J.K. Nelson

Polymer nanocomposite dielectrics-the role of the interface

The incorporation of silica nanoparticles into polyethylene increased the breakdown strength and voltage endurance significantly compared to the incorporation of micron scale fillers. In addition, dielectric spectroscopy showed a decrease in dielectric permittivity for the nanocomposite over the base polymer, and changes in the space charge distribution and dynamics have been documented. The most significant difference between micron scale and nanoscale fillers is the tremendous increase in interfacial area in nanocomposites. Because the interfacial region (interaction zone) is likely to be pivotal in controlling properties, the bonding between the silica and polyethylene was characterized using Fourier transformed infrared (FTTR) spectroscopy, electron paramagnetic resonance (EPR), and x-ray photoelectron spectroscopy (XPS). The picture which is emerging suggests that the enhanced interfacial zone, in addition to particle-polymer bonding, plays a very important role in determining the dielectric behavior of nanocomposites.

Nanocomposite dielectrics—properties and implications

The incorporation of nanoparticles into thermosetting resins is seen to impart desirable dielectric properties when compared with conventional (micron-sized particulates) composites. Although the improvements are accompanied by the mitigation of internal charge in the materials, the nature of the interfacial region is shown to be pivotal in determining the dielectric behaviour. In particular, it is shown that the conditions and enhanced area of the interface changes the bonding that may give rise to an interaction zone, which affects the interfacial polarization through the formation of local conductivity.

Towards an understanding of nanometric dielectrics

Dielectric studies are described aimed at providing an understanding of the charge storage and transport of an epoxy resin containing TiO/sub 2/ nanoparticles. Comparative results for conventionally filled composites are given, and the results discussed in terms of the underlying physics. It is shown that nanometric fillers mitigate the interfacial polarization characteristic of conventional materials with a reduction in the internal field accumulations.

The impact of nanocomposite formulations on electrical voltage endurance

Previous work in which a conventional micron-sized filler was replaced by nanomaterials in an epoxy matrix has shown significant, and encouraging enhancements in the electric strength of the composites. The advantages gained were associated with the mitigation of Maxwell-Wagner polarization and the internal associated space charge, and an optimum particulate loading established. This contribution seeks to extend the previous work, by examining the electrical voltage endurance and partial discharge in a divergent field geometry. The voltage endurance tests demonstrate that significant improvements in endurance are also indicated. Similar results are seen in the partial discharge measurements. In order to gain a mechanistic understanding, the same electrode configuration has also been subjected to electroluminescence experiments in which both the steady-state and temporally-resolved light emission has been compared in these materials. Changes in the magnitude and onset field of the emission suggest that both the enhanced scattering of the nanocomposite and the mitigation of internal charge play a pivotal role in the enhanced voltage endurance obtained. Microscopy, dielectric spectroscopy and free volume measurements are also introduced to provide insight into the possible underlying mechanisms involved.

A review on the importance of nanocomposite processing to enhance electrical insulation

Study of the experimental literature regarding nanodielectrics indicates numerous inconsistencies in the results obtained. In many cases, the likely cause is due to a lack of quality control during nanocomposite processing. By examining examples from the literature along with an alumina/polyamideimide nanocomposite and silica/crosslinked polyethylene nanocomposite, this contribution seeks to shed light on some of the likely causes for these inconsistencies. Measurements of the dielectric breakdown strength and voltage endurance confirm that poor dispersion can lead to poor material performance and the use of quantitative techniques is highlighted. Good dispersion alone is not sufficient to achieve improved properties. The addition of nanoparticles can alter the resulting structure of the nanocomposites, can introduce water into the system resulting in cavity formation and can also result in degradation of the polymer if the processing parameters are not carefully selected. In this review, understanding the effects of nanoparticle addition requires not only characterization of relevant dielectric properties, but also careful control of the processing parameters and characterization of changes in polymer structure, particle dispersion, and water content.

