摘要:

Antioxidants which are 2, 6 di-tert-butyl phenol derivatives exhibit dielectric relaxations at liquid helium temperatures when dispersed in various nondipolar media including polyethylene and polypropylene. Other antioxidants should preferably be used for the insulation tape of AC superconducting power cables. By an appropriate isotopic substitution the effect is shown to be due to tunnelling of the hydroxy hydrogen atoms. These findings prompted further investigations of hydrocarbon media containing phenols and other dipolar additives, from which it appears that the low-temperature dielectric relaxation behaviour can often be understood in terms of the structure and parameters of the additive molecules as revealed by published infrared and microwave spectroscopic data. The low-temperature relaxations of oxidised polyethylene are also summarily reviewed.

DOI:10.1049/ip-a-1.1981.0024

出版商:IEE    

年代:1981

数据来源: IET

摘要:

The time dependence of the charging of insulators on contact with metal electrodes has been examined by determination of the charge after both multiple repeated contacts and time-varying, single contact as well as measuring the influence of applied fields in the charge transfer process. The practical aspects of these investigations have been exhibited by showing that microphony in polymer insulated structures is well described by stored charge effects and further, that injected charges may contribute significantly to the so-called piezo-electric effect in polymers. All the experimental data are described by a logarithmic dependence of the charge density σ on time. A simplified analysis indicates that the field (E) dependence of the rate of charge increase is described by an expression of the form σ =KE1/2Int. It is concluded that the charge transfer process is consistent with a self-field limited injection mechanism but greater refinement is necessary in both experiment and theory to permit its unequivocal specification.

DOI:10.1049/ip-a-1.1981.0025

出版商:IEE    

年代:1981

数据来源: IET

摘要:

Magnesium oxide is widely used as a high-temperature insulant in the form of compacted powder. The reasons for its selection, and the nature of electrical conduction in refractory oxides at high temperatures, are discussed. The literature dealing with conduction in single crystals is reviewed, and results of resistivity measurements on compressed powders are reported. These show the effects on powder resistivity of impurities, specific surface, and of adsorbed or bound water. At 1000°C the resistivity of commercially available fused magnesia powders approaches that of high-purity single crystals.

DOI:10.1049/ip-a-1.1981.0026

出版商:IEE    

年代:1981

数据来源: IET

摘要:

In order to improve our knowledge of the impregnated materials, we have focused our work on polypropylene and capacitor grade papers. In addition to electrical measurements (permittivity, conductivity versus voltage, time, temperature), we made a study of the porous structure of the materials using electron microscopy, the bubble breakthrough method, permeation of gas and liquid under a hydrostatic pressure and mercury porosimetry. The adsorption of electrolytes, the diffusion of ions without voltage through polymer films, and the electrophoresis of microparticles of polymers were also studied. We conclude that paper and swollen polypropylene are porous materials; but in paper the pores are very tortuous and the liquid mainly spreads over the surface or into voids parallel to the surface; in polypropylene the pores seem to be numerous and slightly tortuous. Thus, impregnated paper mainly behaves like a series association of pure cellulose and free liquid, whereas polypropylene has a large parallel component. Another striking difference is that paper is a semipermeable material, able to block ions at its surface (according to the nature of the liquid and of the ions): this physico-chemical interpretation is preferable to the mechanical model proposed by Garton. Polypropylene is not (or but very feebly) selective, and ions of both signs can cross it, but their apparent DC mobility is 10−4to 10−5smaller than in the free liquid: so with AC, ions of both signs are trapped producing the ‘field-enhanced adsorption’ effect described by Tobazeon and Gartner.

DOI:10.1049/ip-a-1.1981.0027

出版商:IEE    

年代:1981

数据来源: IET

摘要:

The theoretical model for electron-beam-induced conductivity in thin insulating films is reviewed and its application to gain measurements in several different insulating materials is discussed. The ideas incorporated in the model suggest a technique for investigating trapping levels in insulators, using the electron beam as a means of populating the traps. It is suggested that, in addition to determining the trap energy, it may be possible to investigate the spatial distribution of the traps. Electron-beam-radiation effects in SiO2are briefly commented upon and the dependence of ΔVFB, the shift in flat-band voltage of MOS capacitors, on beam energy is discussed in terms of the gain model. Finally consideration is given to the problem of observing voltage contrast from the passivated devices in a scanning electron microscope.

