A round robin test aiming at measuring the high-temperature thermoelectric properties was carried out by a group of European (mainly French) laboratories (labs). Polycrystalline skutterudite Co0.97Ni0.03Sb3 was characterized by Seebeck coefficient (8 labs), electrical resistivity (9 labs), thermal diffusivity (6 labs), mass volume density (6 labs), and specific heat (6 labs) measurements. These data were statistically processed to determine the uncertainty on all these measured quantities as a function of temperature and combined to obtain an overall uncertainty on the thermal conductivity (product of thermal diffusivity by density and by specific heat) and on the thermoelectric figure of merit ZT. An increase with temperature of all these uncertainties is observed, in agreement with growing difficulties to measure these quantities when temperature increases. The uncertainties on the electrical resistivity and thermal diffusivity are most likely dominated by the uncertainty on the sample dimensions. The temperature-averaged (300-700 K) relative standard uncertainties at the confidence level of 68% amount to 6%, 8%, 11%, and 19% for the Seebeck coefficient, electrical resistivity, thermal conductivity, and figure of merit ZT, respectively. Thermal conductivity measurements appear as the least accurate. The moderate value of the temperature-averaged relative expanded (confidence level of 95%) uncertainty of 17% on the mean of ZT is essential in establishing Co0.97Ni0.03Sb3 as a high temperature standard n-type thermoelectric material.
The introduction of A-site vacancies in SrTiO 3 results in a glass-like thermal conductivity while Nb substituted samples maintain good electrical conductivity. This unexpected result brings SrTiO 3 one step closer to being a high-performing phonon-glass electron-crystal thermoelectric material.Thermoelectric waste heat recovery is widely expected to be an important component of a sustainable energy future. 1 However, affordable and relatively high-performance materials are lacking. Metal oxides are good candidates because of their abundance, low toxicity, and stability at high temperatures. 2 In addition, metal oxides are used in multilayer capacitors and the infrastructure for large scale device production exists. 3 SrTiO 3 is among the most promising n-type materials because it has the unusual combination (for an oxide) of good electrical conduction (s) and a high Seebeck coefficient (S), yielding power factors S 2 s # 3.5 mW m À1 K À2 in single crystals and epitaxial lms. 4,5 Unfortunately, the overall performance is compromised by a large thermal conductivity k ¼ 12-6 W m À1 K À1 for undoped SrTiO 3 (Fig. 1), 6 which reduces the thermoelectric gure of merit, ZT ¼ (S 2 s/k)T, where T is the absolute temperature. Outstanding thermoelectric materials generally have phononglass and electron-crystal (PGEC) properties, 7 meaning that the electronic transport is characteristic of a crystalline solid, while the k is low and resembles that of a glass. The main focus in the optimisation of SrTiO 3 has therefore been on the reduction of k, which consists of a large lattice contribution (k lat ) and a small electronic component (k el ¼ sLT; L is the Lorenz number). One approach to reduce k lat is to introduce point defects within the perovskite structure. This is for example used in Sr 1Àx La x TiO 3Àd (0 # x # 0.15) and SrTi 1Ày Nb y O 3Àd (0 # y # 0.2) and results in k lat z 3 W m À1 K À1 at 1073 K and a maximum ZT ¼ 0.35 at 1073 K. 5 However, to achieve ZT ¼ 1, a much more substantial reduction to k ¼ 1-2 W m À1 K À1 is needed. One drawback of La 3+ and Nb 4+ substitution is that the conicting requirements of charge carrier doping limit x, y # 0.2. However, even for Sr 1Àx Eu x Ti 0.8 Nb 0.2 O 3Àd with Sr 2+ /Eu 2+ mixtures (0 # x # 1), k was not reduced below $3 W m À1 K À1 for x ¼ 0.5 at 1000 K. 8 Recently, perovskites with A-site vacancies have started to generate attention. Promising S 2 s values and low k were reported in A-site and oxygen decient Sr 1Àx Pr 0.67x TiO 3Àd and Sr 1Àx Ti 0.8 Nb 0.2 O 3Àd , 9 while reductions in k were also observed in Ca 1Àx Nd 0.67x MnO 3Àd perovskites. 10 Here, we report a systematic investigation of the A-site decient Sr 1Àx La 0.67x , 0.33x Ti 1Ày Nb y O 3Àd perovskites. In order to separate the impact of A-site vacancies from oxygen defects we rst prepared the electrically insulating oxygen stoichiometric Sr 1Àx La 0.67x , 0.33x TiO 3 series and measured its thermal conductivity. ‡ Compositions with x ¼ 0, 0.4 and 0.8 were synthesised using standard solid state reactions...
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