From the viewpoint of the most efficient utilization of fossil fuel resources as heat, the combustion temperature should be high as possible to apply to the energy cascading system. Therefore, thermoelectric energy conversion as a topping system is recognized to be very important. High temperature metal silicides such as chromium silicide (M.P.1763K) and tungsten silicide (M.P. 2433K) and so on have been known as a group of metal silicides. The spark plasma sintering method was selected to prepare thermoelectric elements of chromium silicide and tungsten silicide as the examples of the high temperature, transition metal silicides. It is clarified that the spark plasma sintering method has been very useful to make such a high temperature alloy. The temperature dependence of thermoelectric properties such as the Seebeck coefficient, electrical resistivity, power factor, thermal conductivity, carrier concentration and Hall mobility was measured to find out the preferable preparation condition such as doping level and solid solution for these materials systems. The maximum power factor could be obtained O.7x10-O2 W/mK' at 900K for chromium silicide element, and 3 . 0~1 0 -~~

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Ca3Co4O9 samples were made by the sintering method using the slurry consisted of Ca3Co4O9 powder and Ca3Co4(OH)18 hydroxide in binder solution. The preparation conditions for making the crack free samples were examined, and we found that using the slurry having a mixing ratio of Ca3Co4O9 powder to 13 or 18 wt.% Ca3Co4(OH)18 hydroxide in binder solution of 10:1.5, samples with a few crack could be prepared. From SEM observation, it was found that the void density in Ca3Co4O9 samples could be controlled by the slurry sintering method. The electric conductivity (σ), Seebeck coefficient (S), and power factor (PF) of samples prepared by slurry sintering method at room temperature are 53 S/cm, 118 μV/K and 73.8 μW/m · K2, respectively. The power factor (PF) at 600°C was 173 μW/m · K2.

Ca 3 Co 4 O 9 samples were made by the sintering method using the slurry consisted of Ca 3 Co 4 O 9 powder and Ca 3 Co 4 (OH) 18 hydroxide in binder solution. The preparation conditions for making the crack free samples were examined, and we found that using the slurry having a mixing ratio of Ca 3 Co 4 O 9 powder to 13 or 18 wt.% Ca 3 Co 4 (OH) 18 hydroxide in binder solution of 10:1.5, samples with a few crack could be prepared.From SEM observation, it was found that the void density in Ca 3 Co 4 O 9 samples could be controlled by the slurry sintering method. The electric conductivity ( ), Seebeck coefficient (S), and power factor (PF) of samples prepared by slurry sintering method at room temperature are 53 S/cm, 118 V/K and 73.8 W/m ⋅ K 2 , respectively. The power factor (PF) at 600 • C was 173 W/m ⋅ K 2 . K E Y W O R D SCa 3 Co 4 O 9 , oxide thermoelectric material, slurry sintering method, thermoelectric properties Electron Comm Jpn. 2019;102:3-9.