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Micrometer-sized porosity, which cannot be recognized by typical density measurement method, was observed inside transparent polycrystalline alumina prepared by pulsed electric current sintering (PECS). The pores derive from hard agglomerates in starting powder and appear as black dots inside transparent bulk samples. In this study, porosity coming from the agglomeration was studied to degrade by chemical and mechanical treatments. Starting alumina powder was mixed with some surfactants as aqueous solutions. The slurries were ball-milled in 1 d, followed by drying and heat treatment to obtain treated alumina powders. Sintering of those alumina powders was carried out by two-step PECS technique. Transparent alumina specimens were evaluated by the density of black dots and the apparent transmittance. In summary, their treatments of alumina powder can break agglomerates and dramatically reduce the total density of black dots within transparent polycrystalline alumina. Consequently, the transmittance of polycrystalline alumina increased, from 68% for untreated sample to 72­74% for treated samples.

Reduction of black dots in transparent polycrystalline alumina produced by pulsed electric current sintering with various chemical powder treatments