- Email: [email protected]
Preliminary results of combined thermo-luminescence and thermally stimulated current measurements on an additive free polyethylene
R&o/. Ph~s. C/lrn~. Vol. 45. No.
Pergamon
I, pp.3-X. I995
Copyright ,’ 1994 Elsevier Science Ltd Prmred in Greal Britain. All rIghta reserved 0969-X06X/95 17.00 + 0.00
0969-806X(94)EOO16-C
PRELIMINARY RESULTS OF COMBINED THERMO-LUMINESCENCE AND THERMALLY STIMULATED CURRENT MEASUREMENTS ON AN ADDITIVE FREE POLYETHYLENE M. J. Department
of Electronic
GIVEN,
and Electrical
R.
A. FOURACRE
and D. J.
Engineering, University Gl IXW, U.K.
TEDF~RD
of Strathclyde,
George
Street. Glasgow
Abstract-Samples of additive free low density polyethylene have been irradiated at - I80 and - 130 C with I keV electrons. The sample arrangement was such that simultaneous measurements of thermally stimulated discharge currents (TSDC) and luminescence (TL) were possible and these techniques were used to observe effects of surface charge relaxation. The resulting spectra were similar in some respects to those obtained by other workers using either higher electron energies, which create bulk charges, and higher irradiation temperatures. or using X- or g-radiation to create internal charge. In particular some of the TL peaks in the spectra obtained by the present authors were observed in similar regions and a current reversal seen in the TSDC spectrum obtained at the higher irradiation temperature (- I30 C) was also observed by another worker but for material irradiated at above room temperature. The current reversal is most likely to be associated with charge movement in the bulk rather than across the sample surface.
subsequent simultaneous measurement of TL and TSDC. The upper electrode consisted of an annulus in contact with parallel electrode strips which partially filled the area within the annulus (Fig. I). This arrangement enabled the direct irradiation of the polymer surface with electrons from a simple electron gun and allowed the detection of some of the photons produced within or on the sample during the experiments. Compared to a simple annular electrode the arrangement also increased the probability that some charge would decay by surface migration rather than injection into the bulk. This was a consequence of the proximity of a greater fraction of the deposited charge to the electrode as compared to the case of an annular electrode. The lower electrode was a disk of diameter 20 mm, the same diameter as the upper electrode. The electrodes were fabricated by vacuum evaporating aluminium onto opposite surfaces of the nominally additive free low density polyethylene of thickness 75 pm. For measurement purposes, the sample was introduced into a vacuum system where the lower electrode was in contact with a copper surface, which could be both cooled with liquid nitrogen and/or heated using resistance heating. The residual gas pressure was less than 2 x lO-5 torr. The upper surface of the sample was irradiated with low energy electrons (50&l 500 eV) after it had been cooled to a low temperature. During the electron irradiation, the upper electrode was connected to earth through an electrometer. This allowed the flux of electrons arriving at the sample to be monitored. By controlling the
INTRODUCTION
The use of solid state luminescence would appear to be a method of monitoring surface kinetics of reactions occurring on polymers after exposure to corona or glow gas discharge activity (Massines ef al., 1991). These processes can cause surface charging. The irradiation of dielectric surfaces using low energy electrons also causes surface charging with electron penetration limited to the surface region. The decay of such deposited charge, as monitored by thermoluminescence (TL) and thermally stimulated discharge current (TSDC), may provide information on the surface transport of the charge, the transfer of charge from surface to the bulk and the ageing processes occurring to such surfaces. Ageing can be the result of oxidation, exposure to UV light and to gas discharge activity, all factors which may be relevant to the operation of the dielectric material as an electrical insulator. The work reported here describes the results of some preliminary TL and TSDC measurements obtained from a low density polyethylene (LDPE), nominally additive free, after irradiation with I keV electrons at temperatures approaching those of liquid nitrogen.
