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Engineering Materials (Glass)

Introduction

Glass is a non-crystalline solid material. Glasses are typically brittle, and often optically transparent.
The most prevalent type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, made of about 75% silica (SiO2) plus Na2O, CaO, and several minor additives. Often, the term glass is used in a restricted sense to refer to this specific use.

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[pic]Roman Cage Cup from the 4th century A.D.

In science, however, the term glass is usually defined in a much wider sense, including every solid that possesses a non-crystalline (i.e. amorphous) structure and that exhibits a glass transition when heated towards the liquid state. In this wider sense, glasses can be made of quite different classes of materials: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers. Of these, polymer glasses (acrylic glass, polyethylene terephthalate) are the most important; for many applications (bottles, eyewear) they are a lighter alternative to traditional silica glasses.
Glasses play an essential role in science and industry. Their chemical, physical, and in particular optical properties make them suitable for applications such as flat glass, container glass, optics and optoelectronics material, laboratory equipment, thermal insulator (glass wool), reinforcement fiber (glass-reinforced plastic, glass fiber reinforced concrete), and glass art (art glass, studio glass).

Glass transition

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[pic]Glassblowing at temperatures just above the glass transition
Glass transition or vitrification refers to the transformation of a glass-forming liquid into a glass, which usually occurs upon rapid cooling. It is a dynamic phenomenon occurring between two distinct states of matter (liquid and glass), each with different physical properties. Upon cooling through the temperature range of glass transition (a "glass transformation range"), without forming any long-range order or significant symmetry of atomic arrangement, the liquid contracts more continuously at about the same rate as above the melting point until there is a decrease in the thermal expansion coefficient (TEC).
The glass transition temperature, Tg, is lower than melting temperature, Tm, due to supercooling. Tg depends on the time scale of observation which must be defined by convention. One approach is to agree on a standard cooling rate of 10 K/min. Another approach is by requiring a viscosity of 1012 Pa·s. Otherwise, one can only talk about a glass transformation range.
The glassy or vitreous state of matter is typically formed by rapid cooling and solidification from the molten (or liquid) state. If the liquid were allowed to crystallize on cooling, then according to the Ehrenfest classification of first-order phase transitions, there would be a discontinuous change in volume (and thus a discontinuity in the slope or first derivative with respect to temperature, dV/dT) at the melting point. In this context, glass and melt are distinct phases with an interfacial discontinuity having a surface of tension with a positive surface energy. Thus, a metastable parent phase is always stable with respect to the nucleation of small embryos or droplets from a daughter phase, provided it has a positive surface of tension. Such first-order transitions must proceed by the advancement of an interfacial region whose structure and properties vary discontinuously from the parent phase.
Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used. The expansion coefficient for the glassy state is roughly equivalent to that of the crystalline solid. If slower cooling rates are used, the increased time for structural relaxation (or intermolecular rearrangement) to occur may result in a higher density glass product. Similarly, by annealing (and thus allowing for slow structural relaxation) the glass structure in time approaches an equilibrium density corresponding to the supercooled liquid at this same temperature. Tg is located at the intersection between the cooling curve (volume versus temperature) for the glassy state and the supercooled liquid.
Thus, the liquid-glass transition is not a transition between states of thermodynamic equilibrium. It is widely believed that the true equilibrium state is always crystalline. Glass is believed to exist in a kinetically locked state, and its entropy, density, and so on, depend on the thermal history. Therefore, the glass transition is primarily a dynamic phenomenon. Time and temperature are interchangeable quantities (to some extent) when dealing with glasses, a fact often expressed in the time-temperature superposition principle. On cooling a liquid, internal degrees of freedom successively fall out of equilibrium. However, there is a longstanding debate whether there is an underlying second-order phase transition in the hypothetical limit of infinitely long relaxation times.

Transition temperature Tg

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[pic]Measurement of Tg by differential scanning calorimetry
Refer to the figure on the right plotting the heat capacity as a function of temperature. In this context, Tg is the temperature corresponding to point A on the curve. The linear sections below and above Tg are colored green. Tg is the temperature at the intersection of the red regression lines.
Different operational definitions of the glass transition temperature Tg are in use, and several of them are endorsed as accepted scientific standards. Nevertheless, all definitions are arbitrary, and all yield different numeric results: at best, values of Tg for a given substance agree within a few kelvins. One definition refers to the viscosity, fixing Tg at a value of 1013 poise (or 1012 Pa·s). As evidenced experimentally, this value is close to the annealing point of many glasses.
In contrast to viscosity, the thermal expansion, heat capacity, and many other properties of inorganic glasses show a relatively sudden change at the glass transition temperature.

