Thermoelectric properties and electronic structure of Al-doped ZnO

Impure ZnO materials are of great interest for high temperature thermoelectric application. In this work, we present the effects of Al-doping on the thermoelectric properties and electronic structures of a ZnO system. We find that, with increasing Al concentrations, the electrical conductivity increases and the thermal conductivity decreases significantly, whereas, the Seebeck coefficient decreases slightly. Nevertheless, the figure of merit (ZT) increases owing to high electrical conductivity and low thermal conductivity. On the other hand, the electronic band structures show that the position of the Fermi level is moved upwards and the bands split near the valence-band top and conduction-band bottom. This is due to the interaction between the Al3p and Zn4s orbitals, which drive the system towards semimetal. Besides, the Density Of States (DOS) analysis shows that the introduction of Al atom obviously reduces the slope d(DOS)/dE near the Fermi level. Based on the calculated band structures, we are able to explain qualitatively the measured transport properties of the Al-doped ZnO system.     

Electronic transport and specific heat of 1T-V Se2

The low-temperature thermoelectric power and the specific heat of 1T-V Se₂ (vanadium diselenide) have been reported along with the electrical resistivity and Hall coefficient of the compound. The charge density wave (CDW) transition is observed near 110 K in all these properties. The thermoelectric power has been measured from 15 K to 300 K, spanning the incommensurate and commensurate CDW regions. We observed a weak anomaly at the CDW transition for the first time in the specific heat of V Se₂. The linear temperature dependence of the resistivity and thermoelectric power at higher temperatures suggests a normal metallic behavior and electron–phonon scattering above the CDW transition. The positive thermoelectric power and negative Hall coefficient along with strongly temperature-dependent behavior in the CDW phase suggest a mixed conduction related to the strongly hybridized s–p–d bands in this compound.     

Graphene for Thermoelectric Applications: Prospects and Challenges

Thermoelectric power generators require high-efficiency thermoelectric materials to transform waste heat into usable electrical energy. An efficient thermoelectric material should have high Seebeck coefficient and excellent electrical conductivity as well as low thermal conductivity. Graphene, the first truly 2D nanomaterial, exhibits unique properties which suit it for use in thermoelectric power generators, but its application in thermoelectrics is limited by the high thermal conductivity and low Seebeck coefficient resulting from its gapless spectrum. However, with the possibility of modification of graphene's band structure to enhance Seebeck coefficient and the reduction of its thermal conductivity, it is an exciting prospect for application in thermoelectric power generation. This article examines the electronic, optical, thermal, and thermoelectric properties of graphene systems. The factors that contribute to these material properties in graphene systems like charge carriers scattering mechanisms are discussed. A salient aspect of this article is a synergistic perspective on the reduction of thermal conductivity and improvement of Seebeck coefficient of graphene for a higher thermoelectric energy conversion efficiency. In this regard, the effect of graphene nanostructuring and doping, forming of structural defects, as well as graphene integration into a polymer matrix on its thermal conductivity and Seebeck coefficient is elucidated.     

Thermal transport in the intermetallic compound CeNi<Subscript>4</Subscript>Cr

Electrical resistivity, thermopower (TEP), thermal conductivity and the thermoelectric figure of merit are studied for the CeNi₄Cr compound, which has been previously suggested to be a fluctuating valence system with a tendency to the increase of the effective mass at low temperatures. The analysis of the thermoelectric properties confirms such a possibility and provides characteristic parameters like the Debye temperature, Fermi energy and the position of the f band. Both the thermopower and the magnetic part of the electrical resistivity could be analyzed within a similar model assuming a narrow f-band of the Lorentzian form near the Fermi energy. The thermal conductivity shows that the phonon contribution exceeds the electronic one below 220 K.     

Application of one-band concepts to the diborides of group IV transition metals

Results are presented on the electrical conductivity, thermal emf, thermal conductivity, Hall coefficient, and Ettingshausen-Nernst coefficient for the diborides of Ti, Zr, and Hf. The one-band model is used to calculate the concentration, mobility, effective mass, Fermi energy, etc for the conduction electrons. It is concluded that this model is applicable to these compounds.     

