A prerequisite for the preparation of nano glass-ceramics is an increase in viscosity of the residual glassy matrix phase during the course of the crystallization process. This results in a deceleration of the crystal growth process due to increasing stresses, which finally may lead to a total freezing of the crystallization process. In principle, two routes for the preparation of nano glass-ceramics exist: in one of these routes, the first step is a phase separation in which a droplet phase is formed. Inside this droplet phase, the crystals are precipitated. In the other route, a preceding phase separation does not occur and the nucleation takes place in a homogeneous glassy phase. In both cases, the crystal growth velocity is high until the crystals reach a size of some nanometers or some ten nanometers, then the crystal growth velocity decreases strongly.
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