Devitrification or devitrification describes the process in which crystals partially form in a glass , which are still surrounded by an amorphous glass matrix, or in which the glass completely crystallizes and thus loses its glass properties.
In the manufacture of glass, devitrification is usually considered a defect in the molten mass , but it can also be brought about deliberately in order to achieve special properties. The most common example of this are glass ceramics . Devitrification can also occur in natural glasses, such as obsidian .
Crystallization in glasses
Crystallization occurs in glasses under certain temperature-time conditions. The prerequisite for this is the presence or formation of crystallization nuclei . The growth of nuclei occurs through diffusion of crystal components and their build-up into crystals. With conventional glasses, the maximum nucleation number (KB) is at lower temperatures than their maximum growth rate (KG). The latter is largely determined by the diffusion rate, which increases sharply with lower viscosities . The closer the temperatures of the maximum nucleation number and the maximum growth rate are to one another or the larger the overlap area of the two curves, the faster the cooling must take place in order to avoid crystallization. With many metals, these two curves lie directly on top of each other, so there is a temperature range in which the metal suddenly crystallizes out. Therefore, in order to produce metallic glasses from a melt, cooling speeds of up to 10 8 Kelvin per second must be achieved in order to prevent crystallization.
As a rule, in glass production, the cooling of the glass happens quickly enough, which leads to a sharp increase in the viscosity of the melt. The increasing viscosity of the melt increasingly prevents diffusion until the growth of crystals comes to a complete standstill. The rapid cooling of the melt is therefore a basic requirement in order to keep a solid body glassy. The driving force of crystallization is the striving of matter to assume the state of lowest energy. Because of their random structure, glasses are in a metastable - i.e. more energetic - state than crystals.
Homogeneous nucleation and critical nucleus radius
From a thermodynamic point of view, there is a change in free enthalpy during the nucleation of a system . This process will always take place when the free enthalpy is reduced. During the nucleation process, the building blocks arrange themselves into a crystal and thereby give off energy. This process is associated with a phase transition from the liquid melt to the solid crystal. So there is an interface that requires energy to be generated. The total change in the free enthalpy is made up of a volume fraction at which energy is given off and a surface fraction that has to be applied to create the interface. The following relationship arises:
The energy released is proportional to the volume of the nucleus, whereas the energy to be expended is proportional to its surface. Assuming a spherical nucleus with the radius r, the above relation results:
It represents the free volume enthalpy of the nucleus during the phase transition and represents the surface tension of the melt. When considering this relationship, it becomes apparent that energy must first be applied in order to generate a germ. Only when a nucleus with a radius larger than the critical nucleus radius arises through accidental convergence can it grow and thereby minimize its free enthalpy. Germs with a radius smaller than minimize their enthalpy by dissolving again.
Heterogeneous nucleation is a process in which nucleation is facilitated by the presence of a foreign nucleus in the melt. It is found much more frequently than homogeneous nucleation, since the former is not so dependent on the accidental presence of the necessary components of a nucleus. Due to the presence of a sufficiently large nucleus, part of the interfacial energy to be produced has already been produced. In extreme cases, this process can be understood as the growth of a crystal onto a foreign crystal in the melt. This process is called epitaxy and is used specifically to manufacture glass ceramics, for example.
Crystallization in glass production
In general devitrification is regarded as a glass defect that occurs - for example in a glass melt in areas with a low flow of the melt or in particularly susceptible glasses (such as those with a high alkaline earth oxide content, CaO or MgO) - both during cooling of the glass melt and during Heating of glasses can occur. Increased contents of alkaline earth oxides increase the liquidus temperature in most bulk glasses , so that crystallization can start at higher temperatures, such as those found in feeders , for example . Devitrification in the glass processing (as bottom-up - crystallization ) as well as a controlled process for producing a desired color glass, a glass ceramic or a turbidity glass used.
- Werner Vogel: Glass chemistry . 3. Edition. Springer, Berlin 1992, ISBN 3-540-55171-9 .
- Armin Petzold: Inorganic-non-metallic materials . 2nd Edition. German publishing house for basic industry, Leipzig 1986, DNB 870224441 .
- Hans Jebsen-Marwedel, Rolf Brückner (ed.): Glass-technical manufacturing defects . 4th edition. Springer, Berlin 2011, ISBN 978-3-642-16432-3 .