x-ray diffraction, Hall measurement, Seebeck coefficient measurement, scanning electron microscopy, transmission electron microscopy, and thermal diffusivity measurement.

Resume : Device performance is strongly influenced by microstructural characteristics such as stacking fault, phase separation, interface and so on. Therefore, it is quite important for solar cell that understanding and controlling its microstructure because electron movement inside of cells is mainly affected by the microstructure. Transmission electron microscopy (TEM) has been a suitable and unique method to obtain information down to the nanometer scale. The results have greatly helped in understanding the performance of the solar cells. CuInSe2 and its related compounds have been studied extensively because of their promising properties for the photovoltaic energy conversion and thermoelectric applications. The ternary chalcogenide CuIn3Se5 compound is often observed as a secondary phase in the In-rich CuInSe2 films. However, there is controversial description for the possible crystal structure. In this presentation, we will provide TEM studies of CuInSe2 and its related compounds. The structure of off-stoichiometric In-rich Cu-In-Se compounds was studied by electron diffraction and high-resolution TEM.

Resume : In this work, we demonstrated the fabrication and application of a silicon-based thermoelectric device with a sandwich-structure of bulk-silicon/[Ni/Ge-multilayers]/bulk-silicon. The Ni/Ge-multilayers were deposited on the bulk-silicon wafers using a radio-frequency magnetron-sputtering technique. We measured the thermoelectric parameters of sandwich-structures up to 500 K. It was observed that the Ni/Ge-multilayers caused the Seebeck coefficients increased and the thermal conductivities decreased, as compared with the values for bulk silicon thermoelectric devices; the Seebeck coefficient and thermal conductivity were ~ 300 ㎶/K and ~ 0.5 W/cm·K at 500 K, respectively. However, the electrical resistivity showed minimal change up to 500 K. From the thermoelectric parameters, the power factor of ~ 6.0 ㎽/m·K and the dimensionless thermoelectric figure of merit (ZT value) of ~ 0.05 were obtained at 500 K in the 7-set Ni/Ge-multilayer devices. The experimental results point to the possibility of using a silicon-based structure modified with Ni/Ge-multilayers for thermoelectric devices.

Resume : Nanostructured Si1-xGex alloys are of interest as thermoelectric materials because they promise high potential to improve the figure of merit ZT. The ZT depends on several parameters such as the electrical and thermal conductivity, which are strongly related to the crystallinity of the material. To transform an amorphous Si or Ge into a poly-crystalline layer, metal-induced crystallization (MIC) can be applied. It has been shown that a 20 nm Au film drastically reduces the crystallization temperature of Ge and Si to 170 °C [1] and 220 °C [2], respectively. Nevertheless, the MIC process for Si1-xGex alloys on Au has not yet been investigated. Therefore, we utilize a combination of lab-based and synchrotron-based experimental methods including PEEM, XPS, XRD, and SEM to investigate the fundamental processes that occur at the Au/Si1-xGex interface. We detail the composition and morphology of intermediate and final states during MIC of Si1-xGex on Au in order to show the relation between the Si1-xGex crystallinity and the underlying metal morphology. The influence of the Au film thickness on the crystallization process of Si1-xGex is also discussed, as the ratio of Au to Si1-xGex determines whether complete layer exchange occurs. In addition, we present preliminary results for the Seebeck coefficient and electronic conductivity.

Resume : Organic material based thermoelectric generators (OTGs), characterized by their low thermal conductivity, flexibility, and low cost, are considered a promising power source for mobile electronics. One of the most generally used methods to fabricate OTGs is dispersing conductive fillers in the polymer matrix. In this case, most of the previous approaches relied on using conjugated polymers as a matrix to disperse carbon nanotube (CNT). However, such weak physical adsorption of conjugated polymers onto CNT may weaken reliability and operational stability of the resulting OTGs when applied as a power supplier of flexible devices. Therefore, in this research, we introduce 2,4-Hexadiyne-1,6-diol (HD) to simultaneously enhance dispersion of polymer-CNT composite and morphological robustness of the resulting OTG. First, HD is employed as a dispersion agent to disperse carboxylated SWCNT(SWCNT-COOH) by covalently grafting to the surface of CNT, leading to a very stable composite solution at room temperature for 30 days. Second, after generating films, HD plays a role of the molecular binder by initiating UV-assisted polymerization, resulting in a physically robust polymer-CNT network. Such a synergetic contribution of HD enabled to realize high-performance OTG with a high power factor as well as unprecedentedly high shelf stability and bending stability.

Resume : Bismuth telluride is the most commonly used thermoelectric material for refrigeration and thermo-electric generation. Electrodeposition of such films has the advantage of low cost, fast deposition rates and control over film thickness from nanoscale to a few millimetres. We recently electrodeposited Bi2Te3 thin films potentiostatically from a non-aqueous solvent dichloromethane which, unlike aqueous solvents, has a wider potential range allowing us to explore more negative potentials than aqueous solvents. This work investigates the effect of pulsed non-aqueous electrodeposition on surface morphology and repeatability in composition. The pulsed electrodeposition was performed by alternating a constant potential (Eon=-1 V) vs. Ag/AgCl and the open circuit potential (Eoff=OCP). The results show that pulsed electrodeposition can grow smoother and more uniform films on conductive titanium nitride with repeatable compositions in comparison to potentiostatic electrodeposition. Films were transferred onto an insulating substrate for subsequent electrical characterisation using polystyrene as a carrier polymer. Removal of the polystyrene using toluene is shown to be considerably easier than commonly used poly(methyl methacrylate), resulting in a complete transfer of the deposited film. This work is part of the Adept (Advanced Devices by ElectroPlaTing) project funded by a Programme grant from the EPSRC (EP/N035437/1).

Resume : Great progress in polymer synthesis of the last two decades and insights on electron transport mechanisms resulted in dramatic increase in electron mobility making semicrystalline polymer semiconductors promising candidates for thermoelectric applications. On the other hand, heat transport phenomena, governing thermal conductivity, another transport coefficient essential for thermoelectric performance optimisation, have not received as much attention up to date. In spite of simplicity of thermoelectric properties measurements as principle, the combined uncertainty in thermoelectric figure of merit zT could easily reach 50% with the largest uncertainty coming from thermal conductivity measurements. Such high measurement uncertainty often comparable with reported relative changes in thermoelectric efficiency presents a serious issue for thermoelectric research. Relatively large sample dimensions required for practical implementation of accurate thermal conductivity measurement methods make situation even more critical for thin films, which is often the case of polymer semiconductor samples. We present a protocol for thermal conductivity measurements of thin films with reduced measurement uncertainty, which allowed us to investigate effect of microstructural changes on the sample thermal conductivity contributing towards understanding of heat transport mechanisms in semicrystalline polymer semiconductors.

Resume : Nanoscience and nanotechnology fascinate not only the researchers but also the society due to its exciting applications in almost in all the fields such as consumer electronics, environmental engineering, space exploration, renewable energy sectors, medical/healthcare, and so forth. One interest is in the exploration of thermoelectric materials that are potential enough to be developed into a practical energy conversion device. Nanostructured semiconducting materials have widely exhibited a higher thermoelectric figure of merit than their bulk counterparts due to the higher phonon scattering, which contributes to the lower thermal conductivity. Our focus, herein, is presenting the characterization (structural, morphological, compositional, and thermoelectrical) results of bismuth selenide nanomaterials obtained via polyol based synthetic method at various growth durations. Field-emission scanning electron micrographs of Bi-Se samples show the formation of layered two-dimensional hexagonal nanostructures with an estimated short-axis length of ca. 1 µm and a minimum thickness of ca. 20 nm.

Resume : Cu and Ag-based superionic conductors are promising thermoelectric materials due to their good electrical properties, intrinsically low thermal conductivity, high elemental abundance and low toxicity. In this work, n-type pure phase Ag2Te compound is quickly synthesized by grinding raw elemental powders followed by spark plasma sintering. The zTs of 0.8 at 390 K and 0.5 at 550 K are obtained for n-type Ag2Te. Because of the migration of Ag ions and the phase transition, there are holes formed inside the sample during the electrical performance measurement. Meanwhile, the density of the Ag2Te samples decreased with the increase of measurement times, which resulted in poor electrical stability and repeatability of Ag2Te samples. Cation site doping is an effective way to improve stability. Here, Pb-doped Ag2-xPbxTe (x=0, 0.005, 0.01, 0.02) are synthesized by manually mixing and spark plasma sintering. We find that the zT is enhanced the most in x=0.02 sample, achieving the maximum zT=0.75 at 575 K mainly due to an increased power factor. The increase of Pb content doped in Ag2Te significantly improves the stability of n-type Ag2Te, which is characterized via electrical properties measurement during heating process and cooling process.

Resume : Higher manganese silicides (MnSi1,7) are promising alternative materials for mid to high temperature thermoelectric applications, combining low cost, eco-friendliness and high performance with a figure of merit up to ZT=0.7 for the undoped material. Many dopants have been proposed in order to further enhance their thermoelectric efficiency, leading to values of ZT=1, approximately. This study focuses on the synthesis of doped MnSi1.7 powder by the method of pack cementation. The introduction of various dopants in MnSi1.7 powder during synthesis is tested in different experimental conditions (content of dopant, deposition temperature, holding time) and their effect on some of the material’s properties is studied. The structure determination and phase identification are performed by X-Ray diffraction analysis. The morphology and the chemical composition are determined by a Scanning Electron Microscope equipped with an EDS analyzer. Finally, the oxidation resistance of the samples is examined by thermogravimetric analysis. This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research” (MIS-5000432), implemented by the State Scholarships Foundation (ΙΚΥ), and has been supported by EU in the framework of the NetFISiC project (Grant No.PITN-GA-2010-264613).

Resume : P-type half-Heusler compounds MCoSb (M = Ti, Zr, Hf) have attracted many attentions in the thermoelectric community due to their appropriate band gaps generated promising electrical properties [1]. However, their high thermal conductivities obstructs its further application [2]. An intrinsic phase separation achieved by introducing Ti, Zr and Hf at M site can strengthen the phonon scattering, and is an effective way to reduce the lattice thermal conductivity. In this work, starting from the substitution of Sb by 15% Sn (MCoSb0.85Sn0.15) with optimized carrier concentration[3], polycrystalline samples of (Ti/Zr/Hf)CoSb0.85Sn0.15 are synthesized via arc melting method followed by annealing procedure, and the ratio of Ti, Zr and Hf is systematically adjusted to further improve thermoelectric performance. After doping, the lowest thermal conductivity is around 3.5 W m-1 K-1 at room temperature. Additionally, the micro-structuring of intrinsic phase separated samples is studied in detail by DSC and scanning electronic microscopy with energy dispersive X-ray spectroscopy, indicating as-prepared phase separated samples are thermodynamically stable. This phase separation approach saves time and energy, which is a significant alternative to nano-structuring processing [4]. [1] M. Schwall and B. Balke, Appl. Phys. Lett., 2011, 98, 042106. [2] X. Yan, W. Liu, H. Wang, S. Chen, J. Shiomi, K. Esfarjani, H. Wang, D. Wang, G. Chen and Z. Ren, Energy Environ. Sci., 2012, 5, 7543. [3] E. Rausch, B. Balke, T. Deschauer, S. Ouardi, and C. Felser, APL Mater., 2015, 3, 041516. [4] E. Rausch, B. Balke, J. M. Stahlhofen, S. Ouardi, U. Burkhardt and C. Felser, J. Mater. Chem. C, 2015, 3, 10409.

Resume : Highly effective thermoelectric BiSbTe thin films Thermoelectric materials can be used to convert a temperature gradient to an electric potential via diffusion of charge carriers. This phenomena is described as the Seebeck effect and can be used for power generation or temperature measurement applications such as thermopiles. To improve the thermoelectric figure of merit, it is important for these materials to have a high Seebeck coefficient and electrical conductivity as well as a low thermal conductivity. The ternary system bismuth-antimony-telluride is known to provide improvements in the thermoelectric parameters. So far, bismuth-antimony-telluride is usually fabricated by sputtering or hot press sintering methods. As a novel method, we have established a three-source-electron-beam evaporation system, which has been set up for the systematic variation of several process parameters for each material in the ternary system. The focus is set on stoichiometry, substrate temperature, deposition rate, and layer thickness. First results concerning homogeneity and crystal lattice arrangement will be presented. This serves to draw conclusions on the impact of these parameters on the thermoelectric properties of the analyzed materials and allows for the optimization of the thermoelectric figure of merit.

