Devitrification

Devitrification or devitrification is the process in which partially form crystals in a glass, which are still surrounded by an amorphous glass matrix or where the glass completely crystallized. In most cases, devitrification is a failure of the melt, but can also be performed consciously created to achieve specific properties. The most common example of this are glass ceramics.

Crystallization in glasses

Under certain temperature-time conditions, it is in lenses for crystallization. A prerequisite for this is the presence or the formation of crystallization nuclei. The growth of bacteria is done by Herandiffundieren of crystal components and their assembly into crystals. With conventional glasses, the maximum number of nucleation ( KB) is at lower temperatures than the maximum growth rate (KG). The latter is mainly determined by the diffusion rate, which rises sharply at lower viscosities. The closer the temperature of the maximum number of nucleation and the maximum growth rate to each other or the larger is the overlapping area of the two curves, the faster must be the cooling in order to avoid crystallization. For many metals, these two curves lie directly above each other, so that there is a temperature range in which the crystallized metal abruptly. Therefore, must be achieved for the production of metallic glasses from a melt cooling rates of up to 108 Kelvin per second, to prevent crystallization.

As a rule, production of a glass, the cooling of the glass takes place fast enough, resulting in a sharp increase in viscosity of the melt. The increasing viscosity of the melt prevents the diffusion increasingly, to the growth of crystals completely come to a halt. The rapid cooling of the melt is thus a prerequisite to obtain a solid glassy. Driving force for crystallization is the tendency of matter to occupy the lowest energy state. Glasses are because of their irregular structure in a metastable - so energetic - state as crystals.

Homogeneous nucleation and critical nucleus radius

Thermodynamic considered place during the nucleation in the system, a change of the free enthalpy instead. This process is always run when doing the free energy is reduced. During nucleation, the building blocks organize into a crystal and thereby give energy. This process is associated with a phase transition from the liquid melt to solid crystal. Thus, it creates a boundary surface, to the production of energy has to be applied. The total change in the Gibbs free energy is thus composed of a volume fraction is delivered with the energy, and a surface portion that must be applied to the creation of the interface together. This results in the following relationship:

The energy released is proportional to the volume of the nucleus, whereas the energy expended is proportional to its surface. Assuming a spherical seed with radius r, the result for the above relationship:

In this case, the free Volumenenthalpie the seed is at the phase transition and is the surface tension of the melt. When considering this relationship is striking that first energy must be applied to produce a seed. Only when a seed is created by random convergence, whose radius is greater than the critical radius of seed, this can grow and thereby minimize its free energy. Germs whose radius is smaller than that minimize their enthalpy by re-dissolve.

Heterogeneous nucleation

Heterogeneous nucleation is a process in which the nucleation is facilitated by the presence of a foreign seed in the melt. It is substantially more common than homogeneous nucleation, since the former is not to such a great extent depends on the random presence of the necessary components of a seed. Due to the presence of a sufficiently large nucleus, a part of the services to interfacial energy has already been provided. In the extreme case, one can understand this process as a growth of a crystal on a foreign crystal in the melt. This process is called epitaxy and example, be used to produce glass-ceramics.

Crystallization in glass production

Generally devitrification is deemed glass error - for example in a glass melt in areas with low flow of the melt, or in particularly vulnerable glass ( such as those having a high alkaline-earth oxide content, CaO or MgO), - both the cooling of the molten glass, as well as in can occur heating of glasses. Elevated levels of alkaline earth oxides increase in most mass glasses, the liquidus temperature, so that it can use a crystallization at higher temperatures, such as occur for example in the feeders. A devitrification in the glass (A bottom-up crystallization ) also used as a controlled process to produce a desired colored glass, a glass ceramic or a glass turbidity.