Thermoelectric Properties of Polycrystalline SrZn2Sb2 Prepared by Spark Plasma Sintering

Compact polycrystalline samples of SrZn₂Sb₂ [space group $ P\overline{3} m1 $ , a = 4.503(1) Å, c = 7.721(1) Å] were prepared by spark plasma sintering. Thermoelectric performance, Hall effect, and magnetic properties were investigated in the temperature range from 2 K to 650 K. The thermoelectric figure of merit ZT was found to increase with temperature up to ZT = 0.15 at 650 K. At this temperature the material showed a high Seebeck coefficient of +230 μV K^?1, low thermal conductivity of 1.3 W m^?1 K^?1, but rather low electrical conductivity of 54 S cm^?1, together with a complex temperature behavior. SrZn₂Sb₂ is a diamagnetic p-type conductor with a carrier concentration of 5 × 10¹⁸ cm^?3 at 300 K. The electronic structure was calculated within the density-functional theory (DFT), revealing a low density of states (DOS) of 0.43 states eV^?1 cell^?1 at the Fermi level.     

Effect of Initial Bulk Material Composition on Thermoelectric Properties of Bi2Te3 Thin Films

V₂VI₃ compounds and solid solutions based on them are known to be the best low-temperature thermoelectric (TE) materials. The predicted possibility of enhancement of the TE figure of merit in two-dimensional (2D) structures has stimulated studies of the properties of these materials in the thin-film state. The goal of the present work is to study the dependences of the Seebeck coefficient S, electrical conductivity σ, Hall coefficient R _H, charge carrier mobility μ _H, and TE power factor P = S ² σ of Bi₂Te₃ thin films on the composition of the initial bulk material used for preparing them. Thin films with thickness d = 200 nm to 250 nm were grown by thermal evaporation in vacuum of stoichiometric Bi₂Te₃ crystals (60.0 at.% Te) and of crystals with 62.8 at.% Te onto glass substrates at temperatures T _S of 320 K to 500 K. It was established that the conductivity type of the initial material is reproduced in films fairly well. For both materials, an increase in T _S leads to an increase in the thin-film structural perfection, better correspondence between the film composition and that of the initial material, and increase in S, R _H, μ _H, σ, and P. The room-temperature maximum values of P for the films grown from crystals with 60.0 at.% and 62.8 at.% Te are P = 7.5 × 10^?4 W/K² m and 35 × 10^?4 W/K² m, respectively. Thus, by using Bi₂Te₃ crystals with different stoichiometry as initial materials, one can control the conductivity type and TE parameters of the films, applying a simple and low-cost method of thermal evaporation from a single source.     

Effect of Heavy Element Substitution and Off-Stoichiometric Composition on Thermoelectric Properties of Fe2VAl-Based Heusler Phase

The thermoelectric performance of Fe₂VAl-based alloys was improved by using the effects of (a) heavy element substitution and (b) off-stoichiometric (Fe/V ≠ 2) composition. The former method led to a significant reduction of lattice thermal conductivity, whereas the latter to an evolution of the Seebeck coefficient. As a result of sample preparation, we confirmed that the dimensionless figure of merit with n-type behavior was increased up to 0.25 at 420 K for the sample obtained at the optimized composition of Fe_1.98V_0.97Ta_0.05Al_0.9Si_0.1. Electronic structure calculations revealed that the increase of the Seebeck coefficient observed for Fe-poor samples was caused by a reduction of the density of states near the chemical potential.     

Thermoelectric Properties of Polycrystalline Thin Films Under an External Magnetic Field

A theoretical model is proposed to predict the Seebeck coefficient and the electrical conductivity for a polycrystalline thermoelectric (TE) thin film under an external magnetic field. The model considers the distribution of electrons in the microstructure of TE thin-film materials, taking the scattering effect of electrons at the grain boundary as the boundary condition for electron transport in the grain. The transmission coefficient is introduced to describe the probability of electrons passing through the grain boundary potential barrier, while the relationships between the Seebeck coefficient, the electrical conductivity, and the transmission coefficient are studied. Furthermore, the results from the calculations of the Seebeck coefficient, the electric conductivity, and the power factor of TE materials under various applied magnetic fields, transmission coefficients, and grain sizes indicate that the applied external magnetic field has a very significant influence on the TE properties of polycrystalline thin films.     

