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

Abstract

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.

Influences of TiO2 or Y2O3 doping on the homogeneity of polycrystalline Al2O3 produced by pulsed electric current sintering

“…Transparent polycrystalline Al 2 O 3 ceramics have a high potential to replace the single crystal sapphire in many industrial applications. Besides its good mechanical properties as well as good optical transparency, the main advantage of polycrystalline Al 2 O 3 is the simpler production method along with the lower cost by various sintering techniques such as pulsed electric current sintering (PECS) or hot isostatic pressing (HIP) [1][2][3][4][5]. Moreover, with the sintering techniques, the polycrystalline Al 2 O 3 samples can be shaped much more easily while the single crystal sapphire normally requires machining for shaping after the production of a large bulk.…”

“…Using that special punch-die setup has limitation in the sample size. Our group has studied PECS with two-step temperature profile, referred as two-step PECS (TS-PECS) to fabricate transparent polycrystalline Al 2 O 3 [2,3,5]. The meaning of the first step at lower temperature is for additional densification with limited grain growth.…”

FABRICATION OF TRANSPARENT MgAl2O4 SPINEL CERAMICS BY PECS PROCESSING OF COMBUSTION - SYTHESIZED NANOPOWDERS

“…Recently, PECS has emerged as an efficient method to produce transparent ceramics [6][7][8][9], especially transparent MgAl2O4 [10,11]. This method can assist in increasing bulk density and reducing grain size in MgAl2O4 ceramics, which improve both the mechanical properties and the transparency of the materials.…”