EXPERIMENTAL
TECHNIQUES
The electrode configuration was dictated by the requirements of a free dielectric surface for irradiation with low energy electrons and the 3
M. J.
Fig. 1. Schematic
of upper
sample
GIVEN et al.
electrode.
filament temperature of the gun the specimens could be irradiated with a constant flux of electrons, as measured by the electrometer. During irradiation, the measured current was maintained at approximately 0.1 PA. The electrometer was also employed after irradiation to measure the current generated when the temperature of the specimen was raised linearly with time, the so called thermally stimulated discharge current (TSDC). Sample temperatures were measured using thermocouples. A photomultiplier was positioned near to the upper surface of the specimen so that any thermoluminescence (TL) from the polymer occurring during the TSDC measurements was also recorded. The photomultiplier was operated in a photon counting mode and the dark count was less than 30 counts per second. During electron irradiation the photomultiplier was protected from light emission from the electron gun filament by a camera shutter. The requirements that the upper free surface of the sample was to be irradiated with electrons and observed with the photomultiplier precluded heating both sample faces directly as no contact with the upper surface would be possible. Heating was thus through the lower specimen face and had the consequence of an increased thermal gradient across the specimen thickness as compared to the more usual methods of heating. Using an inert gas, after irradiation to increase the thermal contact between specimen and heater, was also excluded because of the risk of gas discharges occurring due to the electrical potentials produced in the region of the charged specimen surfaces. A schematic of the experimental configuration is shown in Fig. 2.
RESULTS
Figure 3 shows spectra obtained
irradiated with I keV electrons at - 180 C. As can be seen the current is negative. This is as would be expected if the excess-electron charge transport is along the polymer surface and is therefore collected by the top electrode. The current peak is broad with possible changes occurring at approximately - 140-C and in the region of -30 C where there is a change in slope of the current temperature curve. Figure 4 shows two TSDC spectra obtained when the sample was irradiated at - 130 C, again using 1 keV electrons. Here the form of the curve is different, showing reversal of the sign of the current during the period of the measurement and, in comparison with the lower temperature irradiation, the measured currents are significantly lower. Initially the current is positive, passing through a negative region between approximately - 90 and - 50 C with a second reversal occurring between 20 and 40C. In one case where the measured currents are very low (10m’4A) the signal in the range -40 to +4O~C is very noisy. Peaks in the spectrum occur at -70, and -20’C. The positive current could conceivably be the result of one of several different processes. The first is that during the irradiation an excess of positive holes are produced in the polymer as a result of secondary electron emission. These holes become mobile at a particular temperature, form the predominant charge carrier at this temperature resulting in a positive current and a TSDC peak. If mobile holes are the cause of the positive peak observed when the irradiation is performed at the higher irradiation temperature, their production has to be predominant at this temperature. suggesting an unlikely change of
AND DISCUSSION
two thermally stimulated current from a polyethylene specimen
Fig.
2. Schematic
of the measurement
system
Preliminary
results of current
measurements
on polyethylene
+ ++ + + + ++
+
++
-H+
+
+
. .
+ . . . . +
.
.+
-200
I
I
-160
ml2o
I -80
Thermocouple Fig. 3. Thermally
stimulated
I -40
temperature
I
I
I
I
0
40
X0
120
(“C)
discharge spectra of low density additive - 180 C with I keV electrons.
charging mechanism. There is no obvious reason why this should happen. In addition due to the sample thickness and the electrode spacing it is more likely that holes would travel to the lower electrode producing a negative current in the external circuit. The second possible process arises if some of the electrons deposited on the surface of the polymer are injected into the bulk of the material where they become trapped. D&rapping of these electrons occurs on raising the temperature. Again because of the electrode geometry and specimen thickness it is more likely for these bulk electrons to move towards the underlying electrode rather than the upper electrode. This type of charge motion would be measured as a positive current in the external circuit. If electron injection is occurring from the surface into the bulk to produce a positive current in the external circuit, then such injection would have to be more significant at the higher irradiation temperature in order to explain the differences between the high and low irradiation temperature results. Since surface electrons would be more mobile at the higher temperature it is plausible that electron injection into the bulk would also be more likely to occur. Such injection and trapping can only be significant during the
free polyethylene
irradiated
at
irradiation otherwise current reversal should be observed with samples irradiated at the lower temperature, since injection and trapping in the bulk would also occur when the sample temperature was raised during a TSDC measurement and produce a current reversal as previously described. The third explanation is that both surface and bulk charges are present but that the surface charge in the case of the high temperature irradiation is significantly less than that occurring for the lower irradiation temperature. If, in the former case the charge relaxes more quickly, the behaviour of the underlying bulk trapped charge will become apparent. The lower surface charge density might be expected because of the greater charge mobility at the higher irradiation temperatures. If initially there is a greater surface charge density at the lower irradiation temperature, then there may still be sufficient negative surface charge present in the temperature region where current reversal should be observed for this effect to be obscured by the migrating surface charge effects. Yang (1993) has observed the TSDC behaviour of polypropylene when irradiated either with 5-30 keV monoenergetic electrons or with a corona discharge. For samples in which the short circuit TSDC current
M. J.
6
8
GIVEN
et
al.
