Study on the Effect of La/Si Ratio on the Microstructures and Electrical Properties of Lanthanum Silicate Electrolyte

A series of $\text{La}_{0.99\times 6\mathrm{x}}\text{Eu}_{0.01\times 6\mathrm{x}}\ \text{Si}_{6}\mathrm{O}_{9\mathrm{x}+12}$ (LSO) with stoichiometric La/Si ratios equal to $\mathrm{x}(\mathrm{x}=1.55,\ 1.58,\ 1.61,\ 1.64, 1.67)$ ceramic electrolytes sintered at 1450°C were prepared and the influence of La/Si ratio on relative densities and electrical properties were investigated. All samples can be characterized as hexagonal apatite structure with good single-phase property. The relative densities of ceramic pellets increase gradually with the increase of La/Si ratio. The surface SEM images of the sample with $\mathrm{x}=1.67$ shows relatively dense microstructure. The oxygen ion conductivities of ceramic pellets increase gradually with the increase of La/Si ratio, which could be ascribed by both relative density and interstitial oxide ion concentration. The sample 1450-1.67LSO has high oxide ion $\sigma_{\text{tot}}$ of $6.62 \times 10^{-4}\mathrm{S}\ \text{cm}^{-1}$ at 600°C. The results of this study may contribute to optimize the electrical conductivity and density of apatite lanthanum silicate electrolyte by modulating La/Si ratio.