GeSbTe is a phase-change material from the group of chalcogenide glasses used in rewritable optical discs and phase-change memory applications. Its recrystallization time is 20 nanoseconds, allowing bitrates of up to 35 Mbit/s to be written and direct overwrite capability up to 106 cycles, it is suitable for land-groove recording formats. It is used in rewritable DVDs. New phase-change memories are possible using n-doped GeSbTe semiconductor; the melting point of the alloy is about 600 °C and the crystallization temperature is between 100 and 150 °C. During writing, the material is erased, initialized into its crystalline state, with low-intensity laser irradiation; the material heats up to its crystallization temperature, but not its melting point, crystallizes. The information is written at the crystalline phase, by heating spots of it with short, high-intensity laser pulses; as the amorphous phase has lower reflectivity than the crystalline phase, data can be recorded as dark spots on the crystalline background.
Novel liquid organogermanium precursors, such as isobutylgermane and tetrakisgermane were developed and used in conjunction with the metalorganics of antimony and tellurium, such as tris-dimethylamino antimony and di-isopropyl telluride to grow GeSbTe and other chalcogenide films of high purity by metalorganic chemical vapor deposition. Dimethylamino germanium trichloride is reported as the chloride containing and a superior dimethylaminogermanium precursor for Ge deposition by MOCVD. GeSbTe is a ternary compound of germanium and tellurium, with composition GeTe-Sb2Te3. In the GeSbTe system, there is a pseudo-line as shown upon. Moving down this pseudo-line, it can be seen that as we go from Sb2Te3 to GeTe, the melting point and glass transition temperature of the materials increase, crystallization speed decreases and data retention increases. Hence, in order to get high data transfer rate, we need to use material with fast crystallization speed such as Sb2Te3; this material is not stable because of its low activation energy.
On the other hand, materials with good amorphous stability like GeTe has slow crystallization speed because of its high activation energy. In its stable state, crystalline GeSbTe has two possible configurations: hexagonal and a metastable face centered cubic lattice; when it is crystallized however, it was found to have a distorted rocksalt structure. GeSbTe has a glass transition temperature of around 100 °C. GeSbTe has many vacancy defects in the lattice, of 20 to 25% depending on the specific GeSbTe compound. Hence, Te has an extra lone pair of electrons, which are important for many of the characteristics of GeSbTe. Crystal defects are common in GeSbTe and due to these defects, an Urbach tail in the band structure is formed in these compounds. GeSbTe is p type and there are many electronic states in the band gap accounting for acceptor and donor like traps. GeSbTe has two stable states and amorphous; the phase change mechanism from high resistance amorphous phase to low resistance crystalline phase in nano-timescale and threshold switching are two of the most important characteristic of GeSbTe.
The unique characteristic that makes phase-change memory useful as a memory is the ability to effect a reversible phase change when heated or cooled, switching between stable amorphous and crystalline states. These alloys have high resistance in the amorphous state ‘0’ and are semimetals in the crystalline state ‘1’. In amorphous state, the atoms have low free electron density; the alloy has high resistivity and activation energy. This distinguishes it from the crystalline state having low resistivity and activation energy, long-range atomic order and high free electron density; when used in phase-change memory, use of a short, high amplitude electric pulse such that the material reaches melting point and quenched changes the material from crystalline phase to amorphous phase is termed as RESET current and use of a longer, low amplitude electric pulse such that the material reaches only the crystallization point and given time to crystallize allowing phase change from amorphous to crystalline is known as SET current.
The early devices were slow, power consuming and broke down due to the large currents. Therefore, it did not succeed as flash memory took over. In the 1980s though, the discovery of germanium-antimony-tellurium meant that phase-change memory now needed less time and power to function; this resulted in the success of the rewriteable optical disk and created renewed interest in the phase-change memory. The advances in lithography meant that excessive programming current has now become much smaller as the volume of GeSbTe that changes phase is reduced. Phase-change memory has many near ideal memory qualities such as non-volatility, fast switching speed, high endurance of more than 1013 read –write cycles, non-destructive read, direct overwriting and long data retention time of more than 10 years; the one advantage that distinguishes it from other next generation non-volatile memory like magnetic random access memory is the unique scaling advantage of having better performance with smaller sizes.
The limit to which phase-change memory can be scaled is hence limited by lithography at least until 45 nm. Thus, it offers the biggest potential of achieving ultra-high memory density cells that can be commercialized. Though phase-change memory offers much promise, there are still certain technical problems that need to be
Ceramic glaze is an impervious layer or coating of a vitreous substance, fused to a ceramic body through firing. Glaze can decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding liquids, sealing the inherent porosity of unglazed biscuit earthenware, it gives a tougher surface. Glaze is used on stoneware and porcelain. In addition to their functionality, glazes can form a variety of surface finishes, including degrees of glossy or matte finish and color. Glazes may enhance the underlying design or texture either unmodified or inscribed, carved or painted. Most pottery produced in recent centuries has been glazed, other than pieces in unglazed biscuit porcelain, terracotta, or some other types. Tiles are always glazed on the surface face, modern architectural terracotta is often glazed. Glazed brick is common. Domestic sanitary ware is invariably glazed, as are many ceramics used in industry, for example ceramic insulators for overhead power lines; the most important groups of traditional glazes, each named after its main ceramic fluxing agent, are: Ash glaze, important in East Asia made from wood or plant ash, which contains potash and lime.
Feldspathic glazes of porcelain. Lead-glazed earthenware, is shiny and transparent after firing, which needs only about 800 °C, it has been used for about 2,000 years around the Mediterranean, in Europe, China. It includes Victorian majolica. Salt-glazed ware European stoneware, it uses ordinary salt. Tin-glazed pottery, which coats the ware with lead glaze made opaque white by the addition of tin. Known in the Ancient Near East and important in Islamic pottery, from which it passed to Europe. Includes faience, maiolica and Delftware. Modern materials technology has invented new vitreous glazes that do not fall into these traditional categories. Glazes need to include a ceramic flux which functions by promoting partial liquefaction in the clay bodies and the other glaze materials. Fluxes lower the high melting point of the glass formers silica, sometimes boron trioxide; these glass formers may be drawn from the clay beneath. Raw materials of ceramic glazes include silica, which will be the main glass former.
Various metal oxides, such as sodium and calcium, act as flux and therefore lower the melting temperature. Alumina derived from clay, stiffens the molten glaze to prevent it from running off the piece. Colorants, such as iron oxide, copper carbonate, or cobalt carbonate, sometimes opacifiers like tin oxide or zirconium oxide, are used to modify the visual appearance of the fired glaze. Glaze may be applied by dry-dusting a dry mixture over the surface of the clay body or by inserting salt or soda into the kiln at high temperatures to create an atmosphere rich in sodium vapor that interacts with the aluminium and silica oxides in the body to form and deposit glass, producing what is known as salt glaze pottery. Most glazes in aqueous suspension of various powdered minerals and metal oxides are applied by dipping pieces directly into the glaze. Other techniques include pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, or applying it directly with a brush or other tool.
To prevent the glazed article from sticking to the kiln during firing, either a small part of the item is left unglazed, or it's supported on small refractory supports such as kiln spurs and Stilts that are removed and discarded after the firing. Small marks left by these spurs are sometimes visible on finished ware. Decoration applied under the glaze on pottery is referred to as underglaze. Underglazes are applied to the surface of the pottery, which can be either raw, "greenware", or "biscuit"-fired. A wet glaze—usually transparent—is applied over the decoration; the pigment fuses with the glaze, appears to be underneath a layer of clear glaze. An example of underglaze decoration is the well-known "blue and white" porcelain famously produced in Germany, the Netherlands and Japan; the striking blue color uses cobalt as cobalt cobalt carbonate. Decoration applied on top of a layer of glaze is referred to as overglaze. Overglaze methods include applying one or more layers or coats of glaze on a piece of pottery or by applying a non-glaze substance such as enamel or metals over the glaze.
Overglaze colors are low-temperature glazes. A piece is fired first, this initial firing being called the glost firing the overglaze decoration is applied, it is fired again. Once the piece is fired and comes out of the kiln, its texture is smoother due to the glaze. Glazing of ceramics developed rather as appropriate materials needed to be discovered, firing technology able to reliably reach the necessary temperatures was needed. Glazed brick goes back to the Elamite Temple at Chogha Zanbil, dated to the 13th century BC; the Iron Pagoda, built in 1049 in Kaifeng, China, of glazed bricks is a well-known example. Lead glazed earthenware was made in China during the Warring States Period, its production increased during the Han Dynasty. High temperature proto-celadon glazed stoneware was made earlier than glazed earthenware, since the Shang Dynasty. During the Kofun period of Japan, Sue ware was decorated with greenish natural ash glazes. From 552 to 794 AD, differently colored glazes were introduced.
The three colored glazes of the Tang Dynasty were used for a period, but were phased out.
Milk glass is an opaque or translucent, milk white or colored glass that can be blown or pressed into a wide variety of shapes. First made in Venice in the 16th century, colors include blue, yellow, brown and the white that led to its popular name. Milk glass contains dispersion of particles with refractive index different from the glass matrix, which scatter light by the Tyndall scattering mechanism; the size distribution and density of the particles control the overall effect, which may range from mild opalization to opaque white. Some glasses are somewhat more blue from the side, somewhat red-orange in pass-through light; the particles are produced via addition of opacifiers to the melt. Some opacifiers only dispersed in the melt. Others are added as precursors and react in the melt, or dissolve in the molten glass and precipitate as crystals on cooling; the opacifiers can be tin dioxide and arsenic and antimony compounds. They are added to ceramic glazes, which can be chemically considered to be a specific kind of milk glass.
First made in Venice in the 16th century, colors include blue, yellow, brown and white. Some 19th-century glass makers called milky white opaque glass "opal glass"; the name milk glass is recent. Made into decorative dinnerware, lamps and costume jewellery, milk glass was popular during the fin de siècle. Pieces made for the wealthy of the Gilded Age are known for their delicacy and beauty in color and design, while Depression glass pieces of the 1930s and 1940s are less so. Milk glass is used for architectural decoration when one of the underlying purposes is the display of graphic information; the original milk glass marquee of the Chicago Theatre has been donated to the Smithsonian Institution. A famous use of milk glass is for the four faces of the information booth clock at Grand Central Terminal in New York City. Milk glass has a considerable following of collectors. Glass makers continue to produce both original pieces and reproductions of popular collectible pieces and patterns. Kanawha Glass Co.
Fenton Glass Company Fostoria Glass Company Imperial Glass Company Mosser Glass Dithridge & Company Westmoreland Glass Company Federal Glass LaOpala RG limited L. E. Smith Glass Company Thai Soojung Glass Company Limited National Milk Glass Collectors Society National Westmoreland Glass Collectors Club
Chalcogenide glass is a glass containing one or more chalcogens. Such glasses may be classified as covalent network solids. Polonium is a chalcogen but is not used because of its strong radioactivity. Chalcogenide materials behave rather differently from oxides, in particular their lower band gaps contribute to dissimilar optical and electrical properties; the classical chalcogenide glasses are strong glass-formers and possess glasses within large concentration regions. Glass-forming abilities decrease with increasing molar weight of constituent elements. Chalcogenide compounds such as AgInSbTe and GeSbTe are used in rewritable optical disks and phase-change memory devices, they are fragile glass-formers: by controlling heating and annealing, they can be switched between an amorphous and a crystalline state, thereby changing their optical and electrical properties and allowing the storage of information. Most stable binary chalcogenide glasses are compounds of a chalcogen and a group 14 or 15 element and may be formed in a wide range of atomic ratios.
Ternary glasses are known. Not all chalcogenide compositions exist in glassy form, though it is possible to find materials with which these non-glass-forming compositions can be alloyed in order to form a glass. An example of this is gallium sulphide-based glasses. Gallium sulphide on its own is not a known glass former. Uses include infrared detectors, mouldable infrared optics such as lenses, infrared optical fibers, with the main advantage being that these materials transmit across a wide range of the infrared electromagnetic spectrum; the physical properties of chalcogenide glasses make them ideal for incorporation into lasers, planar optics, photonic integrated circuits, other active devices if doped with rare-earth element ions. Some chalcogenide glasses exhibit several non-linear optical effects such as photon-induced refraction, electron-induced permittivity modificationSome chalcogenide materials experience thermally driven amorphous-to-crystalline phase changes; this makes them useful for encoding binary information on thin films of chalcogenides and forms the basis of rewritable optical discs and non-volatile memory devices such as PRAM.
Examples of such phase change materials are AgInSbTe. In optical discs, the phase change layer is sandwiched between dielectric layers of ZnS-SiO2, sometimes with a layer of a crystallization promoting film. Other less used such materials are InSe, SbSe, SbTe, InSbSe, InSbTe, GeSbSe, GeSbTeSe and AgInSbSeTe. Intel claims that its chalcogenide-based 3D XPoint memory technology achieves throughput and write durability 1,000 times higher than flash memory. Electrical switching in chalcogenide semiconductors emerged in the 1960s, when the amorphous chalcogenide Te48As30Si12Ge10 was found to exhibit sharp, reversible transitions in electrical resistance above a threshold voltage. If current is allowed to persist in the non-crystalline material, it heats up and changes to crystalline form; this is equivalent to information being written on it. A crystalline region may be melted by exposure to a intense pulse of heat. Subsequent rapid cooling sends the melted region back through the glass transition.
Conversely, a lower-intensity heat pulse of longer duration will crystallize an amorphous region. Attempts to induce the glassy–crystal transformation of chalcogenides by electrical means form the basis of phase-change random-access memory; this technology has been developed to near commercial use by ECD Ovonics. For write operations, an electric current supplies the heat pulse; the read process is performed at sub-threshold voltages by utilizing the large difference in electrical resistance between the glassy and crystalline states. Examples of such phase change materials are AgInSbTe; the semiconducting properties of chalcogenide glasses were revealed in 1955 by B. T. Kolomiets and N. A. Gorunova from Ioffe Institute, USSR. Although the electronic structural transitions relevant to both optical discs and PC-RAM were featured contributions from ions were not considered—even though amorphous chalcogenides can have significant ionic conductivities. At Euromat 2005, however, it was shown that ionic transport can be useful for data storage in a solid chalcogenide electrolyte.
At the nanoscale, this electrolyte consists of crystalline metallic islands of silver selenide dispersed in an amorphous semiconducting matrix of germanium selenide. The electronic applications of chalcogenide glasses have been an active topic of research throughout the second half of the twentieth century and beyond For example, the migration of dissolved ions is required in the electrolytic case, but could limit the performance of a phase-change device. Diffusion of both electrons and ions participate in electromigration—widely studied as a degradation mechanism of the electrical conductors used in modern integrated circuits. Thus, a unified approach to the study of chalcogenides, assessing the collective roles of atoms and electrons, may prove essential for both device performance and reliability. Zakery, A.. R. Elliott. Optical nonlinearities in their applications. New York: Springer. ISBN 9783540710660. Frumar, M.. "4.07: Amorphous and Glassy Semiconducting Chalcogenides". In Pallab Bhattacharya, Roberto Fornari, Hiroshi Kamimura.
Sodium silicate is a generic name for chemical compounds with the formula Na2xSiO2+x or x·SiO2, such as sodium metasilicate Na2SiO3, sodium orthosilicate Na4SiO4, sodium pyrosilicate Na6Si2O7. The anions are polymeric; these compounds are colorless transparent solids or white powders, soluble in water in various amounts. Sodium silicate is the technical and common name for a mixture of such compounds, chiefly the metasilicate called waterglass, water glass, or liquid glass; the product has a wide variety of uses, including the formulation of cements, passive fire protection and lumber processing, manufacture of refractory ceramics, as adhesives, in the production of silica gel. The commercial product, available in water solution or in solid form, is greenish or blue owing to the presence of iron-containing impurities. In industry, the various grades of sodium silicate are characterized by their SiO2:Na2O weight ratio; the ratio can vary between 2:1 and 3.75:1. Grades with ratio below 2.85:1 are termed alkaline.
Those with a higher SiO2:Na2O ratio are described as neutral. Soluble silicates of alkali metals were observed by European alchemists in the 1500s. Giambattista della Porta observed in 1567 that tartari salis caused powdered crystallum to melt at a lower temperature. Other possible early references to alkali silicates were made by Basil Valentine in 1520, by Agricola in 1550. Around 1640, Jean Baptist van Helmont reported the formation of alkali silicates as a soluble substance made by melting sand with excess alkali, observed that the silica could be precipitated quantitatively by adding acid to the solution. In 1646. Glauber made potassium silicate, that he termed liquor silicum by melting potassium carbonate and sand in a crucible, keeping it molten until it ceased to bubble; the mixture was allowed to cool and was ground to a fine powder. When the powder was exposed to moist air, it formed a viscous liquid, which Glauber called "Oleum oder Liquor Silicum, Arenæ, vel Crystallorum". However, it was claimed that the substances prepared by those alchemists were not waterglass as it is understood today.
That would have been prepared in 1818 by Johann Nepomuk von Fuchs, by treating silicic acid with an alkali. The terms "water glass" and "soluble glass" were used by Leopold Wolff in 1846, by Émile Kopp in 1857, by Hermann Krätzer in 1887. In 1892, Rudolf Von Wagner distinguished soda, potash and fixing as types of water glass; the fixing type was "a mixture of silica well saturated with potash water glass and a sodium silicate" used to stabilize inorganic water color pigments on cement work for outdoor signs and murals. Sodium silicates are colorless white powders. Except for the most silicon-rich ones, they are soluble in water, producing alkaline solutions. Sodium silicates are stable in alkaline solutions. In acidic solutions, the silicate ions react with hydrogen ions to form silicic acids, which tend to decompose into hydrated siliconon dioxide gel. Heated to drive off the water, the result is a hard translucent substance called silica gel used as a desiccant. Solutions of sodium silicates can be produced by treating a mixture of silica, caustic soda, water, with hot steam in a reactor.
The overall reaction is 2x NaOH + SiO2 → x·SiO2 + x H2OSodium silicates can be obtained by dissolving silica SiO2 in molten sodium carbonate: x Na2CO3 + SiO2 → x·SiO2 + CO2The material can be obtained from sodium sulfate with carbon as a reducing agent: 2x Na2SO4 + C + 2 SiO2 → 2 x·SiO2 + 2 SO2 + CO2In 1990, 4 million tons of alkali metal silicates were produced. The main applications of sodium silicates are in detergents, water treatment, construction materials; the largest application of sodium silicate solutions is a cement for producing cardboard. When used as a paper cement, the tendency is for the sodium silicate joint to crack within a few years, at which point it no longer holds the paper surfaces cemented together. Sodium silicate is used in drilling fluids to stabilize borehole walls and to avoid the collapse of bore walls, it is useful when drill holes pass through argillaceous formations containing swelling clay minerals such as smectite or montmorillonite. Concrete treated with a sodium silicate solution helps to reduce porosity in most masonry products such as concrete and plasters.
This effect aids in reducing water penetration, but has no known effect on reducing water vapor transmission and emission. A chemical reaction occurs with the excess Ca2 present in the concrete that permanently binds the silicates with the surface, making them far more durable and water repellent; this treatment is applied only after the initial cure has taken place. These coatings are known as silicate mineral paint, it is used in detergent auxiliaries such as complex sodium disilicate and modified sodium disilicate. The detergent granules gain their ruggedness from a coating of silicates. Sodium silicate is used as an iron flocculant in wastewater treatment plants. Sodium silicate binds to colloidal molecules, creating larger agg