Effect of NiTe Nanoinclusions on Thermoelectric Properties of Bi2Te3

Metal nanoinclusions in bulk thermoelectric matrix create metal?Csemiconductor interfaces, which can result in improvement in the thermoelectric power factor due to low-energy electron filtering and a simultaneous reduction in lattice thermal conductivity due to increased phonon scattering at grain boundaries. The combined effect results in enhancement of the thermoelectric figure of merit. We report the effect of NiTe nanoinclusions in a Bi₂Te₃ matrix. The Bi₂Te₃/NiTe nanocomposite was synthesized by planetary ball milling. Different volume fractions of NiTe nanoinclusions were incorporated into the bulk (Bi₂Te₃) matrix and uniaxially hot pressed at 100?MPa and 500°C. The presence of nanoinclusions was confirmed by x-ray diffraction and transmission electron microscopy. The Seebeck coefficient, electrical conductivity, and thermal diffusivity were measured from room temperature to 150°C. The carrier concentration of the matrix (Bi₂Te₃) and the nanocomposites (NiTe/Bi₂Te₃) at room temperature were deduced from Hall-effect measurements. Addition of NiTe decreased the carrier concentration, and the power factor increased in the 1?vol.% NiTe/Bi₂Te₃ compared with inclusion-free Bi₂Te₃ matrix due to an increase in mobility.     

Effect of Biaxial Strain on Electronic and Thermoelectric Properties of Mg2Si

The electronic and thermoelectric properties of biaxially strained magnesium silicide Mg₂Si are analyzed by means of first-principle calculations and semiclassical Boltzmann theory. Electron and hole doping are examined for different doping concentrations and temperatures. Under strain the degeneracy of the electronic orbitals near the band edges is removed, the orbital bands are warped, and the energy gap closes up. These characteristics are rationalized in the light of the electron density transfers upon strain. The electrical conductivity increases with the biaxial strain, whereas neither the Seebeck coefficient nor the power factor (PF) follow this trend. Detailed analysis of the evolution of these thermoelectric properties is given in terms of the in-plane and cross-plane components. Interestingly, the maximum value of the PF is shifted towards lower temperatures when increasingly intensive strain is applied.     

Effect of Chemical Composition on the Thermoelectric Properties of n-Type Bi2Te3 Alloys Fabricated by Gas Atomization and Hot Extrusion

In this research, n-type (Bi₂Te₃)_1?x (Bi₂Se₃) _x -based thermoelectric (TE) materials were produced through a gas atomization process, and subsequently hot extruded with an extrusion ratio of 10:1 at 400 °C. The effect of chemical composition on TE properties was investigated. The microstructure of all extruded bars showed a homogeneous and fine distribution of grains due to the dynamic recrystallization during the hot extrusion process. With increasing Bi₂Te₃ content, from 0.85 to 0.90, both electrical resistivity and Seebeck coefficient values were increased. The maximum figure of merit (ZT) 0.673 was obtained at room temperature for (Bi₂Te₃)₀.₉₀(Bi₂Se₃)_0.10 alloys due to them exhibiting higher seebeck coefficient and lower thermal conductivity than other compositions.     

Effect of Ce Substitution for Sb on the Thermoelectric Properties of AgSbTe2 Compound

We have prepared Ce-doped polycrystalline AgSbTe_2.01 compounds from high-purity elements by a melt-quench technique followed by spark plasma sintering, and their thermoelectric transport properties have been investigated in the temperature range of 300 K to 625 K. The actual concentration of Ce was much less than the initial composition, but roughly proportional to it. Small additions of Ce shifted the composition of the homogeneity range from the nearly ideal atomic ratio Ag:Sb:Te = 0.98:1.02:2.01 toward Sb rich (Ag poor), and led to the reemergence of Ag₂Te impurity in AgSbTe₂ compound. The Ce-doped samples possessed lower electrical conductivity compared with the undoped AgSbTe_2.01 compound at room temperature, but the carrier mobility and effective mass were essentially constant, indicating intact band structure near the covalent band maximum upon Ce substitution for Sb. Due to the decrease of lattice vibration anharmonicity resulting from Ce substitution for Sb, the lattice conductivity of the Ce-doped samples was about 0.1 W m^?1 K^?1 higher than that of the AgSbTe_2.01 sample, and the magnitude spanned the range from 0.30 W m^?1 K^?1 to 0.55 W m^?1 K^?1. A ZT of 1.20 was achieved at about 615 K for the AgSb_0.99Ce_0.01Te_2.01 sample.     

