Preparation and Characterization of γ-NaxCoO2 by Sodium Polyacrylate Gel Method

Chemical synthesis method of sodium polyacrylate (PAAS) gel method was used to prepare gamma-NaxCoO2 powders. The effects of PAAS/Co2+ molar ratio, concentration of raw materials and calcination temperatures on the phase composition and microstructure of product were studied. NaxCoO2 polycrystalline samples with different Na+ concentrations were synthesized by the above method, and the thermoelectric properties were characterized. The results showed that the PAAS/Co2+ molar ratio greatly affected the phase composition. With the increase of PAAS/Co2+ molar ratio, the phase composition of sample was transformed from Co3O4 to gamma-NaxCoO2. The proper molar ratio of PAAS/Co2+ to obtain gamma-NaxCoO2 single phase is 0.8. the phase composition of sample will change slightly with different concentrations of raw materials. gamma-NaxCoO2 single phase was formed at low concentration of raw materials (below 0.025 mol/L), however, when the concentration exceeded 0.025 mol/L, the second phase was formed. The formation of gamma-NaxCoO2 single phase was promoted with the increase of calcination temperature. The gamma-NaxCoO2 powders calcined at 800 degrees C were plate-like particles with about 200 nm in thickness and about 1-4 mu m in width along the plane direction. With the increase of Na content in samples, sigma value and S value were increased while kappa value was decreased, leading to a significant increase in ZT value.