Thermoelectric properties of sintered and textured Nd-substituted Ca ₃Co₄O₉ ceramics

We present the results of low- and high-temperature measurements, from 5 to 800K, of the electrical resistivity and thermoelectric power of polycrystalline and c-axis-oriented textured samples of Ca_3-zNd₂Co ₄O₉ (0 ≤ z ≤ 0.9). Crystallo-graphic structure, microstructure and texture analyses of the samples have also been carried out. The substitution of the lighter rare-earth large ionic Nd for Ca results in an increase in the Seebeck coefficient. However, the resistivity of the samples also increases accordingly. In addition, the Hall effect of the polycrystalline C_3-zNd₂Co₄O₉ sample was studied in the 50-300 K interval, This measurement showed that the variation of Seebeck coefficient and resistivity are caused by a decrease in carrier concentration with Nd content. The decomposition temperatures of the samples with Nd content were observed to increase to values beyond the melting point of Ag, the technically important contact material for the fabrication of thermoelectric modules. The studies on the thermoforged samples indicate that the electrical resistivity of the textured samples decreased by 2.5-fold as compared with that of the conventional sintered samples. The increase in the decomposition temperature and the high Seebeck coefficient with low resistivity in thermoforged samples make the Nd-doped C₃Co₄O₉ oxide as one of the candidate materials for high-temperature thermoelectric modules.