Chapter 8 Glass and Ceramics

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    GLASS AND CERAMICS

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    glass is an inorganicproduct of fusion which

    has been cooled to a rigidcondition without

    crystallizing

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    There is two types of glass

    Glass

    Glass-ceramic

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    Glass-ceramicmaterials share manyproperties with both glass and more

    traditional crystalline ceramics

    It is formed as a glass, and then made to

    crystalize partly by heat treatment glass-ceramics have no pores between

    crystals

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    High mechanical strength Low coefficient of thermal expansion (avoid

    thermal shock)

    very low heat conduction coefficient

    Some can be made nearly transparent (1520% loss in a typical cooktop) for radiationin the infrared wavelengths

    High temperature capabilities Good biological compatibilty

    Good dielectric properties (application inelectronic)

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    A glass-ceramic stove uses radiant heatingcoils or infrared halogen lamps as the heatingelements. The surface of the glass-ceramiccooktop above the burner heats up, but the

    adjacent surface remains cool because of thelow heat conduction coefficient of thematerial.

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    Commercial glass or Soda-lime glass:- bottles and jars, flat glass for windows or for

    drinking glasses is known as commercialglass or soda-lime glass, as soda ash is usedin its manufacture.

    Lead glass:

    - make a wide variety of decorative glass

    objects.

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    Borosilicate glass:

    - used for domestic kitchens and chemistrylaboratories

    Glass Fibre:

    - used in the reinforced plastics to make

    protective helmets, boats, piping, car chassis,ropes, car exhausts and many other items.

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    Glass produced by heating the raw materialsto certain temperature which melting occur.

    Raw material:Sand (SiO2)

    Lime stone (CaCO3)

    Soda ash (Na2CO3)

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    To achieve high optical transparency theglass product must be homogenous andpore free.

    Homogeneity is achieved by complete meltingand mixing of raw ingredients.

    Porosity result from small bubbles.

    To eliminate the bubbles, proper adjustmentof the viscosity of the molten material isrequired.

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    Pressing Blowing

    Drawing

    Sheet Fiber

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    Pressing To produce thick wall pieces of glass

    Such as plate and dishes Form by pressure application in a graphite-coated

    cast iron mold having the desired shape.

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    Blowing Usually done by hand especially for art object Example:

    Glass jar

    Bottles

    Light bulb

    From a raw gob of glass, a parisonor temporaryshape is form by mechanical pressing in a mold.

    The pieces is inserted into a finishing or blow moldand force to conform to the mold contours by thepressure created by a blast of air.

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    Drawing Use to form long glass pieces

    Example : Sheet glass

    Rod glass

    Tube glass

    Fiber glass (using sophisticated drawing operation)

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    The float process of producing sheet glass

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    Heat-treated glass is a general term used in theglass fabrication industry to describe glassthat has been processed through a temperingoven to change its strength and breakagecharacteristics

    (i.e., the size and/or shape of the glass piecesafterbreakage). There are two distinct heat-treated

    Types of Heat Treating Glasses

    1. Annealing2. Glass Tempering

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    The process of heat-treated by heatingannealed glass to a temperature ofapproximately 600 oC then slowly cooling it

    The process remove undesirerable stressesfrom the glass

    Important in retain many of the thermalstresses and decrease the overall strength

    of the glass

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    Tempered glass is made from annealed glassvia thermal tempering process

    Tempered glass is heated above theannealing point

    The glass is then rapidly cooled with forceddraft of air while inner portion of the glassremains free for a short of time.

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    Tempered glass is stronger than annealedglass

    When broken, tempered glass fractures intosmall fragments that reduces serious injuriescompared to annealed glass.

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    Color Color is created by adding to the glass a substance

    such as a metallic oxide. Iron can produce green orbrown glass; copper, light blue or red; cobalt, adeeper blue; tin, white; gold or selenium, red; etc.Colors are added in the form of frit, or crushedglass, or as glass rods or glass powder.

    cobalt oxide has been added toproduce a bluish color

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    Specific volume (1/r) vs Temperature (T):

    Glasses:

    -- do not crystallize

    -- change in slope in spec. vol. curve at

    glass transition temperature, Tg-- transparent - no grain boundaries to

    scatter light

    Crystalline materials:

    -- crystallize at melting temp, Tm-- have abrupt change in spec.

    vol. at Tm

    Adapted from Fig. 14.16,Callister & Rethwisch 3e.

    T

    Specific volume

    Supercooled

    Liquid

    solid

    Tm

    Liquid(disordered)

    Crystalline

    (i.e., ordered)

    Tg

    Glass

    (amorphous solid)

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    Edit your company slogan

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    www.themegallery.com

    An inorganic, non metallic solid prepared frompowdered materials

    Fabricated into products through the application of

    heat. The word of ceramics come from Greek wordkeramikos means pottery.

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    www.themegallery.com

    Structural

    Bricks, pipes, floor and roof tiles Refractories

    Kiln linings, gas fire radiants, steel and glassmaking crucibles

    WhitewaresTableware, wall tiles, decorative art objectsand sanitary ware

    Technical

    Tiles used in the Space Shuttle program, gasburner nozzles, ballistic protection, nuclearfuel uranium oxide pellets, bio-medicalimplants, jet engine turbine blades, andmissile nose cones.

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    MECHANICAL- are usually ionic or covalent bonded materials,and can be crystalline or amorphous.

    - poor toughness

    -porous- decreasing toughness,reducing tensile strength

    ELECTRICAL

    - such as extremely low temperature, some

    ceramics exhibit high temperaturesuperconductivity

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    Compound formed between metallic andnonmetallic elements

    - aluminium and oxygen (Al2O3)- calcium and oxygen (CaO)

    - silicon and nitrogen (Si2N4)

    Silicates (compounds of silicon and oxygen)

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    Ceramics are used inthe manufacture ofknives.

    The blade of the

    ceramic knife willstay sharp for muchlonger than that of asteel knife, althoughit is more brittle andcan be snapped bydropping it on ahard surface

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    Ceramics such asalumina and boroncarbide have beenused in ballistic

    armored vests to repellarge-caliber rifle fire.

    Similar material isused to protectcockpits of somemilitary airplanes,because of the lowweight of the material.

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    The two main categories of ceramics aretraditional and advanced.

    Traditional ceramics include objects made ofclay and cements that have been hardened by

    heating at high temperatures. Traditionalceramics are used in:

    Glasses-dishes

    Clay Products flowerpot, crockery

    Cement

    wall tiles,plaster of paris

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    Advanced ceramics include carbides, such as

    silicon carbide, SiC; oxides, such as aluminumoxide, Al2O3; nitrides, such as silicon nitride,Si3N4; and many other materials, including themixed oxide ceramics that can act assuperconductors.

    Advanced ceramics require modern processingtechniques, and the development of thesetechniques has led to advances in medicine andengineering.

    Types: Microelectromechanical System