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Field assisted thermally stimulated current in low-density polyethylene
Journal of Electrostatics, 18 (1986) 233--241
233
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
FIELD ASSISTED THERMALLY STIMULATED CURRENT IN LOW-DENSITY POLYETHYLENE
D.K. DAS-GUPTA, S. NOEL*
School of Electronic Engineering Sciences, University College of North Wales, Bangor, Gwynedd, LL57 1 UT (Great Britain) and D.E. COOPER
Southern California Edison Company, Research and Development Department, P.O. Box 806, 2244 Walnut Grove, Rosemead, CA 917700 (U.S.A.) (Received April 22, 1985; accepted in revised form December 8, 1985)
Summary The nature and quantity of release of injected charges in electrically pre-stressed lowdensity polyethylene (LDPE) have been studied by a technique in which the discharge current is monitored in the presence of a small drawing voltage while the temperature is slowly increased. Dielectric relaxation strength of (i) virgin, (ii) heat treated and (iii) heat-treated and electrically stressed LDPE have also been measured. The results indicate that at a charging field of 5.3 × 107 V m -1 and at a saturation drawing field of 2 × 106 V m -~ the injected charge density is significantly less than the initial defect and/or impurity density present in the virgin samples used in the present work.
Introduction The relevance of charge injection in insulating polymers on application of an external field is well established [1--3]. The injected charges m a y subsequently be localised, thus providing a persistent polarisation,even in a non-polar semi-crystalline polymer a long time after the removal of the polarizing field. This effect is important in the preparation of electrets for various applications but m a y also have a degrading effect on the insulation properties of the material due to the build-up of high local space charge fields which might induce dielectricbreakdown. Commercially available low density polyethylene ( L D P E ) m a y contain a defect density (localised energy states) [4] of 1019 c m -3 and also unsaturated vinyl groups [5] which can trap injected electrons or impurity ions. It has also been suggested [6] that the observed electroluminescence in polyethylene in the presence of highly divergent 50 Hz a.c. electrical *Present address: Laboratoire de Genie Electrique, E.S.E., Plateau du Moulon, Gif sur Yvette 91190 (France).
0304-3886/86/$03.50
© 1986 Elsevier Science Publishers B.V.
234
stress (point--plane geometry) is due to a recombination of injected carriers at deep trapping centres. The measurement of trapped or injected charges is of the utmost importance in characterising insulations and in determining the breakdown limits of such materials due to internal fields. However, no satisfactory technique exists for measuring the magnitude of this charge. The thermally stimulated discharge current method (TSDC) in which a previously stressed sample is short-circuited through an electrometer while its temperature is slowly increased to a few degrees below its melting point may be useful in providing information on trapping parameters and internal processes such as relaxations and transitions. However, the integrated charge measured in such an experiment may significantly underestimate the actual trap-release of space charges due to the cancellation of charges flowing to opposite electrodes and due to the recombination of ionic species. Indeed, if the polarisation was spatially uniform then no TSDC should be observed in the external circuit as equal numbers of charges would flow to each electrode. In the present work we have attempted to measure the injected charge in LDPE b y a variation of the conventional TSDC m e t h o d (FTSC -- Field and Temperature Stimulated Currents) in which the discharge current, following high field charging, is recorded in the presence of a small drawing voltage, VD, (small in comparison to the polarizing voltage) while the temperature is slowly increased [7--9]. In this way the thermally released charges, either electronic or ionic, may drift preferentially in the direction of this drawing field and will be collected at one electrode. Integration of the charges collected in this way, after subtraction of the temperature dependent conduction charges, will reveal the total trapped charge. We have determined the range of drawing potentials necessary in order to collect the majority of the charges and determine the average spatial depth of the injected charge. We have also estimated the number of defect centres and the charge trapped in the sample initially by measuring the dielectric increment before and after various conditioning processes such as heat treatment and electrical stressing. A comparison of the experimentally determined charge density and the calculated initial number of defect sites may then demonstrate whether injected charges of defects are dominant in commercially available LDPE. Experimental LDPE samples (Monsanto, Type M 301) of 75 um thickness were used in this present work. Aluminium electrodes were vacuum deposited on the sample surfaces, electrode thickness and area being 500 A and 6.25 cm 2 respectively. The electroded sample was located in a stainless-steel sample chamber under short circuit conditions for 15 hours at a reduced pressure
235
of 10 -s Tort at 70°C. The sample was then electrically stressed at +4 kV for 75 minutes at room temperature in the evacuated sample chamber. The electrodes of the sample were th~n short circuited again for 30 minutes at the same temperature. Subsequently the sample temperature was reduced to 0°C in the evacuated chamber. A small d.c. drawing voltage was applied to the sample which was then heated at a uniform rate of 1°C per minute to 70°C. The electrodes of the depolarised sample were short circuited again and the sample temperature was lowered to 0°C for the second time. The thermally stimulated current was again measured with the same drawing field as before while the sample temperature was raised again at the same uniform rate of 1°C per minute to 70°C. All measurements were made at a reduced pressure of 10 -s Tort. The experimental procedure is schematically represented [10] in Fig. 1. Above measurements were repeated with LDPE samples, all pre-stressed at 4 kV (d.c.) for different drawing fields not exceeding -+ 150 V (d.c.) in steps of + 10 V. T(°C )
7o T,
Or
12
H
,[. . . . .