Any such step or kink can be used to define Tg. To make this definition reproducible, the cooling or heating rate must be specified.
The most frequently used definition of Tg uses the energy release on heating in differential scanning calorimetry (DSC, see figure). Typically, the sample is first cooled with 10 K/min and then heated with that same speed.
Yet another definition of Tg uses the kink in dilatometry. Here, heating rates of 3-5 K/min are common. Summarized below are Tg values characteristic of certain classes of materials.
|Material |Tg (°C) |
|Tyre rubber |−70 |
|Polypropylene (atactic) |−20 |
|Poly(vinyl acetate) (PVAc) |30 |
|Polyethylene terephthalate (PET) |70 |
|Poly(vinyl alcohol) (PVA) |85 |
|Poly(vinyl chloride) (PVC) |80 |
|Polystyrene |95 |
|Polypropylene (isotactic) |0 |
|Poly-3-hydroxybutyrate (PHB) |15 |
|Poly(methylmethacrylate) (atactic) |105 |
|Poly(carbonate) |145 |
|Chalcogenide GeSbTe |150 |
|Chalcogenide AsGeSeTe |245 |
|ZBLAN glass |235 |
|Tellurium dioxide |280 |
|Polynorbornene |215 |
|Fluoroaluminate |400 |
|Soda-lime glass |520-600 |
|Fused quartz |~1200 |

These are only mean values, as the glass transition temperature depends on the cooling rate, molecular weight distribution and could be influenced by additives. Note also that for a semi-crystalline material, such as polyethylene that is 60-80% crystalline at room temperature, the quoted glass transition refers to what happens to the amorphous part of the material upon cooling.

Physics of glass

The physics of glass is the science of the glassy or amorphous state of matter as seen from an atomic or molecular point of view. This article provides an overview of research into glass: a solid in which no significant crystallization has occurred. Thus, there is no long-range ordering or extended formation of any Bravais lattice.
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[pic]Schematic representation of a random-network glassy form (top) and ordered crystalline lattice (bottom) of identical chemical composition.
Generally speaking, the atomic or molecular structure of glass exists in a metastable state with respect to its crystalline form. That is, glass would convert into a more stable crystalline form, but the rate of this conversion is slow. This essentially reflects the dynamic nature of the glass transition and the formation of the elastic solid from a non-equilibrium supercooled liquid state.
Much work has been done to elucidate the primary microstructural features of glass-forming substances at both small (microscopic) and large (macroscopic) scales. One emerging school of thought holds that a glass is simply the "limiting case" of a polycrystalline solid at small crystal size. Within this framework, domains, exhibiting various degrees of short-range order, become the building blocks of both metals and alloys, as well as glasses and ceramics. Both within and between these domains, microstructural defects provide the natural sites for atomic diffusion, viscous flow, and plastic deformation in solids.

Structure of glass

Domains and defects

Henry Eyring was among the first to explain the effects of liquid structure on various modes of molecular diffusion. Utilizing the "hole" or "free volume" theory of non-associated liquids, Eyring attempted to explain viscosity, plasticity and diffusion in liquids as the formation and filling of molecular-sized holes. In a manner analogous to that proposed by Einstein for a crystalline solid in his theory of specific heat, each atom is regarded as moving independently of all other atoms.
Mott and Gurney pointed out the absence in these theories of the concept of a varying degree of atomic or molecular order, suggesting the possibility of a continuous transition from the solid to the liquid phase. Bernal, however, had shown on geometrical grounds with various modes of particle packing that no continuous transition from the crystalline to the amorphous or liquid state is possible without passing through the polycrystalline state, in which regular regions are separated by regions of misfit.
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[pic]SEM micrograph of surface of colloidal solid. Structure and morphology consists of ordered domains with both interdomain and intradomain lattice defects.(Amorphous colloidal silica particles of average particle diameter 600 nm).

