882 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 6 No. 6, December 1999

Influence of Radiation Environments on Space Charge Formation

in y-Irradiated LDPE G. Chen, A. E. Davies

Dept. of Electrical Engineering University of Southhampton, Southhampton, UK

H. M. Banford Scottish Universities Research &Reactor Centre, East Kilbriile, UK

ABSTRACT In the present paper the formation of space charge in irradiated low-density polyethylene (LDPE) was investigated using the pulsed electroacoustic (PEA) technique. Specimens of LDPE, -100 pmm thick, were irradiated to various doses at room temperature in a "CO y source in one of three different environments: ambient room air, oxygen-free nitrogen and vacuum. The results obtained indicate that the space charge distribution is dependent on the irradiation environment. There is a large concentration of positive charge evident in the sample irradiated in air compared to very little for material irradiated in either vacuum or nitrogen under similar conditions. This indicates that the presence of oxygen during irradiation, and thereby oxida- tion itself, has a major influence on the mechanism by which space charge evolves. Thus the monitoring of space charge may in itself be an indicator of the degree of oxidation and of aging in a material. The evolution of space charge and its decay are discussed.

, 1 INTRODUCTION OLYMERIC materials are used widely for electrical insulation over P a broad range of applications because of their outstanding proper-

ties such as high breakdown strength, high resistivity and low dielectric loss. The retention of these properties during service is equally impor- tant especially when the cost of manufacture, installation and repair of the insulation is taken into consideration. Thus a thorough understand- ing of the aging processes that bring about either a deterioration of the electrical properties or complete failure of the insulation system is of great importance. There are various environmental stimuli that can in- stigate and promote aging and one of the most deleterious is ionizing radiation. While recognizing that aging is a multi-factor phenomenon, ionizing radiation adds a particular dimension to the problem. This is because it can alter profoundly the molecular structure and thereby the macroscopic properties of polymeric materials through mechanisms like chain scission, crosslinking and oxidation. It may also give rise to the presence of trapped charge within the material, the trapping character- istics of which may be influenced by these radiation-induced structural alterations.

Radiation effects in polymers have been studied for many years to in- vestigate various aspects of material behavior [1-4] and among these is the electrical response to irradiation. Recent work related particularly to the effects of radiation on the electrical behavior of low-density pol-

yethylene (LDPE) has shown that a radiation environment can influence the thermally stimulated discharge current (TSDC) spectra in the high temperature region, with more charge being found within the sample irradiated in air compared to that irradiated in nitrogen [5]; the dc elec- trical conductivity of LDPE irradiated in air is significantly higher than of material irradiated in nitrogen [6]; and the prebreakdown transient current pulse activity in LDPE under direct stress has been demonstrated to be much stronger in material irradiated in air compared to that irradi- ated in nitrogen [7]. It has been considered generally that space charge played a role in the above behavior of irradiated materials, but there has been a difficulty in measuring charge distributions themselves. In recent years the development of specialized techniques like the pulsed electroacoustic (PEA) method [8,9] and the laser induced pressure prop- agation (LIPP) method [lo] have allowed such distributions to be deter- mined non-destructively and their influence on dielectric performance now can be investigated.

Recent investigation by the present authors on the formation and dynamics of charge in y-irradiated thick (-350 pm) LDPE using the LIPP technique indicated that the charge distributions were a function of dose, the applied electric stress and the length of time for which the stress was applied Ill]. It was noted, particularly at high doses, that non-uniform radiation-induced oxidation occurred in a sample with more oxidation at the surface than in the bulk. This led to the propo-

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IEEE Transactions on Dielectrics and Electrical Insulation

sition of a shallow-deep-shallow trapping model across the sample, to explain the observed charge distributions. The previously observed be- havior of LDPE irradiated in air and nitrogen indicated that it would be worth considering the effect of the environment on the formation of space charge.

2 EXPERIMENTAL METHOD

2.1 SAMPLE PREPARATION

In order to minimize the morphological differences and charge trap- ping that may arise from the presence of additives, 100 pm additive free (i.e. free of antioxidants and plasticizers) LDPE film formed by a blowing process, was used in the present study. LDPE samples, having a density of 0.92 g/cm3, melting point of 108°C and crystallinity of -46%, were cut from the film, cleaned with ethanol and subsequently placed in a ''CO y source for irradiation to different doses between 5 and 100 kGy at a dose rate of -1 kGy/h. Different batches of samples were irradi- ated at room temperature in ambient air, oxygen-free nitrogen and vac- uum. For irradiation in nitrogen and vacuum, the samples were placed inside glass tubes which were evacuated for -2 h before being sealed (vacuum) or backfilled with nitrogen gas and sealed. Following irradi- ation, they were provided with sputtered gold electrodes.

