At ordinary temperatures, glass is a nearly perfect elastic solid, an excellent thermal and electrical insulator, and very resistant to many corrosive media. These glasses can be made more transparent to ultraviolet radiation by increasing the silica content. Known as the hackle, these ridges ultimately lead to crack branching. It is also known as electric permittivity. Otherwise, plastic deformation of glass (or ductility), which is generally observed in strength tests as the necking of a specimen placed under tension, is not observed; instead, glass failure is brittle—that is, the glass object fractures suddenly and completely. Thus, glass is an efficient electrical insulator—though this property varies with viscosity, which in turn is a function of temperature. Networks in which the interstitial spaces are less filled with NWM ions possess lower density and allow greater permeation of gases through them. 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. The more or less random order of atoms is ultimately responsible for many of the properties that distinguish glass from other solids. Vitreous silica is the most resistant, but borosilicates and aluminosilicates also offer excellent weathering resistance. In geometric terms, the right angle at which the light ray meets the boundary is called the normal. When glass is subjected to unequal stress components operating on perpendicular planes, it becomes birefringent (that is, doubly refracting). This thermal expansion of glass is critical to its thermal shock performance (that is, its performance when subjected suddenly to a temperature change). Author : Unknown Editor Dielectric Constant: Vacuum: 1: Glass: 5-10: Mica: 3-6: Mylar: 3.1: Neoprene: 6.70: Plexiglas: … Figure 7: The refraction and reflection of light. The dielectric constants of powders and flaked materials are significantly smaller than [the established values for the same materials a solid. Flaws have a stress-concentrating effect; that is, the effective stress at the tip of a flaw can be easily 100 to 1,000 times greater than that applied. Finally, if the critical angle is exceeded, all of the light will be reflected back into the glass without suffering any loss of intensity. Figure 5: The viscosity of representative silica glasses at varying temperatures. Many properties of soda-lime-silica glasses follow such relationships closely. In applications such as high-wattage lamps, where low electrical conductivity is desired, mixed-alkali glasses are useful. The resulting birefringence of a plane-polarized light can be measured by birefringence compensators such as a quartz wedge, and from this measurement the magnitude of the stresses can be estimated. In general, glasses that are low in alkali offer increased weathering resistance. The most noted exception from the additivity relationship here is the mixed-alkali effect, in which glasses containing two or more different types of alkali ions have a significantly lower electrical conductivity than linear additivity would suggest. Glass is exceptionally strong, much stronger than most metals, when tested in the pristine state. Evidence of this “thermal history” is wiped out only after the glass has been reheated to the liquid state. As is evident from Figure 1, glass normally expands when heated and shrinks when cooled. Dielectric Constants of Common Materials MATERIALS DEG. Because of the isotropic nature of glass, only two independent elastic moduli are normally measured: Young’s modulus, which measures the ability of a solid to recover its original dimensions after being subjected to lengthwise tension or compression; and shear modulus, which measures its ability to recover from transverse stress. 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. In chalcogenide glasses, semiconductivity is primarily caused by defective bonds in which a particular atom does not follow its covalent coordination. The primary determinant of chemical durability in glass is an ion exchange reaction in which alkali ions in the glass are exchanged with hydrogen atoms or hydronium ions present in atmospheric humidity or water. Black Friday Sale! Although most glasses contain charged metallic ions capable of carrying an electric current, the high viscosity of glass impedes their movements and electrical activity. This can be accomplished by two methods: fire polishing, a procedure that removes alkali ions by volatilization; or surface treatment with a mixture of sulfur dioxide and steam, which extracts alkali by leaching and converting to washable alkali sulfate. Density follows additivity behaviour closely. When a hot specimen of glass is suddenly cooled—for example, by plunging it in iced water—great tension may develop in the outside layers owing to their shrinking relative to the inner layers. Because electrons in glass molecules are confined to particular energy levels, they cannot absorb and reemit photons (the basic units of light energy) by skipping from one energy band to another and back again. The hardness of glass is measured by a diamond microindenter. Traditional photochromic eyeglasses are generally alkali boroaluminosilicates with 0.01 to 0.1 percent silver halide and a small amount of copper. Other methods of improving chemical durability involve limiting the access of water or humidity to the glass surface. The alkali ions thus leached out of the glass further react with carbon dioxide and water in the atmosphere to produce alkali carbonates and bicarbonates. The annealing point and the strain point lie in the glass transformation range shown in Figure 1; often, the glass transition temperature (Tg) and the annealing point are used synonymously, and the strain point marks the low-temperature end of the range. Borosilicate glass is used because of its unique sealing properties. A ray of light, on passing from one transparent medium to another transparent medium of different density, will be transmitted through the second medium with no loss of intensity or change in direction if it strikes the boundary between the two mediums at a right angle (90°). (Left) When light strikes the boundary between glass and air at less than the critical angle (. Radiation conductivity is also inversely proportional to the absorption coefficient of a glass for specific photon wavelengths. If it arrives at the boundary at the critical angle, then the emerging light will be of diminished intensity and will assume a direction parallel and close to the boundary; most of the light will be reflected. If the light ray meets the boundary at an angle other than the normal, then it will be partially reflected back into the first medium and partially refracted, or deflected, in its path through the second medium. It is evident from Figure 5 that the Tg of vitreous silica is the highest of the commercial glasses and that increasing the amount of alkali additions (and therefore the concentration of NWM ions) lowers Tg. Tensile stresses in excess of a low limit, called the fatigue limit, cause the flaw to undergo a subcritical crack growth. In a polariscope fitted with a tint plate, stressed glass displays colours; the distribution of these colours also may be used for recognizing stress patterns during quality-control operations. Dielectric Constant Table.xls Corn, Refuse 2.3-2.6 Corning Glass 6.5 Cotton 1.3-1.4 Cotton Seed Oil 3.1 Co2 (32° F) 1.6 Creosol (63° F) 10.6 Cresol (75° F) 5.0 Cresol, Liquid 9-11 Crotonic Nitrice (68° F) 28.0 Crystale 3.5-4.7 Cumaldehyde (59° F) 11.0 Cumene (68° F) 2.4 Cumicaldehyde (58° F) 10.7 Cupric Oleate 2.8 Cupric Oxide (60° F) 18.1
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