Electrical ageing of composites: an industrial ... - Schneider Electric

Schneider Electric 2007 - Conferences publicationshence gain much more kinetic energy. We do not needto assume the development of a large scale electronavalanche or even strong, gas phase ionisation because1 photon per second on average is only one per 50cyclesof power frequency voltage which is not very much…Clearly, if voids of less than 40µm existed, avalanchesof up to 10 6 electrons could exist, as described above. Anumber of these electrons could give rise to lightemitting mechanisms in the gas or at least when theybombard the internal surface of the void. Depending onvoid shape, on average they might be able to gain 20eVwith the tail of the statistical distribution containingeven more energetic electrons. ALL these electronsmust give up this kinetic energy on striking the innersurfaces of the void. Under these circumstances onemight expect a considerable amount of emission havinga spectrum corresponding to one, or both, of the twomechanisms mentioned above.Recent experiments with porous polypropylene samplesshows that partial discharges of this type are observable.Once again, at the end of the day we come to theconclusion that even these result do not really help inpinpointing the origin of electrical ageing. We need toAN ALTERNATIVE SCENARIOAt this stage, the convergence of a number of ideas andindications, led us to reconsider the validity of thestandard ageing scenario described above, for our highlyfilled samples. Clearly the extremely heterogeneousnature of the structure is not taken into account, Fig 10.It also neglects the well documented presence of largequantities of absorbed water, (0.1 – 5% weight).The results of dielectric spectroscopy on filled andunfilled samples which have absorbed an identicalamount of H 2 O relative to the resin part, show thevalidity of such concerns. These results point to a multiparametermaxwell-wagner type interfacial polarisationmechanism, Fig.8. Other data lead us to consider thepossibility of an inter-grain mechanism of carriertransport and interface accumulation, Fig10.One of the best known influences of water incomposites is plastification and the correspondingreduction of the glass transition and various physicalproperties. Working with both standard and nano-filledsamples we note a reduction of Tg linked directly to thequantity of water “in the resin part”. This reductionreaches 20°C for saturated samples, (corresponding to aTg reduction of about 30% for our samples, tg = 65°C),Fig.9.Furthermore, the principal mechanism considered to beresponsible for the mechanical failure of “joined”materials is the action of water at the interfaces, [5].Consequently, our latest results, which indicate thegrowth of water layers of about 2nm around each offiller grain, are of particular significance [6] [7].Accordingly we are now concentrating our efforts intesting an “Alternative Ageing Scenario”.This scenario is specific to highly filled materials and isbased on the progressive degradation of the filler/matrixinterfaces by water molecules.Fig.8: Dielectric parameters with absorbed H 2 Oknow exactly the physical mechanism behind thephoton production, before we can decide if it hasanything to do with electrical ageing. To confirm thesethoughts, we have initiated a series of experiments onphoton counting using artificial voids in the 10 – 100µmrange.Fig.9: Glass transition versus atmospheric H 2 O contentN9 =9% nano-filler, LD41=59% micro-fillerOur basic hypothesis is that a small fraction of the waterwhich diffuses into our composites, reaches thepolymer/filler interfaces where it accumulates. The405