Inorganic thermoelectrics - linking material properties and systems engineering for XXI century applications

Resume : Thermoelectrics offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand. Nanoengineering existing low-dimensional thermoelectric materials pertaining to realizing fundamentally low thermal conductivity has emerged as an efficient route to achieve high energy conversion performance for advanced thermoelectrics. The highest ZT of silicon-based structures at room temperature is found to be around 0.6 experimentally, which is about 100 times larger than its bulk counterpart. This significant improvement of ZT of the silicon-based structures is found to be mainly related to decrease of the phonon thermal conductivity. Although the thermal conductivity of these structures is quite low, is still much larger than its amorphous limit. Meanwhile, silicon nanowires (NWs) and membranes (MBs) are two of the most popular candidates for obtaining the high figure of merit (ZT), which can be manufactured in experiments already. Here, by performing Green-Kubo equilibrium molecular dynamics simulations and first principle calculations, we report that the thermal conductivity of Si NWs and MBs can go well below their amorphous limit by nanostructuring: introducing grain boundaries (GBs) in the structures, which are called polycrystalline form here. For the Si NWs in polycrystalline form, their thermal conductivity are found to reach a record low value substantially below the Casimir limit, a theory of diffusive boundary limit that regards the direction-averaged mean free path is limited by the characteristic size of the nanostructures. Such a low value is even only about 1/3 of the value of the purely amorphous Si NW at room temperature. For the polycrystalline Si-based MBs, the lowest thermal conductivity is only 1/2 of its amorphous limit as well. By examining the mode level phonon behaviors including phonon group velocities, lifetime etc., we identify the mechanism of breaking the Casimir limit as the strong localization of the middle and high frequency phonon modes, which leads to a prominent decrease of effective mean free path of the heat carriers including both propagons and diffusons for the structures with small grains, while for the structures with large grains, phonon-grain boundary scattering and phonon-twin scattering are corresponding to the significant decrease of thermal conductivity. Using such a strategy, we can achieve an extremely low thermal conductivity (only 1/3 of its amorphous limit) of Si NWs and MBs for structures with grain size of about 7.5 nm, which can be fabricated quite easily in experiments. Assuming the power factor as a constant, which may be achieved through doping and electron confinement, the ZT of the structures reported here can reach a value above 3 quite easily, which is thought as the value for industrial applications. Our investigation provides a deep insight into the thermal transport in polycrystalline structures and offers a promising strategy to construct thermoelectric materials with high figure of merit.

Resume : Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be infuenced by alloying and nano-structuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructive character of the phase transition that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects are emergent: Lattice thermal conductivity is signicantly lowered with respect to the perfect crystal, and the temperature influence on thermal conductivity is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design. This way the paradigm of domain boundary engineering becomes a transferable principle for improved energy conversionmaterials, combining different and often contrasting property requirements over different lenght scales. On the electric transport, 2D layered materials represents idela candidate for tuning electronic and affecting electric properties. Tuning layer interspaces in 2D materials causes narro-gap semiconductor to transform into bad metals, with a rich variety of properties. Coupled with polymorphic transitions, a overall concept of material (re-)design for a different class of thermoelectric materials is emerging.

Resume : Thermoelectric (TE) materials, which can directly and reversibly convert heat to electricity, provide a promising waste-heat-recovery solution to improve energy sustainability. Despite vast theoretical and experimental efforts, physical mechanisms of thermal transport in some promising high efficiency TE materials, e.g. ordered-disordered systems, have still not been well understood. As one typical ionic crystal with ordered-disordered phase at high temperature, Ag2Te presents an extremely low thermal conductivity. In this talk, we present our investigation of the diffusion properties of Ag2Te in the high temperature range (500K – 1000K) and illustrate the unique lattice dynamics feature of highly disordered cations with liquid-like mobility around the anion sublattice using equilibrium ab initio molecular dynamics (AIMD). These AIMD results and our structural first-principles study demonstrate a high occupancy of the tetrahedral sites for mobile Ag+ and the possible pathways of Ag+ hopping among different interstitial sites. We also predict the lattice thermal conductivities of Ag2Te with non-equilibrium AIMD and discuss how the Ag+ mobility affects the lattice thermal transfer. Furthermore, we calculate dynamical and thermal properties of the similar system Cu2Se for comparison. Our study is expected to advance the understanding of thermal transport in broad ordered-disordered materials and improve the TE performance by reducing the lattice thermal conductivity.

Resume : Thermoelectrics are promising to address energy issues but full potential exploitation requires improvements in their performance (high power factors and low thermal conductivities). We showed recently that highly anisotropic flat-and-dispersive bands can maximize the power factor and at the same time they can produce low-dimensional electronic transport in bulk semiconductors [1]. Using first-principles calculations, we studied SrTiO3 based oxides and their nanostructures of SrTiO3/KNbO3, SrTiO3/LaVO3, SrO[SrTiO3], SrO[SrTiO3]2 and SrO[CoO2F] superlattices identifying those nanostructures which possess highly anisotropic bands [2]. Although most of the considered nanostructures show such highly anisotropic bands, their predicted thermoelectric performance is not improved over that of SrTiO3. Besides highly anisotropic character, we emphasize the importance of the large weights of electronic states participating in transport and the small effective mass of carriers along the transport direction. These requirements may be better achieved in binary transition metal oxides (i.e. Cu2O, TiO2) than in ABO3 perovskite materials. SrTiO3 and related perovskites show low electron mobilities as a result of the polaronic nature of electrical conductivity [2]. 1. D. I. Bilc, G. Hautier, D. Waroquiers, G. M. Rignanese, and Ph. Ghosez, Phys. Rev. Lett. 114, 136601 (2015). 2. D. I. Bilc, C. G. Floare, L. P. Zârbo , S. Garabagiu, S. Lemal, and Ph. Ghosez, J. Phys. Chem. C 120, 25678 (2016).

Resume : In the past decades, efforts to improve the figure of merit ZT of thermoelectric devices have led to tremendous progress accomplished by material nanostructuration in order to decrease the phonon-dominated thermal conductivity. Recently, scientists have been trying to increase the electronic power factor (PF) by various techniques such as band-structure engineering and quantum confinement. A particularly promising alternative to tackle this issue lies in the use of resonant states, as suggested by experimentalists and theorists alike. We propose to discuss the generic effects of resonant states on thermoelectric transport properties by relying on a tight-binding model featuring the essential ingredients for the interplay of a resonant band with the conduction band. We investigate in details the role and effects of various physical parameters. We showcase a significant boost of the PF over a wide range of temperature, essentially resulting from a strong increase of the Seebeck coefficient. As a matter of fact, the electrical conductivity has the opposite effect: it is suppressed in the vicinity of the resonant states. Finally, we discuss resonant effects in more realistic systems such as well-known oxides (Strontium Titanate for instance), that can be studied accurately through a hybrid approach combining ab-initio and tight-binding calculations.

Resume : Efficient thermoelectric (TE) energy conversion requires good TE transport properties, electrical conductivity and Seebeck coefficient, and poor thermal conductivity. Nanocomposites are promising for efficient TE energy conversion. The thermal conductivity decreases down to the amorphous material thermal conductivity in nanocomposites with small characteristic thicknesses and enhanced phonon scattering by non-uniformities, interfaces and boundaries. The improvement of the TE transport properties is more challenging. The electrical conductivity typically decreases in nanostructured materials compared to bulk. Seebeck coefficient may though increase significantly in non-uniform nanostructures due to energy filtering of the charge carriers. We discuss the dependence of energy filtering in the presence of non-uniformity in the composition and the morphology. Particular emphasis is attributed to analyzing energy filtering in distinct transport regimes: (a) degenerate versus non-degenerate carriers transport, and (b) ballistic versus diffusive transport. The various transport regimes may coexist or one of them may dominate, depending on the characteristic sizes and their distribution in the nanocomposite material. The implications of the transport regime on the TE energy conversion efficiency is discussed and design guidelines are provided using the kinetic Monte Carlo simulation technique, Boltzmann transport equation and derived phenomenology.

Resume : The thermoelectric cooling is a robust solution; however, improvements are still needed for large scale applications. The maximum cooling can be improved with the use of segmented or graded materials but full analytical solution is still needed. For this purpose, we investigated an analytical optimization of a graded Peltier cooler. The analysis shows that a local criterion can be obtained on the properties of the graded material. We computed the material properties of an optimized semiconductor based on our criterion. Thirty six percent improvement of the maximum cooling is obtained for a graded semiconductor as compared to a homogeneous semiconductor. The solution has been analyzed both by analytical and numerical means and shows that this improvement is obtained by redistribution of Joule heating and Thomson-Peltier effect through the device. The influence of the equation of state of the electron gas is discussed and the difference in term of entropy production and fluxes between graded and homogeneous systems is discussed.

Resume : Clathrates are perfectly crystalline solids consisting of tetrahedral cage framework and rattling guest atoms and demonstrate great promise to be the phonon-glass and electron-crystal thermoelectric materials. Previous studies focus on functionalizing clathrates via cage atom substitution or carrier type modification to improve the figure of merit ZT. Yet little attention was paid to unravel the effects of rattlers on the relevant physical quantities, i.e., lattice thermal conductivity, electrical conductivity and Seebeck coefficient, that are crucial to determine ZT. In this work we investigate the role of rattlers on the above physical quantities of a representative silicon-based type-I clathrate M8Si46 (M= Cs, Sr, Ba, Tl, Pb) from the atomistic level using first-principles. Comparing the phonon properties of filled and empty clathrates, the rattlers introduce extra phonon modes to scatter the heat-carrying low phonons of Si46 and result in the reduced phonon relaxation time and lattice thermal conductivity. Moreover, by varying rattlers, their influences on phonon dispersion relation, phonon relaxation time, electrical conductivity and Seebeck coefficient are compared, seeking to reveal the underlying physics and then engineer it with better ZT. This work attempts to uncover the atomistic origins of low thermal conductivity of silicon-based type-I clathrate and role of rattling atoms and provide instructions to engineer thermoelectric figure of merit with more sophisticated clathrates.

Resume : Layered materials are the best candidates for thermoelectric application due to their in-plane low thermal conductivity that is a key property to achieve high efficiency. Owing to that, here we present our investigations on electronic as well as thermal transport of bulk and monolayer MX3 compounds (M = Ti, Zr, and Hf and X = S and Se) based on density functional and semi-classical Boltzmann theories. The values of the bandgap is rather similar for bulk and the monolayer, with only a slight change in the shape of bands near the Fermi level that results in a different effective mass. We found that the monolayer MX3 compounds are better thermoelectric materials than bulk. Also, the p-type monolayer of TiS3 has a high power factor at 600 K that is the double of its room temperature value. The monolayer of the Zr/HfSe3 compounds show a promising behavior as a n-type thermoelectric materials at 600 K. In-plane tensile strain could be used to further tune the TE properties of the monolayers in order to obtain high performance TE materials.

Resume : The effect of biaxial strain on the electronic and thermoelectric properties of SnO2 has been examined using state-of-the-art first-principles calculations. All calculations were based on DFT within the Tran-Blaha modified Becke-Johnson exchange potential approximation (TB-mBJ). Under biaxial compression, short covalent Sn-O bonding yields to a wide band gap, while in the case of a tensile strain the elongated Sn-O bond tends to reduce the band gap energy, a mechanism that reveals the ability of strain to modulate the electronic band structure. Under compressive strain, the Seebeck coefficient tends to increase while the opposite is observed in the case of tensile strain resulting in an enhanced electrical conductivity. Also, our figure of merit ZT displays enhanced values at T ≺ 400K under compressive strain, a result that demonstrates the efficiency of an applied strain to enhance the electronic and thermoelectric properties of SnO2 and improve its suitability for thermoelectric applications below T = 400K.

Resume : The binary compound SnSe has emerged as an interesting thermoelectric material due to the announcement of extremely high ZT values of up to 2.6 at 923 K in pristine single crystals [1]. This result has been mainly attributed to the very low lattice thermal conductivity values of the order of 0.25 W m-1 K-1 reached at 923 K. This announcement prompted several groups over the world to determine whether similar values could be achieved in polycrystalline samples [2,3]. Surprisingly, these studies have evidenced lattice thermal conductivity values significantly higher than those measured in single crystals yielding maximum ZT values of the order of 0.5 at 800 K [2,3]. In order to unveil the origin of this surprising difference, we have carried out a detailed reinvestigation of the thermal transport of high-quality single crystals of SnSe grown by a vertical Bridgman method. In this presentation, we shall present measurements of the thermal conductivity performed along the three main crystallographic axes which have revealed values significantly higher than those reported previously by Zhao et al. [1]. The possible reasons of the underestimated values measured in Ref. 1 will be discussed. [1] L.-D. Zhao et al. Nature 508, 373 (2014). [2] S. Sassi et al. Appl. Phys. Lett. 104, 212105 (2014). [3] C.-L. Chen et al. J. Mater. Chem. A 4, 1848 (2014).

