Sol gel synthesis and spark plasma sintering of lanthana-doped alumina glass

Amorphous materials of eutectic composition in the La2O3–Al2O3 system were prepared from powders synthesized by the sol–gel method and then flame sprayed to form amorphous transparent beads ~ 10–150 μm in diameter. These were subsequently consolidated by the spark plasma sintering (SPS) method. The final density of sintered samples showed a dependence on temperature and the applied pressure. The transmittance increased with temperature up to 860 °C, then decreased with further increase in temperature, as a consequence of crystallization. The measured glass transition and crystallization temperatures, Tg and Tx, are 845 and 905 °C, respectively. Crystallization below Tx is likely the consequence of the applied electric field during sintering. The highest observed transmittance in the red-end of the visible region (650 nm) was determined to be about 40%. The transmittance of sintered amorphous samples in the IR region was between about 50% and 60% with a significant broad absorption band at about 2.9 μm. The occurrence of this band is attributed to the stretching of the O―H bond of residual OH species in the samples.