Theoretical study of the influence of carbon contamination on the transparency of spinel ceramics prepared by spark plasma sintering (SPS)

Carbon contamination due to the use of graphite dies, punches and paper is a major challenge in the preparation of transparent ceramics via spark plasma sintering (SPS), but also the elimination of residual porosity is an issue. In this paper, the effect of carbon contamination, alone or in combination with residual porosity, on the transmittance of spinel (MgAl2O4) ceramics is investigated via Mie theory as a function of inclusion/pore size, concentration (volume fraction) and wavelength of light. It is found that there is a critical size of carbon inclusions (around 100 nm), nearly constant with wavelength, above which the transmittance increases with increasing inclusion size to its theoretical limit. However, if the inclusions are smaller than that, the transmittance remains low and nearly constant with decreasing size of inclusions. Carbon contamination accompanied with porosity results in a combination of the typical trends and features caused by carbon inclusions and pores.