Consolidation of amorphous oxide powders using spark plasma sintering

The objective of this study is to synthesize amorphous oxide powders and sinter the systems close to or at theoretical density for optical transparency while retaining the amorphous structure using the Spark Plasma Sintering (SPS) method. In addition, the parameters that govern the formation of these systems were investigated. The main powder processing techniques are sol-gel, flame-spaying and spherodizing.;The systems chosen for this study are alumina-rich rare earth oxides of alumina-lanthana (AL) and alumina-gadolinia (AG). These systems cannot be formed in bulk form by the melt method because of the extremely fast cooling rates needed to maintain the glassy phase. Sintering is an alternative method. Attempts were made by others to form or sinter these systems with little or no success. Amorphous powders of different processing methods and compositions were sintered using SPS at temperatures ranging from 840 to 900°C and pressures from 164 to 742 MPa. For the AL system, the highest transmittance which correlates to the highest relative density without crystallization was obtained with the parameters of 860°C, 742 MPa, and a 200C.min -1 ramp rate and a 4 min hold time for transmittance in the UV/Vis range of more than 40% at 650 nm. In the IR range, the transmittance was above 65%. At lower pressures, e.g. 164 MPA, increasing the hold time increased the transmittance of the sample. Likewise, the AG system showed higher transmittance with increased pressure. Densities of both systems increased with increasing temperature. However, above 880°C, the increase in densities was due to partial crystallization which resulted in the decrease of transmittance. The spherodization process resulted in the crystallization of some of the beads. The transmittance could be improved by removal of the crystallized beads since the amorphous phase sintered to high densities. The flame spray method resulted in nanometric particles which also sintered to high densities. The powders contained silica fibers from the collection process which causes scattering and a reduction of the transmittance. The fast heating rates and short sintering times of the SPS make it possible to sinter amorphous systems at higher temperatures compared to other sintering techniques without crystallization.