Resume : Recently, an extraordinary high thermoelectric efficiency has been reported in single crystalline In4Se3-δ that is resulted from the Peierls distortion in the material system. High power conversion efficiency in In4Se3-δ with nontoxicity and relatively higher earth abundance of its constituent elements makes this material system a potential environment-friendly alternative of conventional materials, which contain toxic and rare elements. However, the weak van der Walls bonding between the layered b-c planes in single crystalline In4Se3-δ causes poor mechanical properties in the material system, which limit its use in practical applications. In order to overcome this problem, we present the possible approaches towards synthesizing polycrystalline counterpart of single crystalline In4Se3-x and to tune its thermoelectric transport parameters for achieving high power conversion efficiency.

Resume : We have developed a series of precursors for effective single source precursors for the low pressure CVD of high quality crystalline thin films of Bi2Te3 and Sb2Te3. Hall conductivity, carrier mobility, carrier density and Seebeck coefficient measurements reveal electronic characteristics comparable with materials deposited by atomic layer deposition or molecular beam epitaxy, suggesting materials quality and performance suitable for incorporation into electronic device structures. Choice of substrate and deposition conditions were found to significantly affect the morphology and enabling deposition of films with various alignment. Use of micro-patterned TiN/SiO2 substrates allows selective deposition of crystalline 2D micro-arrays of both Bi2Te3 and Sb2Te3 onto exposed TiN surfaces only. This high level of selectivity remains even when the TiN hole are reduced to nanoscale for Bi2Te3 and Sb2Te3, resulting in single nanocrystals being deposited in each TiN hole with no deposition on the surrounding SiO2. The selective deposition behaviour provide great potential to simplify the conventional fabrication process of thermoelectric generators. In addition, we have shown that the preferred orientation of the deposited crystals can be controlled both by the hole size as well as deposition temperature. This controlling the orientation of nano-crystals to give the <110> alignment can maximise the anisotropy of the hexagonal crystallites which contribute to higher ZT values. y of the hexagonal crystallites which contribute to higher ZT values. R Huang, SL Benjamin, C Gurnani, Y Wang, AL Hector, W Levason, de Groot Nanoscale arrays of antimony telluride single crystals by selective chemical vapor deposition (2016) Scientific reports 6, 27593

Resume : In the present study, thin films as well as bulk nanocomposites of Bi2Te3, with varying concentration of Graphene (G), Silicon(Si) and Carbon (C) have been synthesized. The effect of concentration of G, Si and C secondary phase segregated along Bi2Te3 crystallite boundaries on electrical and thermal properties of Bi2Te3 nanocomposite has been investigated. The effect of concentration on the thermal conductivity of Bi2Te3 nanocomposites at nanoscale level was investigated using scanning thermal microscopic studies. The value of thermal conductivity for the composite samples was determined using modified Parker’s method. A commercial SThM system was modified by incorporating a microcontroller driven microhotplate. Incorporation of optimized concentration results in change in electronic properties due to modification in crystallite orientation, and the presence of a secondary conducting phase along Bi2Te3 crystallites. This resulted in higher electron transport and increased phonon scattering leading to enhanced ZT ~ 1.4 for Bi2Te3:Si and ZT ~ 0.92 for Bi2Te3:G composite samples. The present study provides an insight into a new route for direct measurement of thermal conductivity for thin films samples. Further this study is important for establishing the role of secondary phase along crystallite boundaries leading to enhanced thermoelectric performance.

Resume : Half-Heusler compounds have been celebrated as one of the most promising families for next generation thermoelectric materials. The large number of potential half-Heusler alloys has led to a few previous screening studies, primarily focusing on their thermal conductivity. We have in this study estimated the complete thermoelectric figure-of-merit from first principles using Boltzmann transport equations within the relaxation time approximation. We investigated 54 different half-Heusler compounds based on TiNiSn and TiCoSb with isoelectronic substitutions on all sites. This has been done at a very high theoretical level, using hybrid functionals (including exact exchange from Hartree-Fock) for the electronic transport properties and temperature-dependent effective potentials for the lattice thermal conductivity. The results give some clear advice concerning the direction of future searches for half-Heusler and other compounds with improved thermoelectric properties.

Resume : Localized cooling in micro- and nano-electronics as well as energy autonomy in applications such as wearable electronics and wireless Internet of Things could be well served by thin film thermoelectric devices fabricated on solid and/or flexible substrates. Bi0.5Sb1.5Te3 is a state-of-the-art p-type thermoelectric material, near room temperature, due to its high power factor value and figure of merit. Nevertheless, the deposition of Bi0.5Sb1.5Te3 thin films with bulk-like thermoelectric properties remains a great challenge. We have grown p-type Bi0.5Sb1.5Te3 thin films onto different types of substrates such as fused silica and Kapton using pulsed laser deposition. The films were grown at different substrate temperature and then were subjected to a post-deposition ex-situ annealing process. In this talk, we will present our recent results on Seebeck coefficient, electrical resistivity, Hall carrier concentration and mobility as a function of temperature (200-390K). We will discuss how the thermoelectric properties of the obtained films are affected by the substrate type and growth temperature. Also, we will address the effect of post-annealing treatment on the structural and thermoelectric properties. We will compare the thermoelectric properties of the Bi0.5Sb1.5Te3 films to those of the bulk material.

Resume : It has been reported crystalline p-type Cu2Se bulk material has been synthesized with excellent thermoelectric properties with a unusual combination of properties leads to an ideal thermoelectric material within the new concept of ‘Phonon-liquid electron-crystal’ [1]. We report a novel approach on the growth of highly efficient thermoelectric materials via pulse controlled reactive magnetron sputtering. This technique allows for a finer control of the stoichiometry, crystallographic orientation, etc. and it is a very fast production technique. It is shown to be fully reproducible and scalable to industry. The fabrication method is a low temperature process, and thus allows the use of organic and/or flexible substrates to deposit the films on them, which opens a whole new field of applications that are not possible with other fabrication techniques. We have focused in the deposit of Cu2-xSe and Ag2-xSe films, obtaining high values in the power factor (1.1 mW•m-1• K-2) for the cubic β-Cu2Se phase and (2.5 mW•m-1• K-2) for the cubic β-Ag2Se phase compared with the highest reported in the literature for both bulk and higher than films. [1] H. Liu et al., Nature Materials 11, 422 (2012).

Resume : Current focus on energy sustainability and stricter legislation on the emission of CO2 has sparked a renewed interest in thermoelectric (TE) power harvesting technologies which can directly convert thermal waste heat into useful electricity. Thermoelectric devices offer advantages over other energy harvesting techniques which include solid-state operation with no moving parts, zero-emission, silent operation, vast scalability and high reliability with no maintenance and long operating lifetimes. Despite these merits the use of TE generators has been vastly limited to niche applications due to their low efficiency and bulky size. Calculations by Dresselhaus et al. [1] on low dimensional structures have predicted that the thermoelectric efficiency in these systems could be dramatically increased which offers an exciting opportunity to engineer novel thermoelectric materials. The currently best performing TE materials in commercial TE devices are based on bulk alloys of bismuth telluride such as n-type Bi2Te3 and p-type Bi0.5Sb1.5Te3 for refrigeration and waste heat recovery up to 200°C. A wide range of different fabrication methods have been employed to prepare nanostructures of n-and p-type bismuth telluride alloys, however these have clear limitations in the size and density of thermoelectric elements that can be prepared whilst not being compatible with silicon microfabrication processes. Ion-track etch lithography on the other hand is a low-cost process that can produce templates with deep vertical and narrow channels suitable for nanowire growth. It employs heavy accelerated ions as a source to damage the material, making it susceptible to chemical etching in the direction defined by the irradiation. This can produce low-cost templates for nanowire growth with diameters < 50 nm and high aspect ratio (> 1000). We report the fabrication of high density arrays of n-type Bi2Te3 and p-type Bi0.5Sb1.5Te3 nanowires by electrodeposition into ion-track etched polyimide Kapton templates. This material offers a number of distinct advantages over porous alumina templates which include high flexibility, a low thermal conductivity (0.12 Wm-1K-1), high chemical and heat resistance (3K to 593K) and compatibility with silicon microfabrication which make it promising for thermoelectric applications. [1] L.D. Hicks and M.S. Dresselhaus, Phys. Rev. B 47, 1272 (1993).

Resume : Cu3SbSe4 based thermoelectric materials are being explored for improved thermoelectric efficiency in the medium temperature range. In the present study, synthesis of Sn doped Cu3Sb1-xSnxSe4 (x = 0.0, 0.005, 0.01, 0.015, 0.02) has been reported employing conventional fusion method followed by spark plasma sintering. A resultant high ZT value of 1.1 at 673K has been exhibited for Cu3Sb1-xSnxSe4 (x = 0.015). This enhancement in ZT value, compared to undoped Cu3SbSe4 (0.48 at 673K), can be attributed to the reduction in the thermal conductivity owing to enhanced phonon scattering at interfaces for the Sn doped Cu3SbSe4 sample. Furthermore, the transport behaviour of undoped Cu3SbSe4 is in good agreement with that predicted theoretically using first-principle density functional theory calculations.

Resume : In this work, we study the effect of biaxial mechanical stresses (pressure and expansion) on the electronic and the thermoelectric properties of Bi2Te3compound. These properties which are band structure, density of state, Seebeck coefficient, electrical and thermal conductivities are investigated by using in both first principles calculation and Boltzmann transport theory, in our calculation we use Generalized Gradient Approximation (GGA). Under pressure, we found Seebeck coefficient increase and band gap reached 0.6 eV, while the thermal and electrical conductivities decrease. However, the expansion produces an opposite behavior, which is summarized sharp decrease in the Seebeck at 3% accompanied by of an overlap of the valance and conduction bands, this result of the anomalies in the electronic structure produces a change in the transport properties. Finally, the pressure effect is able to improve some thermoelectric properties. This has a positive impact on the factor of merit ZT which increase despite the low decrease of the electric conductivity.

Resume : In an attempt to reduce the reliance on fossil fuels, associated with severe environmental effects, the current research is focused on the identification of the thermoelectric potential of n-type PbTe alloys doped with minute amount of Ti (atomic fraction of Ti is less than 3%). An impressive maximal dimensionless thermoelectric figure of merit ZT of 1.2 was obtained upon 0.1 at% Ti doping at 500 °C, indicating a 9% efficiency enhancement compared to undoped PbTe. Density Functional Theory (DFT) calculations demonstrate that Ti concentration of ~1.4 at% in the Pb sublattice maximize the thermoelectric power factor. Using combined DFT and statistical thermodynamic approach, the actual Ti solubility limit in PbTe was found to be ~0.3 at% and in agreement with the experimentally observed appearance of a secondary intermetallic TiTe2 phase. It is demonstrated that Ti doping fractions lower than the solubility limit are expected to provide close to maximal power factor values. This makes doping with Ti a promising opportunity for the generation of highly efficient n-type PbTe-based thermoelectric materials despite the well-known decrease of mobility of electrons due to the resonant states induced by Ti in the conduction band in this system.

Resume : Copper is the interconnect material used in present day IC technology. In attempts to reduce critical dimension on integrated circuit, the interconnect design community encounters issues associated with increased electrical resistivity and lack of suitable material to create a thin diffusion barrier. Because of its inherently low electrical resistivity, graphene has been proposed as an alternative to copper as interconnect. However, due to its low electron concentration, graphene cannot be a replacement for copper. Further it is incomparability with present device fabrication process. A copper-graphene hybrid system is thus a natural solution to these issues. In this manuscript, we report a novel approach in which the hybrid system is accomplished by using graphene as a seed layer for electroplating of a copper top layer. This method is fully compatible with current semiconductor fabrication process. We further show that after thermal annealing, intermixing between graphene and copper takes place. This Cu-C intermixing layer can then serve as a blocking layer for permeation of copper into the underlying silicon substrate. Keyword: Graphene, interconnect, electroplating Reference: Hybrid stacking structure of electroplated copper onto graphene for future interconnect applications, Ya-Wen Su et. al, Applied Physics Letters 107, 093105 (2015);

Resume : The semiconductors with the formula Mg2X (X= Si and Sn), have attract attention as potential high-performance thermoelectric materials, and their electric and thermal properties have extensively investigated [1-4]. They are an indirect band gap semiconductor [5-6]. We carried in this work an ab initio study based on the density functional theory to calculate structural and electronic properties of Mg2SixSn(1-x) (x=0-1). The FP-LAPW method was used with different form of exchange-correlation potential (LDA, GGA, PBEsol and mbj). mBJ give the best result for the lattice parameter, and the best estimation of the band gap energy. Boltzmann transport approach implemented in the BoltzTraP[7] code determine temperature-dependent electron-transport properties such as electrical conductivity σ(T ), thermopower S(T ), electronic thermal conductivity κe(T ). Our results are in good agreement with experiment data available and other theoretical results.

