Congruent dissolution of Al2O3 in a K-disilicate melt, with constraints on the reaction mechanism

Abstract

High resolution O 1s X-ray Photoelectron Spectroscopy (XPS) was employed to determine oxygen speciation in 31 mol% K2O silicate glass containing 0.0, 1.0 and 3.0 mol% dissolved Al2O3. Highly resolved bridging oxygen (BO) and non-bridging oxygen (NBO) peaks were observed. The NBO peak intensity decreased and the BO increased with dissolution of Al2O3. The enhanced BO intensity is localized at two binding energies (BE), one characteristic of a Si-O-Si moiety (e.g., quartz, or v-SiO2), the other of a Si-O-Al moiety (e.g., kyanite, Al2SiO5). The dissolution reaction proceeds in two steps; NBO− of a Q3 species attacks and bonds to an under-coordinated surface Al atom of crystalline Al2O3. The reaction produces a Si-O-Al surface species thus converting the attacking NBO to a BO and altering the reactant Q3 species to a Q4 product species:[Q3]−melt + [AlO1.5]surface → [Q4]0melt + [AlO2]−meltThe second step involves reaction of [AlO2]−melt with K+converting it from a modifier cation to a charge compensator, resulting in the polymerization reaction:[K-Q3]0melt + [AlO2]−melt → [Q4]0 + [KAlO2]0melt + 1/2O2−The stoichiometric reaction is the sum of the two:[Q3]−melt + [K-Q3]0melt + [AlO1.5]surface → 2[Q4]0 + [KAlO2]0melt + 1/2O2−Al species detached from the solid surface initially must be bonded to some NBO. Within the melt, nucleophilic substitution attack of the Al-bearing species by Q3 species converts NBO to BO, resulting in [KAlO2]melt being bonded four BOs. This reaction may not go to completion in all melts, depending on dissolved Al content and melt composition.