Preparation and thermal properties of samarium doped silica xerogels

Samarium doped silica xerogels with different content of Sm3+ up to 0.5 Sm/Si are prepared using a sol–gel method based on acid-catalysed hydrolysation of tetraethylorthosilicate (TEOS) and gelation. The prepared xerogels are investigated by SEM, X-ray diffraction, TG/FTIR, differential scanning calorimetry (DSC) and thermogravimetry (TG). It is proved that triclinic microcrystallites of Sm(NO3)3·6H2O are formed at high Sm contents. Detailed thermal analysis up to 500 °C demonstrates a strong effect of Sm content on the thermal properties of doped xerogels. A peculiar sharp endotherm is observed in the vicinity of 80 °C which position, unexpectedly, shifts to higher temperatures by decreasing of the heating rate. Other two wide endotherms are observed depending on both Sm amount and heating rate. Two different activation energies related to these endotherms are evaluated for the lower- and higher temperature effects: Ea = 38 kJ/mol, and Ea = 210 kJ/mol, respectively. They characterize the corresponding processes of xerogels dehydratation and chemical decomposition of Sm(NO3)3·6H2O. This finding is also in accordance with TG/FTIR analysis of Sm-doped silica xerogels.