Kinetic Neutron Diffraction to Observe the Crystallization of Mica Glass-ceramics

Objectives: It is known that impurities introduced into glasses through feedstock choice can strongly affect the crystallization of glass-ceramics. This study examined the effect of varying the quality of feedstock on the crystallization and properties of a mica glass-ceramic with potential use in CAD-CAM dentistry. Methods: A glass from the barium fluormica: calcium phosphate: cordierite phase diagram (90: 4: 6 mol%) was produced using both high quality laboratory reagents and also much more impure industrial reagents. The glasses so-produced were characterized by kinetic neutron diffraction, which utilizes a high flux neutron beam and the D20 diffractometer at the Institute Laue-Langevin to produce diffraction patterns of bulk samples in real time as they are being heat-treated. Each glass was given an identical heat treatment. Further glass-rods were made and also heat-treated in an identical manner. Discs were produced from these glass-ceramic rods and the biaxial flexural strength measured for each glass-ceramic. Their microstructures were compared by SEM, using both secondary electron and backscattered imaging. Results: Neutron thermograms revealed that crystallization to the desired mica phase was unaffected by feedstock and that the lattice parameters of the mica phase produced were the same for each material. Biaxial flexural strengths measured for each material were not significantly different (p>0.05). Furthermore, the microstructure of each material consisted of a dense mass of interlocking plates of the same size and aspect ratio. Conclusions: Crystallization and the subsequent properties of these glass-ceramics were unaffected by quality of feedstock and the presence of impurities, suggesting that these materials crystallize readily from the bulk, most probably via prior amorphous phase separation. Neutron diffraction provided a unique insight into the crystallization process for these materials. The fluormica glass-ceramics produced were readily machinable and have potential in CAD-CAM dentistry.

The authors would like to acknowledge EPSRC for funding this work