Electrical conduction in irradiated low-density polyethylene

Electrical conduction in irradiated low-density polyethylene

0146-5724/92 $5.00+ 0.00 Copyright © 1992Pergamon Press Ltd Radiat. Phys. Chem. Vol. 40, No. 5, pp. 401-410, 1992 Int. J. Radiat. Appl. lnstrum., Par...

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0146-5724/92 $5.00+ 0.00 Copyright © 1992Pergamon Press Ltd

Radiat. Phys. Chem. Vol. 40, No. 5, pp. 401-410, 1992 Int. J. Radiat. Appl. lnstrum., Part C Printed in Great Britain. All fights reserved

ELECTRICAL CONDUCTION IN IRRADIATED LOW-DENSITY POLYETHYLENE H. M. BANFORD, 1 R. A. FOURACP~,2 G. CHEN 2 and D. J. "I~DFORD 2 tScottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU and 2Centre for Electrical Power Engineering, University of Strathclyde, Glasgow GI IXW, Scotland Abstract--A programme of experiments has been undertaken to examine transient charging/discharging currents and steady state currents in low-density polyethylene (LDPE) under the application of direct fields. This has been undertaken for pristine material and for material which has received doses of radiation between 104 and 106 Gy from either a 6°Co ?-source or a research reactor. The material was irradiated in ambient air or dry nitrogen. Measurements were made for applied fields in the range 6.7 x 105-5.3 x 107 V m -~ and temperatures between ambient and 90°C. With pristine material at low fields, transient charging/discharging currents decreased monotonically with time. However, the mechanism changed at higher fields with a peak occurring in the charging transient indicating a space-charge limited process. Substantial charge injection was also in evidence as demonstrated by anomalous discharging currents. This transient response was echoed by the current/voltage characteristics of steady state behaviour. Gamma and neutron irradiation brought about a change in this situation and the charge transport mechanism altered gradually from space-charge-limited conduction to an ohmic process with increasing dose. The role played by charge traps appears to be significant.

1. INTRODUCTION With polymer electrical insulation being used in a variety of situations where deleterious environmental conditions can accelerate ageing and degradation, an understanding of changes in dielectric behaviour resuiting from such conditions is important from the viewpoints of ensuring an acceptable level of material performance and predicting useful life. There is often a great deal of uncertainty surrounding the charge transport mechanism which operates in a particular type of insulating material. However, if this can be identified, it enables the material to be characterised and, consequently, any changes wrought by an environmental stimulus to be determined. A stimulus such as nuclear radiation can be encountered by both terrestrial and space-borne electrical systems, and the changes would provide information on ageing. 2. EXPERIMENTALDETAILS The samples in this work were 90 mm diameter discs cut from 75/zm-thick, additive-free LDPE film. Before any test, the samples were washed in a detergent, rinsed thoroughly in tap water followed by distilled water, and, when dry, finally wiped with isopropyl alcohol. They were packaged and placed in a 6°Co assembly or a nuclear reactor where they were subjected to doses in the range 104-106Gy at dose rates of approx. 0.25 × 104 and 3 x 10SGy h -~, respectively. It is known that the environment in which the LDPE is irradiated can have a profound influence on consequent chemical reactions. Gamma irradiations

were therefore undertaken in both ambient air and dry nitrogen. In addition, to determine the differences between T and neutron radiation effects, a set of neutron irradiations of the LDPE was also carried out in ambient air. Prior to the electrical conduction measurements, test specimens were provided with vacuum-deposited aluminium electrodes (46 mm in diameter) and then mounted in a guard-ring assembly in a heated air enclosure where the temperature could be varied between ambient and 90°C. All measurements were carded out at a constant temperature. The test circuit consisted of a series combination of a highly-stable HT supply, sample and Keithley 610(2 electrometer, the output of which was connected to a chart recorder. For a given applied voltage, the charging current was recorded, typically for 20 min, after which the test specimen was short-circuited through the electrometer and the discharging current recorded for the same time as the voltage application. In this fashion, charging and discharging currents were measured for a series of incremental voltage steps.

