Temperature Dependence Of Thermoelectric Properties Research Articles

Experimental validation and development of an advanced computational model of a transcritical carbon dioxide vapour compression cycle with a thermoelectric subcooling system

The inclusion of a thermoelectric subcooler as an alternative to increment the performance of a vapour compression cycle has been proved promising when properly designed and operated for low-medium power units. In this work, a computational model that simulates the behaviour of a carbon dioxide transcritical vapour compression cycle in conjunction with a thermoelectric subcooler system is presented. The computational tool is coded in Matlab and uses Refprop V9.1 to calculate the properties of the refrigerant at each point of the refrigeration cycle. Working conditions, effect of the heat exchangers of the subcooling system, temperature dependent thermoelectric properties, thermal contact resistances and the four thermoelectric effects are taken into account to increment its accuracy. The model has been validated using experimental data to prove the reliability and accuracy of the results obtained and shows deviations between the ± 7% for the most relevant outputs. Using the validated computational tool a 13.6 % COP improvement is predicted when optimizing the total number of thermoelectric modules of the subcooling system. The computational experimentally validated tool is properly fit to aid in the design and operation of thermoelectric subcooling systems, being able to predict the optimal configuration and operation settings for the whole refrigeration plant. • Transcritical CO2 vapour compression cycle with thermoelectric subcooling. • Advanced computational model to predict optimal operation settings of the system. • Validation of the model using experimental data with deviations between the ±7%. • Computational optimization of the number of modules in a thermoelectric subcooler. • 13.6% COP improvement using the hybrid solution and optimum number of modules.

Strongly Coupled Tin(IV) Sulfide–MultiWalled Carbon Nanotube Hybrid Composites and Their Enhanced Thermoelectric Properties

Herein, tin(IV) sulfide (SnS2) and multiwalled carbon nanotube (MWCNT) composites are fabricated via a simple solution-mixing method in a hydrothermal reactor. SnS2 is closely coupled to the MWCNT surface, thus forming a coaxial nanostructure. Examination by X-ray photoelectron spectroscopy and scanning transmission electron microscopy indicates that the strong interface between SnS2 and the MWCNTs in the composite material is due to the formation of Sn-O and Sn-S bonds. In addition, an examination of the temperature-dependent thermoelectric (TE) properties demonstrates that the SnS2-MWCNT hybrid composite with 3 wt % MWCNTs exhibits the maximum power factor of ∼91.34 μW/(m·K2) at 500 K, which is ∼50 times larger than that of the pristine SnS2. These results highlight the fabrication and enhanced TE properties of hybrid composites via the coupling of SnS2 and MWCNTs.

Elaboration of the Calculation Algorithm for the Load Diagram of a Thermoelectric Generator that Applies the Temperature Dependent Thermoelectric Layer Properties

The aim of the work is the development of the algorithm for the working parameters calculation of a marine waste heat thermoelectric generator, where the thermoelectric layer properties are considered as temperature dependent. The marine thermoelectric generator must have an extensive operational profile that includes a number of modes with a different power demand and different magnitudes of the waste heat parameters. The working mode alteration involves the change in the thermoelectric properties. To achieve the goal a mathematical model was offered for a load diagram calculation. The load diagram represents the generator parameters alteration with respect to the load current change. The most important result was the proposed new order of calculation that differs from the present models with offered variables, an approach to constructional thermal resistances application and a specific electrical power implementation. Mean temperature and load factor are taken as variables with the load factor being accepted as a relation between the electrical resistances of a load and a generator. Constructional thermal resistances were combined into two complex parameters – the thermal resistances of a hot and cold sides. The significance of the obtained results is determined by shaping a versatile approach to the generator`s working parameters calculation. The methodology is independent of the scheme of the calculated generator`s thermal resistances, which could be transformed into the complex parameters. The specific power calculation allows defining the performance per a cross-sectional unit area. The optimal selection of variables reduces the computational time.

The phonon scattering mechanism and its effect on the temperature dependent thermal and thermoelectric properties of a silver nanowire.