DOI:10.1049/ip-a-1.1981.0028

出版商:IEE    

年代:1981

数据来源: IET

摘要:

The paper describes extensive experimental investigations concerning AC loss and DC conduction in polyethylene (PE) subjected to strong electric fields up to about l00kV/mm. Based on an electronic charge carrier hopping transport model including trap-like and donor-like states it is shown that the current is bulk-limited under both AC and DC conditions. However, the sources of the electronic charge carriers taking part in the AC loss and DC conduction processes, respectively, are different from each other: At direct voltage the electrons are injected from the cathode whereas at alternating voltage they are predominantly activated from donor-like states in the bulk of the dielectric by a Poole-Frenkel-mechanism.

DOI:10.1049/ip-a-1.1981.0029

出版商:IEE    

年代:1981

数据来源: IET

摘要:

The susceptibility of polymeric insulating materials to breakdown by ‘treeing’ has impeded their utilisation as insulation for high voltage cables. This review surveys the results of tests used to investigate tree initiation and propagation and discusses factors which affect the resistance of materials to breakdown by treeing and the mechanisms involved. Several test procedures can provide reproducible results, provided the processing of the test samples and conditioning before tests' are carefully controlled. However, changing the electrode geometry or the morphology of the polymer can greatly affect the electric stress required for tree initiation and the subsequent time to breakdown. The stress at which tree initiation occurs, in a given polymer, depends on the waveform, frequency and time of application of the test voltage, and also on whether the voltage is applied continuously, or periodically interrupted. Usually tree initiation and breakdown occur at lower voltages as the ambient temperature is raised, but the effects of humidity are more complex. In view of the many factors which may affect the treeing resistance of polymers it would seem expedient to recommend standard test conditions so as to facilitate the assessment and comparison of polymers and the effects of additives. Preferably such a recommended test should take account of operating conditions in cables; thus it should be suitable for tests at the maximum temperatures used in cable operation. It may also be wise to demonstrate that additives which are beneficial in small injection or compression moulded samples are equally effective in extruded insulation, where the morphology of the polymer may be very different.

DOI:10.1049/ip-a-1.1981.0030

出版商:IEE    

年代:1981

数据来源: IET

摘要:

It is observed that AC dispersion properties of evaporated silicon oxide film are largely affected by a DC biasing voltage. This anomalous effect is explained in connection with the previously reported experimental results of electrically excited thermally stimulated current (ETSC). Numerical calculation of this effect is based on a two-site hopping model and taking into consideration the internal field gives a hopping distance in a two-site pair of about 5.7 Å, which is very close to the lattice constant of quartz. It is concluded that the two-site hopping pair probably originates from ionic jumps between nearest vacancies. As far as the experimental results are concerned, there are no contraditions in the interpretations of the DC and AC conductances in terms of independent mechanisms.

DOI:10.1049/ip-a-1.1981.0031

出版商:IEE    

年代:1981

数据来源: IET

摘要:

A photographic investigation of luminescence in point-plane specimens of solid dielectric is described. Tests were performed with impulse voltages of both polarities on specimens having ratios of thicknesstto pin radiusrextending from 20 to 250. Whent/r< 100, collapse of voltage accompanied luminescence, whose spatial and temporal development was recorded using a streak camera. In those specimens for whicht/r> 100, treeing occurred. Light emitted during the inital period of tree development proved to be extremely faint and of short duration, and was recorded using a specially sensitive technique. When (t/r)<100, prebreakdown luminescence began at the specimen cathode irrespective of whether this was the point or the plane electrode. Witht/r> 100, luminescence was observed initially at the point electrode and led to the formation of a tree-like structure of microcracks extending into the dielectric from a microscopic tubule at the tip of the point. With the application of many impulses the cracks developed into channels capable of supporting gas discharges, the presence of which marked the establishment of a conventional electrical tree.

DOI:10.1049/ip-a-1.1981.0032

出版商:IEE    

年代:1981

数据来源: IET

摘要:

Future insulation requirements for radio-frequency submarine telephone cables are postulated. A new apparatus is described that enables dielectric loss of low-loss (̃ 50 μrad) materials to be measured under factory conditions over the frequency range of 0.1 to 200 MHz with a repeatability of ̃ oneμrad. Dielectric loss curves for samples of low density polyethylene, high density polyethylene, three propylene-ethylene random copolymers and one propylene homo-polymer are published. A discussion on the relative dielectric, mechanical and extrusion merits is given and an opinion on the preferred candidate for submarine cables operating above 100 MHz is offered.

DOI:10.1049/ip-a-1.1981.0033

出版商:IEE    

年代:1981

数据来源: IET