Resume : Temperature dependence of electric resistivity of bulk pure bismuth has positive temperature coefficient because of a semi-metal. However, the tendency of a narrower bismuth wire differs in the temperature coefficient changing to negative below a certain temperature; because the carrier mobility in the wire is restricted by its surface as a boundary condition. While a model we proposed has been explained the temperature dependence experimentally and analytically, some bismuth wire's temperature coefficient in lower temperature region turned to positive again. To explain the reason based on carrier transport for each carrier, we prepared individual single crystal bismuth wire (diameter is 1.9 µm and length is 1.6 mm) processed local electrodes on the wire surface directly by nano-fabrication and measured electric resistivity, magneto-resistivity, Hall coefficient and Seebeck coefficient in temperature range of 20-300 K. From these results, we demonstrated the temperature dependence of carrier density, mobility, and Fermi energy for each carrier with an assumption of intrinsic condition. The value of the hole mobility increased with decreasing temperature and reached a plateau at 50 K because the mean free path of hole was limited by the wire diameter. On the other hand, the electron mobility kept increasing with decreasing temperature; accordingly, we suppose that electron mobility of bismuth wire includes a component of much larger mobility than that of bulk bismuth.

Resume : The thermoelectric air cooling and power generation systems are available environmentally friendly and miniature-sized system today. In this study, thermoelectric cooler and thermoelectric generator were applied to automobile. We have developed a thermoelectric cooler that can be installed on a ceiling of a car's rear-seat to efficiently cool the interior of the car. A thermoelectric generator was developed to be mounted on an automobile exhaust pipe in order to utilize waste heat of automobiles. The thermoelectric air cooling system consists of thermoelectric coolers, heat sinks, fan and air flow channel. The TECs were evaluated by developed an evaluation system. The thickness of thermoelectric air cooling system was 15 mm. When the system was tested in the insulating chamber (250 mm×250 mm×250 mm), time to be cooled down from 45 ℃ to 26 ℃ was 162 seconds. As the result of practical test in the automobile installed the system, the temperature difference between only HVAC(Heating, ventilation, air conditioning) and HVAC with thermoelectric air cooling system was 2.3 ℃. The thermoelectric power generation system is composed of thermoelectric generator and heat sink. The power generation system was installed on the heat protector for vehicles. Heat protector can protect components and bodywork from heat damage. The temperature difference between exhaust gas pipe and air is key issue to increase efficiency of power generation system. The results showed that the maximum output power of 90.7 W was obtained at △T=100 ℃ from the generator using 24 thermoelectric devices. Finally, we have created an integrated system that can store the electrical energy generated by the thermoelectric power generator in a storage device and use it to turn on the thermoelectric cooler, and confirmed the applicability to the automobile through the performance evaluation of the system.

Resume : Even though thermoelectrochemical cells found some application, especially in waste heat utilization - when and if the temperatures and temperature gradients are not high their efficiency is large enough to be economically viable. The operation of thermoelectrochemical cells is based on the different red-ox activities of red-ox couples at different temperatures. Typically thermoelectrochemical cell consists of liquid electrolyte and two electrodes, with the temperature gradient-driven movement of red-ox species from one electrode to another, accompanied by charge carriers exchange at the electrodes. Until now nobody realized thermoelectrochemical cells in solid state, using solid electrolyte and solid state motion of red-ox species. We developed the first solid state thermoelectrochemical cells, based on mixed valence oxides, as TiOx and solid state electrolyte as modified LIPON. All layers were prepared by Magnetron Sputtering and are effectively thin films. In such case red-ox species are Metal ions having different valence states and charged oxygen vacancies. The operation of thin film thermoelectrochemical cells will be described, separating two cases - continuous operation - true solid state thermoelectrochemical cells and thermoelectrochemical energy harvesters, working as a electrochemical supercapacitor, which is charged and discharged in pulsed mode of operation. The model of solid state thermoelectrochemical cell is proposed.

Resume : Topology, a mathematical concept, became recently a hot topic in condensed matter physics and materials science. The topology of the electronic structure of a material determines the electronic, thermal and magnetic properties of solids. All known materials can be reclassified through the lens of topology. Weyl points, a new class of topological phases were found in NbP, NbAs. TaP, MoP and WP2. In NbP nano wires we have observed the chiral and a mixed gravitational anomaly. Additionally, NbP and WP2 show evidence for a hydrodynamic flow of electrons, which violated strongly the Wiemann Franz law. MoP and WP2 show exceptional properties such as high conductivity, high mobilties and a high magneto-resistance. With thermal and magnetoelectric transport experiments, a transition from a hydrodynamic electron fluid below 15 K into a conventional metallic state at higher temperatures is observed. The hydrodynamic regime is characterized by a viscosity-induced dependence of the electrical resistivity on the square of the channel width. Single crystalline NbP shows an exceptional high Nernst-effect. Even polycrystalline, spark plasma sintered samples of NbP show still a large Nernst thermopower value of ~90 µV/K. Also in magnetic samples, such as Co2MnAl and Co3Sn2S2, we can design giant Nernst effects due to an strongly enhanced Berry curvature. In general, the concept of topology might enable us to design more energy efficient materials for thermoelectric applications and beyond.

Resume : Intermetallic phases reveal typically metallic behaviour in electric and thermal transport, thus they are in pristine form not suitable for thermoelectric purposes. Only several atomic arrangements in intermetallic structures may result in a band structure with a (pseudo)gap, e.g. clathrates, compounds of the types FeGa3, TiSi2, MgAgAs (half-Heusler phases). The reduced VEC does not allow application of the chemical concepts like Zintl-Klemm model for interpretation of composition, crystal structure and (thermoelectric) properties. Quantum chemical bonding indicators in real space help in understanding of the electronic counts necessary to stabilize structural pattern and – at the same time – to result in a band structure with a gap and in a strong DOS gradient at the Fermi level. The inhomogeneity and anisotropy of atomic interactions contribute to the understanding of chemical and physical behaviours of thermoelectric materials.

Resume : Discovery of topological semimetals has motivated plethoric investigations of their electrical transport properties of the relativistic quasiparticles. However the thermoelectric responses of the topological semimetals are less investigated until recently. In this talk I will present our measurements of magneto-thermoelectric properties for several magnetic and non-magnetic topological semimetals. The nonmagnetic semimetals show strong quantum oscillations in their thermoelectric signals which can serve as a powerful tool to probe the fine structure of the Fermi surface. Moreover, the anomalous Nernst effect in magnetic semimetals shed light on their topological nature.

Resume : Chiral fermions exist in an expanding class of electronic materials that display unique transport properties, including negative longitudinal magneto resistance via chiral magnetic effect, anomalous Hall effect, and anomalous Nernst effect that may be exploited for thermomagnetic cooling. Among the contributing factors to these unique properties are linear electronic band dispersion with nontrivial topology, chirality, and Berry curvature. Interesting, a large amount of chiral materials researched today have been known for decades because of their respectable thermoelectric performance upon chemical doping. In this presentation, I will discuss those emergent properties that are the results of chiral fermions transport, and how to utilize them for thermoelectric applications. ?

Resume : We are interested in how to achieve much higher thermoelectric conversion efficiency by effectively manipulating electron-spin degree of freedom. As one possibility, we have been studying Berry-phase-mediated thermoelectric effects, namely anomalous Nernst effect (ANE), which is a heat-to-electricity conversion observed in magnetic materials and directly related to he contribution of the anomalous Hall conductivity (AHC)[1-3]. We discussed AHC mainly driven by an effective magnetic field, Berry curvature, induced by spin-orbit interaction and spin chirality in chiral magnet. We performed first-principles density functional calculations[4] for hypothetical materials, 2D Kagome lattice, 2D square lattice[1], and 3D pyrochlore lattice. We found that the spin-chirality driven Berry curvature indeed generates large ANE. This behavior was clearly understood by the chemical potential dependence of AHC. Based on the gained knowledge, we will discuss how to enhance such Berry-phase-mediated thermoelectric effects in real materials. [1] Y.P. Mizuta and F. Ishii, Sci. Rep. 6, 28076 (2016). [2] Y.P. Mizuta, H. Sawahata, and F. Ishii, Phys. Rev B, 98, 205125 (2018). [3] S. Minami, F. Ishii, Y.P. Mizuta, and M. Saito, Appl. Phys. Lett. 113, 032403 (2018). [4] T. Ozaki et al., OpenMX, http://www.openmx-square.org.

Resume : The search for viable thermoelectric (TE) materials is intensifying, with energy harvesting to power IoT sensors, for example, being an application of high interest [1]. I will report on recent developments regarding our strategy to utilize magnetism to enhance the thermoelectric properties, in particular, the power factor. First of all, we have proposed to utilize magnetic interactions between carriers and magnetic moments to enhance the power factor. This is manifested in two ways. A) Magnetic ion doping into nonmagnetic systems, has led to TE enhancement, if effective coupling like a dilute magnetic semiconductor is created. This was first demonstrated by Mn doping into CuGaTe2 [2]. I will report on other systems where enhancement was realized. B) High power factors at relatively low temperatures near room temperature, were discovered in magnetic semiconductors like CuFeS2-based materials [3] and thiospinels like CuCr2S4 and related materials [4]. We have also realized excellent p, n control in magnetic chromium selenide. As a second strategy, we have also experimentally demonstrated significant enhancement of the Seebeck coefficient via spin fluctuation [5]. As an important point, unlike magnon drag, these TE enhancements via magnetic interaction or spin fluctuation are not solely dependent on ordering, and are effective at relatively higher temperatures also. The possible compatibility with magnetic sensors and devices should also be investigated further. Support from CREST, JST is acknowledged. [1] T. Mori and S. Priya, MRS Bulletin, 43, 176 (2018), I. Petsagkourakis, et al., Sci. Tech. Adv. Mater., 19, 836 (2018). [2] A. Fahim, et al., J. Mater. Chem. A 5, 7545 (2017). [3] Appl. Phys. Express, 6, 043001 (2013), Angew. Chem. Int. Ed., 54, 12909 (2015), Mater. Today Phys., 3, 85 (2017). [4] Chem. Mater., 29, 2988 (2017), Inorg. Chem., 57, 5258 (2018). [5] N. Tsujii, et al., “Observation of enhanced thermopower due to spin-fluctuation in weak itinerant ferromagnet”, Science Advances (2019) in press.

Resume : Established guiding principles based on effective masses, carrier density, and constant relaxation time have been heuristically used for years in the search for thermoelectric materials with enhanced electronic transport properties. In spite of many simplifying assumptions, the Sommerfeld model has became a standard for thermoelectricity. The complexity of many thermoelectric materials, however, is far beyond this interpretative approach and many important effects are often neglected both theoretically and experimentally. We have developed several software tools to challenge the solution of the inverse problem: identify the electronic structure from transport data. We will discuss our approach by drawing examples from recent results obtained in the AFLOW consortium (www.aflow.org) on a variety of materials ranging from oxides to complex sulfides.

