Smart Materials as Intelligent Insulation A. F. Holt, A. C. Topley, R. C. D. Brown, P. L. Lewin, A. S. Vaughan, P. Lang * University of Southampton, Southampton, UK * EDF Energy Networks Ltd. Previous Approaches Introduction Considerations Conclusions Further Work What are smart dielectrics? Smart dielectrics are materials which contain a functional chemical group which produces a measurable response dependant on environmental changes. Electric field sensitive fluorophores can be designed and introduced into the dielectric such that changes in fluorescent properties are indicative of the local electrical environment or dielectric degradation. Such materials can be tailored to the desired application. Be nefits and potential applications Smart dielectrics provide a means of continual condition assessment which is non invasive, easily interpreted and inexpensive to maintain. One such application would be the use of smart dielectrics to remotely monitor the presence of charge on gas insulated switchgear (GIS) spacers. Other uses in outdoor insulation systems to detect the presence of an electric field or as a means of dielectric condition assessment can be envisaged University of Southampton, Highfield, Southampton, SO17 1BJ, UK It was found that a fluorophore such as pyrene could be introduced into bulk PS with only minimal effect on the AC electrical breakdown strength of the material. Clear fluorescence spectra were observed for all blends although the directly doped pyrene blends were visibly more fluorescent. Good dispersion of the fluorophore throughout the polymer is essential in order to obtain reliable experimental results, therefore, soluble fluorescent fillers are preferable due to better dispersion when blending..Table 1: Summary of breakdown data Solid-supported pyrene as a filler Pyrene was attached to small crosslinked polystyrene beads. These were mixed with polystyrene using solution blending techniques. Fluorescence spectra were obtained, as shown in figure 2. In the case of the 5.75 % and 1 % blends attenuators were required to prevent saturation of the spectrometer. Monitor the fluorescence spectra of the materials in real time whilst under electrical stress. Blending of a more polar fluorophore to determine what effect such a modification will have one the sensing and electrical/ dielectric properties of the material. Comparison of electroluminescent and fluorescent properties of blended materials will offer an insight into the most suitable systems for remote monitoring of dielectric materials. Explore the use of fluorescence modified dendritic molecules which are soluble in blending solvents to produce a uniform dispersion of fluorophores amongst the bulk material and protect the active core. Requirements of material Responsive to local electrical environment changes Clear output which can be readily interpreted (e.g. colour change/ fluorescence). Introduction of smart moiety must have minimal effect on electrical properties of bulk material Smart moiety functional group would ideally be cheap to synthesize and easy to introduce into the bulk polymer. Fig. 1. fluorescent insoluble filler Sample Beta Value Eta Value (kV/mm) Virgin PS % Control % Filler % Filler Contact Details Fig. 2. Fluorescence spectra for 5.75 %, 1 % and 0 % filler blends Fig. 3. A cluster of filler beads as observed in the 1% filler blend viewed in reflection mode. During sample manufacture it was noted that in the 5.75 % blend the filler was visually well dispersed. Conversely, filler dispersion was poor in the 1 % blend as shown by a microscope photograph in figure 3, which shows an example of the localized clusters of filler beads observed throughout the sample. Breakdown Testing For AC electrical breakdown testing, a Grazeby Specac press was used to press air and defect free samples at C to a thickness of 100 µm.. The test cell used two 6.3 mm ball bearing electrodes submerged in silicone oil. Calibration ensured that each sample was subject to the same linear AC ramp rate of 50 V/s. A two-parameter Weibull distribution with maximum likelihood confidence bounds was used to analyze the data. Current Approaches Directly-doped pyrene blends Pyrene was dissolved into a solution of polystyrene, owing to the good solubility of pyrene; precipitation of the pyrene-polystyrene mixture was not possible. Samples were instead oven dried for 3 hours at 40 o C Serial dilutions of a stock pyrene solution allowed for concentrations of pyrene as low as %wt. All samples showed strong fluorescence under a UV lamp (254 nm). Fig. 3. Chemcial structure of pyrene Fig. 4. Comparison between polystyrene doped with pyrene (5 %wt.) and unmodified polystyrene (right) Fig %wt. pyrene blend shows weaker fluorescence of a more purple colour compared to the 5 %wt. blend (right) Pyrene concentration BetaEta 0% % % %16162 Table 2. Summary of breakdown data AC Breakdown results Surprising lack of clear trends between the different concentrations Having studied the breakdown sites under a microscope there was no visible reason (such as areas of crystalline pyrene) as to why a lower average breakdown strength was observed for the 0.05 wt % blend compared to the other pyrene blends. Dendrimers Polymeric macromolecules composed of multiple perfectly-branched monomers radially emanating from a central core. Could be used to encapsulate the fluorescent core, offering protection from bulk polymer environment. Head groups of dendrimer could be modified to mimic surrounding polymer, allowing for easier blending. Fig. 6. Schematic representation of a dendrimer