Anisotropic Seebeck coefficient in β-FeSi2 single crystal

To discuss the possibility of improvement in thermoelectric properties of β-FeSi₂, we examined anisotropy of the Seebeck coefficient of the β-FeSi₂ using single crystal specimens grown by temperature gradient solution growth method. Polyhedral shaped bulk crystals with facet planes were obtained, and they were confirmed as single phase of β-FeSi₂ by X-ray diffraction measurements. For measurements of the Seebeck coefficient, we developed an apparatus that is capable of measuring the Seebeck coefficient of a small specimen down to a few mm in size, because commercially available apparatus cannot be used for such small specimens. The Seebeck coefficient measured along one direction on a surface was around 294 μV/K, while that measured along another direction perpendicular to the former one was 218 μV/K. The difference was larger than experimental error, and appeared to attribute to the difference in crystallographic direction.     

Preparation and thermoelectricity of TiO2/V2O5 powders

TiO₂/V₂O₂ powders were prepared by the mixed oxide and sol-gel method. The sols were dried using different procedures. The obtained powders were characterized by X-ray diffraction, scanning electron microscopy, BET-adsorption and laser granulometry. The electric properties were determined by measuring the thermoelectric power. A strict correlation of the thermoelectric power and the V₂O₂ content could be observed, but by contrast to conductivity measurements, the thermoelectric power was not influenced by the powder morphology. Additionally, the thermoelectric power was drastically influenced by the calcination temperature.     

Effect of nonstoichiometry on the thermoelectric properties of Tl<Subscript>4</Subscript>TiS<Subscript>4</Subscript> crystals

Stoichiometric (1), Tl₂S-enriched (2), and TiS₂-enriched (3) Tl₄TiS₄ crystals (with deviations from stoichiometry within the homogeneity range of the ternary compound) have been grown by the Bridgman method, and their thermoelectric properties have been studied by the Harman technique. All of the crystals have a high thermoelectric figure of merit. The highest figure of merit is offered by crystal 1 at positive thermoelectric powers and by crystal 3 at negative thermoelectric powers. The thermoelectric properties of crystal 2 were found to irreversibly degrade over time.     

Fast preparation and thermoelectric properties of Zn4Sb3 by HPHT

In this paper, crack-free bulk thermoelectric material Zn₄Sb₃ was prepared rapidly by high pressure and high temperature (HPHT) method. Near a single-phase Zn₄Sb₃ specimen was obtained using nominal stoichiometric powder mixtures, which were indexed by powder X-ray diffraction. The temperature-dependent thermoelectric properties including the Seebeck coefficient and electrical resistivity were studied. The maximum power factor of Zn₄Sb₃ specimen prepared by HPHT reaches 10.8 ??W/(cmK²) at 637 K, which is comparable to the published data. The results show that the HPHT offers potential processing route to produce the thermoelectric material Zn₄Sb₃ quickly and effectively.     

A theoretical investigation on thermoelectric performance of ternary (Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> compound

The electronic structure and thermoelectric properties of (Bi_1?x Sb _x )₂Te₃ compound are investigated by using full-potential linearized augmented plane-wave method and semiclassical Boltzmann theory. The density of states and band structure are studied in detail. The transport coefficients are then calculated with the assumption of the constant relaxation time approximation. We investigated the thermoelectric properties at full range of Sb compositions from x = 0 to x = 1. By using the calculated thermoelectric properties and an averaged thermal conductivity, the figure of merit ZT is obtained. The theoretical investigations show a good agreement with the present experimental data.     

Improving the Thermoelectric Properties of Sr0.9La0.1TiO3 by Ag Addition

We investigated the effects of Ag added Sr_0.9La_0.1TiO₃ ceramics prepared via a hydrothermal method and sintering. The results indicate that Ag addition is an efficient way to enhance its thermoelectric properties. The maximum thermoelectric figure-of-merit, ZT, is 0.20 at 955 K for x=0.15 sample.     