Resume : Nanocomposites, made of nano-inclusions embedded in a uniform host matrix, have recently come at the forefront of materials research for thermoelectrics, due to the drastic reduction of the thermal conductivity with almost no effect on electronic properties, as reported by a number of theoretical investigations. Such reduction can be understood in terms of phonon scattering at the interfaces between the two phases, and the nanometric dimensions of one of the phases. However, Effective Medium Approaches taking into account these factors, cannot explain some results of molecular dynamic simulations, such as the high energy phonons filtering[1,2], or even the opposite effect of a phonon percolation overcoming the effect of interface scattering and leading to a strong thermal transport enhancement[3]. Here we report our latest achievements in a fundamental understanding of thermal transport in amorphous/nanocrystalline composites. By coupling macroscopic measurements, state of the art numerical simulations and advanced synchrotron techniques in nanocomposites, we relate the changes in thermal conductivity to the modifications of phonon dynamics, which allows us to enlighten the mechanisms at play. Specifically, we show here that the presence of phase mixture at the nanoscale induces glassy-specific features in phonon dynamics, effectively reducing the phase space of efficient propagative transport. [1] A. Minnich et al., RSC Adv., 6, 105154 (2016) [2] T. Damart, V. M. Giordano and A. Tanguy, PRB, 92, 094201 (2015) [3] K. Termentzidis, V. M. Giordano et al., Nanoscale, 10, 21732 (2018)

Resume : Chalcogenide alloys, such as Sb2Te3, have a recognized high potential in several applications, including spintronics, phase change memory devices and thermoelectric conversion [1]. In all these fields, higher performances and new functionalities can be attained by using chalcogenide materials in low-dimensional forms [2], like nanowires, nanoplates and nanopillars (NPs). Developments in the ordered synthesis of these types of Sb2Te3 nanostructures are therefore a key factor for their implementation into advanced thermoelectric devices. In the present work, we show that Metalorganic Chemical Vapor Deposition (MOCVD) can be successfully used to grow arrays of Sb2Te3 NPs inside templates with very high pore density. In particular, we used Au-functionalized Anodized Aluminum Oxide (AAO) membranes with a pores density up to ~5×10^10 cm^-2, to obtain ordered arrays of Sb2Te3 NPs with diameters of 25 nm and height of 200 nm. The microstructure of the NPs and their growth mechanism was investigated by SEM, TEM-EDX, Raman spectroscopy and XRD. Further processing steps, potentially useful for device fabrication, such as mechanical and chemical etch of the AAO/NPs and liberation of the NPs from the template, were also considered. The described results could be extended to other chalcogenide alloys grown by MOCVD and to other types of nanopatterned templates. References. [1] S. Morikawa, et al., Nanotechnol. 29, 75701 (2018). [2] M. Tan, et al., Thin Solid Films 623, 116 (2017).

Resume : Hybrid halide perovskites have recently received exponential interest due to their extraordinary photovoltaic performances.[1] Apart from solar cells, multitude of other applications such as photodetectors, lasers, LEDs, and memory devices have been already demonstrated underpinning the versatile nature of halide perovskites.[2] Despite the unprecedented performance of hybrid perovskites, their other crucial physical properties such as thermal transports have received limited attention. The majority of the works on thermal transport of hybrid perovskites are accomplished using single crystals or polycrystalline pellets with limited information on seebeck coefficient.[3] Furthermore, In-plane thermal conductivity and seebeck coefficient of CH3NH3PbI3 films remains largely unexplored. Thermal transport behaviors strongly correlate with material structure and its meticulous understanding is imperative for thermal management of efficient and high performance devices. In this work, we systematically investigate the thermal conductivity and thermopower of sequential vapor deposited CH3NH3PbI3 films. An ultralow in-plane thermal conductivity of 0.3 Wm-1K-1 at room temperature was recorded for CH3NH3PbI3 using a chip-based 3ω method. Temperature-dependent thermal conductivity measurement of a series of CH3NH3PbI3 films with different degree of methylammonium treatment reveal that the thermal conductivity value is governed by PbI6 octahedron framework. Furthermore, n- and p-type CH3NH3PbI3 films were achieved by compositional tuning of the precursors (CH3NH3I and PbI2) resulting in high negative and positive thermopower. The effect of self-doping and defects on the thermopower along with the implications of the present work on future thermoelectrics based on hybrid perovskites will be discussed. References 1. M. Gratzel, Nat. Mater. 2014, 13, 838. 2. Zhao et. al., Chem. Soc. Rev.2016, 45, 655 3. Pisoni et. al., J. Phys. Chem. Lett. 2014, 5, 2488.

Resume : Thermoelectric energy harvesters can convert directly heat energy into electrical energy based on the Seebeck effect. Recently, thermoelectric energy harvesting technology has attracted great attention to realize wireless power supply for a trillion-sensor network in the forthcoming Internet of Things (IoT) society. In our previous study, we have proposed a miniaturized planar Si-nanowire thermoelectric generator (SiNW-TEG) that can be fabricated by the complementary metal-oxide-semiconductor (CMOS)-compatible technology. Compared with the conventional nanowire TEGs, this SiNW-TEG features the use of an exuded thermal field for power generation. Thus, there is no need to etch away the substrate to form suspended SiNWs, which leads to a low fabrication cost and well-protected nanowires. This SiNW-TEG is composed of several hundreds of SiNW thermoelectric elements connected electrically in series and thermally in parallel. Therefore, a thermally conductive but electrically insulating layer is necessary for the heat spreading from the heat source to the hot side of each thermoelectric element. Furthermore, the thermally conductive layer helps to form a high temperature difference across the thermoelectric elements by reducing the parasitic thermal resistance of the thermoelectric device. Aluminum nitride (AlN) has been a good candidate as the thermally conductive layer due to its high thermal conductivity. However, the thermal conductivity of AlN thin films decreases by nearly two orders of magnitude compared with its bulk counterpart. Thus, AlN thin films are not suitable as a thermally conductive layer in the miniaturized SiNW-TEG. In this study, various metal/insulator thermally conductive layers were fabricated. The insulator layer is firstly deposited on the silicon on insulator (SOI) substrate, the metal layer is subsequently deposited on the insulator layer to form the metal/insulator thermally conductive layer. Al and Cu are used as the metal layer due to their high thermal conductivity. SiO2 and AlN are used as the insulator layer. Furthermore, Ti and Cr are used as bonding layer to improve the adhesion between metal layer and insulator layer. the reference samples with no bonding layer are also fabricated for comparison. The cross-plane thermal resistances of the thermally conductive layers are measured using the frequency-domain thermoreflectance method. Transmission electron microscopy (TEM) together with energy dispersive X-ray spectroscopy (EDS) are used for the characterization of the diffusion and chemical bonding between metal layer and insulator layer. The results show that thermally conductive layers with Cu metal layer have a higher thermal resistance than their counterparts with Al metal layer. Thermally conductive layers with Cr bonding layer have a higher thermal resistance than their counterparts with Ti bonding layer. Furthermore, thermally conductive layers with AlN insulator layer have a higher thermal resistance than their counterparts with SiO2 insulator layer. The underlying physics of the thermal transport in these metal/insulator thermally conductive layers are discussed.

Resume : Bismuth telluride (Bi2Te3) is one of the most efficient thermoelectric materials which find its applications at room temperature. In earlier reports, Bi2Te3 based thermoelectric materials achieved high ZT value using nanoengineering techniques. Recently, Molybdenum Disulphide (MoS2) has also attracted much attention because of its graphene-analogous structure and high mobility. In the present study, the electrical and thermoelectric properties of 2D-3D MoS2/Bi2Te3 (n-n) heterojunctions with varying Bi2Te3 and MoS2 thicknesses have been investigated. Kelvin Probe Force Microscopy (KPFM) was carried out on bilayer samples for analyzing electrical and thermal properties of MoS2/Bi2Te3 hetrojunction. KPFM measurements based on the difference of surface potential (SP) values in surface mode and junction modes have been carried out on Bilayer junction. The 2D-3D heterojunctions with lower MoS2 thicknesses show a large difference in SP values in the two modes, which is observed to increase with a decrease in the MoS2 thickness. In comparison, samples with larger (bulk-like) MoS2 thickness show negligible SP difference value, indicating complete Fermi level alignment, as expected in a normal bulk junction. The difference in the SP value in two modes represents large surface charge accumulation in the 2D layer due to a relatively high value of the depletion width required for achieving equilibrium in comparison to the atomic scale thickness of 2D MoS2. In addition, the power factor observed in Bi2Te3 samples increases from 0.13 mV/mK2 to 70.66 mV/mK2 at the 423 K on decreasing the film thickness from 500 nm to 40 nm. The enhancement of the power factor due to the large Seebeck coefficient is observed in low thickness Bi2Te3 film. The achieved large power factor may be attributed to the topological effect of the thin film which enhanced the Seebeck coefficient value. The observed results show that low thickness Bi2Te3 thin film has unique applications as high efficiency thermoelectric materials for thermoelectric devices.

Resume : Nano-structuration of thermoelectric materials is nowadays one of the routes most explored in order to enhance their efficiency. For instance, the fabrication of nanowires where the high surface to volume ratio tends to reduce the thermal conductivity due to phonon scattering, has been thoroughly studied lately. In this work we present the results on a more complex nano-structure, that is, three-dimensional nanostructured networks based on the growth of materials inside 3D nano-porous templates [1]. In this case, three dimensional bismuth telluride nano-networks were fabricated via template assisted electrochemical deposition, as described in reference [2]. Transport properties of the 3D networks have been measured in an effort to better understand the influence of nanostructuration on the thermoelectric performance, in this particular case, of Bismuth telluride one of the best known and studied thermoelectric materials for room temperature applications.

Resume : A long lifetime of thermoelectric generators in high temperature energy converters requires the full reversibility of thermal phase transitions and redox reactions. Therefore, self-repairing materials have to be developed to produce long-lasting devices. Regenerative perovskite-type ceramics, Heusler compounds and chalcogenides as well as their nanocomposites are prospective candidates for thermoelectric energy conversion processes. Their thermoelectric performance relies on suitable band structures, adjusted charge carrier density, effective mass and - mobility, hindered phonon transport, and electron filtering potentials. The design of thermoelectrics is based on theoretical predictions, a profound knowledge on composition-structure-property relationships and the criticality analysis of used elements. The developed thermoelectric materials are characterised and tested in diverse high temperature applications to improve the efficiency and energy density of the thermoelectric conversion process.

Resume : Among sulfides, the pyrite CoS2 presents a promising and very large power factor reaching 1 mWm-1K-2, and almost constant in a large range of temperature [1]. The major drawback of this material is that the thermal conductivity is too large to achieve interesting ZT values. In this talk, we will present the recent results we have obtained on the reduction of thermal conductivity in this material. Moreover, this pyrite is a ferromagnetic metal and the magnetism can lead to interesting phenomena such as magnetothermopower in oxides or sulfides [2] or spin entropy contribution to the Seebeck coefficient in oxides such as in ruthenates [3]. In CoS2, the magnetic transition has a huge impact on the electronic and thermal transport properties, with an excess of the Seebeck coefficient appearing in the ferromagnetic state, which can be described as a ‘magnonic’ contribution [1]. The impact of magnetism will be discussed in this talk to show the strong interplay between transport and magnetism in these sulfides. [1] : S. Hébert et al., J. Appl. Phys. 114, 103703 (2013). [2] : D. Berthebaud et al., J. Appl. Phys. 124, 063905 (2018). [3] : F. Pawula et al., J. Mater. Chem. C 86, 7 (2019).

Resume : Magnetothermopower (MTEP) measurements were used in layered cobalt oxides to reveal the spin entropy contribution to the Seebeck coefficient [1,2]. In that respect, the magnetic ruthenates behave as such as revealed by S(T) measurements of SrRuO3 perovskites [3] and quadruple perovskites derived from CaCu3Ru4O12 [4]. More recently, the study of Cr substituted hollandites of Ae1.5Ru6.1Cr1.9O16 formula where Ae=Sr, Ba, were also shown to exhibit S values at high T as those of perovskites despite the localized behaviour of their electrical resistivity [5]. Such impact of the spin entropy on S is not limited to metal transition oxides but has been also observed for the first time by MTEP in the CuCrTiS4 spinel sulfide [6]. Also, these results deduced from MTEP measurements showing that spins play an important role in the Seebeck coefficient in ruthenates are supported by modelling [7]. In contrast, the recent unexpected result of a lack of MTEP in the ferrimagnetic (TC >300K) Sr2FeReO6 double perovskite [8] will allow general statements about factors governing the MTEP in these materials to be made. References: 1- Wang et al, Nature 2003, 423, 425. 2- P. Limelette et al, Phys. Rev. Lett. 2006, 97, 0046601. 3- Y. Klein et al, Phys. Rev. B 2006, 731, 052412. 4- S. Hébert, R. Daou and A. Maignan, Phys. Rev. B, 2015,91, 045106. 5- F. Pawula et al, J. Mater. Chem. C 2019, 7, 86. 6- D. Berthebaud et al, J. Appl. Phys. 2018, 124, 063905. 7- J. Mravlje et al, Phys. Rev. Lett. 2016,117, 036401. 8- A. Maignan et al, Chemical Communications (in press).

