Junji Imai, Nobuo Itoh,Tsutayuki Shibata,
Noriyoshi Suzuki, Katsushi Sanda, Kenji Ito

In Japan, Terrestrial Integrated Service Digital Broadcasting (ISDB-T) began in December 2003. Generally speaking, reception quality with mobile terminals is poor in the case of high bit rate transmissions such as HDTV. As part of this study, we developed a diversity reception system and investigated, through field experiments, the effects of diversity techniques on a mobile terminal designed for ISDB-T HDTV services. The results of our experiments show that the reception quality with the newly developed system is significantly improved for mobile applications. It was found that, by using diversity techniques, we could receive HDTV on a mobile terminal across a larger area.

Using muon spin spectroscopy we have found that, for both Na_xCoO₂ (0.6 < χ < 0.9) and 3- and 4-layer cobaltites, a common low temperature magnetic state (which in some cases is manifest as an incommensurate spin density wave) forms in the CoO₂ planes. Here we summarize those results and report an almost dome-shaped relation between the transition temperature into the low-T magnetic state and the composition χ for Na_xCoO₂ and/or the high-temperature asymptotic limit of thermopower in the more complex 3- and 4-layer cobaltites. This behavior is explained using the Hubbard model on two-dimensional triangular lattice in the CoO₂ plane.

Hiroshi Itahara, Toshihiko Tani

The recovery of energy from waste heat by thermoelectric (TE) power generation has been desired as an effective technology to contribute to the reduction of carbon dioxide emissions. Layered oxides have good potential as materials for TE devices, because they are composed of environmentally benign elements and are chemically stable in a high temperature air atmosphere. Layered cobaltites are p-type materials with strong anisotropy in their TE properties and thus fabrication of textured ceramics is required for practical application of the materials. We have proposed for the reactive-templated grain growth (RTGG) method, the use of platelike β-Co(OH)₂ particles as reactive templates that provide an edge-sharing CoO₆ octahedra layer similar to the CoO₂ layer in the crystal structure of layered cobaltites. We prepared ceramic specimens by the in situ reaction between the aligned platelike β-Co(OH)₂ templates and the complementary reactants (oxides or carbonates) for target cobaltite compositions. As a result, the textured ceramics, with a high degree of orientation and improved TE properties, have been prepared as designed for various layered cobaltites. In particular, the value of electrical conductivity (an important factor of TE performance) of the textured [Ca₂CoO₃]_0.62[CoO₂] ceramic reached about 60 % of that of a single crystal. It was shown that the RTGG method is a versatile and effective fabrication technique for the synthesis of textured ceramics of the layered cobaltites.