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Point defects Line Defects - Dislocations Surface Defects - Grain Boundaries Majority of this chapter is on point defect

Defects in Ceramics. Point defects Line Defects - Dislocations Surface Defects - Grain Boundaries Majority of this chapter is on point defects. . Why bother?. Point defects dictate several properties: Diffusion which then determines sintering and sometimes creep Electrical conductivity

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Point defects Line Defects - Dislocations Surface Defects - Grain Boundaries Majority of this chapter is on point defect

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  1. Defects in Ceramics • Point defects • Line Defects - Dislocations • Surface Defects - Grain Boundaries • Majority of this chapter is on point defects.

  2. Why bother? • Point defects dictate several properties: • Diffusion which then determines sintering and sometimes creep • Electrical conductivity • Dielectric constants • Most dramatic example: YBa2Cu3O7 • Metal oxidation • Solid electrolytes

  3. Thermodynamics of Point Defect Formation Or why vacancies are an unavoidable: a thermodynamic necessity… Approach: find an expression for G of perfect crystal and one for defective crystal; the one with the lower (more negative G) wins…

  4. Perfect Crystal Gperf = Hperf - T Sperf S = Scofig + ST

  5. Defective Crystal If it costs, hd to form one defect, how much for nv defects Assume all atoms near a vacancy now vibrate with a different frequency, ’. Thus

  6. Drum roll comes here…. • Lets plot this function ….. Ignore 2nd term.

  7. MISI…

  8. Types of Point Defects in Ionic Solids • Stoichiometric Defects: • Do not change the chemistry of crystal. Egs. Schottky and Frenkel • Non-stoichiometric Defects • Change the chemistry of the crystal • Electronic Defects: electrons and holes • Extrinsic Defects • Aka impurities

  9. Point Defects Complicated Because We Have More than One Sublattice

  10. Kroger-Vink Notation

  11. Examples • Examples: What are defects that can occur in pure Al2O3…. • Vacancy on Al: • Vacancy of O:

  12. Al interstitials ? • O interstitials? • Add CaCl2 to NaCl • Add KCl to NaCl

  13. Rules for Defect Reactions

  14. For an MaXb compound For example in Al2O3: should always be true even in hell.

  15. Stoichiometric Defects and Reactions Stoichiometric defect reactions do NOT change the chemistry of the crystal

  16. Schottky Defect Reaction In MOMO oxide: In M2O3 oxide: In MOOMOO oxide:

  17. Thermo of Schottky Defects • Find minimum in free energy then:

  18. Square brackets denote mole fractions of defects! They are dimensionless!

  19. Frenkel Defects • Frenkel defect for a trivalent cation: Mass balance? Charge balance? Site ratio preserved?

  20. TD of Frenkel Defects

  21. Antistructure Defects • Impossible in ionic solids. • Even if they form are not that important

  22. Nonstoichiometric Defect ReactionsBy definition result in changes in chemistry Mass B? Charge B? Site ratio?

  23. In high oxygen partial pressure

  24. MISI Placing any oxide in a low oxygen partial pressure environment at high temperatures results in the loss of oxygen… You get Swiss cheese… Placing any oxide in a high oxygen partial pressure environment at high temperatures results in the gain of oxygen… You can get Unswiss cheese…

  25. Bookkeeping Operation for Extrinsic Defects

  26. Bookkeeping Operation for Extrinsic Defects

  27. Electronic DefectsBand Theory of Solids

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