Resume : Donor-doped SrTiO3 ceramics are very promising n-type oxide thermoelectrics. We show that significant improvements in the thermoelectric Power Factor can be achieved by control of the nanostructure and microstructure by additions of B2O3 and ZrO2. High quality Sr0.9Nd0.1TiO3 ceramics were synthesised by the mixed oxide route; samples were densified with sintering directly under H2 Ar atmosphere at 1673 K. Diffraction studies revealed an I4/mcm tetragonal symmetry for all specimens. Microstructure development depended on the ZrO2 content; low level additions of ZrO2 (up to 0.3 wt%) led to a uniform grain size with transformation-induced sub-grain boundaries. Zr doping promoted atomic level homogenization and a uniform distribution of Nd and Sr in the lattice, inducing greatly enhanced carrier mobility. Transport property measurements showed a significant increase in PF, mainly resulting from the enhanced electrical conductivity while the Seebeck coefficient values were unchanged. In optimised samples, a power factor of 2.0×10-3 W m-1 K-2 was obtained at 500 K. This is a ~30% improvement compared to the highest values reported for SrTiO3 based ceramics. The highest dimensionless figure of merit (ZT) value for Sr0.9Nd0.1TiO3±δ was 0.37 at 1015 K. This paper demonstrates the critical importance of controlling the structure at the atomic level and the effectiveness of minor dopants in enhancing the thermoelectric response.

Resume : Nano-structuring is recognised as an efficient route for enhancing thermoelectric response. Here we report a new synthesis strategy for nanostructuring oxide ceramics and demonstrate its effectiveness on an important n-type thermoelectric SrTiO3. La doped SrTiO3 ceramics were synthesized by the mixed oxide route. Samples were sintered in air followed by annealing in a reducing atmosphere. Crystallographic data from X-ray and electron diffraction showed Pm3 ̅m cubic symmetry for all the samples. HRTEM showed formation of a core-shell type structures within the grains for the annealed ceramics. The cores contain nanosize inclusions. Atomic level resolution STEM-HAADF-EELS characterization in an aberration-corrected microscope showed the particles to be rich in Ti and the areas around the voids contain high concentrations of Ti3+. Additionally, dislocations were observed, with significantly higher densities in the shell areas. The observed dislocations are combined (100) and (110) edge dislocations. The major impact of the core-shell type microstructures, with nano-size inclusions, is the reduction of the thermal conductivity. La doped SrTiO3 ceramics containing grain boundary shells of size about 1 µm and inclusions in the core of 60 to 80 nm exhibit a peak power factor of 1600 micro W/m.K2 at 540 K; at 1000 K they exhibit a low thermal conductivity (2.75 W/m.K) a power factor of 1050 micro W/m.K2 leading to a high of ZT of 0.39 ± 0.03. This is the highest ZT so far for La doped SrTiO3 compositions. This nanostructuring strategy should be readily applicable to other functional oxides.

Resume : Several decades after Hicks and Dresselhaus' suggestion to nanostructure thermoelectric compounds in order to increase their figure of merit ZT, the search for a high-ZT material is still on. Though increases in ZT have, so far, resulted mainly from a suppression of the phonon-dominated thermal conductivity, it is now expected that a breakthrough should come from an enhancement of the electronic power factor. Resonant doping, which consists in creating dopant impurity states inside and hybridized with the conduction or valence band of the host material, stands out as a particularly promising pathway towards achieving such a boost of the power factor [1]. Using a minimal tight-binding model, we investigate the generic effects of resonant sbstitution doping on thermoelectric properties [2], with an exact treatment of doping disorder using the Chebyshev-Polynomial Green's Function (CPGF) method. Spectacular effects are found, such as a sign inversion of the Seebeck coefficient and a sixfold enhancement of the Power Factor. In order to find resonance effects in a specific material, we then investigate the thermoelectric properties in Vanadium-doped Strontium Titanate [3]. To simulate realistic co-doping of Lanthanum, Niobium and Yttrium, we carry out full ab-initio calculations on supercell systems. To adress the effects of disorder, including Anderson localization, we use a hybrid methodology combining DFT simulations, realistic tight-binding Hamiltonians [4] and the CPGF method. [1] S. Thébaud, Ch. Adessi, S. Pailhès, G. Bouzerar, 2017, Phys. Rev. B 96 075201 [2] S. Thébaud, Ch. Adessi, G. Bouzerar, to be submitted [3] Ch. Adessi, S. Thébaud, D. Bouzerar, G. Bouzerar, to be submitted [4] G. Bouzerar, S. Thébaud, Ch. Adessi, R. Debord, M. Apreutesei, R. Bachelet, S. Pailhès, 2017, EPL 118 67004

Resume : Polycrystalline Eu1–xCaxTiO3– (0 ≤ x ≤ 1) samples were synthesized via a soft chemistry route to investigate the interplay between crystal structure, local structural disorder, and thermoelectric properties. Because Ca2+ is smaller than Eu2+ chemical pressure exists in the perovskite structure. The Ca2+ substitution is locally modifying the crystal structure resulting in distinct differences between long-range and local scale. On local scale, a larger lattice strain is manifested by shorter but more broadly distributed Ca/Eu–Ti distances compared to the long-range scale, implying an enhanced chemical pressure by increasing Ca2+ content. The unit cell volume contraction accompanied by a reduction of the overall symmetry facilitate the accommodation of smaller Eu3+ instead of Eu2+. Hence, the electronic band structure is modified leading to a reduced band gap. The enhancement of lattice defects and the band gap reduction result in a significantly increased electron concentration at RT. The thermal conductivity is dramatically reduced (up to 50 %) compared to the pristine sample. This is most likely attributed to the mass fluctuation due to the large atomic mass difference of Eu and Ca. The average ZT value of Eu0.2Ca0.8TiO3–is increased by almost 100 % compared to pristine EuTiO3. This work demonstrates that controlling lattice deformation offers new ways to improve the thermoelectric performance of titanates.

Resume : The luminescence intensity ratio (LIR) of emission from two thermally coupled excited states is one of the most popular temperature sensing schemes, which has proven to be reliable due to its non-invasive nature, minimal dependence on the measurement conditions, and high temperature-spatial resolution. However, it requires a special design effort to obtain stable luminescence emission, which can be used for any practical application, for example, optical thermometric sensing. In this work, we present our results on the influence of excitation-emission processes on the dynamical behaviour of charges, and their temperature dependence in a wide temperature range (300-870 K), on a single crystal of EuPO4. The EuPO4 host which previously did not appear suitable for temperature sensing, was successfully converted to a highly sensitive optical temperature sensor, by following appropriate experimental strategy. The coupling of two excited states of Eu3+ showed a relative sensitivity of 2.00 %K-1, while, the coupling between two ground states of Eu3+ showed a relative sensitivity of 0.34 %K-1. The results suggest that by optimizing experimental parameters, highly sensitive optical thermometric sensors can be prepared, with ease.

Resume : Waste heat is generated in many different combustion processes as well as industrial production. The potential of waste heat recycling depends on the physical properties like temperatures, on economic considerations like return of invest as well as on the technological aspects of the waste heat recovery system like robustness and the ease of integration. Thermoelectric waste heat recovery has actually low conversion efficiencies. Nevertheless, it has demonstrated already a large numbers of advantages like robustness, being maintenancefree, small volume and lightweight. During the last years thermoelectric materials and modules have been developed which are now implemented in first prototype systems. Fraunhofer IPM has developed a semi-automated lab-scale production process for thermoelectric modules based Half-Heusler modules. These Half-Heusler modules have shown a very good robustness with respect to temperature, autoxidation as well as vibrations. Further optimizations of the modules were performed regarding lower costs and higher efficiencies. The robust modules were finally integrated in a car and tested. Besides these field tests half-Heusler based modules as well as commercial Bi2Te3 based modules for lower temperatures were integrated into heat exchangers of CHPs. In these projects complete thermoelectric generators were developed and tested in the applications. In this presentation the latest results will be shown.

Resume : Typical 18-electron half-Heusler (HH) compounds, ZrNiSn and NbFeSb, have been identified as promising high temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, has recently been reported to also exhibit thermoelectric properties of a heavily doped n-type semiconductor. In this talk we experimentally demonstrate that the nominal 19-electron NbCoSb is actually the composite of 18-electron Nb0.8+xCoSb (0 ≤ x< 0.05) and impurity phases. Single phase Nb0.8+xCoSb with intrinsic Nb vacancies, following the 18-electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ~100%. The similar phenomenon has also been observed in some of other defective 19-electron HH compounds. They all display abnormally low thermal conductivity compared to the normal 18-electron HH. The electron diffraction patterns indicate a complex and interesting crystal structure, in which the short-range order of vacancies coexists with long range atomic order. The disordered arrangement of the atoms combined with the crystalline structure makes defective HH compounds as promising thermoelectric materials with low thermal conductivity and high electrical conductivity simultaneously. This new finding provides important insights into the intrinsic nature of defective HH compounds, and also proposes a possible strategy about tuning the degree of ordering to offer great potential for further improvement of thermoelectrics.

Resume : High thermoelectric figure of merit zT of ~1.0 has been reported in both n- and p-type (Hf,Zr)CoSb-based half-Heusler compounds, and further improvement of thermoelectric performance relies on the insightful understanding of electron and phonon transport mechanisms. In this work, the thermoelectric transport features were analyzed for (Hf0.3Zr0.7)1-xNbxCoSb (x = 0.02 ~ 0.3) with a wide range of carrier concentration. It is found that, although both temperature and energy dependencies of charge transport resemble ionized impurity scattering, the grain boundary scattering is the dominant scattering mechanism near room temperature. With increasing carrier concentration and grain size, the influence of the grain boundary scattering on electron transport weakens. The dominant scattering mechanism changes from grain boundary scattering to acoustic phonon scattering as temperature rises. The lattice thermal conductivity decreases with increasing Nb doping content, due to the increased strain field fluctuations. These results provide the in-depth understanding of the transport mechanisms and the guidance for further optimizing thermoelectric properties of half-Heusler alloys and other thermoelectric systems.

Resume : Narrow band gap half-Heusler (HH) alloys are promising materials for application in thermoelectric devices. Today there is a shortage in p-type HH materials, which may have a compatibility with the HH n-type pair. Sc and Al are good candidates as doping elements for this purpose. Based on DFT calculations it is demonstrated that low level Sc or Al doping leads to p-type conductivity of material. Applying DFT and statistical thermodynamics together with experimental validation, the solubility limit of Sc in HH (Ti1-cScc)NiSn alloy is obtained < 5% below 1000K. Below the solubility limit TiNiSn-rich single phase is expected, while beyond the solubility limit a phase separation into ScNiSn- and p-type TiNiSn-rich phases is favored. For the more complicated case of Al doping combination of CALPHAD and DFT calculations is applied. The predicted solubility of Al is experimentally confirmed. Scheil solidification simulation with CALPHAD database is used to understand the as-cast phase structure. The maximal solubility of Al in the HH phase is estimated as ~1at% at 1400K. Above this composition the alloy decomposes into TiNiSn, Sn(Liquid) and NiAl(B2). These results give the basic route for designing TiNiSn p-type material.

Resume : Thermoelectric modules using p-type Ca3Co4O9 and n-type CaMnO3 have been produced. Outstanding durability against high temperature and heat cycling of thermoelectric modules has been demonstrated. No remarkable degradation of the power output has been observed in the continuous power generation test at 700℃ of hot side temperature in air. The maximum power output at 600℃ of the hot side temperature keeps almost the same value with the initial one after heat cycling between 100 ℃ and 600 ℃ of the hot side temperature. The vibration test has been carried out at room temperature. The maximum power output at 600 ℃ of the hot side temperature is not been changed by the vibration test. Portable power generation units composed of the oxide thermoelectric module as mentioned above. Water circulation are unnecessary for the units. The units can generate 2-5 W using heat energy with temperature of 300-600 ℃. Lighting, sensing, filming, and transmitting data without cables by thermoelectric conversion using waste heat from industrial furnaces, incinerators, engines of automobiles and ships will be possible. This research has been supported by Thermal Management Materials and Technology Research Association (TherMAT).

