CERAMICS

Properties of Ceramic They are distiguished by its bonding which is ionic or covalent bonding High elastic modulus and hardness High melting point Low thermal expansion Good chemical resistance Brittle Thermal insulators Electrical insulators Nonmagnetic

Ceramic Structures : Crystal Structure Factors that affect crystal structure of ceramics : - the magnitude of the electrical charge on each of the component ions and the relative sizes of the cations and anions - the sizes or ionic radii of the cations and anions rC/rA

Ceramic Structures Crystal structures Atomic bonding: ionic bonding Positive charges (metal) cation Negative charges (non metal) anion anion kation

AX-Type Crystal Structures A: cation X: anion Rock Salt Structure ~e.g: NaCl, MgO, MnS, LiF ~The coordination number for both cations&anions: 6 ~Structure: FCC

2. Cesium Chloride Structure ~The coordination number: 8 ~e.g: CsCl Zinc Blende Structure ~The coordination number: 4 ~atomic bonding: covalent bonding ~e.g: ZnS, ZnTe, SiC

AmXp-Type Crrystal Structure When charges of cations and anions are not the same; where m and/or p ≠ 1 AX2 e.g: CaF2, UO2, PuO2

AmBnXp-Type Crystal Structures It is possible for ceramic compounds to have more than one type of cation A & B: cation X: anion Structure: perovskite e.g: BaTiO3 (Barium titanat) cation: Ba2+, Ti4+ anion: O2-

Classification of Ceramics Glasses : 1. Glasses 2. Glass-ceramics Clay Products : 1. Structural clay products 2. Whitewares Refractories : 1. Fireclay 2. Silica 3. Basic 4. Special Abrasives Cements Advanced ceramics

Glasses (1) Glasses They are noncrystalline silicates containing other oxides Glass Type Composition SiO2 Na2O CaO Al2O3 B2O3 Other Characteristics and Application Fused Silica >99.5 High melting temperature, very low coefficient of expansion (thermally schok resistant) 96% Silica (VycorTM) 96 4 Thermally shock and chemically resistant – laboratory ware Borosilicate (PyrexTM) 81 3.5 2.5 13 Thermally shock and cehimcally resistant – ovenware

Glasses (2) 2. Glass-ceramics Formed glass ceramics : We use a process that called crystallization which is the most inorganic glasses can be made to transform from a noncrystalline state to one that is crystalline by the proper high temperature heat treatment Properties : relatively high mechanical strength, low coefficients of thermal expansion, relatively high temperature capabilities, good dielectric properties, good biological compability Applications : ovenware, tableware, oven windows, and rangetops, and etc

Clay Products 1. Structural clay products Applications : building bricks, tiles, and sewer pipes 2. Whitewares Whitewares become white after the high temperature firing Applications : porcelain, pottery, tableware, china and plumbing fixtures (sanitary ware)

Refractories (1) Fireclay refractories The primary ingredients : high purity fireclays, alumina, and silica mixtures usually containing between 25 and 45 wt% alumina. Applications : in furnace construction, to cofine hot atmospheres and to thermally insulates structural members from excessive temperatures. 2. Silica refractories Contains over 95% of SiO 2 and not more than 3% CaO both by weight Well known for their high temperature load bearing capacitiy Applications : arched roof of steel and glass making furnaces

Refractories (2) 3. Basic refractories Have a high propotions of basic content like CaO and MgO. Also contain chromium and iron compounds Applications : use in some steel making open hearth furnaces 4. Special refractories Some of these are relatively high purity oxide materials Applications : For example SiC are used for electrical resistance heating elements

Abrasives The most common examples of abrasives are diamond, silicon carbide, tungsten carbide, corundum, and silica sand The prime requisite of this material is hardness Applications : bonded in grinding wheels, as coated abrasives, as loose grains, etc

Cements Cement used in construction is characterized as hydraulic and non-hydraulic The characteristic feature in this material is when mixed with water, form a paste Cementitious bond develops at room temperature Applications : mortar and concrete

Advanced Ceramics (1) MEMS (Microelectromechanical Systems) Miniature “smart” system consisting of a multitude of mechanical devices that are integrared with a large numbers of electrical elements on a substrate of silicon Applications : accelerometer, microsystem for DNA, chemical detectors, etc

Advanved Ceramics (2) 2. Optical Fibers Is made of extremely high purity silica 3. Ceramic Ball Bearings Consists of balls and races that are in contact with and rub against one another when in use

Fabrication and Processing Of Ceramics (1) Fabrication and Processing Of Glasses and Glass-ceramics Glasses are formed at elevated temperature, hence, we have to consider the temperature viscosity Glass forming techniques : pressing, blowing, drawing and fiber forming After fabrication, glasses may be annealed and/or tempered to improve mechanical characteristics

Fabrication and Processing Of Ceramics (2) 2. Fabrication and Processing of Clay Products Commonly, there are two forming techniques, hydroplastic forming and slip casting After forming, the body must be dried and then fired at elevated temperature 3. Powder Pressing Used to fabricate both clay and non clay compositions Three basic powder pressing : uniaxial, isostatic, and hydro pressing 4. Tape Casting Pouring the slip onto a flat surface A doctor blade spreads the slip into a thin tape of uniform thickness Tape thickness normally range between 0.1 and 2 mm Usually used in the production of ceramic substrates