= PE-08 + PE-06
=
4
Signal’
++$
+ +
+ 4
2
2
+ .
+
2 iY 2
t
v
+
. t+ t* . .
.
.
+
t
7
0
+
t
++
ft + ++
+ .+
-2
.
.
. -4
. -6 -200
-160
-120
-80
Thermocouple PE-06 PE-08 Fig. 4. Thermally
current current
-40
temperature
0
40
80
120
(“C)
x E 13 x El4
stimulated
discharge spectra of low density additive - 130°C with 1 keV electrons.
was measured it was found that a current reversal peak was observed if the sample was irradiated to high dose levels or if the sample was repeatedly exposed to the radiation. In this case the peak occurred in the region of lOO”C, the temperature of irradiation being room temperature. The explanation given was that two trapping levels existed and that the layer of bulk charge broadens and extends towards the lower electrode. TSDC spectra have been obtained by Chen ef al. (1989) for similar additive free LDPE of 75 pm thickness. In this case the spectra were obtained by polarising the sample at a temperature of 80°C using an applied field of between 2.6 x lo6 and 2 x 10’ V/m followed by rapid cooling to - 100°C prior to raising the temperature linearly with time to obtain a TSDC spectrum. The currents observed were small, little structure was observed in the spectra below 0°C and a peak was observed in the region of 45°C whose behaviour at higher polarisation fields suggested the onset of double charge injection from the electrodes. Measurements of this type can observe the behaviour of intrinsic charge polarisation as well as the polarisation of any dipoles present in the polymer. This is in
free polyethylene
irradiated
at
contrast to the present measurements on extrinsically produced electronic charge. Markiewicz and Fleming (1988) made a series of TL and TSDC measurements using samples of LDPE irradiated with X-rays at liquid nitrogen temperatures with an applied field of 9 x lO”V/m. The radiation produced charge which becomes polarised under the influence of the applied field. The relaxation of bulk charge observed in the TSDC spectra were similar to those of Blake et al. (1974) and showed structure indicating the presence of five peaks, some of which could be associated with the TL spectra. The peaks occurred over the temperature range - 180 to IOC. This behaviour of essentially intrinsically generated bulk carriers contrasts with that of the present work on extrinsic electron behaviour. Blake et al. (1974) had previously made similar combined TL and TSDC measurements but on a high density polyethylene in which the radiation employed to produce free charge was y-rays rather than X-rays. Fluctuating currents were obtained for temperatures below -53’C as was the case under particular circumstances in the present work. Perlman and Unger (1974) obtained TSDC spectra for various polymers in thin foil form irradiated with
Preliminary
results
of current
measurements
7
on polyethylene
400
300
v1 E a v1 ; z 0
200
IO0/
-
0I-201tl
I -160
I -120
I -80 Thermocouple
I -40 temperature
I 0
I
I
I
40
80
120
(“C)
Fig. 5. Thermoluminescence spectrum of low density additive free polyethylene irradiated at - 180-C with I keV electrons.
typically 6 keV electrons at various sample temperatures in the range of 25 to 190°C depending on the polymer. The spectra obtained for the decay of these extrinsic electrons were broad and the peak position was dependent on the irradiating temperature. The general shape of the spectra were similar to that obtained for samples irradiated at - 180°C in the present work. Figure 5 shows the thermoluminescence (TL) spectrum obtained in the present work after the polymer had been irradiated at - 180°C. TL and TSDC spectra were obtained from simultaneous measurements on the same sample. There are two peaks, one at - 140°C when corrected for the non-linearity of the thermocouple temperature scale at low temperatures and the other at -20°C. From the position of these peaks the corresponding activation energies would be ~0.3 and ~0.5 eV respectively. The spectrum obtained after irradiation at the
higher temperature of - 130°C is shown in Fig. 6. Again simultaneous measurements of TL and TSDC spectra were obtained from the same sample. Since the irradiation temperature in this case was above that of the lower temperature peak previously observed only one peak would be charged. However positive correlation between the peak position measured by the present authors and others requires some detailed consideration of the measurement of sample temperature in order to come to a definitive conclusion. Two problems arise when the results produced by different authors are compared. These are the calibration of temperature measurements at the specimen and the effect of temperature gradients which are invariably present within the sample. The latter will vary depending on the experimental system and sample thickness and have been recently addressed by Betts ef al. (1993) and by Betts and Townsend (1993).
