Tracking failure of gamma-ray irradiated polybutylene polymers under magnetic field
Introduction
Polymer insulating materials are widely used in a large variety of applications in electrical and electronic devices. Once electric field applied between electrodes reaches the breakdown strength of gas, discharge can be introduced onto polymer surface [1]. As sufficiently intensive discharge lasts for a considerable time, the decomposed carbon products, with some parts of the channel carbonized, are progressive and rapidly deposit on the surface [2]. Once the carbonized products bridge the electrodes, dielectric surface breakdown that is termed tracking failure, occurs with a sudden decrease of resistance between the electrodes and a permanent loss of the insulation property [3]. The tracking failure has been found an important reason for polymer degradation which possibly results in severe damage to the devices [4]. Accordingly, from the viewpoint of safety, it is necessary to gain a good understanding of the behavior of tracking failure. A number of papers with respect to the behavior have been published, and the importance of such information for preventing fires, short-circuit and insulation failure has been proposed [5].
Recently, polymers installed in electrical and electronic devices are required to be used in nuclear reactors, radiation facilities and space equipments [6], [7], [8], where the materials are exposed to high energetic radioactive rays, such as gamma-ray, electrons and protons [9]. For instance, outside and inside secondary shield in the containment vessel of 40-year old nuclear power plants, the maximum radiation dose rates are 0.01 Gy/h and 1 Gy/h. The dose rate in nuclear power plants varies widely from 10 μGy/h to 10 kGy/h with a potential of total exposure of 1000 kGy [10]. The molecular structures of the polymers can be altered as a consequence of the irradiation through mechanisms like chain scission, oxidation and cross-linking [11], which results in the variation of their insulation properties. It was reported that when polyethylene (PE) was exposed to 4 MeV electron beam for a total dose up to 2 MGy, its bulk breakdown strength was approximately twice comparing with PE irradiated under the same condition for a total dose of 1.5 MGy. In either case, the breakdown strength was higher than that of un-irradiated PE [12]. On the contrary, the breakdown strength of polystyrene (PS) was observed decreasing from 63 kV/mm to 51 kV/mm as it subjected to radiation with total dose of 36 MGy [13]. It is proposed that the dielectric bulk strength is improved by irradiation for cross-linking type materials, but the conclusion for degradation type materials is opposite [14].
Although radiation effect on bulk breakdown of polymers has been concerned, previous studies fell short of taking proper account of such effect on tracking failure. Du et al. investigated the resistance to tracking of gamma-irradiated polyethylene and modified polycarbonate under dc voltage, the results revealed that the tracking behavior was dependent upon the radiation dose and was varied as a function of polymer types [15]. Yoshimura et al. has proposed that, for the safe use of polymers in radioactive environment, further studies should be carried out so as to clarify the radiation effect on the tracking failure [5]. Furthermore, it must be brought in mind that, rather than solely under electric field, polymers in practical operation are always subjected to combined electric and magnetic fields. The magnetic field can be introduced by either the application of permanent magnets [16] or the excitation of current flow in electric circuit [17]. The flux density of magnetic field around power equipments varies with an extensively large range from the order of micro Tesla to tens of Tesla [16], [17]. It has been observed that a magnetic field with flux density of 180 mT can significantly change surface discharge characteristics on polymer [18], which is likely to alter the behavior of tracking failure hence influences the reliability of the material. Therefore, designing electrical insulation using polymers, one should take into consideration the effect of magnetic field on tracking behavior as well.
Polybutylene polymers are widely used as insulating materials due to their high breakdown strength, high resistivity and low dielectric loss. PBN is widely used in aerospace electronics, electrical engineering, and medical equipment, while PBT is usually used for electric coupler, blind plug, switch and insulation covering [19]. The present work is carried out with the purpose of understanding the effect of gamma-ray irradiation as well as magnetic field on tracking failure of PBN and PBT. Obtained results show that the behavior of tracking is dependent upon the radiation dose and is varied as a function of .
Section snippets
Test sample and electrode arrangement
PBN and PBT were produced with butanediol by reaction with 2, 6-naphthalic acid and dimethyl terephthalate, respectively. The structural formulas are shown in Fig. 1. The thickness of the sample was 3 mm and with dimension of 20 mm × 20 mm. Prior to the test, both PBN and PBT samples were irradiated in air up to 100 kGy and then up to 1000 kGy with dose rate of 10 kGy/h by using a 60Co gamma source. The sample surface was cleaned with ethyl alcohol and dried in a desiccator at room temperature
Effect of gamma-ray irradiation
During the test, it was observed that surface discharge occurred immediately as the pulse voltage was applied between the electrodes. Subsequently, surface area close to needle electrode was firstly carbonized, which appeared to propagate towards the semicircle-plate electrode. After a period of time, the carbonized path finally bridged the electrodes and the dielectric surface was breakdown. Fig. 4 shows examples of PBN and PBT surfaces after the tracking failure. As can be clearly observed,
Conclusions
Tracking behaviors of gamma-ray irradiated PBN and PBT under magnetic field have been studied by using a negative pulse voltage. The main conclusions can be summarized as follows,
- (1)
With the increase of the radiation dose, the dielectric surface strength is improved for PBN but is worsened for PBT. The tracking behavior is dependent upon the molecular structure of the material that is varied by the radiation induced cross-linking and degradation reactions.
- (2)
With the increase of the relative angle,
Acknowledgements
This work is supported by National Nature Science Foundation of China (NSFC 50777048).
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