Effect of indium doping on thermal stability and dielectric property in sodium beta alumina solid electrolyte

We report the effect of indium doping on thermal stability and ionic conductivity of beta alumina NaInxAl11-xO17 solid electrolyte which is synthesized by sol-gel auto combustion method on varying dopant concentration as x = 0, 0.1, 0.2, 0.3, and 0.4. X-ray diffraction (XRD) pattern and microstructure are investigated on the doped samples calcined at 1100 degrees C for 5 h. XRD confirms the change in unit cell volume on increasing the dopant concentration. Field emission scanning microscope (FESEM) reveals the conversion of cylindrical morphology to small spherical particles at dopant concentration x >= 0.5. Thermal stability is found to improve drastically over a broad temperature range even at small dopant concentrations in beta-alumina as found from thermo-gravimetric analysis (TGA). Electrochemical impedance spectroscopy (EIS) shows a considerable reduction in frequency-dependent dielectric permittivity for doped beta alumina. At frequency 1 kHz, the permittivity of around similar to 10(5) in as-prepared shows non-monotonous dependence and decreases to 10(3) for x >= 0.1. This steep variation is mainly attributed to the change in morphology caused by steric effect and formation of random clusters which reduces the net polarization.