The microstructures of glass-RuO2 composites, with metal oxide volume fractions of between 0.01 and 0.4, were investigated by using scanning electron microscopy, high-resolution electron microscopy, and frequency- and field-dependent conductivity techniques at 4 to 300K. Various transport mechanisms were identified which were controlled by the microstructure of the composite. In the limiting cases of high and low RuO2 contents, metallic or ionic transport prevailed. At concentrations of 0.05 to 0.2, hopping and tunnelling transport were superposed. Hopping transport was identified by frequency-dependent conductivity data, while evidence for tunnelling transport was provided by the electric field dependence of the conductivity at temperatures below 100K. These transport mechanisms depended upon the densely packed microstructures of the composites. Under the present preparation conditions (1100K, 0.25h), mainly RuO2 clusters with a mean size of 250nm (and comparatively short inter-cluster distances) formed within the glass matrix. Moreover, each of these clusters consisted of a large number (more than 1000) of ultra-fine RuO2 particles; separated by a uniform glass layer which was less than 2nm in thickness. That is, the metal oxide particles remained suspended in the matrix.

N.Nicoloso, A.LeCorre-Frisch, J.Maier, R.J.Brook: Solid State Ionics, 1995, 75, 211-6