Meaning of INDUSTRIAL GLASS in English

INDUSTRIAL GLASS

Additional reading D.R. Uhlmann and N.J. Kreidl (eds.), GlassScience and Technology (1980 ), provides an in-depth discussion of glass for the advanced professional. Narottam P. Bansal and R.H. Doremus, Handbook of Glass Properties (1986), is a good compilation excerpted from published literature. Arun K. Varshneya, Fundamentals of Inorganic Glasses (1994), comprehensively covers glass composition, structure, properties, melting technology, and the forming of glass products and includes a chapter on glass transition-range behaviour and annealing/tempering.Samuel R. Scholes, Modern Glass Practice, 7th rev. ed. by Charles H. Greene (1975, reprinted 1992); and P.J. Doyle (compiler and ed.), Glass-Making Today (1979), discuss various aspects of glass manufacturing. Fay V. Tooley (compiler and ed.), The Handbook of Glass Manufacture: A Book of Reference for the Plant Executive, Technologist, and Engineer, 3rd ed., 2 vol. (1984), treats such topics as glass composition, raw materials, furnaces, refractories, fuels, and glasshouse instrumentation. George W. McLellan and Errol B. Shand, Glass Engineering Handbook, 3rd ed. (1984), describes the engineering and technology of glass products, detailing chemical durability, strength, and annealing techniques. ASM International, Handbook Committee, Engineered Materials Handbook, vol. 4, Ceramics and Glasses, ed. by Samuel J. Schneider, Jr. (1991), is an excellent updated compilation of articles written by various experts on a variety of topics covering glass melting and forming technologies.Harry Heltman Holscher, Hollow and Speciality Glass: Background and Challenge (1965); and R.W. Douglas and Susan Frank, A History of Glassmaking (1972), chronicle major historical developments. Arun Kumar Varshneya Glass compositions and applications Oxide glasses Silica-based Of the various glass families of commercial interest, most are based on silica, or silicon dioxide (SiO2), a mineral that is found in great abundance in natureparticularly in quartz and beach sands. Glass made exclusively of silica is known as silica glass, or vitreous silica. (It is also called fused quartz if derived from the melting of quartz crystals.) Silica glass is used where high service temperature, very high thermal shock resistance, high chemical durability, very low electrical conductivity, and good ultraviolet transparency are desired. However, for most glass products, such as containers, windows, and lightbulbs, the primary criteria are low cost and good durability, and the glasses that best meet these criteria are based on the soda-lime-silica system. Examples of these glasses are shown in Table 1. After silica, the many soda-lime glasses have as their primary constituents soda, or sodium oxide (Na2O; usually derived from sodium carbonate, or soda ash), and lime, or calcium oxide (CaO; commonly derived from roasted limestone). To this basic formula other ingredients may be added in order to obtain varying properties. For instance, by adding sodium fluoride or calcium fluoride, a translucent but not transparent product known as opal glass can be obtained. Another silica-based variation is borosilicate glass, which is used where high thermal shock resistance and high chemical durability are desiredas in chemical glassware and automobile headlamps. In the past, leaded crystal tableware was made of glass containing high amounts of lead oxide (PbO), which imparted to the product a high refractive index (hence the brilliance), a high elastic modulus (hence the sonority, or ring), and a long working range of temperatures. Lead oxide is also a major component in glass solders or in sealing glasses with low firing temperatures. Other silica-based glasses are the aluminosilicate glasses, which are intermediate between vitreous silica and the more common soda-lime-silica glasses in thermal properties as well as cost; glass fibres such as E glass and S glass, used in fibre-reinforced plastics and in thermal-insulation wool; and optical glasses containing a multitude of additional major constituents. Nonsilica Oxide glasses not based on silica are of little commercial importance. They are generally phosphates and borates, which have some use in bioresorbable products such as surgical mesh and time-release capsules. Glass formation Volume and temperature changes Cooling from the melt Figure 1: Changes in volume and temperature of a liquid cooling to the glassy or crystalline state. Figure 1: Changes in volume and temperature of a liquid cooling to the glassy or crystalline state. The formation of glass is best understood by examining Figure 1, in which the volume of a given mass of substance is plotted against its temperature. A liquid starts at a high temperature (indicated by point a). The removal of heat causes the state to move along the line ab, as the liquid simultaneously cools and shrinks in volume. In order for a perceptible degree of crystallization to take place, there