The Effect of Adding Nano-Bi2Te3 on Properties of GeTe-Based Thermoelectric Material

The efficient thermoelectric materials (GeTe)_0.85?x (Mn_0.6Sn_0.4Te)_0.15(Bi₂Te₃) _x (0 ≤ x ≤ 0.05), in which Bi₂Te₃ is nanopowder, were prepared by hot pressing. The effect of adding neutral nano-Bi₂Te₃ content on the thermoelectric properties of germanium telluride was investigated. With increasing x, the thermal conductivity of the prepared samples decreased significantly and the Seebeck coefficient declined slightly, while there was no obvious change in electrical conductivity. In both electrical conductivity and Seebeck coefficient curves at different x values, there are inflection points around 600 K. The maximum dimensionless figure of merit ZT of the prepared materials is 1.54, attained in the temperature range from 700 K to 750 K for x = 0.03. The x-ray diffraction (XRD) pattern shows that Bi₂Te₃ has been alloyed into the GeTe-MnTe-SnTe alloy, which is consistent with the high-resolution scanning electron microscopy (HRSEM) images. Adding nano-Bi₂Te₃ to GeTe-based materials could also increase their performance stability at high temperature as a result of decreasing the phase-transition temperature T _c.     

Effect of Ti Substitution on Thermoelectric Properties of W-Doped Heusler Fe2VAl Alloy

Effects of element substitutions on thermoelectric properties of Heusler Fe₂VAl alloys were evaluated. By W substitution at the V site, the thermal conductivity is reduced effectively because of the enhancement of phonon scattering resulting from the introduction of W atoms, which have much greater atomic mass and volume than the constituent elements of Fe₂VAl alloy. W substitution is also effective to obtain a large negative Seebeck coefficient and high electrical conductivity through an electron injection effect. To change the conduction type from n-type to p-type, additional Ti substitution at the V site, which reduces the valence electron density, was examined. A positive Seebeck coefficient as high as that of conventional p-type Fe₂VAl alloy was obtained using a sufficient amount of Ti substitution. Electrical resistivity was reduced by the hole doping effect of the Ti substitution while maintaining low thermal conductivity. Compared with the conventional solo-Ti-substituted p-type Fe₂VAl alloy, the ZT value was improved, reaching 0.13 at 450 K.     

Effect of Thermoelectric and Electrical Properties on the Cooling Performance of a Micro Thermoelectric Cooler

Active cooling has been studied to prevent microprocessor temperature rise due to hot spots, and a micro thermoelectric cooler is a promising candidate for this spot cooling since it can be used to effectively cool the small area near the hot spot. Numerical analysis has been conducted to determine the effect of thermoelectric and electrical properties on the cooling performance of such a micro thermoelectric cooler. In the cooler considered herein, Bi₂Te₃ and Sb₂Te₃ were selected as the n- and p-type thermoelectric materials, respectively. The thermoelectric column considered was 20 μm thick. The coefficient of performance (COP) and cooling rate were the primary factors used to evaluate the performance of the cooler. Although cooling performance varies with thermal conditions such as thermophysical properties and temperature difference, the present study only focuses on the effect of thermoelectric and electrical properties such as the Seebeck coefficient and electrical resistivity.     

Effect of Synthesis and Sintering Conditions on the Thermoelectric Properties of n-Doped Mg2Si

Magnesium silicide (Mg₂Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion in the middle–high temperature range. The detrimental effect of the presence of MgO on the TE properties of Mg₂Si based materials is widely known. For this reason, the conditions used for synthesis and sintering were optimized to limit oxygen contamination. The effect of Bi doping on the TE performance of dense Mg₂Si materials was also investigated. Synthesis was performed by ball milling in an inert atmosphere starting from commercial Mg₂Si powder and Bi powder. The samples were consolidated, by spark plasma sintering, to a density >95%. The morphology, and the composition and crystal structure of samples were characterized by field-emission scanning electronic microscopy and x-ray diffraction, respectively. Moreover, determination of Seebeck coefficients and measurement of electrical and thermal conductivity were performed for all the samples. Mg₂Si with 0.1 mol% Bi doping had a ZT value of 0.81, indicative of the potential of this method for fabrication of n-type bulk material with good TE performance.     

