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


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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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Presentation Transcript
slide1

Defects in Ceramics

  • Point defects
  • Line Defects - Dislocations
  • Surface Defects - Grain Boundaries
  • Majority of this chapter is on point defects.
why bother
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
thermodynamics of point defect formation

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…

perfect crystal
Perfect Crystal

Gperf = Hperf - T Sperf

S = Scofig + ST

defective crystal
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

slide6
Drum roll comes here….
  • Lets plot this function ….. Ignore 2nd term.
types of point defects in ionic solids
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
examples
Examples
  • Examples: What are defects that can occur in pure Al2O3….
    • Vacancy on Al:
    • Vacancy of O:
slide13
Al interstitials ?
  • O interstitials?
  • Add CaCl2 to NaCl
  • Add KCl to NaCl
for an m a x b compound for example in al 2 o 3 should always be true even in hell
For an MaXb compound

For example in Al2O3:

should always be true even in hell.

slide16

Stoichiometric Defects and Reactions

Stoichiometric defect reactions do NOT

change the chemistry of the crystal

schottky defect reaction

Schottky Defect Reaction

In MOMO oxide:

In M2O3 oxide:

In MOOMOO oxide:

thermo of schottky defects
Thermo of Schottky Defects
  • Find minimum in free energy then:
frenkel defects
Frenkel Defects
  • Frenkel defect for a

trivalent cation:

Mass balance?

Charge balance?

Site ratio preserved?

antistructure defects
Antistructure Defects
  • Impossible in ionic solids.
  • Even if they form are not that important
nonstoichiometric defect reactions by definition result in changes in chemistry
Nonstoichiometric Defect ReactionsBy definition result in changes in chemistry

Mass B?

Charge B?

Site ratio?

slide26

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…