Seminar Announcement: Guglielmo Macrelli
Data pubblicazione: 9-ago-2019 7.39.55
Tuesday, 6 August 2019, 10:30
Sala Grande Palazzina B via alla Cascata 56/C
"Strengthened glass by Ion Exchange: Residual stress profile nagging issues"
Isoclima SpA R&D Dept.,
Strengthened glass by ion exchange is a wide studied topic with remarkable applications for transportation, pharmaceuticals, architectural and consumer electronics. Despite the increased production, there are still some nagging issues to be fully resolved. One of the major anomalies, that the observed surface compression magnitude is a factor of 3 to 5 lower than that predicted by Cooper coefficient, was recently explained by Varshneya et. al. (2015). A second anomaly which remains a nagging issue is the ob servation that the maximum surface compression observed is often sub surface. Some Li2O MgO Al2O3 SiO2 glasses after ion exchange exhibit conversion of the surface compression to tension over a period of time followed by a buried inner compression, changin g to the usual central tension (Donald & Hill 1988). Donald & Hill ascribed this to a large differential thermal contraction of surface layers upon cooling. Varshneya’s students refuted this by showing that the relaxation occurred even at temperature of io n exchange. We propose a new explanation. Our approach is based on the calculation of residual stress profile taking into account relaxation effects incorporating fast beta, slow alpha and structural relaxations. We show that a mathematical model of stress build up and relaxation in the ion exchanged glass can be versatile in the prediction of even unusual stress profiles. Correlations between residual stress profile and optical properties (TM and TE refractive index profiles) will be also shortly discussed.