快速急冷法对β-Zn_(4+x)Sb_3材料热电及力学性能的影响

β-Zn4Sb3是一种重要的中温热电材料,但其较差的力学强度和可加工性限制了其实际应用.本文采用熔体旋甩法结合放电等离子烧结技术快速制备了一系列具有高热电性能和高力学强度的β-Zn4+xSb3块体材料.通过调节Zn的含量,优化了其热电性能,随着Zn含量的增加,电导率增大,Seebeck系数有所下降,热导率增加.在700K时,Zn4.32Sb3样品的ZT值达到1.13,相比熔融法制备的样品提高了约40%.该制备方法所得到的样品具有极高的抗压强度,与熔融法制备的样品相比较,所有样品的抗压强度均提高了一倍以上,这种高热电性能和高力学强度的β-Zn4+xSb3块体材料具有很好的应用前景.     

Electronic properties of the Mg2Si thermoelectric material investigated by linear-response density-functional theory

This paper presents Density-Functional Perturbation Theory (DFPT) calculations on the electronic, vibrational, and electron–phonon (EP) coupling properties of the Mg₂Si thermoelectric compound. The DFPT yields very satisfactory results for the electronic and vibrational properties when compared to experiment. Regarding the EP interactions, as far as we know, they have never been reported so far. We show that the EP interactions in Mg₂Si mainly involve the silicon atom. This result explains the improvement of the thermoelectric properties of Mg₂Si using a solid solution Mg₂Si_1?xA_x, where A is a heavier atom than Si. By guiding the choice of the substitution site, the study of the EP coupling properties could be used in the search of new thermoelectric materials based on solid solutions.     

Radial electric field effect on thermoelectric transport properties of Bi2Te3 cylindrical nanowire coaxial structure

Radial electric field effect (REFE) on the thermoelectric figure of merit and Seebeck coefficient S are studied for a coaxial cylindrical capacitor configuration on the basis of bipolar intrinsic semiconductors. Theoretical analysis of REFE nanowire was done based on Poisson's equation in cylindrical geometry with corresponding boundary conditions. Using Newton's method the radial variation of the local Seebeck coefficient, carrier concentration and others transport characteristics are calculated for the bipolar Bi₂Te₃ nanowires neglecting size quantization. The dependence of the thermoelectric parameters on the gate voltage is studied. It is shown that the existence of the transition bipolar–monopolar semiconductor, electric field, differences in carrier masses and mobility essentially affect the thermoelectric properties. The thermoelectric figure of merit can be significantly increased by REFE.     

Thermoelectric properties of Zn4Sb3 prepared by hot pressing

Polycrystalline specimens of the thermoelectric material Zn₄Sb₃ were prepared by the hot-pressing method at various temperatures and pressures and their thermoelectric properties were evaluated in a temperature range from 298 K to 673 K. A single phase of Zn₄Sb₃ was obtained in the samples prepared at 673 K with a pressure above 150 MPa, whereas ZnSb was placed in the Zn₄Sb₃ matrix for the samples prepared at 100 MPa. The electrical transport properties of the single phase compound showed p-type conduction and metallic transport behavior based on the temperature dependence. The sample produced at 673 K under a pressure of 200 MPa exhibited the highest ZT value of 1.36 at 673 K. This study suggests that the dense and single-phase Zn₄Sb₃ compound is a route to achieve a high thermoelectric performance.     

Features of Lifshits Electron Topological Transitions Induced by Anisotropic Deformation in Thin Wires of Doped Bismuth

For the first time, glass-coated single-crystal Bi–0.05Sn wires with d=200 nm ÷ 3 μm with C ₃ oriented along the wire axis were obtained by the recrystallization method of the wires with standard (10 $\bar{1}$ 1) orientation, which made it possible to study the thermopower anisotropy. With the use of ShdH oscillations method, we have determined characteristics of changes in the Fermi surface topology at Lifshits electron topological transitions induced by tension in single-crystal Bi–0.05Sn wires with a standard (10 $\bar{1}$ 1) orientation and with the C ₃ orientation along the wire axis. It is found that the thermopower increase with deformation and thermopower anisotropy of the Bi–0.05Sn wires with trigonal orientation achieves a value of 130–150 μV/K which leads to a considerable increase of thermoelectric efficiency, which is important for practical applications, in particular, for their use as anisotropic thermoelectric energy converters.     

