2010-03-15 Chapter 12 Structures and Properties of Ceramics

2010-03-15 Chapter 12 Structures and Properties of Ceramics...

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Chapter 12 - 1 ISSUES TO ADDRESS. .. Structures of ceramic materials: How do they differ from those of metals? Point defects: How are they different from those in metals? Impurities: How are they accommodated in the lattice and how do they affect properties? Mechanical Properties: What special provisions/tests are made for ceramic materials? Chapter 12: Structures & Properties of Ceramics
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Chapter 12 - 2 Ceramics Compounds of metallic and non-metallic elements mainly oxides, nitrides, and carbides. – Examples: Al 2 O 3 (alumina), SiC (silicon carbide), Si 3 N 4 (silicon nitride), and ZrO 2 (zirconia). Insulative to electricity and heat. Better resistance to high temperatures and harsh environments than metals and polymers. Hard and brittle. Desirable properties of ceramics are achieved through a high temperature heat treatment called firing. Traditional Ceramics: china clay, porcelain, bricks, tiles, glasses, cement. Advanced Ceramics: used in electronic, computer, communication, aerospace, and other industries. IMPORTANT
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Chapter 12 - 3 Space shuttle is protected by ceramic tiles. http://rayjasm.tripod.com/sts Refractory Brickwork in kilns www.fastfire.co.uk/ f_brick.htm High purity alumina and zirconia used in spark plugs www.ceramicstoday.com/ articles/zamek.htm Piston tops are coated with ceramic coatings www.engineceramics.com/ fullservice.htm
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Chapter 12 - 4 Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond , 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Large vs small ionic bond character: Ceramic Bonding SiC: small CaF 2 : large IMPORTANT
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Chapter 12 - 5 Ceramic Crystal Structures Oxide structures oxygen anions much larger than metal cations close packed oxygen in a lattice (usually FCC) cations in the holes of the oxygen lattice Which sites will cations occupy? 1. Size of sites does the cation fit in the site 1. Stoichiometry if all of one type of site is full the remainder have to go into other types of sites. 1. Bond Hybridization
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Chapter 12 - 6 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. Adapted from Fig. 12.1, Callister 7e. - - - - + unstable Charge Neutrality : --Net charge in the structure should be zero. --General form: - - - - + stable - - - - + stable CaF 2 : Ca 2+ cation F - F - anions + A m X p m, p determined by charge neutrality
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Chapter 12 - 7 Coordination # increases with Coordination # and Ionic Radii Adapted from Table 12.2, Callister 7e. 2
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2010-03-15 Chapter 12 Structures and Properties of Ceramics...

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