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GLASS Man made glass has been used for many thousands of years, the first known glass makers being the Egyptians. The glasses used throughout the ages have changed very little in their chemical composition, to the extent that, today, the basic materials are still identical with those used to produced glass hundreds and even thousands of years ago. An interesting and provocative definition of glass is by Morey: Glass is an inorganic substance in a condition which is continuous with, and analogous to, the liquid state of the substance, but which, as the result has been cooled from a fused condition, has attained so high a degree of Viscosity as to be for all practical purposes; rigid. Glass is a non crystalline material produced primarily from inorganic oxides. The principal constituent of most glass is sand, or silica. Since the silica cannot be melted readily, various fluxes are adapted to lower the fusion point of the mixture. Other common glass formers are oxides of boron and phosphorus. Other oxides in glass act as modifiers on the glassy network. STRENGTH As an engineering material, glass is unique. Two of the most significant properties that make it so, are (1) the mechanical strength, and (2) viscosity. Glass is a brittle material but one that is truly elastic with no plastic deformation to failure. The intrinsic strength of glass is extremely high and certain experiments have substantiated that glass may be stressed in the order of 3,000,000 PSI before failure. However, it is difficult to achieve, in current practice, more than a small fraction of these strengths. Measurements of the strength of glass are not true measurements of the strength, but the weakness of the surface. It is the condition of the surface that limits our utilization of this high strength material. The surface irregularities create stress risers, and, as a result, surface stress variations, and by the virtue of the lack of plasticity, these stresses are not relieved by plastic deformation and will generate a catastrophic type of failure by virtue of a crack. In other words, it is extremely notch sensitive. VISCOSITY Actually, at room temperature, glass is a viscous material. At this temperature, however, the viscosity is so high that for all practical purposes, it is considered a solid. The fantastic reduction in viscosity between 500°C and room temperature is significant; however, the difference between 500°C and 1500°C will be approximately 1,000,000,000,000 times as great. It should be pointed out that glasses do not have true melting points. They simply become less viscous at elevated temperatures, and the viscosity reduction is approximately exponential with temperature. THERMAL ENDURANCE One other notable property of glass is its relatively high thermal endurance. The thermal endurance is enhanced when under compressive stress.
ELASTICITY For all ordinary purposes, it can be assumed that glass is perfectly elastic up to the point of fracture. The Young's modulus of elasticity varies from 6,000,000 to 17,000,000 PSI depending on the composition, but most commercial glasses have values between 9,000,000 and 12,000,000 PSI. ELECTRICAL PROPERTIES One of the primary values of glass in electrical applications is its property of electrical insulation. The resistance glass offers to the passage of electricity depends on the type of glass, the temperature of the glass and the surface conditions. On the other hand, glass can be given a metallic oxide coating that conducts electricity. DIELECTRIC STRENGTH The dielectric strength of glass is very high. Thus, in hermetic connector applications, the prime design consideration is to provide sufficient distance over the glass to prevent flashover. Dielectric breakdown voltage decreases with an increase in frequency and temperatures. At elevated temperatures, breakdown is governed mainly by the resistivity of the glass at those temperatures. VOLUME RESISTIVITY Volume resistivity of glass at ordinary room temperature varies widely with the composition from as low as 108 to as high as 1019 ohm centimeters. The resistivity of a given composition at a fixed temperature may vary by a factor of 3, depending upon the degree of annealing or strain that the glass has received. Strained glass has lower resistance than properly annealed glass, which is practically strain-free. SURFACE RESISTIVITY Surface electrical resistivity is of primary importance in all electrical insulation problems with glasses or ceramics since it greatly affects performance under service conditions that involve high humidity. Due to their absorption of a moisture film, the surface electrical resistivity of glasses and ceramics, while high in comparison to that of many other materials, is remarkably lower than volume resistivity. Thus chemical durability is an essential requirement in good electrical glass because film thickness and film conductivity both are increased by the soluble products of unstable glass compositions. GLASS IN COMPRESSION As previously mentioned, glass has a very high capacity to withstand compression stresses. By virtue of inducing and maintaining only compression stresses in the glass, the glass member can be loaded in many ways; as long as the imposed loading does not exceed the compressive preload, the glass will withstand the applied preload without a stress reversal and not generate tensile stresses where its notch sensitivity would limit its usefulness.
Glass: some facts
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