Transport properties of silicon

Electrical conductivity, thermal conductivity, and thermoelectric power of single-crystalline silicon are investigated at temperatures between 2 and 300 K. From the measured data we calculate the mean free path of electrons and phonons and separate diffusion part and phonon-drag part of the thermoelectric power. Using a new method, we evaluate the mean free path of those phonons which are responsible for the phonon drag effect.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Thermoelectric properties of p‐type Mg2Si0.6Ge0.4 fabricated by bulk mechanical alloying and hot pressing

Bulk mechanical alloying (BMA) followed by hot pressing (HP) was used to prepare Mg₂Si_0.6Ge_0.4 thermoelectric material with high densification. Starting from the elemental power mixture, the Mg₂Si_0.6Ge_0.4 solid solution was solid‐state synthesized via BMA. In fact, the peaks for the cubic‐structured Mg₂Si_0.6Ge_0.4 solid solution phase were detected after 300 cycles in BMA. The single phase of Mg₂Si_0.6Ge_0.4 was synthesized at 600 cycles in BMA. Mg₂Si_0.6Ge_0.4 showed p‐type semiconduction without doping. Effects of hot pressing conditions on thermoelectric properties were investigated. With increasing hot pressing temperature from 673 to 773 K and pressure from 500 MPa to 1 GPa, the electrical conductivity increased and the Seebeck coefficient decreased. The maximum figure of merit was obtained with the processing parameter of 600 cycles BMA and hot pressing at 773 K, 1 GPa for 1 h. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Nanostructured thermoelectric oxides with low thermal conductivity

Complex metal oxides, such as e.g. perovskite‐type phases are developed as potential functional materials to improve the efficiency of thermoelectric converters. Among those, cobaltates with p‐type conductivity and n‐type manganates are considered for the realisation of a ceramic thermoelectric converter. Sintered pellets with the composition AMO_3–δ (A = Ln, RE; M = Co, Mn, Ni, Ti) and “Ca₃Co₄O₉ derivates” were synthesized and characterised concerning their thermoelectric properties in a broad temperature range. It was found that the Seebeck coefficient and the electrical conductivity do not depend on the dimensions of the crystallites, while the heat conductivity can be substantially lowered by decreasing the size of the crystalline domains in these systems. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reinvestigation of the thermoelectric properties of Ag8GeTe6

High‐density polycrystalline samples (above 98% of the theoretical density) of Ag₈GeTe₆ were prepared by solid‐state reactions of Ag₂Te, GeTe, and Te, followed by hot‐pressing. The thermoelectric properties were measured at temperatures ranging from room temperature to around 700 K. The thermal conductivity values were extremely low (0.25 Wm^–1 K^–1 at room temperature), and consequently Ag₈GeTe₆ exhibited a relatively high thermoelectric figure of merit, ZT = 0.48 at 703 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectric properties of CoSb3 with maize‐like structure

Maize‐like CoSb₃ powders were obtained via the chemical alloying method. After the consolidation of the nanopowder using hot press, the CoSb₃ compact shows a higher Seebeck coefficient and lower thermal conductivity. For the investigated CoSb₃, a ZT of 0.15 at 673 K is shown. Though the achieved ZT does not reach the optimal value (0.17 to 0.18) for pure CoSb₃, due to its lower electrical conductivity, the novel structure fabrication provides an interesting and promising approach to enhancing the thermoelectric performance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectric properties of the Bi2−xYxRu2O7 (x=0-2) pyrochlores

Electrical conductivity and Seebeck coefficient for the Bi_2−xY_xRu₂O₇ pyrochlores with x=0.0,0.5,1.0,1.5,2.0 were measured in the temperature range of 473-1073 K in air. With increasing Bi content, the temperature dependence of the electrical conductivity changed from semiconducting to metallic. The signs of the Seebeck coefficient were positive in the measured temperature range for all the samples, indicating that the major carriers were holes. The temperature dependence of the Seebeck coefficient for the Y₂Ru₂O₇ indicated the thermal activation-type behavior of the holes, while that for the Bi_2−xY_xRu₂O₇ with x=0.0-1.5 indicated the itinerant behavior of the holes. The change in the conduction behavior from semiconductor to metal with increasing Bi content is consistent with the increase in the overlap between the Ru4d t_2g and O2p orbitals, but the mixing of Bi6s, 6p states at E_F may not be ruled out. The thermoelectric power factors for the Bi_2−xY_xRu₂O₇ with x=1.5 and 2.0 were lower than 10⁻⁵ W m⁻¹ K⁻² and those with x=0.0,0.5,1.0 were around 1-3×10⁻⁵ W m⁻¹ K⁻².     