In this work, the electron-phonon, phonon-phonon, and phonon structure scattering mechanisms and their effect on the thermal and thermoelectric properties of a silver nanowire (AgNW) is investigated in the temperature range of 10 to 300 K. The electron-phonon scattering rate decreases with the increase of temperature. The phonon-phonon scattering rate increases with temperature and becomes greater than the electron-phonon scattering rate when the temperature is higher than the Debye temperature (223 K). The rate of phonon structure scattering is constant. The total phonon scattering rate decreases with temperature when the temperature is lower than about 150 K, and increases when the temperature is higher than 150 K. Correspondingly, the temperature dependent variation trend of the lattice thermal conductivity is opposite diametrically to that of the total phonon scattering rate. The thermoelectric properties of the AgNW are strongly coupled with the thermal conductivity via the phonon and electron transition. The thermoelectric properties of the material are quantified by the figure of merit (ZT). The ZT value of the AgNW is greater than that of bulk silver in the corresponding temperature range, and this difference increases with temperature. The order of the ZT of the AgNW is about 13 times greater than that of bulk silver at room temperature. The large increase of the ZT value of the AgNW is mainly due to the enhanced electron scattering and phonon scattering mechanisms in the AgNW.

Combined effect of donor doping and RGO (reduced graphene oxide) coating in La/Nb-doped SrTiO3 thermoelectrics

SrTiO3 (STO) is one of the most promising oxide thermoelectric (TE) materials with high thermopower, but the low electrical conductivity and high thermal conductivity are still issues to be solved. Recently, grain boundary engineering using reduced graphene oxide (RGO) has been demonstrated to have capable of simultaneously improving both the electrical and thermal properties of STO. In this study, we investigated the combined effect of La/Nb doping and RGO coating in STO to improve further the electrical conductivity and the TE efficiency of STO materials. Two series of La- and Nb-doped Sr1-xLaxTi1-yNbyO3 (x, y = 0–0.02) samples incorporated with RGO were prepared by a combinatorial procedure of solid-state reaction, RGO coating, and the spark plasma sintering (SPS) process. The structural, morphological and temperature-dependent thermoelectric properties of the Sr1-xLaxTi1-yNbyO3/RGO were studied and as a result, the figure of merit was enhanced ∼6.5 times by the combined effect of donor doping and lowering the double Schottky barrier height due to RGO at grain boundaries.

Two-dimensional flexible thermoelectric devices: Using modeling to deliver optimal capability

Two-dimensional flexible thermoelectric devices (2D FTEDs) are a promising candidate for powering wearable electronics by harvesting low-grade energy from human body and other ubiquitous energy sources. However, immature device designs in the parametric geometries of FTEDs cannot provide an optimized output power density because of either insufficient temperature difference or unnecessarily large internal resistance. Here, we theoretically design optimal parametric geometries of 2D FTEDs by systematically considering applied temperature difference, temperature-dependent thermoelectric properties of materials, leg thickness, and thermodynamic conditions. The obtained analytical solution determines the optimal leg length for 2D FTEDs when these parameters are given and, therefore, minimizes the internal device resistance and simultaneously maintains the high temperature difference across the TE legs to maximize the device output power density. According to this design, we use flexible Ag2Se films as thermoelectric legs to assemble a 2D FTED, which displays a maximum power output of 11.2 mW and a normalized output power density of 1.43 μW cm−2 K−1 at a temperature difference of 150 K, outnumbering other 2D FTEDs by threefolds. Our 2D FTED can power up four light-emitting diodes, which shows great potential for harvesting electricity from low-grade heat. The exotic and reliable device design concept of 2D FTEDs reported here can be extended to other thermoelectric systems to boost the practical applications of FTEDs.

