Caloric materials for efficient heat management applications: advances and challenges

Resume : In recent years, several Electrocaloric (EC) heat exchangers have been proposed [1-5], covering different kinds of mechanisms and working principles. However, little has been told about the numerical modelling of these devices and their potential impact on improving the experimental performance. In this work, the authors highlight and prove how finite elements method could exploit and optimize EC heat exchangers. The simulations presented, that match both the transient and the steady state of the device, consist of 2D-representations of the lead scandium tantalate multilayers capacitors (PST MLC) 22mm x 10.4mm x 1mm parallel-plates based active regenerator developed in LIST and are used to explore new prototype’s configurations. The results obtained showed that, for example, by enlarging the length of the regenerator and decreasing the thickness of the parallel-plates, the temperature difference in the device could increase already up to 9 degrees. The performance of other working fluids with better thermal properties, as the case of water, is also attempted, displaying very encouraging results. By guidance of this outcome, some of the proposed new prototype’s configurations were tested in the laboratory. The maximum experimental temperature difference measured was of 7.5 degrees, several factors larger than the value obtained before the numerical modelling was investigated. [1] H. Cu et al., Applied Physics Letters, 102 (2013) [2] U. Plaznik et al., Applied Physics Letters, 106 (2015) [3] Y. D. Wang et al., Applied Physics Letters, 107 (2015) [4] E. Defay et al., Nature communications 9 (1), 1827 (2018) [5] Ma et al., Science, 357, 1130–1134 (2017)

Resume : In comparison to the Carnot efficiency of different cooling technologies, electro-caloric (EC) systems stand out as one of those able to overtake the most used vapour compression systems [1]. However, the energy efficiency of EC materials has barely been studied, particularly directly. EC materials exhibit adiabatic temperature change or isothermal entropy when triggered with electric field. Their dimensionless materials efficiency, called EC efficiency, is the ratio between the heat generated and the electrical work needed to trigger the EC effect. By direct measurements of heat using infra-red (IR) camera, DSC, and electrical work, we report unambiguous quantitative results of the efficiency of Lead Scandium Tantalate (PST).These measurements are done on two different PST samples: 1 mm-thick bulk ceramic and 0.5 mm-thick multi-layer capacitors (MLCs). For both, we observed an adiabatic temperature change ∆Tad of 2.4 K at room temperature. For this ∆Tad, we report an efficiency of 35 and 85 respectively for MLCs and bulk. We present a comparative study on energy efficiency of PST with different caloric materials worked out indirectly in previous articles [1-2]. We reveal a maximum efficiency of 101 for PST (EC), which is four times more than Gd (magneto-caloric) [1] and PbBaZrO_3 (EC) [1] and an order of magnitude more efficient than mechano-caloric materials [1]. Our study gives an insight into the efficiency of EC materials and shows that PST is an excellent candidate for future efficient EC cooling devices. [1] X.Moya, E.Defay, V.Heine and N.Mathur, Too Cool to work, Nature Physics, 2015. [2] E.Defay, et al., Advanced Materials, 2013, 25, 3337-3342.

Resume : Electrocaloric materials are of particular interest as they could open the way for an entirely new range of cooling technologies. The adiabatic temperature change they exhibit, known as electrocaloric effect (ECE), is the first half of a cooling cycle. The second half corresponds to the exchange of heat between the electrocaloric material and its surroundings. Now that several materials with a large ECE have been identified in the literature, heat transfer needs to be studied as well. The first objective of this work was to characterize the specific heat and thermal conductivity of PST, a ceramic presenting a large ECE at room temperature. Specifc heat was measured with DSC and thermal conductivity from laser flash analysis. The measurement were conducted on a large temperature and electrical field range, near the ferroelectric-paraelectric transition. These results also served as a reference to develop new methods of thermal characterization with an infrared camera, whose original purpose is the measurement of ECE. The idea was to obtain every physical property relevant to a cooling cycle with a single measurement apparatus. The specific heat was extracted from the temperature evolution of a sample submitted to a high frequency waveform; the dielectric losses acted as a heat source. The second method required a partially electroded sample where heat was generated by the ECE itself. By monitoring heat diffusion in the bulk, we extracted the material thermal conductivity.

Resume : In materials science, intentional doping has been widely used to improve the properties of a varity of materials. However, such approach si not exploited yet in the fast growing and active field of electrocaloric materials that are serious alternative candidates for cooling refrigeration. Here, we demonstrate on Ba0.9Sr0.1Hf0.1Ti0.9O3 an eco-friendly ferroelectric material that the intentional doping (2% of Cu) which introduces defect-dipoles within the ferroelectric matrix permits i) to enhance the adiabatic temperature change ΔT by up to 54% while maintaining the performances after a large number of electric-field cycles (up to 104), ii) to suppress the parasitic irreversibility in the ferroelectric state between ON-field and OFF-field ΔT response, and iii) to design original refrigeration cycle with pre-poled sample thanks to a two-field step -conventional (ΔT > 0) and inverse (ΔT < 0)- process when the field is sequentially varied. As a by-product, we also show that intentional doping significantly increases the energy storage density (up to 72%). Therefore, the defect engineering approach opens a new path for designing ferroelectrics with improved electrocaloric performances and novel cooling concepts as well as of interest for energy storage applications. Beyond ferroelectrics, this strategy could also be promising in other solid-state caloric materials (magnetocalorics, elastocalorics, etc).

Resume : Both ferroelectric and antiferroelectrics may have large electrocaloric effect due to their multiple types of phase transitions induced domain changes, demonstrating potential in the solid-state cooling technology. In this study, pristine and Gd3+ and Sn4+ doped PLZT compositions Pb0.93La0.07 (Zr0.82Ti0.18)0.9825O3) were designed, fabricated and their ECE are characterized directly, and also compared with the results derived by the Maxwell relations and the Landau – Ginzburg - Devonshire phenomenological theory. The results reveal relatively large directly measured ECE values, i.e., the electrocaloric strengths (ΔT/ΔE) are round 0.42, which are among the largest values in the published data for ceramics. Also, it is found that the doping of Gd3+ ions may enhance the polarization of the antiferroelectric by about 20%, which might be accounted for the larger ECE values than those antiferroelectric ceramics doped with Sn4+ ions.

