Electrical conductivity,thermoelectric power and figure of merit of doped Bi-Sb tapes produced by melt spinning technique

Temperature dependence of electrical conductivity and thermoelectric power are presented for In and Pb doped Bi + 8.28 at % Sb quenched tapes between 77 and 300K. The results are explained in terms of model for disordered semiconductors. Analysis of our data on electrical conductivity indicates the presence of a temperature independent part and a strongly temperature dependent part. While theT independent part originates from band conduction, theT dependent component could be understood considering the presence of localized states. Thermoelectric figure-of-merit of these tapes are also measured at 300K, which shows a large enhancement (∼40%) over that reported earlier on thin Bi-Sb films. This suggests that doped Bi-Sb quenched tapes may be considered as a candidate for material in producing economic and light weight thermoelectric devices.     

Enhancing the electrical conductivity and thermoelectric figure of merit of the p-type delafossite CuAlO2 by Ag2O addition

(CuAlO₂)_1-x(Ag₂O)_x specimens with 0 ≤ x ≤ 0.06 were prepared through the sintering of mixtures of CuO, Al₂O₃ and Ag₂O powders at 1373 K. Hall effect, Seebeck coefficient and electrical conductivity measurements were subsequently employed to assess the electrical transport properties. The electrical conductivity of the as-sintered samples was found to increase with Ag₂O addition as a result of increases in the carrier density. Over the temperature range of 323–623 K, the transport properties can be attributed to thermally activated transitions from the acceptor state to the valence band. In contrast, the variable range hopping theory is applicable over the temperature range of 623–873 K. Ag₂O addition evidently reduces the defect binding energy in the electronic structure of the CuAlO₂. The addition of this compound also obstructs the formation of both a spinel phase and CuO, such that the oxygen off-stoichiometry value and the carrier density are increased with increasing Ag₂O levels. The presence of Ag metal has the main effect on thermal conductivity below 400 K, while above 400 K increases in the phonon concentration affect the conductivity. The highest value obtained for the figure of merit was 0.0044 at 573 K, from a sample containing 0.2 at.% Ag₂O.     

Large transverse thermoelectric figure of merit in a topological Dirac semimetal

The Seebeck effect encounters a few fundamental constraints hindering its thermoelectric(TE)conversion efficiency.Most notably,there are the charge compensation of electrons and holes that diminishes this effect,and the Wiedemann-Franz(WF)law that makes independent optimization of the corresponding electrical and thermal conductivities impossible.Here,we demonstrate that in the topological Dirac semimetal Cd3As2 the Nernst effect,i.e.,the transverse counterpart of the Seebeck effect,can generate a large TE figure of merit zNT.At room temperature,zNT≈0.5 in a small field of 2 T and it significantly surmounts its longitudinal counterpart for any field.A large Nernst effect is generically expected in topological semimetals,benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities.In this case,heat and charge transport are orthogonal,i.e.,not intertwined by the WF law anymore.More importantly,further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport,but also the anomalous Nernst effect arising from a pronounced Berry curvature.A combination of the topologically trivial and nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.     

Enhancement of figure of merit of thermoelectric materials: a new theoretical approach

Thermoelectric materials have attained importance because of the gargantuan energy crisis the world faces today. A thermoelectric material can be used efficiently and frequently, provided, its figure of merit ZT is increased. Also, easy availability, manufacturing, and low cost are the other factors to be considered for a novel thermoelectric material. A theoretical model is proposed in this paper for the enhancement of the figure of merit of thermoelectric materials.     

Enhancement of the thermoelectric figure of merit in a quantum dot due to external ac field

We investigate the figure of merit of a quantum dot (QD) system irradiated with an external microwave filed by nonequilibrium Green?s function (NGF) technique. Results show that the frequency of microwave field influence the figure of merit ZT significantly. At low temperature, a sharp peak can be observed in the figure of merit ZT as the frequency of ac field increases. As the frequency varies, several zero points and resonant peaks emerge in the figure of merit ZT. By adjusting the frequency of the microwave field, we can obtain high ZT. The figure of merit ZT increases with the decreasing of linewidth function Γ. In addition, Wiedemann–Franz law does not hold, particularly in the low frequency region due to multi-photon emission and absorption. Some novel thermoelectric properties are also found in two-level QD system.     

Thermal conductivity of GaS and GaSe layered semiconductors

This paper reports on the results of experimental investigations into the thermal conductivity of GaS and GaSe layered semiconductor crystals in directions parallel and perpendicular to the crystal layers in the temperature range 5–300 K. Specific features of the thermal conductivity of these crystals are analyzed.     

Thermal conductivity of crystalline chrysotile asbestos

The thermal conductivity of crystalline chrysotile asbestos made up of hollow tubular Mg₃Si₂O₅(OH)₄ filaments is measured in the range 5–300 K. The paper discusses the possibility of using this material in studies of the thermal conductivity of thin filaments of metals and semiconductors incorporated into the channels of crystalline chrysotile asbestos tubes.     

