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

In these breakdown mechanisms charge carriers are accelerated by the electric field through spaces in the dielectric. In partial discharge breakdown sparks occur within voids in the insulation causing degradation of the void walls and progressive deterioration of the dielectric. In free volume break down carriers are accelerated through spaces within low-density amorphous regions; the energy thereby gained is lost through collisions and various mechanisms have been proposed as to how this may lead to breakdown. The voids necessary for partial discharges may be thought of as extrinsic since they are artifacts of the process used for manufacturing the insulating system. On the other hand free volume is an intrinsic feature of polymeric insulation in which there are always variations of density between crystalline and uncrystallised regions. Free path lengths of up to a few tens of nanometers may be possible through free volume at room temperature. Voids may range almost up to a millimeter in poorly made material and are generally not recognised below a few tens of nanometers. The latter dimension may therefore be used as a 'rule of thumb' dividing line between voids and free volume. In this chapter we will describe these two breakdown mechanisms.

Partial discharge and free volume breakdown

Effects residues (cross-linking by-products and additives) in polyethylene on space charge accumulation and decay have been investigated using the pulsed electro-acoustic technique. Space charge profiles have shown a great variation both in the charge initiation during the voltage ramping-up process and during long term stressing and decay (volts off). Samples were subjected to different conditioning processes, resulting in different proportions of residues (fresh, 0.5% residue and thoroughly degassed). The results show that residual impurities, including the by-products of cross-linking, play a key role in the space charge accumulation in cross-linked polyethylene. On the removal of impurities by degassing, a small homocharge build up was found in the vicinity of the electrode. It is concluded that space charge accumulation is governed by the charge injection through dielectric/electrode interface when the sample is thoroughly degassed.

The Influence of Residue on Space Charge Accumulation in Purposely Modified Thick Plaque XLPE Sample for DC Application

In this paper, unbiasing procedures for the maximum likelihood method applied to the two-parameter Weibull function are dealt with. The performance of unbiasing methods applied to expected values and point estimates of the Weibull parameters, as well as the way to use the Monte Carlo method for the estimation of the expected values, are discussed. It is shown that the accuracy of the unbiasing methods can be significantly affected by several factors, such as the value of the shape parameter and the estimation of the expected value, and that some methods can be successfully applied to the point estimates of the Weibull parameters.

Unbiasing procedures for the Weibull distribution? Be careful

Charge injection and transport in insulating polymers is reported. Hole transfer is possible however since electron-hole recombination would then occur at the electrode. High fields can reduce both the height and width of this potential barrier. The band structure may be less well defined. In this case there are very few carriers in the higher-energy states which are sufficiently close for carriers to easily move from one to another and there are many carriers trapped in the lower-energy states which are too far apart for carriers to easily move between them.

Charge injection and transport in insulating polymers

It is shown that a complete equivalence exists between static and dynamic methods of breakdown testing. Some preliminary experimental results lend support to the Hill and Dissado theory of a fluctuation controlled breakdown mechanism.

http://ieeexplore.ieee.org/iel7/7411473/7411474/07411524.pdf

John C. Fothergill

Papers

Partial discharge and free volume breakdown

The Influence of Residue on Space Charge Accumulation in Purposely Modified Thick Plaque XLPE Sample for DC Application

Unbiasing procedures for the Weibull distribution? Be careful

Charge injection and transport in insulating polymers

Applications and implications of Weibull statistics in dielectrics