3. RESULTS AND DISCU,~ION

3.1. Pristine samples, transient currents Typical charging transient current profiles at 70°C are depicted in Fig. l(a) and (b). At low applied fields, the current decreased monotonically with time, but the situation changed markedly at high fields when a peak in the current was observed. There was little sample-to-sample variation in current magnitude, and none in the shape of the transient. 401

402

H.M. BAh'FORDet al.

a) ov-. x

~

to charge injection which has taken place during the application of the polarising field. The phenomenon has been modelled with a certain degree of success by Kitani et al. (1984).

\ ~ .

5.3 x 106 Vim

%-o__o_o~-oo

100

1.3 x 1 0 6 ~ ' i ~ a

3.2. Pristine samples, steady-state currents

2.7 x 106

7xlo ~ h - - ~ ~ 1

2

3

Log Time (s)

40O00

b) x

m~°XON

20000:

I 1

\

! 2 Log Time (s)

| 3

Fig. 1. Transient charging currents in LDPE under different applied fields (T = 70°C); (a) 6.7 x 105-5.3 x 106V m -t and (b) 2.7 x 107V rn -I. Such behaviour has been noted by other workers with oxidised polyethylene (Fischer and R6hl, 1976; Mizutani et al., 1982). However, earlier work by the present authors on the same material using i.r. spectroscopy and thermally-stimulated discharge currents (Chen et al., 1989) revealed no obvious oxidation of the pristine material. The present results show that the current peak shifted to shorter times with an increase in either the temperature or applied voltage. The behaviour is similar to the transient space-charge-limited current (SCLC) proposed by Many and Rakavy (1962). According to this theory, a current peak occurs at a time tp given by tp =

0.786 d 2 /~V

Steady-state currents were measured along with the charging/discharging currents, the steady-state value for any applied field being taken as that which obtained at the end of a 20-min charging period. Waiting for longer times than this after voltage application produced no changes whatsoever in the overall shape of the current/voltage characteristics, although the magnitude changed. It is assumed throughout that such isochronal measurements will render meaningful results when interpreted in terms of steady-state behaviour (Walden, 1972; Wintle, 1975). Figure 3 shows a typical current/voltage characteristic obtained at a temperature of 70°C. Some curvature is evident at fields below 5 x 10+V m -], but above this level the current obeys a relationship of the form: / ~ v".

The gradient of the straight-line sector corresponds to approx. 3. Similar results were obtained at other temperatures. In Fig. 4, the same data is plotted as log I v F °'5, where F is the electric field strength. A linear relationship was not obtained. From the evidence, it is fair to say that Schottky and Poole Frenckel mechanisms do not explain the conduction processes in the 0 6.7 x 1 0 5 ~ , ~ ~ o x

-20

~''~

1.3 x 106 +

R -40

*

2.7 x 107

/ ~

-60

~ 5.3x10 6 Vim

-80

! I

(1)

where V is the applied voltage, d the sample thickness and/z the carrier mobility. An estimate of the mobility can therefore be obtained from the time occurrence of the current maximum tp. The mobility values of ,,, 1.5 x 10-~4m:V-'s - ' derived using the above relationship compare favourably with the values in the range 10-~4-10 -~2 m2V-~s - ' obtained by a surface decay method (Davies, 1972). Figure 2(a) and (b) illustrates discharging currents. The usual discharging current decreased monotonically with time, but, if the previously applied polarising field was very high, an anomalous discharging current was obtained and which showed two current reversals. This behaviour has been reported previously by several authors (Mizutani et al., 1979; Kitani and An-i, 1983), and is considered to be due

(2)

I 2

a) ! 3

Log Time(s) 400

~

i

o 1.3 x 1 0 7 1 f f 2.7 x 1 0 7 / / I 5"3 x 107 Wm 0 i I 1 2

b) I 3

LogTime(s)

Fig. 2. Transient discharging currents in LDPE under different applied fields (T = 70°C).

