In the ceramic discipline, the statement glass signifies every amorphous component of a ceramic mixture. More commonly glass is an apparent silica product which may be amorphous or crystalline, based on the heat treatment. Glasses may be either inorganic or organic.
Vitreous material or inorganic glasses are the fusion products that during solidification from a liquid state fail to crystallize. Glass proprieties are unfavorably affected by crystal development in glass liquid. During the cooling process, the glass shows no discontinued alteration at any temperature, and barely a progressive rise in viscosity is noticed. In reality, glass is a hard liquid.
Properties of glasses:
Non-crystalline materials don’t solidify in the same sense as those that are crystalline. During cooling the glass increases its viscosity proportional to decreasing temperature; a specified temperature cannot be determined at which the liquid converts to a solid state with crystalline nature. The difference between all crystalline and noncrystalline materials is distinguished based on specific volume (or volume per unit mass, the reciprocal of density) and on temperature.
For crystalline materials, there is a discontinuous decrease in volume at the melting temperature of Tm. However, for glassy materials volume decrease continuously with temperature reduction a slight decrease in the slope of the curve occurs at what is called the glass transition temperature or fictive temperature Tg.
Below this temperature, the material is considered to be glass; above it is a supercooled liquid and finally a liquid
Types of Glasses:
Commercial glasses can be classified into four categories as
- Soda lime glasses
- Lead glasses
- Borosilicate glasses
- High silica glasses
- Besides these commercial glasses, there are re-crystallized glasses, foam glasses, and fiber glasses.
Soda lime glasses or lime glasses are made by fusing together sand, lime or limestone, and soda ash. Since the raw materials are very cheap and low temperatures may be employed for melting these glasses, soda lime glasses are very cheap and consequently comprise the largest tonnage of glass manufacture. These glasses are used as window glass, and bottles. Lamp globes, ordinary chemical apparatus like the test tube, beakers, and so on
The oxide components in percentage are, SiO2=70-75%, Na2O= 12-18%, K2O 0-1%, CaO=5-4%, Al2O3=0.5-2.5%, MgO=0-4%
Lead glasses also called “flint” glasses are used for high-quality tableware, optical purposes, neon sign tubing, and art objects because of their high luster, and for windows and shields to protect personnel from X-ray radiation. Lead glasses have a relatively low melting point, but they exhibit good hot workability, high electrical resistivity, and high refractory indices. The composition is SiO2=53-68%, Na2O= 5-10%, K2O=1-10%, CaO=0-6%, Al2O3=0-2%, PbO=15-40%
Borosilicate glasses have a part of silica is replaced by boric oxide which confers certain desirable properties to glass such as thermal shock electrical shock and chemical stability. Due to the presence of B2O3 in place of soda and lime its coefficient of expansion is reduced from 9.0X10-6 to 3.5 X10-6. It finds wide application in kitchenware glass pipelines in factories, high-tension insulators, telescope mirrors, and laboratory wares. A special glass of this type is manufactured under the trade name “Pyrex”. Its composition is SiO2=73-82%, Na2O= 3-10%, K2O= 0.4-1%, CaO=0-1%, Al2O3=2-3%, PbO=0-1%, B2O3= 5-20%.
High silica glass (contains 96 percent silica) is mainly used where high-temperature resistance is required and it can be used regularly at temperatures up to about 900 deg cent. High silica glasses have a very low thermal expansion coefficient which accounts for their high resistance to thermal shock. This glass composition would be SiO2=96%, B2O3=3%.
Colored glass is manufactured when various substance is added to the glass fusion purposely to obtain colored glasses. Thus, for instance, chrome oxide Cr3O3 gives the glass a green color, manganese dioxide a reddish-purple color, cobaltous oxide a blue color, etc. in most cases the color of the glass depends on the formation of colored silicates of iron, manganese, cobalt, etc. but sometimes it is due to the fact that the substance added is present in the glass in the form of very finely divided particles.
Thus if a very small quantity of gold is added to glass it acquired a ruby red color due to the presence of minute particles of gold invisible even under the microscope which forms when the melted glass is cooled slowly.
Recrystallized glass also known as polycrystalline glass is commonly produced by adding nucleating agents such as sodium fluoride, phosphorous pentoxide, titanium oxide, or vanadium oxide to the molten glass. The glass is then formed by any of the conventional glass-forming processes and is then heat treated to promote crystallization. Such glasses possess a high hardness and impact strength and better thermal stability compared to ordinary glass. The chief application of these glasses is in the manufacture of the so-called refrigerator- to oven cooking dishes.
Fiberglass, commonly known as glass fiber is glass in fiber form. Molten glass can be drawn through dies into a fiber of 2 to 10-micron diameter and is then known as fiberglass. This is not brittle like ordinary glass and possesses high tensile strength often in excess of 700 MN/m2. Unlike normal glass, glass fibers are almost free from surface defects and thus crack propagation under tensile loads. These fibers are nonflammable, conduct heat nor electrical current are poor conductors of sound, and are chemically inactive.
Fiberglass mixed with various synthetic resins gives new types of building material known as glass fiber laminates. They are three to four times lighter than steel but are just as strong which makes them eligible as substitutes both for metal and wood in mechanical engineering and construction work, etc. glass fiber laminates are widely used for the manufacture of automobile and aircraft parts.
Glass wools are produced by a process entirely different from that of glass fiber; the molten glass is forced through some vents in the process known as a crown process by centrifugal force, forming relatively short fibers of about 0.007 mm in diameter. These are suitable for insulation.
Foam glasses are made by introducing innumerable air cells or pores into molten glass. This on cooling becomes quite light. In fact, it floats in the water as it is lighter than even cork. It is cut into suitable sizes and then used as heat-insulating material.
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