Templated grain growth of textured lanthanum silicate oxyapatite ceramics

Abstract

We have for the first time fabricated the textured ceramics of doped lanthanum silicate oxyapatite (LSO) by templated grain growth method. Tabular template particles of K₂O- and F-doped LSO with well-developed {001} faces were grown by KF flux method, and aligned in BaO-doped LSO matrix by tape casting. The tapes were punched out into disk-shaped pieces, stacked in two layers, and sintered at 1873 K for 50 h. We obtained a disk-shape K₂O- and BaO-doped LSO polycrystal aligned in the c-axis with small amounts of two types of interstitial materials. The texture fraction of apatite {0 0 l} planes was 0.45. The doped LSO was characterized by the heterogeneous distribution of K₂O, the component of which originated from the template particles, within the crystal grains. The chemical formula of the doped LSO was derived from the average composition to be (La_8.43K_0.03Ba_1.43□_0.11)_Σ=10(Si_5.96□_0.04)_Σ=6O₂₆, where □ denotes vacancies in La and/or Si sites. One type of the interstitial materials was composed mainly of BaO and La₂O₃, and the other was rich in SiO₂. As the temperature increased from 773 to 1023 K, the bulk oxide-ion conductivity (σ) steadily increased from 1.89 × 10⁻⁴ to 2.48 × 10⁻³ S cm⁻¹, with the activation energy of conduction (E_a) being 0.78 eV. The σ-values of randomly oriented polycrystal with the same bulk chemical composition steadily increased from 3.89 × 10⁻⁵ to 4.99 × 10⁻⁴ S cm⁻¹ with increasing temperature from 773 to 1023 K (E_a = 0.78 eV). When the σ-values were compared at the same temperatures, they were 6.7 (973 K)–8.2 (923 K) times higher for the former polycrystal than for the latter. The larger σ-values of the c-axis-aligned polycrystal would be principally induced by the substantially higher oxide-ion conductivity along the c-axis than along the other directions in K₂O- and BaO-doped LSO.