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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 In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' ("wit ...
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 Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal. Brazing differs from we ...
but can withstand brazing temperatures up to 700 °C. Glass-ceramics usually have between 30% /mand 90% /mcrystallinity and yield an array of materials with interesting properties like zero porosity, high strength, toughness,
translucency In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...
or opacity, pigmentation,
opalescence Opalescence refers to the optical phenomena displayed by the mineraloid gemstone opalopalescent. 2019. In Noah Webster's 1828 American Dictionary of the English Language. Retrieved January 7, 2019, from https://1828.mshaffer.com/d/word/opalesc ...
, low or even negative thermal expansion, high temperature stability, fluorescence, machinability, ferromagnetism, resorbability or high chemical durability,
biocompatibility Biocompatibility is related to the behavior of biomaterials in various contexts. The term refers to the ability of a material to perform with an appropriate host response in a specific situation. The ambiguity of the term reflects the ongoing de ...
,
bioactivity In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or p ...
, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss,
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
resistance,McMillan, P. W. (1979). Glass-Ceramics (2nd ed.). Academic Press. high
resistivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...
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 seal A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...
ing 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. Second, the glass is put through a controlled heat treatment schedule. In this heat treatment the glass partly
crystallizes Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely de ...
. 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 no pores, unlike sintered ceramics. A wide variety of glass-ceramic systems exist, e.g., the Li2O × Al2O3 × ''n''SiO2 system (LAS system), the MgO × Al2O3 × ''n''SiO2 system (MAS system), the ZnO × Al2O3 × ''n''SiO2 system (ZAS system).


History

Réaumur, a French chemist, made early attempts to produce polycrystalline materials from glass, demonstrating that if glass bottles were packed into a mixture of sand and gypsum, and subjected to red heat for several days, the glass bottles turned opaque and porcelain-like. Although Réaumur was successful in the conversion of glass to a polycrystalline material, he was unsuccessful in achieving the control of the crystallization process, which is a key step in producing true practical glass ceramics with the improved properties mentioned above. The discovery of glass-ceramics is credited to a man named Donald Stookey, a renowned glass scientist who worked at
Corning Inc. Corning Incorporated is an American multinational technology company that specializes in specialty glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The c ...
for 47 years. The first iteration stemmed from a glass material, Fotoform, which was also discovered by Stookey while he was searching for a photo-etch-able material to be used in television screens. Soon after the beginning of Fotoform, the first ceramic material was discovered when Stookey overheated a Fotoform plate in a furnace at 900 degrees
Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...
and found an opaque, milky-white plate inside the furnace rather than the molten mess that was expected. While examining the new material, which Stookey aptly named Fotoceram, he took note that it was much stronger than the Fotoform that it was created from as it survived a short fall onto concrete. In the late 1950s two more glass-ceramic materials would be developed by Stookey, one found use as the
radome A radome (a portmanteau of radar and dome) is a structural, weatherproof enclosure that protects a radar antenna. The radome is constructed of material transparent to radio waves. Radomes protect the antenna from weather and conceal antenna ...
in the nose cone of missiles, while the other led to the line of consumer kitchenware known as
Corningware Corning Ware, also written CorningWare, was originally a brand name for a unique glass-ceramic (Pyroceram) cookware resistant to thermal shock. It was first introduced in 1958 by Corning Glass Works (later Corning Inc.) in the United States. The ...
. Corning executives announced Stookey's discovery of the latter "new basic material" called Pyroceram which was touted as light, durable, capable of being an electrical insulator and yet thermally shock resistant. At the time, there were only few materials which offered the specific combination of characteristics that Pyroceram did and the material was rolled out as the
Corningware Corning Ware, also written CorningWare, was originally a brand name for a unique glass-ceramic (Pyroceram) cookware resistant to thermal shock. It was first introduced in 1958 by Corning Glass Works (later Corning Inc.) in the United States. The ...
kitchen line August 7, 1958. Some of the success that Pyroceram brought inspired Corning to put an effort towards strengthening glass which became an effort by the technical director's of Corning titled Project Muscle. A lesser known "ultrastrong" glass-ceramic material developed in 1962 called Chemcor (now known as
Gorilla Glass Gorilla Glass is a brand of chemically strengthened glass developed and manufactured by Corning, now in its seventh generation. Designed to be thin, light and damage-resistant, the glass gains its surface strength, ability to contain flaws, and ...
) was produced by Corning's glass team due to the Project Muscle effort. Chemcor would even be used to innovate the Pyroceram line of products as in 1961 Corning launched Centura Ware, a new line of Pyroceram that was lined with a glass laminate (invented by John MacDowell) and treated with the Chemcor process. Stookey continued to forge ahead in the discovery of the properties of glass-ceramics as he discovered how to make the material transparent in 1966. Though Corning wouldn't release a product with his new innovation, for fear of cannibalizing Pyrex sales, until the late 1970s under the name Visions.