Resume : Variable designs of TiNiSn/HfNiSn superlattices (SLs) and nonperiodic multilayers were investigated to find the most effective way to reduce the crossplane thermal conductivity at room temperature. The latter property was determined using a differential 3ω method. The research started from investigation of the relation between the thermal conductivity and the SL period while keeping the ratio of the two materials in the SL period equal to unity. Special interest was taken in the minimum of thermal conductivity in dependence on the interface density, caused by a crossover of incoherent and coherent phonon scattering regimes. To obtain information exceeding that from the bare interface density, a series of films with variable amounts of materials in the SL period was investigated. Moreover, different periodic and aperiodic designs and the influence of interface roughness on the thermal properties of TiNiSn/HfNiSn superlattices were studied. For our complex multilayer structures we developed a XRD simulation program that allows an input of non-periodic designs. The performed study shows that the nanostructuring helps to reduce the thermal conductivity and contributes to enhanced figure of merit ZT compared to bare TiNiSn and HfNiSn films*. Furthermore the coherent phonon transport opens new possibilities for additional manipulation of thermal properties. *Reduced thermal conductivity of TiNiSn/HfNiSn superlattices, Paulina Hołuj et al; Phys. Rev. B 92, 125436 (2015)

Resume : Designing greener thermoelectric (TE) power generator means considering the environmental impact of the materials used to make it. Lighter and tougher TE power generators are easier on the automotive application, but sometimes the application isn’t easy on them. Magnesium silicide (Mg2Si) has emerged as one of the most promising thermoelectric materials for automotive application. This is mainly due to the light weight of Mg2Si, the abundance of its constituent elements associated with few national risk to material supply, and to good TE properties at elevated mid-temperatures. To further elevate the TE capability of Mg2Si for the realization of a practical TE generator, improved power factor and figure-of-merit of ZT value are inevitable. Here, we report the updated TE properties, that are, the highest power factor and ZT value were ~4x10-3 W/mK2 at ~800 K and >1.2 at ~850 K, respectively, for co-doped Mg2Si with Sb and Zn. Another important issue of Mg2Si for TE device development is to be as durable and long‑lasting as possible for atmospheric mid-temperature (600 °C) operation. Impurity doping in Mg2Si is an effective approache to stabilize thermal durability of Mg2Si and has already obtained enduring surface condition for ~10,000 hrs at 600 °C in air condition. However, our larger goal is to identify Mg2Si as a mid-temperature waste-heat-recovery system that requires longer and steadier durability. So we have been examined to form several passivation coats on Mg2Si that can help us minimize oxidative degradation. In this report, we also describe the successful surface endurance of Mg2Si with industrially oriented passivation coats, in terms of the thermal stability, interdiffusion and variation in electrical resistance.

Resume : Thermoelectric generation devices are able to convert waste heat into electricity. A typical thermoelectric module consists of paired p- and n-type semiconductor pellets that were sandwiched in between two ceramic plates. Generally, these tiny pellets are joined to metallic conductors on the ceramic plates by soldering. The whole fabrication process including pellet dicing, barrier plating and electrode soldering indeed is very time-consuming and costly. In this study, we present a printing-based process to prepare bismuth telluride thick films on a flexible substrate and demonstrate a solder-free process to make a generation module by connecting thermoelectric film and metallic electrode. Bismuth telluride based compounds were ground into fine powders and mixed with organic binders and solvent. The mixture was coated on a substrate by screen-printing and subsequently sintered using a hot-press technique. The p-type Bi-Sb-Te film exhibits a thermoelectric power factor of 1.15  10-3 W/mK2 at room temperature. To form a thermoelectric module, the contacting electrodes were directly joined to the printed film by forming a layer of intermetallic compound at the contact region through adjustment of sintering pressure and temperature. The thick film module can achieve an output power density of 1 mw/cm3 under a temperature difference of 10 K. The effect of film and contact properties on the performance of thermoelectric modules will be modeled and experimentally evaluated.

Resume : Bi2Te3-based alloys, (Bi,Sb)2Te3 and Bi2(Te,Se)3, are the best thermoelectric materials near room temperature, whose dimensionless figures of merit (ZT) were increased remarkably by employing various nanostructuring techniques. Among the many methods for nanostructuring, nanocomposite synthesis by adding extrinsic nanoscale inclusions is quite simple, effective, and controllable, which are critical for mass production at low cost. It is highly required that the extrinsic nanoinclusions are easily dispersed by using conventional top-down powder process, and the inertness in the matrix is also very important to function as stable pinning centers during sintering. In this study, we have added ZnO nanoparticles (mean particle size ~ 14nm, >99.5% purity) to make Bi2Te2.7Se0.3 (BTS) nanocomposites, and the thermoelectric properties were analyzed. BTS nanocomposites containing x vol% (x≤2.0) ZnO nanoparticles (NPs) were made by powder mixing with high-energy ball milling and hot press sintering at 693 K. The minimum thermal conductivity of BTS–1 vol% ZnO nanocomposites decreased about 7.5% due to the grain refinement, and the resistivity also decreased about 11.8% along the pressing direction, which resulted in the power factor increase of about 25%. The anisotropy of resistivity and power factor decreased by adding ZnO NPs, and ZTmax of the BTS–1 vol% ZnO nanocomposite was increased to 0.89 at 150℃. SEM observation confirmed that ZnO NPs were properly dispersed in the BTS matrix.

Resume : Metal silicides are promising thermoelectric materials developed to create safe, clean and cheap energy. They are characterized by high Seebeck coefficient, low resistance and low toxicity. Chromium, Manganese and Magnesium silicides are lately studied because they present low density, high melting point, low fabrication cost and high thermoelectric figure of merit ZT. The current study focuses on the formation and thermal stability of the above-mentioned silicides. Their growth was succeeded with a novel and low-cost deposition process which is the pack cementation. Bulk Cr-Si and Mn-Si compounds were formed by a two-step deposition while only one step was used for bulk Mg-Si and Cr-Si, Mn-Si powders, respectively. The morphology and the structure of the samples were determined by Scanning Electron Microscopy and X-Ray diffraction analysis, respectively. The thermal stability of the samples was evaluated by thermogravimetric measurements. As a result Mg2Si, MnSi1.7 and CrSi2 compounds were formed which are reported to have significant performance improvements. The formed compounds were free from other substances, such as oxides, which can reduce the performance of the fabricated thermoelectric materials. Finally, the as formed products were found to have remarkable stability especially at temperatures in which they are referred to have maximum Seebeck coefficient. This work has been supported by EU in the framework of the NetFISiC project (Grant No. PITN-GA-2010-264613).

Resume : Low-dimensional materials have been known to give high TE performance by reducing the confinement length of the materials. Recently, we have shown that the TE power factor of low-dimensional semiconductors depends not only on the confinement length, but also on the thermal de Broglie wave length [Phys. Rev. Lett. 117, 036602 (2016)], in which the calculation was performed by assuming the semiconductors to be nondegenerate, i.e. we approximated the Fermi energy to lie only within the energy band gap. Experimentally, the nondegenerate case corresponds to the low doping approximation. Now, in this work, we generalize the previous results considering also the degenerate case, in which the Fermi energy can exist in the valence or conduction bands, thus enabling a full consideration of heavy doping. An analytical formula for the TE power factor is derived to describe the size effects in the power factor of the low-dimensional semiconductors. We find that for both nondegenerate and degenerate cases, the TE power factor is enhanced in one- and two-dimensional semiconductors when the confinement length is smaller than the thermal de Broglie wave length of the semiconductors, with Fermi energy around top (bottom) of valence (conduction) band for the p-type (n-type) semiconductors.

Resume : In the family of two-dimensional (2D) semiconductors, the band structure of monolayer group III chalcogenides such as indium selenide (InSe), galium selenide (GaSe), and galium sulfide (GaS) are rather unusual, having a combination of a flat band in the top of the valence band and a parabolic band in the bottom of conduction band. This leads to appearance of a very sharp electronic density of states (DOS) at the top of the valence band and a finite 2D DOS at the bottom of the conduction band. In this work, we calculate Seebeck coefficient, electrical conductivity, and power factor of monolayer InSe, GaSe, and GaS from first-principles calculations combined with Boltzmann transport theory. The large Seebeck coefficient of about 3 mV/K is found to originate from the moderate (~2 eV) band gaps of the monolayer group III chalcogenides as indirect gap semiconductors, while their large electrical conductivity is due to the metal-like character appearing in the density of states at a limited energy range within 0.3 eV above the bottom of the conduction band. We expect that the monolayer group III chalcogenides are a potential candidate as a thermoelectric material.

Resume : Bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) are among the most efficient thermoelectric materials, which find their application at room temperature. Molybdenum Disulphide (MoS2) has attracted much attention because of its graphene-analogous structure and high mobility. In the present study, the effect of the presence MoS2 on the nano and macro scale electrical and thermal properties of Bi2Te3 and Sb2Te3 have been investigated using AFM based characterizations. Atomic Force Microscopy, Kelvin Probe Force Microscopy and Scanning Thermal Microscopy were carried out on composite samples for analyzing electrical and thermal properties of Bi2Te3:MoS2 and Sb2Te3:MoS2 interfaces. A simultaneous reduction in thermal conductivity and enhanced power factor are observed in Bi2Te3:MoS2 and Sb2Te3:MoS2 composite samples. A maximum ZT of 0.77 is observed for the Bi2Te3:MoS2 sample as compared to 0.23 for the Sb2Te3:MoS2 sample at 427 K. Both composite sample show higher value of ZT as compared to their pristine counterparts. In the case of Bi2Te3:MoS2, the enhancement in ZT is due to decrease in thermal conductivity where as in Sb2Te3:MoS2 the enhancement is due increase in electrical conductivity. This difference is due to the difference in nature of Bi2Te3:MoS2 and Sb2Te3:MoS2 interfaces. This study shows that by incorporating a 2D material at interface, microstructural, electrical and thermoelectric properties of existing thermoelectric materials can be controlled.

Resume : Thermoelectric materials, which can convert heat into electricity or vice versa, are attracting much attention due to their potential applications in the energy conversion industry and providing power for electronics. In this study, we focus on Cu3SbSe4 compound, which has become known as potential TE material due to its excellent electrical transport properties, low thermal conductivity or cheap constituent elements. Further, doping effects with Bi and Sn the TE performance are also studied. Density Functional Theory and the Boltzmann semi-classical transport calculations are performed to determine the electronic structures and main thermoelectric transport parameters of Cu3SbSe4-based compounds. The comparison with experimental provided useful information on the doping effect of TE properties and their relation with the electronic structure.

Resume : The solution of energy security problem by efficiency improving of alternative energy generation, the search for new, clean and renewable energy sources is a key task of both science and the economy. One of the ways to increase the thermoelectric coefficient efficiency (ZT) is the application of skutterudite CoSb3 antimony (conception the G. Slack). In addition, the transition from bulk materials to the nanoscaled allows to addition to increase ZT due to defectiveness in the structure. The paper experimentally was confirmed the theoretical calculations of ZT increase in the transition to nanoscaled materials. The basic laws of phase composition and structure formation, electrical properties in as-deposited Co-Sb layers on SiO2(100 nm)/Si(001) substrates and after annealing in vacuum and nitrogen atmosphere were defined. The CoSb3 films are thermally stable up to ~300°C. At annealing of Co-Sb films at temperatures above 300°C sublimation of not only excessive Sb but also of Sb from the crystalline phase CoSb2 and CoSb3 occurs. Effect of ZT increasing in CoSb3(30 nm) film material at 500°C up to ~1 that is in 8 times higher in comparison to bulk material is determined by nanoscaled factor and due to increased defectiveness of structure. This is the practical significance of the use of these materials for electronic devices with self-powered low-power and at the creation of the refrigerators in element base of the nanoscaled range for computer equipment and infrared sensors.