It was therefore proposed that an amorphous solid or glass is nothing more than the limiting state of a polycrystalline solid in which the individual grains or domains are so small that one cannot draw any sharp distinction between the surfaces or misfit. This suggestion was made in the interest of improvement over the interpretation made by Eyring of the entropy of fusion for the liquid state. Thus, the energy of the aggregate is assumed to be proportional to the total area of the surfaces of misfit, and the entropy approximated accordingly. Subsequent papers by Lennard-Jones et al. on cooperative phenomena emphasized the order-disorder transition of melting, utilizing the statistical approach of the Bragg–Williams model.
This interpretation of boundary motion is similar to that of Mott, who suggests that the fluidity of a liquid is due only to the presence of a large number of such mobile crystal defects. Mott's interpretation of melting or loss of rigidity of a polycrystalline mass is therefore based on the point at which boundaries between individual grains can move freely through the aggregate. This is similar to Frenkel's suggestion that the viscous flow of a liquid and its crystallization both take place by the same mechanism of diffusion.

Dislocations and melting

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[pic]Consider an edge dislocation in a crystal lattice. The Burgers vector is the amount by which the path around the singularity fails to close. The pattern would be a closed circuit on a perfect lattice.

The notion of inter-crystalline or grain boundaries being composed of proper arrays of dislocations led to the next logical step in the sequence of events leading to an understanding of flow mechanisms in liquids as well as in solids. MacKenzie and Mott suggested the possibility that melting results from a sudden proliferation of dislocations. This led Shockley to establish a relationship between the mobility of a dislocation and the fluidity of a liquid in quantitative manner, by assuming a value for the effective dislocation concentration in the liquid state (combined with certain assumptions regarding dislocation width). Sears subsequently proposed that viscosity anomalies in supercooled liquids result from the growth of solid-like nuclei via a screw dislocation mechanism. Cahn elaborated on the concept of crystal growth from the liquid state by proposing that—in sufficiently supercooled systems—a liquid-solid interface can advance normal to itself in the absence of any such heterogeneities.
Kurosawa's study suggested that the contribution of lattice defects is more important than that of lattice vibrations in the melting of ionic crystals. Kuhlmannn-Wilsdorf, emphasizing that the state of a dislocation core is more liquid than solid-like, suggested the pairing of dislocations whose Burgers vectors were of equal magnitude but opposite sign. In this "dislocation dipole", the long-range strain fields of the two members of the dipole cancel each other, resulting in a loss of all resistance to shear forces.

Dynamics of glass

Viscoelasticity

Even most simple liquids will exhibit some elastic response at frequencies or shear rates exceeding 5 x 106 Hz. Alternatively, if the vibrational period of the force is large (low frequency) compared with the relaxation time, then the vibrational motion of the body will partially degenerate into translational motion, and the resulting displacement will be evidenced by viscous flow. Materials which respond to mechanical disturbances by both viscous (or plastic/irreversible) and elastic (or reversible) behavior under distinct ranges of deformation (and rate deformation, or frequency) are referred to as viscoelastic.
Thus, when a mechanical force is applied suddenly to a fluid, the fluid responds elastically at first, just as if it were a solid body. Whether the rigidity or the fluidity predominates in a material under given conditions is therefore determined by the time scale of the experiment relative to the characteristic time of structural relaxation of the material. Zwanzig and Mountain calculated the high-frequency elastic moduli of simple fluids by considering the pressure and internal energy of the fluid. They concluded that the initial response to a sudden disturbance can be characterized by two quantities: 1. The high-frequency limit of the shear modulus G (or modulus of rigidity) 1. The high-frequency limit of the bulk modulus K (or modulus of compression).
The connection between a viscous and an elastic response is made by considering the stress for a disturbance varying periodically in time with a frequency, υ. For consideration of shear flow, it is supposed that the shear viscosity q(υ) is a function of the frequency, and is related to the relaxation time t, which is characteristic of the transition from elastic to viscous response. High-frequency disturbances are identified as those relating to elastic behavior, while low-frequency behavior is identified as ordinary viscous flow. Thus the frequency-dependent viscosity coefficient q(υ) is capable of describing both viscous and elastic phenomena, and can be related to the frequency-dependent elastic moduli, K(υ) and G(υ).

