PHD Defense by Tobias Bechgaard
Tobias Bechgaard, Department of Chemistry and Bioscience will defend his thesis on: "Relaxation Behavior and Mechanical Properties of Aluminosilicate Glasses"
14.12.2018 kl. 09.30 - 12.30
"Relaxation Behavior and Mechanical Properties of Aluminosilicate Glasses"
Glasses are fascinating materials with a wide range of applications. Glasses within the aluminosilicate family are relatively inexpensive to manufacture and can be tailored with superior properties, making them of special industrial interest. Understanding the composition-structure-property relations in aluminosilicate glasses can accelerate the design of new compositions with tailored performances. The goal of this Ph.D. project is to understand the compositional and structural origins of structure, relaxation behavior, and selected mechanical properties of aluminosilicate glasses.
First, we have studied the glass structure of aluminosilicates with three different modifier cations at varying concentration by Raman and nuclear magnetic resonance spectroscopy. The short and intermediate range order of the glasses depends on the modifier-aluminum ratio and the field strength of the modifier.
Second, we have used temperature-modulated differential scanning calorimeters, which function at high temperature (>1000°C), to study relaxation behavior in calcium aluminosilicate glasses. To do so, we first developed experimental protocols for the use of the equipment and then applied them to determine the liquid fragility index and study the enthalpy relaxation through determination of the non-reversing heat capacity. We found that the accuracy of the activation energy approach for the determination of liquid fragility is comparable to the Moynihan method known from linear differential scanning calorimetry. Moreover, a correction is needed to remove the systematic error in the Arrhenius approximation in the fragility determination. By comparing the non-reversing heat capacity to molecular dynamics simulations, we find an apparent correlation between extent of relaxation and network topology in calcium aluminosilicate glasses.
Third, we studied the effect of time and humidity on the crack initiation probability (CIP) upon Vicker’s indentation of aluminosilicate glasses. As expected, humid conditions and longer time result in the initiation of a larger number of radial/median cracks. We show that the CIP increases drastically when the humidity increases and that cracks can form at least 24 h after indentation. The study has quantified the time-scale of indentation cracking and highlighted the need for a general experimental protocol for crack resistance data to be comparable, as the environmental humidity greatly affects the crack resistance. Finally, we studied the pressure dependence of the photoelastic response of aluminosilicate glasses. Today, photoelasticity is predicted by an empirical model, but we found that the model cannot account for the structural changes associated with compression at high temperature and pressure.
- Professor Morten Mattrup Smedskjær
- Associate Professor Casper Steinmann Svendsen, Departemnet of Chemistry and Bioscience, AAU (Chairman)
- Professor Edgar D. Zanotto, Universidade Federal de São Carlos
- Professor Dominique de Ligny, Universität Erlangen – Nürnberg
AFTER THE DEFENSE A SMALL RECEPTION WILL BE HELD AT FREDRIK BAJERS VEJ 7H.
Department of Chemistry and Bioscience, AAU, Section of Chemistry
Fredrik Bajers Vej 7H, 9220 Aalborg Ø, room number: 1.102-1.106