must be a finite amount of supercooling below the freezing point b (which is also the melting point, Tm, of the corresponding crystal). Crystallization is essentially two processes: nucleation (the adoption of a patterned arrangement by a small number of atoms) and growth (extension of that arrangement to surrounding atoms). These processes must take place in the order described, but, since crystal growth kinetics generally precede nucleation with little overlap during cooling, crystallization in a cooling liquid occurs only over a range of temperatures. In Figure 1 this range is shown by the shaded region, with crystallization reaching its maximum probability in the darkest portion, indicated by point c. Figure 1: Changes in volume and temperature of a liquid cooling to the glassy or crystalline state. If cooling is conducted rapidly enough, measurable crystallization will not take place; instead, the mass will continue along line abcf, its volume shrinking with falling temperature and its viscosity rising enormously. Eventually, the supercooled liquid will become so viscous that its volume will shrink at a slower rate, and finally it will become a seemingly rigid solid, indicated in Figure 1 by point g. At this point it is called glass. The glass transformation range Figure 1: Changes in volume and temperature of a liquid cooling to the glassy or crystalline state. Figure 1: Changes in volume and temperature of a liquid cooling to the glassy or crystalline state. The transformation from the seemingly liquid state (the supercooled liquid) to the seemingly solid state (glass) is gradual, with no evidence of any discontinuities in properties. The transition takes place over a range of temperatures called the glass transformation range; in Figure 1 it is shown by the smooth departure of line abcg from line abcf, which is known as the equilibrium liquid line. (Not shown in Figure 1 is the glass transition temperature, or Tg; this would be located at the lower end of the transformation range.) In crystallization, on the other hand, the transition from liquid to solid takes place with essentially a discontinuous change in volume. In Figure 1 this abrupt transition is indicated by a sharp drop, within the shaded crystallization region, from the liquid line abcf to the crystal line de. With further cooling, the solid follows the crystal line to point e. With few exceptions, the volume of the crystal is less than that of the glass, since the orderly arrangement of atoms in a crystalline solid does not permit as great a free volume as occurs in a glassy solid. Cooling a supercooled liquid at slower rates causes the material to shrink to a lesser volume, continuing along the line abcf until a glass is formed at point h. Glass at point h is denser than glass at g (with the known exception of vitreous silica). The structure of glass at h is assumed to be identical to that of the liquid at (Tf)1. Known as the fictive temperature, (Tf)1 is the temperature at which the liquid structure is frozen into the glassy state. (Tf)2 represents the fictive temperature of the glass formed by fast cooling. Glass-forming criteria Kinetic arguments Glass forming Figure 5: The viscosity of representative silica glasses at varying temperatures. All traditional glass products such as tableware, containers, tubes and rods, flat glass, and fibreglass are formed of glass made by the melting process. Viscosity is the key property in glass forming. After melting and conditioning (described in Industrial glassmaking), glass is delivered to a forming machine in a manageable shape at a viscosity of approximately 104 poise. At this viscosity, indicated in Figure 5 as the working point, the glass can be worked on to form the desired object and then released in a near-solid condition. All through the process, heat is extracted in a controlled manner in order to allow the viscosity to increase from the levels typical of a liquid to those of a solid. Beads and microspheres Solid glass beads and microspheres used in blast cleaners, shot peening, and reflective paints can be made simply by passing finely fritted glass through a hot flame. Hollow microspheres, used mostly as low-density fillers, may be produced by one of many processes. In one method, the glassmaking ingredients are dissolved in water, urea is added as blowing agent, and the mixture is fed through the gas or air nozzle of a burner. In another method, a solution of an alkoxide in alcohol is stirred into a liquid dispersant that causes the solution to break up into small droplets. The droplets are allowed to gel and are then separated, dried, and sintered. Glass treating Annealing Figure 5: The viscosity of representative silica glasses at varying temperatures. During the glass-forming process, glasses often develop permanent stresses because various regions of the material pass through the glass transition range at varying cooling rates and at varying times. In order to ensure dimensional stability (for