γ-NaxCoO2粉体的聚丙烯酸钠凝胶法制备及其表征

本研究发展了一种用于制备氧化物热电材料γ-Na_xCoO₂粉体的化学合成方法——聚丙烯酸钠(PAAS)凝胶法。主要研究了PAAS/Co²⁺摩尔比、原料浓度和煅烧温度对产物相组成及微观形态的影响规律, 探讨了物相形成机制, 同时用该方法结合SPS制备了不同Na离子浓度的Na_xCoO₂多晶样品, 并对其热电性能进行了表征。结果表明, PAAS/Co²⁺摩尔比对产物相组成产生了显著影响, 随着PAAS/Co²⁺摩尔比的增加, 样品的相组成由Co₃O₄相向单相γ-Na_xCoO₂转变, 合适的PAAS/Co²⁺摩尔比为0.8~1.1。而反应原料浓度对产物相组成的影响存在一个临界值(0.025 mol/L), 大于临界值抑制单相形成, 小于临界值促进单相形成。煅烧温度的升高有助于γ-Na_xCoO₂单相的形成, 800℃煅烧得到γ-Na_xCoO₂单相, 晶粒形态呈片状, 平均厚度约200 nm, 片状方向的尺寸在1~4 μm之间。随着Na含量的增加, 样品的Seebeck系数增大, 电导率增加, 热导率降低, 最终导致ZT值大幅增加。

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

Chemical synthesis method of sodium polyacrylate (PAAS) gel method was used to prepare γ-Na_xCoO₂ 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. Na_xCoO₂ 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 γ-Na_xCoO₂. The proper molar ratio of PAAS/Co²⁺ to obtain γ-Na_xCoO₂ single phase is 0.8. the phase composition of sample will change slightly with different concentrations of raw materials. γ-Na_xCoO₂ 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 γ-Na_xCoO₂ single phase was promoted with the increase of calcination temperature. The γ-Na_xCoO₂ powders calcined at 800℃ were plate-like particles with about 200 nm in thickness and about 1-4 μm in width along the plane direction. With the increase of Na content in samples, σ value and S value were increased while κ value was decreased, leading to a significant increase in ZT value.