Advanced polymeric dielectrics for high energy density applications

Much effort has been invested for nearly five decades to identify and develop new polymer capacitor dielectrics for higher than ambient temperature applications. Simultaneous demands of processability, dielectric permittivity, thermal conductivity, and dielectric breakdown strength dictated by increasing high power performance criteria limit the number of available materials. The present review first explains the advantages of metallized polymer film capacitors over the film-foil, ceramic, and electrolytic counterparts and then presents a comprehensive review on both past developmental effort of commercial resins and recent research progress on new polymers targeted for operating temperature above 150 °C. Some historical background and discussion on the limitation of the commercially available polymer film dielectrics for high temperature applications are also given. In many cases, further development of promising polymers that appear to possess all or most of the important criteria is limited by lack of large scale market incentives but could be of great value to niche applications in the military or aerospace realm.

Polymer Capacitor Dielectrics for High Temperature Applications.

Flexible dielectrics operable under simultaneous electric and thermal extremes are critical to advanced electronics for ultrahigh densities and/or harsh conditions. However, conventional high-performance polymer dielectrics generally have conjugated aromatic backbones, leading to limited bandgaps and hence high conduction loss and poor energy densities, especially at elevated temperatures. A polyoxafluoronorbornene is reported, which has a key design feature in that it is a polyolefin consisting of repeating units of fairly rigid fused bicyclic structures and alkenes separated by freely rotating single bonds, endowing it with a large bandgap of ≈5 eV and flexibility, while being temperature-invariantly stable over -160 to 160 °C. At 150 °C, the polyoxafluoronorbornene exhibits an electrical conductivity two orders of magnitude lower than the best commercial high-temperature polymers, and features an unprecedented discharged energy density of 5.7 J cm-3 far outperforming the best reported flexible dielectrics. The design strategy uncovered in this work reveals a hitherto unexplored space for the design of scalable and efficient polymer dielectrics for electrical power and electronic systems under concurrent harsh electrical and thermal conditions.

Flexible Temperature-Invariant Polymer Dielectrics with Large Bandgap.

Biaxial-oriented polypropylene (BOPP) thin films are currently used as dielectrics in state-of-the-art capacitors that show many advantages, such as low energy loss and high breakdown strength, but a limited energy density ( 600 MV/m. The PP-OH dielectric demonstrates a linear reversible charge storage behavior with high releasing energy density > 7 J/cm3 (2 - 3 times of BOPP) after an applied electric field at E = 600 MV/m, without showing any significant increase of energy loss and remnant polarization at zero electric field. On the other hand, a cross-linked polypropylene (x-PP) exhibits an e ~ 3, which is independent of a wide range of temperatures and frequencies, slim polarization loops, high breakdown strength (E = 650 MV/m), narrow breakdown distribution, and reliable energy storage capacity > 5 J/cm3 (double that of state-of-the-art BOPP capacitors), without showing any increase in energy loss.

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Functionalization of Polypropylene with High Dielectric Properties: Applications in Electric Energy Storage

The transmission characteristics of underground medium-voltage/broadband over power line (MV/BPL) channels are examined analytically. By appropriately modifying multiconductor transmission-line theory, the bottom-up approach, already used to treat overhead MV/BPL transmission, is extended to analyze BPL transmission in three-phase underground MV lines with common shield and armor. ThIS analysis shows that these cables may support five modes, giving rise to five separate transmission channels which reduce to three if the shield and armor are grounded. The numerical results concerning attenuation in underground MV/BPL channels have been validated against relevant sets of measurements and compared to the relevant results of overhead MV/BPL channels. It has been verified that the attenuation in underground MV/BPL channels exhibits a lowpass behavior and is significantly higher than that of overhead ones.

Broadband Transmission via Underground Medium-Voltage Power Lines—Part I: Transmission Characteristics

This paper provides an overview of the mechanisms of high frequency (HF) loss in shielded power cables and discusses the challenges in the measurements of cable material properties at HF.

High frequency properties of shielded power cable - part 1: overview of mechanisms

Automatic breakdown tester for film samples which employs metalized film electrodes has been developed and demonstrated. Using this system, up to 600 breakdowns have been carried out automatically on capacitor film with a Weibull characteristic breakdown voltage in the range of 5 kV. Breakdown data measured with this system agree with data measured manually using the same test method. Measurements of breakdown field as a function of active area agree well with theory based on an assumed Weibull distribution for the breakdown field. A duplicate of this system has been delivered to the Army Research Laboratory, Adelphi, MD.

Automatic breakdown voltage measurement of polymer films

This paper provides a theoretical context for the types of errors that are likely to occur during the measurement of high frequency cable shield properties. The paper considers a RC dielectric system as it relates to measuring the dielectric properties of cable shield materials.

High frequency properties of shielded power cable. Part 2: sources of error in measuring shield dielectric properties

High field degradation and electric breakdown of dielectrics are extremely complex phenomena as a result of the interplay among the electric field, temperature, material morphology, and extrinsic material properties. Fundamental understanding of carrier mobility related prebreakdown phenomena in dielectrics provides insights into high field transport phenomena as well as associated aging and onset of charge injection induced instability. Investigation of such extreme field conduction has been traditionally limited to the divergent field distribution generated using point-plane electrode configuration, as testing of parallel plate sample configuration under quasi steady-state conditions can only reach around two thirds of the breakdown field as a result of rapid high field aging. A circuit has been developed for transient characterization of conduction through a planar dielectric film during a linear ramp voltage to breakdown via the cancellation of displacement current to facilitate the measurement of small resistive currents down to 10ppm level. The dynamic cancellation of displacement current during an applied voltage waveform is realized through the use of a high frequency sinusoidal “bias” voltage to generate a capacitive current that can be cancelled using a feedback circuit based on a voltage-controlled amplifier with negligible phase shift and a dual-phase digital lock-in amplifier. Such capability of transient characterization of conduction in dielectrics will provide insights into dielectric aging and breakdown mechanism and form a quantitative basis for the extraction of critical transport parameters for conduction under extreme electric fields.

Transient characterization of extreme field conduction in dielectrics

In this paper, we describe the phenomenon and the basis for an analytic approximation for this loss, which we then compare with the loss as computed using finite-element analysis The mathematical solution for the loss is too complicated to be included here As a result, the full mathematical treatment of the problem has been submitted for publication The discussion here will be restricted to a qualitative description of the phenomenon and graphical quantification with comparison to finite-element computations and measured data

Chunchuan Xu

Papers

High frequency properties of shielded power cable - part 1: overview of mechanisms

Automatic breakdown voltage measurement of polymer films

High frequency properties of shielded power cable. Part 2: sources of error in measuring shield dielectric properties

Transient characterization of extreme field conduction in dielectrics

High frequency properties of shielded power cable. 3. Loss from neutral wire-shield interaction