Lecture Notes II Defect Chemistry. Ole Toft Sørensen (Risoe National Laboratory) Ceramic Materials Consultant Email:[email protected] Electroceramics. Electrical properties determined by defects. Knowledge of defect chemistry necessary to understand Electroceramics!. What is a defect?.
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Fundamental definition:Any deviation from the perfect crystal is a defect!
- Macroscopic defects (porosities, cracks)
- oxygen ions,
(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
Relative chargeCharge relative to the charge normally present in the position of the defect
ZrZr – relative charge = zero,
YZr - relative charge = -1
Fe2+ vacancy – rel. ch. = ?
-2 of course!
Formation of oxygen vacancies:
Oxygen atoms are removed from the crystal
Oxygen ions – how many electron in outer orbital ?
Oxygen atoms – how many electrons in outer orbital ?
Oxygen vacancy – how many electrons left ?
Rel. Charge ?
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 !
O2- ions,Oi ?
Cl-1 – ions, Cli, rel. charge = -1
Na+1 – ions, Nai , ?
Zr4+ - ions, Zri, rel. charge = +4
Positive charges: VO•, VO••
Negative charges: VFe″
Three typical areactions:
- ”high” temperatures, INTRINSIC DEFECTS
- reaction with surrounding atmosphere
Pair of defects:
MO: OOx + MMx = VO•• + VM″
M2O3: 3OOx + 2MMx = 3VO•• + 2VM″′
Stoichiomtry must be maintained !
MO2 = MO2-x+ x/2 O2
OOx + 2MMx = VO•• + 2MM′ + 1/2O2
Oxides with cations
MO + y/2O2 = M1-yO
Note – clusters !
Cations easily oxidized!
High oxygen pressures !
1/2 O2 + 2MMx = Oi″ + 2MM•
Oxides where cations are easily oxidized – FeO
Substitution of cations !
ZrO2 doped with CaO:
Y2O3 doped with ZrO2:
CaO(ZrO2) = CaZr″ + VO•• + OOx
2ZrY• + Oi″ + 3OO
Oxygen vacancies formed to
maintain electrical neutrality !
Can defects in a solid be considered as ions in a solution?
Law of mass action can be used.