Effect of high aspect ratio filler on dielectric properties of polymer composites: a study on barium titanate fibers and graphene platelets

High aspect ratio fillers are predicted to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures. Using high aspect ratio fillers is a promising route for creating high dielectric constant, low loss materials at a low filler volume fraction, for use as capacitor and electric field grading materials. In this work, two high aspect ratio fillers were mixed into a polymer matrix, and the dielectric properties of composites were studied. Barium titanate fibers were synthesized by electrospinning a sol-gel, followed by a heat treatment to obtain a perovskite crystal structure. The heat treatment conditions were found to be crucial for obtaining tetragonal barium titanate fibers with high dielectric constant. Graphene platelets were prepared by a thermal shock method, which was found to result in a larger dielectric constant. A combination of barium titanate and graphene platelets yielded the highest dielectric constant when used in a polydimethyl siloxane matrix. The increase in dielectric loss over the pure matrix was small when the volume fraction was below the percolation threshold of graphene platelets. Electric flux density-electric field (D-E) measurements showed a linear dielectric constant in barium titanate filled composites and higher loss when graphene was added. The ac breakdown strength was reduced compared to the neat polymer and was affected by filler aspect ratio. The mechanisms that lead to the observed phenomena are discussed.

Performance of a 1-MVA HTS demonstration transformer

We report on test results for a single phase, 60-Hz, 13.8 kV/6.9 kV, 1-MVA high temperature superconducting (HTS) transformer which was completed in February, 1998. This transformer models in many ways a full scale section of a 30-MVA HTS commercial transformer design. The transformer windings are cryocooled in the range of 25 K and are made with a low-cost, surface-coated BSCCO-2212 conductor. Heat leaks are reduced using a liquid nitrogen thermal ballast and reservoir. The use of high temperature superconductors can substantially reduce transformer losses, weight, size, noise and potential fire and environmental hazards. Designs promise stable operation through faults without thermal degradation, and at temperatures that allow efficient and reliable refrigeration.

Time optimal load shedding for distributed power systems

The formulation and computation of an optimal load shedding time algorithm is presented. This new concept was developed as an enhancement to the application of optimal load shedding for corrective control to support future islanded power systems with anticipated, enhanced communications infrastructure. The methodology combines nonlinear mathematical programming and discretized differential-algebraic power systems equations to estimate the optimal amount of load to be shed as well as the best time to shed it. Several simulated scenarios are studied and results presented.

Role of the interface in determining the dielectric properties of nanocomposites

It has been demonstrated that the electrical breakdown properties of polymer composites can be substantially enhanced when the filler particles are of nanometric dimensions. These benefits are likely related to the mitigation and redistribution of internal charge. Using the example of an epoxy-TiO/sub 2/ nanodielectric (and a comparable conventional composite), this contribution seeks to examine this issue from the physical and chemical viewpoint. It is shown that a reduction in free volume cannot be used to explain the dielectric enhancements. The free volume of nanomaterials is actually higher than that of conventional samples. This conclusion is consistent with recent application of electron paramagnetic resonance spectra which have confirmed earlier speculation that the environment associated with the interface is radically changed when the in-filled particulates are reduced to nanometric dimensions and the associated interfacial area is greatly increased. Through examinations of infrared absorption and EPR measurements, the paper provides some speculation on the part played by an interaction zone surrounding the particulate inclusions. The presence of a highly mobile interlayer is thought to be the key to the electrical property changes seen.

Assessment of deterioration in epoxy/mica machine insulation

Long-time accelerated aging tests in a laboratory environment have been conducted on generator stator insulation to identify meaningful diagnostic measures for the assessment of degradation and identification of aging mechanisms. A plurality of techniques are investigated and diagnostic parameter, the dynamic stagnation voltage, which appears to be correlated with delimination in the epoxy-mica structure, is proposed. The results provide a basis for an online interpretation method for insulation condition. >