Influence of Fe and Ti addition on properties of Na+-β/β″-alumina solid electrolytes

Transition metal (Fe, Ti)-doped Na + –β/β″-alumina samples were synthesized via a solid-state reaction, and the sintered specimens were characterized using X-ray diffraction, scanning electron microscopy, densitometry, and impedance analysis. The results indicated that both the sintered density and β"-alumina fraction were effectively improved by doping with Fe and Ti because of the increased concentration of Al 3+ ion vacancies in the Na + –β/β″-alumina. These vacancies promoted stabilization of the β″-alumina phase and densification by providing a diffusion path for Al 3+ ions. The presence of Fe and Ti in the alumina increased the grain boundary diffusivity, thereby improving the mass transport. Thus, anisotropic grain growth occurred with increasing dopant content. However, excessive liquid-phase formation during sintering occurred when the amount of Ti was greater than 2.0 mol%, and the lowered sintered density decreased the ionic conductivity of the Na + -β/β″-alumina because the ionic conductivity behavior has a closer relationship to the trend of sintered density. The highest ionic conductivities of the Fe- and Ti-doped sintered specimens were 1.4 × 10 -1 S/cm (10.0 mol% Fe-doped) and 1.6 × 10 -1 S/cm (1.5 mol% Ti-doped) at 350 °C.