M. J.
GIVEN
et
al.
350
. 300
.
.
.
. .
. .
250
. . m z a y1 z
.
. 200
..
.
.
.
. . .
: 0
. ..
150
.
. .
. .
. . 100
.
: .
Cmm . .
.
. 50
-200
I
I
-160
-120
I -80
Thermocouple Fig. 6. Thermoluminescence
spectrum
I
I
-40
0
temperature
of low density additive 1 keV electrons.
REFERENCES Betts D. S. and Townsend P. D. (1993) Temperature distribution in thermoluminescence experiments--II. J. Phys. D 26, 849. Betts D. S., Coutier L., Khayrat A. H., Luff B. J. and Townsend P. D. (1993) Temperature distribution in thermoluminescence experiments--I. J. Phys. D 26, 843. Blake A. E., Charlesby A. and Randle (1974) Simultaneous thermoluminescence and thermally stimulated discharge current in polyethylene. J. Phys. D 7, 759. Chen G., Banford H. M., Fouracre R. A. and Tedford D. J. (1989) Electrical conduction in low density
I
I
I
40
80
120
(“C)
free polyethylene
irradiated
at - 130°C with
polyethylene. 3rd Int. Conf. Conducrion and Breakdown in Solid Dieleclrics, p 277, Trondheim, Norway. Markiewicz A. and Fleming R. J. (1988) Simultaneous thermally stimulated luminescence and conductivity in low density polyethylene. J. Phys. D: 21, 349. Massines F., Mary D., Laurent C. and Mayoux C. (1991) Luminescence of plasma treated polymer surfaces: a tool for investigating surface modifications J. Phys. D: 26, 493. Perlman M. M. and Unger S. (1974) Electron bombardment of electret foils. J. Appl. Phys. Left. 24, 579. Yang G. M. (1993) Thermally stimulated discharge of electron beam and corona charged polypropylene films. J. Phys. D: 26, 690.
Pergamon
I, pp.3-X. I995
Copyright ,’ 1994 Elsevier Science Ltd Prmred in Greal Britain. All rIghta reserved 0969-X06X/95 17.00 + 0.00
0969-806X(94)EOO16-C
PRELIMINARY RESULTS OF COMBINED THERMO-LUMINESCENCE AND THERMALLY STIMULATED CURRENT MEASUREMENTS ON AN ADDITIVE FREE POLYETHYLENE M. J. Department
of Electronic
GIVEN,
and Electrical
R.
A. FOURACRE
and D. J.
Engineering, University Gl IXW, U.K.
TEDF~RD
of Strathclyde,
George
Street. Glasgow
Abstract-Samples of additive free low density polyethylene have been irradiated at - I80 and - 130 C with I keV electrons. The sample arrangement was such that simultaneous measurements of thermally stimulated discharge currents (TSDC) and luminescence (TL) were possible and these techniques were used to observe effects of surface charge relaxation. The resulting spectra were similar in some respects to those obtained by other workers using either higher electron energies, which create bulk charges, and higher irradiation temperatures. or using X- or g-radiation to create internal charge. In particular some of the TL peaks in the spectra obtained by the present authors were observed in similar regions and a current reversal seen in the TSDC spectrum obtained at the higher irradiation temperature (- I30 C) was also observed by another worker but for material irradiated at above room temperature. The current reversal is most likely to be associated with charge movement in the bulk rather than across the sample surface.
