Showing papers by "Alun S. Vaughan published in 2013"

On the dielectric response of silica-based polyethylene nanocomposites

Abstract: The dielectric response of silica-based polyethylene nanocomposites is studied by dielectric spectroscopy. The results indicate that nanocomposites absorb significantly more water than unfilled polyethylene, with the consequence that both permittivity and loss tangent increase with increasing duration of water immersion. However, appropriate surface treatment of nanosilica is found to reduce the water absorption effect and to modify the dielectric response of the nanocomposites compared with those containing untreated nanosilica. While water absorption may not be a technologically desirable characteristic, our results indicate that water molecules can act as effective dielectric probes of interfacial factors.

Recyclable power cable comprising a blend of slow-crystallized polyethylenes

Abstract: Crosslinked polyethylene (XLPE) has a successful history as a cable insulation material. Nevertheless, in recent years, as environmental awareness has grown, concerns about the ease with which it can be recycled have emerged. Although technologies have been developed for XLPE recycling, this report concentrates instead on the development of a thermoplastic alternative. Specifically, a 20 : 80 blend of high density and low density polyethylene (HDPE : LDPE) was selected and subjected to a non-isothermal crystallization procedure. It was found that, provided the cooling rate falls between 0.5 and 10 K min-1, the blend exhibits superior breakdown strengths and high temperature mechanical stiffness compared to XLPE. A trial cable was then extruded from this blend using such a cooling rate. The breakdown behavior of the morphologically-designed cable was finally compared with that of LDPE and XLPE reference systems.

Absorption Current Behaviour of Polyethylene/Silica Nanocomposites

Abstract: Absorption current is an important characteristic of polymers with regard to their time-domain response to a direct current (DC) poling field. This is because the results of absorption current measurements can be used to gain understanding of the relationship between space charge accumulation and movement. In semicrystalline polyethylene, for example, charge accumulation is likely to be influenced by the presence of charge trapping sites, which are associated with interfaces between the crystalline and amorphous phases. With the addition of a nanofiller, the charge transport mechanism will become more complicated than in the unfilled polymer, as the inclusion of the nanofiller will introduce nanofiller/polymer interfaces. The presence of such interfaces will affect the current flow due to the introduction or modification of the distribution of trapping sites within the system. In this paper, we report on an investigation into the absorption current behaviour of polyethylene nanocomposites containing 0 wt%, 2 wt%, 5 wt% and 10 wt% of silica nanofiller, either untreated or treated using trimethoxy(propyl)silane coupling agent. Our results indicate that the absorption current behaviour of the polyethylene was affected by the presence of the nanosilica. While the current behaviour through the unfilled polymer decreases with time in a conventional manner, all nanocomposites reveal an initial decrease followed by a period in which the current increases with increasing time of DC field application.

Space charge dynamics in silica-based polyethylene nanocomposites

Abstract: Space charge formation is a well known phenomenon that affects the overall dielectric properties of insulation systems. Space charge occurs when the rate of charge accumulation is different from the rate of removal and involves moving and trapped charges, which modify the electric field distribution within the material. As a result of this, the internal field within the dielectric is locally increased, which then leads to faster degradation and premature failure. Several techniques have been used to probe the space charge behavior of novel dielectric materials, especially in connection with the emerging topic of polymer nanocomposites - material systems that exhibit unique insulation characteristics due to the presence of nanometer-sized inclusions. In this paper, we report on an investigation into space charge dynamics in silica-based polyethylene nanocomposites, where the nanofiller has been modified with respect to its surface chemistry. For this, the pulsed electro-acoustic technique has been used. Experimental observations indicate that the incorporation of nanosilica into polyethylene results in homocharge development near both electrodes. However, with appropriate surface treatment of the filler, homocharges formation was successfully suppressed. Possible relationships between the space charge development and dielectric breakdown properties of the nanocomposites are discussed.

The breakdown strength and localised structure of polystyrene as a function of nanosilica fill-fraction

Abstract: In this work the amorphous matrix of polystyrene provides a homogenous basis into which nanosilica particles are added. Composites are made with four different types of nanosilica particles which are subsequently compared. The DC breakdown strength of the resulting nanocomposite materials is measured as a function of filler fraction with loadings between 0 and 10 %. One advantage of using a polystyrene matrix for this study is its compatibility with permanganic etching. This technique is used to remove part of the polystyrene matrix and render the configuration of the nanofiller particles within the composite amenable to examination by scanning electron microscope (SEM). The simple sample preparation protocol employed here resulted in significant nanofiller agglomeration and the DC breakdown strength was found to decrease with increasing filler fraction.

