منتدى طلبة كلية الهندسه بأسوان

منتدى طلبة كلية الهندسه بأسوان
 
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تاريخ التسجيل : 28/03/2014
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مُساهمةموضوع: كتب    الجمعة 28 مارس - 20:09

glass-ceramic products exceed $500  million a year. The highest volume is in cookware and  tableware consumer items (such as plates and bowls) and domestic ovens (stovetops and stove windows)(1).Glass-ceramics are polycrystalline material formed by controlled crystallization of a glass(2). They consist of a high density (maybe >95 vol%) of small crystals in a glass matrix. Most commercial glass-ceramics products are formed by highly auto-mated glass forming processes and converted to a crystalline product by the proper heat treatment. Glass- ceramics can also be prepared via powder processing methods in which glass frits are sintered and crystallized. The important feature of the processing of glass-ceramics is that the crystallization must be controlled. As usual, crystallization occurs in two stages:-
Ø First the crystals are nucleated.
Ø Then the crystals grow.
The rate at which these two processes occur is a function  of temperature. We can control the nucleation process by adding a nucleating agent
(typically either TiO2 or ZrO2) to the glass batch. Initially the glass batch
is heated to form a homogeneous melt. The shape of the desired object is formed from the glass at the working point by the usual processes such as
pressing, blowing, rolling, or casting. the viscosity of the glass is 104

[justify]dPa-s at the working point. After  annealing to eliminate  internal stresses, the glass object then undergoes a thermal treatment that converts it into a glass-ceramic. [size=19][font='Times New Roman', serif]he first part of the heat treatment is nucleation. The optimum nucleation temperature generally corresponds to a glass viscosity of 1011-1012dPa·s. During this step, which may last for several hours, an extremely high density (1012-1015/cm3)of nuclei forms. Following nucleation,  T is increased to allow growth to occur. The crystal-growth step, like nucleation, may take several hours. The optimum temperature for crystal growth is selected to allow for the maximum development of the crystalline phase without viscous deformation of the object. y]
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