単結晶を利用したNa_xCoO₂配向セラミックスの熱電特性

抄録

NaxCoO₂ single crystal has an anisotropy in thermoelectric properties, in which in-plane thermoelectric performance shows an excellent dimensionless value of ZT (Z=S₂⋅m_-1k_-1T). The high thermoelectric performance of a polycrystalline Na_xCoO₂, however, is not yet realizable because of its high electrical resistivity. Therefore, the highly oriented Na_xCoO₂ polycrystals were synthesized and their thermoelectric properties were measured. For making the highly oriented Na_xCoO₂, the single crystals were used as starting materials with three kinds of powders; for (a) mixed Na₂CO₃ and Co₃O₄, (b) ground Na_xCoO₂ polycrystal and (c) Na₂CO₃. Consequently, it was succeeded in the preparation of the highly oriented sample in each case. The electrical resistivity of the oriented Na_xCoO₂ 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/mK² at 663 K.