Synthesis and thermal behavior of La9.33Si2Ge4O26 apatite for SOFCs

Powders of La2O3, GeO2 and SiO2 were dry milled in a planetary ball mill with different rotation speeds (150–350 rpm) and increasing milling times up to 35 h in order to obtain the La9.33Si2Ge4O26 apatite phase at room temperature. The results showed that the higher the rotation speed the lower the time required for the formation of the apatite phase. No reaction between the starting powders was observed at 150 rpm. Thermal analysis of the unreacted powders milled at 150 rpm showed formation of the apatite phase around 800 °C, with enthalpies ranging from 43.5 and 48.6 kJ mol−1. An activation energy Ea of 65 kJ mol−1 was obtained applying the Kissinger equation. The mean Avrami exponent n calculated was 1.5, indicating that the apatite phase transformation occurs by a diffusion controlled process