I
,.:l
20
'800 I
8 7 5 96"5',950
i i t
7 5 minutes (v) I
,
1020 minutes
i i
i
I ,
V~ I
T {r~m)
Fig. 1. S c h e m a t i c r e p r e s e n t a t i o n o f t h e e x p e r i m e n t .
In addition, the effect of initial conditioning of the sample with heat t r e a t m e n t at 70°C, as described above, and also the effect of subsequent application of an electrical stress have been investigated in this work by measurements of sample capacitance :at two different frequencies, i.e. 10 Hz and l 0 s Hz using General Radio Bridge (Type 1621 with Type 1238 detector and Type 1616 oscillator). Kesults and discussion The resolved TSDC curve " c " of Fig. 2 was obtained by subtracting the FTSC values o f curve " b " from the corresponding values of curve " a " of a pre-stxessed (+ 4 kV d.c.) LDPE sample with + 30 V drawing voltage. A representative set o f similar resolved TSDC curves are shown in Fig. 3 for a few of the drawing voltages emplo~red in this work. It may be observed
236
that there is only one resolved TSDC peak (Fig. 3) at 65°C of one polatiry only.
The origin of thermally stimulated current peaks has usually been attributed to heterocharge motion and depolarization due to dipolar reorientation and/or a motion of homocharges, injected from electrodes or ions distributed non-uniformly, the latter being the more dominant mechanism at high temperatures [11--16]. A thermally stimulated current peak in the temperature region observed in the present work, has also been reported loi
/
a
'0
x 5
0
30
40
50
60
70
T('C)
Fig. 2. Methods of resolving space charge TSDC peaks by FTSC technique, a -- First run; thermally stimulated current after poling but with low bias field; b -- Second run; thermally stimulated current o f the depolarized sample after first run but with the low bias field as in (a); c -- Difference between the first and the second run, i.e. resolved thermally stimulated current spectrum. i
3
/ " ~ V D .
I
i VD= * 5 0 V
~
t
I
I
I
t
' ~
' t vo:
I
t
601
,
I
I
2
~. F
1
Y "
'o i
x
20
÷150V
~
¢/~
:,
~: -1
80
VD : - 3 0 V I I
I I
-2
VD : - 5 0 V I
~
-3 3. Resolved
V
J
~
o
Fig.
*30
I
TSDC
spectra
I
V
D:
-150V
I
for various
drawing
voltages.
237 b y other workers [7--9, 17, 18] and it has been attributed to a release of trapped charges. It is of interest to note that the thermally stimulated current peaks in the temperature range of 70°C to 145°C o f corona charged polypropylene have also been attributed to charge carriers released b y molecular motion from traps [ 2 0 ] . A simultaneous measurement of thermally stimulated current peak [20] and thermoluminescence also shows a peak in this temperature region with irradiated polyethylene. These workers also interpret their results in terms of non-geminate electron motion. Thus the observed broad peak at 65°C (Fig. 3) may be attributed to a motion of negatively charged carriers, released from localised states in the polymer. It was found that a small thermally stimulated discharge current could be obtained even without a drawing voltage i.e. V D = 0. This TSDC was basically of the same form as the resolved spectra drawn in Fig. 3 with a peak at a b o u t 65°C which demonstrates the validity of the present technique in producing resolved spectra. It should be stated that thermally stimulated current peaks should be observable even without the presence of any small external drawing field provided that either there is a non-uniform distribution of charges or that there is a spatial temperature gradient in the sample at all times. In the present work adequate considerations were given in the design of the experimental arrangement to ensure that both the sample faces are at the same temperature in the steady state conditions throughout the range of temperature excursions for the TSDC experiment. A better estimate of the thermally released charges may, however, be obtained with FTSC than with TSDC technique. Present observation of the resolved TSDC peak with V D = 0 indicate that the charge carriers are mostly negatively charged and that they are non-uniformly distributed. The quantity of charge released Q in a thermally stimulated current spectrum may be obtained from the resolved TSDC curves (Fig. 3) thus, t2 T2 Q= f I d t = f I ( d t / d T ) d T t1 TI
(1)
where I is the current, T the temperature and (dt/dT) the inverse of the heating rate. The relaxation process involving the peak at 65°C has been assumed to include charges of one polarity only for both directions of the drawing field. The range of integration extends from T1 = 30°C to T2 = 70°C and the results are shown in Fig. 4 from which it may be observed that the charges released {i.e. Q-values) reach an upper limit as the drawing voltage is increased. The total charge released QT, for any drawing voltage VD, is, of course, given b y QT = Q1 + Q2
(2)
where QI and Q2 refer to released charges with positive and negative polarities of the drawing voltage VD, respectively. With the present technique
238
(FTSC) the efficiency of determining QT will, of course, improve with increasing values of the drawing voltage until a saturation level of QT occurs where space charge limited conduction may occur [9, 17]. It may be obs e r v e d f r 0 m Fig. 4 that QT reaches a saturation value at a drawing voltage of 100 V, thus indicating a space charge limited conduction. Above observation is in good agreement with previous work [21--23].
