MAS NMR study of soda-lime silicate glasses with variable degree of polymerisation

AR Jones, R Winter, GN Greaves, IH Smith; J Non-Cryst. Solids 293 (2001) 87
Abstract. 29Si and 23Na Magic Angle Spinning (MAS) NMR has been used as an atomic probe for structural characterisation of soda-lime-silicate glasses. The glasses studied are based on the composition (CaO)x(Na2Si3O7) where x is varied from x=0 to x=1 (increasing CaO from 0 to 20mol%), finally matching the Na2O fraction. The glass network becomes increasingly depolymerised by increasing CaO, and shows a changing distribution of Qn species. Two Qn species have been observed coexisting for compositions where x=0, 0.2 and 1, while for compositions where x=0.4 and x=0.8 Q2, Q3 and Q4 are present. Progressive changes in chemical shifts of Q2 and Q3 species, and changes in FWHM of all detected Qn species accompany the addition of CaO. The addition of CaO introduces a greater distortion into the structure, and there is evidence to suggest preferential arrangement of Ca in the vicinity of Q2 species rather than Q3.
Fig.1: Compositional trend of <sup>29</sup>Si spectra. Fig. 1: 29Si MAS NMR spectra of the (CaO)x(Na2Si3O7) glass series.
Chemical shift values, line widths and relative abundances of different Qn species were obtained by curve fitting assuming Gaussian line shapes for each component. The top two spectra comprise Q4 and Q3 contributions only, while the remaining spectra are best represented with three components (n=2,3,4). The position of the Q4 line is stable at -106.9ppm. Q3 become deshielded (-94.1ppm to -91.7ppm) while Q2 become shielded (-81.5ppm to -83.9ppm) with increasing CaO content. The line width of the Q4 decreases, that of the other components increases as more CaO is added.

Fig.2: <sup>23</sup>Na spectrum before and after annealing. Fig. 2: 23Na MAS NMR spectra of a) CaO(Na2Si3O7) glass, and b) the corresponding heat treated sample.

Fig.3: <sup>29</sup>Si spectrum before and after annealing. Fig.3: 29Si MAS NMR spectra of a) CaO(Na2Si3O7) glass, and b) the corresponding heat treated sample.

Acknowledgements. We would like to thank ESPRC and Pilkington for funding a CASE studentship, and C. Bracegirdle for assistance with glass preparation.
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