Preparation of grain-aligned polycrystal of doped lanthanum silicate oxyapatite by templated grain growth method and evaluation of its structural and electrical properties

Abstract

The disk-shaped sintered compacts consisting mainly of c-axis-aligned lanthanum silicate oxyapatite (LSO) polycrystals doped with K2O and Al2O3 were prepared by templated grain growth method. The tabular single crystals of K2O- and F-doped LSO were used as template particles, and the Al2O3-doped LSO as matrix powder. The chemical formula derived from the average chemical composition of the constituent LSO crystal grains was (La9.60K0.16□0.24)Σ=10(Si5.48Al0.38□0.14)Σ=6O26, where □ denotes vacancies in La and/or Si sites. One of the doped LSO grains was examined by single-crystal X-ray diffraction to confirm that the crystal structure was isomorphous to those previously reported. Other coexisting phases were 0.16 mol% LaAlO3 and 0.34 mol% SiO2-rich interstitial material (probably in a liquid state at high temperature). With increasing temperature from 773 to 1073 K, the total oxide-ion conductivity (σtotal) along the grain-alignment direction steadily increased from 1.31 × 10−4 to 3.21 × 10−3 S cm−1, each value of which was, at the same temperature, more than 7.7 times larger than that of the randomly grain-oriented polycrystal with the same bulk chemical composition. The larger σtotal-value of the former polycrystal would be due to the significantly higher oxide-ion conductivity along the c-axis direction of the doped LSO. A solid oxide fuel cell (SOFC) using the textured polycrystal as the electrolyte was constructed and tested for power generation. The maximum power density was satisfactorily high at 873 K, indicating that the c-axis-aligned polycrystals of doped LSO would be potentially applicable as electrolytes for the SOFCs operating at medium to low temperatures.