Glass-ceramic
Encyclopedia
Glass-ceramics are polycrystalline material produced through controlled crystallization of base glass. Glass-ceramic materials share many properties with both glass
es and ceramic
s. 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. Glass-ceramics usually have between 30% [m/m] to 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, bio-activity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, 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.
Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass manufacturing process. The glass is cooled down and is then reheated in a second step. In this heat treatment the glass partly crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic. These nucleation agents aid and control the crystallization process. Because there is usually no pressing and sintering, glass-ceramics have, unlike sintered ceramics, no pores.
A wide variety of glass-ceramic systems exists, e.g. the Li2O x Al2O3 x nSiO2-System (LAS-System), the MgO x Al2O3 x nSiO2-System (MAS-System), the ZnO x Al2O3 x nSiO2-System (ZAS-System).
-, silicon
-, and aluminum-oxide
s with additional components e.g. glass-phase forming agents such as Na2O, K2O and CaO and refining agents. As nucleation agents most commonly zirconium(IV)-oxide in combination with titanium(IV)-oxide is used. This important system was studied first and intensively by Hummel and Smoke.
After crystallization the dominant crystal-phase in this type of glass-ceramic is a high-quartz solid solution (HQ s.s.). If the glass-ceramic is subjected to a more intense heat treatment, this HQ s.s. transforms into a keatite-solid solution (K s.s., sometimes wrongly named as beta-spodumene
). This transition is non-reversible and reconstructive, which means bonds in the crystal-lattice are broken and new arranged. However, these two crystal phases show a very similar structure as Li could show.
The most interesting properties of these glass-ceramics are their thermomechanical properties. Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes up to 800–1000 °C. The dominant crystalline phase of the LAS-glass-ceramics, HQ s.s., has a strong negative coefficient of thermal expansion (CTE), keatite-solid solution as still a negative CTE but much higher than HQ s.s.. These negative CTE's of the crystal-phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTE's in the finished composite. Mostly for today's applications a low or even zero CTE is desired. Also a negative CTE is possible, which means, in contrast to most materials when heated up, such a glass-ceramic contracts. At a certain point, generally between 60% [m/m] and 80% [m/m] crystallinity, the two coefficients balance such that the glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero. Also, when an interface between material will be subject to thermal fatigue
, glass-ceramics can be adjusted to match the coefficient of the material they will be bonded to.
Originally developed for use in the mirror
s and mirror mounts of astronomical telescope
s, LAS-glass-ceramics have become known and entered the domestic market through its use in glass-ceramic cooktops
, as well as cookware and bakeware
or as high performance reflectors for digital projectors.
At the same time, it has a very low heat conduction coefficient
and can be made nearly transparent (15–20% loss in a typical cooktop) for radiation in the infrared
wavelength
s.
In the visible range glass-ceramics can be transparent, translucent or opaque and even colored by coloring agents.
, there are two major types of electrical stove
s with cooktops made of glass-ceramic:
Some well-known brands of glass-ceramics are Ceran (cooktops), Eurokera (cooktop, stoves and fireplaces), Zerodur
(telescope mirrors), and Macor
. German
manufacturer Schott
introduced Zerodur in 1968, Ceran followed in 1971. Nippon Electric Glass of Japan is another worldwide manufacturer of glass ceramics, whose related products in this area include Firelite and Neoceram fire-rated glass.
The same class of material was also used, until the late 1990s, in Corningware
dishes, which could be taken from the freezer directly to the oven with no risk of thermal shock.
It is interesting to note that this technology is not entirely new, as glass-ceramic ranges were first introduced in the 1970s using Corningware
tops instead of the more durable material used today. These first generation smoothtops were problematic and could only be used with flat-bottomed cookware.http://www.automaticwasher.org/TD/ARCHIVE/VINTAGE/2005/918x16.htm
Compared to conventional kitchen stoves, glass-ceramic cooktops are relatively simple to clean, due to their flat surface. However, glass-ceramic cooktops can be scratched very easily, so care must be taken not to slide the cooking pans over the surface. Food with a high sugar content (such as jam) should never be allowed to dry on the surface if it spills, otherwise damage will occur. Cleaning is best carried out by using a soft cloth along with a special glass-ceramic cleaner that applies a thin protective film on the glass. http://www.eurokera.com/products/cooking/cleaning.html
For best results and maximum heat transfer, all cookware should be flat-bottomed and matched to the same size as the burner zone.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
es and ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s. 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. Glass-ceramics usually have between 30% [m/m] to 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, bio-activity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, 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.
Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass manufacturing process. The glass is cooled down and is then reheated in a second step. In this heat treatment the glass partly crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic. These nucleation agents aid and control the crystallization process. Because there is usually no pressing and sintering, glass-ceramics have, unlike sintered ceramics, no pores.
A wide variety of glass-ceramic systems exists, e.g. the Li2O x Al2O3 x nSiO2-System (LAS-System), the MgO x Al2O3 x nSiO2-System (MAS-System), the ZnO x Al2O3 x nSiO2-System (ZAS-System).
LAS System
The commercially most important system is the Li2O x Al2O3 x nSiO2-System (LAS-System). The LAS-system mainly refers to a mix of lithiumLithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
-, silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-, and aluminum-oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s with additional components e.g. glass-phase forming agents such as Na2O, K2O and CaO and refining agents. As nucleation agents most commonly zirconium(IV)-oxide in combination with titanium(IV)-oxide is used. This important system was studied first and intensively by Hummel and Smoke.
After crystallization the dominant crystal-phase in this type of glass-ceramic is a high-quartz solid solution (HQ s.s.). If the glass-ceramic is subjected to a more intense heat treatment, this HQ s.s. transforms into a keatite-solid solution (K s.s., sometimes wrongly named as beta-spodumene
Spodumene
Spodumene is a pyroxene mineral consisting of lithium aluminium inosilicate, LiAl2, and is a source of lithium. It occurs as colorless to yellowish, purplish, or lilac kunzite , yellowish-green or emerald-green hiddenite, prismatic crystals, often of great size...
). This transition is non-reversible and reconstructive, which means bonds in the crystal-lattice are broken and new arranged. However, these two crystal phases show a very similar structure as Li could show.
The most interesting properties of these glass-ceramics are their thermomechanical properties. Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes up to 800–1000 °C. The dominant crystalline phase of the LAS-glass-ceramics, HQ s.s., has a strong negative coefficient of thermal expansion (CTE), keatite-solid solution as still a negative CTE but much higher than HQ s.s.. These negative CTE's of the crystal-phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTE's in the finished composite. Mostly for today's applications a low or even zero CTE is desired. Also a negative CTE is possible, which means, in contrast to most materials when heated up, such a glass-ceramic contracts. At a certain point, generally between 60% [m/m] and 80% [m/m] crystallinity, the two coefficients balance such that the glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero. Also, when an interface between material will be subject to thermal fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
, glass-ceramics can be adjusted to match the coefficient of the material they will be bonded to.
Originally developed for use in the mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
s and mirror mounts of astronomical telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s, LAS-glass-ceramics have become known and entered the domestic market through its use in glass-ceramic cooktops
Kitchen stove
A kitchen stove, cooking stove, cookstove, or cooker is a kitchen appliance designed for the purpose of cooking food. Kitchen stoves rely on the application of direct heat for the cooking process and may also contain an oven, used for baking.In the industrialized world, as stoves replaced open...
, as well as cookware and bakeware
Cookware and bakeware
Cookware and bakeware are types of food preparation containers commonly found in the kitchen. Cookware comprises cooking vessels, such as saucepans and frying pans, intended for use on a stove or range cooktop. Bakeware comprises cooking vessels intended for use inside an oven...
or as high performance reflectors for digital projectors.
Cooktops
Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes. It is not, however, totally unbreakable. Because it is still a brittle material as glass and ceramics are, it can be broken. There have been instances where users reported damage to their cooktops when the surface was struck with a hard or blunt object (such as a can falling from above or other heavy items).At the same time, it has a very low heat conduction coefficient
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
and can be made nearly transparent (15–20% loss in a typical cooktop) for radiation in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s.
In the visible range glass-ceramics can be transparent, translucent or opaque and even colored by coloring agents.
