NaxCoO
2 single crystal has an anisotropy in thermoelectric properties, in which in-plane thermoelectric performance shows an excellent dimensionless value of ZT (Z=S
2⋅m
-1k
-1T). The high thermoelectric performance of a polycrystalline Na
xCoO
2, however, is not yet realizable because of its high electrical resistivity. Therefore, the highly oriented Na
xCoO
2 polycrystals were synthesized and their thermoelectric properties were measured. For making the highly oriented Na
xCoO
2, the single crystals were used as starting materials with three kinds of powders; for (a) mixed Na
2CO
3 and Co
3O
4, (b) ground Na
xCoO
2 polycrystal and (c) Na
2CO
3. Consequently, it was succeeded in the preparation of the highly oriented sample in each case. The electrical resistivity of the oriented Na
xCoO
2 ceramics prepared by the (a) powder was about 6.6-8.9 mΩcm. The electrical resistivity of the ceramics, which was prepared by the (b) powder, decreased to 2.5-3.9 mΩcm. The electrical resistivity of the ceramics synthesized by the (c) powder was about 2 mΩcm. The difference of their electrical resistivities was due to the effects of the orientation, the grain boundary resistivity and the content of Na. The maximum power factor was about 500μW/mK
2 at 663 K.
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