CONDUCTION MECHANISMS IN RUO2-GLASS COMPOSITES

The electrical transport and the microstructure of RuO2-glass composit es with metal oxide volume fractions 0.01 less than or equal to f(RuO2 ) less than or equal to 0.4 have been investigated by SEM, HREM and fr equency and electric field dependent conductivity studies at 4-300 K. Different transport mechanisms controlled by the microstructure of the composite have been identified. In the limiting cases of high and low RuO2 contents metallic or ionic transport prevails. For intermediate concentrations, 0.05 less than or equal to f(RuO2) less than or equal to 0.2, hopping and tunneling transport are superposed. Hopping transp ort has been identified by frequency dependent conductivity studies; e vidence for tunneling transport stems from the electric field dependen ce of the conductivity at temperatures < 100 K. The significance of th e latter transport mechanisms rests on the densely packed microstructu re of these composites. Under the preparation conditions involved (110 0 K/15 min), predominantly RuO2 clusters with a mean size of 250 nm an d comparatively short intercluster distances form within the glass mat rix. As importantly, each of these clusters consists of a large number of ultrafine RuO2 particles (n > 10(3)) separated by a uniform glass layer of less than or equal to 2 nm thickness, i.e., the metal oxide p articles remain suspended in the matrix. Based on the statistical eval uation of the SEM- and HREM-data a quantitative description of the ele ctrical transport behaviour of RuO2-glass composites is presented.