Papers by Author: Tsuyoshi Kajitani

Abstract: Oxide thermoelectrics are relatively new materials that are workable at temperatures in the range of 400K≤T≤1200K. There are several types of thermoelectric oxide, namely, cobalt oxides (p-type semi-conductors), manganese oxides (n-type) and zinc oxides (n-type semi-conductors) for high temperature energy harvesting. The Seebeck coefficient of 3d metal oxide thermoelectrics is relatively high due to either high density of states at Fermi surfaces or spin entropy flow associated with the carrier flow. The spin entropy part dominates the Seebeck coefficient of 3d-metal oxides at temperatures above 300K. Introduction of impurity particles or quantum-well structures to enhance thermionic emission and energy filtering effects for the oxide semiconductors may lead to a significant improvement of thermoelectric performance.

Abstract: P- and N-type thermoelectric iron oxides were developed. The p- and n-type thermoelectric iron oxides were based on delafossite-type CuFeO2 and spinel-type Fe3O4, respectively. The dimensionless figure of merit, ZT, of the bulk p- and n-type iron oxides were 0.15 and 0.10 at 1200K, respectively. The ZT values were improved by the introduction of nano-voids. The physical properties of these iron oxides are structurally unique because of the triangular, or “Kagome,” arrangement of FeO6 octahedra. The delafossite-type CuFeO2 becomes anti-ferromagnet at temperatures less than 20K. The inverse spinel-type Fe3O4 is a ferrimagnet　at room temperature. In both crystals, the iron ions are assumed to be in the high-spin state.

Abstract: A partially cobalt-substituted solid solution of Nowotny chimney-ladder phase, (Mn1-xCox)Si􀀂, has been prepared using a tetra-arc-type furnace and a subsequent annealing process. The compounds consist of two tetragonal subsystems of [Mn1-xCox] and [Si], with an irrational c-axis ratio 􀀂 = cMn/cSi ~ 1.7. The crystal structure and thermoelectric properties of (Mn1-xCox)Si􀀂 solid solution were compared with those of the Fe-substituted solid solution, (Mn1-xFex)Si􀀂. In the case of Co-series, extra valence electrons are introduced relative to Fe-series, since the valence electron count is 3d74s2 for Co but 3d64s2 for Fe, respectively. It was naturally expected that the Feand Co-substituted MnSi􀀁 becomes n-type conductor from the p-type one at x > 0.23(5) and x > 0.06(1), respectively. Experimentally, the Fe-substituted samples become n-type at x > 0.28 but it is not the case for the Co-substituted ones. It is thus evident that there is an unknown factor which controls the thermoelectric properties of Co-substituted samples.

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.

Abstract: We have investigated the modulated structure of the misfit-layered crystal Bi1.8Sr2.0Rh1.6Ox by means of electron diffraction and high-resolution electron microscopy. This compound consists of two interpenetrating subsystems of a hexagonal RhO2 sheet and a distorted four-layered rock-salt-type (Bi,Sr)O block. Both subsystems have common a-, c-axes and β-angles with a = 5.28 Å, c = 29.77 Å and β = 93.7º. On the other hand, the crystal structure is incommensurated parallel to the b-axes, among which b1 = 3.07 Å for the RhO2 sheet and b2 = 4.88 Å for the (Bi,Sr)O block. The misfit ratio, b1/b2 ~ 0.63, characterizes the structural analogue as [Bi1.79Sr1.98Oy]0.63[RhO2]. This compound has two modulation vectors, i.e., q1 = – a* + 0.63b1* and q2 = 0.17b1* + c*, and the superspace group is assigned as the Cc(1β0, 0μ1)-type from the electron diffraction patterns. High-resolution images taken with the incident electron beam parallel to the a- and c-axes clearly show displacive as well as compositional modulations.