Exploring pressure induced thermoelectric properties of LiAeH3 (Ae = Ca, Sr, Ba) perovskite hydrides along with optoelectronic features

This report aims to demonstrate the plane wave pseudopotential approached ab initio computation based on density functional method integrated within semi-classical Boltzmann transport theory to investigate the pressure induced electronic, optical and thermoelectric properties of LiAeH3 (Ae = Ca, Sr, Ba) perovskite hydrides. The computed electronic structures of these compounds possess indirect band gaps with mixed covalent and ionic bonding in the whole range of studied pressure (0–50 GPa) in which the band gaps of LiCaH3 decrease while those of LiSrH3 and LiBaH3 increase as the pressure increases. The rational analyses of optical spectra devote these perovskites to optoelectronic applications. For instance, the investigated hydrides reveal plasmonic excitation owing to their unique loss spectra, which are significant in optical sensing devices. The estimated electronic and optical parameters are compared, whenever possible, to the available results and found a reasonable agreement among them. To further assess physical behavior, the pressure and temperature dependent thermoelectric properties in terms of Seebeck coefficients, thermal conductivities, electrical conductivities, power factors and figure of merits are carried out first time as implemented in the BoltzTraP code. The estimated positive Seebeck coefficients of the studied perovskites exhibit p-type semiconducting nature, and thus holes are the majority charge carriers for conduction rather than electrons. The large figure of merits and small thermal to electrical conductivity ratios disclose their suitability in thermoelectric device applications.

Theoretical study of physical properties of Ba3B(Nb,Ta)2O9 (B = Mg, Ca, Sr, Cd, Hg, Zn, Fe, Mn, Ni, Co) perovskites

The electronic structure, optical and thermoelectric properties of Ba3B(Nb,Ta)2O9 are investigated by first-principal calculation based on the density functional theory implemented in Wien2k simulation program. This study is carried out by applying GGA-PBE and GGA-EV potential. We have found that Ba3B(Nb,Ta)2O9(BSr, Mg, Ca, Hg, Zn and Fe) materials are semiconductors and energy band gap varying in the range from 0.73 eV to 5.26 eV, whereas metallic character is observed in Ba3B(Nb, Ta)2O9(B= Ni, Mn and Co) compounds. The structure stability of these materials was confirmed by formation energy. Moreover, the optical properties of the material in question are also examined and discussed. In addition, the temperature-dependent thermoelectric properties have been studied by calculating the electronic thermal conductivity (κ), Seebeck coefficient (S), power factor (PF = σS2/τ), and electrical conductivity (σ/τ). High ZT values of 0.991, 0.998, 0.979, 0.993, 0.995, 0.996, 1.988 and 2.041 at room temperature for Ba3FeNb2O9, Ba3FeTa2O9, Ba3HgNb2O9, Ba3SrNb2O9, Ba3ZnNb2O9, Ba3ZnTa2O9, Ba3HgTa2O9, and Ba3SrTa2O9, respectively. Based on our calculations, predicting Ba3(Sr, Hg)Ta2O9 as a good candidates for thermoelectric applications.

Superior thermoelectric cooling performance by suppressing bipolar diffusion effect and enhancing anisotropic texture in p-/n-type Bi2Te3 based compounds

Thermoelectric technologies can be used for waste heat power generation, and for solid-state cooling without environmentally hazardous refrigerants or moving parts. Thermoelectric cooling materials like the Bismuth Tellurides have been intensively investigated for several decades, but to date, thermoelectric cooling device applications have been limited to niche markets, because of their low cooling efficiency and high power consumption. Here, we demonstrate a thermoelectric module with significantly enhanced cooling performance and low power consumption, with fast cooling, using state-of-the-art p-/n-type Bi2Te3 based thermoelectric materials. Small grain size and anisotropic texture were achieved an exceptional high performance in the p-type Ag0.0006Bi0.46Sb1.5Te3.07 and n-type (CuI)0.002Bi2Te2.7Se0.3 compounds through rapid solidification and hot extrusion processes, resulting in significantly improved thermoelectric performance over a wide temperature range (zTmax = 1.45 for the p-type and 1.40 for the n-type compounds). The observed temperature-dependent thermoelectric properties in the n-type (CuI)0.002Bi2Te2.7Se0.3 compound could be explained in terms of Rashba band splitting rather than the multiband Boltzmann transport equations. We suggest that the enhancement of thermoelectric performance is due to the increase in chemical potential by Ag- (p-type) and CuI-doping (n-type) which suppresses bipolar diffusion, while the anisotropic texture and micro grains decreases lattice thermal conductivity. The thermoelectric module using these state-of-the-art thermoelectric materials demonstrated a reduction in power consumption by 38% and an increase of cooling speed by 170% compared with commercial single crystal-based TE devices, which is promising for practical applications.

