Consolidation of high melting SiO2-Al2O3-La2o3 glass powders with gas pressure vacuum viscous sintering technology

Rare earth (RE) doped silica glasses are widely used as gain medium in optical fibers, providing high efficiency, broad working bandwidth, and high mechanical strength for high power fiber laser applications. However, some of the compositions are limited to the melting temperature of Pt crucibles at about 1700 °C. Therefore, an alternative method is proposed to prepare bulk glasses from high melting crystalline powders by two steps: the vitrification of the crystalline powder and a subsequent consolidation process. Consequently, the GPWS procedure was developed for the consolidation/densification step: to consolidate the already vitrified amorphous powders. In this work, one high melting SAL glass composition has been taken as the model glass. With the here developed viscous stretching measurement as well as the FEM Simulation results, the viscosity and surface tension of the SAL glass at high temperature have been determined. With these parameters, the range of working parameters for this glass composition has been determined using the viscous sintering theory. Finally the GPWS SAL glasses have been characterized in five aspects of inhomogeneities: crystallization, knots, striae, absorption groups and bubbles. Compared to normal vacuum sintering, the GPWS method sinters in higher rate and is able to compensate the loss of sintering rate by using large particles. It provides the ability to suppress bubble formation at high temperature or to close bubbles when the entrapped gases are diffusive. Compared to the SAL glasses prepared from conventional melting, the GPWS SAL glasses are free of OH absorption groups as well as bubbles.