Vitrification

Since the early theoretical and experimental investigations on polymorphism and the various states of aggregation which can be assumed by a given substance, the vitreous state of a substance has been recognized as having the mechanical response of both solid and liquid, depending on the time and spatial scale under consideration. The atomic arrangement of network-forming oxides in the vitreous or glassy state was initially described as exhibiting the disorder (or short-range order) of its liquid precursor and therefore defying description through any crystalline hypotheses.

This point is emphasized in a dynamic interpretation of vitrification. Thus, the kinetic treatment of glass formation by Uhlmann which is based upon the kinetic equations for phase changes developed by Avrami should be contrasted, for example, with a mechanical description of vitrification. The latter emphasizes the localized nature of the distribution of microscopic stress and strain upon vitrification.

Elastic waves

It may be wise to note that the dynamics of any rate-controlled phenomena can only be understood if one considers the dynamic nature of its primary constituents. In this particular case, thermal motion in liquids can be decomposed into elementary longitudinal waves (or acoustic phonons), while transverse waves (or shear waves) were originally described (and observed) only in the crystalline state. This is the fundamental reason why simple liquids cannot support a shearing stress, but rather yield via plastic deformation and macroscopic flow. The inadequacies of this conclusion, however, were pointed out by Frenkel in his revision of the theory of elasticity of liquids. This revision follows directly from the continuous character of the transition from the liquid state into the solid one when this transition is not accompanied by crystallization and long-range atomic and/or molecular ordering (or self-assembly).
The square root of the ratio of the shear modulus G to the density will be equal to the velocity of transverse phonons. Thus, the wave velocities will be given by: [pic] [pic] where the constant of proportionality ρ in both cases is the particle density or reciprocal specific volume.

Heat transport

The velocities of longitudinal acoustic phonons in condensed matter are directly responsible for the thermal conductivity which levels out temperature differentials between compressed and expanded volume elements. The thermal properties of glass are interpreted in terms of an approximately constant mean free path for lattice phonons. Furthermore, the value of the mean free path is of the order of magnitude of the scale of structural (dis)order at the atomic or molecular level.

Thus, heat transport in both glassy and crystalline dielectric solids occurs through elastic vibrations of the lattice. This transport is limited by elastic scattering of acoustic phonons by lattice defects. These predictions were confirmed by the experiments of Chang and Jones on commercial glasses and glass ceramics, where mean free paths were limited by "internal boundary scattering" to length scales of 10−2 cm to 10−3 cm.
The phonon mean free path has been associated directly with the effective relaxation length for processes without directional correlation. Thus, if Vg is the group velocity of a phonon wave packet, then the relaxation length [pic]is defined as: [pic] where t is the characteristic relaxation time. Now, since longitudinal waves have a much greater group or "phase velocity" than that of transverse waves, Vlong is much greater than Vtrans, the relaxation length or mean free path of longitudinal phonons will be much greater. Thus, thermal conductivity will be largely determined by the speed of longitudinal phonons.

Electronic structure

The influence of thermal phonons and their interaction with electronic structure is a topic which was appropriately introduced in a discussion of the resistance of liquid metals. Lindemann's theory of melting is referenced, and it is suggested that the drop in conductivity in going from the crystalline to the liquid state is due to the increased scattering of conduction electrons as a result of the increased amplitude of atomic vibration. Such theories of localization have been applied to transport in metallic glasses, where the mean free path of the electrons is very small (on the order of the interatomic spacing).
Thus, if the electrical conductivity is low, the mean free path of the electrons is very short. The electrons will only be sensitive to the short-range order in the glass since they do not get a chance to scatter from atoms spaced at large distances. Since the short-range order is similar in glasses and crystals, the electronic energies should be similar in these two states. For alloys with lower resistivity and longer electronic mean free paths, the electrons could begin to sense that there is disorder in the glass, and this would raise their energies and destabilize the glass with respect to crystallization. Thus, the glass formation tendencies of certain alloys may therefore be due in part to the fact that the electron mean free paths are very short, so that only the short-range order is ever important for the energy of the electrons.

