Glass is a very fascinating material for its transparency. The glasses can be prepared through the process of supercooling of a liquid phase through glass transition temperature by escaping the crystallization. Glass is thermodynamically unstable with respect to the metastable (undercooled liquid) state, i.e., there is no energy barrier between the glass and its corresponding undercooled liquid. At a first glimpse, the high stability of the glassy state reflects only a relaxation problem; the system cannot evolve to a metastable state due to the kinetic inhibition of this process at low temperatures. On heating, relaxation of the glass structure may occur to reach first a metastable liquid state corresponding to the given temperature and then, eventually, go over into the crystalline state. At room temperature glasses can exist for extremely long duration of time because of their high viscosity inhibits structural rearrangements required for crystal nucleation and growth. However, when a glass is heat-treated for a sufficiently long time at temperatures within or above the glass transition temperature range, devitrification readily starts, from the surface and sometimes in the bulk via heterogeneous or homogeneous nucleation. Nucleation, or the process of formation of the precursors of the crystalline phases, may occur by different mechanisms. Commonly one divides these processes into homogeneous and heterogeneous nucleation. Homogeneous nucleation is a stochastic process occurring with the same probability in any given volume (or surface) element. Alternatively, nucleation occurring on preferred nucleation sites, e.g., such as pre-existing interfaces, previously nucleated phases, and surface defects, is denoted as heterogeneous nucleation [ ]. Depending on the location where nucleation takes places, volume and surface crystallization can be distinguished. Glass-forming melts are interesting models for studies of nucleation, growth and overall crystallization phenomena. Their high viscosities result in relatively low rates of crystallization within the measurable range, which may permit detailed studies of nucleation and growth kinetics. It is obvious that crystallization and glass formation are competitive processes. In this way, in order to avoid uncontrolled crystallization of glassy articles one needs to know the main factors that govern crystal nucleation and growth. On the other hand, controlled nucleation and crystallization of glasses underlay the production of glass-ceramics invented in the mid of 1950s, which are widely used in both domestic and high-technology applications. By the foregoing reasons, the investigation of glass crystallization kinetics is of great interest from both practical and theoretical points of view.
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