Structural relaxation and reheating shrinkage of alkali-free aluminosilicate glasses induced by thermal cycling

Abstract

Glass substrate for organic light-emitting diodes (OLED) requires to undergo several heating processes and thus makes a thermal shrinkage of glass substrate. The shrinkage leads to a pixel shifting and becomes a key factor affecting OLED panel quality. To understand the thermal shrinkage process, it is crucial to study the structural relaxation and thermodynamic characteristics of glass substrate around the glass transition temperature. In this work, the network structure of alkali-free aluminosilicate glasses and their structural relaxation in the glass transition temperature range were studied by both experiment and molecular dynamics (MD) simulations. The alkali-free aluminosilicate glasses with 15.3–22.1 mol% alkali-earth oxides (RO) were prepared, and their reheating shrinkage and enthalpy relaxation were measured to reflect the structural relaxation during thermal cycling. As the RO content increased from 15.3 to 22.1 mol%, the CTE_25–300°C increased from 3.00 × 10⁻⁶ to 4.13 × 10⁻⁶/°C, and the strain point decreased from 780 to 744°C. The reheating shrinkage of 4 × 4 × 40 mm glass rods also increased from 4.01 to 9.51 ppm, and the volume shrinkage primarily occurred in the initial stages of holding at 600°C. The enthalpy relaxation of the glass was measured using differential scanning calorimetry, and there were very small enthalpy relaxation in alkali-free aluminosilicate glasses due to their good thermal stability. Therefore, MD simulations were employed to calculate the potential energy and evaluate the structural relaxation. The potential energy of the glasses during thermal cycling revealed a significant energy hysteresis upon reheating, and the structural relaxation occurred at relatively low temperature with the increasing addition of RO contents. Further analysis confirmed that the structural relaxation of glasses was mainly due to the structural arrangement of alkali-earth metal ions, including the changes in bond length, angles, coordination number, and atomic mean-square distance.