Study On The Preparation And Fluoride Doping Of NaCo₂O₄ Thermoelectric Materials

Thermoelectric (TE) materials can realize coupling and conversion between heat and electric energy. The devices made of TE materials are applicable for refrigeration and power generation with advantages of low weight, no noise, no pollution and no moving parts. In this paper, Oxide NaCo₂O₄ has been selected as this study due to very good thermoelectric performance. A new series of oxides were synthesized by the substitution and their thermoelectric properties were investigated.Preparation method of single-phase fluoride doped and NaCo₂O₄ compounds were explored in this paper. The influences of fluoride doping on the structure and thermoelectric properties of NaCo₂O₄ compounds were investigated systematically. Meanwhile, we also investigated the influences of the preparation craft on the thermoelectric properties of NaCo₂O₄ compounds. Research results indicate that:NaCo₂O₄ compounds synthesized with Na₂CO₃ and Co₂O₃ as raw materials. The effects of solid sintering time, temperature and second sintering technique on microstructure and electric property of materials were investigated. The XRD pattern showed that bulk materials were of single NaCo₂O₄ compounds; The SEM pattern indicated that density would increase with the second sintering technique. The optimal temperature is 800℃, and the most suitable sintering time is 12h. Single NaCo₂O₄ compounds were synthesized by Polymerized Complex(PC) method and Citric Acid Complex(CAC) method, the dimension of grain which synthesized by the PC procedure is well-proportioned.NaCo₂O₄ compound doped fluoride was prepared by solid reaction process and polymerized complex procedure respectively. The effect of doping fluoride on thermoelectric properties was investigated. The result showed that NaCo₂O₄ is a P type semiconductor. And the electrical conductivity and thermal conduction of NaCo₂O₄ synthesized by solid phase reaction decreased while the Seebeck coefficient increased as the addition of doping fluoride rose. The thermal conductivity of all the samples presented the trend of decreased first and increased then with the rising temperature. And when the content of NaF is 0.5wt%, the maximum thermoelectricity performance index of the sample is 0.04 at 873K increasing about 25% compared to the blank sample.The electric conductivity of the sample prepared Sol-Gel method and hot pressed sintering decreased while the Seebeck coefficient increased with the rising temperature. Compared to the sample prepared by solid sintering, the electrical conductivity, Seebeck coefficient and power factor all considerably increased. The sample produced by hot pressed sintering is compact with fewer holes. The maximum power factor of 2.40×10^-4wm^-1k^-2 was obtained at 873K for the 0.5% NaF doping compound.