Until 2006 the performance of confocal Raman spectroscopy depth profiling was typically described and modeled through the application of geometrical optics, including refraction at the surface, to explain the degree of resolution and the precise form of the depth profile obtained from transparent and semicrystalline materials. Consequently a range of techniques, physical and analytical, was suggested to avoid the errors thus encountered in order to improve the practice of Raman spectroscopy, if not the understanding of the underlying mechanisms. These approaches were completely unsuccessful in accounting for the precise form of the depth profile, the fact that spectra obtained from laminated samples always contain characteristic peaks from all materials present both well above and below the focal point and that spectra can be obtained when focused some 40 mum above the sample surface. This paper provides further evidence that the physical processes underlying Raman spectroscopy are better modeled and explained through the concept of an extended illuminated volume contributing to the final Raman spectrum and modeled through a photon scattering approach rather than a point focus ray optics approach. The power of this numerical model lies in its ability to incorporate, simultaneously, the effects of degree of refraction at the surface (whether using a dry or oil objective lens), the degree of attenuation due to scatter by the bulk of the material, the Raman scattering efficiency of the material, and surface roughness effects. Through this we are now able to explain why even removing surface aberration and refraction effects through the use of oil immersion objective lenses cannot reliably ensure that the material sampled is only that at or close to the point of focus of the laser. Furthermore we show that the precise form of the depth profile is affected by the degree of flatness of the surface of the sample. Perhaps surprisingly, we show that the degree of flatness of the material surface is, in fact, more important than obtaining a precise refractive index match between the immersion oil and the material when seeking a high-quality depth profile or Raman spectrum from within a transparent or semicrystalline material, contrary to accepted norms that samples for interrogation by Raman spectroscopy require little preparation.

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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.

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There is a need to develop materials with controlled electrical resistivity, reduced space charge accumulation, higher thermal conductivity, higher dielectric strength and enhanced voltage endurance to cope with DC stresses in High Voltage Direct Current (HVDC) transmission systems in addition to HVAC requirements. If the balance of properties, performance and process requirements are achieved this may lead to HVDC insulation systems and equipment having a reduced footprint, larger power densities, and greater multi-stress resilience with longer service lifetimes. It reports findings of a project that is engaging this challenge and investigates the development and scaling of new thermoset based nanocomposite electrical insulation materials for HVDC power transmission applications. Some of the results such as increased electrical breakdown strength and reduced electrical conductivity for reactively surface functionalised nanosilica, and increased thermal conductivity for nano boron nitride and their significance in regard to the wider application of these electrical insulation materials are also discussed. With sufficient understanding of these properties, their trade-offs and process requirements it is possible to tailor balanced materials for specific use in HVAC or HVDC components.