Highly Textured Na_xCoO_{2 - δ} Ceramics Fabricated by Both Templated Grain Growth and Reactive Templated Grain Growth Methods Using Single-Crystalline Particles as Templates

Abstract

Textured γ-Na_xCoO_{2 - δ} ceramics have been prepared by both templated grain growth (TGG) and reactive templated grain growth (RTGG) methods using single-crystalline particles of γ-Na_xCoO_{2 - δ} as templates. An XRD analysis indicated that the samples were highly c-axis oriented; i.e., the highest Lotgering’s factor was estimated to be 0.93. According to transport property measurements, the highly textured γ-Na_xCoO_{2 - δ} sample exhibited electrical conductivity (σ) = ∼1.85 × 10⁴ S/m, Seebeck coefficient (S) = ∼137 μV/K and power factor (PF ≡ σ·S²) = 0.35 mW·m^-1·K^-2 at ∼800 K. These values of σ and S are smaller than those of single crystals (sc) ; that is, σ_sc = 19.2 × 10⁴ S/m and S_sc = 200 μV/K at 800 K. The magnitude of σ was found to decrease in proportion to the relative density (d) of the sample, while S was independent of d. Furthermore, both σ and S seemed to be sensitive to the Na content of the samples. Both the low density(d∼70%) and the deviation from the optimal Na content were considered to be predominant factors for the low thermoelectric properties of the present textured samples.