x/ x
u
,
i
©
/
× 0
!Q~s i
x
i r m _ a . Z
-150
-1 0
-50
×/!
__
E
5O
100
' V
×//-I i -2i×
x Lx----~x/
Fig. 4. Charge released for different drawing voltages.
The average position of the trapped charges within the thickness of the sample can be estimated from the relative magnitudes of the two saturation charges Qls and Q2s. The normalised spatial depth, d, of the localised states in the bulk is given b y [9] Qls/(Qls + Q2s)
(3)
where Qls and Q2s refer to the saturation values of Q1 and Q2 respectively. A value of d = 0 indicates the position of the cathode, and d = 1 indicates the position of the anode. Using the saturation values of Qls and Q2s from Fig. 4, d appears to be 0.47 which would indicate only a small a s y m m e t r y in the location of trapped charges under the present charging condition, the charge concentration being slightly nearer to the cathode than to the anode. This result is consistent with the analysis of thermal transients [11] which indicated that charge density became more uniform at fields > 3 X 107 V/m (c.f. 5.33 X 107 V/m in the present case). It may also be observed from Fig. 4 that the magnitude of total (saturation) charge released by this technique, which m a y be an indication of the injected charge, is 8 X 10 -8 C. Assuming an electronic charge of 1.6 X 10 -19 C, then this corresponds to an injection of 5 X 101~ electrons. For a sample of electroded area = 6-25 cm 2 and thickness 75/~m the calculated electron density is only 1019/m s. It is of interest to note that the dielectric relaxation strength is given
239 le s - - e , I where e s is the "effective static permittivity" arising from both dipolar and space charge contributions and e . the high frequency permittivity. The results of the capacitance measurement of the sample (i) before heat treatment, (ii) after heat treatment and (iii) after an application of an electrical stress to the heat treated sample at 10 Hz and l 0 s Hz are given in Table 1. It may be observed (see Table 1) t h a t the respective values of the sample capacitances at the two different frequencies were r e d u c e d on heat treatment and further reduced on subsequent electrical stressing, thus providing reductions in the dielectric relaxation strength in comparison with t h a t of the virgin sample. by
TABLE 1 Values o f sample c a p a c i t a n c e and e s - - e ~ o f (i) virgin, (ii) h e a t t r e a t e d and (iii) heatt r e a t e d a n d electrically stressed (--4 k V ) L D P E Sample capacitance (X 10 -9 F )
Virgin s a m p l e Heat-treated sample H e a t - t r e a t e d and electrically stressed
e~ (10 Hz)
e~ (10 s Hz)
e ~- - e Dielectric r e l a x a t i o n
10 Hz value
10 s Hz value
strength
2.7261
2.6171
2.310
2.218
0.092
2.6748
2.5932
2.267
2.198
0.069
2.5791
2.5070
2.186
2.125
0.061
Now the dielectric relaxation strength and the "effective" dipole density N, are related thus N~ 2
e s --e. = ~
3 k T eo
(4)
where ~ is the "effective" dipole m o m e n t , k the Boltzmann's constant and e0 the permittivity of free space. Assuming that the "effective" dipole m o m e n t p, of extruded commercially available polyethylene [24] which may contain impurity ions, is 10 -29 C m and using the value of 0.092 as the dielectric relaxation strength of the virgin sample (see Table 1), the effective density N in the present work is f o u n d to be 1.0 × 1025 m -3 which is in reasonable agreement with the results of other workers [24]. This estimate is six orders of magnitude greater than that of the injected space charges and clearly will dominate the dielectric and electrical behaviour of the sample unless the polarizing field is substantially increased, or applied for a longer time, or applied at an elevated temperature. Indeed the value of the dielectric increment measured in this way may be a lower