Electrical conduction in irradiated LDPE -7

403

/ F~II~I

7

°/a

-11

-13

-13/

I 2 3 Log Voltage (V)

! 4

i

I

i

I

1.3x 10s

1.3x10 6

1.3x 107

1.3x 10s

,

2.7

, 29

,

,

,

3.~

,'-'-., 3.3

IO00/T (I0

Fig. 5. The temperature dependence o f conduction currents under different applied fields.

Field (V/m)

Fig. 3. L o g / - l o g V characteristic o f L D P E at 70°C.

sample. On the other hand, Taylor and Lewis (1971) proposed an electrode-limited process where a general form is assumed for the barrier potential which is given by: ~(x)

=

Ke (ax)"'

(3)

Here, K is a constant which includes the effective charge to be associated with the potential-well and the permittivity of the dielectric, e is the electronic charge, = and n are adjustable constants and x is the radial distance from the centre of the well. The current is then given by: I = Io exp _ [ ~ o - e ,8F"/("+1)]

~-

J

(4)

The conduction current in the sample is thermally activated. Figure 5 shows the measured current under different applied field conditions as a function of the reciprocal of absolute temperature, T. The conduction current, I, obeys a relationship of the form: I = I0 exp - ~ - ~

(6)

where I0 is a pre-exponential factor, E is an activation energy for conduction and k is Boltzmann's constant. The value of E obtained from Fig. 5 is about 0.7 eV at a field of 1.3 x 107V m -1. In a similar way, a dependence of conduction activation energy on the applied field can be obtained, and this is summarised in Table 1. From this, it is readily discernible that the activation energy decreases with increasing applied field, in accord with the results of many other investigations (Crine, 1982). 3.3. Irradiated samples, transient currents

where

, =[:],,,-,, In and ~0 is the zero-field activation energy. Although equation (3) can be made to fit experimental data by adjusting = and n, the transient current beliaviour of the electrode-limited process is unknown. In the present case, however, the transient charging currents show SCLC characteristics (Mott and Gurney, 1948; Helfrich, 1967). Accordingly, it is considered that the current/voltage characteristics also demonstrate SCLC behaviour. -7

-9

/o -11

-13 0

,o/°/°

/ i

Table 1. Influence of electric field on the activation energy

I

i

!

4000

6000

8o0o

Reld I~ (V/m) v~

Fig. 4. Log I vs F °'s.

3.3.1. Gamma irradiation in dry nitrogen. Figure 6(a) and (c) shows some typical results obtained from transient charging current measurements for applied fields in the range of 1.3 × 106-5.3 × 107V m -I at a temperature of 70°C. Two different dose levels are shown (104 and 106 Gy). The transients were similar to those observed in the unirradiated material in that, in certain cases, a transient peak was observed. Figure 6(b) shows such a transient. The peak shifts to a shorter time if the applied field is increased, as is the case for the virgin sample. The behaviour described above for the lower-dosed samples is again indicative of space-charge limited currents. The carrier mobility, calculated from the position of the peak in the current transient, is shown in Table 2 for dose levels of 104 and l0 s Gy and for

Applied fields ( x 106V rn-~)

Activation e n e r g y (eV)

0.67 5.3 9.3 13.3 26.7 53.5

1.0 0.8 0.8 0.7 0.7 0.6

404

H.M. BANFORDet al. 100 F . 5.3 x 107 Vim

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4

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I

2 3 Log Time (s)

Log Time (s)

4

-9.2

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E

5.3 x 107 Vim



x A

%

-9.4

jl

-o

F= 1.3 x 107 Wm

400

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2

3

4

n

-2 0

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Log Time (s)

@~ 8 t

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0

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i -I0000 E

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I

2 Log Time (s)

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c) -10

I 4

10000

o L

1.31107

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Log Time (s)

F m 5.3 X 107 Vim

• ~'~11~.n

1.3 Xn106

2

I

I

3

4

-30000

I

I

I

I

1

2

3

4

LogTime(s)

Fig. 6. Transient charging currents at 70°C for LDPE exposed to different doses of y-radiation in nitrogen; (a) dose 104 Gy, (b) dose 105 Gy and (c) dose 106 Gy.