Introducing Rare Earth Dopants for Controlled Conductivity in Thermoelectric Cobaltites

The conversion of waste heat into electrical energy plays a key role in our current challenge to develop alternative energy technologies to reduce our dependence on fossil fuels. Thermoelectric (TE) materials are the first choice to handle this subject. In TE materials, cobaltites are the material of interest, due to their nontoxic properties. Cobaltites exhibits large TE power, low resistivity, and relatively small thermal conductivity at room temperature. TE material (BiCa_2?x R _x CoO _y ) where R is for rare earth like Nd, (x = 0.0, 0.2) was synthesized in nanoregime by simplified sol–gel method. Simplified sol–gel method was chosen because it gives maximum phase purity and tunable parameters for desired properties. The mechanism used to enhance thermoelectric properties is to reduce thermal conductivity of the material and increase electrical conductivity of the material using nanostructures. Material characterizations were done for structural studies using X-ray diffraction (XRD). XRD revealed monoclinic crystal structure. Temperature-dependent electrical conductivity showed an increase with increase in temperature. Thermal conductivity was also measured. Both electrical resistivity and thermal conductivity decrease as a result of neodymium doping which enhanced its importance as thermoelectric material. The different parameters were correlated to understand the conduction mechanism.     

Electrical properties of Bi2S3 thin films prepared by the dip-dry method

Bi₂S₃ thin films have been prepared by the dip-dry method and their electrical properties such as conductivity, thermoelectric power and spectral response of photoconductivity were investigated. Some uncommon findings, such as a simultaneous increase in conductivity and thermoelectric power with thickness of film, are explained by Seto's model for polycrystalline silicon film duly modified for in homogeneities.     

Properties of Bi2Te3 single crystals doped with Sn

The effect of doping with Sn on the properties of Czochralski-grown Bi₂Te₃ crystals was studied. The effective segregation coefficient for Sn was determined to be 0.6. The thermoelectric power, electrical conductivity, and Hall coefficient of the doped crystals were measured at room temperature. Doping with low Sn concentrations (0.2-0.5 at. %) was found to have only a weak effect on the electrical properties ofp- type Bi₂Te₃. Doping with 0.7-1 at. % Sn reduces the thermoelectric power and increases the electrical conductivity and hole concentration. Lattice thermal conductivity is a nonmonotonic function of Sn concentration. The thermoelectric figure of merit of Bi₂Te₃ doped with less than 0.6 at. % Sn exceeds that of undoped Bi₂Te₃.     

Optoelectronic and thermoelectric response of Ca<Subscript>5</Subscript>Al<Subscript>2</Subscript>Sb<Subscript>6</Subscript> to shift of band gap from direct to indirect

The structural, optoelectronic and thermoelectric properties of Ca₅Al₂Sb₆ under applied external pressures have been studied using the full potential linear augmented plane wave method. WIEN2k code is used with considering the generalized gradient approximation (GGA), modified Becke–Johnson (MBJ) and modified Becke–Johnson?+?spin orbit (mBJ?+?SO) functionals based on density functional theory (DFT). From electronic results, the size of the band gap decreases with increasing pressure and the nature of the band gap shift from direct to the indirect. In high pressure (>35.7 GPa by mBJ?+?SO), the band gap is also completely disappeared and the nature of compound is changed to the metallic. The calculated anisotropic optical properties such as the static dielectric function, increase with decreasing the size of band gap and increasing of pressure. As a novel result, the thermoelectric performance of n-type and p-type doping of Ca₅Al₂Sb₆ is related to the value of pressure. According to the thermoelectric results, the n-type one has the highest ZT in comparison with the p-type Ca₅Al₂Sb₆ material.     

Mechanical properties of thermoelectric Mg2Si using molecular dynamics simulations

In this paper, the mechanical properties of thermoelectric Mg₂Si are investigated using molecular dynamics (MD) method, with Mg₂Si in the forms of bulk, nanofilm, and nanowire, respectively. The effects of the Mg vacancy on the mechanical properties of Mg₂Si in these three forms are studied in details. First of all, the equilibrium state of Mg₂Si is simulated after choosing the proper potential function, boundary conditions, and the speed algorithm. Nondimensionalization is also implemented during the simulations. This part of simulation aims to verify the correctness of the crystal model established via the use of the molecular dynamics analysis. Next, the models of Mg₂Si in the forms of bulk, nanofilm, and nanowire are established with different Mg vacancy proportions, and then the mechanical properties of each model are studied via the uniaxial tensile test. Finally, the stress–strain curve and subsequently the ultimate tensile strength are obtained for each model. Simulation results indicate that the ultimate tensile strength of Mg₂Si in each model is decreased with the increase of the Mg vacancy proportion. Moreover, through the comparison of the ultimate tensile strengths of Mg₂Si bulk, nanofilm, and nanowire, it is found that low-dimensionalization significantly reduces the ultimate tensile strength of thermoelectric Mg₂Si. Results obtained in this paper can provide valuable guidance to the future applications of thermoelectric devices.     

