Improved thermoelectric properties of hole-doped Bi2-x Nax Ba2Co2Oy ceramics

In this work, Bi-2_xNaxBa2Co2Oy (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) ceramic samples were prepared by the conventional solid phase method. The influence of Na element doping on the thermoelectric properties of samples was investigated from 300 K to 950 K. Compared with undoped sample, the Seebeck coefficients of Na-doped samples have only a slight decrease. While, the electrical resistivity of the Na-doped samples reduces, for Bi-2_xNaxBa2Co2Oy ceramic samples with x <= 0.15, the electrical resistivity decreases with increasing Na doping amounts due to the hole-doping effect. In addition, its temperature-dependent resistivity exhibits metallic electrical conductivity behavior. While, the samples with x > 0.20 exhibit semiconductor electrical conductivity behavior at the temperature range of 473-823 K, because of the significant reduction of the electrical resistivity and the slight decrease of Seebeck coefficients, the power factors of the doped samples get a significant improvement. For the most suitable doped sample Bi-1.(a)Nao.2Ba2Co2Oy, its ZT value can reach a better value of 0.2 at 873 K.