Thermoelectric properties of transition-metal oxide NaCo2O4 system

A transition-metal oxide NaCo₂O₄ system has been synthesized by a standard solid-state reaction method, and the thermoelectric properties have been measured. The resistivity ρ and the thermoelectric power α of the as-sintered NaCo₂O ₄ is 2 mΩcm and 100 μV/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°C, and in the hot-pressed Na(Co _0.95Cu_0.05)₂O₄, the maximum value of 0.88×10^-3 K^-1 is achieved around 300°C, with ρ=2 mΩcm, α=147 μV/K, and the thermal conductivity κ=1.3 W/mK. In spite of being a low carrier mobility material, NaCo₂O₄ system shows good thermoelectric performance. This indicates the possibility of applying low-mobility materials to thermoelectric devices, and thus the NaCo₂O₄ system will be a prospective candidate for thermoelectric materials