Resume : A higher figure of merit (ZT) in SnSe-based thermoelectric materials is achieved by reducing the thermal conductivity (κ) while keeping the Seebeck coefficient (S) through the substitution of isoelectric atoms, exfoliation from a bulk ingot, and a transformation process of the material into a porous structure. Specifically, SnSe1-xSx nanosheets (NSs) are prepared from bulk ingots by a hydrothermal Li-intercalation, followed by an exfoliation process. The substitution of S atoms in SnSe, and the fabrication of SnSeS NSs, contribute to the scattering of phonons at a number of atomic disorders and nano-sized boundaries, leading to the effective reduction of the κ value, and an improved ZT. The introduction of a porous structure in the material through a chemical transformation process results in the additional reduction of κ, which leads to a higher ZT. The fabricated porous SnSe0.8S0.2 NS shows the maximum ZT value of 0.13 at 300 K, which is ~27% and ~18% higher than that of pristine SnSe and SnSe0.8S0.2 NS, respectively.

Resume : A DFT study of the electronic properties of d-metallic structures such as bulk surfaces, layers of a few nanometers thick, nanoclusters is presented. Calculations were made in the GGA, LDA in the scalar and fully relativistic approximations. Aproximations, allowing to calculate DOS that fits well the observed features of valence band in experimental XPS spectra of d-metallic (Au, Pt, Ag, Pd) bulk surfaces, layers of a few nanometers thick and nanoclusters of several nm in size were found. Simulations were made with Quantum Espresso code on NRNU MEPhI hpc cluster. The results obtained are considered to be implemented in the development of a new metal nancluster-based highly efficient thermoelectic material.

Resume : Bi2Te3-based alloys possess the highest known ZT near room temperature. The objective of our research is to improve the thermoelectric efficiency of these materials by tuning both the microstructure via hot press/deformation methods and the carrier concentration through varying the Bi/Sb ratio. To this end, we have prepared powders of a series of p-type Bi0.5-xSb1.5+xTe3 solid solutions and compacted them into high density pellets by hot pressing method. Also, by measuring all thermoelectric properties in the same direction, we calculated reliable ZT values. In this poster presentation, we will present our recent results on Seebeck coefficient, electrical resistivity, Hall carrier concentration, thermal conductivity and ZT as a function of temperature. The hot deformation method led to significant enhancement of ZT and, therefore, it is considered to be a promising tool for the fabrication of highly efficient thermoelectric legs.

Resume : Mg2X (X: Si, Ge, Sn) family of compounds and solid solutions has drawn much of attention due to their high ZT, ample availability in nature and also their non-toxicity that is consistent with the priority for friendly technology to the environment and the human. Nano-structured (Mg2Si)1-X-Y(Mg2Si)Y(Mg2Ge)Y pseudo-ternary compounds were synthesized and thermoelectric properties were measured. Materials show a distribution of Si-rich, Sn-rich and Ge-rich phases which spans from micro-sized grains to nano-sized particles. A model is developed to encounter for the different contributions to lattice thermal conductivity. In our model, lattice thermal conductivity is reduced from phonon scattering at grain boundaries and nano-particles, and is limited by alloying and Umklapp and Normal processes. The calculations show excellent agreement with the temperature dependence of the lattice thermal conductivity of the nano-structured Mg2Si-based pseudo-ternary compounds. The use of phonon mean free path diagrams, contributes in the understanding of the different phonon-scattering processes and the phonon frequency range that each of the mechanisms affects. Analysis with our model, for different values of grain-size and nano-particles predicts that for a given grain size there is an optimal value for nano-prticles and vice-versa. This work suggests a general approach in the optimization of lattice thermal conductivity in thermoelectric materials, by controlling both the nano as well as the micro-structural features.

Resume : ZnSb thin-films were deposited by Pulsed Laser Deposition (ArF, 193 nm, 20 ns-pulse duration) according to different percentages of Zn and Sb arranged as a multilayer structure to form ZnxSby compounds. A study as a function of deposition temperature (ranging from 100 to 500 °C) and different substrates (quartz and sapphire) has been carried out. Grazing incidence x-ray diffraction, scanning electron microscopy, and x-ray spectroscopy were used to investigate the characteristics of the deposited thin-films. The thermoelectric analysis was performed by measuring the electrical conductivity and the Seebeck coefficient for evaluating the power factor of the thin-films.

Resume : In the present study, bulk nanocomposites of Bi2Te3, with varying concentration of Graphene (G), and Carbon (C) have been synthesized. Atomic force and conducting atomic force microscopic studies show that G incorporation results in lower electrical conductivity at Bi2Te3:Graphene (Bi2Te3:G) interfaces. The Kelvin probe and scanning thermal images show that Bi2Te3:G and Bi2Te3:C composites samples have different work function and thermal conductivity values compared to Bi2Te3 sample. The decrease in the value of thermal conductivity was further confirmed by macroscopic measurements in the temperature range 300-480 K. The simultaneous increase in the value of power factor and decrease in the value of thermal conductivity of the Bi2Te3/G composite sample led to enhanced value of ZT = 0.92. This increase in the value of ZT is attributed to increased phonon scattering, and limited effect on the electron transport due to large interface area and 2D nature of G. The present study provides an insight into a new path for direct nanoscale measurements of topographical, electrical and thermally conducting characteristics of nanocomposite samples which is important for establishing the role of 2D materials for improving thermoelectric properties.

Resume : One of the most important steps to increase the figure of merit in thermoelectric materials, is the optimization of the relationship between the material’s properties. Several approaches were developed in order to reduce thermal conductivity without changing electrical properties, such as, nanostructuration or complex materials[1]. An important aim was achieved in this work through a new method to obtain doped-Skutterudite/oxide nanocomposites. The effect of the functional interfaces between the Skutterudite material and the oxide inclusions was studied, verifying the importance of these interfaces. These act as effective phonon scattering and trapping centers, thus reducing thermal conductivity. With this low thermal conductivity, by nanocomposite effect, and low electrical resistivity, by doping, a high figure of merit of 1.3 at 790 K for CoSb2.85Te0.15 nanocomposites was achieved[2]. 1. Snyder, G.J. and E.S. Toberer, Complex thermoelectric materials. Nat Mater, 2008. 7(2): p. 105-114. 2. Moure, A., et al., Thermoelectric Skutterudite/oxide nanocomposites: Effective decoupling of electrical and thermal conductivity by functional interfaces. Nano Energy, 2017. 31: p. 393-402.

Resume : In thermoelectric device characterization, differentexperimental methods are used: here, steady-state and fast pulsed techniques are considered. The results of current voltage (I-V) curves, and the extrapolated parameters (power and efficiency), can differ in the above cases expecially for high power devices. As a consequence, a slight difference can arise also in the load matching resistance evaluation. Also, common methods to calculate TEM parameters do not consider thermal drops in passive elements and the influence of current on heat flux. In this work, a model is developed in Matlab to calculate module performance and simulate I V curves of the aforementioned techniques. The model takes into account the passive ele-ments which lead to temperature drops inside the device. It corrects the effective cold and hot side temperature on the thermoelectric materials in a iterative way, calculating the heat flux with and without the current. The numerical I-V simulations show that the Peltier and Joule terms, which changes the heat flux as a function of current, change the thermal drops due to passive elements. Consequently, the integral of Seebeck coefficientdecreases while increas¬ing the current with respect to open circuit voltage, leading to a higher equivalent resistance. Electrical characterizations of a commercial TEM, performed both in steady state condition and with fast pulsed I-V measurements, confirmthe developed numerical analysis.

Resume : In this work, we measure the structural, morphological, compositional, thermal and thermoelectric properties of large-area Si0.8Ge0.2 nanomeshed films manufactured by DC-sputtering of Si0.8Ge0.2 on highly ordered porous alumina matrices [1]. The Si0.8Ge0.2 film replicated the porous alumina structure resulting in nano-meshed films. Very good control of the nanomesh geometrical features (pore diameter, pitch, neck) was achieved through the alumina template, with pore diameters ranging from 294 ± 5nm down to 31 ± 4 nm. The method that we developed is able to provide large areas of nano-meshes in a simple and reproducible way, being easily scalable for industrial applications. Most importantly, the thermal conductivity of the films was reduced as the diameter of the porous became smaller to values that varied from κ = 1.54 ± 0.27 W K−1m−1, down to the ultra-low κ = 0.55 ± 0.10 W K−1m−1 value. The latter is well below the amorphous limit, while the Seebeck coefficient and electrical conductivity of the material were retained. These properties, together with our large area fabrication approach, can provide an important route towards achieving high conversion efficiency, large area, and high scalable thermoelectric materials. Using this approach, it is possible to control thermal transport of these films through nano-engineering. [1] Perez-Taborda, J. A., Rojo, M. M., Maiz, J., Neophytou, N., & Martin-Gonzalez, M. Ultra-low thermal conductivities in large-area Si-Ge nanomeshes for thermoelectric applications. Scientific Reports, 6, 32778 (2016).

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Resume : Nanostructures of Tellurium and its binary or ternary compounds are interesting for thermoelectric micro-devices. 1D nanostructures like nanowires, hollow nanostructures or core-shell nanowires are particularly promising due to the high phonon interface scattering and the blocking of the phonon conduction along the wire axis. This work will present the recent results obtained by our group concerning the synthesis of one-dimensional Te-based nanostructures from various approaches. At first we will discuss some recent results about the thermoelectric properties of electroplated (Bi1-xSbx)2Te3 nanowires obtained by template synthesis. XRD analysis and TEM microscopy characterizations showed a polycrystalline state with preferential orientation perpendicular to (015) planes. High positive Seebeck coefficient (+303.3 µV/K) and optimum output power (0.69 nW for 0.785cm2) for nanowires arrays have been achieved for Bi7.6%Sb26.8%Te65.6% deposited at -100 mV. Additionally, alternative approach was done by template-free electrodeposition of Te 1 D nanostructures in an ionic liquid. SAED and XRD analyses confirm the formation of Te 1D nanowires with a hexagonal single crystalline structure following the [001] direction (1). Recently nanowires of high aspect ratio were obtained (a diameter of 50 nm and a length of 70 μm) (2). 1 Thiebaud, L.; Legeai, S.; Stein, N. Electrochimica Acta 2016, 197, 300. 2 Thiebaud, L.; Legeai, S.; Ghanbaja, J.; Stein, N. Electrochimica Acta 2016. DOI 10.1016/j.electacta.2016.11.005

Resume : Among the most used thermoelectric materials for room temperature applications, Bismuth telluride (Bi2Te3) stands out. Nevertheless, the thermoelectric efficiency in bulk is limited by the parameters of the material. One route to improve the thermoelectric efficiency of Bi2Te3 is to nanostructure it, for instance, producing nanowires. These nanowires are expected to have a reduced lattice thermal conductivity than the material in bulk, thanks to the phonon scattering in the surface of the nanowires. We present arrays of nanowires of Bi2Te3 in porous alumina templates with electrochemical deposition. The transport properties have been measured as a function of the nanowire diameter. Electrical conductivity has been measured with KPFM, Seebeck coefficient with a MicroSeebeck instrument from Panco along the nanowire direction, and the thermal conductivity with 3-omega SThM [1] and Photoacoustic [2]. From the dependence on the radius, a theoretical model proposed for the thermal conductivity in these kinds of nanowires has been validated. [1] Miguel Muñoz-Rojo, Stephane Grauby, J. M. Rampnoux, Olga Caballero-Calero, Marisol Martín-González, Stpehan Dilhaire, J.Appl Phys 113 (2013) 054308. [2] Begoña Abad, Jon Maiz, Alejandra Ruiz-de-Clavijo, Olga Caballero-Calero, Marisol Martín-González, Scientific Reports, 6 (2016) 38595

Resume : Large efforts are being made towards a new energy paradigm grounded in vectors such as energy saving and alternative production/recovery methods [1]. Within this trend, renewable energy sources ranging from wind power to solar photovoltaics must accomplish a target of 20% share in the energy mix by 2020. The conversion of heat into electricity is one of the least exploited fields. This conversion can be attained by using thermoelectric (TE) devices composed of junctions of materials with different Seebeck coefficients (typically combining p and n semiconductors), which generate an electromotive force from a temperature gradient thus providing clean energy from a non-mechanical process. In this work, the study of the deposition applied potential effect on the morphology, stoichiometry and crystallinity of both thin films and nanowires has been conducted [2]. The morphology and stoichiometry was found to highly depend on the deposition potential, where by increasing it one was able to accurately control the Te % content of the deposits. X-ray diffraction measurements have shown the presence of a strong relation between the material’s crystallinity and the deposition potential, where samples ranged from monocrystalline, at very low potentials, to almost completely amorphous, at high potentials. [1] R.V. Noorden, Nature News (7-7-2009). [2] M.P. Proenca, et al Materials and Design 118 (2017) 168–174

