Glass-Ceramics are polycrystalline. They have two phases which
are amorphous phase and crystall phase which can produce spontenous re-xtalisation process (which
is do not wanted in glass manufacturing), (devitrification). They can brazing at 700°C and also they have
zero porosity, high strength, toughness, translucency or opacity, pigmentation,
opalescence, low or even negative thermal expansion, high temperature
stability, fluorescence, machinability, ferromagnetism, resorbability or high
chemical durability, biocompatibility, bio-activity, ion conductivity,
superconductivity, isolation capabilities, low dielectric constant and loss,
high resistivity and break down voltage.
In manufacturing glass-ceramic,
two phase follows: producing glass and adding nucleation agents during heat treatment
to control the re-xtalisation process. Nucleation agents has wide variety
systems like Li2O x Al2O3 x nSiO2-System
(LAS-System), MgO x Al2O3
x nSiO2-System (MAS-System),
ZnO x Al2O3 x nSiO2-System
(ZAS-System),etc.[1]
Only specific glass composites
suitable for precursors for glass-ceramics. As an example, ordinary window
glasses can not crystalise. Glass-Ceramics structure occured by 50vol% to
95vol% crystalline commonly. One or more crystalline phases may form during
heat treatment and as their composition is normally different from the
precursor, these are named parent. These parents make better specialities to
the material such as in LAS-systems, glass ceramics has thermal durability. [2]
[1] http://en.wikipedia.org/wiki/Glass-ceramic
[2] GLASS-CERAMICS: THEIR PRODUCTION FROM WASTES.
A REVIEW (R. D. Rawlings, J. P.
Wu, A. R. Boccaccini)
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