Static and Dynamic Characteristics of Thermoelectric Ceramics
Structural characteristics of the high performance cobaltite thermoelectric semiconductors have been studied intensively by means of X-ray and neutron diffraction measurements and high resolution electron microscopy (HREM). These cobaltites consists of CoO2 triangular conducting sheets and several different types of block layers, i.e., Na, Ca, Sr single layers, three or four layered rock-salt layers, where Co-O2 square lattices are situated at their middle, and Bi-O or Tl-O double layers plus alkaline oxygen layers. Cold neutron scattering technique is employed to search possible low-energy excitation modes, being unique for nearly 1D and 2D crystals, and phonon density of states, DOS, of several high performance cobaltites at temperatures in the range from 10K to the ambient. Low energy, i.e., less than 2meV, excitation modes were found in the three different thermoelectric ceramics, i.e., γ -Na0.7CoO2, [Ca2CoO3]pCoO2, and [Ca2(Cu,Co)2O4]pCoO2. Possible origin of these low energy excitations are discussed in terms of low-energy corrugation mode generated due to weak chemical bondings, for which Van-der-Waals force dominates, between the CoO2 conduction sheets. These characteristics could be the key to realize low thermal conductivity and high-ZT of these ceramics.
Wei Pan and Jianghong Gong
T. Kajitani et al., "Static and Dynamic Characteristics of Thermoelectric Ceramics", Key Engineering Materials, Vols. 336-338, pp. 826-830, 2007