Effects Of Magnetic Fields On Surface Breakdown Of Gamma-Ray Irradiated Polymer Dielectrics

Polymers are widely used as insulating materials because of their excellent dielectric properties. The selection of the proper polymer dielectric for a desired application depends on the operating conditions. The polymer insulations used in space power and nuclear power station are exposed to kinds of radiations which can change their electrical performance. Polymers inevitably operate under radiation with combined environments such as magnetic field. Therefore, it is necessary to investigate the dielectric properties of the polymers under these conditions.Polybutylene naphthalate (PBN) and polybutylene terphthalate (PBT) are employed as experimental samples. The samples were radiated in air up to 100 kGy and then up to 1000 kGy by using a 60Co gamma source. The magnetic flux density (MFD) was 249 mT. The electric field was perpendicular to the magnetic field, which made relative angles between the sample surface and the direction of E×B as 0, 90, 180, 270 and 360 degree respectively.This paper investigates effects of magnetic fields on surface breakdown of gamma-ray radiated polymer dielectrics. Obtained results indicated that the time to tracking failure of both PBN and PBT showed a minimum value and the amount of cumulative charges showed a peak value when the relative angle was 180 degree. The time to tracking failure of both PBN and PBT delayed and the amount of cumulative charges decreased when relative angles were 0/360 and 90/270 degree. The carbonized area was smaller at 180 degree than that at 0/360 degree for PBN whereas the tendency was opposite for PBT. With the increasing of the total dose of the radiation, the time to tracking failure increased for PBN but decreased for PBT; the amount of cumulative charges decreased for PBN but increased for PBT; the carbonized area decreased for PBN but increased for PBT. Comparing the molecular formulas of PBN and PBT, it is suggested that the amount of phenyl in the main chain plays a key role in determining the types of the radiation reactions in which one is crosslinking reaction and the other is degradation reaction.