D.K. Das Gupta

High-field conductivity measurements and search for threshold in PET films

In the paper results of measurements of DC charging current performed at different values of electrical field and temperature on PET films are presented. The aim is to investigate the threshold of space charges trapping, detected on the basis of the appearance of a maximum in the charging current vs. time characteristics. The values of threshold (function of temperature) thus derived are compared with the results of accelerated life tests performed, under DC voltage and temperature, on the same films until the occurrence of electrical breakdown. It is shown that a relationship exists between the threshold of charge trapping, detected by charging current measurements, and that of electrical breakdown, deriving from multi-stress life tests.

An investigation of conduction mechanisms at high-field in PET

The high-field electrical conduction, up to 60 kV/mm, of PET films is investigated in this paper. The behavior of time-to-peak, observed in transient charging current, vs electrical field fits well with the SCLC model, showing a power relationship. An exponential law is obtained between time-to-peak and temperature. Steady state regime provides charging-current density vs. electrical field plots which show clear transition from an ohmic behavior, at low field, to the SCLC mechanism. The threshold for this transition is a function of temperature.

Measurements of space charge on HDPE specimens during long-time depolarisation

Space charge measurements having the purpose to infer long-term depolarisation characteristics of high-density polyethylene (HDPE) specimens are presented and discussed in this paper. Two techniques are used for space charge observation, that is, pulsed electroacoustic (PEA) method and laser intensity modulation method (LIMM). The qualitative and quantitative information coming from the two techniques are fairly close, considering the inherent differences, and show a very slow charge release with time. This is explained by quite deep traps involved in charge transport, as shown by the results of trap-controlled apparent mobility and trap depth distribution obtained from space charge measurements under depolarisation.

Dielectric Properties of Electrically Aged Low Density Polyethylene

Low density polyethylene (LDPE) films kept in a sodium chloride aqueous solution, were aged under a high AC electrical field. The films were prepared from press moulding of LDPE pellets with small amounts of antioxidants. The dielectric spectra at 30 o C in the range of 10 -5 Hz to 10 5 Hz were obtained prior and after ageing. Three different experimental techniques were used to obtain the full spectrum. For the low frequency (LF) region (10 -5 Hz to 10 -1 Hz) the time domain technique was used (charge and discharge currents were also measured). The measuring device used for the 10 -1 Hz to 10 1 Hz medium frequency (MF) region was a lock-in amplifier. While for the high frequency (HF), 10 -1 Hz to 10 5 Hz, RLC bridge measurements were performed. Differences can be seen between aged and unaged PE. The region showing less changes with ageing is the MF region where the peak of the unaged samples seems to become less defined with ageing time. This peak is probably due to additives and impurities (such as antioxidants) that will tend to slowly diffuse out with time. The LF peak is a broad peak related to localised space charge injection driven by the electric field. This peak increases in an earlier stage of ageing decreasing afterwards possibly when the polymer becomes more conductive. Finally the HF shows the beginning of a peak due to and transitions. The later is related to dipolar rotation of carbonyl groups in amorphous polymer regions, while the former is associated to crankshaft motions in the main polymer chain. This peak decreases with ageing disappearing for the most aged samples. This could also be explained if the sample becomes more conductive.

Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30/spl deg/C in the range of 10/sup -5/ Hz to 10/sup 5/ Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10/sup -5/ Hz to 10/sup -1/ Hz) the time domain technique was used. A lock-in amplifier was used for the 10/sup -1/ Hz to 10/sup 1/ Hz medium frequency (MF) region. While for the high frequency (HF), 10/sup -1/ Hz to 10/sup 5/ Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the /spl gamma/ and /spl beta/ transitions. This peak decreases with aging disappearing for the most aged samples.

On the nature of electron energy loss mechanisms in polymeric materials

An electron beam in passing through a thin solid film may suffer an energy loss, the characteristics of which provide information concerning electronic structure, ionization energy, inter- and intra-band transitions, and plasma oscillations. The energy loss peaks in thin metallic films, e.g., aluminum and also alloys have been studied extensively with X-ray photoelectron spectroscopy (XPS or, as it is now often called, ESCA, meaning electron spectroscopy for chemical analysis). The mechanism of the observed loss peaks in metals may be adequately explained by the plasma oscillation theory due to Böhm and Pines1, using the collective behavior of the valence electrons. During the past few years attention has been devoted to the study of the core and valence levels of a number of polymeric insulators using the ESCA technique (Wood et al.2 and Delhalle et al.3). However, there is still a considerable lack of understanding of the energy loss mechanisms in these materials.

A study of electron energy loss in thin polymer films

Spectroscopy is rapidly becoming an established method of research in many branches of science concerned with the study of materials, particularly in chemical analysis where the technique of X-ray electron spectroscopy is widely used at the moment. These instruments are, however, very expensive and it is believed that an electron beam spectrometer used to study transmitted low energy electrons may provide a more economical method with comparable performance.

Electret formation in organic insulating polymers

It has been known for a long time that after application or removal of a field across a metal-dielectric-metal sandwich a current decreasing with time flows in the external circuit. The absorption currents which flow in polymers on application or removal of a field can be attributed to a number of polarization processes such as an orientation of dipoles or trapped space charge. Adamec1 suggested that the absorption current which flows in polyethylene and similar polymeric materials was caused by the formation of a space charge of trapped carriers rather than dipolar rotation.