Resume : Because of its important application in waste heat recovery, thermoelectric(TE) technology has attracted more and more attention from both academia and industry. Since the 21st century, with the rapid development of new thermoelectric transport theory and new compounds, the performance of thermoelectric materials has been greatly improved, and ZT value has achieved above 2.0 in laboratory. However, the conversion efficiency of thermoelectric devices and generator systems lag behind the materials development. To brige the technical gap between material properties in the laboratory and the final power generation system for industry, our recent work focus on the scaling-up fabrication of TE materials such as skutterudites and half-heuslar, the high-throughput screening of diffusion barriers for TE devices, and optimal design of TE module and generator system. Based on the properites of the mass production materials, we realized optimal structure design of single-stage and segmented thermoelectric modules by using finite element simulation method, and achieve high conversion efficiency of 9.8% for SKD single stage module, and 12% for Bi2Te3/SKD segmented module. Further, the systematic study on service reliability of TE module in complex conditions and the final demonstration of kilo-watts thermoelectric generators system integrated with steel rolling line greatly promote the thermoelectric technology from laboratory materials to industrial applications.

Resume : Typically, thermoelectric device placed vertically on the substrate where heat flows from top to bottom or vice versa through the device. The device is easy to obtain temperature difference and high power density per unit temperature difference, but it is complicated to fabricate and less flexible. The planar-type thermoelectric device, laid their thermoelectric legs laterally on the substrates, shows high flexibility and easy fabrication which is needless of bonding process between electrode and thermoelectric legs. In this study, we fabricated a flexible thin-film thermoelectric generation that produce temperature difference created by solar absorber. 1 µm of copper (Cu) layer was first deposited via magnetron sputtering system and 100 nm of chromium (Cr) was deposited on the top of the Cu electrode, which works as diffusion barrier to prevent Cu from diffusion into thermoelectric materials (Bi, Te and Sb). The n-type of Bi2Te3 and p-type of Sb2Te3 thermoelectric films were deposited via co-evaporation techniques. Before fabrication of device, the optimization length and thickness of thermoelectric legs are conducted via Comsol Multiphysics®. Due to smallest resistance in 15 μm of thickness and 10 mm of leg length, it shows highest output power. The experimental results also demonstrated the planar-type thermoelectric power generation performance. The bending test result, controlled the bend radii from 32 to 122 mm, shows the flexibility of devices.

Resume : Radioisotope Thermoelectric Generators (RTG) have been used by NASA to reliably power spacecraft for deep space exploration for over 40 years. Current state of the practice systems are limited to device-level efficiencies of 7.5% or less and system level specific powers of 2.8 to 5.1 W/Kg. NASA’s Radioisotope Power Systems Thermoelectric Technology Development Program (TTDP) is pursuing development of more efficient thermoelectric technologies that could increase performance by a factor of 2 to 4x over these state of the practice systems. NASA’s TTDP is developing high-efficiency segmented couples/modules with the following design goals: a) system conversion efficiency ≥ 11% (≥ 60% improvement over MMRTG at BOL) and b) ≥ 6-8.5 We/kg specific power (2-3 x improvement over MMRTG), for a temperature gradient T = 800 K (TH=1273 K and TC = 473 K). We will be discussing the state of development of the aforementioned couples and the tools that we use to guide this development.

Resume : High temperature thermoelectric materials are emerging for energy efficient heat waste recovery. The bidirectional gradient heater technique has been established as a powerful tool for characterization of thermoelectric bulk materials. Its full measurement potential is, however, limited by the choice of contact materials, that are detachable solders and alloys, ensuring optimum thermal and electrical contact between the gradient heaters and the sample, respectively. Of importance is the transient degradation of the interface between the contact and thermoelectric material. Here we report on the characterization of thermoelectric model materials, including skutterudites, by applying the gradient heater technique. In particular, we measured the most important material properties, namely, the ZT value, the Seebeck coefficient, and thermal and electrical conductivity in a single temperature cycle with unprecedented accuracy and repeatability. Simultaneously we monitored the interface properties by orthogonal voltage probes. Our study is complemented by microscopic and spectroscopic techniques. We show that the bidirectional gradient heater technique combined with monitoring the interface properties is applicable for quality control on an industrial scale. Our results are therefore not only important for research on novel thermoelectric materials but highlight also critical performance parameters in practical thermoelectric generators in general.

Resume : The ocean floor has extensive natural resources including energy, solid minerals (Cu, Co, Ni, and Mn), and biogenic resources that can provide significant global economic benefits. Robotic exploration of Earth’s oceans using unmanned underwater vehicles (UUVs) poses significant power challenges due to the high specific energy and long life power sources required to support the UUV operation. Hydrothermal vents present in Earth’s ocean seabed can provide a source of high-quality thermal energy for power generation using known static or dynamic thermal-to-electric conversion systems. Underwater hydrothermal vents on Earth produce steep local thermal gradients from a fluidic thermal energy source (temperatures up to 400°C at “black smoker” vents) flowing ~2 m/sec into cold seawater (~4°C). This temperature gradient exists at reasonable depths (i.e., 3-5 km) with available thermal enthalpy >1MW. This paper discusses definition and development of key components of a thermoelectric (TE)-supercapacitor (SC)-based power system to enable hydrothermal vent science on Earth, thereby laying foundations for potential hydrothermal vent exploration on Icy Moons and in Ocean Worlds of Jupiter and Saturn moons in NASA’s potential missions roadmap. Thermal, thermoelectric and supercapacitor design tradeoff results, including TE power output and supercapacitor storage capacity, will be presented highlighting optimized design points and interdependencies in achieving system design goals.

Resume : Higher manganese silicide (HMS)-based thermoelectric (TE) materials have recently attracted renewed interest as they consist of naturally abundant and less-toxic elements. HMSs exhibit reasonable TE performance at around 800 K, but they have a serious problem to form MnSi (monosilicide) striations during the solidification. Such striations can be dissipated by the partial substitution of several atoms, e.g., V, Ge, etc. Simultaneously, typical diffraction peaks split into lower-angle sharp peaks and higher-angle broad shoulders. The shape, integrated intensity and width of both peaks reflect variety of nanostructure of HMS. By appropriately selecting the substituting elements and compositions, we have obtained relatively high power factor 2.4 mW/K2m at 800 K. This study is supported by New Energy and Industrial Technology Development Organization (NEDO), Japan.

Resume : Among the various thermoelectric materials, silicides are promising for such applications in the intermediate temperature range, due to their excellent thermoelectric properties, low-cost, abundance and non-toxicity. However, scalable synthesis is a critical issue for this type of materials due to their large difference in melting points and high vapor pressure, that limit the type of techniques that can be applied. In this work, Bi-doped Mg2Si0.6Sn0.4 material was prepared by mechanical alloying combined with hot press sintering, for optimization aiming to achieve good thermoelectric properties comparable to the promising materials prepared via solid state synthesis and also to increase the mass capabilities of the synthetic route. Our recent results on the compositional and structural analysis of materials as well as their thermoelectric and mechanical properties will be presented.

Resume : Facilities in the steel, non-ferrous and glass industries use a lot of energy, 50% of which gets lost during the production process. We focused our attention on thermoelectric materials with potential industrial applications at high temperature (above 500°C) to harvest waste heat and convert it into usable energy. It is critical to focus attention on thermoelectric materials that are non-toxic, practicable in size, vibration-tolerant and scalable for industry. Among potential materials respecting the above-mentioned requirements, the manganese silicide MnSiγ [1,2] is one of the top candidates as it is also made of cheap and abundant elements. MnSiγ and iron disilicide compounds β-FeSi2 were respectively selected as p-type and n-type candidates for the fabrication of thermoelectric legs for thermoelectric devices. Thermal stability and contact resistance between the active material and the electrodes are some of the major challenges to improve efficiency of thermoelectric devices (TE). Here we will focus our interest on the structural properties of these silicides especially at high temperature and also in comparison with the evolution of their thermoelectric properties versus temperature. References [1] Y. Miyazaki, Y. Saito, K. Hayashi, K. Yubuta, T. Kajitani, Jpn. J. Appl. Phys. 50 (2011) 035804 [2] Y. Miyazaki, D. Igarashi, K. Hayashi, T. Kajitani, K. Yubuta, Phys. Rev. B 2008, 78 (21), 214104.

Resume : Magnesium silicide (Mg2Si) has emerged as one of the more promising thermoelectric (TE) generator materials for use in automotive exhausts and combustion furnaces. The features of Mg2Si that make it an attractive material for thermoelectric applications are its light weight, the abundance of its constituent elements with little risk to the material supply, its good TE properties and its durability at mid-temperatures. To further develop the capability of Mg2Si as a TE material in order to realize a practical TE power generator, we report here on improvements made to the properties of Mg2Si. These improvements include obtaining a good ZT value, operation in air with long-term durability, finding an appropriate electrode material matched to Mg2Si with sufficiently low contact resistance, and improving the mechanical properties by incorporating second phase nanoparticles. We examined double impurity doping of Mg2Si to improve the power factor and reduce the thermal conductivity. As a result, we obtained a maximum ZT value of ~1.2, while the doped matrix was resistant to oxidation for ~5,000 hrs at 600 °C in air. This was improved with the addition of a glass coating with a thermal expansion coefficient matched to Mg2Si, which increased the oxidation resistance to ~10,000 hrs at 600 °C in air. The resistance of the ohmic contact at the interface between the Ni electrode and the Mg2Si was less than <1x10-9 ohm-m, which is sufficient for the realization of a practical thermoelectric generator. Incorporating SiC nanoparticles improved the mechanical properties, especially the fracture toughness, which was as high as ~2 MPa m1/2. The process for introducing the nanoparticles into the Mg2Si matrix and the amount introduced were designed to minimize any deterioration in the TE properties.

Resume : As it was recently established, effective and compact thermoelectric converters for solving energy saving and energy efficiency problems can be made on the basis of semiconductor nanostructures, among which thin layers of silicon nanowires (SiNWs) obtained by metal-assisted chemical etching (MACE) are especially promising. Despite a significant amount of work, the influence of porosity, the level of doping and the crystallographic orientation of nanowires on their thermoelectric characteristics are still completely unclear. Since the technique of MACE is inexpensive, flexible and scalable, it can be used to produce SiNWs with given thermoelectric characteristics over large areas and various surface geometries, including flexible media (for example, metal tapes) that can be of great practical use in the development of thermoelectric converters of a new generation that are easily adaptable to various tasks of increasing energy efficiency. According to the Nyquist diagram, it can be said that the impedance of this system decreases with temperature. The chemical etching leads to a formation of additional “resistor-capacitor” part in equivalent circuit that represents the contribution of SiNWs. In this report the nature of structural changes, thermoelectric and electron transport properties of SiNWs was studied depending on the conditions of MACE. This work was supported by the Russian Science Foundation (Grant № 17-12-01386).

Resume : Although nanoscale semiconductors have a great potential for electronic applications, generated heat during the device operation usually deteriorates a device performance. When the nanostructure is applied to the thermoelectric devices, the thermal conductivity in a figure of merits is a key parameter and should be reduced by increasing a phonon boundary scattering. We have succeeded to introduce a highly periodic Si nanopillar (Si-NP) structure embedded in Si(0.7)Ge(0.3) for thermoelectric device application. By using a bio-nanotemplate and a neutral beam etching techniques, we have fabricated a high-density uniform array of Si-NPs with a 10-nm diameter. The observed thermal conductivity measured by a 2-omega method was 4 W/mK and this is about 40 times smaller than that of bulk Si. In this study, to confirm a significant reduction of thermal conductivity of nanoscale Si from the point of view of electronic structure and carrier scattering, we applied photoluminescence (PL) and piezoelectric photo-thermal (PPT) methods, where they can detect a radiative and a non-radiative recombination of photoexcited carriers, respectively. From the PL measurements, no difference was found between Si-NP and bulk Si samples. On the other hand, the observed PPT signal intensities for Si-NP were drastically decreased. From the frequency analyses of signal intensity and its phase, we confirmed that the thermal conductivity of Si-NP structure was smaller than one-tenth of that of bulk Si.

Resume : Bringing silicon to express high thermoelectric efficiency would be a breakthrough for thermoelectricity, since Si is a low-cost geo-abundant non-toxic material that may be easily integrated in microelectronic devices. Silicon figure of merit may be increased either by decreasing the thermal conductivity of the single-crystalline material (while preserving its electrical resistivity, as in Si nanowires and nanolayers) or by increasing the power factor (PF) of polycrystals. Enhancements of the PF of nanocrystalline silicon through the precipitation of second phases have been reported over the past years. This communication provides clues on why such an effect was not previously observed in standard nanocrystalline Si films. Heavily boron-doped nanocrystalline Si thin films were analysed by transmission electron microscopy, high-resolution scanning electron microscopy, and atomic probe tomography along with Seebeck, Hall mobility and electrical conductivity measurements. Analyses could prove that PF enhancement requires extended high-temperature annealing to fully outdiffuse hydrogen, dissolved in the film upon deposition. Since H hinders B precipitation at grain boundaries, this explain why the PF enhancement has never been observed in production-scale wafers. Fully hydrogen-depleted films display colossal PFs at 350 K, ranging from 14 up to 40 mW/m2K depending on thermal processing conditions.