Transport Properties of Si and Ge Liquid Semiconductor Metals

In the present article, we study the electrical resistivity ρ, the thermoelectric power (TEP) α, thermal conductivity σ, Knight-Shifts and temperature coefficient of the Knight-Shifts of the liquid Si and Ge using the well known model potential for the first time. The structure factor used in the present work is derived from the Percus-Yevick (PY) theory. Various local field correction functions are used to study the screening influence. The present results of resistivity are found in qualitative agreement with available experimental and theoretical whenever exists.     

Synthesis and thermoelectric properties of YbSb2Te4

The study of the ternary phase diagram Yb–Sb–Te has led to the synthesis of YbSb₂Te₄ as a pure phase by way of high energy ball milling followed by annealing, whereas typical high temperature powder metallurgy leads to multiphase sample with impurities of the very stable YbTe. The Hall mobility, Seebeck coefficient, electrical resistivity and thermal conductivity of the layered compound YbSb₂Te₄ were measured in the range of 20–550 °C. The thermoelectric figure of merit peaks at 525 K and reaches 0.5. Of particular interest is the very low lattice thermal conductivity (as low as a glass) which makes YbSb₂Te₄ and related compounds promising thermoelectric materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of liquid PbS,PbSe and PbTe

We present here a study of the various properties of liquid PbS, PbSe and PbTe. In the first part of the presentation, we discuss the various possible models in the light of the semiconducting properties of these liquids. The electrical conductivity studies lead us to believe that the temperature dependence of electrical conductivity for these liquids is in accord with our earlier reported⁽¹⁾ studies of the temperature dependence of the energy gap for crystalline PbS, PbSe and PbTe. We also give an interpretation for σ₀, and discussion of the Hall coefficient and the Hall mobility is presented. The next part of the presentation deals with thermal conductivity, thermoelectric power and other related properties. The negative values of the thermoelectric power for these liquids are qualitatively analysed. Our studies of the magnetic susceptibility for crystalline and liquid PbTe indicate that the ratio of the transition term to the energy gap is a constant at all temperatures so that the Van-Vleck component is temperature independent, thereby explaining the constancy of the total susceptibility. Electronegativity and ionicity studies are presented. Finally, we report our studies of the density, viscosity and other physio-chemical properties.     

Unraveling the effect of uniaxial strain on thermoelectric properties of Mg_2Si: A density functional theory study

In this work, the effect of uniaxial strain on electronic and thermoelectric properties of magnesium silicide using density functional theory(DFT) and Boltzmann transport equations has been studied. We have found that the value of band gap increases with tensile strain and decreases with compressive strain. The variations of electrical conductivity,Seebeck coefficient, electronic thermal conductivity, and power factor with temperatures have been calculated. The Seebeck coefficient and power factor are observed to be modified strongly with strain. The value of power factor is found to be higher in comparison with the unstrained structure at 2% tensile strain. We have also calculated phonon dispersion, phonon density of states, specific heat at constant volume, and lattice thermal conductivity of material under uniaxial strain. The phonon properties and lattice thermal conductivity of Mg_2Si under uniaxial strain have been explored first time in this report.     

Anomalously large contributions of vacancies and dislocations to Hall coefficient and thermoelectric power of copper films

Effect of vacancies and dislocations on Hall coefficient (R_H) and thermoelectric power (TEP) of copper films has been deduced from annealing behaviour of various transport properties. The large contributions of these defects may be attributed to variations in the density of hole-like states and/or the energy dependence of conductivity (at the Fermi surface), caused by changes in the extent and/or degree of curvature of the necks at Fermi surface.     

Transport coefficients and flux motion in Bi2Sr2CaCu2Ox single crystals

We present measurements of the electrical resistivity, thermal conductivity, and Hall, Nernst, and Seebeck effects in the mixed state of single crystalline Bi₂Sr₂CaCu₂O_x. It is shown that the sign of the Hall voltage changes twice as temperature decreases below T_c. From the Nernst effect we estimate the transport entropy S_φ to be about 10⁻¹⁰ erg/K cm. S_φ is equal to zero in the normal state, increases and passes through a maximum at the mixed state as expected. The temperature dependences of the thermoelectric power in magnetic fields are analogous to the resistive transition curves. These phenomena are discussed in terms of flux flow. The contribution of the flux flow to the thermal conductivity is estimated to be negligible. Lowering of the thermal conductivity at temperatures below T_c by a magnetic field is attributed to phonon scattering by the vortex lines.     