240 estimate since the 10 Hz value of capacitance will be less than the true static value. Hence the defect density calculated in eqn. {4) may also be a lower estimate. Nevertheless, this value is also in agreement with the concentration density of polar groups which may exist in LDPE [25]. On this basis it may be stated that the present practice of heat treatment alone and an appropriate electrical stressing {i.e. field cleaning) following the heat treatment reduce the original "effective" dipole density of the virgin sample b y 25% and 34% respectively. The latter observation is also in agreement with previous work [11, 2 6 ] , in which it has been shown that with increasing charging potential there is a progressive field cleaning effect which may be attributed to the impurity ions being driven out or neutralized by recombination with the injected carriers. At still higher fields the blocking electrode effects due to impurity ions are removed and the charge injection becomes more dominant. However, such injected charges may get localized in deep traps from which they may be released at high temperatures. It is also established that heat treatment (or annealing) may produce increases in the various physical dimensions within the spherulitic structure of LDPE. These dimensions include the average crystallite size, the lamellar thickness and the actual crystallinity [19, 27, 2 8 ] . It has been reported that the polymer chain-ends that lie on the surfaces of lamellae move into the crystalline regions at high temperatures [29]. Such chain ends are polar in nature and their interaction with an electric field can contribute significantly to dielectric loss measurement. Hence, it may be suggested that the observed reduction in dielectric relaxation strength after heat treatment may be due to a general increase in the crystallinity of the polymer (or a decrease in the amorphous content). It is, of course, possible that a similar improvement in electric strength of cross-linked polyethylene (XLPE) may occur due to heat treatment which may improve the ageing characteristics of high voltage power transmission cables. Further work is in progress aimed at determining the magnitude and relative position of injected charges in XLPE and developing techniques to improve the loss characteristics of such insulations in service. Conclusions It has been shown that the Field and Temperature Stimulated Current technique can be used to study the charge release from an insulating p o l y m e r such as LDPE after polarisation in a high d.c. field. It may also be employed to estimate the magnitude of the injected charges and their average relative position within the bulk of the sample. In LDPE at room temperature and for a field of 5.33 X 107 V/m a charge of 8 X 10 -8 C is injected which provides an electron density significantly less than the initial defect (or impurity) density present in the virgin samples as revealed by measurements of the dielectric increment. Dielectric ageing due to injected charges may
241
only become relevant at applied stress :>:> 107 V/m though metallic inclusions and inhomogeneous samples may well produce highly divergent fields locally where accelerated ageing may occur.
Acknowledgement This work is financed by Rosemead California, U.S.A. the Royal Society of Great European Science Exchange Doughty for many valuable
the Southern California Edison Company of One of the authors (S.N.) is also grateful to Britain for a Research Fellowship under the Programme. It is a pleasure to thank Dr. K. discussions during the progress of this work.
References 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