Fig. 7. Transient discharging currents at 70°C for LDPE exposed to different doses of y-radiation in nitrogen; (a) dose 104 Gy, (b) dose 105 Gy and (c) dose 106 Gy.

different applied voltages. It should be noted that the peak does not always occur, unlike the situation in the pristine material. Transient discharging currents were also monitored, and typical results are shown in Fig. 7(a)-(c). At dose levels of 104 and 105Gy, anomalous discharging currents were observed under high field conditions, with the current reversing polarity after a period of time before finally decreasing to zero. This effect is very marked in the sample exposed to a dose of 10s Gy. At 104 G y the observed transients are not anomalous but decrease monotonically to zero. 3.3.2. G a m m a irradiation in air. Sample results of transient charging currents at 70°C, for samples irradiated in air, are illustrated in Fig. 8(a) and (b). Generally these currents increase with both increasing applied field and dose, as expected (Banford et al.,

1987). For a dose of 104 Gy, a peak is observed in the transient [Fig. 8(c)] and appears to be related to space-charge limited currents. This behaviour is similar to that of pristine samples, although the peak occurs at a later time. Increasing the field reduces the time to reach the peak. At the other dose levels, no peaks were observed. Mobility results are indicated in Table 3 for measurements at 90°C with different applied voltages. As with the charging currents, the discharging currents are greater for the irradiated samples [Fig. 9(a)-(c)] compared to the unirradiated material. Current magnitudes increase with dose except at 106 Gy where the current is less than at lower doses. The curves also show a marked change in shape with both dose and applied voltage. At 104 Gy there is evidence of anomalous discharging current for the highest applied voltage, the current changing polarity

Table 2. Carrier mobility in samples after 3''irradiation in a nitrogen atmosphere /a (m2V - ~s- i)

400 V

700 V

1000 V

2000 V

4000 V

Dose lOgGy 30°C 50°C 70°C 90°C

7.37)<10 14

9.21 x 10 -14

1.051 10 L3

1.11×10

is

1.47× 10 13

Dose lOgGy 30°C 50°C

2.21 x 10 ~4

70°C

90°C

7.02 x 10 -14

3.68 x 10 -ts 9.83 x 10 -14

1.84 x 10-1a 2.21 x 10 -13

1.11 x 10 is

"I

Electrical conduction in irradiated LDPE

5.3 x 107 Wm l ~ l " l l ' - I I ~II,E41B

~

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405

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0

a)

0

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I

I

I

1

2

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Log Time (s)

0

6 , , o__W;, / o x -20000

5.3 x 107 Vim I1~ I I - B - I I ~ I I , II i ~

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8

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Log Time (s)

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o

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<_

i

-60000

6.7 x I

5.3 x 107 Vim

-8.0

a -8.4

4> 1.3 x 107

E

O

c) I 1

I 2 Log Time (s)

I 3

| 4

Fig. 8. Transient charging currents for LDPE exposed to different doses of ~,-radiation in air; (a) 104 Gy, T = 70°C, (b) 103Gy, T = 7 0 ° C and (c) 104Gy, T=90°C, field = 1.3 x 10-?V m -I.

before finally returning to zero. At higher exposure levels no such anomalous behaviour is apparent. The magnitude of the discharging current at a particular time after commencing the discharge is not proportional to the voltage originally applied to the sample, as would be expected from a dipole relaxation phenomenon. F o r samples polarised at high fields, the current is often smaller than that observed in samples polarised at lower voltages. Generally, the magnitude of the discharge current increases with temperature. 3.3.3. Neutron irradiation in air. A typical transient charging current for samples exposed to various doses is shown in Fig. 10(a)-(c). The current magnitude increases both with dose and applied field and, at 70°C, the current decreases monotonically with time. A current peak is observed only at 90°C and only for the sample exposed to 104 Gy. F o r this case,

-800OO

I

1

t

c) I

1

2 3 Log Time (s)