Modeling of the permittivity of epoxy nanocomposites

Abstract: Polymer nanocomposites exhibit a number of desirable dielectric properties, like increased resistance to partial discharge, electrical treeing, dc breakdown strength, or a decreased intake of space charges. The permittivity is one of the properties at which a direct impact from the introduction of nanoparticles can be observed. Many applications would benefit from the ability to tailor the permittivity of their insulation. Both a reduction, for e.g. cable systems and GIS spacers, and an increase, for e.g. capacitors, would be desirable if they can be controlled and exploited accordingly. Hence there is an interest in predicting how a filler material would alter the permittivity. There are two main groups of theoretical approaches to the problem of composite permittivities. Effective medium theories, which utilize average fields or polarizabilities and induced dipole moments, and integral methods, which use low concentration formulae and integrate them to higher concentration. When modeling the permittivity of nanocomposites both methods struggle to deliver satisfying results. This exploratory work tries to make estimations about the bulk permittivity by taking into account the host-polymer interface, to which many of the astounding properties of polymer nanocomposites are ascribed. As a starting point for the modeling, nanocomposites are considered which are based on bisphenol-A type epoxy resin with aluminum oxide and magnesium oxide nanofiller. The selected filler materials are spherical or quasi-spherical, to minimize the influence of additional parameters like the aspect ratio.

Comparison of laser ablation and inclined plane tracking tests as a means to rank materials for outdoor HV insulators

Abstract: As the popularity of polymeric composites for outdoor high voltage (HV) use continues to increase, surface degradation through electrical discharge activity is becoming a more serious issue. In order to design composites offering improved resistance to degradation, and hence improved component lifetimes, tests need to be performed under controlled conditions in order to rank different materials. Inclined plane tracking tests are often used to determine the relative performance of various material formulations used in their construction. An alternative approach is to employ a high power laser to deposit known amounts of energy at a known rate to the sample surface. In this paper a variety of material formulations were subjected to inclined plane tracking tests as well as to laser ablation testing. The results indicate that the two test methods compare favorably and that laser ablation offers a quick and cost effective way of screening candidate material formulations, prior to more detailed study.

On nanosilica surface functionalization using different aliphatic chain length silane coupling agents

Abstract: The use of polymer nanocomposites in electrical insulation is claimed to be capable of enhancing the dielectric performance of insulation systems. This is believed to be related to the much smaller size of the filler particles, which leads to the presence of extensive interfacial areas. In this regard, nanocomposites are expected to possess unique dielectric properties that reflect the resulting interphase regions. Since the surface state of the nanofiller is closely related to the interphase regions, surface functionalization of the nanofiller has been shown by many workers to be an important factor in determining the nanocomposite's interphase structure and, hence, the macroscopic physical properties of the system. While many comparisons of nanocomposites containing functionalized and unfunctionalized fillers have been reported, few systematic studies of this area have been undertaken with a view to understanding the mechanisms underpinning the concept of filler functionalization in nanodielectrics. In this paper, we begin to address this by using a series of functionalizing agents, which differ with respect to their aliphatic chain length; all systems were based upon trimethoxysilane and, in all cases, the nature of the thermodynamic interactions with the polyethylene matrix should be comparable. This paper reports on the effect of aliphatic chain length on the structure, breakdown and space charge behavior in the resulting nanocomposites. The possible mechanisms leading to the observed property changes are discussed.

Dielectric properties of epoxy/BN micro- and meso-composites: Dielectric response at low and high electrical field strengths

Abstract: By incorporating boron nitride (BN) in epoxy resin, we aimed to affect the morphology of the underlying composites. Two different filler sizes, one micrometric with an average grain size of 9 μm and a sub-micrometric one with 0.5 μm, have been used to form composites. The amount of each type of BN in the matrix has been varied from 1 to 5 wt%. Dielectric response measurements at ambient temperature revealed slightly lower real permittivity values of BN composites over neat epoxy and also an increase in imaginary permittivity. In addition, the BN composites had improved thermal stability of properties as compared to neat epoxy, due to lower values in real permittivity as well as in dielectric loss, for temperatures above glass transition temperature. Further dielectric spectroscopy showed significantly decreased values in both real and imaginary permittivity of BN composites, when subjected to high field strengths and very high temperatures compared to measurements at low field strengths. Differential Scanning Calorimetry (DSC) measurements have shown that reticulation was likely to be hindered due to numerous agglomerations in the matrix for the case involving the sub-micrometric BN composite with 5 wt% filler content.

A dielectric spectroscopy study of the polystyrene/nanosilica model system

Abstract: In this study a simple solvent blending technique is used to produce silica/polystyrene nanocomposites. Dielectric spectroscopy is then used to measure both the real and imaginary permittivity of the samples. The nanosilica/polystyrene system is characterized over a range of different filler loadings, and additionally, as a function of temperature. To supplement this, absorbed water is used as a dielectric probe to explore molecular relaxation processes at the nanoparticle interfaces.