Resume : Caloric cooling is an emerging and environmentally friendly technology within the field of solid state cooling for replacing the other conventional vapor-compression refrigeration technologies making use of liquid refrigerant. Recently, ElectroCaloric Effect (ECE) has attracted attention after the discovering of a giant ECE [1] making this effect useable for solid state cooling. The performant EC cooling device require materials which show high EC temperature change, large cooling capacity, small dielectric loss for the working range, generally around the ferroelectric-paraelectric phase transition temperature. A large amount of work is dedicated to the search of efficient Electrocaloric materials. The barium titanate is one of the most well-known lead-free ferroelectric material, however its phase transition temperature is too high for the EC cooling application in ambient temperature. This phase transition temperature can be decreased by the substitution of barium (the A-site in ABO3 perovskite structure) by other cations, especially rare-earth. A site doping of the BaTiO3 can also improve the electrocaloric properties of the materiel, and leads to a relaxor behavior which characterized by the high values of the polarization observed in large temperature range [2]. In this work, an original lead-free ferroelectric material based on a substituted barium titanate BaTiO3 with the substitution of the A-site by samarium (Sm3 ) ions for different amounts of substitution has been elaborated by solid-state reactions in the form of bulk ceramics. The EC properties have been measured with a direct measurement of the adiabatic EC temperature change and the isothermal EC change of the entropy as function of the different parameters (amount of substitution, temperature, applied electric field intensity) and compared with other substituted BaTiO3 ceramics EC properties. [1] A. Mischenko et al, Science, 311 (2006) 1270. [2] Si Xie et al, Journal of alloys and compounds, 724 (2017) 163-168. Keywords: Electrocaloric Effect, Ferroelectric material, Relaxor, Solid-state refrigeration

Resume : We report a large electrocaloric effect with an adiabatic temperature change(delta T) of 0.47 K under 20 kV/cm combined with a broad temperature span (Tspan) of 23 K within 90% delta T(maximum) attenuation in a (Ba0.87Ca0.13)(Ti0.87Hf0.13)O3 ceramic by indirect measurement. Here, in this (Ba0.87Ca0.13)(Ti0.87Hf0.13)O3 ceramic, Ca2+ and Hf4+ are introduced into BaTiO3 to obtain a multiphase coexisting region with a diffuse phase transition character. The large electrocaloric effect benefits from a larger number of possible equilibrium orientations of polar states due to multiphase coexistence, and the broad temperature span comes from the diffuse phase transition character. Compared to the control sample, BaTi0.82Hf0.18O3 with only diffuse transition but no multiphase coexistence, (Ba0.87Ca0.13)(Ti0.87Hf0.13)O3 displays a DT value nearly 1.5 times larger than that of BaTi0.82Hf0.18O3 (delta T~ 0.33 K), although they exhibit similar Tspan values. Our work may shed light on developing high performance electrocaloric materials with broad temperature ranges.

Resume : The electrocaloric effect (ECE) offers a unique mechanism to realize environmentally friendly and highly efficient solid-state cooling that completely differs from the conventional vapor-compression refrigeration. We report a new class of hybrid films composed of ferroelectric polymer nanowire array and anodic aluminum oxide (AAO) membrane, which displays pronounced ECE driven by relatively low electric fields. Under confinement and orientation of AAO channels on the crystallization of the polymer, the polymer nanowire array shows substantially enhanced ECE that is about three times that of the corresponding thin films. Simultaneously, the integrated AAO membrane forms thermally conducting channels for the polymer nanowires, enabling the efficient transfer of cooling energy and operation of the EC materials under high frequencies, which are unattainable based on the currently available EC structures. Consequently, the integrated polymer nanowire-AAO hybrid film exhibits the state-of-the-art cooling power density, outperforming the current ferroelectric polymers, ceramics, and composites. This work opens a new route for the development of scalable, high-performance EC materials for next-generation refrigeration.

Resume : Ferroelectric materials, such as lead zirconate titanate (PZT) or barium titanate (BT) are technically attractive ceramics because of their special properties. They are often used for actuators or sensors in the precision range. Problems arise due to irreversible domain wall motion, leading to self - heating associated with changes in the material properties, thermal stresses and sometimes even phase transformations, whereupon the devices finally are inoperative. In low Curie temperature materials, such as BT, even depolarization is possible. In this work, the theoretical background of an irreversible thermo - electromechanical field - problem is presented. Based on a micromechanically and physically motivated nonlinear constitutive model, two methods to solve such problems, a Finite Element (FE) approach and a condensed - method (CM) approach are developed. The CM is suitable for the efficient implementation of various constitutive behaviors, taking into account interactions between grains or different components of a material compound with low computational complexity and high numerical stability [1,2]. It was developed to calculate e.g. hysteresis loops and residual stresses for polycrystalline materials without spatial discretization of the grain structure. To predict the temperature evolution in ferroelectric materials on the basis of a single polycrystalline material point, the CM is extended by the nonlinear bilateral caloric - electromechanical couplings including the switching - induced heat source in the material. Thus enables the simulation of the temperature change of the adiabatic behavior and the temporary heating for a local material point. For the FE model, weak formulations based on a generalized Hamilton - Jourdain - type variational approach are introduced [3]. Both methods show the stepwise heating due to domain switching and the associated development of temperatures with increasing number of load cycles, depending on the environmental conditions and the material. Numerical calculations of both approaches show the effects of temperature on the electromechanical field quantities and vice versa. They also reveal switching processes in ferroelectrics and associated heating, taking into account their dependence on temperature changes. For the calculations, the temperature dependencies of the essential material parameters are also taken into account. For verification, the theoretical results from the CM and FEM are compared to the experimental results according to [4] and to own experiments. [1] S. Lange and A. Ricoeur, Int. J. Solids Struct. 54, pp. 100 - 110 (2015). [2] S. Lange and A. Ricoeur, Arch. Appl. Mech. (2019). [3] M. Wingen and A. Ricoeur, Continuum Mech. Thermodyn. 31, 549 - 568 (2019). [4] H.~S. Chen, Y. M. Pei, B. Liu and D. N. Fang, Appl. Phys. Lett. 102, 242912 (2013).

