Lanthanum silicate oxyapatites (LSO) are promising and alternative electrolyte materials for SOFC applications. The LSO materials exhibit high ionic conductivity and the possibility to operate at a relatively low temperature range, in comparison with classical zirconia-based electrolytes [1]. They also present an anisotropic ionic conductivity, which contribute to better performances, and lead to the development of specific processing techniques to obtain oxyapatites-type lanthanum silicates highly oriented along their c-axis [2]. In this study, the synthesis of a textured LSO was carried out by reactive sintering between two reagents: SiO₂ and La₂SiO₅. Different bilayers with these reagents were elaborated in sandwich-types to obtain a La₂SiO₅/SiO₂ diffusion couple. The experimental parameters that govern the preferential orientation along the c-axis of LSO during the solid-state reaction at high temperature, have been identified.

These bilayers were prepared from silica or La₂SiO₅ supports obtained respectively by electrophoresis and uniaxial pressing, which were pre-sintered and then covered with a suspension of the contrary reagent. The crystalline nature of the reagent, the pre-sintering temperature and the relative density of the supports have an impact on the thickness of the oriented LSO layer obtained. The influence of the temperature and reactive sintering time, were studied, by X-Ray diffraction, SEM observations, EDS and RAMAN spectroscopy. This work allowed to suggest a reaction mechanism model based in four main steps, to explain the formation of LSO phase oriented along the c-axis. 

The experimental results showed that La^+III and O^-II were the predominant diffusing species in this bilayer system. The formation of a La₂Si₂O₇ intermediate phase was detected, which plays an important role in the interdiffusion of species and in the nucleation-growth of oriented apatite crystals. It was also observed that the diffusion processes is enhanced by the formation of a glassy phase between silica grains because of the allotropic transformation of silica type-quartz into cristobalite. Finally, it was possible to obtain a LSO material of high purity, with an oriented and textured microstructure, presenting a porosity gradient, which opens new perspectives in the development of novel electrolytes materials for SOFC applications.
 

References:
[1]  S. Nakayama and M. Highchi, “Electrical properties of apatite-type oxide ionic conductors Re_9.33(SiO₄)₆O₂ (RE = Pr, Nd and Sm) single crystals,” J. Mater. Sci. Lett., vol.20, no. 10, pp. 913–915, 2001.
[2]  K. Fukuda et al., “Oxide-ion conductivity of highly c-axis-oriented apatite-type lanthanum silicate polycrystal formed by reactive diffusion between La₂SiO₅ and La₂SiO₇,” Chem. Mater., vol. 23, no. 24, pp. 5474–5483, Dec. 2011.