Preparation And Properties Of NaCo₂O₄ Thermoelectric Oxide

With the decrease of energy resource that can be exploited and the severity of the environment pollutions caused by coal, oil and natural gas, it has become urgent that develops new clean energy source. As potential candidates for a new thermoelectric material several electronic conductive oxides have attracted more and more research interests. These oxides have many advanced properties, such as, can be used at high temperature for long time, nontoxic, without pollutions and can be prepared easily.In this work, NaCo₂O₄ polycrystal materials were synthesized by the PC (polymerized complex) and CAC (citric acid complex) methods. The influences of the process parameters on the thermoelectric properties of NaCo₂O₄ were investigated. Based on the results, a reasonable process technics was obtained. Then different elements were induced into the NaCo₂O₄ to improve its thermoelectric performance.In addition, the electronic structures of the (Na7Co14O48)-47cluster and cluster comprised different dopants were carried out using a DV-Xα(Discrete-variational- Xα)method.The optimized process for preparing the NaCo₂O₄ by the PC and CAC methods includes heating circumfluence for 4h, dried at 250℃for 2h, calcined at 800℃for 6h and then at 900℃for 1.5h, finally sintered at 900℃for 18h. The Seebeck coefficient of the sintered NaCo₂O₄ is about 1.17×10^-3V/K, the electronic conductivity is about 73.74S/cm. To improve the thermoelectric performance, Ca and Ni were induced into the NaCo₂O₄ as dopants. The results indicate the electric conductivity increased with the content of the dopant increasing. A maximum is reached when the dopent content is 0.15. Especially, the NaCo₂O₄ shows better thermoelectric performance when the temperature is above 500℃, so the material is suitable to work at higher temperature. The Seebeck coefficient is improved more effectively by adding Ni than by adding Ca.Using the DV-Xαmethod, the energy level, density of state, bond order, energy gap and net charge were evaluated. The electronic structure of NaCo₂O₄ presents the characters of transition-metal oxides. The electrical conductivity comes from Co-3d, O-2p, Na-2s and Na-2p. After doping by Ca and Ni, electronic density has been increased and energy gap has been decreased. All these maybe contribute to the improvement of the electronic conductivity.