Resume : Caloric materials encompass reversible thermal effects triggered in solids by magnetic, electric, and/or stress fields. Taken separately or together, caloric effects are in the foundation of transformative solid-state cooling technologies that have the potential to realize substantial energy savings upon adoption and deployment by heating, ventilation, air conditioning, refrigeration, and gas liquefaction industries. A common feature observed in all materials that exhibit the giant magnetocaloric effect is the coupling of magnetic and elastic effects. In addition to the interplay between magnetic and lattice entropies, both of which are intrinsic materials’ parameters that in principle can be modelled from first principles, extrinsic parameters, such as microstructure, play a role in controlling both the magnetostructural transition(s) and magnetocaloric effect. The role of different control parameters, the potential pathways towards materials exhibiting advanced magnetocaloric effect, and a few examples describing newly discovered magnetocaloric materials families will be discussed. This research is supported by the Office of Energy Efficiency and Renewable Energy of the United States Department of Energy through the Advanced Manufacturing Office and Building Technologies Office. Ames Laboratory is operated for the United States Department of Energy by Iowa State University of Science and Technology under Contract No. DE-AC02-07CH11358.

Resume : We study the magnetocaloric effect and critical behavior of Co2Cr1?xMnxAl (x = 0.25, 0.5, 0.75) Heusler alloys across the ferromagnetic (FM) transition with Mn concentration. The Rietveld refinement of x-ray diffraction pattern exhibits single phase cubic structure for all the samples. The temperature dependent magnetic susceptibility ?(T) data show a systematic enhancement in the Curie temperature and effective magnetic moment with Mn concentration, which is consistent with the Slater-Pauling behavior. The M(H) isotherms also exhibit the FM ordering and the analysis of ?(T) data indicate the nature of the phase transition to be a second order, which is further supported by scaling the entropy curves and Arrott plot. Further, Mn substitution causes an increase in the magnetic entropy change and hence an increased relative cooling power for multi-stage magnetic refrigerators. In order to understand the nature of the magnetic phase transition we examine the critical exponents ?, ?, ? for the x = 0.75 sample by the modified Arrott plot and the critical isotherm analysis, which further confirmed by Kouvel-Fisher method and Widom scaling relation, respectively. The estimated values of ? = 0.507, ? = 1.056, ? = 3.084 as well as their effective exponents ?eff and ?eff are close to the mean field theoretical values. The renormalized isotherms (m vs h) corresponding to these exponent values collapse into two branches, above and below TC that validates our analysis. Our results suggest an existence of the long-range FM interactions, which decays slower than power law as J(r) ? r ?4.5 for a 3 dimensional mean field theory.

Resume : The reliable determination of the adiabatic temperature change ΔT is a key challenge in the research on caloric materials. We have developed a novel method for direct measurements of ΔT that allows for the investigation of the magnetocaloric effect in thin samples and on time-scales inaccessible with traditional calorimetry. Our technique is based on the application of temporally oscillating magnetic fields and detection of the resulting change of the thermal radiation emitted by the sample. We achieve a unique sensitivity of better than 1 mK and a time-resolution of 10 µs using magnetic field modulation frequencies exceeding 1 kHz. The ΔT dynamics of the first-order phase transition material La(Fe,Mn,Si)13 shows a peculiar temporal self-quenching effect at temperatures slightly below the peak maximum. This behavior can be attributed to the first-order nature of the phase transition and diffusion of latent heat within the sample, which takes place at the phase boundary between the paramagnetic and ferromagnetic sample regions. The setup is easily converted to the application of high-frequency AC electric fields, thus allowing us to study also electrocaloric samples. First measurements of ΔT were performed on a 1.3 mm thick sample of the relaxor dielectric PMN-0.075PT. A pronounced aging behavior of ΔT is found that is caused by the nonergodic nature of the relaxor. Heating the sample above a certain threshold temperature reverses the degradation of the electrocaloric effect.

Resume : Most of the current research on alternative cooling technologies is oriented towards either electrocaloric (EC), magnetocaloric (MC) or mechanocaloric effects, where the entropy of the material, and thus its temperature, changes under external stimuli, i.e., electric, magnetic or mechanical, respectively. There is an increased interest in multicaloric materials where the application of two or more stimuli could enhance the total caloric effect or extend the operating temperature range of cooling device. The existence of EC and MC properties in the same material was already demonstrated in Pb(Fe0.5Nb0.5)O3 (PFN) ceramics. While this material appears promising, it possesses a relatively small EC and MC changes at room temperature (RT). Additionally, the maximum MC effect persists at very low temperatures. In general, the highest caloric effects are obtained near ferroic phase transitions. In order to increase the temperature of the magnetic transition and consequently to shift the maximum of MC effect closer to RT, PFN was modified with BiFeO3 (BFO). Seven different (1-x)PFN-xBFO solid-solutions were prepared (x = 0-0.5) and their EC and MC properties were investigated. The addition of BFO successfully shifted the paramagnetic-antiferromagnetic transition of PFN to the higher temperatures, while at the same time, did not drastically effect on the paraelectric-ferroelectric phase transition, which stays in the vicinity of RT. 0.8PFN-0.2BFO exhibits both ferroic phase transitions close to RT and is therefore one of the first such multiferroics. In this contribution, the EC and MC effects as well as the electrical conductivity, dielectric and ferroelectric properties of the prepared (1-x)PFN-xBFO ceramics will be presented.