Different types of Glass

Borophosphosilicate glass

Borophosphosilicate glass, commonly known as BPSG, is a type of silicate glass that includes additives of both boron and phosphorus. Silicate glasses such as PSG and borophosphosilicate glass are commonly used in semiconductor device fabrication for intermetal layers, i.e., insulating layers deposited between succeedingly higher metal or conducting layers.
BPSG has been implicated in increasing a device's susceptibility to soft errors since the Boron-10 isotope is good at capturing thermal neutrons from cosmic radiation. It then undergoes fission producing a gamma ray, an alpha particle, and a lithium ion. These products may then dump charge into nearby structures, causing data loss (bit flipping, or single event upset).
In critical designs, depleted boron consisting almost entirely of Boron-11 is used to avoid this effect as a radiation hardening measure. Boron-11 is a by-product of the nuclear industry.

Borosilicate glass

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[pic]Borosilicate glassware (two beakers and a test tube)
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion (~5 × 10−6 /°C at 20°C), making them resistant to thermal shock, more so than any other common glass.

Borosilicate glass was first developed by German glassmaker Otto Schott in the late 19th century and sold under the brand name "Duran" in 1893. After Corning Glass Works introduced Pyrex in 1915, it became a synonym for borosilicate glass in the English-speaking world.
In addition to the quartz, sodium carbonate, and calcium carbonate traditionally used in glassmaking, boron is used in the manufacture of borosilicate glass. Typically, the resulting glass composition is about 70% silica, 10% boron oxide, 8% sodium oxide, 8% potassium oxide, and 1% calcium oxide (lime). Though somewhat more difficult to make than traditional glass (Corning conducted a major revamp of their operations to make it), it is economical to produce; its superior durability, chemical and heat resistance finds excellent use in chemical laboratory equipment, cookware, lighting and, in certain cases, windows.

Manufacturing process

Borosilicate glass is created by adding boron to the traditional glassmaker's frit of silicate sand, soda, and ground lime. Since borosilicate glass melts at a higher temperature than ordinary silicate glass, some new techniques were required for industrial production. Borrowing from the welding trade, burners combining oxygen with natural gas were required.

Usage

During the mid-twentieth century, borosilicate glass tubing was used to pipe coolants (often distilled water) through high power vacuum tube–based electronic equipment, such as commercial broadcast transmitters.
Glass cookware is another common usage. Borosilicate glass is used for measuring cups, featuring screen printed markings providing graduated measurements, which are widely used in American kitchens.
Aquarium heaters are sometimes made of borosilicate glass. Due to its high heat resistance, it can tolerate the significant temperature difference between the water and the nichrome heating element.
Many high-quality flashlights use borosilicate glass for the lens. This allows for a higher percentage of light transmittance through the lens compared to plastics and lower-quality glass.
Specialty marijuana and tobacco pipes are made from borosilicate glass. The high heat resistance makes the pipes more durable.
Most premanufactured glass guitar slides are also made of borosilicate glass.

Flint glass

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[pic]An achromatic doublet, which combines crown glass and flint glass.
Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00. A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet lens because of their compensating optical properties, which reduces chromatic aberration (color defects).
With respect to glass, the term flint derives from the flint nodules found in the chalk deposits of southeast England that were used as a source of high purity silica by George Ravenscroft, circa 1662, to produce a potash lead glass that was the precursor to English lead crystal.
Traditionally, flint glasses were lead glasses containing around 4–60% lead oxide; however, the manufacture and disposal of these glasses are sources of pollution. In many modern flint glasses, the lead can be replaced with other additives such as titanium dioxide and zirconium dioxide without significantly altering the optical properties of the glass.
Flint glass can be fashioned into rhinestones which are used as diamond simulants.

