Highly textured Na_xCoO_(2-delta) ceramics fabricated by both templated grain growth and reactive templated grain growth methods using single-crystalline particles as templates

摘要

Textured gamma-Na_xCoO_(2-delta) ceramics have been prepared by both templated grain growth (TGG) and reactive templated grain grwoth (RTGG) methods using single-crystalline particles of gamma-Na_xCoO_(2-delta) 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 gamma-Na_xCoO_(2-delta) sample exhibited electrical conductivity (delta)=approx1.85*10~4 S/m,Seebeck coefficient (S)=approx137 muV/K and power factor (PFident todelta·S~2)=0.35 mW·m~(-1)·K~(-2) at approx800 K. These values of delta and S are smaller than those of single crystals (sc); that is,delta_(sc)=19.2*10~4 S/m and S_sc=200 muV/K at 800 K. The magnitude of delta was found to decrease inproportion to the relative density (d) of the sample,while S was independentof d. Furthermore,both delta and S seemed to be sensitive to the Na content of the samples. Both the low density (dapprox70%) and the deviation from the optimal Na content were considered to be predominant factors for the low thermoelectric properties of the present textured samples.