Various dielectric characteristics of polymer nanocomposites

CIGRE Task Force D1.16.03 was set up in 2002 to investigate emerging advanced polymer nanocomposites as dielectrics and electrical insulation. Polymer nanocomposites are defined as polymers filled with a few wt% inorganic nano-fillers, defined as particles <100 nm. They are superior in the physical, chemical, mechanical and electrical properties over base polymers and conventional micro-filled polymers. It was elucidated that electrical and dielectric properties can be improved, when dielectric polymers are nanostructured. The paper reviews all the characteristics of dielectric permittivity, dielectric loss, dc conductivity, high field conduction, space charge, electroluminescence (EL), thermally stimulated currents (TSC), partial discharge (PD) resistance, treeing resistance, tracking resistance, breakdown (BD) strength and the like. Materials investigated include polyethylene (PE), polypropylene (PP), ethylene vinyl acetate (EVA), polyamide (PA), polyimide (PI), epoxy, silicone rubber with layered silicate, silica, alumina, titanate and ZnO. Effects of nano-filler addition are described as below: (1) Permittivity decreases, if properly nanostructured. (2) Dielectric loss may decrease at 50/60 Hz, and exhibits complicated dependence on frequency. (3) Low field dc conductivity increases in PP and EVA/layered silicate nanocomposites. But it decreases in polyamide/layered silicate and polyimide/silica nanocomposites. (4) High field conductivity decreases. Threshold to transition decreases. (5) Space charge decreases especially at high field. Threshold field decreases. (6) TSC peak shifts to higher temperature for PA/silica nanocomposites. (7) Threshold field for EL increases for epoxy/titania nanocomposites. Response becomes faster. (8) Breakdown strength increases. Time to BD increases much in treeing experiments. (9) PD resistance improves much. (10) Tracking resistance increases for silicone rubber. (11) Thermal endurance increases. Change in free volume, carrier trap depth, thermal conductivity and glass transition temperature is also discussed. Concepts of interaction zones and a multi-core model have been proposed to explain the effects of nanofillers. It is concluded from the data obtained thus far that polymer nanocomposites are promising as future advanced dielectrics and electrical insulating materials.