Study of Sodium Aluminoborosilicate Glasses for Photomultiplier Tube Glass Application in Neutrino Detection Using Experimental and Molecular Dynamics Techniques

Abstract

In this thesis, we present a comprehensive study of structural, mechanical properties, and chemical durability of commercial and experimental aluminoborosilicate glasses with potential for photomultiplier tube application using both experimental and molecular dynamics (MD) techniques.

The mechanical properties, measured using Vickers indentation and ring-on-ring (R-O-R) biaxial flexural strength test, showed similar performance for two commercial PMT glasses. The chemical durability test under simulated neutrino detector conditions revealed dealkalized and deboronated layer at the glass surface. In addition, the normalized mass loss indicated pH and temperature dependent ion-release behavior. The effect of high-purity water on the R-O-R flexural strength of the glass was minor as the test measures the bulk strength of the glass. Furthermore, we verified the release of gadolinium (Gd) ions in high-purity water from a Gd-doped aluminoborosilicate glass, which could be used as a controlled Gd-source in future WCDs to enhance neutrino detection.

Two MD projects were completed. In the first project, the composition-structure-property relationship of sixty-nine different multicomponent aluminoborosilicate glasses within the compositional space of commercial PMT glasses were studied. The glasses were simulated using classical MD. The structural and mechanical properties such as bond distance, coordination number (CN), Qn speciation, bridging oxygen (BO) and non-bridging oxygen (NBO) concentration, and Young's Modulus (E) were calculated. The glasses were further differentiated in terms of low-E and high-E glasses and subjected to temperature and pressure simulations. The densities and E values calculated showed no statistical variations for the glasses simulated at different temperature and pressure conditions. In the simulated glasses, silicon was almost entirely 4-coordinated, boron was observed in 3-and 4-coordination and aluminum was found in 4- and 5-coordination. The BO concentration of 93.4±0.84% suggests highly polymerized simulated glasses. The high-E glasses contained higher BO and lower NBO concentrations as compared to low-E glasses. Furthermore, low-E glasses consisted of higher amount of depolymerized glass former structural units. The polymerization of the simulated glasses was affected by the presence of sodium, which takes on multiple roles of charge compensating ion for tetra-coordinated Al and B, and modifies the glass network by creating NBO sites at Si and B structural units.

In the second MD project, the bulk and surface structures, and the glass surface-water reactions of sodium aluminosilicate glass were simulated using a combination of classical Buckingham and reactive force field (ReaxFF) potentials. On comparison of the simulated glass structures generated using ReaxFF and Buckingham potentials, our results showed that the atomic density profiles calculated for the surface glass structures indicate a bond-angle distribution dependency, and higher concentrations of NBOs and sodium ions at the glass surface. Additionally, our results showed the formation of silanol species and diffusion of water molecules at the glass surface using ReaxFF.