Anisotropy of local electrical conductivity of hyper-stoichiometric uranium dioxide revealed by current-sensing atomic force microscopy (CS-AFM)

The local electrical conductivity and chemical composition of hyper-stoichiometric uranium dioxide were studied using localized Raman and current-sensing AFM techniques. Distinct types of surface features, representative of either stoichiometric or hyper-stoichiometric uranium dioxide, and remarkably different in their electrical conductivity, were identified. Stoichiometric UO2 formed smooth featureless grains and showed little or no electrical conductivity, while considerably more conducting hyper-stoichiometric UO2+x formed characteristic ridges and/or facetted structures. Surprisingly, such hyper-stoichiometric regions also showed anisotropic electrical conductivity and combined relatively non-conducting flat terraces with a highly conducting inclined facets. Our CS-AFM measurements clearly indicate that the highly conducting grain boundaries, as well as facetted grains and grain regions, are the prime locations for local corrosion.