FULL PAPER Templated grain growth of textured lanthanum silicate oxyapatite ceramics Koichiro FUKUDA 1,³ , Fuminori MAEKAWA 1 , Takuya EGUCHI 1 , Yuki TSUNODA 1 , Daisuke URUSHIHARA 1 , Toru ASAKA 1 and Hideto YOSHIDA 2 1 Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Nagoya 466–8555, Japan 2 Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113–0033, Japan 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 K 2 O- 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 K 2 O- and BaO-doped LSO polycrystal aligned in the c-axis with small amounts of two types of interstitial materials. The texture fraction of apatite {00 l} planes was 0.45. The doped LSO was characterized by the heterogeneous distribution of K 2 O, 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 8.43 K 0.03 Ba 1.43 □ 0.11 ) -=10 (Si 5.96 □ 0.04 ) -=6 O 26 , where □ denotes vacancies in La and/or Si sites. One type of the interstitial materials was composed mainly of BaO and La 2 O 3 , and the other was rich in SiO 2 . As the temper- ature increased from 773 to 1023 K, the bulk oxide-ion conductivity (·) steadily increased from 1.89 © 10 ¹4 to 2.48 © 10 ¹3 S cm ¹1 , with the activation energy of conduction (E a ) being 0.78eV. The ·-values of randomly oriented polycrystal with the same bulk chemical composition steadily increased from 3.89 © 10 ¹5 to 4.99 © 10 ¹4 S cm ¹1 with increasing temperature from 773 to 1023 K (E a = 0.78 eV). When the ·-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 ·-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 K 2 O- and BaO-doped LSO. ©2020 The Ceramic Society of Japan. All rights reserved. Key-words : Ionic conductivity, Polycrystalline materials, Tape casting, Impedance spectroscopy [Received August 19, 2020; Accepted September 11, 2020] 1. Introduction Lanthanum silicate oxyapatite (LSO) is one of the most promising materials for electrolytes of solid oxide fuel cells because of its relatively high oxide-ion conduc- tivity at relatively low temperatures and low oxygen partial pressures. 1),2) Since the oxide-ions preferentially conduct along the c-axis in the crystal structure (space group P6 3 /m), 3) it is required to fabricate the single crystals and/ or c-axis-aligned polycrystals for the enhancement of conductivity. The conductivity has been also increased by doping of foreign oxides (e.g. BaO) 4) and introduction of vacancy in Si site. 5) Actually, the c-axis-aligned poly- crystal of (La 9.32 Ba 0.28 □ 0.40 ) -=10 (Si 5.87 □ 0.13 ) -=6 O 26 has demonstrated the superior conductivity to that of (La 9.33 □ 0.67 ) -=10 Si 6 O 26 , where □ denotes vacancies in La and/or Si sites. 6) These grain-aligned polycrystals, including those of lanthanum germanate oxyapatite, sodium titanogallate, and sodium titanoaluminate, have been readily prepared by a reactive diffusion technique, in which the appropriate combinations of diffusion couples are simply heated in air at high temperatures where atomic diffusion occurs. 5)-10) Although the methodology itself is quite conventional and therefore suitable for the research phase, this technique is disadvantageous in that the result- ing grain-aligned polycrystals are relatively small (nor- mally less than 3 cm in diameter for disc-shaped samples) and in some cases contain significant amounts of micro- cracks. Thus, in power generation tests aimed at practical use, it is necessary to prepare the large-sized and high- strength crystal-aligned electrolytes. The templated grain growth (TGG) method 11),12) is advantageous over the reactive diffusion technique in that the resulting textured polycrystals are in most cases relatively large in size and include few microcracks. This method requires two types of starting materials; one is the anisotropic template seeds that control texture develop- ment (template particles) and the other is the fine-grained source materials (matrix powders). In TGG, the template ³ Corresponding author: K. Fukuda; E-mail: fukuda.koichiro@ nitech.ac.jp Journal of the Ceramic Society of Japan 128 [11] 954-961 2020 DOI http://doi.org/10.2109/jcersj2.20171 JCS - Japan ©2020 The Ceramic Society of Japan 954 This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by-nd/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.