2.2 MEASUREMENTS

Measurements of space charge were carried out using the PEA tech- nique [8,9]. Test specimens were stressed electrically under dc at two levels (30 and 60 kV/mm) and for a variety of times, the maximum du- ration being 2 h. Charge distributions were recorded immediately after the removal of the'applied stress. All the measurements were made at room temperature.

3 RESULTS AND DISCUSSION

Before tests were carried out on irradiated samples, reference space charge distributions were obtained with unirradiated control samples at the two experimental stress levels of 30 and 60 kV/mm and with an unstressed sample that had been irradiated in air to 100 kGy The charge distribution in the pristine control sample following stressing for 2 h at 60 kV/mm is displayed in Figure 1. No charge was found at 30 kV/mm. The homocharge evident indicates that charge injection occurred at the higher of the two electric stresses. However, the charge density is small and not sufficient to modify the stress distribution across the sample to any great extent.

With zero applied voltage, no space charge was observed in the ir- radiated sample as shown in Figure 2, thus indicating that the mate- rial remained neutral following y irradiation. When a dielectric is ir- radiated, ion pairs are produced, and it has been reported [12,13] that a space charge profile can be formed in insulation when photons pass through it. In the present work, however, samples were irradiated in a uniform flux of y photons and consequently all ion pairs formed will in turn be distributed uniformly. As a result of this, no resultant space charge distribution can be observed across the sample, a finding that is in agreement with the previous observations [ll, 14,151.

Vol. 6 No. 6, December 1999

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Figure 1. Charge distribution in a control sample after stressing at 60 kV/mm for 2 h.

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Figure 2. Charge distribution in an unstressed sample irradiated in air to 100 kGy

3.1 IRRADIATION IN AIR

Figure 3 shows the charge distribution for two different stresses in a sample irradiated to 5 kGy. There is a considerable amount of positive charge adjacent to the cathode. In considering this situation it must be borne in mind that while the majority of electrons from ion-pair produc- tion are geminate, in that they remain within the sphere of influence of their parent ions, some are non-geminate and are free to move around in the material by means of thermal excitation and/or the action of an elec- tric stress. In these circumstances, trapping and detrapping will play an important role in the population dynamics. In the particular case of Fig- ure 3 it would appear that electrons have been swept by the electric field towards the anode and left the more massive and less mobile positive ions at the cathode. It is evident that the applied stress has a significant influence on the charge distribution and at 60 kV/mm the density of pos- itive charge is lower than that at 30 kV/mm. This perhaps unexpected behavior is reasonable when the result of Figure 1 is taken into account. This showed that electronic charge injection takes place at 60 kV/mm, but not at the lower value of the applied stress. In the case of a 5 kGy sample, there will be a stress enhancement at the cathode deriving from the presence of positive charge adjacent to the cathode. This stress en- hancement will further increase charge injection and any injected elec- trons will tend to neutralize the positive charge. The resultant effect is to reduce the positive charge density adjacent to the cathode. A dynamic system will therefore be in operation, with the density of space charge

884 Chen et al.: Radiation and Space Charge Formation in 7-Irradiated LDPE

varying with time until equilibrium is established. This is demonstrated in Figure 4, where the time dependence of the interfacial stress deriv- ing from the space charge within the material is portrayed. While the interface stress for 30 kV/mm stabilizes fairly quickly because there is essentially no charge injection resulting from the applied stress accord- ing to Figure 1, the stress for 60 kV/mm changes continually during the complete time period of 2 h for which observations were made.

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Figure 3. Air irradiation, 5 kGy Charge distribution in the sample after stressing for 2 h.

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Figure 4. Air irradiation, 5 kGy Time dependence of interface stress at the cathode due to space charge.

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Figure 5. Air irradiation, 100 kGy. Charge distribution in the sample after stressing at 30 kV/mm for 2 h.

If the dose is increased, space charge profiles through the material

0 20 40 60 80 100 120 140 time (min)

Figure 6. Air irradiation, 100 kGy. Time dependence of interface stress at the cathode due to space charge.