Resume : Bismuth telluride is one of the best performing thermoelectric materials at room temperature. The thermoelectric figure of merit, defined as ZT = S2σT/κ determines the efficiency of a thermoelectric material, where S is the Seebeck coeffcient, σ is the electrical conductivity, κ is the thermal conductivity and T is the absolute temperature. It is generally believed that by nanostructuring, the thermoelectric figure of merit, and thus, the thermoelectric efficiency of Bismuth telluride can be enhanced, due to a decrease in the thermal conductivity, caused by the increased of phonon scattering at the interfaces and grain boundaries. In this work we present 3D nanostructured bismuth telluride networks. Three-dimensional nanoestructured alumina membranes fabricated by a two steps anodization process were used as templates [1]. Then, these templates were filled with bismuth telluride via electrochemical deposition. The replica of this unconventional 3-D nanowire like network allows the development of a new range of properties and applications. Therefore, we also present a study on the thermoelectric properties of the resulting highly ordered [110] Bi2Te3 3D nanowire networks [2]. [1] Jaime Martín, Marisol Martín-González, José Francisco Fernández, Olga Caballero-Calero, Nature Communications, 5 (2014) 5130 [2] Begoña Abad, Jon Maiz, Alejandra Ruiz-de-Clavijo, Olga Caballero-Calero, Marisol Martín-González, Scientific Reports, 6 (2016) 38595

Resume : To date, PbTe-based materials and devices hold the highest thermoelectric figure of merit ZT and highest conversion efficiency, respectively, over the intermediate temperature range of 650 K-850 K. [1] However, the high toxicity of Pb and scarcity of Te hinder the use of these materials in commercial applications. Copper sulfides are an intriguing new class of thermoelectric materials, because both Cu and S are environment-friendly and cost-effective elements [2,3]. This talk will address the enhancement of ZT in p-type thermoelectric sulfides Cu26A2E6S32 (A = Nb, Ta; E = Sn, Ge) called colusites. The samples were prepared by melting the mixture of pure elements in an evacuated quartz tube followed by hot pressing. The complex crystal structure of colusites yields an extremely low lattice thermal conductivity, ranging from 0.3 W K-1 m-1 to 0.8 W K-1 m-1 over the temperature range of 300-670 K. The electrical resistivity decreases with decreasing Sn and Ge contents, leading to a high power factor. For example, a value of 800 micro W K-2 m-1 at 670 K was obtained for Cu26Ta2Sn5.5S32. The ZT for this sample was achieved 1.0 at 670 K. This work was supported as part of Bilateral Joint Research Projects between the JSPS and MAEDI (SAKURA Program) and the International Joint Research Program for Innovative Energy Technology funded by METI, Japan. [1] X.K Hu, P. Jood, M. Ohta et al., Energy Environ. Sci., 2016,9, 517. [2] P. Jood and M. Ohta, Materials, 2015, 8, 1124. [3] Y. Kikuchi, Y. Bouyrie, M. Ohta et. al., J. Mater. Chem. A, 2016,4, 15207.

Resume : The bottom-up assembly of semiconductor colloidal nanoparticles into macroscopic multi-compound materials is a particularly versatile methodology to precisely design thermoelectric materials. Beyond the control over crystal domain size, shape, crystal phase and composition during nanoparticle synthesis, solution-processed nanoparticles allow an exquisite surface engineering, which provides additional means to modulate transport properties. The exchange of native surface organic ligands by short inorganic molecules can be envisioned as carriers of foreign ions which may diffuse to the nanoparticle lattice to tune the type and concentration of majority carriers or to modify the electronic band structure. Herein, we report the thermoelectric performance of consolidated surface modify SnTe nanoparticles. CdSe complexes were selected as ligands to suppress the excess of holes arising from the intrinsically large number of Sn vacancies, to converge the light- and heavy- bands, and to generate nanoinclusions of a secondary phase to further reduce the lattice thermal conductivity. The SnTe-CdSe nanocomposites produced allowed us to obtain figures of merit up to 1.3 at 850 K which is, to the best of our knowledge, the highest thermoelectric figures of merit reported for Sn-based solution processed chalcogenides, and comparable to the highest figure of merit stated for SnTe.

Resume : Worldwide, small temperature gradients are available from many power generation/consuming systems. In fact, it is estimated that many gigawatt of energy are wasted annually in the form of heat [1]. Thermoelectric effect (TE) can offer a simple technology to convert this waste heat into useful energy. In fact, recently the US department of energy [2] report that if a thermoelectric generator with 2.5% conversion efficiency is implemented into manufacturing process a total of 1880-4700 GWh/yr of waste heat could be recovered. However, nowadays the TE devices have a limited application mainly due to its low energy-conversion efficiency and high material cost [3]. The efficiency can be improved by size reduction and enhancement of the phonon scattering [4,5]. TE efficiency are defined by the figure of merit ZT and is defined as ZT=(S2σ/к)T, where S, σ, к and T are the Seebeck Coefficient, electrical and thermoelectric conductivity, and absolute temperature, respectively. In this work, the Ion Beam Sputtering is used to growth Bi2Te3 and Sb2Te3 thin films, with different: substrates (flexible – PET and Kapton, and rigid – Si and Glass) and thickness (VBeam: 300 to 1000 V and tdeposition time =30 min). A dependence on deposition parameters is observed, yielding different crystallinity and electric properties. The improvement of this properties is analyzed by performing thermal treatments to the prepared thin films, and the respective results discussed. References: [1] Boston R, et al. Chem Mater 2017;29:265–80. [2] Energy UD of. Quadrennial Technology Review. 2016. [3] He W, et al. Appl Energy 2015;143:1–25. [4] Lin J-M, et al. J Nanomater 2015;2015:1–6. [5] Lin J-M, et al. J Nanomater 2013;2013:1–6.

Resume : INTRODUCTION Thermoelectric modules interconvert thermal gradients for power generation through Seebeck effect. Restricted by its low efficiency, which is, measured by a dimensionless parameter ZT (function of Seebeck coefficient, electrical and thermal conductivities), it finds niche applications. Thermal conductivity in semiconductors is dominated by the phonon contribution implying that the electrical and thermal conductivities can be decoupled 1. Unfortunately, bulk silicon, the most abundant and widely used semiconductor, has a very high value of thermal conductivity (~150W m^-1K^-1 at 300K) which leads to low efficiency. (ZT ≈ 0.01 at 300K) 2. Nanomaterials allow tailoring of the interdependent parameters, which opens up new avenues to enhance efficiency. When the dimensions of the material are comparable to the phonon mean free path, the thermal conductivity drops significantly due to surface scattering. Nanowires with modulated diameter would further decrease the thermal conductivity substantially as the corrugations would act as a trap for phonons and reduce heat transmitivity 3. EXPERIMENTAL/THEORETICAL STUDY Dense forest (10^9/cm^2) of diameter-modulated silicon nanowires is catalytically grown using Chemical Vapor Deposition (CVD) through a nanoporous alumina template-. Sensitive 3-omega measurements are carried out on the forest of nanowires to confirm the reduction in the thermal conductivity of Silicon. The nanoporous alumina template is fabricated by anodizing an aluminium layer in acidic electrolytes previously deposited on a highly doped silicon wafer. The diameter of the Si nanowire segment is predefined by the internal pore structure of porous alumina during the template assisted CVD following the Vapor-Liquid-Solid (VLS) mechanism. Structural engineering of the Si nanowires is achieved by deliberately designing the cylindrical pore structure with the potential pulse sequences during aluminium anodization. CONCLUSION As the diameter of the nanowires is less than the phonon mean free path at room temperature, substantial decrease in thermal conductivity of silicon is expected. Comparison between the thermal conductivities of thin straight and comparatively thicker but modulated diameter nanowires will be done with 3-omega measurements. Large scale potential applications in the fabrication of real thermoelectric module will be discussed. REFERENCES 1. D. K. C. MacDonald, Thermoelectricity: an introduction to the principles. Courier Corporation, 2006. 2. L. Weber et al. Applied Physics A, 53(2), 136-140, 1991. 3. X. Zianni et al., Journal of Electronic materials, 42(7), 1509–1513, 2013.

Resume : The ability to fabricate Si-based thermoelectrics with improved performance metrics is expected to be a game-changer for harvesting waste heat for electricity generation or active cooling in integrated chips. However, it is required to substantially improve the thermoelectric figure of merit, ZT of Si via decreasing thermal conductivity (κ) while increasing its electrical conductivity (σ) and Seebeck coefficient (S). Amorphous (a-)Si has inherently low κ and high S due to the disorder in the material. Nevertheless, due to the same reason, the flow of charge carriers is hindered resulting in rather low σ and hence poor thermoelectric power factor. The ZT of amorphous Si is thus very low and unsuitable for use as a thermoelectric material. In the present work, we report an improvement of ZT of n-type a-Si wrought by enhancement of σ while maintaining the advantage of suppressed κ and higher S of the amorphous phase. We have implanted an a-Si thin film with Arsenic and activated the dopants while avoiding crystallization of the a-Si. The doping results in six orders of magnitude improvement of σ while κ and S remains comparable to its amorphous values, giving a large improvement of the thermoelectric figure of merit, ZT. Experimental results related to optimized film growth, implantation, dopant activation, characterization of the film and method of measurement of S using Raman thermometry will be presented.

Resume : This study aims to investigate the effect of an original way for materials nanostructuring that could enhance the efficiency of thermoelectric materials (TE). Indeed, we proposed the surface nanostructuring using ultrashort laser beams as a method to improve the surface TE properties. In fact, laser induced periodic surfaces structures (LIPSS) have attracted extensive interest in recent years for their potential in surface nanostructuring process leading to enhance some surface properties of these materials such as wettability, color and reflectivity. In the case of thermal and electrical transport, the laser assisted monitoring of nanostructures is expected to induce a better confinement of phonons leading to the reduction of the thermal conductivity without destroying the electrical and Seebeck coefficients. In this work, two TE promising materials have been studied: Mesoporous silicon (50 µm) and titanium oxide thin films (300 nm) deposited on Si substrates. The first step consists to optimize the laser parameters to obtain large and homogenous dots and ripples surfaces (up to 25 x 25 cm²). The second step aims to study the effect of these formed structures on the evolution of TE properties such as the power factor S²σ (where S is the Seebeck coefficient and σ is the electrical conductivity) and the thermal conductivity Kth. TE properties have been determined by the use a micro-scale device homemade called “ZT-meter” based on the modulation of CO2 laser beam for heating and characterizing of samples. The first results have shown a significant enhancement of Seebeck coefficient (up to 30% in the case of mesoporous Si) due to the laser nanostructuring. Also, thermal characterizations are in progress, and the first estimation on the mesoporous Si substrate shows a sensitive reduction of the thermal conductivity.

Resume : The 1st application of thermoelectric power generation in Japan was a candle radio using beta-FeSi2 modules that was commercialized in 1990. Development of thermoelectric devices or modules was activated in 2002, when the 5-year NEDO project titled by “Development of thermoelectric energy conversion system with high efficiency” started. Bi-Te power generation devices with high energy conversion efficiency of 7.2% under a temperature gradient of 303-553K was developed. Ministry of Economy, Trade and Industry (MITI) of Japan has launched the 10-year project of "Development on the innovative utilization technologies of unused heat energy" since 2013. The administrator of this project has been changed from MITI to NEDO since 2015. In this presentation, the recent progress of thermoelectric devices or modules in Japan will be introduced.