Stove
A stove is an enclosed heated space. The term is commonly taken to mean an enclosed space in which fuel is burned to provide heating, either to heat the space in which the stove is situated or to heat the stove itself, and items placed on it...
s with cooktops made of glass-ceramic:
- A glass-ceramic stove uses radiantRadiant-Computers and software:* Radiant , a content management system* GtkRadiant, a level editor created by id Software for their games-Ships:* HMS Radiant , a destroyer of the British Royal Navy launched in 1916 and sold in 1920...
heating coils or infrared halogenHalogenThe halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
lamps as the heating elements. The surface of the glass-ceramic cooktop above the burner heats up, but the adjacent surface remains cool because of the low heat conduction coefficient of the material. - An induction stoveInduction cookerAn induction cooker uses induction heating for cooking. Unlike other forms of cooking, heat is generated directly in the pot or pan , as opposed to being generated in the stovetop by electrical coils or burning gas...
heats a metalMetalA metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
pot's bottom directly through electromagnetic inductionElectromagnetic inductionElectromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
.
Some well-known brands of glass-ceramics are Ceran (cooktops), Eurokera (cooktop, stoves and fireplaces), Zerodur
Zerodur
Zerodur®, a registered trademark of Schott Glass Technologies, is a lithium aluminosilicate glass-ceramic produced by Schott AG since 1968. It has been used for a number of very large telescope mirrors including Keck I and Keck II. With its very low coefficient of expansion it can be used to...
(telescope mirrors), and Macor
MACOR
MACOR is a machineable glass-ceramic developed and sold by Corning Incorporated. It is a white material that looks somewhat like porcelain. MACOR has excellent thermal characteristics, acting as efficient insulation, and stable up to temperatures of 1000 °C, with very little thermal expansion or...
. German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
manufacturer Schott
Schott Glass
SCHOTT AG is a German manufacturer of high-quality industrial glass products, its main markets are household appliances, pharmaceutical industries, solar energy, electronics, optics as well as automotive...
introduced Zerodur in 1968, Ceran followed in 1971. Nippon Electric Glass of Japan is another worldwide manufacturer of glass ceramics, whose related products in this area include Firelite and Neoceram fire-rated glass.
The same class of material was also used, until the late 1990s, in Corningware
Corningware
CorningWare was originally a brand name for a unique pyroceramic glass cookware resistant to thermal shock, that was first introduced in 1958 by Corning Glass Works. CorningWare is notable for the fact that it can be used directly on the stovetop.- History :In 1953, Dr. S...
dishes, which could be taken from the freezer directly to the oven with no risk of thermal shock.
It is interesting to note that this technology is not entirely new, as glass-ceramic ranges were first introduced in the 1970s using Corningware
Corningware
CorningWare was originally a brand name for a unique pyroceramic glass cookware resistant to thermal shock, that was first introduced in 1958 by Corning Glass Works. CorningWare is notable for the fact that it can be used directly on the stovetop.- History :In 1953, Dr. S...
tops instead of the more durable material used today. These first generation smoothtops were problematic and could only be used with flat-bottomed cookware.http://www.automaticwasher.org/TD/ARCHIVE/VINTAGE/2005/918x16.htm
Compared to conventional kitchen stoves, glass-ceramic cooktops are relatively simple to clean, due to their flat surface. However, glass-ceramic cooktops can be scratched very easily, so care must be taken not to slide the cooking pans over the surface. Food with a high sugar content (such as jam) should never be allowed to dry on the surface if it spills, otherwise damage will occur. Cleaning is best carried out by using a soft cloth along with a special glass-ceramic cleaner that applies a thin protective film on the glass. http://www.eurokera.com/products/cooking/cleaning.html
For best results and maximum heat transfer, all cookware should be flat-bottomed and matched to the same size as the burner zone.
Literature
- McMillan P.W., "The glass phase in glass-ceramics", Glass Technology, 1974, Vol. 15 (1), P. 5-15
- Bach H. (Editor), "Low thermal expansion glass ceramics", Springer-Verlag (1995).
- Holand, Wolfram and Beall, George H. Glass-Ceramic Technology. Wiley, 2002. ISBN 978-1-57498-107-0