Crystallization inhibitors during sintering of Pyrex borosilicate glass

摘要

In a previous paper, it was demonstrated that a crystalline phase of cristobalite is continuously precipitated out of an initial amorphous Pyrex borosilicate glass (Corning code 7740) at temperatures from 700 to 1000 °C [1], Devitrification results show that the precipitation of cristobalite exhibits a characteristic incubation period, which decreases with increasing temperature. Activation energy analysis of devitrification shows that the transport kinetics of Na+ in Pyrex borosilicate glass is the rate-limiting step. In subsequent investigations, we devised and successfully demonstrated a strategy to prevent the formation of cristobalite during sintering of Pyrex borosilicate glass powder [2-4]. The strategy lies in diverting the transport of alkali ions from the desired target to a more attractive but harmless target (crystallization inhibitor) and to do so in a time frame shorter than the characteristic incubation period for cristobalite growth. Acording to the random network model of glass structure [5], moreover, the doping cations of crystallization inhibitor should be a network former with a valence number of three (M3+) and tetrahedrally coordinated to oxygen. Therefore, the negative charge created by the substitution of Si4+ by M3+ in the glass network provides an electrochemical potential for alkali ions (M+) to segregate to M3+ to maintain electroneutrality at the local level. To verify the above principles, a wide variety of oxides including trivalent metal ions such as A12O3, Ga2O3, Y2O3, La2O3 and Sc2O3 were chosen and separately added to Pyrex borosilicate glass during sintering.