instance, for space-based telescope mirrors) and to avoid the development of excessive tension in critical regions, these stresses must be reduced by the process of annealing. As is explained in Properties of glass, the atomic structure of a glassy solid undergoes a process of relaxation as it is cooled through the transition range. The time required for relaxation to be sufficient to reduce internal stresses can range from only a few minutes when the glass is held at its annealing point to a few hours when it is held at the lower temperatures of its strain point. (The strain point and annealing point of several oxide glasses are shown in Figure 5.) Practical annealing is achieved generally by holding the product approximately 5 C (9 F) above its annealing point for 5 to 15 minutes, followed by slowly cooling it through the glass-transition range and the strain point and finally to room temperature. In dead-annealing, glass is so well annealed that the internal tension is almost undetectable. As is explained in Optical properties, internal stresses are examined by using the photoelastic property of glass. Strengthening Glass may be strengthened using one of several processes: temporarily reducing the severity of flaws by fire polishing or etching (i.e., chemical polishing); introducing surface compression by overlay glazing, thermal tempering, or ion exchange; and toughening by lamination. Glassmaking in the laboratory Glassmaking requires a carefully weighed selection of raw materials. For laboratory melting, a batch is prepared from reagent-grade chemicals such as floated silica, sodium carbonate, calcium carbonate, alumina, and boraxall of which are assumed to convert to equivalent amounts of oxides after decomposition. The mixed batch is placed in a covered crucible and heated generally inside an electric resistance furnace. The crucible is made of suitable refractory materialsfor instance, fireclay (inexpensive but contaminating), fused silica (for good thermal shock resistance), and high-density alumina. In order to avoid contamination of the molten glass by refractory materials, it is often recommended that crucibles be made of platinumeither the pure metal or alloyed with 2 to 20 percent rhodium or 5 percent gold. Because of the expense associated with these noble metals, the laboratory glassmaker must be careful not to mix a batch that, upon melting, would undergo chemical reaction with the crucible materials. Convenient electric-resistance furnaces are temperature-controlled, with programming capabilities. Heating elements may be made of molybdenum disilicide with low thermal mass insulation. Glass may be poured in graphite or steel molds or, alternatively, rolled (using a metal roller) into thin flakes while being poured onto a steel or aluminum chill plate. If fritting, or breaking into small particles, is desired, the molten glass stream may be dropped into water. Blocks of glass can be cut or drilled with diamond-impregnated saws and drills. Glass also may be ground using diamond-impregnated rotating wheels, silicon carbide paper, or silicon carbide slurry. It can be polished using cloths loaded with finer-grained abrasives such as diamond, iron oxide, or ceria. History of glassmaking Development of the glassmaker's art The ancient world Glass as an independent object (mostly as beads) dates back to about 2500 BC. It originated perhaps in Mesopotamia and was brought later to Egypt. Vessels of glass appeared about 1450 BC, during the reign of Thutmose III, a pharaoh of the 18th dynasty of Egypt. A glass bottle bearing Thutmose's hieroglyph is in the British Museum in London. From Mesopotamia and Egypt, glassmaking using the basic soda-lime-silica composition traveled to Phoenicia, along the coast of present-day Lebanon. From there the art spread to Cyprus, Greece, and, by the 9th century BC, the Italian peninsula. After the conquests of Alexander the Great in the 4th century BC, glassmaking skills spread to the East, including the Indian subcontinent. Glass beads and bangles characteristic of the Hindu culture of about 200 BC have been discovered in Nevasa excavations. Glassmakers in Syria prospered during this time, specializing in plain bowls of single colours. In Alexandria about 100 BC the millefiori (thousand flowers) process for making open beakers and shallow dishes was developed. In this process a shaped core was made, perhaps of mud, to which sections of coloured glass canes were attached. The core and canes were placed into an outer mold to keep the shape while the glass fused in an oven. After removing the mold and core, the glass surfaces were ground smooth. Cross sections of the coloured rods showed a striking mosaic effect. Near the beginning of the Christian era, the Phoenicians learned how to blow glass with a blowing iron. The blowing iron was an iron tube about 1.5 metres (5 feet) long, with a mouthpiece at one end and a knob for holding soft glass at the other end. A blob of molten glass was collected