Investigation of ionic transport in composites by nuclear magnetic resonance

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Abstract
The addition of large-surface-area powdered oxides to finely divided ionic salts has been known to enhance ionic conductivity, sometimes by several orders of magnitude. The ionic transport mechanisms is not well established. In this study, lithium bromide mixed with alumina has been investigated by 7Li nuclear magnetic resonance (NMR) and compared with the behaviour of pure lithium bromide. The ionic conduction of the samples has previously been investigated. We report here a preliminary investigation of NMR line shape, spin-spin (T 2) relaxation and spin-lattice (T 1 and T 1ρ) relaxation which have been measured for several well characterized samples over a temperature range 300–500 K. Evidence for enhanced lithium ion mobility is clearly found in the composites but the spectral line-shape and spin-relaxation behaviour are complex. The data can be interpreted on the basis of two Li+ environments, one more mobile than the other. The number of ions in the mobile regions increases with increasing temperature. A preliminary analysis of these results suggests that diffusion rates of a substantial proportion of the lithium ions (30% to 60%) are enhanced and these are not confined to the interface regions but exist in a high density of dislocations or grain boundaries.

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