Ionic conduction mechanisms in CaF2 and CaF 2-Al2O3 nanocomposite films on Al2O 3 substrates.

摘要

Thin films of pure CaF(sub 2) and nanocomposite mixtures of Al(sub 2)O(sub 3) with CaF(sub 2) were sublimated on Al(sub 2)O(sub 3) substrates. Interdigital electrodes allowed in situ measurements of the electrical conduction vs thickness, deposition rate, composition, time, and temperature. Conductivity in pure CaF(sub 2) adjacent to an Al(sub 2)O(sub 3) interface sometimes exceeded the bulk CaF(sub 2) conductivity (i.e., value at more than 50 nm distance) by as much as a factor of 6700 at 200C. The high conductivity is characterized by an activation energy of 0.6(plus minus)0.1 eV, significantly lower than about 1.0 eV for bulk conduction. However, this high conductivity is thermally unstable and diminishes in time. A high but stable conductivity was obtained in CaF(sub 2) films containing about 10 mole percent Al(sub 2)O(sub 3) as a dispersed second phase. At 200C, a 2-phase film gave a factor of 360 enhancement over the measured bulk CaF(sub 2) conductivity and a factor of 7 improvement over the best previously reported conductivity for CaF(sub 2)-Al(sub 2)O(sub 3) composite materials. The origin of enhanced conduction in CaF(sub 2) is attributed to ion transport along dislocations. Dislocations anneal with a characteristic log of time dependence that is recognizable in the annealing behavior of the electrical conductivity. Presumably, the addition of a dispersed second phase of Al(sub 2)O(sub 3) to CaF(sub 2) serves both to generate and to pin dislocations; the electrical conductance is thereby enhanced and stabilized.