ENGR_220_Ch12.ppt_Callister8e_RK_2

ENGR_220_Ch12.ppt_Callister8e_RK_2 - 10/19/11 1 Chapter 12...

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Unformatted text preview: 10/19/11 1 Chapter 12 - 14 Site Selection II 2. Stoichiometry: If all of one type of site is full the remainder have to go into other types of sites e.g.: FCC unit cell has 4 O H and 8 T D sites If, for a specific ceramic, each unit cell has 6 cations and these prefer the O H sites, then: - 4 will go into O H- 2 will go into T D Chapter 12 - 15 Site Selection III 3. Bond Hybridization significant covalent bonding: the hybrid orbitals can have an impact if significant covalent bond character present For example in SiC: L X Si = 1.8 and X C = 2.5 (electronegativities) % ionic character = 100 {1-exp[-0.25( X Si-X C ) 2 ]} = 11.5% L Therefore SiC has ~89% covalent bonding L Both Si and C prefer sp 3 hybridization L Therefore in SiC we get Si occupying T D sites 10/19/11 2 Chapter 12 - 16 L &< B63 0/A7A =4 B63 ionic radii , what crystal structure would you predict for 3& ? L <AE3@ r cation r anion = 0.077 0.140 = 0.550 Based on this ratio: - CN = 6 since 0.414 < 0.550 < 0.732 - Structure = NaCl Data from Table 12.3, Callister & Rethwisch 8e. Example: Predicting Structure of FeO Ionic radius (nm) 0.053 0.077 0.069 0.100 0.140 0.181 0.133 Cation <7=< : 3+ Fe 2 + Fe 3+ Ca 2+ & 2- Cl- F- Chapter 12 - 17 AX Crystal Structures Same concepts can be applied to ionic solids in general: Equal #s cations and anions several possible structures e.g.: NaCl (rock salt) structure r Na = 0.102 nm r C /r A = r Na+ / r Cl- = 0.564 CN = ?...
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ENGR_220_Ch12.ppt_Callister8e_RK_2 - 10/19/11 1 Chapter 12...

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