Determination of the local electrical properties in ceramic materials gained by microcontact impedance spectroscopy

We used microcontact impedance spectroscopy with spatial resolution of about few micrometers to study the influence of microstructure on electrical conductivity of two selected ceramic materials: (I) polycrystalline AgBr with highly conductive grain boundaries and well-known bulk properties and (II) LiMn2O4-carbon black composite which is of technological importance for lithium rechargeable batteries. In the quantitative study of AgBr polycrystals we could easily separate grain conductivity from grain boundary conductance. We found that the grain conductivity followed the behavior of single crystals, while the grain boundary conductance exhibited single activation energy in the whole temperature range studied. Comparing the results obtained by microimpedance spectroscopy with conventional impedance spectroscopy, we showed that the brick layer model relates accurately the local electrical properties to the overall sample's conductance. In the study of LiMn2O4-carbon black composite we showed that if the carbon black coating was prepared by controlled deposition from dispersion, the electronic conductivity was higher than that of the conventional composites prepared by mixing. Microcontact impedance spectroscopy shows that in the former case the distribution of the interfacial conductances (carbon black) is much narrower which explains the percolation threshold observed already at very low carbon black fractions. (C) 2001 Elsevier Science Ltd. All rights reserved.