Lead glass

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[pic]Lead crystalware

Lead glass is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40 weight% lead(II) oxide (PbO), while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO. Lead glass is desirable owing to its decorative properties.
The fluxing and refractive properties valued for lead glass also make it attractive as a pottery or ceramic glaze. Lead glazes first appear in first century BC to first century AD Roman wares, and occur nearly simultaneously in China. They were very high in lead, 45–60% PbO, with a very low alkali content, less than 2%.[8] From the Roman period, they remained popular through the Byzantine and Islamic periods in the Near East, on pottery vessels and tiles throughout medieval Europe, and up to the present day. In China, similar glazes were used from the twelfth century for colored enamels on stoneware, and on porcelain from the fourteenth century. These could be applied in three different ways. Lead could be added directly to a ceramic body in the form of a lead compound in suspension, either from galena (PbS), red lead (Pb3O4), white lead (2PbCO3·Pb(OH)2), or as lead oxide. The second method involves mixing the lead compound with silica, which is then placed in suspension and applied directly. The third method involves fritting the lead compound with silica, powdering the mixture, and suspending and applying it.[8] The method used on a particular vessel may be deduced by analysing the interaction layer between the glaze and the ceramic body microscopically.
Tin-opacified glazes appear in Iraq in the eighth century AD. Originally containing 1–2% PbO, by the eleventh century high-lead glaze had developed, typically containing 20–40% PbO and 5–12% alkali. These were used throughout Europe and the Near East, especially in Iznik ware, and continue to be used today. Glazes with even-higher lead content occur in Spanish and Italian maiolica, with up to 55% PbO and as low as 3% alkali.[8] Adding lead to the melt allows the formation of tin oxide more readily than in an alkali glaze, which precipitates into crystals in the glaze as it cools, creating its opacity.

Crown glass

Crown glass is type of optical glass used in lenses and other optical components.
Crown glass is produced from alkali-lime (RCH) silicates containing approximately 10% potassium oxide. It has low refractive index (≈1.52) and low dispersion (with Abbe numbers around 60); crown glass is one of the earliest low dispersion glasses.
As well as the specific material named crown glass, there are other optical glasses with similar properties that are also called crown glasses. Generally, this is any glass with Abbe numbers in the range 50 to 85. For example, the borosilicate glass Schott BK7 is an extremely common crown glass, used in precision lenses. Borosilicates contain about 10% boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage. Other additives used in crown glasses include zinc oxide, phosphorus pentoxide, barium oxide, fluorite and lanthanum oxide.

Soda-lime glass

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[pic]Reusable soda-lime glass milk bottles
Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers (bottles and jars) for beverages, food, and some commodity items. Glass bakeware is often made of tempered soda-lime glass.
Soda-lime glass is prepared by melting the raw materials, such as sodium carbonate (soda), lime, dolomite, silicon dioxide (silica), aluminium oxide (alumina), and small quantities of fining agents (e.g., sodium sulfate, sodium chloride) in a glass furnace at temperatures locally up to 1675 °C. The temperature is only limited by the quality of the furnace superstructure material and by the glass composition. Green and brown bottles are obtained from raw materials containing iron oxide. Relatively inexpensive minerals such as trona, sand, and feldspar are used instead of pure chemicals. The mix of raw materials is termed batch.
Soda-lime glass is divided technically into glass used for windows, called float glass or flat glass, and glass for containers, called container glass. Both types differ in the application, production method (float process for windows, blowing and pressing for containers), and chemical composition (see table below). Float glass has a higher magnesium oxide and sodium oxide content as compared to container glass, and a lower silica, calcium oxide, and aluminium oxide content. From this follows a slightly higher quality of container glass concerning the chemical durability against water (see table), which is required especially for storage of beverages and food.

Silicate glass

Silicate glasses are amorphous and have no crystalline structure. Silicate glass is useful for conducting x-ray crystallography because the x-rays will pass through the silicate pipette holding the sample under examination without being reflected by crystals in the glass itself; thus the resulting measurement is assured to be from the sample. Silicate glasses have also been commonly used in the field of semiconductor device fabrication as an insulator between active layers of the semiconductor device. Also, some airbags in cars react SiO2 with harmful byproducts of nitrogen gas producing reactions to produce Silicate glass to remove the harmful substances (K2O and Na2O).
These materials have relatively low melting temperatures, and can be flowed by heating in order to "planarize" the semiconductor layers. There will typically be contact holes or vias etched into the glass layers using wet etching or dry etching, and the silicate glasses can then be reflowed by heating in order to make smoother tops to the contact holes or vias, which makes the metal connections into the contact holes or vias more durable.
The silicate glasses are typically formed of phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG). The boron and/or phosphorus impurity levels used can be adjusted to affect the silicate glass's melting point.

[pic]Cast glass

Cast glass windows, albeit with poor optical qualities, began to appear in the most important buildings in Rome and the most luxurious villas of Herculaneum and Pompeii.

