Preparation by Poly(Acrylic Acid) Sol–Gel Method and Thermoelectric Properties of γ-Na x CoO2 Bulk Materials

γ-Na x CoO 2 single-phase powders have been synthesized by a poly(acrylic acid) (PAA) sol–gel (SG) method, and γ-Na x CoO 2 bulk ceramic fabricated using spark plasma sintering. The effects of the PAA concentration on the sample phase composition and morphology were investigated. The thermoelectric properties of the γ-Na x CoO 2 bulk ceramic were also studied. The results show that the PAA concentration did not significantly affect the crystalline phase of the product. However, agglomeration of γ-Na x CoO 2 crystals was suppressed by the steric effect of PAA. The Na x CoO 2 bulk ceramic obtained using the PAA SG method had higher crystallographic anisotropy, better chemical homogeneity, and higher density than the sample obtained by solid-state reaction (SSR), leading to improved thermoelectric performance. The PAA SG sample had power factor (in-plane PF =  σS 2 ) of 0.61 mW m −1  K −2 and dimensionless figure of merit ( ZT ) along the in-plane direction of 0.19 at 900 K, higher than for the SSR sample (in-plane PF = 0.51 mW m −1  K −2 , in-plane ZT  = 0.17). These results demonstrate that a simple and feasible PAA SG method can be used for synthesis of Na x CoO 2 ceramics with improved thermoelectric properties.