Quantitative Evaluation of Glass-Forming Impurities in Alumina: Equivalent Silica Concentration (ESC)

Equivalent silica concentration (ESC) is proposed as a quantitative measure of the total impurity concentration that contributes to intergranular glass formation in nominally pure alumina. The method involves, in its determination, the processing and sintering of the alumina under contamination-free conditions, as well as measuremet of the average grain size at the moment that abnormal grains first appear. This latter value can be converted to an ESC value from the inverse relationship between grain size and silica concentration, which can be established via an identical method using ultrapure alumina. The ESC values for four different commercial alumina powders of varying purity have been measured and compared with their chemical-analysis data.