subsequent simultaneous measurement of TL and TSDC. The upper electrode consisted of an annulus in contact with parallel electrode strips which partially filled the area within the annulus (Fig. I). This arrangement enabled the direct irradiation of the polymer surface with electrons from a simple electron gun and allowed the detection of some of the photons produced within or on the sample during the experiments. Compared to a simple annular electrode the arrangement also increased the probability that some charge would decay by surface migration rather than injection into the bulk. This was a consequence of the proximity of a greater fraction of the deposited charge to the electrode as compared to the case of an annular electrode. The lower electrode was a disk of diameter 20 mm, the same diameter as the upper electrode. The electrodes were fabricated by vacuum evaporating aluminium onto opposite surfaces of the nominally additive free low density polyethylene of thickness 75 pm. For measurement purposes, the sample was introduced into a vacuum system where the lower electrode was in contact with a copper surface, which could be both cooled with liquid nitrogen and/or heated using resistance heating. The residual gas pressure was less than 2 x lO-5 torr. The upper surface of the sample was irradiated with low energy electrons (50&l 500 eV) after it had been cooled to a low temperature. During the electron irradiation, the upper electrode was connected to earth through an electrometer. This allowed the flux of electrons arriving at the sample to be monitored. By controlling the
INTRODUCTION
The use of solid state luminescence would appear to be a method of monitoring surface kinetics of reactions occurring on polymers after exposure to corona or glow gas discharge activity (Massines ef al., 1991). These processes can cause surface charging. The irradiation of dielectric surfaces using low energy electrons also causes surface charging with electron penetration limited to the surface region. The decay of such deposited charge, as monitored by thermoluminescence (TL) and thermally stimulated discharge current (TSDC), may provide information on the surface transport of the charge, the transfer of charge from surface to the bulk and the ageing processes occurring to such surfaces. Ageing can be the result of oxidation, exposure to UV light and to gas discharge activity, all factors which may be relevant to the operation of the dielectric material as an electrical insulator. The work reported here describes the results of some preliminary TL and TSDC measurements obtained from a low density polyethylene (LDPE), nominally additive free, after irradiation with I keV electrons at temperatures approaching those of liquid nitrogen.
EXPERIMENTAL
TECHNIQUES
The electrode configuration was dictated by the requirements of a free dielectric surface for irradiation with low energy electrons and the 3
M. J.
Fig. 1. Schematic
of upper
sample
GIVEN et al.
electrode.
filament temperature of the gun the specimens could be irradiated with a constant flux of electrons, as measured by the electrometer. During irradiation, the measured current was maintained at approximately 0.1 PA. The electrometer was also employed after irradiation to measure the current generated when the temperature of the specimen was raised linearly with time, the so called thermally stimulated discharge current (TSDC). Sample temperatures were measured using thermocouples. A photomultiplier was positioned near to the upper surface of the specimen so that any thermoluminescence (TL) from the polymer occurring during the TSDC measurements was also recorded. The photomultiplier was operated in a photon counting mode and the dark count was less than 30 counts per second. During electron irradiation the photomultiplier was protected from light emission from the electron gun filament by a camera shutter. The requirements that the upper free surface of the sample was to be irradiated with electrons and observed with the photomultiplier precluded heating both sample faces directly as no contact with the upper surface would be possible. Heating was thus through the lower specimen face and had the consequence of an increased thermal gradient across the specimen thickness as compared to the more usual methods of heating. Using an inert gas, after irradiation to increase the thermal contact between specimen and heater, was also excluded because of the risk of gas discharges occurring due to the electrical potentials produced in the region of the charged specimen surfaces. A schematic of the experimental configuration is shown in Fig. 2.
RESULTS
Figure 3 shows spectra obtained
irradiated with I keV electrons at - 180 C. As can be seen the current is negative. This is as would be expected if the excess-electron charge transport is along the polymer surface and is therefore collected by the top electrode. The current peak is broad with possible changes occurring at approximately - 140-C and in the region of -30 C where there is a change in slope of the current temperature curve. Figure 4 shows two TSDC spectra obtained when the sample was irradiated at - 130 C, again using 1 keV electrons. Here the form of the curve is different, showing reversal of the sign of the current during the period of the measurement and, in comparison with the lower temperature irradiation, the measured currents are significantly lower. Initially the current is positive, passing through a negative region between approximately - 90 and - 50 C with a second reversal occurring between 20 and 40C. In one case where the measured currents are very low (10m’4A) the signal in the range -40 to +4O~C is very noisy. Peaks in the spectrum occur at -70, and -20’C. The positive current could conceivably be the result of one of several different processes. The first is that during the irradiation an excess of positive holes are produced in the polymer as a result of secondary electron emission. These holes become mobile at a particular temperature, form the predominant charge carrier at this temperature resulting in a positive current and a TSDC peak. If mobile holes are the cause of the positive peak observed when the irradiation is performed at the higher irradiation temperature, their production has to be predominant at this temperature. suggesting an unlikely change of
AND DISCUSSION
two thermally stimulated current from a polyethylene specimen
Fig.
