ICT2014 Abstracts

Thermoelectric (TE) oxides offer many advantages, among them are structural and chemical stability at high temperatures, non-toxicity, and low-cost. Whereas the figure of merit of both cobaltates Na_xCoO₂ and Ca₃Co₄O₉ p-type oxides approaches unity, which is the criterion of applicability, the ZT-values of their n-type counterparts are significantly lower (~0.3 on average) and should, therefore, be improved. Herein, we focus on CaO(CaMnO₃)_m n–type Perovskites (m=1,2,3,4,5,¥), which are modulated structures, formed of m perovskite CaMnO₃ layers separated by a rock-salt CaO layer. We aim at drawing the correlation between thermal conductivity and the planar defects periodicity, denoted by m, in CaO(CaMnO₃)_m- based compounds.

We prepare several pure and Nb- doped CaO(CaMnO₃)_m compounds with different periodicities (m-values) by mixing of oxide powders and sintering at temperatures up to 1673 K. We perform basic materials characterization procedures employing x-ray diffraction and electron microscopy in concert with TE measurements. The later include laser flash analysis (LFA) for thermal conductivity measurements as well as electrical conductivity and thermopower measurements at the range of 150 K through 750 K. We observe a remarkable reduction of thermal conductivity as a result of increasing the lattice periodicity for Nb-doped compounds. These values decrease from ca. 3.5 W/mK for m=¥ to ca. 1.3 W/mK for m=1 at 298 K. This trend is, however, accompanied by a respective reduction of electrical conductivity values from ca. 200 to 0.10 S/cm, respectively). Correspondingly, the power factor is enhanced with increasing the m-vales. We, therefore, focus our ongoing efforts on optimizing the m-values to obtain maximum ZT and extending our TE measurements to temperatures as high as 1100 K, i.e. in the applicability range of CaO(CaMnO₃)_m compounds.