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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 ) • Dif¡cult 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 dif¡cult to grip ceramic materials to measure their...
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- Spring '10