Ca3Co4O9 is a promising material for thermoelectric generation, as it is stable up to 1173
K in the air, and shows good thermoelectric properties. Recently, it was found that Ca3Co2O6 was
stable up to 1300 K in the air. The Ca3Co2O6 is decomposed phase of Ca3Co4O9 and the temperature
limit is higher than one of Ca3Co4O9. The electrical resistivity of Ca3Co2O6 was, however, higher
than the one of Ca3Co4O9. Not only high power generation performances but also excellent strength
is required for practical use of the thermoelectric oxide materials. Polycrystalline samples of
Ca3Co2O6 were prepared by solid-state reaction (SSR) and hot pressing (HP). Relative density of
Ca3Co2O6 (HP) was over 98%, which is larger than the one of Ca3Co2O6 (SSR). Ca3Co2O6 (HP)
showed larger strength and lower resistivity than Ca3Co2O6 (SSR). The resistivity (ρ) of Ca3Co2O6
(HP) in perpendicular to the pressurized direction decreased from 64 Ωcm to 4.0×10-2 Ωcm at the
temperature range between 373 and 1173 K. In addition, the resistivity of this sample was decreased
by heat treatment in the air. The Seebeck coefficients (S) of Ca3Co2O6 (HP) was positive value and
more than 160 μVK-1 at the temperature range between 373 and 1173 K. Ca3Co1.8M0.2O6 (M= Mn
or V) were prepared by solid state reaction and hot pressing. The resistivity of Mn-substituted
Ca3Co2O6 (HP-Mn) and V-substituted Ca3Co2O6 (HP-V) were lower than the one of
non-substituted Ca3Co2O6 (HP) at the temperature below 523 K for the Mn-substituted sample or
723 K for V-substituted sample. The latter showed the lowest value 1.53 Ωcm of all specimens at
383 K. The power factor (S2ρ-1) of Ca3Co2O6 (HP) was 88.3 μWm-1K-2, which is the largest of all
specimens at 1176 K, but S2ρ-1 of V-substituted Ca3Co2O6 (HP-V) is the largest of all specimens up
to 773 K.