D.K. Davies, J. Sci. Instrum., 44 (1967) 521. D.K. Davies, Br. J. Appl. Phys., 2 (1969) 1017. D.K. Davies, J. Phys. D: Appl. Phys., 6 (1973)1017. D.K. Davies, J. Phys. D: Appl. Phys., 5 (1972) 162. D.K. Davies and P.J. Lock, J. Electrochem. Soc., 6 (1973) 1371. C. Laurent, C. Mayoux and S. Noel, J. Appl. Phys., 54 (1983) 1532. G. Sawa, M. Kawade and M. Ieda, J. Appl. Phys., 44 (1973) 5397. G. Sawa, M. Kawade, D.C. Lee and M. Ieda, Jpn. J. Appl. Phys., 13 (1974) 1547. T. Hino, IEEE Trans. Electr. Insul., EI-15 (1980) 301. S. Noel, D.K. Das-Gupta and D.E. Cooper, Annual Report, Conference on Electrical Insulation & Dielectric Phenomena (CEIDP), 1983, 263. D.K. Das-Gupta, J.S. Duffy and D.E. Cooper, J. Electrostat., 14 (1983) 99. M.M. Perlman, Electrochem. Tech., 6 (1968) 95. R.A. Creswell and M.M. Perlman, J. Appl. Phys., 41 (1970) 2365. M.L. Miller, J. Polym. Sci., A-2, 4 (1966) 685. F.E. Karasz, Dielectric Properties of Polymer, Plenum, New York, 1972, p. 295. J. van Turnhout, in G.M. Sessler (Ed.), Topics in Applied Physics, Vol. 33, Electrets, Springer-Verlag, Berlin, 1980, pp. 81--215. T. Mizutani, T. Oomura and M. Ieda, Jpn. J. Appl. Phys., 21 (1982) 1195. T. Hashimoto, M. Shiraki and T. Sakai, J. Polym. Sci., Polym. Phys. Ed., 13 (1975) 2401. A. Baba and K. Ikezaki, J. Appl. Phys., 57 (1985) 359. A.E. Blake, A. Charlesby and K.J. Randle, Polym. Lett., 11 (1973) 83. D.K. Das-Gupta and M.K. Barbarez, J. Phys. D: Appl. Phys., 6 (1973) 867. D.K. Das-Gupta, M.E. TindeU, R.D. Wingrove and K. Joyner, Electrostatische Aufladung, Dechema Monogr., 72 (1973) 73. R.S. Brockley, M.Sc. Dissertation, University of Wales, 1976. R.W. Greaves, E.P. Fowler and A. Goodings, J. Mater. Sci., 9 (1974) 1602. L. Alexander, X-ray Diffraction Methods in Polymer Science, Interscience, New York, 1969. D.K. Das-Gupta, K. Doughty, J.S. Duffy and D.E. Cooper, J. Electrostat., 14 (1983) 165. A. Peterlin, J. Polym. Sci. C., 9 (1965) 61. G.S.Y. Yeh, R. Hoseman, J.L. Loboda-Cackovic and H. Cackovic, Polymer, 17 (1976) 309. M. Sakaguchi and H. Kashiwara, Polymer, 23 (1982) 1594.
233
Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands
FIELD ASSISTED THERMALLY STIMULATED CURRENT IN LOW-DENSITY POLYETHYLENE
D.K. DAS-GUPTA, S. NOEL*
School of Electronic Engineering Sciences, University College of North Wales, Bangor, Gwynedd, LL57 1 UT (Great Britain) and D.E. COOPER
Southern California Edison Company, Research and Development Department, P.O. Box 806, 2244 Walnut Grove, Rosemead, CA 917700 (U.S.A.) (Received April 22, 1985; accepted in revised form December 8, 1985)
Summary The nature and quantity of release of injected charges in electrically pre-stressed lowdensity polyethylene (LDPE) have been studied by a technique in which the discharge current is monitored in the presence of a small drawing voltage while the temperature is slowly increased. Dielectric relaxation strength of (i) virgin, (ii) heat treated and (iii) heat-treated and electrically stressed LDPE have also been measured. The results indicate that at a charging field of 5.3 × 107 V m -1 and at a saturation drawing field of 2 × 106 V m -~ the injected charge density is significantly less than the initial defect and/or impurity density present in the virgin samples used in the present work.
Introduction The relevance of charge injection in insulating polymers on application of an external field is well established [1--3]. The injected charges m a y subsequently be localised, thus providing a persistent polarisation,even in a non-polar semi-crystalline polymer a long time after the removal of the polarizing field. This effect is important in the preparation of electrets for various applications but m a y also have a degrading effect on the insulation properties of the material due to the build-up of high local space charge fields which might induce dielectricbreakdown. Commercially available low density polyethylene ( L D P E ) m a y contain a defect density (localised energy states) [4] of 1019 c m -3 and also unsaturated vinyl groups [5] which can trap injected electrons or impurity ions. It has also been suggested [6] that the observed electroluminescence in polyethylene in the presence of highly divergent 50 Hz a.c. electrical *Present address: Laboratoire de Genie Electrique, E.S.E., Plateau du Moulon, Gif sur Yvette 91190 (France).
0304-3886/86/$03.50
© 1986 Elsevier Science Publishers B.V.