Resume : One of the promising multicaloric candidate with enchanced multicaloric (MC) effect at room temperature is proposed to be material based on Pb(Fe1/2Nb1/2)O3 (PFN) - BiFeO3 (BFO) solid solution. The ideal MC material candidate should not only exhibit multiferroic properties but should also possess low electric conductivity. In this study we will focus on BFO component which use is known to be hinder by its conductivity mainly coming from nm-sized local features known as domain walls (DWs). DWs are created spontaneously at the Curie temperature and separate two regions of uniform polarization in ferroelectric materials. Recently, we explained that higher DW electrical conductivity is due to the segregation of charged defects at the DWs. [1] We will show that DWs conductivity in bulk BFO can be tailored by controlling the type and concentration of point defects using different processing conditions (temperature, atmosphere, cooling rates). With the support of different scanning transmission electron microscopy methods down to the atomic level we will explain how different processing conditions affect the local DW structure and concentration of the defects at DW and thus local DW conductivity. Methodology developed on the BFO material will serve as a basis for domain structural study of a PFN- BFO solid solutions. 1. T. Rojac et al, Nat. Mat. (2017) 16 322-327.

Resume : The technology of magnetic cooling has been successfully used to attain ultra-low temperatures for more than 80 years, however for the last decade it is also regarded as a promising refrigeration technique working at ambient temperatures. From this point of view, magnetic compounds with first-order transition, exhibiting large magnetocaloric effect (MCE) in the temperature range of 270–320K and under magnetic field changes of Δμ0H=1–2 T are attracting much attention due to their potential application in this emerging refrigeration technology. The main issue related to application of giant MCE materials is the hysteresis of the first-order transition, which is one of the main factors delaying the development of this novel and disruptive cooling technology. The central focus of this talk is the overview of magnetocaloric materials with first order transition, such as FeRh, Gd5(SiGe)4, La(FeCoSi)13, La(FeMnSi)13Hx and Heusler alloys. Cyclic measurements of the adiabatic temperature change together with calorimetric data allow us to determine the reversible magnetocaloric effect of these first-order materials with thermal hysteresis. This method provides a basis for a comparison of the suitability of different hysteretic magnetocaloric materials for their application in a magnetic refrigerator. One of our goals is to thoroughly understand such a complicated phenomenon as the magnetostructural first-order transition and then then optimise the material for a particular technical application. In this invited talk we will show the correlation between changes occurring in magnetization, magnetostriction, resistivity and the resulting MCE in different classes of advanced magnetocaloric materials.

Resume : Magnetocaloric materials based on field-induced first-order transformations are promising for more environmentally friendly cooling devices. An interesting candidate is the Ni-Mn-Ga-Co Heusler alloy, which exhibits an inverse magnetocaloric effect during a martensitic transition between a high temperature ferromagnetic phase and a low temperature phase with lower magnetization. As this transformation is of first order, it proceeds by nucleation and phase growth. The associated hysteresis reduces the efficiency of a possible cooling cycle. Here we use epitaxial thin films as a model system to tailor the microstructure and examine the consequences on hysteresis. By depositing Ni-Mn-Ga-Co films on different substrates with DC magnetron sputtering, we control film orientation and microstructure. The resulting microstructure, which is either single crystalline or containing large angle grain boundaries, is examined by a combination of local and integral texture measurements. Thermo-magnetic measurements allow to correlate a broader martensitic transition and hysteresis with the presence of large angle grain boundaries. We attribute this to the increased total interface energy which is required to complete the transition. Funded by DFG through SPP1599 www.FerroicCooling.de.

Resume : Ni-Mn-Ga-based Heusler shape-memory alloys that undergo a martensitic-structural transition around room temperature are well known for exhibiting large magnetic entropy change and shape-memory effect. Here, we report the observation of a large adiabatic temperature change of 18 K in a Ni-Mn-Ga system by using direct adiabatic temperature-change measurements in static and pulsed magnetic fields. We show that a large heat transfer at isothermal magnetization can be achieved in the region of magnetostructural transition at room temperature. The features of the formation of a martensitic structure in a magnetic field under adiabatic and isothermal conditions are determined. The field-dependent irreversibility of magnetocaloric effect is analyzed based on in-situ observations of the formation of martensitic structure in adiabatic isothermal conditions. Based on the experimental data obtained, a magnetic phase diagram was constructed. The magnetic phase diagram allowed us to analyze the process of adiabatic magnetization and the associated temperature change. The present study expand the understanding of irreversible magnetocaloric effects in materials with magnetostructural transition. The work was supported by the National Science Center, Poland through the SONATA Program under Grant No. 2016/21/D/ST3/03435. We acknowledge the support of the HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL).

Resume : A Magnetic Refrigerator (MR) with an Active Magnetic Regenerator (AMR) consists of a porous Magneto-Caloric Material (MCM) through which a Heat Transfer Liquid (HTL) passes. A new MR is proposed, in which a suspension of MCM in a HTL is used to build a system with no moving parts (increased system reliability) by utilizing Ferro-Hydro-Dynamic Pumping (FHDP). This enables higher heat transfer rate between MCM and HTL, when compared with an AMR. The increased system reliability is at the cost of pumping efficiency. Galinstan, owing to its high thermal conductivity (30 times that of water), and low specific heat (1/14th of water), is considered as a base liquid for the suspension. The high cost of Galinstan is partly balanced by the compactness of the system it offers. The effect of Galinstan on the structural, and thereby the magnetic properties of - MnFe(P,Si) based, and La(Fe,Mn,Si)H based compounds, are not yet known. This study discusses these characteristics for the MCM suspensions of 4 wt% to 45 wt%, over a period of 1 month to assess the changes in the characteristics with time. Furthermore, the feasibility to include FHDP in MR will be discussed in details.

