Silica-Polypropylene Nanocomposites for Film Capacitors: Structure–Property Studies and the Role of Biaxial Stretching Conditions

Abstract

Biaxial-stretching-induced morphology development anddielectric properties of hydrophobic fumed silica-BOPPnanocomposite films were investigated. The precursor (cast)film morphology, pre-heating as well as biaxial stretchingtemperature and stretch ratio were found to profoundly affect thesilica-BOPP film bulk morphology, surface texture and theformation of particle/agglomerate-induced cavitation uponbiaxial stretching. Isothermal high-field conductivity andthermally stimulated depolarization current measurementsindicated decreasing high-field/high-temperature conductivityand modification of trap density of states upon incorporation ofnanosilica in comparison to neat BOPP. Large-area DCdielectric strength is, however, sensitive to thermal/mechanicalstresses upon biaxial stretching, hence necessitating carefuloptimization of the nanocomposite formulation and processing.

Biaxially oriented silica–polypropylene nanocomposites for HVDC film capacitors: morphology-dielectric property relationships, and critical evaluation of the current progress and limitations

“…The initial high-field charging current at 100 V/µm exhibited a slowly decaying trend characterized by an inverse power law relationship 𝑖 𝑐 ~𝑡−𝑛 with the exponent 𝑛 being in the range of ~0.6, similarly as observed for commercial capacitor-grade BOPP films 2,96 . Within the typical short-term test conditions (measurement for up to 24 h under electro-thermal stress of 80-100 °C and 100 V/µm), a decrease in apparent conductivity with increasing silica content was often observed for 0-4.5 wt-% silica-BOPP films, as is demonstrated in Figure 11a for the 1.0 wt-% silica-BOPP film (see also our previous publications 34,36 ). We associate the above decrease of bulk conductivity under moderate stresses and time to the modified (deep) trap density of states and reduced charge mobility brought by the nanosilica, as evidenced by the trap spectroscopy methods in the preceding sections.…”

“…29,30 On the other hand, the latter approach, focusing on increasing the maximum permissible electric field stress of PP films via inclusion of permittivity-matching nanoparticles to tailor the charge trapping and transport properties at the nanometric level, has been shown to be a promising route towards practical film capacitor applications. Previously, pathways to the modification of the trap density of states and indications of bulk dielectric property improvement in BOPP films through incorporation of low amounts of surface-functionalized inorganic nanoparticles such as SiO 2 , Al 2 O 3 , TiO 2 and POSS have been demonstrated, 31–33 including decreased electrical conductivity and space charge accumulation, 34–36 increased short-term dielectric breakdown strength, 33,37,38 and increased resistance against partial discharges (PD), 38 for instance.…”