MAS NMR investigation of atomic rearrangements during batch melting of glass

AR Jones, R Winter, GN Greaves, C Targett-Adams, IH Smith; Glass Technology 43C (2002) 52
Abstract. Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy has been used to investigate atomic rearrangement during the batch melting of a soda-lime silicate glass. The changes in the local environment of 29Si in the glass forming system 76 SiO2 - 20 Na2O - 4 CaO at various stages of melting was followed from the initial quartz to the final glass composition. Spectra were collected ex situ from quenched samples held at 1450oC for up to an hour. The glass formed in the early stages of melting is modifier-rich and dominated by Q3 species, and contains undissolved quartz. The undissolved quartz is identified by a narrow resonance peak at 115ppm which decreases in intensity with melting time, until all the quartz has dissolved after 26min. Q4 species become more abundant in the glass network as more quartz dissolves and this is accompanied by a progressive change in the Q4 chemical shift towards higher shielding. The Q4 in the final glass is de-shielded with respect to that in the quartz, due to interactions with modifiers in distant coordination spheres. FWHM and chemical shift values for the Q3 species suggest that these are least affected by the dissolution of quartz, presumably because their cation environment in the glassy phase are stable through the melting process. Intermediate crystalline phases formed during the early stages of batch reaction have also been detected by MAS NMR.
Fig.1a: <sup>29</sup>Si spectra after various melting times (glass only). Fig.1b: <sup>29</sup>Si spectra after various melting times (glass and crystal). Fig. 1: 29Si MAS NMR spectra of the a) glass fraction and b) glass and crystalline fractions of a 76 SiO2 - 20 Na2O - 4 CaO batch as a function of melting time at 1450oC. The glass fraction was differentiated from residual quartz by shorter T1 values resulting from Fe3+ doping.

Fig.2: Trends of NMR fit parameters with melting time. Fig. 2: Qn percentage abundance, chemical shift and FWHM for 29Si sites in the vitreous phase as a function of melting time.

Fig.3: NMR spectrym of intermediate stage of batch melting. Fig. 3: 29Si MAS NMR spectra of the 76 SiO2 - 20 Na2O - 4 CaO batch after t<18min melting. The narrow peaks at 93, -95 and 101 ppm are due to intermediate crystalline phases and are assigned to alpha- and beta-Na2Si2O5.

Acknowledgements. We acknowledge EPSRC and Pilkington plc for funding a CASE studentship.
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