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1、1Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering1Chapter Outline: CeramicsChapter 13: Structure and Properties of Ceramics Crystal Structures Silicate Ceramics Carbon Imperfections in CeramicsOptio
2、nal reading: 13.6 13.10Chapter 14: Applications and Processing of Ceramics Short review of glass/ceramics applications and processing (14.1 - 14.7)Optional reading: 14.8 14.18Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Material
3、s Science and Engineering2 keramikos - burnt stuff in Greek - desirable properties of ceramics are normally achieved through a high-temperature heat treatment process (firing). Usually a compound between metallic and non-metallic elements Always composed of more than one element (e.g., Al2O3, NaCl,
4、SiC, SiO2) Bonds are partially or totally ionic, and can have combination of ionic and covalent bonding Generally hard and brittle Generally electrical and thermal insulators Can be optically opaque, semi-transparent, or transparent Traditional ceramics based on clay (china, bricks, tiles, porcelain
5、), glasses. “New ceramics” for electronic, computer, aerospace industries.Ceramics2Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering3Electronegativity - a measure of how willing atoms are to accept e
6、lectrons (subshells with one electron - low electronegativity; subshells with one missing electron -high electronegativity). Electronegativity increases from left to right.Bonding in Ceramics (Review of Chapter 2)The atomic bonding in ceramics is mixed, ionic and covalent, the degree of ionic charac
7、ter depends on the difference of electronegativity between the cations (+) and anions (-).Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering4Crystal structure is defined by Magnitude of the electrical
8、 charge on each ion. Charge balance dictates chemical formula (Ca2+and F-form CaF2). Relative sizes of the cations and anions. Cations wants maximum possible number of anion nearest neighbors and vice-versa.Crystal Structures in Ceramics with predominantly ionic bondingStable ceramic crystal structu
9、res: anions surrounding a cation are all in contact with that cation. For a specific coordination number there is a critical or minimum cation-anion radius ratio rC/rAfor which this contact can be maintained.3Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversi
10、ty of Tennessee, Dept. of Materials Science and Engineering5Coordination NumberThe number of adjacent atoms (ions) surrounding a referenceatom (ion) without overlap of electron orbitals. Also called ligancy Depends on ion size (close packed) Ideal: Like-sized atoms = 12 Calculated by considering the
11、 greatest number of larger ions(radius R) that can be in contact with the smaller one (radius r).R=1.0r =0.2CN = 1 possible CN = 2 possibleCN = 4 possible30Cos 30=0.866=R/(r+R) r/R = 0.155Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept
12、. of Materials Science and Engineering6Example: Al2O3Al+3r=0.057nm, O-2R=0.132 nm, r/R = 0.43, CN = 6However for O-2CN= (2/3) (6) = 4 (2/3) = (cation/ion) ratioExample: KClK+r=0.133nm, Cl-R=0.188nm, r/R = 0.71, CN = 6Example: CsClCs+r=0.165nm, Cl-R=0.188nm, r/R = 0.91, CN = 8CN r/R2 0r/R0.1553 0.155
13、r/R0.2254 0.225r/R0.4146 0.414r/R0.7328 0.732r/R112 1CN numbers for ionic bondingCoordination Number4Introduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering7Introduction to Materials Science, Chapter 13, S
14、tructure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering8 0.155 0.155-0225 0.225-0.414 0.414-0.732 0.732-1.0C.N. rC/rA Geometry The critical ratio can be determined by simple geometrical analysisCos 30= 0.866= R/(r+R)r/R = 0.155305Introduction to Materia
15、ls Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering9NaCl structure: rC=rNa= 0.102 nm, rA=rCl= 0.181 nm rC/rA= 0.56From the table for stable geometries we see that C.N. = 6Crystal Structures in Ceramics Example: Rock Salt Str
16、uctureTwo interpenetrating FCC latticesNaCl, MgO, LiF, FeO have this crystal structureIntroduction to Materials Science, Chapter 13, Structure and Properties of CeramicsUniversity of Tennessee, Dept. of Materials Science and Engineering10Cesium Chloride Structure: rC= rCs= 0.170 nm, rA=rCl= 0.181 nm rC/rA= 0.94From the table for stable geometries we see that C.N. = 8More ex
