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Journal of Physics D: Applied Physics Volume 47 Number 7, 2014. Meng H Lean and Wei-Ping L Chu.
QEDone LLC, Santa Clara, CA 95054, USA.
Abstract
This paper describes a hybrid algorithm to study the effect of a gaseous void on bipolar charge transport in layered polymer film. This hybrid algorithm uses a source distribution technique based on an axisymmetric boundary integral equation method to solve the Poisson equation and a fourth order Runge–Kutta (RK4) method with an upwind scheme for time integration. Iterative stability is assured by satisfying the Courant–Friedrichs–Levy stability criterion. Dynamic charge mapping is achieved by allowing conducting and insulating boundaries and material interfaces to be represented by equivalent free and bound charge distributions that collectively satisfy all local and far-field conditions. This hybrid technique caters to bipolar charge injection, field-dependent mobility transport, recombination, and trapping/de-trapping in the bulk and at material and physical interfaces. The resulting charge map is the taxonomy of the different charge types and their abundance, and presents a dynamic view of the temporal and spatial distributions. The paper is motivated by images of breakdown experiments that point to the role of gaseous void in delamination growth. For the test configuration, the high field at the edge of the gaseous void act as a sink first for positive and then negative charge. The net effect is to increase delamination stress at the edge leading to further growth of the defect and increasing the potential for partial discharge within the void.
Significance statement
Metalized film polymer capacitors are used in power electronic circuits to enable efficient power conditioning, energy storage, energy conversion, and providing pulsed power for propulsion, protection, and directed energy systems. Multi-layering of polymer films increases energy density and breakdown strength. Alternate layering of two polymers, one with high breakdown strength and the other with high dielectric constant, results in a tiered structure with higher reliability, higher energy density, and increased capacity to withstand higher voltage fluctuations.
Breakdown is influenced by charge injected from both electrodes which migrate and accumulate at interfaces leading to electric field modification. Gaseous voids may form at the interfaces as a by-product of the layering process during microlayer co-extrusion. This flaw serves as a stress concentrator under mechanical load, or an electric field concentrator in the presence of an externally applied electrical potential. One major cause of failure in polypropylene film high voltage capacitors is partial discharges (PD) which can occur in the cavities or gas pockets. Prior to PD, the void experiences time-staggered attraction of the bipolar mobile charge species with the first specie in response to the high field, and the second specie in response to the resulting concentration of opposite polarity charge, leading to the creation of hetero-charge distributions. The presence of hetero-charge creates local field modifications that increase local stress leading to accelerated breakdown and aging. Charges that are not recombined take the shortest path to the adjacent interface, creating conduction paths that are precursors to the eventual breakdown pathway. The high field at the edge of the gaseous void may see compressive stress that leads to further delamination and growth of the void.
This paper describes the axisymmetric simulation of bipolar charge injection, transport, trapping/de-trapping, and recombination in layered polymer films using the continuum solution to the drift-diffusion equations to map temporal and spatial charge distributions.
