Monitoring crystallization in lithium silicate glass-ceramics using 7Li → 29Si cross-polarization NMR

• New NMR method for quantifying crystallinity in lithium silicate ceramics
• The method has very high sensitivity for low crystal contents.
• Crystallization conditions influence 7Li relaxation behavior.

Quantification of the crystalline versus amorphous fraction is an important objective for the structural characterization of glass-ceramics. Owing to its well-documented ability of differentiating between crystalline and amorphous materials and its inherently quantitative character, magic-angle-spinning solid state nuclear magnetic resonance (MAS-NMR) spectroscopy is an excellent method for this objective. For the technologically important lithium disilicate glass-ceramics, the applicability of 29Si MAS-NMR is, however, seriously impeded by poor signal to noise ratios and extremely long spin–lattice relaxation times (on the order of magnitude of hours). The detection sensitivity problem can be overcome by magnetization transfer from 7Li nuclei to the 29Si spins (7Li → 29Si cross-polarization). While this method is inherently non-quantitative owing to the influence of various relaxation processes involved, we show that it can be successfully calibrated to yield quantitatively reliable crystalline fractions that are similar to those determined by alternative methods (optical microscopy and X-ray powder diffraction). In addition, this method can be used to detect very low (~ 1%) crystallized volume fractions.