|Ph.D Thesis||Department of Materials Science and Engineering|
|Supervisor:||Assoc. Prof. Berger Shlomo|
The electrical properties of composite materials composed of a mixture of conducting and insulating phases are of great technological and scientific interest. The dielectric constant and the electrical conductivity of such materials can be altered by few orders of magnitude by increasing the conducting phase content above the percolation threshold. The goal of this research was to take advantage of the dramatic changes in the electrical properties in order to tailor these properties at will according to specific needs. In particular, we wanted to understand the correlation between microstructure and electrical properties, in order to form a composite material with high capacitance and low conductivity.
The materials studied in this research were composed of nano-sized amorphous silica powder and micron-sized tungsten powder. Their electrical properties and microstructure were studied as a function of composition, frequency and heat treatment temperature.
The dielectric constant and the electrical conductivity of the nanocomposites were higher than those of pure silica powder at all compositions. Higher dielectric constants and lower conductivities were observed at lower frequencies. The percolation threshold was about 20% vol. W. Above this composition a sharp increase in the dielectric constant and in the electrical conductivity occurred. This increase became even sharper after heat treatments at various temperatures for 1 hour. It was found that the amount and nature of the W/SiO2 interfaces dominate the electrical properties of the nanocomposites. The electrical conduction mechanism was found to be a variable range hopping, with an activation energy in the range of 0.3-0.95eV for dc conditions and 0.28eV for ac conditions. In addition, a thermally activated behavior was found for the polarization, with an activation energy of 0.2eV.
The results of this research could be implemented best in systems that are operated at low frequencies, up to 10kHz; in this frequency range a relatively high dielectric constant is achieved together with a relatively low conductivity.