ALUMINA AS A DEVITRIFICATION INHIBITOR DURING SINTERING OF BOROSILICATE GLASS POWDERS

Cristobalite is formed when borosilicate glass (Corning Code 7740) is sintered at temperatures ranging from 700-degrees to 1000-degrees-C. T he precipitation kinetics, determined by XRD analysis, exhibit a chara cteristic incubation period which decreases with increasing sintering temperature, from 60-120 min at 700-degrees-C to 3-5 min at 1000-degre es-C. Activation analysis of precipitation shows an activation energy of 75 kJ/mol, which is close to that for the diffusion of Na+ in boros ilicate glass, suggesting mass-transport-controlled kinetics.1,5 With added alumina content greater than a critical value, however, the cris tobalite formation in the borosilicate glass is completely prevented a t the sintering temperatures investigated. The critical alumina conten t is found to decrease with decreasing alumina particle size but with increasing sintering temperature. The above result, similar to observa tions previously made in a binary glass mixture containing a low-softe ning borosilicate glass (BSG) and a high-softening high silica glass ( HSG)3 is attributed to a strong coupling between Al3+ from alumina and Na+ from borosilicate glass. The coupling reaction causes segregation of Na+ in borosilicate glass to alumina, thus forming a Na+- and Al3-rich reaction layer around alumina particles far too rapid for cristo balite formation.