Highly efficient bismuth telluride doped p‐type Pb0.13Ge0.87Te for thermoelectric applications

Bi₂Te₃ doped p‐type Pb_0.13Ge_0.87Te samples were prepared by hot pressing. We report on very high power factor values of ～30 μW/cm K² at 500 °C, as were determined from Seebeck coefficient and electrical resistivity measurements. From dilatometric characterization, the phase transition from the low temperature rhombohedral to the high temperature cubic NaCl structures, takes place at 373 °C. This transition is accompanied by a continuous and gradual change of the lattice parameters, as was observed by hot stage XRD, suggesting a good mechanical durability upon thermal cycling and operating in large thermal gradients.

     

Nonlinear electrical characteristics and dielectric properties of Ca,Ta-doped TiO<Subscript>2</Subscript> varistors

Ca,Ta-doped TiO₂ varistors with high nonlinear coefficients are obtained by a ceramic sintering. The nonlinear electrical and dielectric properties of the samples doped with 0.5mol% Ca and various concentrations of Ta (0.05∼2.0mol%) were investigated. The samples sintered at 1350 °C have nonlinear coefficients of α=5.1∼42.1 and high relative dielectric constants approach 10⁵. The effects of Ta-doping on the nonlinear and dielectric properties of the Ca,Ta-doped TiO₂ varistors are studied in greater detail. When the concentration of Ta is 0.5mol%, the sample possesses the highest nonlinear coefficient and a comparatively lower dielectric constant. The effects of Ta and the nonlinear electrical behavior of the TiO₂ system are explained by analogy to a grain-boundary atomic defect model. Received: 24 October 2001 / Accepted: 8 January 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/826-49531, E-mail: wangwanyan@yahoo.com.cn     

Structure and thermoelectric properties of nanocomposite bismuth telluride prepared by melt spinning or by partially alloying with IV–VI compounds

Bismuth telluride samples are compared with respect to the evolution of their thermoelectric material parameters like thermal and electrical conductivity. The Seebeck coefficient is discussed in dependence on the melt spinning fabrication technique. The melt spinner used is only able to produce small thin ribbon shaped specimens, some as thin as 10 μm. This limits melt spinning to mainly production of research specimens for alloys with high critical cooling rate, which are difficult to fabricate with other techniques. Additional parameters are alloying or doping of the base material by comparing the properties as prepared to different annealing conditions. The intrinsic p‐ and n‐doped material was alloyed with up to 0.5% lead telluride by rapidly cooling the bulk material to improve the thermoelectric properties analysed from RT up to about 600 K. A Seebeck coefficient of well above 200 µV/K could be obtained for p‐ and n‐type materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Electronic structure of the Co4Sn6Te6 ternary skutterudite phase

The electronic structure of Co₄Sn_6–x Te_6+x (x = 0, 1) ternary skutterudite systems has been investigated using ab ‐initio band structure computation. The x = 0 system is a semiconductor like the binary Co₄Sb₁₂, but with a lower band gap. The best dopant concentration for Co₄Sn₆Te₆ is estimated to be lower than that of Co₄Sb₁₂, with the highest electronic fig‐ ure‐of‐merit Z_eT for the n‐doped system. Finally, the increased charge transfer between the 8c Co and 24g Sn and Te atoms in Co₄Sn₆Te₆ compared to that of Co₄Sb₁₂ could be one reason for the observed decrease of thermal conductivity in ternary skutterudite systems. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ab‐initio thermodynamic calculations of Mg2BIV (BIV = Si,Ge, Sn) solid solutions

