Influence of pre- and post-annealing on discoloration of MgAl2O4 spinel fabricated by spark-plasma-sintering (SPS)

In order to discuss the influence of carbon contamination on the transmission, the effect of pre- and post-annealing treatments was investigated in the spark-plasma-sintered (SPSed) MgAl2O4 spinel. During the SPS process, the carbon phases transformed from CO32−, which is pre-existing in the powder, remains along grain junctions and caused discoloration. The pre-annealing of the starting powder can reduce the impurities and improve in-line transmission Tin by 10%. Post-annealing can remove the discoloration from the spinel, but it changes the spinel to whitish color with the annealing temperature and degraded Tin. This degradation can be explained by the increase of a scattering coefficient caused by the pore. During the post-annealing, the carbon phases generate high pressure CO/CO2 gas by reacting with oxygen and forms many pores along the grain boundaries. This suggests that reducing the carbon contamination is important for attaining highly transparent spinel by the SPS processing.