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 PEA technique is used to measure the distribution of space charge in an epoxy resin after polarisation for one week at an applied field of 7.14 kV/mm over a range of temperatures. The decay of the space charge is followed for times up to 114 h after removal of the voltage and analysed in terms of a number of alternative decay mechanisms. It is shown that the rate-determining stage of the decay mechanism is that of a thermally activated process that has been associated with charge de-trapping. At times greater than 102 s the de-trapping process behaves as though the space charge field does not exist and the retention time of the space charge depends only upon the depth of the deepest occupied traps and the temperature

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Decay of space charge in a glassy epoxy resin following voltage removal

Space charge in solid dielectrics

The formation of space charge packets in crosslinked polyethylene (XLPE) tapes from un-aged cable insulation has been studied utilizing the pulsed electroacoustic (PEA) technique. The 150 /spl mu/m thick sheets were studied under constant applied DC field of 120 kV/mm at a temperature of 20/spl deg/C for a period of 48 h. After an inception period of /spl sim/3.5 h, during which heterocharge accumulates at the anode and increases the local field there, a sequence of positive charge packets was observed to transit the sample starting from near the anode. Calculation of the internal field showed that the packets required a field of /spl ges/140 kV/mm for their initiation. Reduction of the applied field step-wise from 120 to 80 kV/mm indicated that the charge packet would keep moving as long as the local field at its front was >100 kv/mm, but with a reducing magnitude. A return to an applied field of 120 kV/mm confirmed that the local field required to initiate a new packet was >135 kV/mm. The results are discussed in terms of current theories of charge packet formation. The first packet appears to be a moving front of field ionization. The generation of subsequent packets is governed by the field at the anode and the balance of charge injection and extraction process, which occur there. The nature of the negative charges produced at the ionization front is not clear, but they are unlikely to be electrons.

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Electric field criteria for charge packet formation and movement in XLPE

This position paper, prepared by the IEEE DEIS HVDC Cable Systems Technical Committee, illustrates a protocol recommended for the measurement of space charges in full-size HVDC extruded cables during load cycle qualification tests (either prequalification load cycles or type test load cycles). The protocol accounts for the experimental practices of space charge measurements in the thick insulation of coaxial cables in terms of poling time, depolarization time, heating and cooling of specimens, as well as for the experience gained very recently from such kind of measurements performed in the framework of qualification tests relevant to ongoing HVDC cable system projects. The goal of the protocol is not checking the compliance with any maximum acceptable limit of either space charge or electric field. Rather, this protocol aims at assessing the variation of the electric field profile in the cable insulation wall during poling time at the beginning and at the end of load cycle qualification tests for full-size HVDC extruded cables. Indeed, in the design stage the electric field distributions are determined by the cable geometry and by temperature gradient in the insulation. Thus, the design is based on macroscopic parameters conductivity and permittivity and how they depend upon temperature. Any disturbance of the electric field due to space charge accumulation will only be revealed during space charge measurements either in as-manufactured state or in the aged state after load cycle qualification tests.

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A protocol for space charge measurements in full-size HVDC extruded cables

Techniques for estimating the parameters of the 2-parameter Weibull distribution from data obtained from uncensored tests are compared. This will allow the most convenient method to be chosen by considering the data's characteristics and the level of algorithm complexity. It is shown that common techniques, such as least squares regression and maximum likelihood, may give rise to very significant errors in terms of the bias of the estimated Weibull parameters. The estimator suggested by Jacquelin (1993) for /spl alpha/ and the correction factor suggested by Ross (1994) for /spl beta/ usually give errors of <5 and <1%, respectively and are to be recommended, because they are straightforward to implement. Techniques are available which will eliminate virtually all bias in the estimation of the parameters, but these may be at the expense of considerable complexity in implementation.

John C. Fothergill

Papers

Decay of space charge in a glassy epoxy resin following voltage removal

Space charge in solid dielectrics

Electric field criteria for charge packet formation and movement in XLPE

A protocol for space charge measurements in full-size HVDC extruded cables

In search of convenient techniques for reducing bias in the estimation of Weibull parameters for uncensored tests