Nucleation and Crystal Growth

The key to engineering a glass-ceramic material is controlling the nucleation and growth of crystals in the base glass. The amount of crystallinity will vary depending on the amount of nuclei present and the time and temperature at which the material is heated. It is important to understand the types of nucleation occurring in the material, whether it is homogeneous or heterogeneous. Homogeneous nucleation is a process resulting from the inherent thermodynamic instability of a glassy material. When enough thermal energy is applied to the system, the
metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...
glassy phase begins to return to the lower-energy, crystalline state. The term "homogeneous" is used here because the formation of nuclei comes from the base glass without any second phases or surfaces promoting their formation. The rate of homogenous nucleation in a condensed system can be described with the following equation, proposed by Becker in 1938. ::I\ =\ A \exp \left( -\frac \right) Where Q is the activation energy for diffusion across the phase boundary, A is a constant, and F^* is the maximum activation energy for formation of a stable nucleus, as given by the equation below. ::\Delta F^* = \frac Where \Delta f_v is the change of free energy per unit volume resulting from the transformation from one phase to the other, and \Delta f_s can be equated with interfacial tension. Heterogeneous nucleation is a term used when a nucleating agent is introduced into the system to aid and control the crystallization process. The presence of this nucleating agent, in the form of an additional phase or surface, can act as a catalyst for nucleation and is particularly effective if there is epitaxy between the nucleus and the substrate. There are a number of metals that can act as nucleating agents in glass because they can exist in the glass in the form of particle dispersion of colloidal dimensions. Examples include copper, metallic silver, and platinum. It was suggested by Stookey in 1959 that the effectiveness of metallic nucleation catalysts relates to the similarities between the crystal structures of the metals and the phase being nucleated. The most important feature of heterogenous nucleation is that the interfacial tension between the heterogeneity and the nucleated phase is minimized. This means that the influence that the catalyzing surface has on the rate of nucleation is determined by the contact angle at the interface. Based on this, Turnbull and Vonnegut (1952) modified the equation for homogenous nucleation rate to give an expression for heterogenous nucleation rate. ::I_c\ =\ A^1 \exp \left( -\frac \right) If activation energy for diffusion is included, as suggested by Stokey (1959a), the equation then becomes: ::I_c\ =\ A^1 \exp \left( -\frac \right) From these equations, heterogeneous nucleation can be described in terms of the same parameters as homogeneous nucleation with a shape factor, which is a function of θ (contact angle). The term f(\theta) is given by: f(\theta)=\frac if the nucleus has the form of a spherical cap. In addition to nucleation, crystal growth is also required for the formation of glass ceramics. The crystal growth process is of considerable importance in determining the morphology of the produced glass ceramic composite material. Crystal growth is primarily dependent on two factors. First, it is dependent upon the rate at which the disordered structure can be re-arranged into a periodic lattice with longer-range order. Second, it is dependent upon the rate at which energy is released in the phase transformation (essentially the rate of cooling at the interface).


