Study of flexible nanodielectric materials (FNDMs) with high permittivity is one of the most active academic research areas in advanced functional materials. FNDMs with excellent dielectric properties are demonstrated to show great promise as energy-storage dielectric layers in high-performance capacitors. These materials, in common, consist of nanoscale particles dispersed into a flexible polymer matrix so that both the physical/chemical characteristics of the nanoparticles and the interaction between the nanoparticles and the polymers have crucial effects on the microstructures and final properties. This review first outlines the crucial issues in the nanodielectric field and then focuses on recent remarkable research developments in the fabrication of FNDMs with special constitutents, molecular structures, and microstructures. Possible reasons for several persistent issues are analyzed and the general strategies to realize FNDMs with excellent integral properties are summarized. The review further highlights some exciting examples of these FNDMs for power-energy-storage applications.

Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage

A multi-core model, i.e. a simplified term of a multi-layered core model, is proposed as a working hypothesis to understand various properties and phenomena that polymer nanocomposites exhibit as dielectrics and electrical insulation. It gives fine structures to what are called "interaction zones". An interfacial layer of several tens nm is multi-layered, which consists of a bonded layer, a bound layer, and a loose layer. In addition, the Gouy-Chapman diffuse layer with the Debye shielding length of several tens to 100 nm is superimposed in the interfacial layer to cause a far-field effect. Nano-particles may interact electrically with the nearest neighbors each other due to this effect, resulting in possible collaborative effect. Such a multi-core model with the far-field effect is discussed, for example, to explain partial discharge (PD) resistance of polyamide layered silicate nanocomposites, and is verified to demonstrate its effectiveness.

Proposal of a multi-core model for polymer nanocomposite dielectrics

Perovskite lead-free dielectrics for energy storage applications

Polymer nanocomposites possess promising high performances as engineering materials, if they are prepared and fabricated properly. Some work has been recently done on such polymer nanocomposites as dielectrics and electrical insulation. This was reviewed in 2004 based on the literatures published up to 2003. New significant findings have been added since then. Furthermore, a multi-core model with the far-distance effect, which is closely related to an "interaction zones", has been proposed from consideration of mesoscopic analysis of electrical and chemical structures of an existing interface with finite thickness. It is speculatively examined in the paper how the model works for various properties and phenomena already found in nanocomposites as dielectrics focusing on electrical characteristics, resistance to high voltage environment, and thermal properties.

Dielectric nanocomposites with insulating properties

Polymer nanocomposites are defined as polymers in which small amounts of nanometer size fillers are homogeneously dispersed by only several weight percentages. Addition of just a few weight percent of the nanofillers has profound impact on the physical, chemical, mechanical and electrical properties of polymers. Such change is often favorable for engineering purpose. This nanocomposite technology has emerged from the field of engineering plastics, and potentially expanded its application to structural materials, coatings, and packaging to medical/biomedical products, and electronic and photonic devices. Recently these 'hi-tech' materials with excellent properties have begun to attract research people in the field of dielectrics and electrical insulation. Since new properties are brought about from the interactions of nanofillers with polymer matrices, mesoscopic properties are expected to come out, which would be interesting to both scientists and engineers. Improved characteristics are. expected as dielectrics and electrical insulation. Several interesting results to indicate the foreseeable future have been revealed, some of which are described on materials and processing in the paper together with basic concepts and future direction.

Polymer nanocomposites as dielectrics and electrical insulation-perspectives for processing technologies, material characterization and future applications

The probability of electrical breakdown in a stressed dielectric is related to the strength and time of application of the electric field. It is the aim of cable manufacturers to increase the dielectric field strength and service life without increasing the probability of failure. In this respect it is necessary to improve the quality of the insulation and prevent in-service degradation, without prohibitively increasing the cost of the product. A method which may achieve this in polymeric insulation without creating new chemical compos1t1ons is to alter the polymer morphology in such a way as to increase the “breakdown strength” and reduce the susceptibility of the material to “treeing”. Treeing appears to be the most important physical long term pre-breakdown phenomenon that occurs in dielectrics due to the externally applied electric field. Although these areas of study are obviously important, no generally accepted physical treeing model exists and there is little published work on the interrelationship between trees, electrical breakdown strength and polymer morphology. In this paper we report some interesting initial results from preliminary experimental and theoretical work relating all three subjects.