Resume : Magnetocaloric thin films have been proposed for cooling microelectronics [1]. In such systems, the magnetic, structural and magnetocaloric properties at the surface can be different to that of the bulk [2-5], and this can compromise the performance of any cooling device. Here we combine bulk magnetometry (SQUID) and photoemission electron microscopy with contrast from x-ray magnetic circular dichroism (PEEM-XMCD) to demonstrate that the magnetocaloric response of thin films of the Heusler alloy Ni-Mn-Ga is not suppressed at their surface. Our findings should inspire imaging studies of other giant magnetocaloric thin films.

Resume : Two-dimensional inorganic-organic (2D-IO) hybrid systems are Ruddlesden-Popper phase-like materials which can be tailored to achieve desired and tunable thermoelectric, optoelectronic and multiferroic properties. The lead-free inorganic-organic hybrids are the demand of the day for real life applications. Much importantly, these 2D-IO hybrids are easy to synthesize and have shown excellent structural and chemical stability against heat, humidity, temperature and external environment conditions as compared to other inorganic-organic hybrid counterparts. Here, we report magnetic properties of lead-free inorganic-organic hybrids. Specifically, (C12H25NH3)2Cu(Br1-xClx)4 and (C6H9C2H4NH3)2Cu(Br1-xClx)4 have been synthesized by solution processing and have been investigated in detail for their suitability in low temperature magnetic refrigeration applications, an environment-friendly cooling technology. All the 2D-IO hybrids investigated in our work reveal ferromagnetic-paramagnetic transition having the corresponding Curie temperature shifting towards lower values with the increasing Cl/Br ratio. For (C12H25NH3)2CuCl4 a large magnetic entropy change is detected from the isothermal magnetization curves around the Curie temperature. Critical exponents have been determined from the field dependent magnetic entropy change and the results for (C12H25NH3)2CuCl4 indicate good correspondence with the 3D-Heisenberg model.

Resume : Barocaloric effect (BCE) associated with the reversible change in the entropy/temperature under pressure variation under the isothermal/adiabatic conditions is a common caloric characteristic for substances of different physical nature. We performed the analysis of the extensive and intensive BCE in some complex oxyfluorides at successive structural phase transitions. The high sensitivity of these compounds to a change in the chemical pressure allows one to vary the succession and parameters of the transformations (temperature, entropy, baric coefficient) over a wide range and obtain optimal values of BCE. The complicated T-p phase diagrams of different types including the triple points observed experimentally are analyzed. It was found which parameters of the phase transitions and phase diagrams should be taken in consideration for improving BCE. Complex oxyfluorides demonstrate very important advantages: first, the maximum values of the extensive and intensive BCE can be realized at rather low pressure (0.1–0.3 GPa), second, in a narrow temperature range around the triple points conversion from conventional BCE to inverse BCE is observed, which is followed by a gigantic change of both effects. Due to the large magnitude of BCE, complex oxyfluorides can be considered as new competitive solid refrigerants.

Resume : Most current activity in cooling research is looking at one of the caloric effects; electrocaloric (EC), magnetocaloric or mechanocaloric, where the materials entropy changes under the application of external stimuli (electric, magnetic or mechanical). Among them, the EC effect is triggered by voltage, which is easily available around us. Currently, one of the most promising inorganic EC materials is relaxor-ferroelectric (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT). Namely, the PMN-0.1PT exhibits EC temperature change of 3.45 K at electric field of 160 kV/cm. In this contribution, we will discuss the feasibility of using PMN-xPT ceramics in active cooling elements for cooling the electronic components in niche applications such as medical accelerators, nuclear reactors and space technologies, where the material is exposed to high radiation. For this purpose, three different compositions of PMN-xPT ceramics (x = 0, 0.1 and 0.35) were prepared and their dielectric, ferroelectric and EC properties were investigated prior and after the neutron and gamma irradiation with different doses (1015 cm-2 to 1017 cm-2 of 1 MeV equivalent neutron fluence for silicon and gamma doses from 12 kGy to 1.2 MGy). The functional properties of irradiated samples show no major differences in comparison to the virgin samples, therefore we can conclude that the prepared materials could be used as working materials in EC coolers exposed to such harsh environments.

Resume : Solids exhibiting high caloric effects (CE) attract a great attention of both researches and engineers due a possibility to use them as effective solid state refrigerants at designing alternative cooling cycles. Among CE of different physical nature, piezocaloric effect (PCE) associated with the temperature/entropy change under uniaxial pressure is investigated to the least extent. In recent years, much attention has been paid to PCE in shape memory alloys undergoing the martensitic transformations. Effect was considered only for one direction in the sample. However, it is obvious that strongly anisotropic materials are of considerable interest due to the possibility of implementing PCE of different signs in them. In the present work, we analyzed the effect of anisotropy of thermal expansion on the intensive and extensive piezocaloric efficiency of ferroelectric NH4HSO4 undergoing the succession of two phase transitions P21/c ↔ Pc ↔ P1 of the second and first order, respectively, which are differ greatly from each other by entropy parameters and sensitivity to hydrostatic pressure. It was shown that ammonium hydrogen sulphate can be considered as a model solid-state refrigerant for the design of a combined cooling cycle using uniaxial pressures applied alternately along two different crystallographic axes. Acknowledgments This work was supported by the Russian Science Foundation (RSF) grant (No. 19-72-00023).