Permittivity mismatch and its influence on ramp breakdown performance

Abstract: Ramp breakdown tests remain a useful method for characterizing polymeric materials. In the current paper the effects of sample and oil permittivity and electrode radius on measured DC breakdown strength are explored, with the aid of finite element modeling. Localised field enhancement appears to explain apparent decreases in electrical breakdown strength whereas the shape parameter in the Weibull distribution appears to be related to the size of electrically stressed area relative to the electrode radius. Care needs to be taken to choose an oil of equal or higher permittivity than the sample to be tested and to use a fixed electrode geometry and a fixed voltage ramp rate to ensure such tests are valid for comparative studies.

Conduction currents and time to frequency domain transformation for epoxy resin nanocomposites

Abstract: This paper concerns several epoxy resin nanocomposites. A DER 332 epoxy resin was chosen as matrix and nanosilica and/or Boron Nitride were chosen as fillers. Conduction currents results obtained using Polarization and Depolarization Current (PDC) tests at room temperature are presented and discussed. Different conduction phenomena were observed following the analysis of the variation of the current density versus the applied electric field. Using the depolarization currents obtained at room temperature under several applied electric fields, time to frequency domain transformation was performed. For this purpose, the currents were fitted using a general time response function based on Curie-von Schweidler law and on the transform proposed by Hamon. The empiric law proposed by Helegeson was also investigated. The time to frequency domain transformed spectra were compared with those obtained by Dielectric Spectroscopy.

Surface resistance of epoxy/BN micro- and Meso-composites exposed to electrical discharges

Abstract: Boron nitride (BN) has shown in the past to improve DC breakdown strength as well as augmenting thermal conductivity. Two different filler sizes, one micrometric with an average particle size of 9 μm and a sub-micrometric one, 0.5 μm in size, have been used to form composites. The amount of each type of BN in the matrix has been varied from 1 to 5 wt%. The samples have been exposed to electrical discharges using a point-to-plane geometry operated in open air. The experiment was conducted at an enhanced frequency of 300 Hz to accelerate the erosion process. The surface erosion has been evaluated after several distinct time steps using a mechanical profilometer. It has been found that introducing BN into epoxy decreases the amount of eroded volume of the organic matrix with decreasing filler amount. In addition, micrometric BN composites performed better than their sub-micrometric counterparts. The dielectric response has been evaluated using broadband dielectric spectroscopy (BDS) at 20°C before the erosion, as well as after each time step. An increase of dielectric losses at very low frequencies for all samples after erosion has been observed, accompanied by a slight increase in real permittivity. Besides the β-peak, the formation of an additional loss peak, located around 1 Hz, has been found.

Developing an experimental method for a cavity PD based life model

Abstract: Despite the large amount of activities undertaken in both the area of partial discharge (PD) in cavities and polymeric insulation degradation, degradation of polymeric insulation induced by cavity PD remains a subject where there are more questions than answers. In this paper, an experimental approach is discussed which has the aim of allowing observation of degradation induced by cavity PD. A model based on electron scattering mechanisms in both the cavity gas gap and polymeric insulation is proposed. From observing electron avalanche behavior and its damage mechanisms to the insulation, it may be possible to develop approaches that improve our ability to estimate remaining life of polymeric insulation systems in high voltage plant.

Process for producing polypropylene blends for thermoplastic insulation

Abstract: Polymer blends of polypropylene homopolymer and propylene-α-olefin interpolymer. Processes for producing polymeric compositions comprising control-cooling heated blends of polypropylene and propylene-α-olefin interpolymer. Such polymeric compositions can be employed in forming coated wires and cables.

Nano at nineteen

Abstract: Although the exploitation of nanostructured materials can be traced back hundreds of years, many authors cite John Lewis' 1994 paper, “Nanometric Dielectrics” [1], as a convenient start of the current interest in nanodielectrics. As such, nanodielectrics are 19 this year and, like many teenagers, they are not easy to deal with. This paper begins with a brief overview of how the subject of nanodielectrics has developed in the subject's formative years and then considers the implications and challenges that are emerging. To illustrate this, two examples have been chosen from recent work at Southampton that specifically relate to interfaces and interphases. First, chemical interactions between a nanofiller and an epoxy matrix are explored by varying the number of functional groups attached to the nanosilica surface. Second, the effect of nanosilica on structure and breakdown in polyethylene-based systems is described, in which the length of the aliphatic chains attached to the nanofiller surface was adjusted. In both cases, pronounced and systematic variations in properties are revealed.

