Lecture 10 - ENGR 4250 – Advanced Materials Engineering...

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Unformatted text preview: ENGR 4250 – Advanced Materials Engineering Lecture 10 Applications & Processing of Ceramics Callister’s Chapters 12 &13 Dr. A. Keshavarz - ENGR 4250U keramikos- burnt stuff in Greek Desirable properties of ceramics are normally achieved through a high temperature heat treatment process (firing). Dr. A. Keshavarz - ENGR 4250U Ceramics Ionic Character of Ceramics Dr. A. Keshavarz - ENGR 4250U Ionic and Covalent Nature % ionic character = {1-exp[ - (0.25)(X A – X B ) 2 ]} x 100 (2.10) X A and X B are electro-negativities for the respective elements Dr. A. Keshavarz - ENGR 4250U Ionic Bonding & Structure 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 Dr. A. Keshavarz - ENGR 4250U • Coordination # increases with Coordination # and Ionic Radii Adapted from Table 12.2, Callister 7e. 2 r cation r anion Coord # < 0.155 0.155 - 0.225 0.225 - 0.414 0.414 - 0.732 0.732 - 1.0 3 4 6 8 linear triangular T D O H cubic Adapted from Fig. 12.2, Callister 7e. Adapted from Fig. 12.3, Callister 7e. Adapted from Fig. 12.4, Callister 7e. ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium chloride) r cation r anion Issue: How many anions can you arrange around a cation? Dr. A. Keshavarz - ENGR 4250U The critical ratio can be determined by simple geometrical analysis α cos = AO AP C A r r AO + = A r AP = 2 3 30 cos = = + = C A A r r r AO AP 155 . = A C r r Dr. A. Keshavarz - ENGR 4250U Tetrahedral vs Octahedral Dr. A. Keshavarz - ENGR 4250U Rock Salt Dr. A. Keshavarz - ENGR 4250U Mechanical Properties We know that ceramics are more brittle than metals. • Consider method of deformation – slippage along slip planes • in ionic solids this slippage is very difficult • too much energy needed to move one anion past another anion Dr. A. Keshavarz - ENGR 4250U Silicate Ceramics Most common elements on earth are O & Si • SiO 2 (silica) structures are quartz, crystobalite, & tridymite • The strong Si-O bond leads to a strong, high melting material (1710ºC) Si 4+ O 2- Adapted from Figs. 12.9-10, Callister 7e. crystobalite Dr. A. Keshavarz - ENGR 4250U Crystalline & Non-Crystalline Silica Dr. A. Keshavarz - ENGR 4250U Layered Silicates • Layered silicates (clay silicates) – SiO 4 tetrahedral connected together to form 2-D plane • (Si 2 O 5 ) 2- • So need cations to balance charge = Adapted from Fig....
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This note was uploaded on 06/02/2011 for the course ECONMICS ECN204 taught by Professor Brenan during the Spring '08 term at Ryerson.

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Lecture 10 - ENGR 4250 – Advanced Materials Engineering...

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