Advanced polymeric dielectrics for high energy density applications

Better understanding of the fundamental electrical properties of polymeric materials is a key to their further progress in the practical application, not only as electrical insulating materials, but also as functional elements. In this paper, recent topics and their important results will be reviewed on electrical conduction and related problems including carrier traps and electrical breakdown in various polymers. At first, the electronic nature, e.g. photoconduction and electron transport are explained in insulating, semi-insulating, and conducting polymers. Also, interfacial phenomena at both metal-polymer and polymer-polymer interfaces, carrier traps (their physico-chemical nature) and impurity effects on electrical conduction are introduced with their results and experimental techniques. Finally, interesting topics on electrical breakdown related to polymer morphology will be given.

Electrical Conduction and Carrier Traps in Polymeric Materials

The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress.

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Electrodes and charge injection in low-density polyethylene using the pulsed electroacoustic technique

Electrical conduction phenomena in polyimide (Kapton) films were studied with particular attention devoted to the separation of interface and bulk phenomena. The measurements were carried out with a variety of methods in the temperature range of 50 to 270 °C at electrical fields of 104 to 6×105 V/cm and at time intervals of up to 2×104 s after voltage application. Biased, two‐side metallized samples yield, after sufficiently long voltage application, interface‐controlled steady‐state currents described by Schottky injection, modified by space‐charge layers in the vicinity of the electrodes. The effective work function for aluminum‐polyimide is estimated to be 1.7 eV in the temperature range between 100 and 270 °C. A distinct dependence of these currents on electrode material is observed. Bulk phenomena were studied on one‐side metallized samples subject to positive‐corona charge injection. At temperatures below 200 °C, significantly larger currents than those for biased, two‐side metallized samples were observed. The current‐voltage characteristics are ohmic at low fields and space‐charge limited at high fields. From these data, trap modulated mobilities for positive carriers of 4×10−12 cm2/V s at 50 °C and 10−9 cm2/V s at 200 °C, ohmic bulk conductivities of 10−16 (Ω cm)−1 at 100 °C and 10−14 (Ω cm)−1 at 200 °C and an intrinsic carrier density of 5×1013 cm−3 independent of temperature are obtained. Activation energies for the mobility are between 0.2 and 0.8 eV for the temperature range between 50 and 200 °C.

Electrical conduction in polyimide films

Intrinsic photoemission and photoconduction of polyethylene in the photon energy range 7<E<14 eV have been experimentally investigated. Both phenomena are parabolic in E with the same threshold energy EG=8.8 eV. The main features of a model that accounts for the observations are: (a) EG is the intrinsic electron-hole band gap. (b) Strong absorption for 7.6<E<8.8 eV is due to exciton creation. (c) The bottom of the conduction band lies an energy A above the vacuum level, where 0<A<1.2 eV. (d) The photocurrents are due to rapid motion of carriers through a short distance approximately=10 nm along individual polyethylene chains. (e) The carriers subsequently travel through the polyethylene with a very low mobility.

https://iopscience.iop.org/article/10.1088/0022-3719/6/21/014/pdf

Intrinsic Photoconduction and Photoemission in Polyethylene

Electrical conduction is studied in polyethylene terephthalate and to a limited extent in polyethylene over a range of fields up to 12×106 V cm−1. In common with previous investigations, it is found that the current decays with time according to an inverse power law, but this can be obviated by a conditioning procedure which then allows steady values of current to be measured over a range of temperature and field. The results suggest that the current has an activation energy which is field dependent, but attempts to fit the results to a Schottky-type law are not very successful. By adopting a more generalized form of potential barrier than the coulombic one usually chosen in the Schottky law, it is possible to obtain consistent agreement with experiment. The form of potential barrier chosen appears to refer to the cathode-dielectric interface and is probably determined by space charge in the dielectric. Values of the metal-dielectric contact potential for the samples of polyethylene terephthalate and polyethylene investigated were 258 and 214 eV respectively for aluminium electrodes. It is suggested that the time dependence observed before conditioning is completed is due to the slow build-up of the full interfacial barrier.

https://iopscience.iop.org/article/10.1088/0022-3727/4/9/315/pdf

T J Lewis

Papers

Electrical conduction in polyethylene terephthalate and polyethylene films