Effects of Al2O3/Na2O ration on the rate of ion-exchange in PbO containing opticalglasses.

Abstract

Effects of chemical composition on the Na+→←K+ ion exchange in the mixed alkali optical glasses containing PbO were studied by EPMA and optical microscopy. It is generally known that the rate of ion exchange increases with increase in Al2O3/Na2O ratio within a ratio of unity. In the present study, however, it was found that the effect of Al2O3/Na2O ratio on the rate of ion exchange was almost negligible for the optical glasses containing a large amount of PbO, and Pb2+ may suppress the migration of K+ ion in the process of Na+→←K+ ion exchange. By increasing the amount of Al2O3 (and Al2O3/Na2O ratio) and decreasing the amount of PbO the apparent diffusion coefficient of the glasses increased up to about 5 times comparing with that of a base glass. Therefore, it can be concluded that, even if the Al2O3/Na2O ratio increases, the rate of ion exchange in the glasses containing a large amount of PbO is almost invariable and that the rates increase by the decrease of the amount of PbO and the increase in Al2O3/Na2O ratio. As the results of Na+→←K+ ion exchange of the glasses containing PbO at 400°C for 24h, the observed compressive stress and the thickness of the compressive layer were in the ranges of 0.5-16kg/mm2 and 10-70μm, respectively.