Resume : Zintl pnictides are attractive for thermoelectric applications owing to their favorable charge carrier transport properties and low lattice thermal conductivities. In this work, we have computationally assessed the potential for thermoelectric performance of 145 Zintl pnictides using our previously-developed descriptor beta [1] and predicted that many of these Zintls, if doped n-type, can outperform their p-type counterparts. However, most Zintls exhibit asymmetric doping with a strong proclivity for p-type conduction, including Yb14MnSb11, Sr3GaSb3, and Ca5Al2Sb6. The skutterudite CoSb3 is a notable exception that can be doped both p- and n-type, which is desirable for practical devices. We have recently experimentally realized two other Zintl pnictides, KAlSb4 [2] and KGaSb4, which can be doped n-type and exhibit relatively high thermoelectric performance (zT~1). These two materials were previously identified as n-type candidates with large beta. From our experimental findings and detailed defect calculations, we predict that KAlSb4 and KGaSb4 can also be doped p-type. Based on these results, we find that the doping tendencies are intimately related to the behavior of native defects in the material. Our analysis of the defect chemistry in these two and other Zintl pnictides suggests the existence of design principles governing the dopability of this class of materials. [1] J. Yan, P. Gorai, B. Ortiz, V. Stevanovic, and E. S. Toberer, "Material Descriptors For Predicting Thermoelectric Performance," Energy Environ. Sci. 8 (2015) 983. [2] B. Ortiz, P. Gorai, V. Stevanovic, and E. S. Toberer, "Potential for High Thermoelectric Performance in n-type Zintl Compounds: A Case Study of Ba doped KAlSb4," J. Mater. Chem. A (2017) DOI: 10.1039/C6TA09532A.

Resume : Abstract Bornite, a relatively abundant natural mineral, has recently been categorized as a novel ‘clean’ p-type thermoelectric material. Mechano-chemical synthesis[1] and parent element doping[2] has been used in an attempt to fine tune the thermoelectric properties of this ultra-low thermal conductivity material. Copper in iron site doping (Cu5+xFe1-x◌2S4, where ◌ denotes a vacant site) has shown that significant enhancements in the power factor (From P.F.max=0.22 mW m-1 K-2 to P.F.max=0.52 mW m-1 K-2) can be achieved. Vacancy in copper site doping (Cu5-xFe◌2+xS4) has shown that it is possible to observe slight reductions in the thermal conductivity (From T.C.min=0.26 W m-1 K-1 to T.C.min=0.23 W m-1 K-1). More importantly, using a combination of these techniques (Cu4.94+xFe1-x◌2.06S4) allows for significant enhancement of overall thermoelectric performance (ZTmax=0.44 to ZTmax=0.79) of bornite, which peaks at a relatively low temperature of 550K. [1] - G. Guélou, A. V. Powell and P. Vaqueiro, J. Mater. Chem. C, 2015, 3, 10624–10629. [2] - P. Qiu, T. Zhang, Y. Qiu, X. Shi and L. Chen, Energy Environ. Sci., 2014, 7, 4000.

Resume : Transition metal nitrides well known for their mechanical properties and wide range of electrical properties have shown potentialities for thermoelectricity applications [1,2,3]. ScN is one of the explored material with its high Seebeck coefficient and low electrical resistivity. However, it exhibits a relatively low figure of merit ZT due to its thermal conductivity. In this study, the n-type semiconductor “ScN” was studied in its thin film form. The thin films were deposited on c-plane sapphire by dc-magnetron sputtering at 900C. The (111) epitaxial thin film exhibited similar thermoelectric properties (S,, and k) as the ScN thin films reported in the literature with “low” ZT values. Different approach of increasing ZT can be chosen between increase the Seebeck, reduce the electrical resistivity or decrease the thermal conductivity. Theoretical calculations showed a possibility of improving the overall thermoelectric properties of ScN by insertion of defect in the matrix such as vacancies of nitrogen or dopants, as Mg in our case [4]. Doping of the thin film were chosen to be achieved by ion implantation. Ion implantation, more than doping of the thin film can create defects which may reduce the propagation of the phonons, and so the thermal conductivity. Different percentages of Mg+ were implanted with different conditions of implantation (Temperature, implantation profile). Implantation of few percent Mg were achieved while keeping the crystal structure and the morphology of the initial film. At the maximum, the Implantation of Mg+ allowed to reduce 2.5 of the thermal conductivity, to increase of the Seebeck values while keeping a reasonable value of the electrical resistivity. [1] Eklund, P. et al, J. Mater. Chem. (2016) [2] Burmistrova, P. V. et al, J. Appl. Phys 113 153704 (2013) [3] Quintela, C. X. et al, Adv. Mater. 27 3032 (2015) [4] Kerdsongpanya, S et al, Phys. Rev. B 86 195140 (2012)

Resume : We present a novel nano structuring approach that enables us to fabricate bismuth based nanowire composites in a cheap and scalable manner. Mesoporous silica with an interconnected pore system was used as template for the synthesis of bismuth based nanowires inside its pore channels. A new impregnation technique was developed to impregnate bismuth salt into the template. The influence of various reduction methods and the template’s surface chemistry on cluster formation and leaching behaviour were investigated by means of TEM. Post modification treatments were performed and studied to prevent oxidation of the nanowires and to ensure good electrical conductivity. The nano composite material was characterized by means of N2 sorption measurements, XRD analysis, XRF analysis, XPS analysis, TEM and STEM – EDX.

Resume : Although the crystal structure of Cu2Ge(S1-xSex)3 with 0 < x < 1 has been recently reported by several authors, the dependence of the thermoelectric properties of these compounds as a function of x have not been reported yet. In this work, we have employed ampoule synthesis followed by Spark Plasma Sintering (SPS) in order to prepare samples of Cu2Ge(S1-xSex)3 covering the whole range of x. Each specific stoichiometry has been characterized by XRD, Hall effect measurements at room temperature, and power factor measurements versus temperature. Moreover, we have measured the thermal diffusivity and the heat capacitance versus temperature for each cases in order to quantify their respective thermoelectric figure-of-merit (zT). All samples have been found to exhibit a p-type conductivity as attested by Hall effect measurements. Among the different compounds investigated, Cu2GeSe3 (x=1) has shown the highest thermoelectric performance with a zT of 0.31 at a temperature of 460°C. This zT could be increased up to 0.42 at same temperature by using an excess of germanium. In the case of Cu2GeS3 (x=0), a decrease of the resistivity by 3 orders of magnitude could be achieved by an adequate doping with indium. We will thereby report the first zT values ever published on p-type doped Cu2GeS3.

Resume : The most relevant outcomes of the joint project “Tailoring Electronic and Phononic Properties of Nanomaterials: Towards Improved Thermoelectricity (nanoTHERM)” will be presented, a Spanish research Consolider project focused on the understanding of the thermoelectric materials and the tailoring of both their electronic and phononic properties toward the optimization of the efficiency of thermoelectric devices working at low and high temperatures. Reference: Olga Caballero-Calero and Roberto D'Agosta Review—Towards the Next Generation of Thermoelectric Materials: Tailoring Electronic and Phononic Properties of Nanomaterials ECS J. Solid State Sci. Technol. 2017 volume 6, issue 3, N3065-N3079

Resume : Half-Heusler compounds are one of the most promising candidates for thermoelectric materials for automotive and industrial waste heat recovery applications. In this talk, we will give an overview about our recent investigations in the material design of thermoelectric half-Heusler materials. Since the price for Hafnium was doubled within the last 18 months, our research focusses on the design of half-Heusler compounds without Hafnium. We will present a very recent calculation on ZT per € and efficiency per € for various materials followed by our latest very promising results for n-type half-Heusler compounds without Hafnium resulting in 20 times higher ZT/€ values. Additionally and even more important are the investigations about the upscaling possibilities. Any high temperature TE material will only be suitable for the mass market if the material production and the module production is industrial upscalable. Therefore, we will focus on various upscaling approaches, their challenges, how we tackle these challenges and our recent results with the different upscaling approaches. These results strongly underline the importance of phase separations as a powerful tool for designing highly efficient materials for thermoelectric applications that fulfill the industrial demands for a thermoelectric converter.

Resume : Intermetallics with Half-Heusler (HH) structure have attracted major interest as efficient thermoelectric (TE) materials in the temperature range around 700 K, which is close to the temperature range of most industrial waste heat sources. Here we will summarize our recent progress in improving the thermoelectric performance of HH compounds via nanostructuration. This approach enables the decoupling of thermal and electrical transport properties in HH compounds. By controlling the in situ formation of InSb [1-2], MnSb [3] and full-Heusler nanoinclusions [4] in the half-Heusler matrix, it was shown experimentally that the Seebeck coefficient, electrical conductivity and thermal conductivity can be independently manipulated in a manner that significantly enhances the figure of merit (ZT) of these materials. References [1] W. J. Xie, Y. G. Yan, S. Zhu, M. H. Zhou, S. Populoh, K. Gałązka, S. J. Poon, A. Weidenkaff, J. He, X. F. Tang, T. M. Tritt, Acta Materialia 61, 2087–2094 (2013). [2] W. J. Xie, J. He, S. Zhu, X. L. Su, S. Y. Wang, T. Holgate, J. W. Hubbard, V. Ponnambalam, S. J. Poon, X. F. Tang, Q. J. Zhang, T. M. Tritt, Acta Materialia, 58, 4705–4713 (2010). [3] W. J. Xie, A. Weidenkaff, in preparation. [4] W. J. Xie, J. Feng, T. H. Zou, X. X. Xiao, M. Widenmeyer, A. Weidenkaff, in preparation.

Resume : Engineering lattice thermal conductivity commonly implies controlling the heat flow carried by waves of atomic vibrations called phonons. Our understanding and the modelling of thermal transport requires to get a full comprehension of phonon properties at all energies and temperatures: the experimental investigation of the individual properties of phonons is essential and shedding light into the mechanisms underlying the single phonon lifetime fundamental for new materials engineering. However, while phonon energies and group velocities are experimentally mastered properties, the measurement of acoustic phonons lifetime turns out to be difficult even in simple systems. In this talk, we will first present the techniques of spectroscopy currently used for measuring thermal phonon energies, velocities and lifetimes (inelastic neutron and X-rays scattering) and show the experimental limitations, which at this day prevent from a measurement of long phonon lifetimes in most crystals . Based on our results published in [1-3], we will then focus on the studies of phonon dynamics in a family of complex thermoelectric crystal structure, the so-called clathrate, renowned for their puzzling glass-like thermal conductivity. Here, we present the first quantitative measurement of phonon lifetimes in a single crystal of the clathrate-I phase Ba7.81Ge40.67Au5.33 obtained using the Neutron Resonant Spin-Echo technique (NRSE) [4]. [1] H. Euchner, S. Pailhès et al., Physical Review B 86: 224303 (2012) [2] S. Pailhès et al., Phys. Rev. Letters 113: 025506 (2014) [3] Y. Bouyrie, C. Candolfi, S. Pailhès et al, Phys. Chem. Chem. Phys., 2015,17, 19751-19758 [4] P. F. Lory, S. Pailhès et al., submitted.

Resume : Copper-based chalcogenides that comprise abundant, low-cost, and environmental friendly elements are excellent materials for a number of energy conversion applications, including thermoelectrics (TE). In TE the use of solution-processed nanocrystal (NC) to produce thin films or bulk nanomaterials has associated several potential advantages, such as high material yield and throughput, and composition control with unmatched spatial resolution and cost. Here we report on the production of Cu based chalcogenides NCs with tuned amounts of dopants. After proper ligand removal, these NCs were used to produce dense bulk nanomaterials for solid state TE energy conversion. By adjusting the amount of extrinsic dopants, dimensionless TE figures of merit (ZT) above 1.3 at 670 K were reached. Such high ZT values are related to an optimized carrier concentration, a minimized lattice thermal conductivity due to efficient phonon scattering at point defects and grain boundaries, and to an increase of the Seebeck coefficient obtained by a modification of the electronic band structure with different dopants chosen, as demonstrated by extensive and precise Density Functional Theory (DFT) calculations.

Resume : Approximately 36,000 possible ternary alloy systems can be formed from 61 commercially viable elements. Clearly, it is impossible to perform systematic experimental investigation among such large number of materials. Although the predication of thermoelectric properties from the first principle becomes possible owing to recent progress in computational materials sciences, the time, efforts and resources required for computation are almost as exhaustive as experimental approach. A fast and reliable screening procedure is needed. The data analysis shows that the thermoelectric power factor appears to be related to the electronegativity difference of their constituent elements, while the thermal conductivity is associated with atomic weight, atomic radii and electronegativity. A phenomenological thermoelectric property diagram (PTPD) is constructed that has potential to serve as an initial screening produce to identify promising candidate materials from the available information on the periodic table. This can lead to accelerated discovery of new thermoelectric materials.

