23Na multiple-quantum (MQ) MAS NMR is applicable for monitoring the chemical and structural changes resulting from atmospheric exposure of a series of alkali aluminoborosilicate glasses with compositions RNa2U: 1B2O3:1SiO2:0.25Al2O3 (where R = 0.5-2.5). Glasses with high alkali concentrations possess greater numbers of nonbridging oxygens within the bulk structure and presumably at the initial surface of a fresh sample, and for three samples with R ≥ 1.5 sharp resonances are revealed in the isotropic dimension of an MQMAS NMR experiment conducted after prolonged atmospheric exposure. The MQMAS NMR experiments, combined with 1H → 23Na cross-polarization magic-angle spinning (CPMAS) NMR measurements, indicate that these resonances arise from sodium cations no longer participating in the glass network. Two new phases are formed as corrosion products and have been identified as an anhydrous Na2CO3 phase and a NaBO2·O phase through comparison with 23Na MQMAS and 1H →23Na CPMAS NMR spectra of crystalline samples. Due to an inherent difficulty with direct quantification of populations based on MQMAS spectra, a simplified approach for quantification of the amount of the new carbonate phase is presented. Values are then calculated for relative amounts of corrosion product formation for different exposure times and bulk glass compositions.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry