Glass

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Glass is a transparent, non-crystalline, amorphous solid that has extensive practical, technological and decorative uses in, for example, window panels, tableware, and optoelectronics. The most familiar, and historically oldest, type of glass is the "silicate glass" based on a chemical compound of silica (silicon dioxide, or quartz), the main constituent of sand. The term glass , in popular use, is often used to refer only to this type of material, which is familiarly used as window glass and in glass bottles. Of the many silica-based glasses present, glaze glass and ordinary containers are formed from a special type of soda-lime glass, composed of about 75% silicon dioxide (SiO ₂ ), sodium oxide (Na). ₂ O) of sodium carbonate (Na ₂ CO ₃ ), calcium oxide (CaO), also called lime, and some small additives.

Many silicate glass applications come from their optical transparency, thus increasing their primary use as window panels. Glass will radiate, reflect, and reflect light; This quality can be enhanced by cutting and polishing to create optical lenses, prisms, fine cups, and fiber optics for high speed data transmission by light. Glass can be stained by adding metal salts, and can also be painted and printed with vitreous enamel. This quality has led to the use of extensive glass in the manufacture of art objects and in particular, stained glass windows. Although fragile, silicate glass is very durable, and many examples of glass fragments exist from early glass-making cultures. Since glass can be formed or shaped into any shape, it has traditionally been used for ships: bowls, vases, bottles, bottles and drinking glasses. In its most congested form it has also been used for paperweights, marbles, and beads. When extruded as glass fibers and crumpled as glass wool by trapping air, it becomes a thermal insulation material, and when these glass fibers are embedded into organic polymer plastics, they are a major structural reinforcing part of fiberglass composite material. Some objects are historically so commonly made of silicate glass that they are only called by the name of materials, such as glasses and glasses.

Scientifically, the term "glass" is often defined in a broader sense, including any solid having non-crystalline (ie, amorphous) structures on the atomic scale and which exhibit glass transitions when heated to a liquid state. Porcelains and many of the polymer thermoplastics known from everyday use are glasses. Such glasses can be made of different types of materials from silica: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. For many applications, such as glass bottles or glasses, polymer glasses (acrylic glass, polycarbonate or polyethylene terephthalate) are a lighter alternative than traditional glass.

Video Glass

Silicate Glass

Materials

Silica (SiO ₂ ) is a common basic constituent of glass. In nature, quartz vitrification occurs when lightning strikes the sand, forming hollow, branched off like a root structure called fulgurites.

Quartz melt is a glass made of pure chemical silica. It has excellent resistance to thermal shock, able to survive in low water while red hot. However, its high melting temperature (1723 ° C) and viscosity make it difficult to do. Usually, other substances are added to simplify processing. One is sodium carbonate (Na ₂ CO ₃ , "soda"), which lowers the glass-transition temperature. Soda makes water-soluble glasses, which are usually undesirable, so chalk (CaO, calcium oxide, generally obtained from limestone), some magnesium oxide (MgO) and aluminum oxide (Al ₂ O ₃ ) were added to provide better chemical resistance. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass. Sodium-lime glasses produce about 90% glass produced.

The most common glass contains other materials to change its properties. Stained glass or glass stones are more "brilliant" because of the increased refractive index leading to more speculative reflections and increased optical dispersion. Adding barium also increases the refractive index. Thorium oxide provides high refractive index glass and low dispersion and was previously used in producing high quality lenses, but because its radioactivity has been replaced by lanthanum oxide in modern glasses. Iron can be inserted into the glass to absorb infrared radiation, for example in a heat-absorbing filter for a film projector, while cerium (IV) oxide can be used for glass that absorbs ultraviolet wavelengths.

The following is a list of the more common types of silicate glasses and their materials, properties, and applications:

• Fused quartz , also called glass-silica glass , vitreous-silica glass : silica (SiO ₂ ) in the form of glass, or glass, (ie, the molecule is irregular and random, without a crystal structure). It has very low thermal expansion, very hard, and high temperature resistance (1000-1500 ° C). It is also the most resistant to weathering (caused by other glasses by alkaline ions coming out of the glass, tempting them). Fused quartz is used for high temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.
• Glass of soda-lime-silica , window glass : silica sodium oxide (Na ₂ O) lime (CaO) magnesia (MgO) alumina (Al ₂ O ₃ ). Transparent, easily shaped and best suited for window glass (see flat glass). It has a high thermal expansion and poor heat resistance (500-600 ° C). It is used for windows, some low temperature incandescent lamps, and tableware. Glass containers are soda-lime glasses of slight variation on flat glass, which uses more alumina and calcium, and less sodium and magnesium, which are more soluble in water. This makes it less susceptible to water erosion.
• Sodium borosilicate glass , Pyrex : boron trioxide silica (B ₂ O ₃ ) soda (Na < sub> 2 O) alumina (Al ₂ O ₃ ). Standing heat expansion is much better than window glass. Used for beaker, cooking glass, car head lamp, etc. Borosilicate glasses (eg Pyrex, Duran) have the major constituents of silica and boron trioxide. They have a fairly low thermal expansion coefficient (7740 Pyrex CTE is 3.25 ÃÆ' - 10 ^{- 6} /Ã, Â ° C compared to about 9 ÃÆ' - 10 ^{- 6} /Ã, Â ° C for typical soda-lime glasses), making it more dimensionally stable. The lower coefficients of thermal expansion (CTE) also make them less subject to stress caused by thermal expansion, making it less susceptible to cracking of thermal shock. They are generally used for reagent bottles, optical components and household cookware.
• Lead-oxide glass , crystal glass , lead glass : silica lead oxide (PbO) potassium oxide (K ₂ O) soda (Na ₂ O) zinc oxide (ZnO) alumina. Due to its high density (generating high electron density), it has a high refractive index, making the glass display more brilliant (called "crystal", though of course it is glass and not crystal). It also has high elasticity, making "ring" glass. It is also more applicable in the factory, but can not stand well. Glass of this type is also more fragile than other glasses and more easily cut.
• Aluminosilicate glass: magnesium barium oxide (BaO) oxide boric acid silica (Ba ₂ O ₃ ). Widely used for fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.) and for halogen bulb glass. The aluminosilicate glasses are also resistant to weather and water erosion.
• Oxide-germanium glass : germanium dioxide alumina (Geo ₂ ). Very clear glass, used for optical fiber waveguides in communication networks. Light loses only 5% of its intensity through 1 km of glass fiber.

Other common glass materials are crushed alkaline glass or 'cullet' ready for recycled glass. Recycled glass saves raw materials and energy. Dirt in the cullet can cause product and equipment failure. Fillers such as sodium sulfate, sodium chloride, or antimony oxide may be added to reduce the amount of air bubbles in the glass mixture. The glass batch calculation is a method by which the correct mixture of feedstocks is determined to achieve the desired glass composition.

Maps Glass

Physical properties

Optical properties

Glass is widely used mainly because of the production of glass compositions that are transparent to visible light. In contrast, polycrystalline materials generally do not transmit visible light. Individual crystals may be transparent, but the facets (grain boundaries) reflect or diffuse light that results in diffuse reflection. Glass contains no internal subdivisions associated with grain boundaries in polycrystals and therefore does not diffuse light in the same way as polycrystalline materials. Glass surfaces are often subtle because during the formation of glass the super-cold liquid molecules are not forced to be discarded in rigid crystalline geometry and can follow the surface tension, which imposes a very fine surface. These properties, which give the glass its clarity, can be maintained even if the glass absorbs some of the light - that is, colored.

Glass has the ability to bias, reflect, and transmit light following optical geometry, without spreading it (due to the absence of grain boundaries). It is used in the manufacture of lenses and windows. The general glass has a refractive index of about 1.5. This can be modified by adding low-density materials such as boron, which lowers the refractive index (see the crown glass), or increases (up to 1.8) with high-density materials such as (classically) lead oxides (see flint). glass and lead glass), or in the use of modern, less toxic zirconium, titanium, or barium oxides. These high index glasses (not known as "crystals" when used in glass vessels) cause more chromatic light dispersion, and are valuable for optical properties such as diamonds.

According to the Fresnel equation, the reflectivity of the glass sheet is about 4% per surface (in normal occurrence in the air), and the transmissivity of one element (two surfaces) is about 90%. Glass with high germanium oxide content also finds applications in optoelectronics - for example, for light-emitting optical fibers.

Other properties

In the process of making, silicate glass can be poured, molded, extruded and molded into shapes ranging from flat sheets to very complex shapes. The finished product is fragile and will break unless it is laminated or specially treated, but very durable under various conditions. It erodes very slowly and most can withstand water action. It is largely resistant to chemical attack, does not react with food, and is an ideal material for the manufacture of containers for groceries and most of the chemicals. Glass is also a substantial inert substance.

Corrosion

Although glass is generally corrosion resistant and more corrosion-resistant than other materials, it can still rust. The materials that make up certain glass compositions have an effect on how fast the glass is rusty. An alkaline or alkaline glass with a high proportion is less corrosion resistant than any other type of glass.

Glass fragments have an application as an anti-corrosive coating.

Strength

Glass usually has a tensile strength of 7 megapascals (1,000 psi), but theoretically can have a strength of 17 gigapascals (2,500,000 psi) due to the strong chemical bonds of glass. Some factors such as imperfections such as scratches and bubbles and the chemical composition of glass have an impact on the tensile strength of the glass. Some processes such as toughness can increase the strength of glass.

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Contemporary production

After the preparation of glass batch and mixing, the raw material is transported to the furnace. Soda-lime glass for mass production is melted in fired gas units. Small-scale furnaces for specialty glasses include electric melter, pot stove, and day tank. After melting, homogenization and purification (bubble removal), glass is formed. Flat glass for windows and similar applications are formed by a float glass process, developed between 1953 and 1957 by Sir Alastair Pilkington and Kenneth Bickerstaff of England's Pilkington Brothers, who created a continuous glass tape using a liquid tin bath where liquid glasses flow unhindered below the influence of gravity. The top surface of the glass is subjected to nitrogen under pressure to get the finish polished. Glass containers for common bottles and bottles are formed by blowing and pressing methods. The glass is often slightly chemically modified (with more alumina and calcium oxide) for greater water resistance. The glass forming technique is further summarized in Table Glass forming technique.

Once the desired shape is obtained, the glass is usually annealed to remove pressure and to increase the hardness and resistance of the glass. The surface treatment, coating or lamination may follow to increase chemical resistance (glass container lining, internal care glass container), strength (toughened glass, bulletproof glass, windscreen), or optical properties (insulated glass, anti-reflective coating).

Color

Colors in glass can be obtained by addition of distilled electrolyte (or color centers), and by precipitation of finely dispersed particles (such as in photochromic glasses). Normal soda-lime glass appears colorless to the naked eye when thin, although iron impurities (II) oxide (FeO) up to 0.1% by weight produce a green color, which can be seen in thick pieces or with scientific help. instrument. Furthermore, the additional FeO and chromium (III) oxide (Cr ₂ O ₃ ) can be used for green bottle production. Sulfur, along with carbon and iron salts, are used to form iron polysulfides and produce amber glass from yellowish to almost black. A molten glass can also obtain a yellow color from a reduced combustion atmosphere. Manganese dioxide can be added in small amounts to remove the green color given by iron (II) oxide. Art glass and studio glass pieces are colored using a carefully guarded prescription that involves a specific combination of metal oxide, melting temperature and "cook" time. Most colored glass used in the art market is produced in volume by vendors serving this market, although there are some glass makers with the ability to create their own colors from raw materials.

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History of silicate glass

Natural glass, especially volcanic glass obsidian, is used by many Stone Age communities around the world for the production of sharp cutting tools and, due to their limited source area, is being traded extensively. But in general, archaeological evidence suggests that the first true glass was made on the north coast of Syria, Mesopotamia or ancient Egypt. The earliest known glass objects, from the middle of the third millennium BC, are beads, probably originally created as accidental by-products of working metal (slags) or during the production of faience, the vitreous pre-glass material made by the process which is similar to glazing.

Glass remains a luxury, and the disaster overtaking the Late Bronze Age Civilization seems to have made glass stalled. The development of glass technology in South Asia may have started in 1730 BC. In ancient China, however, glass manufacture seems to have a late start, compared to ceramics and metal work. The term glass evolved in the late Roman Empire. It was at the center of Roman glass making in Trier, now in modern Germany, that the late Latin term glesum originated, probably from the Germanic word for transparent and shiny substance. Glass objects have been found throughout the Roman Empire in the context of domestic, funerals, and industry. Examples of Roman glass have been found outside the former Roman Empire in China, the Baltics, the Middle East and India.

Glass was used extensively during the Middle Ages. Anglo-Saxon glass has been found in England during the archaeological digs of settlements and burial sites. Glass in the Anglo-Saxon period is used in the manufacture of various objects including ships, windows, beads, and also used in jewelry. From the 10th century onwards, glass was used in stained glass windows of churches and cathedrals, with notable examples at the Chartres Cathedral and Saint Denis Basilica. In the 14th century, architects designed buildings with stained glass walls such as Sainte-Chapelle, Paris, (1203-1248) and the eastern end of Gloucester Cathedral. Stained glass has a great revival with Gothic Revival architecture in the 19th century. With the Renaissance, and a change of architectural style, the use of large stained glass windows became less common. The use of domestic stained glass is increased until the largest houses have glass windows. This was originally a small panel put together, but with technological changes, glass can be produced relatively cheaply in larger sheets. This leads to larger window glasses, and, in the 20th century, to the much larger windows in ordinary domestic and commercial buildings.

In the 20th century, new types of glass such as laminated glass, reinforced glass, and glass bricks increased the use of glass as a building material and produced new applications of glass. Tall buildings are often built with curtain walls that are almost entirely made of glass. Similarly, laminated glass has been widely applied to vehicles for window glass. Optical glasses for eyeglasses have been used since the Middle Ages. The production of lenses has become increasingly proficient, helping astronomers as well as having other applications in medicine and science. Glass is also used as an opening cover in many solar energy collectors.

From the 19th century, there was a revival in many ancient glass making techniques including cameo glass, which was achieved for the first time since the Roman Empire and was originally mostly used for pieces in neo-classical style. The Art Nouveau movement greatly leverages the glass, with Renà © à © Lalique, ÃÆ'â € ° GallÃÆ'Ã… ©, and Daum of Nancy produces colored vases and similar pieces, often in cameo glass, and also using the luster technique. Louis Comfort Tiffany in America specializes in stained glass, both secular and religious, as well as its famous lamps. The beginning of the 20th century saw the production of large-scale glass art by companies such as Waterford and Lalique. From about 1960 onwards, there has been an increase in the number of glass artworks that produce small glasses, and glass artists began to classify themselves as sculptors working on glass, and their work as part of pure art.

In the 21st century, scientists observed the properties of ancient stained glass windows, where clogged nanoparticles prevent UV rays from causing chemical reactions that alter the color of the image, developing a photography technique that uses the same stained glass to capture true color images from Mars for 2019. The mission of ESA Mars Rover.

Chronology of progress in architectural glass

• 1226: "Wide Sheet" was first produced in Sussex.
• 1330: "Crown Glass" for the first artwork and ship produced in Rouen, France. "Sheet Area" is also produced. Both are also provided for export.
• 1500s: The glass mirror-making method developed by a Venetian glass maker on the island of Murano, which covers the back of the glass with lead-tin amalgam, obtains an almost perfect and undistorted reflection.
• 1620: The first "Blown plate" was produced in London. Used for mirrors and trainer plates.
• 1678: The first "crown glass" was produced in London. This process dominated until the 19th century.
• 1843: Early form of "buoy glass" discovered by Henry Bessemer, pours glass into a liquid can. Expensive and not a commercial success.
• 1874: Tempered glass was developed by Francois Barthelemy Alfred Royer de la Bastie (1830-1901) from Paris, France by extinguishing almost liquid glass in a bath or heated oil.
• 1888: Rolled glass machine introduced, allowing patterns.
• 1898: The first cast-cast glass was produced commercially by Pilkington for use in which safety or security was a problem.
• 1959: Floating glass launched in England. Invented by Sir Alastair Pilkington.

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Other types

New chemical glass compositions or new maintenance techniques can be initially investigated in small-scale laboratory experiments. Raw materials for laboratory-scale glass melting often differ from those used in mass production because cost factors have low priority. In the laboratory most pure chemicals are used. It should be noted that the raw material does not react with moisture or other chemicals in the environment (such as alkaline or alkaline earth oxides or hydroxides, or boron oxides), or that the impurities are quantified (ignition loss). Loss of evaporation during glass melting should be considered during the selection of raw materials, for example, sodium selenite may be preferable to vaporize SeO ₂ easily. Also, easier reacting of raw materials may be preferred over relatively inert ones, such as Al (OH) ₃ above Al ₂ O ₃ . Typically, melting is done in platinum crucibles to reduce contamination of the container material. The homogeneity of glass is achieved by homogenizing a mixture of raw materials (glass batch), by stirring melt, and by crushing and melting the first melt. The obtained glass is usually annealed to prevent damage during processing.

To make glass from materials with poor glass forming tendencies, new techniques are used to increase the cooling rate, or reduce the triggering of nucleation crystals. Examples of these techniques include aerodynamic levitation (cooling the melt while floating on the gas stream), splat quenching (pressing melt between two metal anvils) and cooling the roller (pouring melt through the roller).

Fiberglass

Fiberglass (also called glass-reinforced-plastic) is a composite material made of glass fiber (also called fiberglass or fractal glass) embedded in a plastic resin. This is made by melting the glass and stretching the glass into fiber. These fibers are woven together into a cloth and allowed to stick in a plastic resin.

Fiberglass filaments are made through a pultrusion process in which raw materials (sand, limestone, kaolin clay, fluorspar, colemanite, dolomite and other minerals) are melted in large stoves into liquids extruded through very small holes (5-25). micrometres in diameter if glass is E-glass and 9 micrometers if glass is S-glass).

Fiberglass has lightweight and corrosion-resistant properties. Fiberglass is also a good insulator, allowing it to be used to protect buildings. Most fiberglasses are not alkaline resistant. Fiberglass also has properties become stronger with age glass.

Network glasses

Some types of glass that does not include silica as the main constituent may have physico-chemical properties useful for its application in optical fibers and other specialized technical applications. These include fluoride glass, alumina and aluminosilicate glass, phosphate glass, borate glass, and chalcogenide glass.

There are three classes of components for oxide glass: network formers, intermediates, and modifiers. Network formers (silicon, boron, germanium) form a network of highly interlinked chemical bonds. Intermediates (titanium, aluminum, zirconium, beryllium, magnesium, zinc) may act as tissue formers and modifiers, according to glass compositions. The converters (calcium, lead, lithium, sodium, potassium) alter the structure of the tissues; they are usually present as ions, compensated by the nearest non-bridging oxygen atoms, bound by a covalent bond to the glass network and holding one negative charge to compensate for nearby positive ions. Some elements can play many roles; eg leads can act as either a previous network (Pb ⁴ replace Si ⁴ ), or as a modifier.

The presence of non-bridging oxygen decreases the relative amount of the strong bond in the material and disrupts the tissue, reducing the viscosity of the melt and lowering the melting temperature.

Alkali metal ions are small and moving; Their presence in glass enables the level of electrical conductivity, especially in liquid or at high temperatures. Their mobility lowers the chemical resistance of the glass, enabling washing by water and facilitating corrosion. The alkaline earth ion, with two positive charges and requirements for two non-bridging oxygen ions to compensate for its charge, is much less self-propagating and also inhibits the diffusion of other ions, especially alkalis. The most common commercial glass types contain alkaline and alkaline earth ions (usually sodium and calcium), for easier processing and satisfactory corrosion resistance. The corrosion resistance of glass can be increased by dealkalization, removal of alkaline ions from the glass surface by reaction with sulfur or fluorine compounds. The presence of alkali metal ions also has a detrimental effect on the loss of tangents of glass, and on electrical resistance; glass produced for electronics (sealing, vacuum tubes, lamps...) should take this into account.

The addition of tin (II) oxide lowers the melting point, decreases melt viscosity, and increases the refractive index. The lead oxide also facilitates the dissolution of other metal oxides and is used in colored glass. The decrease in the viscosity of lead melting glass is very significant (approximately 100 times compared with glasses of soda); this allows easier removal of bubbles and works at lower temperatures, then this is often used as an additive in vitreous glass enamel and glass. The high ionic radius of the Pb ² ion makes it very immobile in the matrix and prevents the movement of other ions; Therefore, tin glasses have high electrical resistance, about twice as high as soda-lime glass (10 ^8.5 vs. 10 ^6.5 Ã, cm , DC at 250 ° C). For more details, see lead glass.

The addition of fluorine decreases the glass dielectric constant. Fluor is highly electronegative and attracts electrons in the lattice, reducing material polarizability. Such silicon dioxide-fluoride is used in the manufacture of integrated circuits as insulators. High levels of fluorine doping cause the formation of volatile SiF ₂ O and the glass is then thermally unstable. The stable layer is achieved with a dielectric constant down to about 3.5-3.7.

Amorphous metal

In the past, small quantities of amorphous metals with high surface area configurations (ribbons, cables, films, etc.) have been generated through the application of extremely rapid cooling rates. It was originally called "splat cooling" by doctoral student W. Klement at Caltech, which showed that the cooling rate at the order of millions of degrees per second was enough to block the formation of crystals, and the metal atoms became "locked into" a glass state. Amorphous metal cables have been produced by molten metal that obscures to the spinning metal disks. Recently a number of alloys have been produced in layers with thickness exceeding 1 millimeter. These are known as mass metallic glasses (BMG). Liquidmetal technology sells a number of zirconium-based BMGs. Batches of amorphous steel have also been produced which exhibit far greater mechanical properties than those found in conventional steel alloys.

In 2004, NIST researchers presented evidence that the isotropic non-crystalline metal phase (dubbed "q-glass") can grow from melting. This phase is the first phase, or "main phase", which is formed in the Al-Fe-Si system during rapid cooling. Interestingly, experimental evidence suggests that this phase is formed by first-order transitions . Transmission electron microscopy images (TEMs) show that q-glass berinti of melt as discrete particles, which grow spherically with uniform growth rate in all directions. The diffraction pattern shows it to be an isotropic glass phase. But there is a nucleation barrier, which implies interface discontinuity (or internal surface) between glass and melt.

Electrolytes

Electrolytes or liquid salts are mixtures of various ions. In a mixture of three or more ionic species of different sizes and shapes, crystallization can be so difficult that the liquid can be easily cooled into a glass. The best examples studied are Ca _0.4 (K ₃ ) _{1, 4} . Glass electrolytes in the form of Ba-doped Li-glass and Ba-doped Na-glass have been proposed as solutions to problems identified with organic liquid electrolytes used in modern lithium-ion battery cells.