Resume : During the last decade, obtaining new materials in which at least two ferroic orders (magnetic, electric, or elastic) coexist in the same phase (the so-called multiferroic materials) has been a hot topic in materials science. [1] This interest relies in the huge potential of these materials for technological applications, especially as caloric materials and even multicaloric materials, for solid-state cooling devices. [2] We have reported giant barocaloric (BC) effects in a hybrid organic−inorganic perovskite driven by accessible pressures. [3] These compounds show a wide diversity, depending on the “building-blocks” of the different positions, namely A (generally organic molecules), B (typically divalent transition metals cations), and X (different halides or bidentate-bridge ligands). [3] In view of these interesting results, we have scrutinized the literature searching for hybrid perovskites that experience large and reversible entropy changes related to first-order phase transitions close to room temperature. We identified different families of hybrid perovskites as potential emergent barocaloric materials, in particular those with azide ligands in the X position. [4] In this work, we have focused on the multiferroic azide-perovskite [(CH3)4N][Mn(N3)3], which undergoes a first-order phase transition at T = 310 K with an associated magnetic bistability, ferroelastic and dielectric properties. Here, we report the structural characterization and the isothermal entropy change driven by applied pressure at different temperatures. The obtained results are very promising and encourage to further explore this interesting family of organic−inorganic hybrids for solid-state cooling applications. [1] J. F. Scott, Nat. Mater. 2007, 6, 256. [2] X. Moya et al. Nat. Mater. 2014,13, 439. [3] Bermúdez-García, J. M. et al. Nat. Commun. 2017, 8, 15715 [4] Bermúdez-García, J. M. et al. J. Phys. Chem. Lett. 2017 , 8, 4419 −4423. [5] L.C. Gómez‐Aguirre et al. Chem. Eur.J. 2016 ,22, 7863

Resume : Spin crossover (SCO) materials have been intensively studied due to their switchable physical properties controlled by external stimuli (such as temperature, pressure, light or magnetic field) of interest for potential technological applications [1]. SCO materials are typically transition metal compounds that display a phase transition related to a change in the spin state, which is induced by an external stimuli. Recently, Sandeman has reported that SCO materials show a great potential to be considered as a new class of giant mechanocaloric materials [2]. This finding extends the scarce family of caloric materials with suitable properties to be useful for solid-state cooling devices. In this work, we have focused on the SCO material with general formula [Fe(Htrz)2(trz)][BF4] (Htrz: 1,2,4-4H-triazole; trz: 1,2,4-triazole) [3], which exhibits two phase transitions near room temperature, that can be modulated as a function of the thermal treatment history of the material. Here, we report the structural characterization and the isothermal entropy change driven by applied pressure at different temperatures. The obtained results can enlarge the horizons on the investigation of SCO materials for solid-state cooling applications. [1] Halcrow, Malcolm A., ed. Spin-crossover materials: properties and applications. John Wiley & Sons, 2013. [2] Sandeman, Karl G. et at. Apl. materials, 2016, 4, 111102. [3] Kroeber, J. et al. Chemistry of materials 6.8, 1994, 1404-1412.

Resume : For magnetic cooling technology, magnetocaloric effect (MCE) plays a major role. Extensive investigations on materials exhibiting MCE has been done on intermetallics (e.g. LaCo13, MnAs, Gd5Ge2Si2), Heusler alloys, Fe2P type compounds and such like. Among oxide materials, the manganites have been broadly explored. However, AFe2O4 spinels where A = Mn, Fe, Co, Ni, Cu and Zn have emerged exhibiting competitive cooling power as observed e. g. in Zn0.6Cu0.4Fe2O4 and Zn0.2Ni0.4Cu0.4Fe2O4 with high relative cooling power (289 and 233 J/kg, respectively) [1]. The CoFe2O4 ferrite with inverse spinel structure has unique physical properties such as high magneto–crystalline anisotropy, high Currie temperature (793 K), and high magnetostriction, which gives the material great application potential in the field of energy storage, ferrofluids, microwave devices and drug delivery [2]. Our work is focused on the preparation of cobalt ferrite doped with Mn2 , Ni2 and Tb3 ions in the form of nanostructured bulk ceramics. The bulk ceramics AB2O4 with different stoichiometry (A = Co, Mn, Ni, and B = Fe, Tb) was prepared using the sol–gel Pechini method. The phase composition and structure of the final material was characterized by XRD. The particle size was evaluated from the diffraction full profiles, TEM, and dynamic light scattering method. The effect of a nanosizing effect on magnetic and thermal properties (measured by PPMS) is discussed. The structural and magnetic measurements of the prepared samples show promising properties of the spinel ferrite. [1] N.R. Ram, et. al., J. Supercond. Novel Magn., 31 (7), 1971 (2018). [2] V. Bartůněk, et. al., Materials, 11 (7), 1241 (2018). Financial support from specific university research (MSMT No 21-SVV/2019)

Resume : ElectroCaloric (EC) cooling technique is an emerging environmentally friendly solid-state refrigeration technology that has been recently demonstrated to be a promising alternative to conventional vapour-compression refrigeration technologies making use of fluid refrigerant. Several approaches using EC effect to make cooling systems have been reported. Basically, the caloric effect is not enough large for application and needs to be amplified. Two possible amplification processes have been proposed [1]: regeneration process is based on the use of a thermodynamic cycle continuously repeated to transfer the heat from the cold side to the hot side, the heat exchange being insured by the motion of a fluid or a solid. The second one is a cascade process in which the EC cooling device consists in several layers of EC active material separated by thermal diodes, stacked between a heat source and a heat sink. In this work, we study the performance of an Active Electrocaloric Regenerator AER using the heat transfer fluid motion. The AER is composed of barium titanate BaTiO3 MultiLayer Capacitors (MLCs) [2] separated by gaps through which a fluid move periodically to transfer the heat from the cold reservoir to the hot reservoir, the fluid motion being assured by bidirectional pump. The influence of different parameters (fluid velocity, electric field, frequency…) will be presented and experimental results compared with numerical simulation. [1] Blumenthal, P., and A. Raatz. "Classification of electrocaloric cooling device types." EPL (Europhysics Letters) 115.1 (2016): 17004. [2] Moya, Xavier, et al. "Electrocaloric effects in multilayer capacitors for cooling applications." MRS Bulletin 43.4 (2018): 291-294. Keywords: Electrocaloric Effect, Solid-state refrigeration, Heat pumps