A preliminary investigation into treeing, morphology and the electrical properties of polymers

The Kramers-Kronig (K-K) transform relates the real and imaginary parts of the complex susceptibility as a consequence of the principle of causality. It is a special case of the Hilbert transform and it is often used for estimation of the DC conductance from dielectric measurements. In this work, the practical limitations of a numerical implementation of the Kramers-Kronig transform was investigated in the case of materials that exhibit both DC conductance and quasi-DC (QDC) charge transport processes such as epoxy resins. The characteristic feature of a QDC process is that the real and imaginary parts of susceptibility (permittivity) follow fractional power law dependences with frequency with the low frequency exponent approaching −1. Dipolar relaxation in solids on the other hand has a lower frequency exponent <1. The computational procedure proposed by Jonscher for calculation of the K-K transform involves extrapolation and truncation of the data to low frequencies so that convergence of the integrals is ensured. The validity of the analysis is demonstrated by performing K-K transformation on real experimental data and on theoretical data generated using the Dissado-Hill function. It has been found that the algorithm works well for dielectric relaxation responses but it is apparent that it does not work in the case of a low frequency power law in which the low frequency exponent approaches −1, i.e. in the case of QDC responses. In this case convergence can only be guaranteed by extrapolating the low frequency power law over many decades towards zero frequency.

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Limitations of kramers-kronig transform for calculation of the DC conductance magnitude from dielectric measurements

Polyethylene is a material that is widely used as insulation for power cables. In this paper, the space charge decay of high density polyethylene under different temperatures and accumulation fields is described. On voltage application both positive and negative space charge was found to accumulate rapidly in the bulk of the samples. The amount of positive space charge present increased with increasing voltage. An application of 5KV at temperatures 25-60°C gave mainly negative space charge, whereas voltages 8-23kV gave much more positive charge than negative charge. Charge accumulation was found to be rapid after voltage application. On short-circuit the main part of the space charge disappeared within a few tens of seconds. The decay time of the positive space charge decreased either with increase of the magnitude of applied voltage before short circuiting or with increase of temperature. It was suggested that the decay rate of positive space charge in HDPE was enhanced by temperature and the field produced by the accumulated space charge in the bulk.

The space charge decay of high density polyethylene under different temperatures and accumulation fields

The advent of experimental techniques capable of directly measuring the density of space charge in insulators under voltage in a non-destructive manner has revealed the existence of charge packet formation at high DC fields. In these circumstances space charge is observed to cross the insulation in the form of a packet rather than through the steady advance of a charge front. Such packets have been observed both in cable insulation, and in thin films. Since they occur at high fields before an electric failure occurs they are of interest as a possible pre-breakdown phenomenon. For this reason there is an interest in understanding the phenomena. In the experiments reported here we explore the electrical conditions required for the initiation of very slow moving charge packets in XLPE.

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Electric field requirements for charge packet generation and movement in XLPE

The movement of residual positive charges along the walls of tree tubules and into the surrounding polymer is a critical determining factor in the shape (bush or branch) of electrical trees. Positive charge that has a higher mobility along tubule walls promotes single discharges propagating to the tree tip. Lower mobility positive wall charge tends to more, but perhaps smaller, discharges. The former may favour branch tree growth, the latter bush trees. Spread out positive wall charge may occur when there are a number of discharges per half cycle. The resulting damage may favour bifurcation (transverse extension).

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John C. Fothergill

Papers

A preliminary investigation into treeing, morphology and the electrical properties of polymers

Limitations of kramers-kronig transform for calculation of the DC conductance magnitude from dielectric measurements

The space charge decay of high density polyethylene under different temperatures and accumulation fields

Electric field requirements for charge packet generation and movement in XLPE

Discharges, space charge, and the shape of electrical trees