The thermoelectric properties and stability of Al-substituted ZnO as a potential high-temperature n-type material were studied in heating-cooling cycles. Zn 1Àx Al x O(x = 0.02, 0.06) was prepared by soft chemistry and solidstate reaction synthesis methods. Cycling during the thermoelectric measurement leads to an increase of the electrical resistivity and Seebeck coefficient values. The reason for this aging phenomenon can be assigned to a change in composition due to oxygen uptake along with modification in the defect concentrations. The aging is enhanced if the cycling is performed in oxygen. ZT value of 0.21 is reached at 1275 K for samples with 2% Al substitution made by soft chemistry synthesis.
Abstract:In this work, we synthesized cubic perovskite ceramics of the whole La 1-x Sr x CoO 3 (0 ≤ x ≤ 1) solid solution for the first time. Synthesis was carried out by solid state reaction and conventional sintering to reach dense ceramics. For x > 0.8, it was necessary to substitute 3% cobalt by silicon to stabilize the cubic perovskite structure. Electrical conductivity increased with Sr content to reach 3×10 5 S·m -1 at 330 K for x = 0.3. However, the optimum electrical properties have been found for x = 0.05 at 330 K with PF max = 3.11×10 -4 W·m -1 ·K -2 . Indeed, the Seebeck coefficient was decreasing when x increased to reach values close to 0 for x ≥ 0.3. Thermal conductivity was low at low temperature (≈ 2.5 Wm -1 ·K -1 ) and increased up to 6.5 W·m -1 ·K -1 when temperature increased. As the highest power factor was reached at low temperature as well as the lowest thermal conductivity, La 1-x Sr x CoO 3 compounds with low x values appeared as very promising thermoelectric materials around room temperature, on the contrary to layered cobalt oxides. For high x values, Seebeck coefficient values close to zero made these materials unsuitable for thermoelectric applications.
The spark plasma sintering (SPS) behaviour of pure SnO2 has been studied. Two different SnO2 powders have been studied: a commercial 50-200 nm one and 4-6 nm nanoparticles obtained by precipitation. It has demonstrated that it is not possible to keep pure SnO2 above 1223 K by SPS. Indeed, at 1248 K, SnO appears whereas at higher temperatures, samples are composed by SnO2 and metal Sn. Three different cycles have been
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