Computational Search for Better Thermoelectric Performance in Nickel-Based Half-Heusler Compounds.

Half-Heusler alloys have recently received extensive attention because of their promising thermoelectric (TE) properties and great potential for applications requiring efficient thermoelectricity. Although the conversion efficiency of these materials can be greatly improved by doping, it is still far away from the real-life applications. Therefore, search for better parent TE compounds is deemed urgent. Using a high-throughput search method based on first-principles calculations in newly proposed 378 half-Heusler alloys, we identify nine nickel-based half-Heusler semiconductors as candidates and systematically study their mechanical, electronic, and transport properties. Their mechanical and dynamical stabilities are verified based on the calculated elastic constants and phonon spectra. The electronic structure calculations indicate the existence of direct energy gaps in the NiVZ (Z = Al, Ga, and In) and indirect energy gaps in the NiTiZ (Z = Si, Ge, and Sn) and NiScZ (Z = P, As, and Sb) compounds. Among them, NiVAl, NiVGa, and NiVIn exhibit a sharp slope of density of states near the Fermi level, which is predicted to be essential for a high TE performance. Further investigation on carrier concentration and temperature dependence of TE properties shows the high power factors of NiVAl, NiVGa, and NiVIn, which are responsible for their high figure of merit values. The highest maximum power factor of 5.152 mW m–1 K–2 and figure of merit of 0.309 are predicted for pristine half-Heusler NiVIn, which are larger than the values of some known pristine and doped half-Heusler TE materials. Our work opens up new avenues for rationally searching better TE materials among half-Heusler alloys for applications in fields requiring efficient thermoelectricity.

Analytical model-based optimization of the thermoelectric cooler with temperature-dependent materials under different operating conditions

This paper presents a novel general single/multi-parameter optimization model for the thermoelectric cooler (TEC) based on the 1D resistance method, which precisely captures all the thermoelectrical effects and the impacts of temperature-dependent thermoelectric properties, thermal operating conditions, and contact resistance at the solder layer, respectively. The explicit analytical expressions of designed optimal parameters, including the applied current, thermoelectric leg length, the cross-section area ratio of P/N-type thermoelectrical legs, and the number of leg pairs, are derived to maximize TEC performances (such as cooling capacity, coefficient of performance, or temperature difference between the cold/hot sides) under different operating conditions. The validation of the developed model is conducted through comparisons with the reported experimental data or 3D simulation results. The inherent correlations between the designed parameters are demonstrated and used to clarify the conflicting results in the literature. The ratio of leg length and its area cannot be assumed as an independent optimal parameter for large contact resistance. The filling ratio of thermoelectrical legs should be chosen as large as possible and limited by the manufacturing technology. It is demonstrated that the multiplication of applied current and number of leg pairs could be considered an independent optimal parameter, which provides a new strategy to design the available applied current for practical applications. The correlation between applied current and leg length is also discussed in detail. Multi-parameter optimization could remarkably improve the TEC performance compared with the single-parameter model, such as a significant improvement by 31% for the optimal cooling capacity. The cooling capacity is enhanced by 98% when the convective heat transfer coefficient of 1000 W/m2K at the cold/hot sides increases to 2000 W/m2K. A decrease of 15% for cooling capacity induced by the contact resistance is also observed. The present work provides a promising comprehensive tool for intensifying TEC performance maximization and clarifying its impact factors and the multi-parameter inherent correlation.

Binary and Ternary Colloidal Cu-Sn-Te Nanocrystals for Thermoelectric Thin Films.