remain similar to what takes place at low doses, but the density of the space charge increases as illustrated in Figure 5. The calculated stress enhancement at the cathode caused by this space charge is shown in Fig- ure 6. It increases rapidly at the outset and then stabilizes after 5 min at a value that is almost 4x that of the applied stress. This is a quite dif- ferent situation compared to what happens at low dose, where the rate of change of the interface stress is much slower. The reason for this is that the density of ion pairs produced by y irradiation increases with increasing radiation dose [16] and therefore they are far more prevalent at 100 kGy than at 5 kGy. Ionization of the material also gives rise to radicals and to the subsequent alteration of the structure of the material via chemical radical reactions and the mechanisms of crosslinking, chain scission, gas production and oxidation, cited earlier. Thus, as well as re- leasing charge within the material, radiation causes it to change from its original state. This happens not only while irradiation takes place, but also afterwards when oxygen, for example, diffuses into the bulk and produces oxidation products such as ketones, alcohols and hydroper- oxides among many others, depending on the material in question. In the presence of oxygen, one oxidation product that is readily observable with irradiated LDPE: is the carbonyl group (C=O). It is well established that the concentration of carbonyl groups increases with radiation dose [17] and that the carbonyl groups generate a shallow trap that readily assists charge transport [18,19]. It is also known that oxidation can en- hance charge injection [20]. What is probably having a strong effect on the speed at which the interface stress stabilizes at 100 kGy is the fact that at a high dose, and consequently a higher level of oxidation, there is an enhanced injection of charge along with a much higher abundance of radiation-induced shallow traps available to assist charge movement compared to what takes place at low dose. This will obviously have a marked effect on the speed at which the space charge itself stabilizes.

The decay of space charge and its resultant interface stress follow- ing stressing are shown in Figures 7 and 8 respectively. The behavior demonstrated in these Figures derives from charge moving under the action of the space charge field alone. As has been discussed earlier, the positive ions have a very low mobility and the enhanced field at the in- terface causes the injection of electronic charge that neutralizes the pos- itive charge. This can only continue for a finite time, however, because at 30 kV/mm charge injection does not take place as evidenced by the results of Figure 1. It is also demonstrated in Figure 8 that the interface

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 6 No. 6, December 1999 885

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p -100

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m

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Figure 7. Air irradiation, 100 kGy Decay of space charge after stressing at 30 kV/mm fdr 2 h.

the sample Figure 9. Vacuum irradiation, 100 kGy. Charge distribution in Sample after stressing for 2 h.

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ode due to space charge. Figure 10. Nitrogen irradiation, 100 kGy. Charge distribution in the

sample after stressing at 30 kV/mm for 2 h.

stress deriving from the space charge stabilizes at -30 kV/mm.

3.2 IRRADIATION IN VACUUM

A typical distribution of space charge in a sample exposed to 100 kGy in vacuum is shown in Figure 9 for the two different stresses of 30 and 60 kV/mm. There is a very small amount of positive charge present in the sample stressed at 30 kV/mm which may be created by irradiation. However, the stress enhancement due to these positive charges is neg- ligible. Homocharge was found in the sample stressed at 60 kV/mm and the characteristic is essentially identical to the unirradiated control sample. Similar measurements were also carried out on low-dose Sam- ples and no differences could be seen when compared with the control sample.

In the absence of oxygen, LDPE undergoes predominantly crosslink- ing under irradiation. The results suggest that the crosslinking by irra- diation has little effect on the charge formation in LDPE.

3.3 IRRADIATION IN NITROGEN

Figure 10 shows the charge distribution in the sample exposed to 100 kGy in nitrogen gas at a stress of 30 kV/mm. Again, only a small amount of positive charge is present across the sample. At 60 kV/mm only injected charge could be found and the behavior was essentially the same as for the irradiation in vacuum and unirradiated material.

Compared to the results obtained from irradiation in air, the results for both vacuum and nitrogen show that the presence of oxygen is a vital factor in the evolution of space charge in irradiated material.

4 CONCLUSIONS PACE charge profiles have been observed directly in irradiated S LDPE using the PEA technique. There are marked differences be-

tween the space charge characteristics of material irradiated in air in comparison with LDPE irradiated in either vacuum or oxygen-free ni- trogen gas. It is evident that the presence of oxygen during irradiation is essential for a significant amount of space charge to form. At low applied stress, a large amount of positive charge is present in material irradiated in air while only a small amount of charge exists in samples irradiated in both vacuum and nitrogen. The amount of positive charge increases with the radiation dose and the interface stress caused by the space charge can be as much as 4x that of the applied stress. The rate at which the interface stress stabilizes is much faster in high-dose samples compared to low-dose ones. Only a small amount of positive charge can be observed in the 100 kGy sample irradiated in either vacuum or nitrogen gas. In these cases stress enhancement caused by space charge is negligible.

The manner in which space charge distributions vary with dose, stress and irradiation environments is clearly a function of the structural changes taking place within the LDPE. It is particularly evident as being related to the degree of oxidation and therefore the monitoring of space

886 Chen et al.: Radiation and Space Charge Formation in y-Irradiated LDPE

charge may in itself be a useful indicator of aging in insulating materials in general.

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Manuscript was received on 18 February 1999, in revised fom 9 July 1999.