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Tuesday, 29 December 2015

GLASS

Glass:
Introduction:

          Glass is any substance or mixture of substances that has solidified from the liquid state without crystallisation. The term "Glass" as ordinarily used referred as an amorphous, hard, brittle, transparent or translucent super cooled liquid of infinite viscosity, having no definite melting point obtained by fusing a mixture of number of metallic silicates or borates of sodium, potassium, calcium and lead. Many of its properties corresponding to those of supercooled liquid whose ingredients can't be identified because they have not separated from the solution in in crystallisation form.
Glass is an amorphous solid having a homogeneous texture.

Structure of Glass:


          The glass is a random arrangement of molecules, the great majority of which are oxygen ions bonded together with the network forming ions of silicon, boron or phosphorous. If the network was perfectly regular, each silicon atom would be surrounded by four Oxygen atoms arranged towards the corner of a tetrahedron. Because each oxygen atom in this network is shared by two silicon atoms, the empirical formula of this solid would be SiO2 and the material would have the structure of quartz. A glass made of silica alone has many desirable characteristics but unfortunately the high temperature involved make it expensive, and difficult to prepare. In order to reduce the temperature, required network-modifying ions are added. Sodium, Potassium, Titanium and Calcium are the most common. The network-modifying ions increase the competition for the oxygen ions, thus loosing the Si-O-Si bonds. Certain other ions may substitute for either the network-modifying ions: Aluminium, lead are the few of these intermediate ions.
         
         The amorphous structure of glass makes it brittle. Because glass doesn't contain planes of atoms that can slip past each other, there is no way to relieve stress. Excessive stress therefore forms a crack that starts at a point where there is a surface flaw. Particles on the surface of the crack become separated. The stress that formed the crack is now borne by particles that have fewer neighbours over which the stress can be distributed. As the crack grows, the intensity of the stress at its tip increases. This allows more bonds to break, and the crack widens until the glass breaks. thus, if you want to cut a piece of glass, start by scoring the glass with a file to produce a scratch along which it will break when stressed. 

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