Electrical properties of Al-doped oxyapatites at intermediate temperature

In order to obtain acceptable performances in SOFC, relatively high operating temperatures (∼850–1000 °C) are required. That is mainly due to the low ionic conductivity of YSZ electrolyte. These temperature conditions are at the origin of the cell degradation, which justify the search for alternative electrolytes presenting the same performances but at lower temperature (∼800 °C). It is in this context that the oxyapatites of general formula La10−xSi6−yAlyO27−3x/2−y/2 (x = 0, 0.67; y = 0.25, 0.50, 0.75) are studied in this work. The solid solutions are prepared by sintering the oxide powders at 1600 °C. All samples have shown a hexagonal structure with an increase in the cell parameters with the aluminium content. Electrical properties are determined by impedance spectroscopy between 169 and 790 °C. The results are treated by separating the bulk and grain boundary conductivities between 169 and 500 °C and in the form of total (bulk + grain boundary) conductivity between 500 and 790 °C. The respective influences of the activation energy and the pre-exponential factor on conductivity are analyzed. Activation energies are about 0.65 eV suggesting an interstitial mechanism. No variation of conductivity is observed with the oxygen partial pressure. Finally, La10Si5.5Al0.5O26.75 shows conductivity higher than that of YSZ at intermediate temperature.