Improved thermoelectric figure of merit of self‐doped Ag8–xGeTe6 compounds with glass‐like thermal conductivity

Nonstoichiometric Ag_8–xGeTe₆ (x = 0, 0.01, 0.02, 0.04) compounds with complex crystal structure are demonstrated to exhibit very low thermal conductivities of <0.28 W/m K, comparable with the calculated theoretical minimum thermal conductivity κ_min. Ag deficiency leads to the improved electrical properties and a maximum thermoelectric figure of merit ZT of 0.85 has been obtained at 623 K for Ag_7.99GeTe₆, about 30% increase compared to that of stoichiometric Ag₈GeTe₆. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

粗糙界面对Bi2Te3/PbTe超晶格热电优值影响的理论分析

以Bi2Te3/PbTe超晶格薄膜为例,分析电子在Bi2Te3量子阱中的输运过程,综合了薄膜的经典散射效应和理想量子效应,并以此混合效应为基础,在PbTe障碍层厚度一定时,模拟计算了两种混合效应中量子效应占不同比例时,Bi2Te3/PbTe超晶格热电优值的变化.在镜面反射占混合效应的0 3时,得到的热电优值与当前报道的量子阱超晶格的实验值接近.     

Highly enhanced thermoelectric figure of merit of a β-SiC nanowire with a nanoelectromechanical measurement approach

We developed a reliable and highly reproducible way of fabricating a one-stop measurement platform for characterizing the thermoelectric properties of individual nanowires (NWs) using a focused ion beam and a nanomanipulator. 3-ω and 1-ω signals obtained by the four-point-probe method were used in measuring the thermal and electrical conductivities of the NW. Subsequently, the Seebeck coefficient was measured by using additional nanoelectrodes including a nanoheater. The thermal conductivity of the single β-SiC NW was obtained at 86.5±3.5 W/mK. The Seebeck coefficient was obtained to be −1.21 mV/K by using the same measurement platform. Thus, the dimensionless figure of merit, ZT=σS ² T/k, was measured to be ∼0.12. This value is around 120 times higher than the reported maximum value of bulk β-SiC.     

Thermal conductivity of thermoelectric Al‐substituted ZnO thin films

ZnO:Al thin films with a low electrical resistivity were grown by magnetron sputtering on sapphire substrates. The cross‐plane thermal conductivity (κ = 4.5 ± 1.3 W/mK) at room temperature is almost one order of magnitude lower than for bulk materials. The thermoelectric figure of merit ZT at elevated temperatures was estimated from in‐plane power factor and the cross‐plane thermal conductivity at room temperature. It is expected that the thermal conductivity drops with increasing temperature and is lower in‐plane than cross‐plane. Consequently, the thin film ZT is at least three times higher than for bulk samples at intermediate temperatures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal conductivity of nanofluids and size distribution of nanoparticles by Monte Carlo simulations

Nanofluids, a class of solid–liquid suspensions, have received an increasing attention and studied intensively because of their anomalously high thermal conductivites at low nanoparticle concentration. Based on the fractal character of nanoparticles in nanofluids, the probability model for nanoparticle’s sizes and the effective thermal conductivity model are derived, in which the effect of the microconvection due to the Brownian motion of nanoparticles in the fluids is taken into account. The proposed model is expressed as a function of the thermal conductivities of the base fluid and the nanoparticles, the volume fraction, fractal dimension for particles, the size of nanoparticles, and the temperature, as well as random number. This model has the characters of both analytical and numerical solutions. The Monte Carlo simulations combined with the fractal geometry theory are performed. The predictions by the present Monte Carlo simulations are shown in good accord with the existing experimental data.     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

On the enhancement of the figure of merit in bulk nanocomposites

In this paper we make a detailed comparison of the thermoelectric properties of quantum dot superlattices with those of equivalently doped bulk material and show that a major contribution to the enhancement of the figure of merit comes from the increase of the thermoelectric power over that of bulk, in addition to the lattice thermal conductivity reduction in quantum dot superlattices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Texture of bismuth telluride-based thermoelectric semiconductors processed by high-pressure torsion

P-type Bi₂Te₃-based thermoelectric semiconductors were prepared, having a grain-refined microstructure and a preferred orientation of anisotropic crystallographic structure. Disks with a nominal composition of Bi_0.5Sb_1.5Te_3.0 were cut from an ingot grown by the vertical Bridgman method (VBM) and deformed at 473 K under a pressure of 6.0 GPa by high-pressure torsion (HPT). The crystal orientation was characterized by X-ray diffraction. The microstructures were characterized using optical microscopy and scanning electron microscopy (SEM). It was found that the HPT disks had a fine and preferentially oriented grain compared to that of the VBM disks. Further, the power factor of the HPT disks was about twice as large as that of the VBM disks. These results indicate that HPT is effective in improving the thermoelectric properties of Bi₂Te₃-based thermoelectric semiconductors.     

States of hydrogen in crystalline semiconductors

Abstract

The stable configurations of hydrogen in undoped, and in n- and p-semiconductors are reviewed. We compare the experimentally determined configurations with theoretical predictions made by a variety of calculational methods. In undoped Si, as an example, hydrogen appears to occupy a energy occurring at a tetrahedral interstitial (T) site. In B-doped, p-type Si, hydrogen at a near BC site accounts for acceptor passivation. By contrast in n-type Si, hydrogen at the antibonding (AB) position appears to be responsible for donor passivation. The possible configurations in other semiconductors (Ge, GaAs and AlGaAs) are also summarized.