Resume : Silicon is the most important material in microelectronics. Due to its high abundance in the earth's crust and technological development, it is an interesting candidate for future applications with a large number of devices. Such devices are e.g. micro thermoelectric generators (µTEGs) converting the thermal energy into electrical. They can provide electrical energy to electronic components in IOT networks by "harvesting" the wasted heat from the environment. To optimize the thermoelectric properties of single crystal silicon, manipulations on nanoscale e.g. on the surface of the silicon structures are required. We present the simulation and fabrication of micro-sized platforms to analyze the thermoelectric transport properties of top-down etched crystalline silicon beams. The platforms and micro single crystalline beams are fabricated from SOI and bulk silicon wafers using state-of-the-art CMOS technology. The beams and platforms are manufactured in the same process because of the advantage of monolithic contacts. Simultaneous fabrication of many devices on one wafer helps to achieve the required homogeneity of the layers and junctions. A large number of devices allows a good statistical evaluation of the results and the indication of the measurement related accuracies. Initial measurements to determine the thermal conductivity of silicon beams of different sizes are presented. The simulations are discussed taking into account the measurement results.

Resume : Nowadays, commercially used thermoelectric materials are based on Pb-Te or Bi-Te compounds, which are toxic and expensive. Therefore, it is crucial to find new materials for thermoelectric applications. Lately, materials based on sulphur, as one of the cheapest and most common element, draw scientists’ attention due to promising performance. In this work, thermoelectric properties of thiospinel CuCr2-xTixS4 (x=1, 1.05, 1.1) have been investigated both experimentally and theoretically. The examined compounds were synthesized by a solid state method. Measurements of the electrical, thermal and structural properties of CuCr2-xTixS4 have been conducted in order to investigate its thermoelectric properties. The cubic structure with the space group Fd-3m was determined from X-ray diffraction. The electrical conductivity increases with Ti doping, while the Seebeck coefficient changes its sign with doping level. Consequently, the maximum power factor reaches 90 μW/mK^2 at 600 K for x=1. The same sample also exhibits the highest ZT value of 0.03 at 600 K, which is more than 50% enhancement as compared to pure CuTi2S4. For better understanding of the electronic properties, the density functional theory (DFT) calculations were employed. Transport properties were provided using Boltzmann’s transport theory. The DFT calculations were based on the general gradient approximation in the parametrization of Perdew-Burke-Ernzerhof for the exchange-correlation potential with Dudarev’s approach.

Resume : SrSi2 is a narrow-gap semiconductor [1]. It has values of dimensionless figure of merit ZT ranging from 0.05 to 0.09 [2, 3]. For improvement of its ZT value, it need to decrease thermal conductivity. In this study, we examine a Ba substitution effect on thermoelectric properties. We synthesized Sr1-xBaxSi2 with x (x = 0.0, 0.1 and 0.2). The arc-melted samples with x of 0.0 and 0.1 consist of a single SrSi2 phase. The sample with x of 0.2 consists of SrSi2 and BaSi2 phases. By spark plasma sintering of arc melted samples, we obtained single-phase samples with x of 0.0, 0.1 and 0.2. The ZT value of SrSi2 is 0.15 at 300 K, which is larger than that previously reported. Thermal conductivity decrease from 4.7 down to 2.9 Mm-1K-1 at 300 K when x changes from 0.0 to 0.2. As a result, ZT value increase 0.15 to 0.19 at 300 K when x changes from 0.0 to 0.2. References [1] M. Imai et al., Appl. Phys. Lett. 86, 032102 (2005). [2] K. Hashimoto et al., J. Appl. Phys. 102, 063703 (2007), [3] Y.K. Kuo et al., Front. Chem. 2, 106 (2014).

Resume : Nowadays, the research to find semiconducting materials for energy conversion applications has raised a huge interest in the scientific community. Particularly, a high number of investigations are focused to find suitable compounds for thermoelectric applications[1]. In this work, we report a detailed investigation about the deposition of 4 different Cu-Se chalcogenides films (orthorhombic CuSe, hexagonal CuSe, orthorhombic Cu2Se, cubic Cu2Se) on flexible polycarbonate substrates by using PHRMS, a novel semiconductor thin film deposition technique developed in our lab that allows the growth at room temperature (RT) and in a few minutes of many metal selenides (Ag-Se, Cu-Se, Sn-Se, …) with different compositions[2,3]. The obtained films were characterized by X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDX), micro-Raman spectrometry and scanning electron microscopy (SEM-FEG). The transport properties (Seebeck coefficient, resistivity and mobility carriers) were measured from room temperature up to 100ºC. All the four compounds investigated show p-type conductivity with different values of the Seebeck coefficient (ranging from 10V/K to 350V/K), resistivities measured at RT from 10-4 to 100 cm and a maximum power factor (PF) of the order of 0.16mW m-1 K-2. [1] M. Martín-González, O. Caballero-Calero, P. Díaz-Chao, Renew. Sustain. Energy Rev. 2013, 24, 288. [2] J. A. Perez-Taborda, L. Vera, O. Caballero-Calero, E. O. Lopez, J. J. Romero, D. G. Stroppa, F. Briones, M. Martin-Gonzalez, Adv. Mater. Technol. 2017, 2, 1700012. [3] J. A. Perez-Taborda, O. Caballero-Calero, L. Vera-Londono, F. Briones, M. Martin-Gonzalez, Adv. Energy Mater. 2018, 8, 1702024.

Resume : We demonstrated the enhancement of thermoelectric power (TEP) performance of molybdenum disulfide (MoS2) by exposure to H2 pressure up to 75 bar. MoS2, the chalcogenide semi-conducting material, has a hexagonal structure and basically shows a high TEP value. In hydrogen condition, sulfur in MoS2 detached as hydrogen sulfide, which is hydrodesulfurization process. It has been reported that the electrical conductivity of MoS2 is enhanced when it exposed to high H2 pressure. This is attributed to the generation of mid-gap state in the band-gap. This phenomenon also enhance the TEP of MoS2. In this study, we measured TEP of 2D and bulk MoS2 in high H2 pressure. As a result, TEP of 2D MoS2 was enhanced from 19.839 μV/K at vacuum to 99.526 μV/K at 10 bar of H2 pressure, and 536.753 μV/K at 20 bar. TEP of bulk MoS2 was also enhanced from 22.896 μV/K at vacuum to over 300 μV/K when exposure to high H2 pressure, and remained 260 μV/K after hydrogen was removed. Here we provide the easy way to enhance the TEP of MoS2 using exposure to molecular hydrogen gas.

Resume : In this study we demonstrate the use of elemental precursors (Cu,Sb,S) to synthesize tetrahedrite Cu12Sb4S13 using an industrial excentric vibrational mill followed by spark plasma sintering. Mechanochemical synthesis of tetrahedrite in such industrial mill lead also to the formation of covellite (CuS) and famatinite (Cu4SbS4). However, the composite product can be modified in favour of high purity tetrahedrite when the spark plasma sintering treatment is applied after milling. The as-synthesized product is composed of nanosized particles. The thermoelectric measurements performed for the optimal sample (milling time of 60 min) reveal a figure-of-merit value zT of 0.67@700K, as a consequence of relatively high power factor (1.07 mW K-2 m-1) and low thermal conductivity (1.12 W m-1 K-1). The obtained zT values for samples synthesized in an industrial mill are comparable to the ones obtained in samples prepared by laboratory mills. The synthesis of ternary sulfides by a scalable and industrializable milling process represents a prospective route for mass production of termoelectric materials.

Resume : Nowadays, the depletion of fossil fuels and the increase in mobile electronic devices require the development of renewable energy. Among them, thermoelectric (TE) devices offer an alternative way to generate electricity from waste heat and have many advantages such as eco-friend and cost-efficiency. The performance of the TE device is evaluated by the figure of merit (ZT) which is expressed as ZT = S^2σT/κ where S, σ, and κ is the Seebeck coefficient, electrical and thermal conductivity, respectively. In order to achieve a high ZT, TE devices exhibit high electrical conductivity and Seebeck coefficient, and low thermal conductivity. Extensive researches have been exploited on conducting polymers towards TE practical applications because of the improvement in the electrical conductivity and the intrinsic low thermal conductivity. In this study, we introduced the printing process (inkjet printing) for flexible poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) TE devices on the plastic substrate. The systematic study with processing condition and the relating mechanism and TE properties are addressed. The details will be explained in the presentation.

Resume : Conducting polymers have demonstrated technological potentiality in the field of plastic electronics and are also promising materials in renewable energy harvesting technology due to their thermoelectric properties. Indeed, their specific structures make them good electrical and poor thermal conductors, which is the perfect combination to compete with inorganic thermoelectric materials in the rush to high figure of merit (ZT). Previous studies already showed that the control of the doping level is playing an important role. In this work, we will focus on poly(3,4-ethylenedioxythiophene) as reference polymer and control its doping level by using a super-undoping molecule allowing to prepare the fully undoped polymer. The thin films have been characterized by UV-Vis and IR spectroscopy as well by X-ray diffraction and microscopy. The electrical and thermoelectric transport properties have been measured with a PPMS in thin films as a function of the temperature and have displayed strong doping level dependences. Finally, we propose correlations between the electronic doping, the morphology and the thermoelectric properties allowing a better understanding of the thermoelectric parameters. Keywords : thermoelectricity, PEDOT-Tos, transport properties, thin film, super-undopant

Resume : Thermal transport is an important consideration in many applications from thermoelectrics to optoelectronics to power electronics. Thermal transport by phonons is quantified by the lattice thermal conductivity (kL) - a key transport property, especially of semiconductors and insulators. In anisotropic materials such as those with layered motifs, kL can vary significantly with direction of heat transport. For instance, in-plane kL is 50x higher than cross-plane kL in MoS2. Traditional computational approaches for predicting kL involve computationally-intensive calculations (phonon dispersion, third-order force constants). We present a new semi-empirical model to predict the direction-dependent kL. The inputs to the model are obtained from simple first-principles DFT calculations. We have validated the model extensively against experimental measurements of anisotropic kL in single-crystal materials. The model is especially attractive because it predicts the 3D direction-dependent kL within an average factor difference of 1.8 compared to experimental measurements (across 5 orders of magnitude in kL) while still maintaining computational tractability. We demonstrate the utility of this model by performing a large-scale study of the kL anisotropy in 2261 layered materials. This exploratory study has revealed many layered materials with unique anisotropic thermal transport properties. The database of computed anisotropic lattice thermal conductivity is publicly available (as part of TEDesignLab). The database also offers interactive web-based tools to visualize the direction-dependent kL.

Resume : The best commercial thermoelectric materials for applications near room temperature are still bismuth telluride-based alloys and according to recent reported results, it is worthwhile to “revisit” this material system. The aim of this work is to optimize the thermoelectric properties of bismuth telluride-based materials by tuning both the microstructure and the carrier concentration. To this end, p-type Bi0.3Sb1.7Te3 bulk materials have been prepared by different methods: melting and mechanical alloying. Powders prepared by hand grinding and ball milling were compacted into high density pellets by hot pressing. The temperature dependence of all thermoelectric properties (electrical conductivity, thermal conductivity and Seebeck coefficient) was measured along the same in-plane direction and reliable thermoelectric power factor and dimensionless figure of merit (ZT) values were calculated. In this poster presentation, our recent results on electrical resistivity, Seebeck coefficient, thermal conductivity, Hall carrier concentration, and ZT will be presented as a function of temperature. Also, it will be discussed how thermoelectric properties are affected by the grain size and will be demonstrated that high ZT values can be achieved in nanostructured bulk materials, suggesting that Bi0.3Sb1.7Te3 alloy is a promising candidate for future industrial applications.

