Module 3- Refractories, Glasses, Optical Fibers PDF

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Engineering chemistry 2
Basic subject to all polytechnic college students for sem 2...

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REFRACTORIES Refractories :- These are materials that can withstand high temperatures without softening, melting or deformation and are used for the construction of the linings of furnaces, tanks, kilns, crucibles etc. employed for manufacture of metals, cement, glass, ceramics, steel etc. Examples:- Alumina bricks, silica bricks, carborundum refractories, magnesia bricks etc Properties or characteristics of refractories 1) Refractoriness:- it is the ability of the material to withstand high temperature without appreciable softeninfg under working conditions. It is measured as softening or melting temperature of the material. A r efractory material should have a softening temperature much higher than the operating furnace temperature. 2) Strength under load;- A refractory material should have high mechanical strength even at the operating temperature to bear maximum possible load without breaking. 3) Porosity:- Porosity is the ratio of its pore’s volume to the bulk volume. A good refractory should have low porosity. 4) Thermal spalling:- It is the breaking, cracking, peeling off or fracturing of the refractory brick at high temperature or under rapid fluctuation in temperatures. 5) Chemical inertness:- A refractory material should be chemically inert towards slag, fuel, furnace gases etc 6) Thermal expansion:- A good refractory material should have least possible thermal expansion 7) Electrical conductivity:- A refractory material should have low electrical conductivity 8) Resistance towards abrasion or corrosion:- A good refractory material should r esist abrasion and erosion due to hard charge , fuel gases, carbon particles etc. 9) Dimensional stability:- It is the resistance of the material to volume changes which may occur due to exposure to high temperature and a good refractory material should maintain the dimensional stability Classification of refractories On the basis of chemical nature refractories are classified into acid refractories, basic refractories and neutral refractories Acid refractories:- They consist of acidic materials like alumina (Al2O3) and silica (SiO2)Basic refractories. They are not attacked by acidic materials but easily attacked by basic materials. eg. Alumina, silica and fire clay refractories. Basic refractories:- They consist of basic materials like CaO, MgO etc. They are not attacked by basic materials, but easily attacked by acidic materials. eg. Magnesite and dolomite refractories Neutral refractories:- They are made from weakly basic or acidic materials like carbon, chromite(FeO.CrO2), etc. They are not attacked by slightly acidic or basic materials. eg. Graphite, chromite and carborundum Classification of refractories based on physical nature Based on the physical form, refractories are classified into shaped refractories and unshaped refractories or monolithic refractories. Shaped refractories:-They are in the form of bricks of some standard dimension. These refractories are machine pressed and have uniform pr operties. Special shapes with required dimensions are hand molded and are used for particular kilns and furnaces. Monolithic refractories or Unshaped refractories:- Monolithic refractories are replacing conventional brick refractories in steel making and other metal extraction industries. Monolithic refractories are loose materials which can be used to for m joint free lining. Monolithic refractories can be installed by casting, spraying etc.

GLASSES Glass is an amorphous, hard, transparent, super cooled liquid of infinite viscosity, obtained by fusing a number of silicates, most commonly of Na, K, Ca and Pb. It possesses no sharp melting point, definite formula or crystalline structure. It can be represented as x R2O. y MO. 6SiO2 where R is an atom of monovalent alkali metals like Na, K etc. M is an atom of bivalent metals like Ca, Pb, Zn etc. x and y are whole numbers. General properties of glasses 1) Glass is amorphous 2) It has no definite melting point 3) It can be moulded into any shape 4) It can absorb, reflect or transmit light 5) It is very brittle 6) It can take high polish 7) It is a good electrical insulator 8) It softens on heating 9) It is not attacked by air, water, acids and chemical reagents except HF 10) It is affected by alkalies Types of glasses Soda glass or soft glass :It is a mixed silicates of calcium and sodium with approximate composition Na2O.CaO.6SiO2. The raw materials consisting of silica (sand), calcium carbonate (lime stone ) and soda ash (sodium carbonate) in required proportion are fused together to get soda –lime glass or soda glass. Uses:- Soda glass is used for making window glasses, plates, glasses, bottles jars, bulbs, table wares where high temperature resistance and chemical stability are not required. Borosilicate glass or pyrex glass It is a mixture of sodium aluminium borosilicate. The substitution of sodium oxides (Na 2O) and calcium oxide (CaO) of the soda glass by boron and aluminium oxides results in the formation of borosilicate glass. It can with stand high temperature and can resist the action of chemicals. It has low thermal coefficient of expansion. Uses:- It is used for making laboratory glass-wares (flasks, beakers etc ), kitchen wares, television tubes, pipelines for carrying corrosive liquids. Safety glass It is a type of glass which when breaks does not allow its broken pieces to fly apart . It is made by taking two or three flat sheets of glass and in between them alternate thin layer of vinyl plastic is introduced and the whole is subjected to slight pressure. It is then heated till the glass layers and plastic layers merge into one another to give a sandwitch. On cooling, glass becomes quiet tough. Uses:- It is mostly used in automobiles, aeroplanes and railway trains as wind shields. Insulating glass It is a transparent unit of glass, prepared by two or more plates of glass separated by 6 to 13mm gap filled with dehydrated air and sealing around edges. This gives high insulation against heat. If such a glass is used for separating apartments, the apartment will remain cool during summer and warm during winter.

OPTICAL FIBERS An optical fibre is a transparent flexible fibre made by drawing glass or plastic to a diameter slightly thicker than that of a human hair and used as medium for telecommunications by transmission of light by the principle of total internal reflection. An optical fibre consists of three parts, a central core surrounded by a cladding layer of low refractive index and a protective outer sheath. Advantages: 1) Immunity to electromagnetic interference:- Since optical fibres are made of glass or plastic they are free from electromagnetic interference and radiofrequency interference and so not susceptible to lightening strikes. 2) Low transmission loss:- Compared to metallic wires, signals travel with lesser amounts of transmission loss 3) Signal security:-Optical fibres provide a high degree of signal security , since light from the optical fibre does not radiate significantly 4) Small size and weight;- They have very small diameter and light weight compared to copper cables 5) Improved speed of transmission, information density and transmission distance :6) Electrical insulation:- Since optical fibres are fabricated from glass or plastic , they are electrical insulators and so suited for communications in electrically hazardous environments, since optical fibres create no spark hazard at abrasions or short circuits. 7) Enormous potential width:- Optical fibres transmit light at higher band width (1013 GHz to 1016 GHz) compared to metallic cable system ( around 500MHz) Uses 1) In telecommunication industry for transmitting communication over longer distance 2) As endoscopes in medical applications to view object through a small hole 3) For illumination in decorative applications like toys and artificial Christmas trees. 4) As sensors to measure strain, temperature, pressure, wavelength etc. 5) As light guides in medical applications 6) To transmit power using a photovoltaic cell to convert the light into electricity...