Densification and characterization of transparent polycrystalline spinel produced by spark plasma sintering

Polycrystalline transparent magnesium aluminate spinel (MgAl2O4) is an important optical material due to its good mechanical properties combined with a high light transmittance over the visible and near infrared wavelength range. It shows promise for many applications including high energy windows and light-weight armors. Being a difficult-to-sinter material, the fabrication of transparent spinel often requires a pressure-assisted sintering method with the use of a sintering aid. Thanks to its high heating rate combined with a moderate pressure, spark plasma sintering (SPS) is capable of producing a fine-grain spinel without using a sintering aid. The densification dynamics of spinel in SPS processing and its influence on the transmittance and the microstructure of the material were of great interest. Early in the course of study, it was determined that the mobility of the magnesium ions in spinel under an electric field at elevated temperatures was the major factor that caused discoloration in SPS-processed spinel. Controlling the SPS heating rate and soaking time after the sample reach 60% of its theoretical density was the decisive factor in attaining a high transmittance and minimizing discoloration. Later it was found that increasing the SPS processing time would diminish the transmittance of spinel in the visible region, and at the same time exacerbated discoloration. Based on these observations, the SPS conditions for transparent spinel were determined and optimized. Enhancement of the hardness of spinel by doping was also studied. A simple surface precipitation process was developed to coat alumina onto the spinel particles. Doping caused a reduction the transmittance of the material in the visible region but inserting an alumina layer between the graphite die and the powder compact to be sintered helped to solve this problem. By doping, the hardness of the material was increased by 3 – 6% compare to other SPS-processed spinels, and by about 8.5% compare to HIP-processed spinel of the same grain size. The findings of this dissertation explained why SPS processed spinel could not reach its theoretical transmittance in the visible wavelength range. Simple solutions to minimize discoloration of SPS-processed spinel were determined. The experimental designs served as a platform to explore potential densification dynamics and interactions of other nonconductive ceramic materials in SPS processing.Ph.D.Includes bibliographical referencesby Minh T. V