The thermodynamic properties of ternary Mg₂B^IV (B^IV = Si, Ge, Sn) solid solutions were first calculated by the ab‐initio density functional method. The results showed that there exist composition regions with d²G /dx² < 0 in Mg₂Si_1–x Sn_x and Mg₂Ge_1–x Sn_x systems, implying the possibility of spinodal decomposition of the pseudobinary solid solutions. It is suggested that the spinodal decomposition would be a potential way to obtain Mg₂B^IV based bulk in‐situ nanocomposites with reduced grain sizes and enhanced phonon scattering, and hence an improved thermoelectric figure of merit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ∼v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Local structure of interstitial Zn in β‐Zn4Sb3

The low thermal conductivity of the thermoelectric material β‐Zn₄Sb₃ has been linked to disorder arising from multiple interstitial Zn sites. Here we investigate the energetics and local distortions associated with these interstitial sites via DFT calculations. Our results show the β‐Zn₄Sb₃ structure is able to distort into many inequivalent geometries of similar energies, suggesting a topology rich with transport pathways through energetically accessible metastable states. The occurrence of such a shallow energy landscape may explain the recently discovered liquid‐like diffusivity of Zn in β‐Zn₄Sb₃ – comparable to that found in superionic conductors. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spin-state transition, magnetic, electrical and thermal transport properties of the perovskite cobalt oxide Gd0.7Sr0.3CoO3

The magnetization, resistivity ρ, thermoelectric power (TEP) S, and thermal conductivity κ in perovskite cobalt oxide Gd_0.7Sr_0.3CoO₃ have been investigated systematically. Based on the temperature dependence of susceptibility χ_g(T) and Seebeck coefficient S(T), a combination of the intermediate-spin (IS) state for Co³⁺ and the low-spin (LS) state for Co⁴⁺ can be suggested. A metal-insulator transition (MIT) caused by the hopping of σ* electrons (localized or delocalized e_g electrons) from the IS Co³⁺ to the LS Co⁴⁺ is observed. Meanwhile, S(T) curve also displays an obvious phonon drag effect. In addition, based on the analysis of the temperature dependence of S(T) and ρ(T), the high-temperature small polaron conduction and the low-temperature variable-range-hopping conduction are suggested, respectively. As to thermal conduction κ(T), rather low κ values in the whole measured temperature range is attributed to unusually large local Jahn-Teller (JT) distortion of Co³⁺O₆ octahedra with IS state.     

Room temperature synthesis of Cu2O nanocubes and nanoboxes

Monodisperse Cu₂O nanocubes are synthesized by reducing freshly prepared Cu(OH)₂ with N₂H₄·H₂O in water at room temperature. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show that most of these nanocubes are uniform in size, with the average edge length of ∼500 nm. Selected area electron diffraction (SAED) investigation reveals that these nanocubes are single crystalline. Further, Cu₂O nanoboxes are obtained by etching Cu₂O nanocubes with acetic acid solution at room temperature. The nanoboxes retain the size and external morphology of the nanocubes.     

Electron transport with mobility above 10–3 cm2/Vs in amorphous film of co‐planar bipyridyl‐substituted oxadiazole

We demonstrate that a bipyridyl substituted oxadiazole (Bpy‐OXD) shows high electron mobility that reached above 10^–3 cm²/Vs. We believe that the high mobility results from both the hybrid molecular structure of the two electron‐accepting units: bipyridyl and oxadiazole, and the planar molecular structure based on its lack of sterically hindered bulky substituent. The computational analysis elucidates that the amorphous nature of Bpy‐OXD in thin‐film state probably results from the polymorphic effect in isolated state and the volume effect in solid state. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and properties of Mn1−XFeXS single-crystals at room temperature

Results of the first synthesis of Mn_1−XFe_XS single-crystals and its structural, electrical and magnetic properties at room temperature are presented. The Mn²⁺→Fe²⁺ substitution in Mn_1−XFe_XS solid solutions is accompanied by a compression of the NaCl lattice and a small deformation of the octahedral environments, and the concentration transition from dielectric to semimetal. Single-line Mossbauer spectra indicate the paramagnetic state Mn_1−XFe_XS sulfides at room temperature.