Resume : Work on electrocaloric (EC) materials has been focused on finding materials with a high EC response. However, several other parameters play a role in the path to integration of these materials in actual cooling devices. For example, long-term stability of the EC response and material's electrical losses. In this contribution, we explore EC fatigue and electrical losses and their interrelation. Upon cycling of lead magnesium niobate (PMN), we observed an increase in sample?s electrical conductivity and an electric-field-induced phase transformation to a supercritical ferroelectric state, which both increased the electrical losses of PMN. The increase of losses was accompanied by a significant self-heating of the samples. To uncover and study the origin of the self-heating, we employed modified lead zirconate titanate in a "soft" and "hard" composition. Differently modified PZTs enabled us to separate and understand contributions of electrical conductivity and ferroelectric domains to self-heating. The results show that the high electric fields used in EC excitation lead to severe fatigue. Fatigue in the case of PMN is manifested in increased electrical losses and, consequently, self-heating. However, with careful control of excitation conditions the degradation of the cooling properties can be tamed or even avoided.

Resume : Barocaloric materials show reversible thermal changes when subjected to changes in applied hydrostatic pressure. These barocaloric effects hold potential to displace the vapour-compression technology that is currently used in refrigeration and air-conditioning, but also promise applications at high temperatures where vapour compression is not suitable. Here we present quasi-direct barocaloric measurements on the superionic conductor AgCuS, which shows large pressure-driven isothermal changes in entropy near its orthorhombic-to-hexagonal transition at 355 K.

Resume : There is currently great interest in replacing the harmful hydrofluorocarbon fluids that are used in refrigeration and air-conditioning with solid materials that display barocaloric effects. However, pressure-driven thermal changes in barocaloric materials have traditionally fall short with respect to their fluid counterparts. Here we show that organic materials can display extremely large pressure-driven thermal changes near room temperature and that these changes can be comparable with those exploited commercially in hydrofluorocarbons. Our discovery of colossal barocaloric effects in organic materials should bring them to the forefront of research and development in order to achieve environmentally friendly cooling without compromising performance.

Resume : The International Energy Agency highlights the necessity to provide new solutions for the growing refrigeration demand that currently accounts for ~20 % of the global energy consumption, which is expected to triplicate by 2050.[1] In the recent years, barocaloric materials with giant thermal changes under hydrostatic pressure have become a promising alternative to traditional gas refrigerants that are toxic (ammonia, carbon dioxide), flammable (hydrocarbons), or strong greenhouse gases (hydrofluorocarbons). [2] Nevertheless, most of barocaloric materials require large operating pressures (>1000 bar), which preclude their immediate application using existing commercial compressors that normally reach ~100 bar. In this work, we present a new family of barocaloric hybrid organic-inorganic perovskites that operates under small pressures (p ~ 70 bar) and at room temperature, therefore matching applications needs.[3-5] Interestingly, these compounds show multiple phase transitions that lead to both conventional and inverse barocaloric effects over a wide range of operational temperatures that include room temperature. Our results show that hybrid perovskites are a very versatile family of barocaloric materials with potential for a wide range of refrigeration applications. [1] International Agency of Energy, The Future of Cooling (2018, https://www.iea.org/cooling/) [2] Moya, X. et al. Nat. Mater. 13, 439-450 (2014). [3] Bermudez-Garcia, J. M. et al. Nat. Commun., 8, 15715 (2017). [4] Bermudez-Garcia, J. M. J. Phys. Chem. Lett. 8, 4419-4423 (2017). [5] Bermudez-Garcia, J. M. et al. J. Mater. Chem. C, 6, 9867-9874 (2018).

Resume : Caloric materials are under intense study due to the prospect of new-generation refrigerators and air-conditioners that are energy efficient and environment friendly. Most caloric research is performed in solid materials with limited field-induced changes in entropy. By contrast, liquid crystals may exhibit all the key ingredients required to achieve outstanding caloric responses: (i) extremely large (colossal) thermally driven changes in entropy, (ii) small thermal hysteresis, and (iii) large shifts of transition temperatures with external field (electric, magnetic or stress). Here, I will focus on large pressure-driven thermal changes in lyotropic and thermotropic liquid crystals, which show large latent heats associated with order-disorder phase transitions and low hysteresis. This rather unique combination of properties in the same material promises pressure-driven thermal changes close to those observed in commercially exploited fluid refrigerants.

Resume : Natural rubber spearheads a new group of elastocaloric polymers that show giant thermal changes when stretched/unstretched using small uniaxial forces. Moreover, natural rubber is non-toxic and inexpensive, making it an ideal candidate for new environmentally friendly and energy efficient cooling solutions. However, before any applications take place, it is important to understand in detail the mechanistic of its elastocaloric response. Here we combine infra-red imaging, Raman spectroscopy and finite-element modelling to explain the origin of the large adiabatic temperature changes observed in natural rubber on loading and unloading. Our findings should lead to the improvement of the elastocaloric response of elastic polymers, and their ultimate implementation into cooling devices.

Resume : Superelastic behavior and elastocaloric effect were investigated in a Ti-44Ni-5Cu-1Al (at%) alloy subjected to various thermomechanical treatments. The specimen heat-treated at 673 K for 5 minutes after hot rolling and subsequent cold rolling exhibited excellent superelastic strain of 4.9% with a small stress hysteresis of 90 MPa when the maximum tensile stress was 500 MPa. This specimen also exhibited a large elastocaloric effect with a temperature decrease of 17 K when the stress of 500 MPa was removed adiabatically. No remarkable deterioration was observed for the superelastic strain and elastocaloric effect up to 5000 mechanical cycles. The maximum superelastic strain obtained was 6.8% under a tensile stress of 750 MPa. Transmission electron microscope observation and in-situ X-ray diffraction analysis under tensile stress revealed that the average grain size of the specimen is about 40 nm, and the specimen exhibits a successive B2-B19-B19’ transformation.

