High-Temperature Phase Transformation of Er~(3+)-doped Boehmite (γ-AlOOH) Gels during Sintering

The 0～20mol%Er~(3+)-doped γ-AlOOH gels were prepared by using sol-gel methods from the aluminium isopropoxide [Al(OC_3H_7)_3]-derived Al_2O_3 sols. The phase structures and these transformation processes of Er~(3+)-doped γ-AlOOH gels sintered at 950～1400℃ were investigated by using X-ray diffraction (XRD) and differential thermal analysis (DTA). The experimental results show that Er~(3+) doping lead to restraining the phase transformation of θ→α for alumina, and decreasing the growth rate of a phase with an increase of 50～70℃ for the completed transformation temperature. The precipitation of the two Er-Al-O compounds, i.e. Al_(10)Er_6O_(24) and ErAlO_3 phases, is dependent on the Er~(3+) doping concentration and the sintering temperature. The ErAlO_3 phase precipitates in the 1～5mol%Er~(3+)-doped Al_2O_3 sintered at 1100～1200℃. The Al_(10)Er_6O_(24) phase precipitates in the 5～20mo1%Er~(3+)-doped Al_2O_3 sintered at 1000～1100℃, and is in all Er~(3+)-doped Al_2O_3 with further increasing the sintering temperature. The nucleation of phase transformation for θ→α is accelerated due to the presence of ErAlO_3 phase, resulting in a decrease of 30～50℃ for the beginning temperature of phase transformation correspondingly. It has no greatly influence on the growth rate of a phase. The Al_(10)Er_6O_(24) phase does not affect the phase transformation for θ→α.