Thermoelectric Properties of RuSb2Te Ternary Skutterudites

Polycrystalline samples of the RuSb₂Te ternary skutterudite compound were prepared by the powder metallurgy method, and the influence of various types of doping on its thermoelectric properties was studied. The phase purity of the prepared samples was checked by means of powder x-ray diffraction, and their compositions were checked by electron probe x-ray microanalysis. Hot-pressed p-type samples were characterized by measurements of electrical conductivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. Various doping strategies, i.e., cation substitution (Ru_0.95Fe_0.05Sb₂Te), anion substitution (RuSb₂Sn_0.1Te_0.9) or partial filling of voids of the ternary skutterudite structure (Yb_0.05RuSb₂Te), were investigated, and the influence of the dopants on the changes of the resulting transport, thermoelectric, and thermal properties is described.     

制备工艺对新型热电材料制冷性能的影响

通过取点法得到了由Ingot法、BM法、S-MS法和Te-MS法制备的四种新型p型热电材料(Bi0.5Sb1.5)Te3的变物性参数拟合公式,分析了温度对不同方法制备的热电材料的影响,得到了热电材料无量纲优值与绝对温度的关系曲线.从热力学方面研究了制备工艺对基于新型热电材料的热电制冷器最大制冷系数的影响.结果表明:由Te-MS法制备的新型p型热电材料(Bi0.5Sb1.5)Te3具有最大的优值系数,基于该材料的热电制冷器最大制冷系数可达2.49,较其他三种方法制备的热电材料分别提升了 34.59％,37.57％和25.76％.     

Thermoelectric Properties of Li-Intercalated ZrSe2 Single Crystals

Zirconium diselenide (ZrSe₂) is one of many members of the layer-structured transition-metal dichalcogenide family. The structure of these materials features a weakly bonded van der Waals gap between covalently bonded CdI₂-type atomic layers that may host a wide range of intercalants. Intercalation can profoundly affect the structural, thermal, and electronic properties of such materials. While the thermoelectric potential of layer-structured transition-metal dichalcogenides has been formerly studied by several groups, to our best knowledge, neither the thermoelectric properties of ZrSe₂ nor the impact of intercalation on its thermoelectric properties have been reported (specifically, the full evaluation of the dimensionless figure of merit, ZT, which includes the thermal conductivity). In this proof-of-principle study, ZrSe₂ single crystals have been synthesized using an iodine-assisted vapor transport method, followed by a wet-chemistry lithium intercalation process. The results of resistivity, thermopower, and thermal conductivity measurements between 10 K and 300 K show that Li intercalation induced additional charge carriers and structural disorder that favorably affected the thermoelectric properties of the material. As a result, a dimensionless figure of merit ZT ≈ 0.26 has been attained at room temperature in a Li-intercalated sample, representing nearly a factor of three improvement compared with the pristine sample. These improvements, along with the abundance, relatively low toxicity, and low cost of such materials, merit further thermoelectric investigations of intercalated zirconium diselenide, especially in conjunction with a substitutional doping approach.     

Thermoelectric Properties of Mn-Doped Cu2SnSe3

We report the thermoelectric properties of Mn-doped Cu₂Mn _x Sn_1?x Se₃ compound, with x ranging from 0.005 to 0.1 at temperature ranging from 80?K to 723?K. All samples maintain cubic zincblende-like structure, and no impurity phase was detected. The electrical resistivity decreases rapidly when Mn⁴⁺ replaces Sn²⁺ in the matrix. The excess Mn impurities in the x?=?0.05 and x?=?0.1 samples also affect the Seebeck coefficient. The total thermal conductivity is increased for Mn-doped samples except for the x?=?0.005 sample. In all, both power factor and figure of merit are improved by Mn doping over the entire temperature range. The ZT value of the x?=?0.02 sample reaches 0.035 at 300?K, and for x?=?0.01 reaches 0.41 at 716?K, which are comparable to the best thermoelectric performance for ternary Cu-based compounds.     

Effect of Composition on Thermoelectric Properties in PbTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Composites

The solidification of alloys in the Bi₂Te₃-PbTe pseudobinary system at off- and near-eutectic compositions was investigated for their microstructure and thermoelectric properties. Dendritic and lamellar structures were clearly observed due to the phase separation and the existence of a metastable ternary phase. In this system, three phases with different compositions were observed: binary Bi₂Te₃, PbTe, and metastable PbBi₂Te₄. The Seebeck coefficient, electrical resistivity, and thermal conductivity of ternary alloys as well as binary compounds were measured. The phonon thermal conductivities of Pb-Bi-Te alloys were lower than those in binary PbTe and Bi₂Te₃, which could have resulted from the increased interfacial area between phases due to the existence of the metastable ternary phase and the resultant phase separation.