2. Schematic
of the measurement
system
Preliminary
results of current
measurements
on polyethylene
+ ++ + + + ++
+
++
-H+
+
+
. .
+ . . . . +
.
.+
-200
I
I
-160
ml2o
I -80
Thermocouple Fig. 3. Thermally
stimulated
I -40
temperature
I
I
I
I
0
40
X0
120
(“C)
discharge spectra of low density additive - 180 C with I keV electrons.
charging mechanism. There is no obvious reason why this should happen. In addition due to the sample thickness and the electrode spacing it is more likely that holes would travel to the lower electrode producing a negative current in the external circuit. The second possible process arises if some of the electrons deposited on the surface of the polymer are injected into the bulk of the material where they become trapped. D&rapping of these electrons occurs on raising the temperature. Again because of the electrode geometry and specimen thickness it is more likely for these bulk electrons to move towards the underlying electrode rather than the upper electrode. This type of charge motion would be measured as a positive current in the external circuit. If electron injection is occurring from the surface into the bulk to produce a positive current in the external circuit, then such injection would have to be more significant at the higher irradiation temperature in order to explain the differences between the high and low irradiation temperature results. Since surface electrons would be more mobile at the higher temperature it is plausible that electron injection into the bulk would also be more likely to occur. Such injection and trapping can only be significant during the
free polyethylene
irradiated
at
irradiation otherwise current reversal should be observed with samples irradiated at the lower temperature, since injection and trapping in the bulk would also occur when the sample temperature was raised during a TSDC measurement and produce a current reversal as previously described. The third explanation is that both surface and bulk charges are present but that the surface charge in the case of the high temperature irradiation is significantly less than that occurring for the lower irradiation temperature. If, in the former case the charge relaxes more quickly, the behaviour of the underlying bulk trapped charge will become apparent. The lower surface charge density might be expected because of the greater charge mobility at the higher irradiation temperatures. If initially there is a greater surface charge density at the lower irradiation temperature, then there may still be sufficient negative surface charge present in the temperature region where current reversal should be observed for this effect to be obscured by the migrating surface charge effects. Yang (1993) has observed the TSDC behaviour of polypropylene when irradiated either with 5-30 keV monoenergetic electrons or with a corona discharge. For samples in which the short circuit TSDC current
M. J.
6
8
GIVEN
et
al.
= PE-08 + PE-06
=
4
Signal’
++$
+ +
+ 4
2
2
+ .
+
2 iY 2
t
v
+
. t+ t* . .
.
.
+
t
7
0
+
t
++
ft + ++
+ .+
-2
.
.
. -4
. -6 -200
-160
-120
-80
Thermocouple PE-06 PE-08 Fig. 4. Thermally
current current
-40
temperature
0
40
80
120
(“C)
x E 13 x El4
stimulated
discharge spectra of low density additive - 130°C with 1 keV electrons.