First principles investigation of electronic, optical and transport properties of α- and β-phase of arsenic telluride

Crystalline arsenic telluride exists in two stable phases. The monoclinic α-phase transforms to rhombohedral β-phase under high pressure. The electronic, optical and transport properties of the two phases has been investigated using full potential linear augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of DFT with generalized gradient approximation (GGA). We present the energy bands, density of states and optical properties like the complex dielectric functions and absorption coefficients. From the dynamic dielectric constant, the structural anisotropy for the monoclinic α-phase is clearly observed, whereas the longitudinal and transverse components are almost identical for the β-phase. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the infrared and visible regions compared to the α-phase. The rhomohedral phase which has the Bi₂Te₃ type structure has the possibility of thermoelectric properties, therefore transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results.     

The thermoelectric properties of Co4Sb12-xTex synthesized at different pressure

Skutterudite compounds Co₄Sb_12-xTe_x (0.1 ≤ ×≤0.8) was synthesized successfully by high temperature and high pressure (HTHP) method and characterized with X-ray diffractometry and thermoelectric properties measurements. The samples prepared by HTHP are nearly with the single phase CoSb₃. The electrical resistivity, Seebeck coefficient and thermal conductivity were all depending on synthetic pressure and the Te content of the Skutterudite compounds were performed at room temperature. As our expected, the Seebeck coefficient increased with an increase of the synthetic pressure and the thermal conductivity decreased with an increase of the synthetic pressure. These results indicated that HTHP technique may be helpful to prepare thermoelectric materials with enhanced thermoelectric properties.     

Strain effects on thermoelectric properties of two-dimensional materials

Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (hBN), phosphorene, transition metal dichalcogenides (e.g., MoS₂, WS₂, etc.), metal oxides (e.g., MoO₃) have attracted much attention recently due to their extraordinary structural, mechanical and physical properties. In particular, 2D materials have shown great potential for thermal management and thermoelectric energy generation due to their fascinating electrical and thermal transport properties, which can lead to a significantly large figure-of-merit. Also due to their large stretchability, 2D materials are promising for using strain engineering to tune and modulate their electronic and thermal properties, which can further enhance their figure-of-merit. In this article, we give a review on the recent advances in the study of strain-engineering on the thermoelectric properties of 2D materials. We first review some important aspects in thermoelectric effects, such as Peltier effect, Seebeck effect, the coefficient of performance and figure-of-merit (ZT) and discuss why 2D materials are ideal candidates for thermal management and thermoelectric applications. We then briefly discuss the strain (stress) generation in 2D materials and their structure integrity under strain (stress). Next, we discuss how strain affects the electronic properties of 2D materials, followed by the discussion on the effects of strain on the thermal properties of 2D materials. Subsequently, we discuss the strain effects on two important thermoelectric properties, Seebeck coefficient and figure-of-merit ZT. Finally, we present our conclusions and future perspective.     

Theoretical Investigation of Cubic BaVO<Subscript>3</Subscript> and LaVO<Subscript>3</Subscript> Perovskites via Tran–Blaha-Modified Becke–Johnson Exchange Potential Approach

The full potential linearized augmented plane wave method of density functional theory has been used to investigate the structural, electronic, magnetic and thermoelectric properties of cubic perovskites BaVO₃ and LaVO₃. The ferromagnetic ground state has been found to be stable by comparing the total energies of non-spin-polarized and spin-polarized calculations performed for optimized unit cells. For both compounds, the bond length and tolerance factor are also measured. From the band structures and density of states plots, it is found that both compounds are half-metallic. We found that the presence of V at the octahedral site of these perovskites develops exchange splitting through p-d hybridization, which results in a stable ferromagnetic state. The observed exchange splitting is further clarified from the magnetic moment, charge and spin of the anion and cations. Finally, we also presented the calculated thermoelectric properties of these materials, which show that half-metallic BaVO₃ and LaVO₃ materials are potential contenders for thermoelectric applications.