Effective Thermal Strengthening of Glass by Enhanced Configurational Entropy at Its Supercooled State

Glass is a representative of fragile material. But it can be used as a good protective cover when strengthened. As a method of such strengthening, so-called “physical tempering” is widely used for large glass plates. In this method, heated glass is rapidly quenched so that the outside part of the glass is frozen while the inside cools slowly. Such difference in frozen temperatures causes the excessive shrinkage of the inside than the outside, which generates surface compressive stress. Thus, for better strengthening, higher thermal expansion can be expected. In this research, we investigate glass which has similar expansion coefficient while it is in glassy state but is much larger when they are in the supercooled liquid state. We successfully found that the borosilicate glass system possesses systematically controllable thermal expansion at high temperatures while keeping that at a temperature below glass transition to be similar. It was available due to the systematic but drastic change in configurational entropy related to the alkali-assisted-Si-O-B bonding in the series of borosilicate glasses. Two types of tempering demonstrations show that the developed glass can be strengthened more than twice as large as that of the window glass when applied to a similar strengthening condition.