High-resolution electrical characterization of RuO2-borosilicate glass composites

Abstract

RuO2-borosilicate glass composites representing nuclear glasses show a sudden increase in electrical conductivity when the RuO2 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 RuO2-borosilicate glass composites that comprised clusters of precipitated RuO2 particles and "apparently” RuO2-free borosilicate regions. As expected, differences between the relative permittivity of the two zones, borosilicate matrix, and RuO2 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 RuO2 content. Such an increase can be explained by a local enrichment of the borosilicate matrix in RuO2, 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 RuO2 phase necessary for the electrical percolation.