Recent advances in copper chalcogenide-based nanocrystals (NCs), copper sulfide, and copper selenide derived nanostructures, have drawn considerable attention. However, reports of crystal phase and shape engineering of binary or ternary copper telluride NCs remain rare. Here, a colloidal hot-injection approach for producing binary copper/tin telluride, and ternary copper tin telluride NCs with controllable compositions, crystal structures, and morphologies is reported. The crystal phase and growth behavior of these tellurides are systematically studied from both experimental and theoretical perspectives. The morphology of Cu1.29 Te NCs is modified from 1D nanorods with different aspect ratios to 2Dnanosheets and 3D nanocubes, by controlling the preferential growth of specific crystalline facets. A controllable phase transition from Cu1.29 Te to Cu1.43 Te NCs is also demonstrated. The latter can be further converted into Cu2 SnTe3 and SnTe through Sn incorporation. Temperature dependent thermoelectric properties of metal (Cu and Sn) telluride nanostructure thin films are also studied, including Cu1.29 Te, Cu1.43 Te, Cu2 SnTe3 , and SnTe. Cu2 SnTe3 is a low carrier density semimetal with compensating electron and hole Fermi surface pockets. The engineering of crystal phase and morphology control of colloidal copper tin telluride NCs opens a path to explore and design new classes of copper telluride-based nanomaterials for thermoelectrics and other applications.

Universality and thermoelectric transport properties of quantum dot systems

We discuss the temperature-dependent thermoelectric transport properties of semiconductor nanostructures comprising a quantum dot coupled to quantum wires: the thermal dependence of the electrical conductance, thermal conductance, and thermopower. We explore the universality of the thermoelectric properties in the temperature range associated with the Kondo crossover. In this thermal range, general arguments indicate that any equilibrium property's temperature dependence should be a universal function of the ratio ${T}^{*}=T/{T}_{K}$, where ${T}_{K}$ is the Kondo temperature. Considering the particle-hole symmetric, spin-degenerate Anderson model, the zero-bias electrical conductance has already been shown to map linearly onto a universal conductance through a quantum dot embedded or side-coupled to a quantum wire. Employing rigorous renormalization-group arguments, we calculate universal thermoelectric transport coefficients that allow us to extend this result to the thermopower and the thermal conductance. We present numerical renormalization-group results to illustrate the physics in our findings. Applying the universal thermoelectric coefficients to recent experimental results of the electrical conductance and thermovoltages versus ${V}_{\mathrm{gate}}$, at different temperatures in the Kondo regime, we calculate all the thermoelectric properties and obtain simple analytical fitting functions that can be used to predict the experimental results of these properties. However, we cannot check all of them, due to the lack of available experimental results over a broad temperature range.

Simplified calculation model for the effect of nonlinear temperature dependence of thermoelectric properties on the conversion efficiency

Thermoelectric (TE) materials play an important role in energy production and utilization, but the temperature dependence of material properties makes theoretical analysis challenging. In this work, the TE differential equation is solved by introducing the volume average of TE material parameters, and a simplified TE element model is established for a TE element with a linear temperature profile. The temperature dependence of the Seebeck coefficient, electrical resistivity and thermal conductivity are incorporated into the thermal conductivity equation, and the TE conversion efficiency is solved. This proposed model is applied to a high-performance N-type half-Heusler alloy, and the obtained maximum conversion efficiency agrees well with the numerical results (with relative error 1.36%) for an average temperature Tm = 510 K and temperature difference ΔT = 440 K. The results confirm that the proposed model, which incorporates the Thomson effect, is simple and accurate, providing valuable guidance for optimal performance design of TE devices.

A comparative study on thermoelectric properties of ZnO bulks and thin films

Introduction: Zinc oxide (ZnO) is well-known as a promising thermoelectric material owing to its safety, inexpensiveness, and thermal stability. This research provides an overview of thermoelectric potentials, including structure, electrical conductivity, Seebeck coefficient, and power factor of pure ZnO semiconductor synthesized in bulk and thin-film forms.
 Methods: The ZnO bulk was synthesized by solid-state reaction at high temperature, while the thin film was prepared by d.c. magnetron sputtering technique. The temperature-dependent thermoelectric properties of all the samples were measured by the Seebeck LSR-3 system. The crystallographic and surface morphological information of the samples were obtained by using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), respectively.
 Results: The XRD results confirm that both the bulk and thin-film have polycrystalline structure and characteristics of hexagonal-wurtzite ZnO. Through the FESEM observation, the bulk is well densified under high-temperature condition, while the thin-film achieve good orientation and close-packed grains. At 573 K, the obtained thermoelectric properties (electrical conductivity, Seebeck coefficient, and power factor) are respectively 352.4 S/cm, -89.5 µV/K and 282.5 µW/mK2 for the ZnO bulk; and 289 S/cm, -113.8 µV/K and 374.3 µW/mK2 for the ZnO film.
 Conclusion: The comparative study shows the good thermoelectric potential of ZnO material in both forms of bulk and thin film. Among them, the thin film has better results, especially in the Seebeck coefficient and power factor than one.

