Silicone rubber (SiR) is widely used in various electric equipment as insulating material. However, electrical tree initiation mechanisms in SiR are not clear. In our previous study, SiR specimens dewatered by vacuum heating treatment(10 Pa at 160°C for 8 hours, 2 times) show the higher tree initiation voltage than that of untreated specimens. However, SiR specimens impregnated with distilled water show even higher tree initiation voltage than that of dewatered specimens. These results can be explained in terms of the water states as follows. We consider that H2O gas molecule in SiR forms hydrogen bond with oxygen molecule of the Si-O bond of SiR. This hydrogen bond may weaken the Si-O bond resulting in easier break by the electron bombardment due to voltage application. On the other hand, in SiR specimens impregnated with H2O liquid, H2O liquid molecules exist in the free volume of SiR, and remove acceleration space for electrons. These H2O liquid molecules may trap H2O gas molecules by cohesion. It is also possible that the electric field is relaxed due to high permittivity of liquid water. In addition, we did a simulation to exam the effect of H2O gas molecule on the Si-O bond, where electric charge distribution and bond distance was calculated by “GAMESS”.
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