Resume : Catalysts affect processes of chemical reactions. Catalysis provide alternative reaction mechanism with a lower activation energy than non-catalyzed mechanism. Therefore, catalysts are utilized to make petrochemical feedstocks which are the basic raw materials for the manufacture of plastics, synthetic rubber and polyester fiber. Moreover, Automobile industry have used a catalytic converter which converts toxic pollutant (NOx, CO, HC) into less-toxic pollutant (CO2, N2, H2O) by the catalyzed redox reactions. Typically, the catalytic converter have been produced by palladium(Pd)-based alloy catalysts, which are alloyed with Silver(Ag) for increasing a redox reaction. The deactivated Pd-based alloy catalysts are discarded but the natural resources of Pd and Ag are limited. The Ag metal from Pd catalyst wastes can be separated by precipitation or solvent extraction. Ag is a noble metal which has high electronical performances. Therefore, the separated Ag from the leach solution can be utilized as electrodes for various electric devices including thermoelectric devices (TEDs). The Flexible TEDs (f-TEDs) made with Bi2Te3-based materials can be used to generate electricity at low temperature range, whose applications can be wearable electronics. In order to realize f-TEDs the flexible electrodes should be also implemented in the devices, for which the Ag paste can be used. We have studied fabrication of the silver pastes in which Ag was recovered from leaching solutions of catalyst wastes. Ag in the Pd-based catalyst wastes was leached by the pH-controlled liquid. And then, the silver particles were synthesized by solvent reduction method and the shape of the Ag particles was controlled by various surfactants. We evaluated the electrical resistivity of the paste obtained within various fabrication procedures and then the procedures were optimized.

Resume : The thermoelectric measurement of nanomaterials, such as nanowires and nanoribbons, are challenging due to their dimensions and low signal-to-noise ratio. To tackle this, we developed a microfabricated measurement platform enabling a full-theromelectric characterization. In this work, we evaluated the uncertainty of ZT measurement for the microdevice technique. We measured the Seebeck coefficient, electrical conductivity, and thermal conductivity of Bi2Te3 nanowires. The measurement uncertainty was evaluated based on the law of uncertainty propagation. As a result, the uncertainty of ZT was determined to be ~30 % at 300 K.

Resume : Despite many efforts to exploit phenomena occurring in nanomaterials, their practical uses remain limited. Here, we propose an industrially applicable inorganic-polymer nanowire with a backbone exclusively composed of copper and chlorine atoms. Our synthetic approach facilitates the mass production of the nanowires because the simple mixing of inexpensive chemicals at room temperature completes the overall reaction. Due to the polymeric nature of the nanowires, these materials give rise to unusual phenomena that have never been observed. The nanowires absorb a considerable amount of various atoms and molecules at the atomic scale. Moreover, these nanowires are amorphous and can readily be crystallised into different crystalline structures depending on the external energy sources. In this manuscript, we use the scalable, harmless, and cost-effective nanowires as a thermoelectric material.

Resume : The development of energy-harvesting applications for high temperatures based on the tetrahedrite mineral offer great promise. Naturally occurring tetrahedrite series consists of earth-abundant and relatively non-toxic elements and can be generically expressed as Cu6[Cu4(Fe,Zn)2]Sb4S13. Besides that, tetrahedrites show p-type semiconductor material behavior with high Seebeck coefficient, a complex cubic crystal structure and extremely low thermal conductivities at moderate temperatures, reaching zT~0.7 around 700K after adequate doping. Owing to these properties they are considered as a suitable and promising thermoelectric material. However, the development of cost-effective and high yield sustainable technologies to produce synthetic tetrahedrite is still a major and relevant issue. In this work, we report a fast-solid‐state synthesis method of Cu12-x-yZnxFeySb4S13-z tetrahedrites, with x, y ≤ 1 and z ≤ 0.3, based on powder sintering. Key concepts in the proposed research methodology are the direct synthesis of nanocrystalline tetrahedrites with a fast-high energy milling step and maintaining the nanostructure during the subsequent densification step through hot pressing in order to produce thermoelectric components consisting of nanocrystalline tetrahedrite. A comprehensive characterization will be presented including chemical, structural, microstructural and thermoelectrical properties. The thermal stability of the processed tetrahedrites materials will also be reported.

Resume : Thermoelectric materials that can directly convert waste heat into electricity are attracting much attention as a solution for sustainable energy. Recently, nanostructured materials have proven to be effective in enhancement of the energy conversion efficiency of thermoelectric materials. Antimony telluride is a well-known p-type semiconductor material and considered to be one of the best candidates for thermoelectric applications. As interest in nanostructured materials has increased, methods for synthesizing various types of antimony telluride nanomaterials have been developed. However, compared with the synthesis of Bi-Te, a well-known n-type thermoelectric material, there is a lack of research on shape and size changes according to synthesis conditions. We report the synthesis of the Sb2Te3 hexagonal nanoplates via a facile solvothermal process, along with their morphology changes according to the synthesis conditions. The amount of ligand, reaction pH, and precursor species were varied, and the resulting Sb2Te3 nanoplates were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Furthermore, the synthesized nanoplates were sintered to form bulk pellets, and their thermoelectric properties were measured. It was confirmed that the thermal conductivity was greatly reduced due to the low density. These results suggest that controllable synthesis of Sb2Te3 nanoplates and thermoelectric properties is feasible, and this methodology can be applied to other materials with a layered structure.

Resume : Thermoelectric materials, capable of directly and reversibly converting thermal energy into electricity, present a promising direction for renewable energy generation. The effectiveness of a thermoelectric material is measured using the dimensionless figure of merit ZT, with a good thermoelectric material presenting a value of ZT ~ 1.5. Efficient thermoelectrics typically possess high electrical conductivity and low thermal conductivity, with the ZT of a material often being limited by the strong correlation between these properties. Layered and porous materials with complex crystal structures have recently been explored in greater depth, with the aim of exploiting the interesting structural properties to yield low thermal conductivity whilst maintaining high electrical conductivity. Despite extensive efforts over the past 50 years’ to find promising thermoelectrics containing earth abundant and non-toxic elements, bismuth telluride (Bi2Te3) and lead chalcogenides remain the champion materials. Oxide-based materials present valuable properties for thermoelectric applications, due to their earth abundance, low cost and chemical stability. However, oxides have failed to compete with the traditional thermoelectric materials thus far, largely due to the materials’ exhibiting high lattice thermal conductivity. In this work, we present a study of novel mixed-anion thermoelectric materials identified using rational chemical design principles.

Resume : Among the different thermoelectric materials, bismuth telluride stands out as one of the most used for room temperature applications. In the last years, there have been many efforts to nano-structure this material in order to enhance its thermoelectric properties, in the form of thin films [1], nanowires [2], 3D nano-structures [3], etc. In this work we present a route to obtain nano-structured bismuth telluride in the form of spongy material, via template assisted electrochemical deposition. Then, the main properties of this nano-structured spongy bismuth telluride structure will be studied from the morphological, crystallographical and transport point of view. [1] Olga Caballero-Calero, Diana-Andra Borca-Tasciuc, Rut Martínez-Moro, András Gorog, Melissa Mohner, Theodorian Borca-Tasciuc, Marisol Martín-González, Elecrtrochimica Acta, 269, 490-498 (2018). [2] Miguel Muñoz Rojo, Stéphane Grauby, J-M Rampnoux, Olga Caballero-Calero, Marisol Martín-González, Stefan Dilhaire, Journal of Applied Physics, 113 (5), 054308 (2013). [3] Alejandra Ruiz-Clavijo, Olga Caballero-Calero, Marisol Martín-González, Nanomaterials 8 (5), 345 (2018).

Resume : Although a number of high thermoelectric (TE) performance materials such as Te-based, Se-based and intermetallic compounds have been developed so far, most of them are suffered from toxicity, rarity in earth and high cost to find universal application. Moreover, they are limited in their ability to harvest electricity from solar, automotive and industrial waste heat, due to their decomposition and volatilization at elevated temperatures. As a result, new, nontoxic, more stable, naturally abundant, light-weight and low-cost materials based on oxides have to be developed despite that oxides are of low carrier mobility and high thermal conductivity in general. Thus, p-type oxides based on cobaltates have been found very promising especially in high temperature range. To date, n-type oxides with equivalent TE figure of merit ZT have yet to be discovered to produce effective p-n TE modules. With this work we have proven that the microstructural engineering of SrTiO3-based materials by powder morphology and sintering cycle design in conventionally sintered ceramics can result in an enhanced ZT TE oxide of n-type in addition to grain nano-structuring, lattice defect and grain boundary engineering approaches. SrTi0.95Nb0.05O3±δ ceramics produced using powders with average particle size of ~760 nm by the conventional sintering in air at 1450 °C and further in H2/N2 at 1400 °C present relative density of 89.8% only and average grain size of 1.1 µm, whereas those prepared from powders with particle size of ~320 nm reveal the 95.5% density and grains growing to 2.2 µm average size. The origin of such difference is related to the fact that the densification and grain growth temperature decreases for the smaller powder particle size. As result, ZT value is increased from 0.01 for conventionally prepared coarse-particle-size derived ceramics to 0.12 at 970 K for the fine-size-powder ceramics. Thus, the results here obtained on donor-doped SrTiO3 ceramics contribute to the fields of high-temperature thermoelectrics for the automotive and manufacturing energy-harvesting sectors and enhanced performance of n-type oxides.

Resume : We present a systematic first-principles study of the structural, magnetic and optical properties of perovskite-structure EuTiO3. This compound exists in different structures: cubic, tetragonal and presents multiferroic properties. Comparing the formation energy between tetragonal and cubic structures, the system has a tendency to symmetry lowering structural deformations composed of rotations of the oxygen octahedral, especially the I4/mcm phase is the most stable structure. Our calculations of the high symmetry cubic structural prototype show an antiferromagnetic order type G. We discuss the dynamical stability of Pm-3m, P4mm and I4-mcm structures, and the influence of some parameters on the magnetic coupling and the electrical polarization.

Resume : Conductive polyaniline (PANI) is incorporated with porous tin sulfide nanosheets (pSnS NSs) to achieve a highperformance thermoelectric material. The SnS NSs are chemically exfoliated from an SnS ingot, thus producing two-dimensional thin structures with sizes from 200 to 500 nm. Numerous pores are introduced into the SnS NSs via solution-phase transformation with the aid of tartaric acid. Dodecylbenzenesulfonic acid-doped PANI is coated onto the surface of the pSnS NSs, resulting in the fabrication of an organic/inorganic hybrid thermoelectric material. The thermoelectric power factor (σ·S2) of the PANI-coated pSnS NSs (PANI-pSnS NSs) is optimized by controlling the number of PANI coatings, and the thermoelectric properties of the PANI-pSnS NSs are investigated and discussed as a function of the carrier transport properties. The PANI-pSnS NS sample coated twice with PANI produces an outstanding ZT of 0.078 at 450 K, which is significantly higher than that of the pristine pSnS NSs.

Resume : The lattice thermal conductance of DNA like systems made by base-pair that follows periodic and aperiodic sequences are addressed within the Boltzmann formalism through the transference matrix method by using a 2D coarse grain model of two sites per nucleotide plus Born interaction potential [1]. Such coarse grain model, consists of one site representing the nucleobase and the other symbolizing the sugar-phosphate molecule. The Born potential is used to describe the interaction between grains by central and non-central restoring force constants, which were calculated from DNA longitudinal and transversal sound velocities estimated by the inelastic X-ray scattering. The aperiodicity is introduced through Fibonacci, Thue-Morse and double-period sequences, while the transference matrix method allows introduce them in natural way and address systems of macroscopic length through a renormalization process. The results reveal that the lattice thermal conductance of aperiodic DNA sequences decreases in comparison to periodic ones, due to the long-range disorder gradually blocks the phonon transference, even the low-frequency acoustic phonons, which are insensitive to local defects [2]. The results gives the possibility to implement DNA systems as good thermoelectric materials. [1] J. E. González, V. Sánchez, and C. Wang, MRS Communications 8, 248-256 (2018). [2] J. E. González, V. Sánchez y C. Wang, Journal of Electronic Materials 46, 2724-2736 (2017).

