Chemical stability of Ca3Co4-x O9+δ /CaMnO3-δ p-n junction for oxide-based thermoelectric generators.

An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. Ca3Co4-x O9+δ and CaMnO3-δ exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 °C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca3Co1+y Mn1-y O6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca3Co1+y Mn1-y O6 varies and is close to 1 (y = 0) at the Ca3Co1+y Mn1-y O6-CaMnO3-δ boundary. The existence of a wide homogeneity range of 0 ≤ y ≤ 1 for Ca3Co1+y Mn1-y O6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.

The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X = Hf, Zr, Ti) half-Heusler alloys.

Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κ Lat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κ Lat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κ Lat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κ Lat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.

Versatile apparatus for thermoelectric characterization of oxides at high temperatures.

An apparatus for measuring the Seebeck coefficient and electrical conductivity is presented and characterized. The device can be used in a wide temperature range from room temperature to 1050 °C and in all common atmospheres, including oxidizing, reducing, humid, and inert. The apparatus is suitable for samples with different geometries (disk-, bar-shaped), allowing a complete thermoelectric characterization (including thermal conductivity) on a single sample. The Seebeck coefficient α can be measured in both sample directions (in-plane and cross-plane) simultaneously. Electrical conductivity is measured via the van der Pauw method. Perovskite-type CaMnO3 and the misfit cobalt oxide (Ca2CoO3)q(CoO2) are studied to demonstrate the temperature range and to investigate the variation of the electrical properties as a function of the measurement atmosphere.