Aqueous solution

Some aqueous solutions can be cooled to a glassy state, eg LiCl: R H ₂ O (lithium chloride salt solution and water molecule) in the composition range of 4 & lt; R & lt; 8. Aqueous solutions containing sugars have a glassy state and may be used as surfactants.

Molecular liquid

A molecular fluid consists of molecules that do not form covalent tissues but interact only through weak van der Waals forces or through transient hydrogen bonds. Many liquid molecules can be cooled into glass; some are good glass formers that usually do not crystallize.

An example of this is a glass of sugar.

Under extreme pressure and dense temperatures can indicate large structural and physical changes that can lead to a polyamorphic phase transition. In 2006 Italian scientists created an amorphous phase of carbon dioxide using extreme pressure. The substance is called amorphous carbonia (a-CO ₂ ) and shows the atomic structure resembling silica.

Polymers

Essential polymer sunglasses include amorphous pharmaceutical compounds and glass. This is useful because the solubility of the compound is greatly increased when amorphous compared to the same crystal composition. Many of the emerging medicines are practically insoluble in their crystalline form.

Colloid glasses

The concentrated colloid suspension may indicate different glass transitions as a function of particle concentration or density.

In cell biology, there is recent evidence to suggest that the cytoplasm behaves like a colloidal glass approaching a glass-liquid transition. During periods of low metabolic activity, such as in dormancy, cytoplasmic vitrifies and prohibit movement to larger cytoplasmic particles while allowing smaller diffusion across cells.

Glass-ceramic

These glass-ceramic materials share many properties with non-crystalline glass and crystal ceramics. They are formed as glass, and then partially crystallized by heat treatment. For example, ceramic microstructure whiteware often contains amorphous phases and crystals. Grain crystals are often embedded in the intergranular phase of non-crystalline grain boundaries. When applied to ceramic whiteware, vitreous means the material has a very low permeability to the liquid, often but not always water, when determined by a particular test regime.

The term refers primarily to a mixture of lithium and aluminosilicates which produces a variety of materials with attractive thermomechanical properties. The most important of these commercially has a difference that is resistant to thermal shock. So, glass-ceramics have become very useful for cooking tables. The negative thermal expansion coefficient (CTE) of the crystalline ceramic phase can be offset by a positive CTE of the glass phase. At some point (~ 70% crystal) the glass-ceramic has a net CTE close to zero. This glass-ceramic type exhibits excellent mechanical properties and can maintain rapid and rapid temperature changes of up to 1000 ° C.

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Structure

As with other amorphous solids, the glass atomic structure has no long-term periodicity observed in crystalline solids. Due to the characteristics of chemical bonds, the glasses do have a high degree of short distance according to the local atomic polhedra.

Formation of super cold liquid

In physics, the standard definition of a glass (or vitreous solid) is a solid formed by rapid cooling, although the term glass is often used to describe amorphous solids showing the glass transition temperature T _g . For cooling melt, if cooling is sufficiently rapid (relative to characteristic crystallization time) then crystallization is prevented and instead the irregular atomic configuration of super cold fluid is frozen into solid state at T _g . The tendency of the material to form glass when extinguished is called the ability of glass formation. This ability can be predicted by the theory of stiffness. Generally, the glass is in a state which can structurally metastasize with respect to its crystalline form, although in certain circumstances, for example in an atactic polymer, there is no crystalline analogue in the amorphous phase.

Glass is sometimes regarded as a liquid due to the absence of a first-order phase transition in which certain thermodynamic variables such as volume, entropy and enthalpy are intermittent through the glass transition range. The glass transitions can be described as analogous to second-order phase transitions in which intensive thermodynamic variables such as thermal expansion and heat capacity are disconnected. Nevertheless, the equilibrium theory of phase transformation is not fully applicable to glass, and hence the glass transition can not be classified as one of the classic equilibrium phase transformations in solids.