Resume : Since their re-discovery for thermoelectric applications in the early 1990’s, skutterudite materials have attracted much attention worldwide both from a fundamental research point of view and a technology/application point of view. The variety of chemical and physical characteristics of skutterudites offers an array of options to control their electronic and phonon transport properties which make them attractive thermoelectric materials. A very large number of studies have been dedicated to improving their thermoelectric figure of merit, ZT, and much progress has been made and ZT above unity have been consistently achieved. Much attention has also been given to the integration of skutterudite materials into thermoelectric modules and generators. This includes the development of stable metallization and interconnects as well as ways to minimize the inevitable degradation expected when operating at elevated temperatures including sublimation and thermal-mechanical degradation. This papers presents a brief historical overview of the key fundamental and technological development for skutterudite materials since the 1990’s and discusses some of the key remaining challenges to large-scale use of skutterudite for thermoelectric applications.

Resume : Entropy is a thermodynamic quantity representing the degree of disorder in a system. A closely related macroscopic parameter is the thermal conductivity that directly measures the entropy flow and creation. However, it remains a daunting task to visualize and quantify the configurational entropy at the atomic scale and correlate it with macroscopic heat transport properties. We herein, by a concerted effort of aberration-corrected scanning transmission electron microscopy integrated with newly developed atomic column intensity analysis technique, for the first time, quantify the atomic resolved configurational entropy and nano, mesoscopic fluctuations in single crystalline GeSb2Te4 phase-change material, which afforded such a hierarchical entropy’s perspective to its intriguing lattice thermal conductivity. It is uncovered that the increasing configurational entropy promoted anharmonicity, yielding reduced phonon mean free path and phonon lifetime and thus an ultralow lattice thermal conductivity as well as high thermoelectric performance. These results pave a route of high spatially resolved visualizing and quantifying configurational entropy for designing high entropy alloys and materials at atomic level.

Resume : In thermoelectric materials, both electrical conductivity σ and Seebeck coefficient S are direct dependences of carrier concentration, and strongly correlated with each other to generally limit the power factor S^2 σ. By the presented study, we report that a metallic copper selenide, Cu2Se, shows two distinct sign reversals, colossal and many times reproduced values of S even beyond ±2 mVK-1 in a restrict temperature range, 340 K < T < 400 K, where an order-disorder structure phase transition takes place. This phenomenon is only observed, when the temperature gradient is simultaneously applied to both parallel and perpendicular directions of measurement, never been observed before. The metallic behavior of σ, having values up to 600 Scm-1, ensure unprecedented values of S^2 σ to be as high as 2.3 Wm–1K–2. By combining the staged outcome of S^2 σ, and very low thermal conductivity, being measured to be less than 1.8 Wm-1K-1, the estimated ZT value exceeds extreme values of 450, totally unexpectedly obtained being the highest ever achieved. This strongly unusual behavior together with the mentioned extreme values of both S^2 σ and ZT, was presumably brought about by the self-tuning carrier concentration effects in the low-temperature phase which presence were triggered and/or assisted by the high-temperature phase as situated beneath the low-temperature phase.

Resume : GeSbTe (GST)-based compounds, the pseudo-binary alloys (intermetallic phases) of GeTe and Sb2Te3, are state-of-the-art phase-change materials (PCMs). The stable phases of GST compounds adopt a quasi-two-dimensional trigonal or hexagonal structure with high electrical conductivity and low thermal conductivity that are also favorable to thermoelectric application. Nonetheless, studies on thermoelectric properties of GST compounds are scarce and incomplete. In this presentation, I will report our recent work on these materials. Firstly, we fabricated polycrystalline bulks of three typical compounds Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7, and systematically studied their electrical and thermal transport properties. Interestingly, a large anisotropy in Seebeck coefficient was observed, which is uncommon in thermoelectric polycrystals. Combining experimental work and calculations, we ascribed this phenomenon to the asymmetry of the band structure rather than the commonly conceived scattering mechanisms. Furthermore, alloying and doping strategies were employed in Ge2Sb2Te5 to optimize the excessively high carrier concentration. Possible variations in band structure were suggested and analyzed. Due to the enhanced electrical performance and suppressed thermal conductivity, maximum zT values of 0.73 were obtained at 800 K, suggesting the potential of GST compounds as promising mid-temperature thermoelectric materials.

Resume : The increasing energy demand from the rapid economic development and technological innovation highlights the importance to seek and use eco-friendly energy sources rather than conventional carbon based sources. Despite greener energy technologies such as photovoltaics, aeolian and geothermal being established as alternative energy sources in recent years, the quest to render fossil fuel combustion more efficient by recovering otherwise dissipated waste heat remains. Thermoelectric generators hold significant promise to recover waste heat, but in order to efficiently recover low temperature waste heat, it is paramount to develop new thermoelectric materials. Due to the so-called Seebeck effect, a charge density gradient will be established between an n-type and p-type semiconductor, generating an electric voltage when a temperature differential across a the materials is applied. It is still challenging to recover low-temperature waste heat via thermoelectric generators, mainly because current technologies are primarily based on inorganic semiconductors including rare, often toxic elements (i.e. Bi, Te, Sb, Pb …). Consequently, the resulting thermoelectric modules are rather expensive and can only be efficiently operated at higher temperatures. To meet the requirement of large-scale manufacture and wide application, it is crucial to develop new thermoelectric materials to harvest low-temperature waste heat efficiently. Ideal thermoelectric materials should simultaneously possess both a high electrical conductivity and a low thermal conductivity, while finding materials fuifil both requirements is very much difficult. Organometallic coordination polymers are an intriguing class of materials for thermoelectric using as they have shown promising electrical conductivities and low thermal conductivities. This presentation focuses on several synthetic methods giving different organometallic coordination polymers with different geometries. Apart from the study on the effect of the molecular geometry on the thermoelectric properties, the relevant optimization and the effects of the coordinated metal cations will also be investigated.

Resume : Recently, hybrid 2D inorganic/organic materials have attracted a lot of scientific attention as they are potentially good candidates for flexible, wearable or maintenance-free power sources for IoT applications.1,2 Moreover, these hybrid materials are produced by low-cost and scalable processes such as printing. In this paper, IMRA Europe is going to present thermoelectric properties of thin films printed from inks that contain exfoliated TiS2 and amines. The ink formulation process consists on the intercalation of amines into TiS2 followed by sonication and centrifugation steps. The influence of various ink formulation parameters on the n-type thermoelectric properties are going to be discussed. We will demonstrate that depending on such parameters the thermoelectric power factor of the printed TiS2/Amines thin films can be enhanced from about 200 µW.K-2.m-1 up to 1000 µW.K-2.m-1 in the temperature range from 60°C to 160°C. 1. R. Tian, C. Wan, Y. Wang, Q. Wei, T. Ishida, A. Yamamoto, A. Tsuruta, W. Shin, S. Li, and K. Koumoto, “A Solution-Processed TiS2/Organic Hybrid Superlattice Film towards Flexible Thermoelectric Devices.” Journal of Materials Chemistry A 5, n°. 2 (2017), 564–70. 2. L. Wang, Z. Zhang, L. Geng, T. Yuan, Y. Liu, J. Guo, L. Fang, J. Qiu, and S. Wang, “Solution-Printable Fullerene/TiS2 Organic/Inorganic Hybrids for High-Performance Flexible n-Type Thermoelectrics.” Energy & Environmental Science 11, n°. 5 (2018), 1307–17.

Resume : Nowadays, the requirements of sustainability and large-scale production makes organic thermoelectric generators (OTEG) gaining high attention within the thermoelectric community as based for a new class of modules. In fact, OTEGs not only represent the alternative to the present metal-compound thermoelectric devices, but also fulfil prerequisites that the present technology does not satisfy, allowing the fabrication of flexible and lightweight modules implementable where adaptability to curved and irregular surfaces, and to surfaces changing in motion is needed. Moreover, flexibility results in easier adaptability of the devices, and therefore lower costs of integration. This project has the objective to fabricate thin-film OTEGs based on conjugated polymers by inkjet printing method. This manufacturing non-contact technique is suitable for low-cost and large-area fabrication of flexible organic devices. Here, we report the state-of-art of our research on both the screening and assessment of efficient thermoelectric organic materials, and on the first OTEGs proof-of-concept. The materials and the devices are tested in homemade custom setups for the thermoelectric measurements. . Moreover, in collaboration with the Smart Materials Lab (@IIT Genova), cotton fiber based materials were functionalized using both carbon nanofibers and graphene nanoplatelets. Here, we report the measurements of the thermoelectric properties of these new interesting materials as a function of different parameters.

Resume : Heat flux and temperature are key parameters to measure in order to regulate any physical, chemical, and biological processes. New emerging technologies, e.g. electronic-skin and interactive buildings, are in the need of accurate temperature or heat flux sensors, that are also mechanically flexible and easily projected onto large areas. Thermopiles can provide accurate and stable heat flux and temperature reading but the technology today is based on inorganic materials that have low Seebeck coefficient (100 µV K–1) and are brittle and difficult to scale up to large areas. Recently, polymer electrolytes have been proposed for thermoelectric devices because of their giant ionic Seebeck coefficient, high flexibility and ease of manufacturing. [1, 2] However, only “p-type” (positive Seebeck coefficient) polymer electrolytes are typically reported, while “n-type” (negative Seebeck coefficient) polymer electrolytes lag behind, hampering the design of ultra-sensitive ionic thermopiles. Here we report a solid “ambipolar” ionic liquid polymer gel showing a giant negative ionic Seebeck coefficient (-4,000 µV K–1). The Seebeck coefficient can be tuned from negative to positive (up to +14,000 µV K–1) by adjusting the polymer matrix composition. Nuclear magnetic resonance spectroscopy, Raman and infrared spectroscopy analysis shows that the interaction between the ion and the polymer is the key factor in controlling the sign and magnitude of the ionic Seebeck coefficient. We demonstrate the first ionic thermopile by connecting 18 pair of legs, the module showing a Seebeck coefficient of 0.333 V K–1. The ambipolar ionic gel can be easily screen printed, enabling low-cost device manufacturing for large-area applications. 1. Zhao, D., Wang, H., Khan, Z. U., Chen, J. C., Gabrielsson, R., Jonsson, M. P., Berggren, M., Crispin, X., Ionic thermoelectric supercapacitor, Energy Environ. Sci., 9, 1450-1457 (2016). 2. Zhao, D., Fabiano, S., Berggren, M., Crispin, X., Ionic thermoelectric gating organic transistors, Nat. Commun., 8, 14214 (2017).

Resume : Despite a recent surge of interest in organic thermoelectric materials owing to their intrinsically low thermal conductivity, the community has had difficulties in formulating the charge transport mechanism in the presence of a significant degree of disorder. In this study, we investigate the thermoelectric response of various conducting polymers doped by an efficient doping method based on solid-state diffusion of dopant molecules that yielded favorable charge transport properties with one of the highest Hall mobilities for conducting polymer [1]. A fine control of the degree of doping via post-doping annealing provides an accurate empirical evidence of a strong energy dependence of the carrier mobility in the conducting polymers [2]. Furthermore, we show that the doping by solid-state diffusion can achieve a superior thermoelectric power factor than a conventional solution-doping due to their higher intrinsic mobility and higher free carrier concentration. Our results are highly relevant for developing thermoelectric power generation technology based on low-cost organic materials. Reference 1. K. Kang et al. Nat. Mater. 15, 896 (2016). 2. K. Kang et al. manuscript under revision.

Resume : The assessment of thermoelectric thin films requires precise and reliable techniques for the measurement of the in-plane thermal conductivity κ. Although several approaches have been proposed in literature, almost all of them rely on the possibility of suspending thin films by removing or anyway thinning the substrate over relatively large areas. This may be especially critical for strained thin films (e.g. poly and nanocrystalline Si) that may not tolerate the micromechanical processing. An alternate technique was developed and tested for films grown on oxidized silicon. It uses two metal tracks that are placed on top of the film. A polymer layer, deposited by spinning, holds the film together and enables the opening of micrometric windows to laterally etch the oxide. This fully avoids thermal shunts by the substrate while requiring the film to remain suspended by just a few microns, remarkably reducing problems of mechanical stability. The heat produced by Joule heating one track diffuses along the plane of the film. The temperature reached by the second metal track is found to depend only on the distance between the tracks and on the thermal conductivity of the film. Thus, no information about the dependence of the electrical conductivity of the film on the temperature is needed either. The technique was validated on single-crystalline Si thin film and then used to measure κ on nanocrystalline Si films.