Characterization of the alkoxide-based sol-gel derived La_9.33Si₆O₂₆ powder and ceramic

Abstract

In this study, we report on the acid-catalysed synthesis of La_9.33Si₆O₂₆ from lanthanum nitrate or acetate and silicon ethoxide (TEOS) in the ethanol solvent, upon the transition from liquid to amorphous and crystalline phases. The similarity of the Fourier transform infrared spectra of the lanthanum-salt solutions and lanthanum-silicon sols indicates that the lanthanum environment is not changed in the reaction of the La-salt with TEOS. In the nitric-acid catalysed synthesis, the hydrolysis reaction was almost instantaneous, as a consequence of a higher amount of water in this system, which contributed to a high level of chemical heterogeneity in the product. The acetic acid-based synthesis ensured a good mixing of the reagents at the nanometre level, which gave rise to the formation of the pure La_9.33Si₆O₂₆ powder upon heating at 900 °C, and single phase ceramics with 94% relative density after sintering at 1400 °C for 3 h in air, which is 200 °C lower temperature then usually reported for the apatite material.