A review of recent developments made in the understanding of the electrical tree mechanism is presented. The life of the tree is covered from initiation, through propagation, to long-term changes in shape. The initiation process is examined in terms of the injection of space charge and its ability to transfer energy to the polymer to create damage. Theoretical models for the processes involved are assessed in terms of the experimental data and an outline for the sequence of events in tree initiation developed. The inter-relationship between tree discharges, tree propagation, and tree shape is discussed. Theoretical models for these processes are evaluated in terms of their ability to reproduce experimental data, especially tree shapes and discharge sequences in time and space. The chaotic nature of the tree propagation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model. Some special features of electrical trees such as the existence of conducting trees, acceleration at long times and slow growth in thick insulation are briefly touched upon. Finally a summary of the state of the art is presented.

Understanding electrical trees in solids: from experiment to theory

The primary event which occurs when an X-ray photon of energy less than 30 keV is absorbed in a protein crystal (or other organic material) is the production of a photoelectron with a similar energy to that of the absorbed photon. The electron then scatters inelastically off the surrounding material losing energy in the process. This reduction in energy takes place over track lengths of a few µm for 20 keV electrons. The vector distances between the initial and final positions of the photoelectrons are less than the track lengths owing to the non-linear tracks followed by the electrons. For crystals with smaller dimensions than the vector distances, a significant proportion of the energy could leave the crystal with the high-energy electrons. This could provide an advantage in terms of reduced radiation damage. In order to estimate the possible benefits, calculations of the electron tracks are given, initially using the continuous slowing-down approximation. A Monte Carlo approach is then used to provide more accurate values of the vector distance travelled by electrons inside a protein crystal. The calculations indicate that significant reductions in radiation damage could occur for crystals of a few µm in size. The benefits would be greater when operating at higher energies. In addition, a scheme for realising the possible benefits in a practical situation is described. This could then form the basis of trial experiments.

Will reduced radiation damage occur with very small crystals

The interface between the polymer and the particle has a critical role in altering the properties of a composite dielectric Polymer-ceramic nanocomposites are promising dielectric materials for many electronic and power devices, combining the high dielectric constant of ceramic particles with the high dielectric breakdown strength of a polymer Self-assembled monolayers of electron rich or electron poor organophosphate coupling groups were applied to affect the filler–polymer interface and investigate the role of this interface on composite behavior The interface has potential to influence dielectric properties, in particular the leakage and breakdown resistance The composite films synthesized from the modified filler particles dispersed into an epoxy polymer matrix were analyzed by dielectric spectroscopy, breakdown strength, and leakage current measurements The data indicate that significant reduction in leakage currents and dielectric losses and improvement in dielectric breakdown strengths resulte

Dielectric properties of polymer-particle nanocomposites influenced by electronic nature of filler surfaces.

One key to optimizing the performance of polymer nanocomposites for high-tech applications is surface ligand engineering of the nanofiller, which has been used to either tune the nanofiller morphology or introduce additional functionalities. Ligand engineering can be relatively simple such as a single population of short molecules on the nanoparticle surface designed for matrix compatibility. It can also have complexity that includes bimodal (or multimodal) populations of ligands that enable relatively independent control of enthalpic and entropic interactions between the nanofiller and matrix as well as introduce additional functionality and dynamic control. In this Spotlight on Applications, we provide a brief review into the use of brush ligands to tune the thermodynamic interactions between nanofiller and matrix and then focus on the potential for surface ligand engineering to create exciting nanocomposites properties for optoelectronic and dielectric applications.

https://www.benicewiczgroup.com/UserFiles/benigroup/Documents/BB102.pdf

Ligand Engineering of Polymer Nanocomposites: From the Simple to the Complex

Most modern extruded high voltage cables employ cross-linked polyethylene (XLPE) as the insulation material. XLPE has excellent thermo-mechanical properties, is relatively cheap and has a low dielectric loss, which make it an ideal material for this application. Unfortunately, XLPE is not easily recycled at the end of its lifetime leading to questions concerning its long-term sustainability. A previous investigation in this series considered the potential of a range of ethylene-based systems to provide suitable recyclable alternatives to XLPE. Whilst blending could allow systems having similar thermo-mechanical and electrical properties to XLPE to be designed, it was not possible to obtain better performance than XLPE using these systems. Polypropylene offers, potentially, a route to improved insulation systems by virtue of its higher melting point and excellent dielectric properties. However, traditional isotactic polypropylenes have always had the problem of being too brittle for inclusion into practical cable designs. Recently a broad range of propylene co-polymers having improved ductility have become available, which may prove more suitable. The current study compares traditional isotactic and syndiotactic polypropylenes to a range of commercially available propylene co-polymers and focuses on their morphology, thermal, thermo-mechanical and electrical properties. These parameters were then taken together to identify the most suitable candidate materials for future cable applications. The use of blending as a means to further optimise the various material properties was also explored.

An investigation of the potential of polypropylene and its blends for use in recyclable high voltage cable insulation systems

The relation between crystallinity and the mean free path of high-energy electrons in low-density polyethylene and their effects on dielectric strength was studied. Thin LDPE films obtained by vacuum evaporation were annealed at temperatures between room temperature and 100 degrees C. Crystallinity was measured by X-ray diffraction. Electrons were injected into the film by photo-emission and the energy of electrons emitted into vacuum through the film was analyzed to obtain their energy distribution. The mean free path was then calculated from the energy distribution. It was found that the crystallinity increases and the mean free path becomes longer as the annealing temperature increases. Dielectric strength was measured by applying an impulse voltage of 1.2/50 mu s at room temperature and -50 degrees C. It was found that the breakdown field decreases with an increase in annealing temperature at both measurement temperatures. Thus, increases crystallinity makes electron transport easier and the mean-free path longer, resulting in a lower dielectric strength. >

Effects of crystallinity and electron mean-free-path on dielectric strength of low-density polyethylene

The mean free path of photoinjected hot electrons in LDPE was obtained experimentally, and based on this result the relation between crystallinity and dielectric strength is discussed. The change in X-ray diffraction spectra of 50- mu m-thick hot-pressed LDPE films annealed at various temperatures is shown. The crystallinity of the film is improved as the annealing temperature increases. The relation between the dielectric strength and the annealing temperature of the hot-pressed film is shown. The dielectric strength decreases as the annealing temperature increases, as shown in measurements at room temperature and -50 degrees C. The heat treatment improves the crystallinity and makes the mean free path longer, resulting in a lower dielectric strength. >

Effects of crystallinity and electron mean-free-path on dielectric strength of low density polyethylene

Space-charge-formation characteristics in polyethylene, polypropylene, and their blends are discussed based on the results of DC conduction, thermal pulse current, and thermally stimulated current Negative space charge is formed throughout the bulk of polypropylene, while positive space charge is formed in polyethylene In polymer blends of the two homopolymers, the amount of negative space charge increases with an increase in polypropylene content, while that of positive space charge increases with an increase in polyethylene content By blending the two homopolymers, DC conduction current becomes larger than that of homopolymers The dielectric strength at low temperature which is electronic in nature decreases by blending The dielectric strength at room temperature which is governed by a thermal process increases by blending the two homopolymers, because the spherulite boundaries of polypropylene are reinforced to improve the mechanical properties >

K. Katsunami

Papers

Effects of crystallinity and electron mean-free-path on dielectric strength of low-density polyethylene

Effects of crystallinity and electron mean-free-path on dielectric strength of low density polyethylene

Dielectric properties of polymer blend of polypropylene and polyethylene