Ctirad Uher

Thermoelectric properties of the n-type filled skutterudite Ba0.3Co4Sb12Ba0.3Co4Sb12 doped with Ni

Synthesis and electrical and thermal transport properties are reported for several filled skutterudite compounds doped with Ni: Ba0.3NixCo4−xSb12 with 0<x<0.2. Divalent Ba readily fills the cages of the skutterudite structure and is effective in reducing the thermal conductivity of the structure. The presence of a small amount of Ni increases the electron concentration, further reduces the thermal conductivity, and enhances the thermoelectric power factor. Hall mobility studies indicate that the addition of Ni to the system has the effect of increasing the relative strength of ionized impurity scattering as compared to acoustic phonon scattering. These results suggest that doping with Ni is an attractive avenue to optimization of filled skutterudites. The dimensionless thermoelectric figure of merit ZT was observed to increase from a value of 0.8 at 800 K for Ba0.3Co4Sb12 to a value of 1.2 for the sample with x=0.05. These materials show considerable potential as n-type legs in thermoelectric power genera...

Observed properties and electronic structure of RNiSb compounds (R = Ho, Er, Tm, Yb and Y). Potential thermoelectric materials

The RNiSb compounds (R=Ho, Er, Tm, Yb and Y) and some selected solid solution members such as (Zr 1-x Er x )Ni(Sn 1-x Sb x ) and ErNiSb 1-x Pn x (Pn=As, Sb, Bi) have been studied. They all crystallize in the MgAgAs structure type, which can be considered as a NaCI structure type in which half of the interstitial tetrahedral sites are occupied by Ni atoms. The measured values of the Seebeck coefficients, at room temperature, are positive for RNiSb (R=Ho, Er, Yb and Y) compounds and ErNiSb 1-x Pn x (Pn=As, Sb, Bi) solid solutions, but for (Zr 1-x Er x )Ni(Sn 1-x Sb x ) members vary from negative to positive values when 0

Complex bismuth chalcogenides as thermoelectrics

A solid state chemistry synthetic approach towards identifying new materials with potentially superior thermoelectric properties is presented. Materials with complex compositions and structures also have complex electronic structures which may give rise to high thermoelectric powers and at the same time possess low thermal conductivities. The structures and thermoelectric properties of several new promising compounds of K-Bi-S, K-Bi-Se, and Cs-Bi-Te are discussed.

Lattice thermal conductivity of K2(Bi1−zSbz)8Se13K2(Bi1−zSbz)8Se13 solid solutions

The family of solid solutions of the type β-K2(Bi1−zSbz)8Se13 (0<z⩽1) was studied with respect to thermal conductivity as a function of temperature and stoichiometry. At low temperature, the variation of lattice thermal conductivity with composition shows a transition from a typical crystalline to glasslike behavior. Analysis of the high-temperature data shows a contribution due to the mixed occupation of Bi/Sb crystallographic sites as well as an additional contribution due to point defects.

Thermal conductivity of bismuth at ultralow temperatures

Abstract Our measurements indicate that the phonon mean free path is shortened below 1 K due to enhanced scattering by electrons. For one sample, the predominant thermal conduction by carriers near 0.1 K was suppressed with a small transverse magnetic field.

Thermoelectric properties of the cubic family of compounds AgPbBiQ3 (Q = S, Se, Te). Very low thermal conductivity materials

The AgPbBiQ 3 class of compounds and their solid solution members are related to the NaCl structure type, where Ag, Pb and Bi atoms are statistically disordered on the Na site and Q atoms occupy the Cl site. These compounds were synthesized by combining the elements in the appropriate ratio and heating under static vacuum at 900° C for 3 days. They are narrow gap semiconductors with band gaps in the range of 0.6 to 0.28 eV. The charge-transport properties were measured on ingots as a function of temperature. The compounds AgPbBiTe 3 , AgPbBiSe 3 , AgPbBiTe 2.75 Se 0.25 and AgPbBiTe 2 Se, undoped, possess an electrical conductivity in the range of 70 S/cm to 400 S/cm. These materials exhibit negative thermopower ranging from -40 μV/K to -160 μV/K at room temperature and thermal conductivity less than 1.30 W/mK.

