Preparation of Mullite Whiskers from Powder of Topaz Pure and Doped with Rare Earths

Mullite whiskers were obtained by thermal decomposition of powders of natural topaz, pure and doped with 3 and 5% in weight of La2O3 and Y2O3, at the temperatures of 1300 and 1400 degrees C, for 1h, in air. Pure and doped mullite whiskers showed a molar ratio Al2O3: SiO2 close to 2:1, rich in alumina. The aspect ratio (AR) of whiskers varies with dopant concentrations and temperatures. The ARs for pure whiskers were 28 and 31.5, at the temperatures of 1300 and 1400 degrees C, respectively, showing no glassy phase. For the doped whiskers, there were observed a reduction in the ARs, ranging from 14.13 to 16.10 (3 and 5% La2O3) and from 6.32 to 7.35 (3 and 5% Y2O3), for the temperatures of 1300 and 1400 degrees C, respectively. The Y2O3 was not entirely incorporated, and part of it formed Y2Si2O7 and Y2O3-delta, with delta = 0.33 according to quantitative X-ray diffraction analysis (XRD analysis) with the presence of small amount of vitreous phase.