Résumé
RuO 2-borosilicate glass composites representing nuclear glasses show a sudden increase in electrical conductivity when the RuO 2 content reached a concentration of around 1% in volume. This percolation threshold is astonishingly low compared to a geometrical percolation threshold, i.e. 15%, considering the random distribution of phases. Herein, electrostatic force microscopy (EFM) analyses were used to investigate this anomalous behavior by performing local dielectric characterization of the RuO 2-borosilicate glass composites that comprised clusters of precipitated RuO 2 particles and "apparently" RuO 2-free borosilicate regions. As expected, differences between the relative permittivity of the two zones, borosilicate matrix, and RuO 2 particles, were found. Most interesting, the sensitivity of the EFM technique combined with numerical modeling based on a finite element method revealed that the relative permittivity of the borosilicate matrix was enhanced with the overall RuO 2 content. Such an increase can be explained by a local enrichment of the borosilicate matrix in RuO 2, which increases the overall relative permittivity due to its much higher electronic polarizability. The enrichment of the matrix could play a role in the conduction mechanism, changing the effective volume of the conductive medium and, therefore, decreasing the minimal volume of the RuO 2 phase necessary for the electrical percolation.