Time to Relax: Relaxation Behavior of Sodium Aluminosilicate Glasses from Modulated Differential Scanning Calorimetry
Open Access
- Author:
- Hauke, Brittney
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 18, 2025
- Committee Members:
- John Mauro, Program Head/Chair
Michael Lanagan, Outside Unit & Field Member
Venkatraman Gopalan, Major Field Member
John Mauro, Chair & Dissertation Advisor
Joshua Robinson, Major Field Member - Keywords:
- glass
relaxation
sodium aluminosilicate glass
thermal analysis
modulated differential scanning calorimetry
MDSC
glass relaxation - Abstract:
- One of the most important glass compositional families for advanced industrial applications is the sodium aluminosilicate (Na2O-Al2O3-SiO2) or NAS family. For example, after undergoing chemical strengthening through the ion exchange process, sodium aluminosilicate glasses are used as protective coverings for screens and windshields. Glass undergoes relaxation toward the supercooled liquid state during both annealing and ion exchange processes. This relaxation involves a spontaneous lifting of kinetic constraints that were imposed during cooling through the glass transition region. Relaxation is an intrinsic property of all glasses due to their inherently non-equilibrium nature. At room temperature, the relaxation phenomenon appears to be frozen on typical observation time scales, while at higher temperatures measurable relaxation can be observed. By understanding how different modes of relaxation impact the stress response of NAS glasses, they can be topologically designed with improved properties. Currently, relaxation is being studied in a variety of glass families, particularly calcium aluminosilicates, sodium aluminoborosilicates, and chalcogenide systems, but less work has been devoted to the NAS family and the atomic scale origin of relaxation remains mostly unknown. An added challenge is that in NAS glasses, the role of Al3+ and Na+ in the structure and its effect on relaxation have not been thoroughly examined. Expanding this understanding allows for predicting and designing glasses with a minimum in relaxation, mitigating undesirable changes to structure and creating glasses with improved properties. Modulated differential scanning calorimetry (MDSC) has received much attention to study the relaxation behavior of different glasses but, due to equipment limitations, has almost exclusively been used on non-oxide and low glass transition temperature (Tg) compositions. While MDSC allows for the deconvolution of overlapping kinetic and thermodynamic signals, the addition of a sinusoidal modulation to the heating rate introduces more experimental parameters that are nontrivial to determine. Additionally, the relaxation of NAS glasses, which are important in industrial applications, has not been thoroughly studied, especially by MDSC. There are two competing analysis techniques for MDSC; the non-reversing heat flow and complex heat capacity methods, and there has been much debate about the merits of both, but especially the non-reversing heat flow method. This dissertation includes the first known parametric study of modulation parameters for MDSC on sodium aluminosilicate glass system. A variety of heating rates, modulation amplitudes, and modulation periods were compared to determine the parameters that gave the best linearity for each composition. The complex heat capacity analysis was also used to determine the shapes of the phase angles and imaginary heat capacity, which gives insight into the relaxation behavior of each composition. This work also provides novel best practices for integrating the enthalpy of relaxation from MDSC based on the non-reversing heat flow method. Currently, most enthalpy of relaxation data reported from MDSC does not include error bars; thus, it is difficult to determine whether there are measurable minima in the enthalpy of relaxation or if they are just artifacts from the equipment. Based on the results published in this dissertation and the lack of standardization, the non-reversing heat flow method needs more investigation, especially on high-Tg glasses. The complex heat capacity method is more mathematically rigorous and can provide more detailed information about relaxation behavior, provided the correct modulation conditions are used. Lastly, equations were developed that allow industries to determine specific Tg values and quench rates for determining when a static stretching exponent beta or a dynamic, temperature dependent beta should be used to model the complexity of relaxation behavior.