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 (w/w), 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 samples containing only the dispersant (0.4%) was approximately five times larger than that in LDPE samples without additive, but approximately an order of magnitude larger than that in samples containing 10% nanoparticles and 0.4% dispersant. Addition of 10% nanoparticles and 0.4 % dispersant increased ε′, the real part of the relative permittivity, from approximately 2.34 to around 2.58 at 30°C, in air and in vacuum. 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 for LDPE with 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.