Semi-metallic behaviour of i-AlCuFe quasicrystals

We report on new measurements of the electrical conductivity, Hall effect and thermoelectric power in the temperature range from 2 K to 440 K for AlCuFe quasicrystals of different compositions and annealing treatments. Particularly, the Hall coefficient and the thermopower show a strong dependence on composition and also on heat treatment. The increase of sample perfection on annealing between 810° C and 825° C is demonstrated by analysing X-ray measurements.All measured quantities can be explained in terms of a semimetal containing both electrons and holes at low temperatures, at least for samples near the ideal quasicrystalline structure and composition.     

Electrical transport properties of CuS single crystals

Electrical resistivity, transverse magnetoresistance and thermoelectric power measurements were performed on CuS high quality single crystals in the range 1.2-300 K and under fields of up to 16 T. The zero field resistivity data are well described below 55 K by a quasi-2D model, consistent with a carrier confinement at lower temperatures, before the transition to the superconducting state. The transverse magnetoresistance develops mainly below 30 K and attains values as large as 470% for a 16 T field at 5 K, this behaviour being ascribed to a band effect mechanism, with a possible magnetic field induced DOS change at the Fermi level. The transverse magnetoresistance shows no signs of saturation, following a power law with field Δρ/ρ(0) ∝ H(1.4), suggesting the existence of open orbits for carriers at the Fermi surface. The thermoelectric power shows an unusual temperature dependence, probably as a result of the complex band structure of CuS.     

Thermoelectric properties of Bi-Sb semiconducting alloys prepared by quenching and annealing

Bi_100−xSb_x (x=8-17) alloys were prepared by direct melting of constituent elements, which was followed by quenching and annealing. The synthesis of high-homogeneity alloys was confirmed by X-ray diffraction, differential thermal analyses and electron microprobe analysis. The semiconducting and thermoelectric properties of the samples were investigated by measuring Hall coefficient, electrical resistivity and Seebeck coefficient in the temperature range from 20 to 300 K for both the as-quenched and annealing samples. The properties change gradually with the Sb concentration x, which is attributed to the variation of the energy gap. The Hall mobility was enhanced by annealing, which leads to a small electrical resistivity and a large Seebeck coefficient. Consequently, large values of about 8.5 mW/mK² for the power factor were obtained in the annealed alloys of x=8,12, and 14.     

Synthesis and low temperature transport properties of Mg2Ge1–y Sby

We report on the synthesis and low temperature transport of Mg₂Ge_1–y Sb_y with 0 ≤ y ≤ 0.33. In these materials Sb substitutes for Ge in the antifluorite structure. Electrical and thermal transport measurements indicate that as the Sb content increases, vacancies are formed on the Mg sites thereby contributing to variations in the transport properties. With increasing Sb content both the absolute Seebeck coefficient and electrical resistivity first decrease and then increase, while the thermal conductivity decreases monotonically. Hall measurements indicate this tendency is associated with vacancy formation at higher Sb concentrations. The lattice thermal conductivity is fitted using the Debye approximation in order to elucidate the effect of alloying. We discuss these results in terms of potential for thermoelectric applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tuning the thermal conductivity of strontium titanate through annealing treatments

Strontium titanate(SrTiO_3) is a promising n-type material for thermoelectric applications. However, its relatively high thermal conductivity limits its performance in efficiently converting heat into electrical power through thermoelectric effect.This work shows that the thermal conductivity of SrTiO_3 can be effectively reduced by annealing treatments, through an integrated study of laser flash measurement, scanning electron microscopy, Fourier transform infrared spectroscopy, x-ray absorption fine structure, and first-principles calculations. A phonon scattering model is proposed to explain the reduction of thermal conductivity after annealing. This work suggests a promising means to characterize and optimize the material for thermoelectric applications.     