was measured it was found that a current reversal peak was observed if the sample was irradiated to high dose levels or if the sample was repeatedly exposed to the radiation. In this case the peak occurred in the region of lOO”C, the temperature of irradiation being room temperature. The explanation given was that two trapping levels existed and that the layer of bulk charge broadens and extends towards the lower electrode. TSDC spectra have been obtained by Chen ef al. (1989) for similar additive free LDPE of 75 pm thickness. In this case the spectra were obtained by polarising the sample at a temperature of 80°C using an applied field of between 2.6 x lo6 and 2 x 10’ V/m followed by rapid cooling to - 100°C prior to raising the temperature linearly with time to obtain a TSDC spectrum. The currents observed were small, little structure was observed in the spectra below 0°C and a peak was observed in the region of 45°C whose behaviour at higher polarisation fields suggested the onset of double charge injection from the electrodes. Measurements of this type can observe the behaviour of intrinsic charge polarisation as well as the polarisation of any dipoles present in the polymer. This is in
free polyethylene
irradiated
at
contrast to the present measurements on extrinsically produced electronic charge. Markiewicz and Fleming (1988) made a series of TL and TSDC measurements using samples of LDPE irradiated with X-rays at liquid nitrogen temperatures with an applied field of 9 x lO”V/m. The radiation produced charge which becomes polarised under the influence of the applied field. The relaxation of bulk charge observed in the TSDC spectra were similar to those of Blake et al. (1974) and showed structure indicating the presence of five peaks, some of which could be associated with the TL spectra. The peaks occurred over the temperature range - 180 to IOC. This behaviour of essentially intrinsically generated bulk carriers contrasts with that of the present work on extrinsic electron behaviour. Blake et al. (1974) had previously made similar combined TL and TSDC measurements but on a high density polyethylene in which the radiation employed to produce free charge was y-rays rather than X-rays. Fluctuating currents were obtained for temperatures below -53’C as was the case under particular circumstances in the present work. Perlman and Unger (1974) obtained TSDC spectra for various polymers in thin foil form irradiated with
Preliminary
results
of current
measurements
7
on polyethylene
400
300
v1 E a v1 ; z 0
200
IO0/
-
0I-201tl
I -160
I -120
I -80 Thermocouple
I -40 temperature
I 0
I
I
I
40
80
120
(“C)
Fig. 5. Thermoluminescence spectrum of low density additive free polyethylene irradiated at - 180-C with I keV electrons.
typically 6 keV electrons at various sample temperatures in the range of 25 to 190°C depending on the polymer. The spectra obtained for the decay of these extrinsic electrons were broad and the peak position was dependent on the irradiating temperature. The general shape of the spectra were similar to that obtained for samples irradiated at - 180°C in the present work. Figure 5 shows the thermoluminescence (TL) spectrum obtained in the present work after the polymer had been irradiated at - 180°C. TL and TSDC spectra were obtained from simultaneous measurements on the same sample. There are two peaks, one at - 140°C when corrected for the non-linearity of the thermocouple temperature scale at low temperatures and the other at -20°C. From the position of these peaks the corresponding activation energies would be ~0.3 and ~0.5 eV respectively. The spectrum obtained after irradiation at the
higher temperature of - 130°C is shown in Fig. 6. Again simultaneous measurements of TL and TSDC spectra were obtained from the same sample. Since the irradiation temperature in this case was above that of the lower temperature peak previously observed only one peak would be charged. However positive correlation between the peak position measured by the present authors and others requires some detailed consideration of the measurement of sample temperature in order to come to a definitive conclusion. Two problems arise when the results produced by different authors are compared. These are the calibration of temperature measurements at the specimen and the effect of temperature gradients which are invariably present within the sample. The latter will vary depending on the experimental system and sample thickness and have been recently addressed by Betts ef al. (1993) and by Betts and Townsend (1993).
M. J.
GIVEN
et
al.
350
. 300
.
.
.
. .
. .
250
. . m z a y1 z
.
. 200
..
.
.
.
. . .
: 0
. ..
150
.
. .
. .
. . 100
.
: .
Cmm . .
.
. 50
-200
I
I
-160
-120
I -80
Thermocouple Fig. 6. Thermoluminescence
spectrum
I
I
-40
0
temperature
of low density additive 1 keV electrons.
REFERENCES Betts D. S. and Townsend P. D. (1993) Temperature distribution in thermoluminescence experiments--II. J. Phys. D 26, 849. Betts D. S., Coutier L., Khayrat A. H., Luff B. J. and Townsend P. D. (1993) Temperature distribution in thermoluminescence experiments--I. J. Phys. D 26, 843. Blake A. E., Charlesby A. and Randle (1974) Simultaneous thermoluminescence and thermally stimulated discharge current in polyethylene. J. Phys. D 7, 759. Chen G., Banford H. M., Fouracre R. A. and Tedford D. J. (1989) Electrical conduction in low density
I
I
I
40
80
120
(“C)
free polyethylene
irradiated
at - 130°C with
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