A confocal Raman microprobe analysis of partial discharge activity in gaseous voids N A Freebody 1*, A SVaughan 1, G C Montanari 2 and L Wang 2 1 University of Southampton, UK 2 University of Bologna, Italy Raman Results Micrograph and SEM Results Introduction Samples and Method Conclusions Electrical ageing in polymeric insulators used in high voltage cables and transformers is often thought to originate in small gas filled voids within the bulk of the material. Understanding this ageing process is vital as it can lead to electrical treeing and, in some cases, complete electrical failure of the material as an insulator. This paper investigates the chemical processes in electrically aged voids via confocal Raman microprobe spectroscopy (CRMS), a technique well suited to this analysis due to its ability to probe sub surface artefacts in the sample and a resolution of approximately 2 μm in the optical plane. The surfaces of the voids were analysed using CRMS in conjunction with both air and oil immersion lenses and data were compared to and discussed in conjunction with SEM images obtained of the surfaces of the voids which were exposed via microtomy. N Freebody, University of Southampton, Highfield, Southampton, SO17 1BJ, UK Contact details :  CRMS is a useful tool in the determination of chemical variations beneath the surface of a sample and would be an effective tool in the analysis of ageing and breakdown in dielectric materials.  Depth profiles of voids show the spectral peaks of PE within the bulk of the sample but not within the void and the presence of a range of peaks between 200 and 600 cm -1.  Aged samples show traces of fluorescence (a phenomena previously linked with ageing) which increases in intensity at the surface of the void.  Oil immersion enables a more accurate Raman study of the void surface due to the reduction in refraction effects.  Non destructive imaging of the void surface can be achieved with the use of oil immersion however SEM imaging is still recommended when possible.  Micrographs and SEM imaging revealed the presence of debris and the possibility of micropiting.  Further study into the application of oil immersion is needed in order to determine the exact nature of the peaks within the Raman spectra. As well as this further tests are needed in order to determine the origin and composition of the debris found within the aged voids as seen with the SEM in order to see if it is caused by micropitting as the literature suggests.  Data were collected using a Lecia microscope coupled to a Renishaw Raman RM1000 system using a 785 nm CW diode laser of power 25 mW, set up in line with Renishaw’s recommendations for confocal operation.  Spectra were obtained using an extended scan and were built up of the accumulation of 25 scans of 10 s with a laser power of 25% to minimize sample damage.  Samples were probed at various intervals along the z-axis and at a number of points in the horizontal plane, for comparison, using both air and oil immersion lenses.  Samples were then cut open using an RMC MT-7 ultra microtome equipped with a CR-21 cryo-system set at -50 o C in order to expose the internal walls of the voids. The microtomed surfaces were sputter coated with gold and examined by scanning electron microscopy (SEM). Figure 1: Schematic showing structure and dimensions of layered void samples Figure 6: optical micrograph of aged (157h) void surface using an oil immersion objective Figure 7: SEM images of unaged void, a) exposed void, b) edge of void. Figure 8: SEM images of aged void (156h), a) exposed void, b) edge of void. Figure 2: Raman depth profiles (in air) of a) unaged void and b) aged (157h) non void Figure 5: Raman spectra of the aged (157h) void surface (in air) Figure 4: Raman depth profiles of aged (157h) void, a) in air, b) in oil  With the unaged void, the spectral peaks of PE can be seen up to 150 μm into the surface of the sample.  Once the surface of the void can be found approximately 100 μm into the surface of the sample.  Between 600 and 200 cm -1 a range of spectral peaks can be seen of varying intensity.  When an aged non void is profiled, the same spectral features as before can be seen throughout the sample.  When oil immersion lens and a suitable immersion oil (silicone oil) is used, it is possible to eliminate refraction effects at the sample surface.  Using oil immersion it is possible to image the top surface of the void in a non destructive way.  Evidence of micropitting on void surface, with pits of approximately 1-2 μm in diameter.  Although a destructive method for obtaining data, SEM provides valuable complimentary information.  SEM images of exposed voids showed the following:  No evidence of a boundary/change in morphology between the XLPE and LDPE.  With the non aged void the walls of the void are very smooth with very few small (< 1μm) features on the surface  When the sample is aged, the void appears to contain ‘debris’ and features 8-10 μm can be found protruding from the void surface  The size of the void appears unchanged regardless of ageing time leading to ambiguity to the nature and origin of the debris  Voided samples were made by hot pressing 2 sheets of cross linked polyethylene (XLPE) on either side of a layer of LDPE in which a microcavity was embedded and then aged under an AC field. Figure 3: Raman depth profile of aged (157h) void in air  In air, similar results to the non aged void can be found with the addition of fluorescence.  Spectra obtained using an oil immersion objective enables a more accurate depth profile due to a reduction in noise  Depth profiles in air and oil show:  PE within the sample,  increased fluorescence within the void.  a range of peaks (possibly due to oxygenated bi products) throughout the sample at lower wavenumbers.  Raman spectra of the void surface revealed large amounts of fluorescence and evidence of the presence of carbon. 10 μm