Glass Ceramics in Medical Applications

Glass-ceramics are used in medical applications due to their unique interaction, or lack thereof, with human body tissue. Bioceramics are typically placed into the following groups based on their biocompatibility: biopassive (bioinert), bioactive, or resorbable ceramics. Biopassive (bioinert) ceramics are, as the name suggests, characterized by the limited interaction the material has with the surrounding biological tissue. Historically, these were the "first generation" biomaterials used as replacements for missing or damaged tissues. One problem resulting from using inert biomaterials was the body's reaction to the foreign object; it was found that a phenomenon known as "fibrous encapsulation" would occur, where tissues would grow around the implant in an attempt to isolate the object from the rest of the body. This occasionally caused a variety of problems such as necrosis or sequestration of the implant. Two commonly used bioinert materials are alumina (Al2O3) and zirconia (ZrO2). Bioactive materials have the ability to form bonds and interfaces with natural tissues. In the case of bone implants, two properties known as osteoconduction and osteoinduction play an important role in the success and longevity of the implant. Osteoconduction refers to a material's ability to permit bone growth on the surface and into the pores and channels of the material. Osteoinduction is a term used when a material stimulates existing cells to proliferate, causing new bone to grow independently of the implant. In general, the bioactivity of a material is a result of a chemical reaction, typically dissolution of the implanted material. Calcium phosphate ceramics and bioactive glasses are commonly used as bioactive materials as they exhibit this dissolution behavior when introduced to living body tissue. One engineering goal relating to these materials is that the dissolution rate of the implant be closely matched to the growth rate of new tissue, leading to a state of dynamic equilibrium. Resorbable ceramics are similar to bioactive ceramics in their interaction with the body, but the main difference lies in the extent to which the dissolution occurs. Resorbable ceramics are intended to gradually dissolve entirely, all the while new tissue grows in its stead. The architecture of these materials has become quite complex, with foam-like scaffolds being introduced to maximize the interfacial area between the implant and body tissue. One issue that arises from using highly porous materials for bioactive/resorbable implants is the low mechanical strength, especially in load-bearing areas such as the bones in the legs. An example of a resorbable material that has seen some success is tricalcium phosphate (TCP), however, it too falls short in terms of mechanical strength when used in high-stress areas.


LAS System

The commercially most important system is the Li2O × Al2O3 × ''n''SiO2 system (LAS system). The LAS system mainly refers to a mix of lithium, silicon, and
aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It h ...
oxides 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. An interesting property of these glass-ceramics is their thermomechanical durability. 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 Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
(CTE), keatite-solid solution as still a negative CTE but much higher than HQ s.s. These negative CTEs of the crystalline phase contrasts with the positive CTE of the residual glass. Adjusting the proportion of these phases offers a wide range of possible CTEs 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% /mand 80% /mcrystallinity, 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 mirrors and mirror mounts of astronomical
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
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 Cookware and bakeware is food preparation equipment, such as cooking pots, pans, baking sheets etc. used in kitchens. Cookware is used on a stove or range cooktop, while bakeware is used in an oven. Some utensils are considered both cookware ...
or as high-performance reflectors for digital projectors.


Ceramic Matrix Composites

One particularly notable use of glass-ceramics is in the processing of
ceramic matrix composite In materials science, ceramic matrix composites (CMCs) are a subgroup of composite materials and a subgroup of ceramics. They consist of ceramic fibers embedded in a ceramic matrix. The fibers and the matrix both can consist of any ceramic mate ...
s. For many ceramic matrix composites typical sintering temperatures and times cannot be used, as the degradation and corrosion of the constituent fibres becomes more of an issue as temperature and sintering time increase. One example of this is SiC fibres, which can start to degrade via pyrolysis at temperatures above 1470K. One solution to this is to use the glassy form of the ceramic as the sintering feedstock rather than the ceramic, as unlike the ceramic the glass pellets have a softening point and will generally flow at much lower pressures and temperatures. This allows the use of less extreme processing parameters, making the production of many new technologically important fibre-matrix combinations by sintering possible.


