Glass:
•A state of matter as well as a type of ceramic
•As a state of matter, the term refers to an amorphous (noncrystalline) structure of a solid material. The glassy state occurs in a material when insufficient time is allowed during cooling from the molten state for the crystalline structure to form.
• As a type of ceramic, glass is an inorganic, nonmetallic compound (or mixture of compounds) that cools to a rigid condition without crystallizing. They may have a wide range of compositions, but have the property in common that during manufacture they are cooled quickly enough to prevent crystallization from taking place, so that the glass (amorphous) state is retained at ambient temperatures. This happens because the chemical ‘unit’ in glasses is very large, being formed of large silicate networks, bonded internally covalently, and held together by ionic bonds provided by metallic ions within the structure. Movement of these ‘units’ is difficult even when they are thermally activated, so it is easy to cool the material past its normal melting point without crystallization taking place. As shown in Figure 1, the material cooled slowly will crystallize at the normal melting point. Typically this will not happen, and more rapid cooling results in the formation of a supercooled liquid. Even in this state, some contraction takes place, until temperature reaches a point where a sudden change occurs because there is no further possibility of molecular readjustment. This temperature is referred to as the ‘glass transition temperature’: as with polymers this is not a well-defined temperature, and depends on the cooling rate.
Why So Much SiO2 in Glass?
•Because SiO2 is the best glass former
- Silica is the main component in glass products, usually comprising 50% to 75% of total chemistry.
- It naturally transforms into a glassy state upon cooling from the liquid, whereas most ceramics crystallize upon solidification.
Other Ingredients in Glass:
•Sodium oxide (Na2O), calcium oxide (CaO), aluminum oxide (Al2O3), magnesium oxide (MgO), potassium oxide (K2O), lead oxide (PbO), and boron oxide (B2O3).
•Functions:
- Act as flux (promoting fusion) during heating.
- Increase fluidity in molten glass for processing.
- Improve chemical resistance against attack by acids, basic substances, or water.
- Add color to the glass.
- Alter index of refraction for optical applications.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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