4

Fig. 9. Transient discharging currents at 70°C for LDPE exposed to different doses of 7-radiation in air; (a) 104 Gy, (b) 105 Gy and (c) 106 Gy.

the mobility of the charge carriers may be derived, and values appear as part of Table 3. Transient discharging currents for samples exposed to more than 10SGy increase with increasing magnitude of the field initially used to polarise the sample and decreases monotonically with time. At 104Gy exposure, the current increases with field, but exhibits current reversal if the initial field is high, i.e. an anomalous discharging current is observed. Both types of transient current increase with increasing temperature. The change in transient response of the irradiated material derives from the general changes in the physical and chemical state of the polymer compared to its pristine condition. Oxidation will have taken place for those cases where radiation exposure occurred in the presence of air, giving rise to carbonyl groups (Chen et al., 1991): additional trapping centres will have been created (Okkalides, 1984) and

Table 3. Carrier mobility /~ (m2V-~s-~) in samples exposed to radiation under different conditions at 90°C Dose (Gy) 400 V 700 V 1000V 2000 V 0 8.0 x 10-15 1.8 x 10-14 6.4 x 10-]4 104 (7 + N2) 9.2 x 10-14 1.1 X 10 -14 1.5 X 10 -13 104 (R + air) 1.6x 10-]4 1.8 x 10-14 3.2 x 10-14 1.9 x 10-13 104 (7 + air) 1.3 x 10-14 1.8 x 10-14 3.2 x i0 -14 1.9x 10-13 10s (7 + Nz) 7.0 x 10-]4 9.8 x 10-]4 2.2 x 10-13 > 104 (air) Not possible to obtain SCL transients R = reactor radiation; y = gamma radiation.

406

H . M . BANFORD et al.

charge will have been released with a proportion becoming trapped (Sakharov and Gromov, 1984; Keyser et al., 1972). The trapped charge can influence and create space charge fields within the dielectric and affect the potential barrier at the electrode/dielectric interface (Lewis, 1984; Lampert and Mark, 1970). The above processes affect measured transient and steady state currents where possible mechanisms are: (i) migration of charge carriers created via irradiation, (ii) migration of electrode injected charge carriers, (iii) polarisation dipoles produced by oxidation. In the cases where anomalous discharging current behaviour is observed, it is likely that the cause is similar to that occurring in the pristine material, namely, due to double charge injection as postulated and modelled by Kitani et al. (1984). Previous TSDC measurements (Chen et al., 1991) indicated that a peak resulting from double charge injection was observed in pristine samples under high field conditions. This supports the hypothesis. The peak was no longer present after irradiation. The cause of this is either that the process is dominated by radiationinduced charge effects, unlikely because of the magnitude of the peak seen in the pristine material, or that radiation-induced changes in the injection mechanism -7 S 3 x 1~7 VT~ II II

Z

E <~

II IIIiii

1.3 x 107

-9



6

'I''I''IP ee~l,,

(3",,. O~

°'a~o-r~rl n

13 x 106 -11

I

1

A

"7r

I

2 Log Time (s)

l=~l..

a)

I

I

3

4

5 3 x 107 Vim

~ -sL

.~ " "--'''BB / 1.3 x lO7 " " • - - , t,

~

9l

0

°"°'o-=. 1

"

2 Log Time (s)

3

-7

Reactor

E -9 O

0Gy

6

O,==g-g-~--_o-o-o-o -- ~-v~O--O-O-.O Gamma + nitrogen

-11 0

I

I

i

i

1

2

3

4

Log Time (s)

Fig. 11. Transient charging currents in LDPE exposed to different conditions; dose 105 Gy, T = 70°C.

