Paper on Engineering Materials (Glass)

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  • Topic: Glass, Glass transition, Float glass
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  • Published : February 18, 2012
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Engineering Materials (Glass)


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.

[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


[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...
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