Cylinder glass

In this manufacturing process glass is blown into a cylindrical iron mould. The ends are cut off and a cut is made down the side of the cylinder. The cut cylinder is then placed in an oven where the cylinder unrolls into a flat glass sheet.

Drawn Sheet glass (Fourcault process)

Drawn Sheet glass was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat - this is known as the Fourcault process. This film or ribbon was pulled up continuously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. This glass may still be seen in older houses. Float glass replaced this process.

Cast plate glass

Developed by James Hartley, 1848. The glass is taken from the furnace in large iron ladles, which are carried upon slings running on overhead rails; from the ladle the glass is thrown upon the cast-iron bed of a rolling-table; and is rolled into sheet by an iron roller, the process being similar to that employed in making plate-glass, but on a smaller scale. The sheet thus rolled is roughly trimmed while hot and soft, so as to remove those portions of glass which have been spoiled by immediate contact with the ladle, and the sheet, still soft, is pushed into the open mouth of an annealing tunnel or temperature-controlled oven called a lehr, down which it is carried by a system of rollers.

Polished plate glass

The polished plate glass process starts with sheet or rolled plate glass. This glass is dimensionally inaccurate and often created visual distortions. These rough panes were ground flat and then polished clear. This was a fairly expensive process.
Before the float process, mirrors were plate glass as sheet glass had visual distortions that were akin to those seen in amusement park or fun-fair mirrors.

Rolled plate (figured) glass

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[pic]Figure rolled glass
The elaborate patterns found on figured (or 'Cathedral') rolled-plate glass are produced in a similar fashion to the rolled plate glass process except that the plate is cast between two rollers, one of which carries a pattern. On occasions both rollers can carry a pattern. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.

Float glass

90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces.
A very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass. The tin side of glass is also softer and easier to scratch.

Prism glass

Prism glass is architectural glass used around the turn of the century to provide lighting to underground spaces and areas that would otherwise be too difficult to light. Prism glass uses a unique convex lens design to help illuminate more than ordinary glass. Prism glass can sometimes be found on sidewalks and in this form is known as vault lighting.

Laminated glass

[pic]

[pic]Broken tempered laminated glass "wet blanket effect"
Laminated glass is manufactured by bonding two or more layers of glass together with layers of PVB, under heat and pressure, to create a single sheet of glass. When broken, the PVB interlayer keeps the layers of glass bonded and prevents it from breaking apart. The interlayer can also give the glass a higher sound insulation rating.
There are several types of laminated glasses manufactured using different types of glass and interlayers which produce different results when broken.
Laminated glass that is made up of annealed glass is normally used when safety is a concern, but tempering is not an option. Windshields are typically laminated glasses. When broken, the PVB layer prevents the glass from breaking apart creating a "spider web" cracking pattern.
Tempered laminated glass is designed to shatter into small pieces, preventing possible injury. When both pieces of glass are broken it produces a "wet blanket" effect and it will fall out of its opening.
Heat strengthened laminated glass is stronger than annealed, but not as strong as tempered. It is often used where security is a concern. It has a larger break pattern than tempered, but because it holds its shape (unlike the "wet blanket" effect of tempered laminated glass) it remains in the opening and can withstand more force for a longer period of time, making it much more difficult to get through.

Toughened glass (tempered glass)

Toughened (or tempered) glass is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations. Using toughened glass on automobile windshields would be a problem when a small stone hits the windshield at speed, as it would shatter into small squares endangering the driver and passengers. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and door lites and vision lites adjacent to doors. Toughened glass is typically four to six times the strength of annealed glass.

Heat-strengthened glass

Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses.

Chemically strengthened glass

Chemically strengthened glass is a type of glass that has increased strength. When broken it still shatters in long pointed splinters similar to float (annealed) glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. Chemically strengthened glass is typically six to eight times the strength of annealed glass.
The glass is chemically strengthened by submerging the glass in a bath containing a potassium salt (typically potassium nitrate) at 450 °C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution.

Low-emissivity glass

Glass coated with a low-emissivity substance can reflect radiant infrared energy, encouraging radiant heat to remain on the same side of the glass from which it originated, while letting visible light pass. This often results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside, while infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.

References

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