Structural studies in aluminophosphosilicate glasses and in cold-pressurized borosilicate glasses by Nuclear Magnetic Resonance in the solid state

• Autor(a) principal: Santos, Millena Logrado dos
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/76/76134/tde-02082023-101358/
• topic: Aluminofosfossilicato; Aluminophosphosilicate; Borosilicate; Borossilicato; Double resonance; Dupla ressonância; Estado sólido; NMR; Pressurized glasses; RMN; Solid-state; Vidros pressurizados

Technologically important glasses can have their properties fine-tuned by understanding the role of each element in the glass structure. However, elucidating properties, such as heat-, weathering- and crack-resistance, based on the glass structure is a challenging task due to the multiple factors influencing these macroscopic properties. In this study, we investigate the structure-property correlation for three distinct system: (i) an aluminophosphosilicate heat resistant optical fiber with composition: xNa2O (22.50-x)Al2O3 7.50P2O5 70.00 SiO2 (NAPS), 2.50 x 17.50; (ii) an aluminophosphosilicate weathering resistant optical fiber, with composition xSiO2 (59-x)P2O5 13Al2O3 28Na2O (SPAN), 0 x 13; and (iii) cold-pressurized glasses which can shed light on crack resistant properties, with the composition 20Na2O xB2O3 (70-x)SiO2, x = 10, 20, 30. The short- and medium-range order of the three distinct systems were investigated by 11B, 23Na, 27Al, 29Si, 31P single and double resonance solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. The glass structures are elucidated in terms of two terminologies: X(n), which refers to the atom X bonded to n bridging oxygen atoms connected to like or unlike network former units, and X(N), which refers to the isotope X coordinated by N atoms (N being a Roman numeral). (i) (NAPS glasses), the dual role of sodium, either as a charge balancer or as a network modifier was investigated. The structure of these glasses is dominated by a significant bonding preference between anionic four-coordinate Al(IV) species and cationic P(4) units (Al-O-P linkages). The gradual replacement of alumina by sodium oxide initially increases the number of Si-O-Al linkages, which are charge balanced by Na+ introduced by Na2O. Higher levels of x ([Na]/[P] > 1) lead to a partial depolymerization of Al-O-P linkages and to the formation of neutral P(3) and ionic P(2) and P(1) units strongly interacting with the sodium species. Furthermore, the presence of P-O-P linkages in the glasses with the highest sodium concentration was confirmed by 31P double quantum excitation experiments. The overall scenario can be described by the dual role of sodium ions in the glass structure serving as a charge balancer for anionic Al(IV) network former, and as a neutralizer to the non-bridging oxygens of newly formed P(n) units (3 n 1). In the system (ii) (SPAN glasses), the spectra of 29Si Magic Angle Spinning (MAS)-NMR indicates that no significant proportion of hexacoordinated Si, expected for glasses with high [P]/[Si] ratios, was formed. This structure is likely suppressed by the strong interaction between phosphorus and aluminum, which was measured by 27Al-31P double resonance dipolar recoupling experiments. Dipolar recoupling techniques also facilitated the analysis of the 31P MAS-NMR spectra in terms of two distinct components with different chemical shifts: one interacting more strongly with Na, the other one interacting more strongly with Al. The NMR-based structural information can provide insights about the non-linear composition dependence of the weathering resistance of the glasses. In (iii) the cold pressurized borosilicate glasses, 11B MAS- and 23Na MAS-NMR illustrated together a counterintuitive structural effect: the average B coordination number decreases upon compression. This effect exposes a behavior not previously reported in the literature: the applied pressure induces a closer approximation between Na+ ions and oxygen species bridging between tetrahedral boron and silicon, causing a breaking in B(IV)-O-X (X = B, Si) linkages. As a result, new B(III) units are formed, as well as new non-bridging oxygens predominantly attached to silicon (later compensated by Na+ ions). These results clearly document an insightful detailed structural picture provided by solid-state NMR, which was used to suggest possible and multiple structural origins for each macroscopic effect investigated.