Improvement in high-temperature thermoelectric properties of NaCo2O4 through partial substitution of Ni for Co

We have prepared polycrystalline Na(Co-l-Ni-z(z))(2)O-4 (0 <= z <= 0.25) samples using a solid-state reaction process and investigated the Ni substitution effect on the high-temperature thermoelectric properties. With the Ni substitution, the electrical conductivity (sigma) decreased mainly because of a decrease in both density and grain size as well as ail increase in NaNiO2 content. On the other hand, the Ni substitution led to a significant increase in the Seebeck coefficient (alpha). The calculated power factor (sigma alpha(2)) was substantially improved by the partial substitution of Ni for Co in NaCo2O4. The highest power factor (2.36 x 10(-3) Wm(-1) K-2) was obtained for Na(Co0.95Ni0.05)(2)O-4 sample at 1073 K. It is demonstrated that the Ni substitution is highly effective for enhancing high-temperature thermoelectric properties. (c) 2005 Published by Elsevier B.V.