Glass Ceramics in Cooktops

Glass-ceramic from the LAS-System is a mechanically strong material and can sustain repeated and quick temperature changes. However, it is not 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). The material has a very low heat conduction coefficient, which means that it stays cool outside the cooking area. It can be made nearly transparent (15–20% loss in a typical cooktop) for radiation in the infrared wavelengths. 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 A stove or range is a device that burns fuel or uses electricity to generate heat inside or on top of the apparatus, to be used for general warming or cooking. It has evolved highly over time, with cast-iron and induction versions being develope ...
s with cooktops made of glass-ceramic: * A glass-ceramic stove uses
radiant heating Radiant heating and cooling is a category of HVAC technologies that exchange heat by both convection and radiation with the environments they are designed to heat or cool. There are many subcategories of radiant heating and cooling, including: ...
coils or infrared halogen 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 stove heats a
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
pot's bottom directly through
electromagnetic induction Electromagnetic or magnetic induction is the production of an electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1831, and James Clerk ...
. This technology is not entirely new, as glass-ceramic ranges were first introduced in the 1970s using
Corningware Corning Ware, also written CorningWare, was originally a brand name for a unique glass-ceramic (Pyroceram) cookware resistant to thermal shock. It was first introduced in 1958 by Corning Glass Works (later Corning Inc.) in the United States. The ...
tops instead of the more durable material used today. These first generation smoothtops were problematic and could only be used with flat-bottomed cookware as the heating was primarily conductive rather than radiative. 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. If food with a high sugar content (such as jam) spills, it should never be allowed to dry on the surface, otherwise damage will occur. For best results and maximum heat transfer, all cookware should be flat-bottomed and matched to the same size as the burner zone.


Industry and Material Variations

Some well-known brands of glass-ceramics are Pyroceram, Ceran, Eurokera,
Zerodur Zerodur (notation of the manufacturer: ZERODUR®), registered trademarkof Schott AG, 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 GTC, Keck ...
, and Macor.
Nippon Electric Glass , also known as NEG, is a Japanese glass manufacturer. The company is a manufacturer of glass for flat panel displays (FPD). It has about 20% share in the world's production of glass for liquid crystal displays (LCD). The company is listed on th ...
is a predominant worldwide manufacturer of glass ceramics, whose related products in this area include FireLit

and NeoCera

ceramic glass materials for architectural and high temperature applications respectively
Keralite
manufactured b
Vetrotech
Saint-Gobain, is a specialty glass-ceramic fire and impact safety rated material for use in fire-rated applications. Glass-ceramics manufactured in the Soviet Union/
Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eight ...
are known under the name '' Sitall''. Macor is a white, odorless, porcelain-like glass ceramic material and was developed originally to minimize heat transfer during manned spaceflight by Corning Inc. StellaShine, launched in 2016 by Nippon Electric Glass Co., is a heat-resistant, glass-ceramic material with a thermal shock resistance of up to 800 degrees Celsius. This was developed as an addition to
Nippon Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
's line of heat-resistant cooking range plates along with materials lik
NeoceramKangerTech
is an ecigarette manufacturer which began in Shenzen, China which produces glass ceramic materials and other special hardened-glass applications like vaporizer modification tanks. The same class of material is also used in Visions and
CorningWare Corning Ware, also written CorningWare, was originally a brand name for a unique glass-ceramic (Pyroceram) cookware resistant to thermal shock. It was first introduced in 1958 by Corning Glass Works (later Corning Inc.) in the United States. The ...
glass-ceramic cookware, allowing it to be taken from the freezer directly to the stovetop or oven with no risk of thermal shock while maintaining the transparent look of glassware.


Sources


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. {{DEFAULTSORT:Glass-Ceramic American inventions 01 Ceramic materials Glass engineering and science Glass chemistry