Structural, electronic, vibrational, mechanical and thermoelectric properties of 2D and bulk BaX (X=O, S, Se and Te) series under DFT and BTE framework

Abstract A thorough study on the structural, electronic, vibrational, mechanical, and thermoelectric properties of 3D (rock-salt) and 2D (hexagonal) barium chalcogenides (BaX; X = GrVI or O, S, Se, and Te) is carried out in the present work. For the first time, transport properties with respect to temperature (ranging from 50 to 800 K) for 3D (rock-salt or rs) and 2D (hexagonal or h) barium chalcogenides are studied. Together with their wide indirect and direct energy bandgaps, the dynamical and mechanical stability of both the rock-salt (rs) and hexagonal (h) BaX compounds motivates us to look at their temperature-dependent thermoelectric properties, viz. thermal or heat conductivity (κ), specific conductance or electrical conductivity (σ), thermopower or Seebeck coefficient (S) and figure of merit (ZT), under the framework of semi-classical Boltzmann transport equations (BTE). Phonon dispersion findings demonstrated the dynamic stability of BaX compounds, while via different elastic parameters, viz. Young's modulus and Poisson's ratio, the mechanical stability and elastic nature of these compounds are evaluated. As per the present investigation, it is noted that the GGA-PBE measurement provides stability in all the rs and h phases of BaX, which is in contrary to the previous report in which h-BaO, h-BaS and h-BaSe compounds were identified as unstable at the DFT-LDA (local density approximation) computation. For all the BaX except for rs-BaO and h-BaTe, the electronic band structure reveals indirect bandgap nature. The evaluation of effective mass, mobility and relaxation time has also been included in the electronic properties and, as per our observation, the maximum mobility for rs-BaSe, rs-BaTe and h-BaTe are confirmed. The BaSe in both of its rs (ZT > 0.8) and h (ZT > 0.75) phases has revealed to be outstanding thermoelectric materials in the present work. This investigation reveals excellent figure of merit (ZT) in the range of 0.5–1.1 for all the considered BaX compounds at the temperature of 800 K, suggesting its viability in thermoelectric generators and thermoelectric coolers.

Direct measurement of the thermoelectric properties of electrochemically deposited Bi2Te3 thin films

The best known thermoelectric material for near room temperature heat-to-electricity conversion is bismuth telluride. Amongst the possible fabrication techniques, electrodeposition has attracted attention due to its simplicity and low cost. However, the measurement of the thermoelectric properties of electrodeposited films is challenging because of the conducting seed layer underneath the film. Here, we develop a method to directly measure the thermoelectric properties of electrodeposited bismuth telluride thin films, grown on indium tin oxide. Using this technique, the temperature dependent thermoelectric properties (Seebeck coefficient and electrical conductivity) of electrodeposited thin films have been measured down to 100 K. A parallel resistor model is employed to discern the signal of the film from the signal of the seed layer and the data are carefully analysed and contextualized with literature. Our analysis demonstrates that the thermoelectric properties of electrodeposited films can be accurately evaluated without inflicting any damage to the films.