234
stress (point--plane geometry) is due to a recombination of injected carriers at deep trapping centres. The measurement of trapped or injected charges is of the utmost importance in characterising insulations and in determining the breakdown limits of such materials due to internal fields. However, no satisfactory technique exists for measuring the magnitude of this charge. The thermally stimulated discharge current method (TSDC) in which a previously stressed sample is short-circuited through an electrometer while its temperature is slowly increased to a few degrees below its melting point may be useful in providing information on trapping parameters and internal processes such as relaxations and transitions. However, the integrated charge measured in such an experiment may significantly underestimate the actual trap-release of space charges due to the cancellation of charges flowing to opposite electrodes and due to the recombination of ionic species. Indeed, if the polarisation was spatially uniform then no TSDC should be observed in the external circuit as equal numbers of charges would flow to each electrode. In the present work we have attempted to measure the injected charge in LDPE b y a variation of the conventional TSDC m e t h o d (FTSC -- Field and Temperature Stimulated Currents) in which the discharge current, following high field charging, is recorded in the presence of a small drawing voltage, VD, (small in comparison to the polarizing voltage) while the temperature is slowly increased [7--9]. In this way the thermally released charges, either electronic or ionic, may drift preferentially in the direction of this drawing field and will be collected at one electrode. Integration of the charges collected in this way, after subtraction of the temperature dependent conduction charges, will reveal the total trapped charge. We have determined the range of drawing potentials necessary in order to collect the majority of the charges and determine the average spatial depth of the injected charge. We have also estimated the number of defect centres and the charge trapped in the sample initially by measuring the dielectric increment before and after various conditioning processes such as heat treatment and electrical stressing. A comparison of the experimentally determined charge density and the calculated initial number of defect sites may then demonstrate whether injected charges of defects are dominant in commercially available LDPE. Experimental LDPE samples (Monsanto, Type M 301) of 75 um thickness were used in this present work. Aluminium electrodes were vacuum deposited on the sample surfaces, electrode thickness and area being 500 A and 6.25 cm 2 respectively. The electroded sample was located in a stainless-steel sample chamber under short circuit conditions for 15 hours at a reduced pressure
235
of 10 -s Tort at 70°C. The sample was then electrically stressed at +4 kV for 75 minutes at room temperature in the evacuated sample chamber. The electrodes of the sample were th~n short circuited again for 30 minutes at the same temperature. Subsequently the sample temperature was reduced to 0°C in the evacuated chamber. A small d.c. drawing voltage was applied to the sample which was then heated at a uniform rate of 1°C per minute to 70°C. The electrodes of the depolarised sample were short circuited again and the sample temperature was lowered to 0°C for the second time. The thermally stimulated current was again measured with the same drawing field as before while the sample temperature was raised again at the same uniform rate of 1°C per minute to 70°C. All measurements were made at a reduced pressure of 10 -s Tort. The experimental procedure is schematically represented [10] in Fig. 1. Above measurements were repeated with LDPE samples, all pre-stressed at 4 kV (d.c.) for different drawing fields not exceeding -+ 150 V (d.c.) in steps of + 10 V. T(°C )
7o T,
Or
12
H
,[. . . . .
I
,.:l
20
'800 I
8 7 5 96"5',950
i i t
7 5 minutes (v) I
,
1020 minutes
i i
i
I ,
V~ I
T {r~m)
Fig. 1. S c h e m a t i c r e p r e s e n t a t i o n o f t h e e x p e r i m e n t .
In addition, the effect of initial conditioning of the sample with heat t r e a t m e n t at 70°C, as described above, and also the effect of subsequent application of an electrical stress have been investigated in this work by measurements of sample capacitance :at two different frequencies, i.e. 10 Hz and l 0 s Hz using General Radio Bridge (Type 1621 with Type 1238 detector and Type 1616 oscillator). Kesults and discussion The resolved TSDC curve " c " of Fig. 2 was obtained by subtracting the FTSC values o f curve " b " from the corresponding values of curve " a " of a pre-stxessed (+ 4 kV d.c.) LDPE sample with + 30 V drawing voltage. A representative set o f similar resolved TSDC curves are shown in Fig. 3 for a few of the drawing voltages emplo~red in this work. It may be observed
236
that there is only one resolved TSDC peak (Fig. 3) at 65°C of one polatiry only.
The origin of thermally stimulated current peaks has usually been attributed to heterocharge motion and depolarization due to dipolar reorientation and/or a motion of homocharges, injected from electrodes or ions distributed non-uniformly, the latter being the more dominant mechanism at high temperatures [11--16]. A thermally stimulated current peak in the temperature region observed in the present work, has also been reported loi
/
a
'0
x 5
0
30
40
50
60
70
T('C)
Fig. 2. Methods of resolving space charge TSDC peaks by FTSC technique, a -- First run; thermally stimulated current after poling but with low bias field; b -- Second run; thermally stimulated current o f the depolarized sample after first run but with the low bias field as in (a); c -- Difference between the first and the second run, i.e. resolved thermally stimulated current spectrum. i
3
/ " ~ V D .
I
i VD= * 5 0 V
~
t
I
I
I
t
' ~
' t vo:
I
t
601
,
I
I
2
~. F
1
Y "
'o i
x
20
÷150V
~
¢/~
:,
~: -1
80
VD : - 3 0 V I I
I I
-2
VD : - 5 0 V I
~
-3 3. Resolved
V
J
~
o
Fig.
*30
I
TSDC
spectra
I
V
D:
-150V
I
for various
drawing
voltages.
