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Ca 3 Co 4 O 9 ceramics powders were synthesized by the citric acid complex (CAC) method and consolidated by the hydrothermal hotpressing (HHP) technique. The observation by the scanning electron microscope indicated that the powders obtained by the CAC process showed plate-like grains. The density and the Lotgering factor, which was estimated from the X-ray diffraction data, of the sintered body increased with an increase of the operating pressure during the HHP process. The electrical resistivity was much reduced with an increase of the operating pressure, but the Seebeck coefficient was hardly affected by the HHP conditions. As a result, the sample treated with the HHP under the condition of 573 K, 200 MPa and 3 h showed a maximum power factor.

The effects of rare earth oxide dispersion on the thermoelectric properties of the Mn-doped p-type -FeSi 2 (Fe 0:9 Mn 0:1 Si 2 ) were investigated. The Fe 0:9 Mn 0:1 Si 2 powders were mechanically alloyed with Y 2 O 3 powder (0.5-4 mass%) and subsequently hot pressed. The added Y 2 O 3 was dispersed in the phase matrix as a fine particles around 10 nm in size. The thermal conductivity of the sample was significantly reduced by Y 2 O 3 addition, which was caused by enhancing phonon scattering due to fine Y 2 O 3 dispersion. It was also found that the added Y 2 O 3 was partially decomposed in the sample, resulted in solution of elemental Y in the phase as a p-type dopant. The Seebeck coefficient was slightly improved when a small amount of Y 2 O 3 (0.5 and 1 mass%) was added. The improvement of the Seebeck coefficient could not be explained only by the fine dispersion of Y 2 O 3 and the Y doping as a p-type dopant. The slight increase in the Seebeck coefficient was also observed in the case of the samples with addition of small amounts of La 2 O 3 powder.

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