Electronic transport in tungsten and iron-doped tungsten below 1 K

The electrical resistivity ϱ and the thermoelectric ratio G have been measured for zone-refined single crystals of both tungsten and iron-doped tungsten from 5 K down to 40 mK. The samples had residual resistance ratios RRR ranging from 1750 to 90,000. The observed behavior is conveniently divided into two classes, “normal” and “anomalous.” Completely normal behavior was displayed by only three W samples with high RRRs. The Fe-doped W and the remaining W samples contained one or more anomalies. Normal behavior is that which would be expected for W containing impurities with no internal degrees of freedom. In normal behavior ϱ decreased monotonically with decreasing temperature and was consistent with the equation θ = θo + AT2 below about 1.5 K. This form for ϱ is taken as evidence that electron-electron scattering dominates electron-phonon scattering in ϱ for W at such low temperatures. For the range of sample purities studied, A increased slightly with increasing ϱ0, but did not vary systematically with either the sample diameter d or with the ratio ϱ(273 K)/dϱ(T). In normal behavior, G was positive and constant below about 0.5 K, increased in magnitude as T rose to 4 or 5 K, and then began to decrease, becoming negative above about 7 K. An explanation is provided for this behavior. Those samples that fell into the anomalous class displayed at least one of three anomalies: (1) a minimum in the electrical resistivity, with an approximately logarithmic variation with T at temperatures below the minimum; (2) a positive contribution to G which increased in magnitude with decreasing temperature approximately as T−1/2from about 4 K down to at least 0.5 K; and (3) a negative contribution to G which set in at about 0.5 K, varied approximately as log T, and dominated G at the lowest temperatures. These anomalies are presumably due to one or more impurities dissolved in the W, possibily including Fe. However, chemical and spectroscopic analyses of pieces from several samples, including the Fe-doped W, failed to establish a clear link between any specific impurities and the observed anomalies. At the moment we neither know the source nor understand the nature of these anomalies.

Thermoelectric properties of Co[sub 0.9]Fe[sub 0.1]Sb[sub 3]-based skutterudite nanocomposites with FeSb[sub 2] nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher figure of merit due to effects arising from the nanostructure. In this paper, we report thermoelectric properties from 80 K to 800 K of Co0.9Fe0.1Sb3 based skutterudite nanocomposites containing FeSb2 nanoinclusions. The nanoscale FeSb2 precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from the nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to the regular iron substituted skutterudite samples. As a result, our best nanocomposite sample reached a dimensionless figure of merit of 0.59 at 788 K, a factor of two higher than that of the control sample Co0.9Fe0.1Sb3.

Thermoelectric properties of K2Bi8-xSbxSe13 solid solutions and Se doping

Our efforts to improve the thermoelectric properties of β-K2Bi8Se13, led to systematic studies of solid solutions of the type β-K2Bi8-xSbxSe13. The charge transport properties and thermal conductivities were studied for selected members of the series. Lattice thermal conductivity decreases due to the mass fluctuation generated in the lattice by the mixed occupation of Sb and Bi atoms. Se excess as a dopant was found to increase the figure-of merit of the solid solutions. INTRODUCTION Our investigations of ternary and quaternary compounds of bismuth chalcogenides [1] have shown that several multinary compositions containing alkali metals show promising thermoelectric properties. One of these phases, β-K2Bi8Se13 [2], possesses low thermal conductivity (~1.4 W/m⋅K) and high power factor. The room temperature ZT value is 0.22 [2] and can be substantially improved on doped β-K2Bi8Se13 mainly by raising the power factor [3]. A solid solution series of β-K2Bi8Se13 was prepared in order to study the effects on the general properties such as electrical transport and the lattice thermal conductivity. β−K2Bi8Se13 was alloyed with its isostructural K2Sb8Se13 analog in order to generate extensive mass fluctuations in the lattice with the Sb/Bi substitution. The Bi/Sb distribution in the structure was also studied by single crystal X-ray diffraction analysis of selected crystals of K2Bi8-xSbxSe13 [4]. The Sb is not uniformly distributed as would be expected from a true solid solution but it affects disproportionally the various metal sites in the structure. We find that the band gaps and the melting points vary systematically as a function of x [5]. Various K2Bi8-xSbxSe13 solid solutions were prepared with the Bridgman technique [6] in order to grow large oriented ingots samples. In this work, charge transport properties and thermal conductivity measurements of selected members of K2Bi8-xSbxSe13 solid solutions are presented. Doping studies using excess of Se were also performed in order to study its influence on the thermoelectric properties. RESULTS AND DISCUSSION β-K2Bi8Se13 and its isostructural Sb solid solutions are anisotropic three-dimensional monoclinic structures that propagate along the b-axis [2,4]. In the structure, Bi2Te3-type rods form layers perpendicular to the c-axis. The NaCl-type rods connect the layers to build a 3-D framework, which creates the needle-like crystal morphology along the b-axis, with tunnels filled with K cations. The anisotropic structure of these materials also causes strong anisotropy in their thermoelectric properties [6]. Mat. Res. Soc. Symp. Proc. Vol. 691

Thermoelectric Measurements at Millikelvin Temperatures

In the late 1950s and early 1960s physicists at the National Research Council (NRC) in Canada made a pioneering series of thermoelectric measurements on metals to temperatures below 0.1 K.1–7 A fitting tribute to their ultralow temperature work is the fact that until recently not a single measurement they made below 0.3 K had been superceded.