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For bulk thermoelectrics, improvement of the figure of merit ZT to above 2 from the current values of 1.0 to 1.5 would enhance their competitiveness with alternative technologies. In recent years, the most significant improvements in ZT have mainly been due to successful reduction of thermal conductivity. However, thermal conductivity is difficult to measure directly at high temperatures. Combined measurements of thermal diffusivity, specific heat, and mass density are a widely used alternative to direct measurement of thermal conductivity. In this work, thermal conductivity is shown to be the factor in the calculation of ZT with the greatest measurement uncertainty. The International Energy Agency (IEA) group, under the implementing agreement for Advanced Materials for Transportation (AMT), has conducted two international round-robins since 2009. This paper, part II of our report on the international round-robin testing of transport properties of bulk bismuth telluride, focuses on thermal diffusivity, specific heat, and thermal conductivity measurements. [ABSTRACT FROM AUTHOR]

Ca3-xDyxCo4O9+δ (x=0, 0.15, 0.3 and 0.45) polycrystalline samples have been prepared using a sol–gel process followed by SPS sintering. Their thermoelectric properties have been carefully studied from room temperature to 1000 K. The substitution of Dy for Ca results in increasing of both thermopower and electrical resistivity, which could be attributed to the decreasing of carrier concentrations. The Dy substituted samples (x=0, 0.3) have lower thermal conductivity than Ca3Co4O9+δ due to their lower electronic and lattice thermal conductivities. The dimensionless figure of merit ZT reaches 0.27 at 1000 K for the sample of Ca2.7Dy0.3Co4O9+δ. [Copyright &y& Elsevier]

The low weighted carrier mobility has long been considered to be the key challenge for improvement of thermoelectric (TE) performance in BiTeI. The Rashba-effect-induced two-dimensional density of states in this bulk semiconductor is beneficial for thermopower enhancement, which makes it a prospective compound for TE applications. In this report, we show that intercalation of minor Cu-dopants can substantially alter the equilibria of defect reactions, selectively mediate the donor-acceptor compensation, and tune the defect concentration in the carrier conductive network. Consequently, the potential fluctuations responsible for electron scattering are reduced and the carrier mobility in BiTeI can be enhanced by a factor of two to three between 10 K and 300 K. The carrier concentration can also be optimized by tuning the Te/I composition ratio, leading to higher thermopower in this Rashba system. Cu-intercalation in BiTeI gives rise to higher power factor, slightly lower lattice thermal conductivity, and consequently improved figure of merit. Compared with pristine BiTe0.98I1.02, the TE performance in Cu0.05BiTeI reveals a 150% and 20% enhancement at 300 and 520 K, respectively. These results demonstrate that defect equilibria mediated by selective doping in complex TE and energy materials could be an effective approach to carrier mobility and performance optimization. [ABSTRACT FROM AUTHOR]

Abstract: TiCoSb-based half-Heusler compounds were prepared by solid-state reaction and their thermoelectric properties were studied. The un-doped TiCoSb compound shows n-type conduction and demonstrates high Seebeck coefficient at high temperatures. The partial substitution of Co by Ni caused great increase in electron concentration and electrical conductivity, while the Seebeck coefficient and the thermal conductivity showed little change when Ni content (x) is below 5%. A maximum power factor of 16μW/K2 cm and a maximum dimensionless figure of merit (ZT) of 0.27 have been obtained for the composition of TiCo0.95Ni0.05Sb at 900K. [Copyright &y& Elsevier]

The intrinsic ultra-low lattice thermal conductivity ( κ L ) of high performance thermoelectric Cu 2 S may arise from its multiform complex crystal structures while the further sharp drop of κ L observed at elevating temperatures close to the phase transition to hexagonal high-chalcocite is of considerable interest. In this work, in-depth in-situ transmission electron microscopy investigations have been performed by either heating or electron irradiation to analyze the structural evolution of Cu 2 S. Multiple including new phase structures have been observed in Cu 2 S. Upon increasing the temperature, drastic structural changes took place including phase transition among different phases, formation of an intermediate phase, switching of crystal orientation and inverse phase transitions, which indicate multiple forms of Cu ordering with high temperature-sensitivity. Such tremendous structural evolution along with formation and movement of phase boundaries are believed to strongly scatter the phonons. [ABSTRACT FROM AUTHOR]

Abstract: Ab-initio calculations of the resonant modes and frequencies for a number of possible fillers in p-type RFe3CoSb12 and RFe4Sb12 were carried out. The results indicate that, although the exact values of fillers’ resonant frequencies in p-type skutterudites are somewhat different from those in n-type Co-based skutterudites, the Einstein-like resonant modes of the fillers are similar to those in n-type materials. Experimentally, several pairs of the fillers were selected and double-filled p-type skutterudite compounds RxMy Fe3CoSb12 (R, M = Ba, Ce, Nd, and Yb) were successfully synthesized. The reduction in the lattice thermal conductivity was realized by extending the range of resonant frequencies. As a result, enhanced ZT values above unity were achieved in these double-filled p-type skutterudites. [Copyright &y& Elsevier]

Abstract: n-type filled skutterudite Yb0.26Co4Sb12 composites in which p-type GaSb nanostructured inclusions (5–20nm) were dispersed were fabricated by an in situ method involving the introduction of metastable void-filling impurity Ga and enrichment of Sb in the synthesis procedure. With the homogeneously dispersed GaSb nanoinclusions, which probably result from the scattering of low-energy electrons by the GaSb-related boundary energy barriers, the power factor is enhanced due to the significant enhancement of the Seebeck coefficient. The total thermal conductivity was decreased with the depression of electronic thermal conductivity. As a result, the dimensionless thermoelectric figure of merit ZT value was improved over a wide working temperature range of 300–850K, and the expected optimal thermal-electric conversion efficiency ηopt 15.0% was obtained for the Yb0.26Co4Sb12/0.2GaSb nanocomposite. [Copyright &y& Elsevier]

Abstract: Ba8Ga16Ge30 clathrate crystals were grown by Czochralski method. The influence of growth condition on the composition and thermoelectric property of the obtained crystals was investigated. The lower pulling rate and higher pressure resulted in a larger Ga/Ge ratio in the grown crystals, and the elemental composition was homogeneous. The sample with a higher Ga/Ge ratio showed the larger electrical conductivity than the sample with a lower Ga/Ge ratio, but the Seebeck coefficient was not sensitive to the Ga/Ge ratio, which led to a larger power factor. The higher Ga/Ge ratio also resulted in a lower lattice thermal conductivity and as a result led to a higher ZT value, which was measured to be 0.93 at 850K and could reach 1.3 as extrapolated to 1000K. [Copyright &y& Elsevier]

Abstract: Ge-doped TiCoSb-based half-Heusler compounds, TiCoGe x Sb1−x (x =0–0.2), were prepared by arc melting and spark plasma sintering (SPS). The electrical conductivity, Seebeck coefficient and thermal conductivity were measured from room temperature to 850K. The Seebeck coefficient changed from negative to positive by Ge doping, and the electrical conductivity increased with increasing Ge content. The lattice thermal conductivity for TiCoGe x Sb1−x is considerably reduced, because the point defects induced by Ge substitution for Sb intensively scatter the thermal phonons. The mass fluctuation part Γ mass and strain field fluctuation part Γ stra were estimated, and it was found that the reduction of lattice thermal conductivity is mainly attributed to the strain field fluctuation. Ge doping improved the thermoelectric figure of merit ZT of TiCoSb and the maximum ZT value of 0.16 was obtained for TiCoGe0.15Sb0.85 sample at 850K. [Copyright &y& Elsevier]