Glass is an amorphous solid. It shows the atomic structure close to that observed in the super cold liquid phase but displays all the mechanical properties of a solid. The notion that glass flows into a large enough rate over a long period of time is not supported by empirical research or theoretical analysis (see viscosity of amorphous materials). Measurement of room temperature glass flow laboratory show consistent movement with material viscosity on the order of 10 ¹⁷ -10 ¹⁸ Pa s.

Although the atomic structure of glass has structural characteristics in very cold liquids, glass tends to behave like solids below its glass transition temperature. The superfluid liquid behaves like a liquid, but it is below the freezing point of the material, and in some cases it will crystallize almost instantaneously if the crystal is added as the nucleus. The change in heat capacity in glass transitions and melted transitions of comparable materials usually of the same order of magnitude, indicating that changes in the degree of active freedom are also proportional. Neither in the glass nor in the crystal is largely the degree of vibration of freedom that remains active, while the rotational and translational motion is captured. This helps to explain why crystalline and non-crystalline solids show stiffness on most experimental time scales.

Antique glass behavior

Observations that old windows are sometimes found thicker at the bottom than at the top are often offered as supporting evidence for the view that glass flows over centuries-time scales, the presumption that glass has shown liquid properties to flow from one form to another. This assumption is not true, because once compacted, the glass stops flowing. The reason for the observation is that in the past, when glass panels were generally made by glassblowers, the technique used was spinning liquid glasses thus creating a round, mostly flat and even a plate (the crown glass process, described above). The plate is then cut to fit the window. The pieces are not really flat; the edge of the disc becomes a different thickness when the glass is rotating. When mounted on the window frame, the glass will be placed with a thicker side under both for stability and to prevent water from accumulating in the main cames at the bottom of the window. Sometimes, the glass has been found installed with a thicker side at the top, left or right.

The mass production of window glass in the early twentieth century caused the same effect. In the glass factories, the liquid glass is poured onto a large cooling table and allowed to spread. The resulting glass is thicker at the cast site, which is located in the middle of a large sheet. These sheets are cut into smaller window panels with uniform thickness, usually with a cast location centered on one panel (known as "bull's eye") for a decorative effect. Modern glass intended for windows is produced as float glass and is very uniform in thickness.

Some other points may be considered contrary to the theory of "cathedral glass":

• Writing in the American Journal of Physics , the materials engineer Edgar D. Zanotto states "... Ã, the predicted relaxation time for Geo ₂ in the room temperature is 10 ³² year.Therefore, the period of relaxation (flow time characteristics) of cathedral spectacles will be longer. "(10 ³² years is much longer than the estimated age of the universe. )
• If the medieval glass has been flowing clearly, then ancient Roman and Egyptian objects should flow more proportionally - but this is not observed. Similarly, a prehistoric obsidian knife should lose its edge; this is not observed (although obsidian may have a different viscosity than glass windows).
• If the glass flows at a rate that allows changes to be seen with the naked eye after centuries, then the effect should be seen on antique telescopes. Any slight deformation of antique telescopic lenses will cause a dramatic decrease in optical performance, an unobserved phenomenon.

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Gallery

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See also

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References

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External links

• Ã, "Glass". EncyclopÃÆ'Â|dia Britannica . 12 (issue 11). 1911 Ã,
• The Canadian Glass Making Story from the Canadian Museum of Civilization.
• "How Your Glass Ware Is Created" by George W. Waltz, February 1951, Popular Science.
• All About Glass from Corning Museum of Glass :. A collection of articles, multimedia, and virtual books are all about glass, including the Glass Dictionary
• Encyclopedic Glass from 20th Century Glass: a comprehensive guide to all types of antique and collectible glass, with information, images and references
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• The National Glass Association is the largest trade association representing flat glass (architecture), windshields, and windows & amp; door industry

Source of the article : Wikipedia