on the knob end and rolled into a suitable shape on a flat surface of iron or stone called the marver. The shape could then be blown inside a mold or freely in air with occasional reheating. A solid iron rod called the pontil was used to wrap, twirl, or pinch glass into desired complexities. Handle, stem, or bottom also could be fused to the vessel when desired. The Romans and Egyptians probably used sand mixed with ground seashells as raw materials for silica and lime and hardwood ash as the source of soda. They also showed astonishing skill in the way they used metallic oxides as colorizers. Very small differences in oxide content can drastically affect the final colour of a glass; yet colours and tints were reproduced time and again with remarkable consistency. Copper was used to make green and ruby-red glass; iron produced black, brown, and green; antimony, yellow; manganese was employed to make purple and amethyst glass. An opaque white glass, made by using tin, was important in glass cameo work, of which the famous Portland vase, made in 1st-century Rome, is an outstanding example. To make this vase, a layer of white glass was superimposed on a darker material and afterward sculpted, pierced, and cut away to leave the white figures in relief against the darker background. Roman attempts to make flat glass by pouring slabs about 12 millimetres (1/2 inch) thick were unrewarding. Proper transparency could not be achieved by such means without grinding and polishing the cast material; the lack of transparency and the difficulty encountered in making any but small panes by this method led to the introduction of stained-glass windows, first used in the Eastern Roman Empire in the early 12th century. The Middle Ages and the Renaissance Glassmaking skills in Europe declined after AD 200; for about a thousand years, standards remained far below those of the Romans. The range of articles, as well as the quality of the material, was poor; the glass was of inferior colour and marred by streaks and bubbles. The stained-glass windows that began to appear in the new Gothic churches in Europe in the 12th century reached their full splendour in the 13th and 14th centuries. Many of the colours were produced by the method of fusing stains to the surface of the glass. Clear, colourless glass proved extremely difficult to achieve, however. The real revival of glassmaking skills in Europe came by way of Venice through contact with the Eastern Roman Empire (Byzantium). The Venetians made discoveries and innovations of their own, learning, for example, to eliminate all accidental colorizers from a glass melt by adding pyrolusite, a manganese mineral known as glassmaker's soap. The natural result was a gray glass, the overall transparency of which was even less than that of the otherwise slightly tinted glass. However, so long as the amounts of the original colorizer and of its antidote were small and the thickness of the finished article was slight, the loss of transparency was less noticeable than the unwanted colour would have been. The Venetians eventually redeveloped all the skills of the Romans. Their products and their secrets were so much sought after that regulations were passed forbidding the emigration of workers. The glassworks, said to be more than a kilometre long, was moved in its entirety to the island of Murano in 1291 because of the fires it had caused. Most of the glass produced was soda-lime glass. Satisfactory for most purposes because it was very stable chemically and of reasonable hardness, soda-lime glass was also easily made. Its moderate softening temperature made it very workable, and it could be readily resoftened a number of times if necessary to complete an article. Above all, the materials needed for its manufacture were plentiful: sand and limestone were ubiquitous, and soda ash was readily obtainable from the hardwood forests that also provided fuel for the furnaces. Desirable but scarcer materials included potash, produced by burning seaweed and favoured by the Venetians above soda ash, and calcined and crushed river pebbles (selected for their whiteness), which most Italian glassmakers preferred to common sand. Figure 13: Medieval European glassworks. (Top) Sand is collected and carried to the By the 15th century, Venetians had learned to make a fine rock crystal known as Venetian cristallo. Beautiful coloured glasses and techniques for decoration, such as gilding, also were developed. The Venetian product was exported to all over Europe and the Byzantine Empire. After Constantinople was captured by the Turks in 1453, however, the Venetian glass trade fell. Glassmakers emigrated from Venice and helped other Europeans set up their glass houses. Germans and Bohemians concentrated on the preparation and selection of newer, purer raw materials. Glass quality improved by the addition of low-iron potash and purer quartz. By 1680, Bohemian crystalbasically a potash-lime glasswas developed. In 1674 George Ravenscroft