Electrical- Ionic Conduction Flashcards
Way of getting ionic conduction in crystalline solids
Create vacancies in the crystal structure using aliovalent doping
Isovalent doping
Dopant ion is of the same charge but different ionic radius. No defects are required for charge compensation.
Aliovalent doping
Dopant ion of different charge and ionic radius. Defects are required for charge compensation
Types of ionic compensation
Cation vacancies, cation interstitials, anion vacancies, anion interstitials.
Describe the fluorite structure
It is cubic close packed structure. It is a fcc unit cell of metal ions with the tetrahedral holes filled with 8 anions. The larger octahedral sites are empty. Each metal ion has coordination number of 4.
How does yttria-stabilised zirconia work?
ZrO2 has fluorite structure. Can partially replace Zr4+ with Y3+. To maintain electroneutrality, oxygen vacancies are created.
Zr4+ +1/2O2- -> Y3+
The cation sites contain a random mixture of Zr and Y. The cation to anion ratio becomes 1:<2 as the oxygen sites are not fully occupied. O2- ions can migrate between tetrahedral sites via the empty (larger) octahedral sites. Oxide ion conductivity is optimised for x=0.16 so
Zr(0.84)Y(0.26)O(1.92)
What is yttria-stabilised zirconia classed as and how does its conductivity change?
It is an oxide ion conducting ceramic electrolyte. Conductivity obeys Arrhenius law with Ea being the activation energy for O2- ions to hop through the crystal structure and roughly 1eV. At high temperature, the oxygen vacancies are mobile.
How do fuel cells work?
YSZ used as the solid electrolyte. The fuel electrode involves oxidation of hydrogen to make water using O2- ions and creating 2e-. The air electrode involves reduction of oxygen to make O2- ions using 2e-. These O2- ions can move across the YSZ electrolyte. The gases on either electrode must be kept separate.