Thermoelectric properties of PbTe-based graphene nanocomposite

PbTe is a classic thermoelectric being used commercially in interplanetary and deep space missions due its high reliability and availability for power generation, however, the low efficiency limits its broader applications. The advancement in nanostructured powder processing with low-dimensional constituents has evolved as an effective strategy for the development of high-performance materials. In this work, coarse PbTe was transformed into nanostructured material through ball milling in an inert environment. Subsequently, 1 vol% graphene was uniformly dispersed in PbTe fine powder in order to translate the favorable properties of 2D sheets to the bulk matrix. The coarse PbTe and nanostructured PbTe/graphene composite powders were consolidated by the high-frequency induction heated sintering. The temperature-dependent thermoelectric properties of the bulk samples were evaluated. The electronic transport of pure PbTe demonstrated semi-metallic conducting behavior and the composite with 1 vol% of graphene indicated somehow semiconducting trend. The Seebeck coefficient changed from positive for PbTe to negative for PbTe/graphene composite thus exhibiting transformation of majority carriers. The power factor of the composite dominated by the electrical conductivity increased at higher temperature ~ 490K from pristine PbTe. The thermal conductivity of the composite decreased substantially over the entire temperature range suggesting multiscale phonon scattering. Consequently, reduction in thermal conductivity synchronized with increase in power factor leads to enhancement of thermoelectric figure of merit of the composite above ~ 490K from pristine PbTe.

Computational Study of Structural, Electronic, Elastic, Half-Metallic and Thermoelectric Properties of CoCrScZ (Z=Al, Si, Ge, and Ga) Quaternary Heusler Alloys

The structure, electronic, half-metallic, elastic, and thermoelectric properties of the innovative quaternary Heusler (QH) CoCrScZ (Z=Al, Si, Ge, and Ga) alloys are studied by using density functional theory (DFT). CoCrScZ (Z=Al, Ga, and Ge) materials are found to be magnetically active with 100% spin polarization where partially filled 3d-orbitals of Cr confirm the major contribution to the magnetic moment. The results of half-metallic robustness show that the mechanical stability and half-metallicity of CoCrScZ (Z=Ga and Al) can be well maintained in the range of 5.79 to 6.28 A and 5.84 to 6.33 A, respectively. The QH CoCrScSi has a magnetic moment of 4.0 μB at the equilibrium lattice constant a = 6.04 A. The calculated Cauchy pressure, Pugh, and Poisson ratio prove that all alloys are hard, elastically ductile, and anisotropic. In addition, the temperature-dependent thermoelectric properties have been studied by calculating the electronic thermal conductivity (κ), Seebeck coefficient (S), power factor (PF), and electrical conductivity (σ/τ). High ZT values of 0.15, 0.12, 0.77, and 0.83 were obtained for CoCrScSi, CoCrScGe, CoCrScAl, and CoCrScGa, respectively. Based on our calculations, CoCrScZ (Z=Al, Ge, and Ga) combine both good spintronic and thermoelectric behaviors that may be used in spin injection applications.

Effect of Zn-Doping on the Phase Transition Behavior and Thermoelectric Transport Properties of Cu2Se

We report on the phase transition behavior and thermoelectric transport properties of Zn-doped Cu2Se. Cu2Se is comprised of nontoxic, low-cost and abundant elements, and has been attracting attention because of its very high thermoelectric performance, ZT, at high temperatures. Many studies have reported enhanced ZT in impurity doped Cu2Se, however, little is known about the effect of dopants on the phase transition behavior of Cu2Se. In this study, we prepared (Cu1-xZn)2Se (x = 0 ~ 0.03) compounds by spark plasma sintering, and investigated the phase transition behavior in terms of the temperature-dependent thermoelectric transport properties. Undoped Cu2Se consists of monoclinic α-Cu2Se and berzelianite at room temperature. However, the crystal structure of (Cu1-xZnx)2Se compounds is changed into a single phase of α-Cu2Se with increasing Zn content. As the Zn content increased, electrical conductivity decreased and Seebeck coefficient increased due to the donor role of Zn. The phase transition temperatures of the compounds were characterized as a function of Zn content, based on the temperature-dependent electrical conductivities and Seebeck coefficients. The phase transition temperature was increased by increasing Zn content, and it was attributed to the disappearance of the berzelianite phase whose phase transition temperature is lower than that of α-Cu2Se. Detailed effects of Zn-doping on both the phase transition behavior and thermoelectric transport properties were discussed.