Dielectrics

Enhancement of dielectric breakdown strength in glass using polymer coatings

High temperature power electronic and pulsed power applications will drive the development of circuit components that operate under extreme environmental conditions. There are several types of capacitors and the choice depends upon factors including frequency, temperature and cost. Electrolytic capacitors have high series resistance and self-heating at high ripple currents reduces the capacitor's operating lifetime [1]. Ceramic capacitors have excellent performance and meet a majority of specifications - high ripple current, low stray inductance, and high temperature, for pulsed power and power electronics [2]; however, available capacitor size and the failure mode are impediments to their use as DC link capacitors in electric vehicle power converters. Polypropylene film capacitors have high reliability and self-healing capability, but they do not have sufficient high temperature performance for future automotive applications [3]. New dielectric materials for capacitors are needed for high temperature and high ripple current applications.

The volumetric efficiency of a capacitor is directly related to the energy density of the dielectric. Electrostatic energy storage in capacitors is quantified in two ways - energy density and total stored energy. The total energy, in Joules, is related to capacitor volume and energy density. Energy density, expressed as Formula (units of J/cm3) is a material parameter which captures permittivity Formula and dielectric breakdown strength Formula as key material properties. Because dielectric breakdown strength and energy density generally decrease as capacitor size increases, the total energy of a capacitor does not scale proportionally with the large volumes of materials required for capacitors. It is important to maintain high energy density values as the capacitor volume increases from the lab scale to practical capacitor sizes.

Glass capacitors are candidates for high temperature applications because of their excellent insulating properties and possible self-healing behavior during dielectric breakdown. Previous research has shown that alkali-free glass has high breakdown strength (12 MV/cm) and energy density of 35 J/cm3 at room temperature and low dielectric loss for temperatures up to 200°C for lab-scale capacitors [4], [5]. The dielectric breakdown strength has been shown to decrease as the glass layer thickness increases from 12 MV/cm at Formula to 4 MV/cm at Formula [6]. In addition, breakdown strength decreases with increasing dielectric area as the probability of encountering a critical defect increases. This paper will address glass/polymer/metal structures which significantly increase the breakdown strength of alkali-free glass sheets, compared to uncoated glass.

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