Multi-patch model for transport properties of cuprate superconductors

A number of normal state transport properties of cuprate superconductors are analyzed in detail using the Boltzmann equation. The momentum dependence of the electronic structure and the strong momentum anisotropy of the electronic scattering are included in a phenomenological way via a multi-patch model. The Brillouin zone and the Fermi surface are divided in regions where scattering between the electrons is strong and the Fermi velocity is low (hot patches) and in regions where the scattering is weak and the Fermi velocity is large (cold patches). We present several motivations for this phenomenology starting from various microscopic approaches. A solution of the Boltzmann equation in the case of N patches is obtained and an expression for the distribution function away from equilibrium is given. Within this framework, and limiting our analysis to the two patches case, the temperature dependence of resistivity, thermoelectric power, Hall angle, magnetoresistance and thermal Hall conductivity are studied in a systematic way analyzing the role of the patch geometry and the temperature dependence of the scattering rates. In the case of Bi-based cuprates, using ARPES data for the electronic structure, and assuming an inter-patch scattering between hot and cold states with a linear temperature dependence, a reasonable agreement with the available experiments is obtained. Received 3 August 2001 and Received in final form 1st November 2001     

A comparison study on the electronic structures,lattice dynamics and thermoelectric properties of bulk silicon and silicon nanotubes

In order to investigate the mechanism of the electron and phonon transport in a silicon nanotube(SiNT),the electronic structures,the lattice dynamics,and the thermoelectric properties of bulk silicon(bulk Si)and a SiNT have been calculated in this work using density functional theory and Boltzmann transport theory.Our results suggest that the thermal conductivity of a SiNT is reduced by a factor of 1,while its electrical conductivity is improved significantly,although the Seebeck coefficient is increased slightly as compared to those of the bulk Si.As a consequence,the figure of merit(ZT)of a SiNT at 1200 K is enhanced by 12 times from 0.08 for bulk Si to 1.10.The large enhancement in electrical conductivity originates from the largely increased density of states at the Fermi energy level and the obviously narrowed band gap.The significant reduction in thermal conductivity is ascribed to the remarkably suppressed phonon thermal conductivity caused by a weakened covalent bonding,a decreased phonon density of states,a reduced phonon vibration frequency,as well as a shortened mean free path of phonons.The other factors influencing the thermoelectric properties have also been studied from the perspective of electronic structures and lattice dynamics.     

Physical properties of cage-like compound UB12

Boron and uranium form three metallic borides having the chemical formulae UB₂, UB₄ and UB₁₂. In this study, we present the temperature variations of magnetic susceptibility, specific heat, electrical resistivity (performed in magnetic fields of 0 and up to 9 T), thermoelectric power and thermal conductivity measured on the bulk sample of UB₁₂. This dodecaboride behaves as a typical metal, being a Pauli paramagnet and exhibiting a large variety of physical properties due to its specific close-packed structure containing B₁₂ groups. We describe also an uncommon phenomenon observed in UB₁₂, that is, a fairly large scattering of the experimental resistivity data under application of a magnetic field at low temperatures and its systematic vanishing during heating of the sample. This effect is probably caused by inharmonious movement (rattling) of the uranium atoms inside the oversized coordination cage, B₂₄, reflected by applying the magnetic field. The specific heat, resistivity, thermoelectric power and heat transport data have been analysed in the framework of the low-frequency Einstein modes, which are mainly responsible for the phonon spectra behaviour in the system studied here.     

β型锑烯热电输运性质的第一性原理研究

利用第一性原理与半经典玻尔兹曼方程，计算并分析β型锑烯的声子色散、声子群速度、声子弛豫时间、晶格热导率及不同温度下的塞贝克系数、电导率和电子热导率随化学势的变化；结果表明：β型锑烯由于非平面六角结构，三支声学声子在Γ点附近均呈线性变化；声学声子对整个晶格热导率的贡献高达96.68%，而光学声子仅仅占到3.32%；由于较大的声光带隙（a-o gap）导致LA支在声子群速度和弛豫时间中占据主导地位，从而增大了LA支声子对整个热导的贡献；热电优值随温度的升高而增大，在费米面附近其绝对值最大可达0.275.