Resume : In the quest of finding new materials for thermoelectric applications, the question of “how selecting promising materials?” is raised. Since low thermal conductivity (κ) is one of the components that can lead to good thermoelectric materials, ideal candidates would be materials fulfilling this specification. Many studies showed that low dimensional structures associated to disorder are conditions needed to obtain low lattice thermal conductivities (κL). It was then chosen to focus our research on finding new phases inside families that present low dimensional features. The successfully synthesized quasi one-dimensional pseudo-hollandite Ba0.5Cr5Se8 [1] and the known compound TlIn5Se8 [2] show thermal conductivities as low as 0.8 and 0.45 W.m-1.K-1, respectively. Additionally to these studies, a new layered compound of the MnPSe3 family was characterized. A transmission electron microscopy in HREM mode analysis shows a high rate of stacking fault inside the material leading to low thermal conductivity of 0.35 W.m-1.K-1. [1] Lefèvre, R.; Berthebaud, D.; Perez, O.; Pelloquin, D.; Hébert, S.; Gascoin, F. Polar Transition-Metal Chalcogenide: Structure and Properties of the New Pseudo-Hollandite Ba0.5Cr5Se8. Chem. Mater. 2015, 27 (20), 7110–7118. [2] Lefèvre, R.; Berthebaud, D.; Perez, O.; Pelloquin, D.; Boudin, S.; Gascoin, F. Ultra low thermal conductivity of TlIn5Se8 and structure of the new complex chalcogenide Tl0.98In13.12Se16.7Te2.3. J. Solid State Chem. Submitted

Resume : Thin film thermoelectric generators (TFTG) are a promising via in thermoelectric field, especially on constrained to micrometric and large area applications such as sensors and smart-windows [1]. In this context, the substrate dimensions and their physical properties (thermal and mechanical stability, etc.) may play a significant role to determine the efficiency of the final device. In this work, the influence of the substrate thermal properties on the efficiency of an ideal TFTG was investigated by combining fundamental thermoelectric theory, comsol simulation and experimental measurements. An “effective” decoupling of thermal and electrical parameters of the films/substrate is observed in TFTG with planar geometry turning out in an increase of the output power compared with those exhibiting orthogonal geometry and bulk under certain conditions. Therefore, films with high power factor deposited onto low thermal conductors substrates could be used as thermoelectric materials. I-V curves of semimetal films with different thickness deposited onto different substrates were measured to validate the previous calculations. [1] P. Fang et al., Appl. Phys. Lett. 2015 102, 033904 .

Resume : We report the synthesis, chemical and transport properties of p-type polycrystalline silicon germanium for use as the high temperature (300 K – 900 K) Seebeck coefficient National Institute of Standards and Technology (NIST) Standard Reference Material (SRM®). Chemical composition and purity analyses include XRD and XRF; low temperature transport measurements (2 K – 390 K) include electrical resistivity, Seebeck coefficient, thermal conductivity, and carrier concentration; and high temperature transport measurements include electrical resistivity and Seebeck coefficient. In addition, we have characterized the thermocyclic variability of the Seebeck coefficient to define dopant stable temperature boundaries. This SRM will complement SRM 3451 “Low Temperature Seebeck Coefficient Standard: 10 K - 390 K”, enabling researchers the means by which to calibrate instrumentation and to facilitate reliable data comparisons. In this talk, we will also highlight our work in both developing and promoting standardized measurement protocols, with the goal of assisting the thermoelectric community in the development of more efficient materials and devices. These protocols have been evaluated using our custom high temperature thermoelectric measurement apparatus, which is uniquely capable of in situ comparison of commonly applied probe arrangements and measurement techniques. We demonstrate that the probe arrangement is the primary influence on measurement accuracy at high temperature due to the thermal errors in measuring temperature by direct surface contact.

Resume : Thermal conductivity measurements using the 3? method with scanning thermal microscopy (SThM) have been carried out with a SiN probe. The 3??SThM technique is able to obtain simultaneously the topographical and thermal images of the analyzed sample. These thermal images are related to the 3??voltage that is proportional to temperature variations in the probe and therefore to the heat flux between the sample and probe [1]. Thereby, the thermal conductivity of the sample can be determined. In this work, the thermal conductivity of TiO2 nanotubes filled with polycarbonate is analyzed. The low thermal conductivity and Magnèli phases of titanium oxides make of this a prominent material for thermoelectric applications [2]. In order to study the influence of both the air and the filler material in the nanotubes thermal conductivity, TiO2 nanotubes without polycarbonate and polycarbonate in the AAO template will be also studied with the 3? technique [3]. References: [1] Cahill, D. G., Rev. Sci. Instrum. 1990, 61, 802. [2] Harada S, Tanaka K, Inui H. J. Appl. Phys. 2010, 8: 83703-83709. [3] Rojo, M. M., Grauby, S., Rampnoux, J-M, Caballero-Calero, O., Martín-Gonzalez, M., Dilhaire, S, J. Appl. Phys. 2013, 113 (5), 054308

Resume : In the last 10–15 years, organic materials have become important in optoelectronics, both in solar cells and light emitting diodes and, more recently, in thermoelectricity, especially as thin films, where the polymer chains remain basically in two dimensions, which improves the conductivity. In the last few years, several intrinsically conducting polymers (ICPs) have been successfully used in the field of thermoelectricity and the dimensionless figure of merit ZT (ZT=α2σT/κ where α, σ and κ are the Seebeck coefficient, the electrical and thermal conductivities, respectively) has been improved several orders of magnitude, until values very close to those of inorganic materials. Polymers present, in addition, many advantages over inorganic materials: non scarcity of raw materials, lack of toxicity, lower cost of production and many others. The incorporation of conducting inorganic nanofillers, as CNTs, can be used to control the conductivity and thus the thermoelectric performance of ICPs. In this work, the objective is to develop new hybrid inorganic-organic materials to improve the efficiency of thermoelectric devices. So, ICPs can be conveniently mixed with some inorganic materials such as carbon nanofillers and the films obtained have been characterized. In addition, a new method for the fabrication of thermoelectric modules (TEG) has been with these materials.

Resume : Radioisotope Thermoelectric Generators (RTGs) have proven to be extremely reliable components of space power systems, enabling the scientific exploration of deep space, Mars, and the moon. These systems are based on technological advances completed in the 1960’s and 1970’s. RTGs have relied on thermoelectric couple technology based on materials identified and developed over 50 years ago. These “single point design” generators use a converter array configuration with hundreds of discrete thermoelectric (TE) couples interconnected on the cold side in a series-parallel “laddering” pattern to achieve high redundancy and eliminate single point failures while meeting their output voltage requirement under maximum output power condition. NASA constantly seeks the development of more capable and high-performing flight systems in support of future science and exploration missions, and it is desirable to identify and develop common energy converter technology “building blocks” that could span a wide range of potential next-generation power systems. The use of arrays of discrete couples to assemble into converters becomes impractical either at high power levels (> 1 kW) due to the large quantities and constraints posed by system integration, or at low power levels when requirements dictate a high module output voltage (e.g. 32V), which translates into a large number of couples with high aspect ratio. The best approach for practical and efficient thermal and mechanical integration with the heat source and heat rejection system components is to develop robust modular TE device architectures. This provide maximum versatility for application to system concepts ranging from terrestrial high-grade waste heat recovery to space power. We discuss the key scientific and engineering challenges associated with developing and validating the performance of novel high temperature thermoelectric devices based on higher performance materials such as skutterudites and Zintl phases. Progress in the development of suitable electrical, thermal and mechanical interfaces, and results from performance testing in relevant environments are presented.

Resume : A thermoelectric sensor of far-infrared radiation (FIR) can be fabricated from a thermopile by adding a suitable absorber for the range of wavelengths to be detected on the hot junction. Recently, arrays of thermopiles for a thermoelectric generator have been fabricated on silicon membranes using a CMOS-compatible process [1]. Thermoelectric sensors of FIR may lead to the low-cost fabrication of arrays for applications in the home-appliances and automotive industry [2]. Here, we are investigating sensors where hot and cold junctions are part of a suspended membrane. Apart from enhanced sensitivity, such a design has potential for miniaturization and the advantage of an easier fabrication process [3]. A multilayer structure including a SiGe layer was deposited by molecular-beam epitaxy on (100) oriented silicon-on-insulator (SOI) wafers. Metallic contacts were deposited and patterned to form an array of SiGe/metal thermopiles. Silicon was selectively etched below the hot junctions to form suspended membranes. Hot junctions were covered with either a polymer or gold-black as radiation-absorbing materials. We measured the absorption spectra of these materials and found that they perform well in the FIR range. Sensitivity and responsivity of devices were measured in a setup with a large-area heat source with temperature controlled from ambient up to 100 ºC. Our original design allows for a simple fabrication process which is compatible with standard CMOS technology. We intend to use these devices as a test-bench to improve thermoelectric materials by incorporating a variety of nanostructures in the membrane such as modulation-doped quantum wells and quantum dots. [1] A. P. Pérez-Marín et al., Nano Energy 4 (2014) 73. [2] M. Hirota et al., Proc. of SPIE, volume 6940, (2008) 694032. [3] P.O Vaccaro et al., ECT-2016, Lisbon, Portugal, September 20-23, 2016.

Resume : Nanotechnology has opened a door to enhance the figure of merit of thermoelectric materials. However, despite the spectacular improvements achieved in the last decades, the practical implementation of the proofs of concept has been extremely limited. The main reason is the absence of cost effective high scale techniques able to produce stable nanostructured materials. For this reason, an important goal is to find strategies to get access to the aforementioned outstanding properties, looking for cheap high scale production techniques. In this work, a new material based on flexible fibers of doped silicon and silicon-germanium nanotubes is proposed. Macroscopic easy to handle fabrics of several square centimeters were produced by means of scalable fabrication techniques: electrospinning and chemical vapor deposition. The diameter of the constituting tubes is around 500 nanometers, presenting a nanometric nature thanks to a wall thickness in the range of 10 to 300 nm. Thermal and electrical characterizations were carried out at the produced fabrics, showing a huge potentiality as low cost efficient material for thermal energy harvesting. A synergic enhancement is observed thanks to the sum of a low thermal diffusion along the nanostructured nanotube walls and a decrease of the air convection across the micro-cavity arrangement of the fibers. Remarkably large temperature differences can be achieved in short distances (c.a. 150 µm) without the need of using complex encapsulations. After the excellent results provided by this silicon-germanium case of study, the proposed fabrication approach can be envisaged as a general procedure to obtain large area nanostructured paper-like materials that can be easily produced, handled and implemented in large areas for waste heat recovery.

Resume : Thermoelectric bolometers suitable for fast and sensitive detection of weak levels of thermal power are presented. These bolometers are fabricated by patterning a 40 nm thick silicon membrane [A. Shchepetov et al., Appl. Phys. Lett. 102, 192108 (2013)] into 50 µm x 50 µm and 100 µm x 100 µm areas suspended by beams. The thermal signal is transduced into electric voltage using thermocouple consisting of highly-doped n and p type Si beams. The devices operate in a Johnson-Nyquist noise limit with the measured noise equivalent power below 20 pW/rtHz, voltage responsivity in the order of 1000 V/W, and thermal time constant in the range of 1-10 ms. Reducing the thickness of the Si membrane improves the performance (i.e. sensitivity and speed) as thermal conductivity and thermal mass of Si membrane decreases with decreasing thickness [S. Neogi et al., ACS Nano 9, 3820 (2015)]. This and other performance improvements and use of these devices in scanning thermal microscopy and optical applications, such as uncooled detectors in long-wave IR sensing, are discussed.

Resume : Nanostructured and low-dimensional systems have long been of considerable research interest from the perspective of thermoelectricity, known to benefit from the quantum and classical size effects for electrical and thermal conduction, respectively. In the recently established family of two-dimensional materials, especially graphene has shown potential for thermoelectric application due to its unique electrical properties, as well as economical and ecological advantage over the current high-efficiency thermoelectric materials. Several theoretical studies have strived to circumvent the anomalously high thermal conductivity of graphene together with the modest Seebeck coefficient originating from the semimetallic character in order to maximize the thermoelectric figure of merit, ZT. However, reports on succesfully translating these strategies into viable large-area applications remain scarce. We present a simple solution-based method for producing large-area flexible all-graphene films for thermoelectric application. We show that the solution-based deposition scheme is instrumental in producing thin films of comparable sheet resistance to graphene grown by chemical vapour deposition while dramatically reducing the thermal conductivity of the structure, constituting transport characteristics of the phonon-glass electron-crystal approach. Consequently, the thermoelectric performance of the thin films ranks in the range of graphene-based organic nanocomposites, surpassing prior reports on large-area all-graphene structures. Furthermore, a novel large-area distributed thermocouple sensor concept is presented, utilizing a single flexible thermoelectric thin film as a common leg for an array of individual thermocouples [1]. This facile and scalable device principle provides real-time spatiotemporal information of the thermal environment of the device. While the operational principle is liberally applicable to, e:g:, human interface devices, biometry, and heat distribution mapping, we demonstrate its operation as a touch panel with atomic layer deposited ZnO as the active material, combining transparency and flexibility with reasonable thermoelectric performance. [1] M. Ruoho, T. Juntunen, T. Alasaarela, M. Pudas, and I. Tittonen, Adv. Mater. Technol. 1 (2016) 1600204.

