γ-Na_xCoO_2粉体的聚丙烯酸钠凝胶法制备及其表征

Preparation and Characterization of γ-Na_xCoO_2 by Sodium Polyacrylate Gel Method

本研究发展了一种用于制备氧化物热电材料γ-Na_xCoO_2粉体的化学合成方法——聚丙烯酸钠(PAAS)凝胶法。主要研究了PAAS/Co^(2+)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_xCoO_2多晶样品,并对其热电性能进行了表征。结果表明,PAAS/Co^(2+)摩尔比对产物相组成产生了显著影响,随着PAAS/Co^(2+)摩尔比的增加,样品的相组成由Co_3O_4相向单相γ-Na_xCoO_2转变,合适的PAAS/Co^(2+)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_xCoO_2单相的形成,800℃煅烧得到γ-Na_xCoO_2单相,晶粒形态呈片状,平均厚度约200 nm,片状方向的尺寸在1~4μm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。

Chemical synthesis method of sodium polyacrylate （PAAS） gel method was used to prepare γ-NaxCoO2 powders. The effects of PAAS/Co^2＋ 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/Co^2＋ molar ratio greatly affected the phase composition. With the increase of PAAS/Co^2＋ molar ratio, the phase composition of sample was transformed from C0304 to γ-NaxCoO2. The proper molar ratio of PAAS/Co^2＋ to obtain y-NaxCoO2 single phase is 0.8. the phase composition of sample will change slightly with different concentrations of raw materials, y-NaxCoO2 single phase was formed at low concentra- tion 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 7-NaxCoO2 single phase was promoted with the increase of calcination tem- perature. The y-NaxCoO2 powders calcined at 800℃ were plate-like particles with about 200 nm in thickness and about 1-4μm in width along the plane direction. With the increase of Na content in samples, σ value and S value were increased while K value was decreased, leading to a significant increase in ZT value.