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Lecture Notes II Defect Chemistry

Lecture Notes II Defect Chemistry. Ole Toft Sørensen (Risoe National Laboratory) Ceramic Materials Consultant Email:otsdk@yahoo.dk. Electroceramics. Electrical properties determined by defects. Knowledge of defect chemistry necessary to understand Electroceramics!. What is a defect?.

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Lecture Notes II Defect Chemistry

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  1. Lecture Notes IIDefect Chemistry Ole Toft Sørensen (Risoe National Laboratory) Ceramic Materials Consultant Email:otsdk@yahoo.dk

  2. Electroceramics • Electrical properties determined by defects • Knowledge of defect chemistry necessary to • understand Electroceramics!

  3. What is a defect? Fundamental definition:Any deviation from the perfect crystal is a defect! - Macroscopic defects (porosities, cracks) • Atomic defects • Electronic defects

  4. Atomic (point) defects in Oxides! Missing ions: - oxygen ions, oxygen vacancies - cations, cation vacancies Substituted ions Interstitiel ions

  5. Electronic defects in oxides • Atomic (point) defects – type, properties depend on position ! • Electronic defects – type, properties depend on energy levels • available for the electrons • electrons • positive holes

  6. Defect notations subscript (Symbol for type)position VO = vacancy on oxygen position VM = vacancy on metal position, VFe Oi = interstitial oxygen ion Mi = interstitial cation YZr = Y-ion on Zr-ion position

  7. Charges of defects Relative chargeCharge relative to the charge normally present in the position of the defect Examples: ZrZr – relative charge = zero, but YZr - relative charge = -1 FeO: Fe2+ vacancy – rel. ch. = ? -2 of course! ZrY ? Rel. ch.+1

  8. Charges of oxygen vacancies Formation of oxygen vacancies: Oxygen atoms are removed from the crystal Oxygen ions – how many electron in outer orbital ? 8 Oxygen atoms – how many electrons in outer orbital ? 6 2 Oxygen vacancy – how many electrons left ? Rel. Charge ? Zero! But these electrons can easily migrate to neighbouring ions forming vacancies with one or zero electrons present. Thus VO with rel. charges of zero, +1 and +2 can be formed !

  9. Relative charges of interstitial ions -2 O2- ions,Oi ? Cl-1 – ions, Cli, rel. charge = -1 Na+1 – ions, Nai , ? +1 Zr4+ - ions, Zri, rel. charge = +4

  10. Nomenclature: relative charges • Relative charges are indicated by a superscript: • neutral - x • positive charges – black dots • negative charges - apostrophes Examples: Neutral: VOx Positive charges: VO•, VO•• Negative charges: VFe″

  11. Practise: Nomenclature

  12. Answers to practise

  13. Formation of defects Three typical areactions: - ”high” temperatures, INTRINSIC DEFECTS - reaction with surrounding atmosphere - substitution

  14. Intrinsic defects Pair of defects: • Frenkel defect: • cation vacancy and interstitial cation • Anti-Frenkel defect: • oxygen vacancy and interstitial oxygen ion • Schottky defect: • oxygen vacancy and cation vacancy MO: OOx + MMx = VO•• + VM″ M2O3: 3OOx + 2MMx = 3VO•• + 2VM″′ Stoichiomtry must be maintained !

  15. Defects formed in an reaction with surronding atmosphere. Reduction MO2 = MO2-x+ x/2 O2 OOx + 2MMx = VO•• + 2MM′ + 1/2O2 Oxides with cations easily reduced!

  16. Defects formed in a reaction with a surrounding atmosphere - 2 Oxidation MO + y/2O2 = M1-yO Note – clusters ! Cations easily oxidized!

  17. Formation of interstitial oxygen ions- Oi High oxygen pressures ! 1/2 O2 + 2MMx = Oi″ + 2MM• Oxides where cations are easily oxidized – FeO

  18. Defects formed by substitution! Substitution of cations ! Lower valency: Higher valency: ZrO2 doped with CaO: Y2O3 doped with ZrO2: CaO(ZrO2) = CaZr″ + VO•• + OOx 2ZrO2(Y2O3) = 2ZrY• + Oi″ + 3OO Oxygen vacancies formed to maintain electrical neutrality ! Same valency?

  19. PracticeFormation of Defects

  20. AnswersPractice – Formation of Defects

  21. Dependence on oxygen pressure 1 Can defects in a solid be considered as ions in a solution? • Yes if these conditions are fulfilled: • random distribution of defects • no interactions • high mobility Law of mass action can be used.

  22. Rules which must be obeyed • ratio between cation and anion positions – Constant! • the total number of positions can be changed, • but not the ratio! • neutrality must be maintained

  23. Formation of oxygen vacancies

  24. [VO]  log pO2 1 2

  25. Brouwer plots - VO

  26. Practise:Brouwer plots oxygen vacancies

  27. Answers to practice:Brouwer plots oxygen vacancies

  28. [VM]  log pO2 1 2 3 4 5

  29. Brouwer plots - VM

  30. Practice:Formation of cation vacancies

  31. Answers to practice:Formation of cation vacancies

  32. Brouwer plot for Oi 1 2 3

  33. Brouwer plot:many defects • Construction: • - Log Conc. defect vs log(pO2)- 3 p(O2) regions; • one type of defect dominates in each region • - sharp transition between regions, approximation

  34. Calculation of defect concentrations • Deviation from the stoichiometric composition. • Fraction of defects • Number of defects per cm3

  35. Deviation from stoichiometric composition

  36. Site Fractions MO2-x Fe1-yO MO2+x

  37. Number of defects per cm3

  38. PracticeCalculation of defect concentrations

  39. AnswersCalculaion of defectconcentrations

  40. Content

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