Conductivity and space charge in LDPE/BaSrTiO³ nanocomposites

Nano-sized BaSrTiO₃ particles and a dispersant were incorporated in samples of low density polyethylene (LDPE). The nanoparticle loading was 2% or 10% by weight, and the dispersant loading was 4 parts per hundred relative to BaSrTiO₃. dc conductivity measurements were made in the temperature range 30-70°C in vacuum, and in air at 30 °C. The vacuum dc conductivity in LDPE with 0.4% dispersant was approximately five times larger than in LDPE, but in LDPE containing 0.4% dispersant and 10% nanoparticles it was approximately a factor of 6-7 smaller than in LDPE. Since the conductivity of the latter composite was smaller than that of its least conductive component, it may be that the interface regions between the nanoparticles and the LDPE control the dc conductivity. The dispersant and the nanoparticles both increased ε', the real part of the relative permittivity. The ε' values for the nanoparticles, implied by the Lichtenecker-Rother equation for a three-part composite, were of order several hundred, much smaller than the value of at least 10,000 quoted by the nanoparticle suppliers. Space charge measurements were made in air using the Laser Intensity Modulation Method (LIMM), and the profiles calculated using the LIMM Monte Carlo method. The maximum space charge density adjacent to the negative poling electrode was 600 C/m³, for the pure LDPE and the LDPE with dispersant, and 200 C/m³ near the positive electrode. The corresponding maxima in samples with dispersant and nanoparticles were approximately an order of magnitude smaller. The Monte Carlo method is more accurate than other methods of analyzing LIMM data, and this may account for the very large densities close to the electrodes.