come about due to the creation of different trapping levels or changed conditions at the electrode/polymer interface. The most likely reason for the disappearance of the anomalous transient with increasing dose is that two competing processes are occurring. The first process is the anomalous transient current resulting from double charge injection which may be modified by radiation, as indicated above, and the second is a normal transient which has been enhanced by the presence of radiation-produced charge. The magnitude of the negative component of the radiation-produced charge may be greater than the positive anomalous current component. This produces a net current transient which no longer reverses polarity. Figure 11 shows the charging transient for samples exposed to 105Gy under different exposure conditions. The current magnitude at a particular time shows the decreasing order: 2: in air, neutron in air, pristine sample, 7 in nitrogen. This order for the irradiated samples also represents the order of decreasing chemical change, as the absence of oxygen precludes oxidation products such as carbonyl groups which are electrically active. The chemical change produced by the reactor radiation will be less than that produced by ~, because of the disparity in exposure times required to produce a given dose. In the case of the reactor radiation, exposure is brief; hours compared to many days for 7-irradiation- This limits the extent of diffusion-controlled processes. Figure 12 shows the effect of radiation dose on the carbonyi concentration measured using infrared spectroscopy.

4 300

1.3 x 107 Vim

-8.4

== a

~" loo

c) I

1

I

I

I

2

3

4

Log Time (s)

Fig. 10. Transient charging currents for LDPE exposed to different doses of neutron radiation in air; (a) 104Gy, T = 70°C, (b) 105Gy, T = 70°C and (c) 104Gy, T = 90°C.

o.~-~0

o~

/

-8.0

~-8.8

Gamma + air

I~,~ o.

E

J

o , 0.2

Reactor

, 0.4 Dose (M Gy)

, 0.6

Fig. 12. Effects of radiation dose on the concentration of carbonyl groups in LDPE irradiated in air.

Electrical conduction in irradiated LDPE

407

For a given dose, the concentration is greater for T and l0 s Gy). Such an explanation cannot be used to compared to neutron irradiation. It is thus consistent explain why, for neutron irradiation in air, a transient to ascribe the current transient changes to chemical peak is observed only at high temperature where the effects, perhaps through the creation of traps which intrinsic carrier contribution would be enhanced. The may then be filled with radiation-produced charge. charge injection enhancement at this higher temperaThe greater the dose, the greater the amount of ture must, therefore, be more significant; both protrapped charge which will be thermally activated cesses are exponentially dependent on temperature. during the conduction process, resulting in the larger Two conclusions can be drawn from Table 3 transient currents normally observed. concerning the mobility results at 90°C for all exThe fact that the pristine sample exhibits a slightly posure conditions. First, in all cases, the mobility is enhanced conductivity compared to the T samples field dependent, increasing with increasing field, and irradiated in nitrogen can be explained in terms of the attributed to field-assisted detrapping (Crine, 1982). log I / V characteristics for the different types of Secondly, there is a radiation effect on the mobility. exposure. An idealised version is shown in Fig. 13. The following are observed: The ohmic low-voltage region is a result of thermallygenerated charge released from traps. The magnitude (i) in all cases, the charge mobility measured of the resultant current increases with increased trap for irradiated samples is greater than for the density produced as a result of chemical degradation. pristine material The magnitude of this current influences the voltage (ii) in the presence of air, there is no difference at which SCL currents become dominant. It can be in charge mobility between T and neutron seen that a voltage such as V~ results in the pristine irradiated samples sample having a larger current than the y-irradiated (iii) mobilities measured for samples irradiated in nitrogen are larger than in air and sample in nitrogen. This occurs provided that the (iv) in nitrogen, an increased dose from 104 to irradiated material has a higher conductivity in the lOSGy reduces the mobility by a small ohmic region, as would be expected from radiationinduced creation of charges and traps, and that the amount. SCL current has been reduced by the radiation. The Qualitatively, these results may be explained. For latter effect would displace the SCLC characteristic to high voltages. The transient currents would also be pristine material, the most likely limitation on mobility is trapping, particularly at interfaces between affected by such a mechanism. The disappearance of the space-charge-limited amorphous and crystalline regions. Radiation introconduction peak in the charging transient can also be duces increased numbers of traps some of which will considered to be due to the two types of charge be shallower than those present in the virgin material. carder. These carriers are either intrinsic carders Charge escapes more readily from these traps, and thermally excited from the normally trapped charge spends less time in them. This produces an increase also present in the pristine material and, in addition, in charge mobility. Alternatively, the traps present created from traps produced by the radiation or may be filled with radiation-induced carders, thus carriers injected from the electrodes. At low fields, the limiting the possibility of capture of injected charge intrinsic carriers dominate, producing ohmic conduc- resulting in higher mobility. To differentiate between tion, but, at high fields, SCLC occurs and can these possibilities, knowledge of trap distributions produce the characteristic peak in the charging- and occupancy is needed. The difference between T and neutron irradiation is current transient. The increases in the proportion of intrinsic carders caused by the irradiation results in not apparent from the mobility measurements. It the onset of SCLC conditions occurring at higher would be expected that, on considerations of radiapplied voltages (Fig. 13), and explains why SCLC is ation exposure time, there would be fewer carbonyl only seen for relatively low exposure conditions (104 traps in neutron-irradiated samples (Fig. 12). Thus, it would appear that the differences in the numbers of such traps between the T and neutron irradiated material are not controlling the mobility, although it is still possible that the presence of such traps is /~ SCLC region sufficient to affect the value measured. The difference between the air and nitrogen en//~/ ~ Transition from 8 vironments is that, in the former case, carbonyl ~ ohmicto ,~LC groups are formed which act both as electron and hole traps. These have trap energies of 1 eV (elec.-'~ /I ~ ¥ innitrooon trons) and 0.5-1 eV (holes) (Takai et al. 1979; Hilczer i and Malecki, 1986). Charge transport can thus occur through a process of release from, and capture by, V~ Log Voltage Ohmic region traps. If the carbonyl trap is relatively deep compared to other traps produced, then it is probable that the Fig. 13. Idealiscd log/-log V characteristics.

