Glass ceramics are materials that are made of glass melts by means of controlled crystallisation. The processing of the melt is analogous to the processing for glasses, finally the glass is but usually transferred by a special heat treatment in a partially polycrystalline and partly glassy, ceramic state. The result is a glass-like product having novel properties.
Properties and Applications
There are many different ceramic systems. Some of the most important are the MgO x Al2O3 x nSiO2 system (MAS ) system, the ZnO x Al2O3 x nSiO2 system ( ZAS - System), glass ceramics, lithium disilicate glass and ceramics with phlogopite as a basic system.
However, for by far the most important system as the main components lithium oxide, aluminum oxide and silicon dioxide are used. This most important for the ceramic industry system is also referred to as the LAS system and exists in many variations. As a nucleating agent is usually added epitaxial zirconium ( IV) oxide in combination with titanium ( IV) oxide. To be encountered in this system, the main crystal phases, a high- quartz solid solution ( HQMK ) and a keatite mixed crystal ( KMK), Hummel and Smoke have done groundbreaking work.
LAS glass ceramics of the system with HQMK as the main crystal phase through its own low coefficient of thermal expansion of about 0.1 x 10-6 1 / K (in the range 20 to 700 ° C) a very good thermal shock resistance. If the chemical composition of pure Li2O x Al2O3 x nSiO2 system at n> 3.5, the HQMK converts to keatite mixed crystal from about 950 ° C. The phase transition is irreversible and reconstructive, so coupled with the eruption of bonds. Nevertheless, the two crystal phases, such as Li was able to show in their structures are very similar. According to the conversion coefficient of thermal expansion of the glass ceramic increases due to the higher coefficient of thermal expansion of the CMC to about 1.10 -6 1 / K (in the range 20 to 700 ° C).
Characteristic of this glass-ceramic is to be understood as a composite material of glass and crystals, so a very low or even negative thermal expansion coefficient in different temperature ranges, whereby a failure is prevented by thermal shock. It can be realized glass ceramics with excellent thermal shock properties also very good mechanical strength, therefore, with these phases. Due to the volume ratio of the glass phase to the crystal phase of the thermal expansion coefficient of glass-ceramic can be adapted to many different applications.
Applications arise in a variety of ways as a material for laser gyroscopes or as protective glasses with high thermal shock resistance, and the domestic sector as a hob and cooking utensils. Mirror support large telescopes today are made of glass-ceramics, as well as high-performance reflectors for digital projectors. In laboratories, glass-ceramic plates asbestos wire nets have replaced as support during the heating.
The largest manufacturers of glass ceramics - Schott, Nippon Electric Glass and Corning ( EUROKERA ) - have mainly focused on these applications. Product names are known, such as ceramic and KeraBlack in the range hobs and Zerodur for telescope mirror substrate and the transparent glass ceramic Robax and Pyroceram for fireplace panels, as well as Fire Lite and Neoceram.