Glass-ceramic

Glass-ceramics are polycrystalline materials produced through controlled crystallization of base glass, producing a fine uniform dispersion of crystals throughout the bulk material. Crystallization is accomplished by subjecting suitable glasses to a carefully regulated heat treatment schedule, resulting in the nucleation and growth of crystal phases. In many cases, the crystallization process can proceed to near completion, but in a small proportion of processes, the residual glass phase often remains. Glass-ceramic materials share many properties with both glasses and ceramics. Glass-ceramics have an amorphous phase and one or more crystalline phases and are produced by a so-called "controlled crystallization" in contrast to a spontaneous crystallization, which is usually not wanted in glass manufacturing. Glass-ceramics have the fabrication advantage of glass, as well as special properties of ceramics. When used for sealing, some glass-ceramics do not require brazing but can withstand brazing temperatures up to 700 °C. Glass-ceramics usually have between 30% [m/m] and 90% [m/m] crystallinity and yield an array of materials with interesting properties like 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, bioactivity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, corrosion resistance, high resistivity and break-down voltage. These properties can be tailored by controlling the base-glass composition and by controlled heat treatment/crystallization of base glass. In manufacturing, glass-ceramics are valued for having the strength of ceramic but the hermetic sealing properties of glass. Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass-manufacturing process, after which the glass is cooled down.