Applications of liquid crystals in intelligent insulation

Abstract: The integrity of electrical plant becomes increasingly suspect over time due to various ageing processes. Numerous devices and spectroscopic techniques are available to analyze plant condition and recently, research has been carried out into the design of materials which can perform some form of self-diagnosis. These mainly include smart materials which can respond visibly, predictably and in a reproducible manner to stimuli such as heat, mechanical stress and electrical fields. A multi-layer device has been devised for detection and reporting of electric fields, a key component of which is a layer of electric field responsive material. This paper describes the investigation carried out to determine the suitability of liquid crystals as an electric field responsive medium for passive electric field detection.

A new process for preparing insulation materials for high voltage power applications and new insulation materials

Abstract: The present invention relates to a process for preparing insulation materials for high voltage power applications and new insulation materials. The materials comprise a mixture of at least one C2- 8olefin polymer and an a-nucleating agent and/or β-nucleating agent. The process comprises the steps a) mixing and melting ingredients at a temperature between Tm and Tm plus 150°C, whereby Tm is the melting temperature of the polymer; b) cooling and pelletizing the mixture; c) feeding the pelletized mixture in an extruder at a feeding speed between 100 and 800 kg/h, whereby the extruder barrel temperature is between Tm minus 50 and Tm plus 150°C; d) cooling the extruded materials in a single step between room temperature and below Tm; and e) optionally collecting the extruded materials at a collecting speed between 10 and 40 m/min. The invention also relates to new HV power applications comprising the new insulation materials.

Nanocomposite thermosetting materials for HVDC applications

Abstract: This paper reviews the objectives of a new project that is engaging the challenge to develop and scale the processing of nanocomposite thermosetting electrical insulation materials for HVDC power transmission applications. We discuss some of the work being carried out by the project and potential deployment in HVDC schemes.

Thermal and dielectric properties of clay/epoxy nanocomposites with low percentage of graphite oxide

Abstract: This work is concerned with hybrid nanocomposite materials that contain both clay and graphite oxide incorporated in an epoxy matrix. In order to assess the challenge of improving the thermal conductivity while not changing or even improving the electrical properties, a new composite system was designed by applying an additional thermal conductive phase represented by the graphite oxide. In this paper, extremely low quantities of graphite oxide were used (going from 0.0012 to 0.0025 wt%) in order to avoid the forming of an electrical conduction percolation network. The impact of the additional filler on the thermal and electrical properties of the clay/graphite oxide/epoxy nanocomposites were investigated. Using thermal conductivity measurements, it was found that even for low quantities of GO filler added to the clay/epoxy nanocomposite material, the thermal conductivity is improved significantly. Moreover, using dielectric characterization techniques (Dielectric Spectroscopy, Space Charge or Dielectric Breakdown measurements), it was found that the electrical properties of the material remain unchanged or are slightly improved by the extra graphite oxide filler.

Polypropylene blends and processes for producing polymeric compositions

Abstract: Polymer blends of polypropylene homopolymer and propylene-α-olefin interpolymer. Processes for producing polymeric compositions comprising control-cooling heated blends of polypropylene and propylene-α-olefin interpolymer. Such polymeric compositions can be employed in forming coated wires and cables.

Modelling PD in cavities and PD-based degradation mechanisms

Abstract: Micro cavities are considered to be unavoidable during manufacturing processes of polymeric insulation materials. Partial discharge initiated by micro cavities can induce various levels of damage and degradation, sometimes leading to global breakdown. Thus, developing an understanding of PD activities in such cavities and damage caused is essential. This project commenced in May 2012 and contains experimental validation and development of simulation models. The focus is on PD activities in micro cavities, damage and degradation resulted, and final breakdown mechanisms. Experimental work aims to observe degradation process by stressing five identical samples simultaneously until one fails, so that the different levels of degradation of the other samples that have yet to suffer catastrophic breakdown can be studied. Different insulation materials will be involved, such as epoxy resin, LDPE, and XLPE. Moreover, three types of methods are used to create cavities inside the samples, including the traditional sandwich structure, syringe injection, and use of a foaming agent. Predicted experimental results are the initiation and growth conditions of degradation and final breakdown mechanisms. Among all mechanisms, thermal ageing and breakdown, pitting, and treeing are the major interests of this work. The experimental results will be simulated, based on some existing models and theories, the major ones are Niemeyer’s PD model, and its Matlab version by Illias that uses COMSOL for field simulation [1]; Sanche’s hot electron theory [2], and its Matlab version by Testa to analyse energy and speed spectrums of PD avalanches and the resultant damage caused [3]. Please note that throughout the experiments, PD data will be recorded to study possible relationships between PD pattern and degradation status, as well as to prove that the experimental method is valid against multiple sample data superposition and interaction. To conclude, this project aims to provide more complete knowledge for PD and related degradation process, by distinguishing the major damage type, identifying the conditions for it to initiate and grow in different insulating materials, and providing simulation models as a conclusion of experimental results and theories