Lecture handout including QS - Department of Materials Science ...

BH44 Course B: Materials for Devices BH44
Solid Ionic Conducting Materials
Ceramics, which are compounds between metallic and non-metallic elements (e.g. oxides, nitrides,
carbides), are generally electrically insulating (all electrons are bound to atoms or bonds):
e.g. Al 2
O 3
: resistivity, ρ = ~10 10 Ωm or conductivity, σ =10 -10 Ω −1 m −1 or Sm -1
c.f. Cu: ρ = 1.6×10 -8 Ωm σ = 6×10 7 Ω −1 m −1
However, some ceramics can conduct via ionic conduction.
e.g. the ionic conductivity of (ZrO 2
+ 8 mol.% Y 2
O 3
) is ~ 0.1 Ω −1 m −1 @ 500° C
An example of a ceramic structure:
interstitial
+
vacancy
anion
(Frenkel defect)
+
cation
vacancies
(Shottky
defect) → charge neutrality
Ions / atoms aren’t completely stationary on their lattice sites; the higher the temperature, the larger
the amplitude with which they vibrate. They can migrate through the lattice by swapping position with
other ions / atoms, and this migration is greatly enhanced by the existence of vacant sites.
Ions migrate by hopping into vacant lattice sites:
Ionic mobility depends upon:
- whether an adjacent site is empty, and
- the energy barrier between lattice sites
An ionic current flow can result from:
(a) a concentration gradient (of ions, or vacancies): diffusion current
(random diffusion evens out the gradient) and / or
(b) an electric field, E: drift current