237 b y other workers [7--9, 17, 18] and it has been attributed to a release of trapped charges. It is of interest to note that the thermally stimulated current peaks in the temperature range of 70°C to 145°C o f corona charged polypropylene have also been attributed to charge carriers released b y molecular motion from traps [ 2 0 ] . A simultaneous measurement of thermally stimulated current peak [20] and thermoluminescence also shows a peak in this temperature region with irradiated polyethylene. These workers also interpret their results in terms of non-geminate electron motion. Thus the observed broad peak at 65°C (Fig. 3) may be attributed to a motion of negatively charged carriers, released from localised states in the polymer. It was found that a small thermally stimulated discharge current could be obtained even without a drawing voltage i.e. V D = 0. This TSDC was basically of the same form as the resolved spectra drawn in Fig. 3 with a peak at a b o u t 65°C which demonstrates the validity of the present technique in producing resolved spectra. It should be stated that thermally stimulated current peaks should be observable even without the presence of any small external drawing field provided that either there is a non-uniform distribution of charges or that there is a spatial temperature gradient in the sample at all times. In the present work adequate considerations were given in the design of the experimental arrangement to ensure that both the sample faces are at the same temperature in the steady state conditions throughout the range of temperature excursions for the TSDC experiment. A better estimate of the thermally released charges may, however, be obtained with FTSC than with TSDC technique. Present observation of the resolved TSDC peak with V D = 0 indicate that the charge carriers are mostly negatively charged and that they are non-uniformly distributed. The quantity of charge released Q in a thermally stimulated current spectrum may be obtained from the resolved TSDC curves (Fig. 3) thus, t2 T2 Q= f I d t = f I ( d t / d T ) d T t1 TI
(1)
where I is the current, T the temperature and (dt/dT) the inverse of the heating rate. The relaxation process involving the peak at 65°C has been assumed to include charges of one polarity only for both directions of the drawing field. The range of integration extends from T1 = 30°C to T2 = 70°C and the results are shown in Fig. 4 from which it may be observed that the charges released {i.e. Q-values) reach an upper limit as the drawing voltage is increased. The total charge released QT, for any drawing voltage VD, is, of course, given b y QT = Q1 + Q2
(2)
where QI and Q2 refer to released charges with positive and negative polarities of the drawing voltage VD, respectively. With the present technique
238
(FTSC) the efficiency of determining QT will, of course, improve with increasing values of the drawing voltage until a saturation level of QT occurs where space charge limited conduction may occur [9, 17]. It may be obs e r v e d f r 0 m Fig. 4 that QT reaches a saturation value at a drawing voltage of 100 V, thus indicating a space charge limited conduction. Above observation is in good agreement with previous work [21--23].
x/ x
u
,
i
©
/
× 0
!Q~s i
x
i r m _ a . Z
-150
-1 0
-50
×/!
__
E
5O
100
' V
×//-I i -2i×
x Lx----~x/
Fig. 4. Charge released for different drawing voltages.
The average position of the trapped charges within the thickness of the sample can be estimated from the relative magnitudes of the two saturation charges Qls and Q2s. The normalised spatial depth, d, of the localised states in the bulk is given b y [9] Qls/(Qls + Q2s)
(3)
where Qls and Q2s refer to the saturation values of Q1 and Q2 respectively. A value of d = 0 indicates the position of the cathode, and d = 1 indicates the position of the anode. Using the saturation values of Qls and Q2s from Fig. 4, d appears to be 0.47 which would indicate only a small a s y m m e t r y in the location of trapped charges under the present charging condition, the charge concentration being slightly nearer to the cathode than to the anode. This result is consistent with the analysis of thermal transients [11] which indicated that charge density became more uniform at fields > 3 X 107 V/m (c.f. 5.33 X 107 V/m in the present case). It may also be observed from Fig. 4 that the magnitude of total (saturation) charge released by this technique, which m a y be an indication of the injected charge, is 8 X 10 -8 C. Assuming an electronic charge of 1.6 X 10 -19 C, then this corresponds to an injection of 5 X 101~ electrons. For a sample of electroded area = 6-25 cm 2 and thickness 75/~m the calculated electron density is only 1019/m s. It is of interest to note that the dielectric relaxation strength is given
239 le s - - e , I where e s is the "effective static permittivity" arising from both dipolar and space charge contributions and e . the high frequency permittivity. The results of the capacitance measurement of the sample (i) before heat treatment, (ii) after heat treatment and (iii) after an application of an electrical stress to the heat treated sample at 10 Hz and l 0 s Hz are given in Table 1. It may be observed (see Table 1) t h a t the respective values of the sample capacitances at the two different frequencies were r e d u c e d on heat treatment and further reduced on subsequent electrical stressing, thus providing reductions in the dielectric relaxation strength in comparison with t h a t of the virgin sample. by