!

H. M.

408

BANFORD

migrating charge will spend more time in them, thus decreasing the measured mobility. The mobility differences observed between air and nitrogen irradiated samples can be explained in this manner. The decrease in mobility with increased dose, which is found for ~ irradiated samples exposed in nitrogen, supports this thesis that mobility is limited by trapping since the higher the dose the greater the number of traps. 3.4. Irradiated samples, steady-state currents 3.4.1. Gamma irradiation, dry nitrogen. Figure 14(a) shows a typical log I~ V characteristic for a sample exposed to 104 Gy in dry nitrogen. The slope of the characteristic is 1 for fields less than 1.3 × 107 V m ~, and increases with applied fields greater than this to reach approx. 2. Similar behaviour was exhibited at other doses. Figure 14(b) is a typical Arrhenius plot for currents measured at a field strength of 1.3 × 107V m ~. Activation energies derived from such plots as a function of dose are shown in Table 4. 3.4.2. Gamma irradiation in air. Log I / V characteristics for samples exposed to 104 and 106Gy are shown in Fig. 15(a) and (b), respectively. The material exhibits similar behaviour to that described in Fig. 14(a), except at 10 6 Gy where the conduction is purely ohmic. Activation energies for the conduction process are shown in Table 4.

et al.

Table 4. Activation energies for the d.c. conduction process after exposure to different radiation environments and doses Activation energy (eV) Dose (Gy)

G a m m a in N 2

G a m m a in air

N e u t r o n in air

0.7 0.6 0.6

0.7 1.0 0.6

0.7 0.7 0.5 0.5

0.5

0.5 0.8

0 104 10 5 5 x 10 5 10 6 3 × 10 6

3.4.3. Neutron irradiation in air. Log I / V characteristics [Fig. 16(a) and (b)] are similar to those obtained for ~,-radiation. Activation energies for the conduction process are given as a function of dose in Table 4. The material behaviour is similar to that described in Fig. 14(a). In steady-state conduction, the charge carriers can be considered to consist of two components, intrinsic charge which can be increased by irradiation effects and injected carriers from the electrode. The current density in the material in which SCLC occurs may be expressed as: J = ~ (noiel~V/d) + JSCLC

where no is the density of thermally generated carriers and JSCLC takes the form of equation (2). The summation sign contains the charge generated in the pristine material and the radiation-produced charge.