Resume : Elastocaloric cooling shows high potential for various cooling and heat-pumping applications, as it (theoretically) shows high efficiency and is environmentally friendly. It is based on the elastocaloric effect (eCE) that is closely related to the superelasticity of shape memory alloys and their latent heat during the stress-induced martensitic transformation. When an elastocaloric material is mechanically loaded at (or close to) the adiabatic conditions it heats up and when it is unloaded (released) it cools down. These temperature changes can be used for cooling and/or heat-pumping. In the recent years several demonstrators of elastocaloric cooling devices were designed, built and tested. In general they can be divided into three groups: miniature single-stage devices, regenerative devices and active heat pipe devices. In this talk, the most interesting elastocaloric materials will be reviewed and different concepts of utilization of the eCE will be presented and discussed. In the second part of the talk, future challenges of elastocaloric technology (fatigue-life and driver mechanism), that needs to be solved in order demonstrate real market potential of elastocaloric technology will be addressed. Acknowledgement This work is supported by European research Council (ERC) under Horizon 2020 research and innovation program (ERC Starting Grant No. 803669).

Resume : Gas compressor refrigeration devices are widely used both for household and industrial applications although suffering from several issues, like their environment unfriendliness and their difficult miniaturization. In addition, the need for small sized components for integration purpose make solid state cooling system very attractive. The principle of the solid state cooling being based on the entropy of a caloric material that can be controlled by an external quantity. Three challenges must be addressed: the understanding of the caloric effect mechanisms within the material, the development of analytical models for a better conception and optimization of the refrigerating system, the control of the heat transfer kinetics between the hot source and the cold source. This communication will present the contribution of partners from various expertise fields gathered to address the issue mentioned above in order to develop solid state cooling devices based on the elastocaloric effect in rubber. Firstly, the analytical model of a less complex cooling system will be discussed to point out the main characteristics that must be taken into account for better refrigeration performances. Secondly, the measurements of microstructural, mechanical and thermal characteristics of natural rubber under stress will be presented. Finally, methods to choose the best materials and to enhance the elastocaloric coupling in natural rubber will be investigated.

Resume : Global heating demands a breakthrough in refrigeration technology. In this context, increasing energy efficiency and replacing harmful refrigerants in vapour-compression refrigerators becomes crucial. Giant barocaloric materials have recently emerged [1-4] as promising solid refrigerants, and they can now even match the thermal performance of commercially exploited hydrofluorocarbons [5]. However, their implementation into a working cooler faces a number of challenges from materials and design viewpoints, which have not hitherto been explored. Here, we will first overview state-of-the-art barocaloric materials, and then key aspects in heat transfer methods and energy efficiency to be addressed by barocaloric prototypes. [1] Moya, Xavier, et al. "Caloric materials near ferroic phase transitions." Nature Materials 13 (2014): 439. [2] Lloveras, Pol, et al. "Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate." Nature Communications 6 (2015): 8801. [3] Bermúdez-García, Juan M., et al. "Giant barocaloric effect in the ferroic organic-inorganic hybrid [TPrA][Mn (dca) 3] perovskite under easily accessible pressures." Nature Communications 8 (2017): 15715. [4] Aznar, Araceli, et al. Giant barocaloric effects over a wide temperature range in superionic conductor AgI. Nature communications 8 (2017): 1851. [5] Lloveras, Pol, et al. "Colossal barocaloric effects near room temperature in plastic crystals of neopentylglycol." Nature Communications 10 (2019): 1803.

Resume : The aerosol deposition (AD) is a relatively new method for rapid deposition of thick films of a variety of functional materials at room temperature. The complete consolidation of functional films happens in AD due to the impacting of high velocity particles with the substrate. The AD method offers an excellent way of integrating different materials together, such as ceramics, metals, polymers and glasses. For that reason, the AD method opens up new solutions when designing various applications, for example in preparation and integration of caloric elements for new solid-state cooling devices. In this contribution, we will present a few examples of integrating different materials by utilizing the AD method. We will demonstrate how the AD method enables preparation of alumina surface coatings for the protection and the electrical isolation of caloric elements. In addition, the possibilities and prospects of integrating caloric materials on a variety of low-cost substrates will be considered. Furthermore, the electric and electrocaloric properties of prepared aerosol-deposited layers will be discussed.

Resume : Today's state-of-the-art caloric technologies, such as magnetocalorics, electrocalorics or elastocalorics, are based on the so called Active Caloric Regeneration (ACR) principle. The ACR is based on the reciprocating movement of the fluid through a porous caloric structure. Such system usually comprises a large amount of caloric material and a fairly complex hydraulic system, which is more suitable to be implemented in large cooling, refrigeration or heat pump devices. On the other hand, miniaturized electronics also produce vast amounts of heat that need to be efficiently managed. In this manner, an alternative research approach is emerging in the fields of caloric technologies. It involves new concepts of devices, which would apply so called thermal switches. The application of thermal switches could lead to drastic improvements in the heat transport from/to the caloric material and consequently to the miniaturization of caloric devices. An interesting domain, to look for thermal switch mechanisms, is microfluidics, which has enabled the development of integrated lab-on-chip devices. Although most microfluidic devices are based on a continuous flow of liquids in microchanells, there has been an increasing interest for the past couple of years in devices that rely on manipulation of discrete droplets using surface tension effects. One such technique is ElectroWetting On Dielectric (EWOD), which is based on wettability of liquids on a dielectric solid surface by varying the electrical potential. In this contribution we will present a new concept of caloric device which couples caloric effect and EWOD droplet actuation as thermal switch mechanism. We will show different potential designs of such devices and their operation. Furthermore, the materials and its properties which constitute the whole device will be discussed.