Thermoelectric properties of transition-metal oxide NaCo/sub 2/O/sub 4/ system

Abstract

A transition-metal oxide NaCo/sub 2/O/sub 4/ system has been synthesized by a standard solid-state reaction method, and the thermoelectric properties have been measured. The resistivity /spl rho/ and the thermoelectric power /spl alpha/ of the as-sintered NaCo/sub 2/O/sub 4/ is 2 m/spl Omega/cm and 100 /spl mu/V/K at room temperature, respectively. By hot-pressing, the electrical resistivity is reduced to about 60%, and by Ba- or Bi-substitution for Na, Cu- or Bi-substitution for Co, the thermoelectric power is enhanced up to 1.4 times. For all the samples, the figure-of-merit shows a flat temperature dependence over a wide range from 100 to 400/spl deg/C, and in the hot-pressed Na(Co/sub 0.95/Cu/sub 0.05/)/sub 2/O/sub 4/, the maximum value of 0.88/spl times/10/sup -3/ K/sup -1/ is achieved around 300/spl deg/C, with /spl rho/=2 m/spl Omega/cm, /spl alpha/=147 /spl mu/V/K, and the thermal conductivity /spl kappa/=1.3 W/mK. In spite of being a low carrier mobility material, NaCo/sub 2/O/sub 4/ system shows good thermoelectric performance. This indicates the possibility of applying low-mobility materials to thermoelectric devices, and thus the NaCo/sub 2/O/sub 4/ system will be a prospective candidate for thermoelectric materials.