-8

a)

/

E

o



E -10

;~/o

_J

/o

8 o~

/ ¢ . o "° a) f

t

2 3 Log Voltage (V)

I

4

I

i

1.3 x 10 5

1.3x10 6

I

i

1.3x 10 7

1.3 x 10 8

o

,io" °

I

I

2 3 Log Voltage (V) I

i

i

1.3 x 10 5

1.3 × 10 6

1.3 x 10 7

i

4 i

1.3 x 10 8

Field (V/m)

-6

Field (V/m)

b)

-9 ~-7

/o

~" -10

a

/

o~ °

-I I -12

(7)

i

b)

~-8

/

/o/°°

o/°

-11 -9 -12 2.7

i

i 2.9

i

i 3.1

l

I 3.3

I 2

I 3 Log Voltage (V)

J

1ooo/l" (K)

Fig. 14. (a) Log l-log Y characteristic for LDPE after exposure to ),-radiation in nitrogen. Dose 104 Gy, T = 70°C. (b) Arrhenius plot of conduction current in LDPE after exposure to 7-radiation in nitrogen.

I 4

I

I

I

I

1 . 3 x 10 5

1.3x 10 6

1 . 3 x 10 7

1.3x 10 8

Field

(V/m)

Fig. 15. Log/-log V characteristics for LDPE exposed to ),-radiation in air (T = 70°C); (a) dose 104 Gy and (b) dose 10 6 G y .

Electrical conduction in irradiated LDPE -7

a)

/

<_ ==

409

trapping energies, then it is likely that the apparent conduction activation energy will decrease in the way observed.

rn

I"1 4. CONCLUSIONS

8

-11

I

t

2

3

I 4

LogVoltage(V) i

I

1.3 x 10 5

I

1.3 x 10 6

1.3 x l O 7

I

1.3 x 10 8

Field (V/m)

E o/ o ./ of

8 o~ -9

I

i 1.3x

I

I

2 3 Log Voltage (V)

4

I 10 5

1.3x

I 10 6

1.3x

I 10 7

1.3x

10 8

Reid ( V / m )

Fig. 16. Log/-log V characteristics for LDPE exposed to neutron radiation in air (T = 70°C); (a) dose 104Gy and (b) dose 106Gy.

Electrical conduction in the pristine LDPE of the present investigation is dominated by a space-chargelimited mechanism in the high field region. The mobility of the charge carriers derived from the transient charging current characteristic is --, 10 -~410-13 mW - is- l comparing favourably with the work of other authors. The magnitude of the mobility is radiation dependent both in terms of dose and radiation conditions. The activation energy for the conduction process is in the range 0.5-1 eV, and decreases with field and dose. Charge injection appears to be substantial at high fields. Anomalous charging currents have been observed in both virgin and irradiated material. A double injection model has been used to explain this phenomenon. Increase in intrinsic charge as a result of irradiation provides plausible explanations for the disappearance of transient charging peaks and the transition from ohmic to space-charge-limited behaviour in steady-state current characteristics. The results correlate strongly with earlier thermallystimulated discharge measurements.

REFERENCES

Under low field conditions, injection from the electrodes is of minor importance, and the main source of carriers is that thermally generated. The current flow is thus determined by the summation term in equation (7). Ohmic conduction occurs: I oc V.

(8)

For high fields, the second term in equation (7) can dominate as it increases more rapidly than linearly with voltage [equation (2)]. Thus, the I / V characteristic will change from ohmic to a space-charge limited form as the applied voltage increases. The significance of the radiation is that the contribution from the first term in equation (7) increases progressively with dose, as indicated by the increased conductivity in the ohmic region of the I / V characteristic, thus causing the transition between the two conduction mechanisms to occur at higher voltages. This is precisely what is observed experimentally. From the variation of the activation energies as a function of dose (Table 4), it is apparent that the trend is for the activation energy to decrease with increasing dose, although the accuracy of the results perhaps precludes a definite conclusion on this aspect. If it is considered that the consequence of increasing radiation dose is to produce an increased number of traps, and that the distribution of traps becomes increasingly biased towards smaller

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