Abstract
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.
Publisher
Trans Tech Publications, Ltd.
Subject
Mechanical Engineering,Mechanics of Materials,General Materials Science