Synthesis and characterization of Al3+-doped La9.33Ge6O26 intermediate temperature electrolyte for SOFCs

The apatite-type La9.33+x/3Ge6−xAlxO26 (x = 0, 1.0, 2.0) and La9.33Ge6−yAlyO26−y/2 (y = 0.5, 1.0, 1.5) nanopowders with an average grain size of 50 nm have been synthesized via a sol–gel auto-combustion process. It has been found that these nanopowders possess high sintering activities and allow their pressed pellets to be fired in 4 h up to an apparent density higher than 94% of the theoretical values at the temperatures from 1150 °C to 1300 °C, indicating at least 200 °C lower than that for the powders obtained by solid state reactions. The sintered samples were structurally characterized using XRD and SEM and a well-sintered microstructure formed by single-phase component grains of diameters around 5 μm has been identified, with no impurity phases that may often occur due to the volatilization of Ge when the sintering temperature is high. The thermal expansion coefficients and the AC impedance spectrum behaviors of the samples were also measured in the temperature range from 400 °C to 800 °C and discussed. In the studied composition range, La9.67Ge5AlO26 was found to have an oxide ion conductivity of 0.013 S cm−1 at 800 °C and a good sintering activity to match the co-firing technology with electrodes.