Crystallization of Silicon Pyrophosphate from Silicophosphate Glasses as Monitored by Multi-Nuclear NMR
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Crystallization of Silicon Pyrophosphate from Silicophosphate Glasses as Monitored by Multi-Nuclear NMR Randall Youngman, Carrie Hogue, and Bruce Aitken Science & Technology, Corning Incorporated, SP-AR-02-4, Corning, NY, 14830
ABSTRACT Binary silicophosphate glasses containing between 7 and 35 mol% P2O5 were made by conventional melt quenching techniques and characterized by 29Si and 31P NMR spectroscopy. 31 P and 29Si NMR data show that, as the P2O5 content is raised in these glasses, the network connectivity is decreased by a dilution of the silicate network with less polymerized Q3 phosphate groups. In addition to such changes in the network structure, 29Si NMR spectra for the most P2O5-rich glasses reveal the presence of silicon atoms with 5- and 6-fold coordination. These species increase in concentration with increasing P2O5 above approximately 30 mol% P2O5. Self-nucleating SiP2O7 glass-ceramics can be formed from the binary silicophosphate glasses with >25 mol% P2O5. 29Si and 31P NMR results were used to monitor the crystallization of SiP2O7 by providing details on amount of crystallinity, relative grain sizes and even composition of the residual glassy matrix. Stabilization of octahedral Si as well as the onset of phase separation correlates with propensity for crystallization of SiP2O7 during subsequent heat treatment. INTRODUCTION Since their discovery in the 1950’s, glass-ceramics, i.e. composite materials in which a crystalline phase has been internally nucleated and grown within the confines of a glassy matrix, have found application in many different commercialized technologies [1]. A key attribute of glass-ceramics is the ability to combine the physical properties of conventional polycrystalline ceramics with the favorable processing characteristics of glasses. These materials are typically formed from a precursor glass by controlled crystallization of at least one crystal phase. Therefore an understanding of glass structure and compositional effects on the resulting glassceramic are critical in the development of new materials. In this study, we describe glass-ceramic materials based on binary SiO2-P2O5 glasses in which crystalline SiP2O7 is grown without the aid of a nucleating agent. 29Si NMR results on P2O5-rich precursor glasses demonstrate the presence of Si atoms with high-coordination numbers (5 and 6), with growth of internally-nucleated SiP2O7 upon heat treatment. Likewise, 31 P NMR data allow us to track SiP2O7 crystallization as well as changes in the residual glass composition as a function of heat treatment. NMR of both nuclides is sensitive to crystallite size and composition of the residual glass, and the data suggest that phase separation and/or intermediate range order structures similar to those in SiP2O7 are prerequisites for SiP2O7 crystallization during heat treatment of high P2O5 glasses.
EXPERIMENTAL DETAILS Binary SiO2-P2O5 glasses were made over a range of approximately 7 to 35% P2O5. As a first step, a 30% P2O5 glass was made by melting a silica and phospho
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