Resume : Nanotechnology opened the way for the production of more efficient thermoelectric (TE) materials. But to definitively boost the applications spectra of this technology and reach additional and larger markets, it is necessary to advance in parallel in two directions: development of efficient nanomaterials at the large scale and design of new TE devices with optimized thermal contacts. The design and engineering of nanomaterials by nanoparticle-based solution processing technologies opens up countless opportunities to produce metamaterials with controlled functional properties. Furthermore, taking advantage of the versatility of this technique, it is possible to assemble them into conformable or rigid but shape-adapted TE devices, maintaining low manufacturing costs. In this work, we will prove that this methodology provides the necessary level of compositional and morphological control to produce high performance inorganic TE materials and devices, reaching different real-scenario applications: from the fabrication of shape-adaptable ring-shaped TE generators that offer a simple design with good thermal contact for automobile applications, to conformable TE devices produced by means of ink-based additive manufacturing technologies.

Resume : With the emergence of low power communication protocols for the Internet of things, there is a real demand for autonomous micro-sources of energy that can supply the battery and deliver power in the order of 100 μW. These ranges of power can be supplied by a standard thermoelectric module with a temperature gradient of ~ 10 °C under stationary operating conditions. However, these conditions of use are not necessarily adapted for the connected objects. For example, the energy inputs can be very intermittent, or the volume available in an object too limited to be able to arrange a TE module and its heat sink. The expertise developed at the Institut Néel in the development of suspended sensors dedicated to thermal measurements at very low temperatures has been used to designe a suspended thermoelectric micro-module based on a network of micro-membranes. The duplication of this small thermoelectric module thanks to standard clean room techniques enabled us to obtain power of approximately 0.4 μW.K-1.cm-2. Under production conditions, these powers should reach the value of 1 μW.K-1.cm-2. These values are sufficiently high to contemplate the use of these micromodules in support of a battery in order to supply sober sensors whose autonomy would then be made infinite.

Resume : We discuss the structural as well as thermoelectric properties of the layered compounds K(x)RhO(2) in comparison to isostructural and isovalent Na(x)CoO(2). The optimized structure of K(1/2)RhO(2) exhibits a remarkable deviation of the c/a ratio from the experimental result as well as from c/a ratios of related compounds. This indicates that a hydrated phase of K(x)RhO(2) exists and the experimental structure determination refers to this hydrated phase. The calculated Seebeck coefficient of pristine K(1/2)RhO(2) amounts to 50 μV/K at 300 K, which is close to the experimental value 40 μV/K. Importantly, we observe high values for the Seebeck coefficient and power factor for hydrated K(x)RhO(2) in the whole temperature range from 0 to 700 K. At 100 K, we find for hydrated K(7/8)RhO(2) a value Z = 3·10(−3) K(−1), which is the highest power factor observed at this temperature. It exceeds also the exceptionally high value of Na(0.88)CoO(2) by more than 50%. Our results, hence, demonstrate that hydration is an effective approach to modify the lattice parameters and, as a result, enhance the thermoelectric performance. The Na(x)RhO(2) oxides are found to form a new class of materials with exciting thermoelectric features, even outperforming the 2H phases of the K(x)RhO(2) system. In the latter the optimal thermoelectric performance is achieved at low temperature, whereas the modified stacking of the atomic layers in the 3R phases of Na(x)RhO(2) results in a reduced interlayer coupling and, in turn, in a dramatically enhanced thermoelectric response in the technologically relevant high temperature range. We find that Na vacancies in Na(x)RhO(2) avoid clustering and that the RhO(6) octahedra are modified depending on the amount of Na deficiency. Analysis of the induced changes in the DOS close to the Fermi level indicates that the Rh(3+δ) d(3z2−r2) states control the transport properties of the compounds. A substantial figure of merit of 0.35 at 580 K is found in hydrated Na(0.83)RhO(2) due to the enhanced effective mass of the charge carriers. In general, the figure of merit can be further increased by reduction of the Na vacancy concentration to increase the resistivity. Journal References: Adv. Funct. Mater. 22, 2792 (2012); Sci. Rep. 4, 4390 (2014).

Resume : We study the thermoelectric performance of two different La(1−x)Sr(x)CoO3 sample series. All studied samples have been produced through the same chemical route. Though, alternative heat treatments lead to the production of two different sample series, with respect to their microstructure. In particular, the first series (called A-series samples) consists of highly dense samples with inhomogeneously-mixed grains of different sizes, while the other series (B-series samples) includes porous samples made from grains of uniform size. We show that the A-series samples exhibit Seebeck coefficient values higher than B-series ones. The enhancement could be plausibly related to the “energy-filtering” mechanism that is related to the energy barrier of the grain boundary. On the other hand, the thermal conductivity for the porous B-series compounds is significantly reduced in comparison to the dense A-series ones. As a consequence, it is suggested that a fine-manipulation of grain size ratio combined with a fine-tuning of porosity could considerably enhance the thermoelectric performance of oxides, towards energy harvesting applications, at temperatures near or above room temperature.

Resume : The thermoelectric properties of Sr1-xLax/2Smx/2TiO3-δ (0.0≤x≤0.30) ceramics have been investigated with compositions batched, synthesised by solid state reaction, and sintered in 5% H2/N2 at 1500 oC for 6 hrs. All peaks in X-ray diffraction patterns could be indexed according to a single perovskite phase. Scanning electron microscopy revealed a homogeneous grain structure and confirmed relative density ≥ 89 %. The electrical conductivity of ceramics with x≤0.15 were < 1000 S/cm and metallic in nature, whereas with increasing dopant concentration they became semiconducting. The Seebeck coefficients of all compositions were negative and thus exhibited n-type behaviour. A maximum power factor of 1400 μW/K2.m was obtained for x = 0.10 at 573 K. The lowest thermal conductivity was ~ 3 W/m.K at 973 K for Sr0.8La0.1Sm0.1TiO3-δ. The highest dimensionless figure of merit (ZT) achieved to date was 0.24 for x = 0.15 at 875 K.

Resume : As the performance of a thermoelectric device depends on the figure of merit (ZT) of the employed materials, efforts are usually made in order to improve the Seebeck coefficient and the electrical conductivity and reduce the thermal conductivity. In this context, a purposeful microstructural design might be beneficial for achieving the desired properties in polycrystalline materials. Starting from these considerations, obtaining a percolating three-dimensional conductive network surrounding insulating SiAlON particles may result in higher power factor than a conventional particle-reinforced composite consisting of the very same phases with the same volume fractions. For this purpose, two different sets of samples were prepared in this study. Firstly, for the particle reinforced composites (PRC), 10 vol.% TiC0.7N0.3 with an average particle size <150 nm was mixed with SiAlON powder. Secondly, in order to obtain a percolating three-dimensional network, spray-dried SiAlON granules were coated with TiC0.7N0.3. As a consequence, enhanced values of power factor, as well as the figure of merit, were achieved between 300 and 800 K in the comprising the TiCN three-dimensional percolating network. The findings obtained from this model system are encouraging for designing materials with enhanced thermoelectric properties.

Resume : Layered mixed anion chalcogenides have received increasingly attention due to their complex crystal structure and rich chemical/physical properties. For instance, the layered Bi2O2S is a good photoelectric material, while the LnOBiS2 (Ln = La, Ce, Pr, Nd, Sm), are famous superconductors. Recently, the layered BiOCuSe is reported to be a promising thermoelectric material with ZT upto 1.4. Electrons and phonons transport occurred in distinct two type of layers ([Cu2Se2]2- and [Bi2O2]2+ ), and hence decoupled with each other. Based on the prototype BiOCuSe, we designed and synthesized a new layered oxyselenide, namely the SmCrSe2O. The structure of the SmCrSe2O features 2-dimentinal [CrSe2O]3- layers, composed of [CrSe6]9- and [CrSe4O2]9- octahedra. The Sm3+ ions reside between the [CrSe2O]3- layers. SmCrSe2O shows metallic properties at room temperature with low Seebeck coefficient. However, it turns to be a narrow band semiconductor after Bi/S codoping. Therefore, the power factor has been significantly enhanced.

Resume : Layered cobalt oxide, Bi2Sr2Co2Oy, is a promising candidate for the thermoelectric applications because of its high thermopower, combined with metallic electrical conductivity, and the low thermal conductivity characteristic of the misfit crystal structure. However, the origin of the high large thermopower of misfit cobaltites is still under debate. For example, although the magnitude of the temperature independent thermopower at high temperature was explained in terms of an additional spin-orbital degeneracy term to the Heike’s formula and other authors suggest that multiple carriers (itinerant and polaronic) are instead responsible for the large thermopower (Ref. Beatriz et. al. Sci Rep, 2015; DOI:10.1038/srep11889 ). In this work, we have grown highly c-axis oriented Bi2Sr2Co2Oy thin films on LAO (100) substrates by PLD. Through different annealing protocols in oxygen, we controlled the oxygen stoichiometry of the films, and therefore the carrier density, as measured by Hall effect. With the carrier density measured in each film, we have calculated the expected thermopower by using Heikeʼs formula. We have found that these values are in good agreement with the measured thermopower at room temperature. This shows that the thermopower in misfit cobalt oxides can be understood without including the spin and orbital contribution to the entropy in the Heike’s formula.

Resume : With the emergence of flexible thermoelectrics the growth of high performance transferable and flexible thin film thermoelectric materials has become a serious challenge. Here, we present a novel approach for the growth of preferentially nanopatterned and transferable thin Ca3Co4O9 films for flexible thermoelectric applications. The film is grown by sequentially depositing CaO-CoO film on mica substrate by rf-magnetron reactive sputtering, followed by post annealing to form the final phase of Ca3Co4O9. The presence of randomly distributed holes of nanometer dimension (10 – 200 nm) on the film surface is expected to drastically reduce its thermal conductivity. Despite the near 25 % coverage of the film surface by nanoholes its thermoelctric performance is found to be remarkably high, achieving the highest power factor nearly 0.2 mWm-1K-2 near 600 K, which is attributed to the good crystalline quality of the film. The room temperature value of electrical resistivity and Seebeck coefficient of the film are measured to be nearly 9 mOhm.cm, and 123 uV/K, respectively. The most interesting part of the film is that its interfacial region look like perforated paper, which allow the easy isolation of the film from parent mica substrate just by mechanical stripping by some carrier or receiver layer. Further enhancement of power factor is possible by tuning the size of the naoholes in the film, while size of the nanoholes can be controlled by tuning thickness of alternate layers of CaO and CoO in as-deposited films.

Resume : High-efficiency thermoelectric (TE) materials are important for power-generation devices that are designed to convert waste heat into electrical energy or to use in solid-state refrigeration. These applications require innovative materials which not only possess high conversion efficiency (related to high dimensionless number called figure of merit, "ZT" which is a combination of three material properties: Seebeck coefficient, electrical conductivity and thermal conductivity , but should also be no toxic and have high chemical stability in air, over a wide temperature range such as oxide materials. From last decade, a major breakthrough in the field of TE came by designing nanostructures that scatter phonons more effectively than electrons, so that the thermal conductivity is reduced more than the electrical conductivity. Herein we present a cheap and thermodynamically driven process to produce intra-granular nanostructures in bulk materials: the spinodal decomposition in the Nb5+-doped SnO2-TiO2 system via an innovative molecular approach. Such in-situ partitioning takes advantage to produce nanostructuration with coherent interfaces. While classical approach based on SnO2 (nano)powders mixing faces trouble to sinter dense ceramics, our bottom-up approach, trough synthesis of mixed TixSn1-xO2 (x = 0.25; 0.5; 0.75) rutile nanoparticles, suppress pure SnO2 grains and allow full densification at low temperature by SPS process. Impact of the pressure, heating rate and soaking time of SPS process onto the densification, the nano-structuration and the dopant distribution will be addressed towards the thermoelectric properties.