Effect of Crystallographic Anisotrophy and Lanthanum Stoichiometry on Microstructural Evolution of Lanthanum Silicate Electrolytes

Abstract

Lanthanum silicates with different lanthanum concentrations La9.33+x(SiO4)6O2+1.5x were prepared by solid state reaction using milling procedures of different degrees and sintered at 1700°C or 1750°C. The phase composition and the microstructural evolution were shown to depend strongly on the heat‐treatment temperature and history. The presence of the liquid phase in the lanthanum‐rich compositions at sintering temperatures is clearly evidenced, which, however, is not in accordance with the known phase relations of the material. Exaggerated growth of plate‐shaped grains in the c‐axis direction was induced by the inhomogeneous presence of the liquid phase. The solid solution region with excess lanthanum appears to extend with increasing temperature, as evidenced by intragranular second phase precipitation on heat treatments at lower temperature as well as the X‐ray powder diffraction results of differently treated samples. Lanthanum‐deficient compositions were shown to exist as a two‐phase mixture with La2Si2O7 in equilibrium, although the powders prepared by solid state reaction at 1300°C do not indicate the significant presence of a secondary phase, regardless of the composition. The groove profiles with both a maximum and minimum (or hump) indicate the surface diffusion mechanism controlled by bulk defect chemistry and defect migration in the material. This phenomenon can be explained by the transport anisotropy in La diffusion which controls the matter transport by the bulk ambipolar diffusion mechanism, supported by recent theoretical as well as experimental studies. Electron backscattered diffraction mapping clearly correlated the grains perpendicular to the c‐axis with the strong development of thermal grooving and vice versa by the transport anisotropy. There is little indication of the surface and grain‐boundary energy anisotropy regardless of the compositions.

Combustion Synthesis of Aluminum Nitride: A Review

“…Many combustion synthesis methods have been developed for synthesis of AlN [20,21,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55]. In this work, these methods are firstly classified into two categories according to the phases involved in the reaction (see Fig.…”

“…In this work, these methods are firstly classified into two categories according to the phases involved in the reaction (see Fig. 1): solid-gas reaction [20,21,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] and gas phase reaction [55]. These are described in the following.…”

“…Therefore, the major phases involved in the SHS reaction should be liquid (molten Al) and gas (N 2 gas)). Aluminum (and additives) powder is either pressed into a compact [20,36,37,39,40,48,53] (referred to as compaction process) or contained in a container [21,[41][42][43][44][45][46] (usually a graphite crucible and will be referred to as crucible process). The compact or the container (containing the reactant powders) is then placed in a SHS reactor.…”

Chemical solution deposition of the highly c-axis oriented apatite type lanthanum silicate thin films

“…Actually, high temperature operation of SOFCs can cause several problems, such as limitations on the component materials (collector, housing, etc. 1600°C) required for sintering, [14][15][16][17][18] and the other is difficulty in the fabrication of thinner films with thicknesses of few micrometers for SOFC electrolyte applications. Therefore, new electrolyte materials have been explored to enable efficient operation at lower temperatures (<500°C).…”

“…One is the high temperatures (ca. 1600 °C) required for sintering, [14][15][16][17][18] and the other is difficulty in the fabrication of thinner films with thicknesses of few micrometers for SOFC electrolyte applications.…”