Mechanisms of enhanced ionic conduction at interfaces in ceramics

摘要

A large enhancement in the ionic conductivity of certain compounds occurs when the compound is produced as a composite material containing a finely-dispersed non-conductor such as SiO(sub 2) or Al(sub 2)O(sub 3). This paper describes recent experiments which proposes that extended defects such as dislocations and grain boundaries, either resulting from or stabilized by the interface, are responsible for the enhancement. The ionic conductivities of LiI and CaF(sub 2) thin films grown on sapphire(0001) substrates were monitored in-situ during deposition as a function of film thickness and deposition conditions. LiI films grown at 27(degrees)C exhibited a region of enhanced conduction within 100 nm of the substrate and a lesser enhancement as the film thickness was increased further. This conduction enhancement was not stable but annealed out with a characteristic log(time) dependence. The observed annealing behavior was fit with a model based on dislocation motion which implies that the increase in conduction near the interface is due to extended defects generated during the growth process. CaF(sub 2) films grown at 200(degrees)C showed a behavior similar to the 27(degrees)C LiI films, with a region of thermally unstable enhanced conduction that occurs within 10 nm of the substrate. Amorphous Al(sub 2)O(sub 3) films deposited over the CaF(sub 2) layers created no additional enhancement but did increase the stability of the conduction, consistent with an extended defect model. Simultaneous deposition of CaF(sub 2) and Al(sub 2)O(sub 3) produced films consisting of very-fine-grained CaF(sub 2) and particles of amorphous Al(sub 2)O(sub 3) (5-10 nm grain and particle size) and a high defect density which was stable even well above the growth temperature. Measured conduction in the composite at 200(degrees)C was approximately 360 times that of bulk CaF(sub 2).