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Structure and Properties of Glasses
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Definition of a Glass A glass is a ceramic material in that it is made from inorganic materials at high temperatures. However, it is distinguished from other ceramics in that its constituents are heated to fusion and then cooled to a rigid state without crystallization. Thus, a glass can be defined as an inorganic product of fusion that has cooled to a rigid condition without crystallization. Glass has a non-crystalline or amorphous structure. The molecules in a glass are not arranged in regular repetitive long-range order as exists in crystalline solid.
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Structure of Glasses Glass-Forming Oxides The most inorganic glasses are based on the glass- forming oxide silica, SiO 2 . The fundamental sub-unit in silica-based glasses is the tetrahedron in which a silicon (Si 4+ ) atom (ion) in the tetrahedron is covalently ionically bonded to four oxygen atoms (ions)
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In crystalline silica, for example, cristobalite, the Si-O tetrahedra are joined corner to corner in a regular arrangement, producing long- range order. In a simple silica glass the tetrahedra are joined corner to corner to form a loose network with no long-range order.
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Boron Oxide, B 2 O 3, is also a glass-forming oxide and by itself forms subunits that are flat triangles with the boron atoms slightly out of the plane of the oxygen atoms. However, in borosilicate glasses that have additions of alkali and alkaline earth oxides, triangles can be converted to tetrahedra. Boron oxide is an important addition to many types of commercial glasses such as borosilicate and aluminoborosilicate glasses.
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Glass-Modifying Oxides Oxides that can break up the glass network are known as network modifiers. Alkali oxides such as Na 2 O and K 2 O and alkaline earth oxides such as CaO and MgO are added to silica glass to lower its viscosity so that it can be worked and formed more easily.
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  • Fall '16
  • Unkwnow
  • Glass, Glass transition, Silicon dioxide, Amorphous solid, C. Thermal

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