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Unformatted text preview: 1 Ceramics Ice-Frisbee Experiment Ice Ice with shredded paper Before After 5 cm Tough Ice 2 3 Ceramic Crystal Structures Atomic packing obeys two rules: 1) Electrical neutrality 2) Maximum coordination, which depends on the ratio of cation to anion radius ( r C /r A ) When 0.225 < r C /r A < 0.414, CN = 4 When 0.414 < r C /r A < 0.732, CN = 6 4 Crystal Structures When 0.414 < r C /r A < 0.731, CN = 6 This corresponds to the packing in a simple cubic unit cell (rock salt structure) 5 Crystal Structures (2r A ) 2 + (2r A ) 2 = 4 (r A + r C ) 2 r A 2 + r A 2 = (r A + r C ) 2 ! 2 r A = r A + r C r C / r A = ! 2 - 1 = 0.414 2 r A 2 (r A + r C ) 6 Crystalline Point Defects Frenkel defect Schottky defect antisite defect 7 Impurity Effects Subject to the same solubility rules as in metals, e.g., Ti 4+ ion radius = 0.068 nm in TiO 2 and Al 3+ ion radius= 0.05 nm in Al 2 O 3--> no mutual solubility Produce defects, e.g., addition of CaCl 2 into NaCl produces cation vacancies 8 9 Brittle Fracture of Ceramics Brittle nature due to two reasons: Few slip systems available for plastic deformation ( slip system defned by slip plane and direction oF slip For the dislocation ) Difcult to generate required dislocations 10 Brittle Fracture of Ceramics (100)  (110)  Positive ion Negative ion 11 Brittle Fracture of Ceramics Because of low dislocation activity, no crack blunting Therefore, ceramics are weaker under tension than compression K ic ~ 1-5 MPa-m 1/2 for most ceramics cf. 30-100 for many metallic alloys 12 Flexural Strength Because of their brittleness, it is difcult to grip ceramic materials to measure their...
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- Spring '10