of London experimented with flint glass, a lead-crystal composition made with a large proportion of calcined flints and potash. By using 15 percent lead oxide, quartz pebbles imported from the Po River in Italy, and purer potash, he produced a fine, lustrous glass, soft enough to be cut and engraved easily and of greater refractive power than the common soda-lime glass. By the end of the century, there were 11 houses in London producing leaded crystal. During the same period, Johann Kunckel in Germany developed a reliable formula for producing ruby-red glass using gold chloride. Gold was dissolved in aqua regia and mixed with the batch, which was then melted, formed, and subsequently reheated to strike the precipitation of the ruby-red colour. Kunckel also developed a phosphate opal glass, also called porcelain glass or Milchglas, by adding burned bone or horn to the soda-lime batch. Properties of glass At ordinary temperatures, glass is a nearly perfect elastic solid, an excellent thermal and electrical insulator, and very resistant to many corrosive media. (Its optical properties, however, vary greatly, depending on the light wavelengths employed.) The more or less random order of atoms is ultimately responsible for many of the properties that distinguish glass from other solids. One unique attribute of special importance may be called the isotropicity of properties, meaning that such properties as tensile strength, electrical resistance, and thermal expansion are of equal magnitude in any direction through the material. As a glass-forming melt is cooled through the transition range, its structure relaxes, or changes continuously, from that of a liquid to that of a solid. The properties of solid glass reflect the extent of this structural relaxation. Indeed, glass can be said to retain a memory of the temperature-time schedule through the transition. Evidence of this thermal history is wiped out only after the glass has been reheated to the liquid state. Figure 2: The irregular arrangement of ions in a sodium silicate glass. Most properties of glassexcept for elastic and strength behaviour in the solid stateare sensitive to its chemical composition and, hence, its atomic structure. (The role of composition and structure in the formation of the glassy state is described in Glass formation: Atomic structure.) In oxide glasses, the specific composition-structure-property relationships are based upon the following factors: (1) the coordination number of the network-forming (NWF) ion, (2) the connectivity of the structure, as determined by the concentration of nonbridging oxygens, which, in turn, is determined by the concentration and nature of network-modifying (NWM) ions, (3) the openness of the structure, determined, again, by the concentration of NWM ions, and (4) the mobility of the NWM ions. Thus, tetrahedrally connected networks, such as those formed by silicates and illustrated in Figure 2, are more viscous than triangularly connected networks, such as those formed by borates. In silicates, the addition of network-modifying alkali ions would raise the concentration of nonbridging oxygens, and the resulting lowered connectivity would lead to a lowering of viscosity. Networks in which the interstitial spaces are less filled with NWM ions possess lower density and allow greater permeation of gases through them. Since alkali ions are the most mobile species through interstices of oxide glasses, the higher the alkali concentration, the lower the chemical durability and electrical resistivity of the material. Because glass generally acts as if it were a solution, many of its properties can be estimated by applying what are known as additivity relationships over a narrow range of compositions. In additivity relationships, it is assumed that each ingredient in a glass contributes to the properties of the glass by an amount equal to the concentration of that ingredient multiplied by a specific additivity factor. Many properties of soda-lime-silica glasses follow such relationships closely. Physical properties Density In the random atomic order of a glassy solid, the atoms are packed less densely than in a corresponding crystal, leaving larger interstitial spaces, or holes between atoms. These interstitial spaces collectively make up what is known as free volume, and they are responsible for the lower density of a glass as opposed to a crystal. For example, the density of silica glass is about 2 percent lower than that of its closest crystalline counterpart, the silica mineral low-cristobalite. The addition of alkali and lime, however, would cause the density of the glass to increase steadily as the network-modifying sodium and calcium ions filled the interstitial spaces. Thus, commercial soda-lime-silica glasses have a density greater than that of low-cristobalite. Density follows additivity behaviour closely. (The densities of representative oxide glasses are shown in Table 2.)

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