TABLE 1 Values o f sample c a p a c i t a n c e and e s - - e ~ o f (i) virgin, (ii) h e a t t r e a t e d and (iii) heatt r e a t e d a n d electrically stressed (--4 k V ) L D P E Sample capacitance (X 10 -9 F )
Virgin s a m p l e Heat-treated sample H e a t - t r e a t e d and electrically stressed
e~ (10 Hz)
e~ (10 s Hz)
e ~- - e Dielectric r e l a x a t i o n
10 Hz value
10 s Hz value
strength
2.7261
2.6171
2.310
2.218
0.092
2.6748
2.5932
2.267
2.198
0.069
2.5791
2.5070
2.186
2.125
0.061
Now the dielectric relaxation strength and the "effective" dipole density N, are related thus N~ 2
e s --e. = ~
3 k T eo
(4)
where ~ is the "effective" dipole m o m e n t , k the Boltzmann's constant and e0 the permittivity of free space. Assuming that the "effective" dipole m o m e n t p, of extruded commercially available polyethylene [24] which may contain impurity ions, is 10 -29 C m and using the value of 0.092 as the dielectric relaxation strength of the virgin sample (see Table 1), the effective density N in the present work is f o u n d to be 1.0 × 1025 m -3 which is in reasonable agreement with the results of other workers [24]. This estimate is six orders of magnitude greater than that of the injected space charges and clearly will dominate the dielectric and electrical behaviour of the sample unless the polarizing field is substantially increased, or applied for a longer time, or applied at an elevated temperature. Indeed the value of the dielectric increment measured in this way may be a lower
240 estimate since the 10 Hz value of capacitance will be less than the true static value. Hence the defect density calculated in eqn. {4) may also be a lower estimate. Nevertheless, this value is also in agreement with the concentration density of polar groups which may exist in LDPE [25]. On this basis it may be stated that the present practice of heat treatment alone and an appropriate electrical stressing {i.e. field cleaning) following the heat treatment reduce the original "effective" dipole density of the virgin sample b y 25% and 34% respectively. The latter observation is also in agreement with previous work [11, 2 6 ] , in which it has been shown that with increasing charging potential there is a progressive field cleaning effect which may be attributed to the impurity ions being driven out or neutralized by recombination with the injected carriers. At still higher fields the blocking electrode effects due to impurity ions are removed and the charge injection becomes more dominant. However, such injected charges may get localized in deep traps from which they may be released at high temperatures. It is also established that heat treatment (or annealing) may produce increases in the various physical dimensions within the spherulitic structure of LDPE. These dimensions include the average crystallite size, the lamellar thickness and the actual crystallinity [19, 27, 2 8 ] . It has been reported that the polymer chain-ends that lie on the surfaces of lamellae move into the crystalline regions at high temperatures [29]. Such chain ends are polar in nature and their interaction with an electric field can contribute significantly to dielectric loss measurement. Hence, it may be suggested that the observed reduction in dielectric relaxation strength after heat treatment may be due to a general increase in the crystallinity of the polymer (or a decrease in the amorphous content). It is, of course, possible that a similar improvement in electric strength of cross-linked polyethylene (XLPE) may occur due to heat treatment which may improve the ageing characteristics of high voltage power transmission cables. Further work is in progress aimed at determining the magnitude and relative position of injected charges in XLPE and developing techniques to improve the loss characteristics of such insulations in service. Conclusions It has been shown that the Field and Temperature Stimulated Current technique can be used to study the charge release from an insulating p o l y m e r such as LDPE after polarisation in a high d.c. field. It may also be employed to estimate the magnitude of the injected charges and their average relative position within the bulk of the sample. In LDPE at room temperature and for a field of 5.33 X 107 V/m a charge of 8 X 10 -8 C is injected which provides an electron density significantly less than the initial defect (or impurity) density present in the virgin samples as revealed by measurements of the dielectric increment. Dielectric ageing due to injected charges may
241
only become relevant at applied stress :>:> 107 V/m though metallic inclusions and inhomogeneous samples may well produce highly divergent fields locally where accelerated ageing may occur.
Acknowledgement This work is financed by Rosemead California, U.S.A. the Royal Society of Great European Science Exchange Doughty for many valuable
the Southern California Edison Company of One of the authors (S.N.) is also grateful to Britain for a Research Fellowship under the Programme. It is a pleasure to thank Dr. K. discussions during the progress of this work.
References 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
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