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The influence of annealing time on thermally stimulated discharge current of corona-charged polymers
September 1994
MaterialsLetters
(1994) 119-125
The influence of annealing time on thermally stimulated discharge current of corona-charged polymers Eugen Neagu Technical University Iasi, lasi 6600, Romania
Received 26 April 1994; in final form 29 June 1994; accepted 30 June 1994
Abstract The influence of annealing time on thermally stimulated discharge current of negatively and positively corona-charged Tefzel has been studied. For negatively charged samples the peak position and temperature are strongly dependent on annealing time. The TSDC spectrum for short annealing time is more complex than that for long annealing time. If the annealing time is long enough, the peak remains in the same position and it corresponds to the deeper energy level for traps, the traps being distributed exponentially in energy. The stored charge decays exponentially in time and the relaxation time is longer than the Maxwell relaxation time. The activation energy increases as the annealing time increases. For positively corona-charged Tefzel the peak positions do not depend on the annealing time. The stored charge decays fast. The activation energy for positively coronacharged samples is higher than that for negatively corona-charged samples.
1. Introduction
Thermally stimulated discharge current (TSDC) analysis of non-metallic solids was firmly established as an alternative method of studying molecular motion, charge trapping and transport and other physical and chemical effects. Before a TDSC analysis, a dielectric may be charged positively or negatively by: (i) corona discharge; (ii) an electron beam; (iii) liquid contact; (iiii) the conventional thermal poling. Following the charging process, the sample electrodes are short circuited to allow isothermal charge discharge for a suitable period of time (i.e. annealing period) before it is heated at a low and uniform rate (typically lo C/min ). As the temperature raises, the detrapping and movement of the charge injected into the sample during formation or the induced polarization decay produce a current flowing in the external circuit. A plot of this current versus temperature is a thermogram that may contain current peaks re-
lated to the molecular relaxation processes and delocalization of charges injected in the polymer during the polarization process [ 1,3]. Although the TSDC technique has been employed extensively in the characterization of the macromolecule states there is little information available on the effect of annealing time on the nature of the thermograms. In a previous paper [4], we studied the TSDC spectrum of coronacharged Tefzel that is a high temperature thermoplastic, produced by Du Pont. Chemically, Tefzel is a copolymer of ethylene and tetrafluorethylene. The volume resistivity of Tefzel is higher than 1016Q cm and its relative dielectric permitivity E= 2.6 and tan 6= 10V4. Tefzel has a chemical inertness approaching that of Teflon. The melting point of Tefzel is 27 1‘C and it can be used for continuous operation at 150°C. The present work provides the results of the influence of annealing time period on the TSDC spectrum of negatively and positively corona-charged Tefzel.
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SsDIO167-577x(94)00151-0
120
E. Nea~/~ateri~s~tters21(1994)
The initial rise method is applied to some characteristic TSDC peaks in order to estimate the activation energy.
119-125
20
60
100
140
lt3OTK)
L
-15 Z-30 g
1z-45
2. Experimental procedures The short-circuit TSDC measurements were performed with Tefzel samples of 25 pm thickness. An aluminium electrode of 5 cm in diameter and 7 X 1O-’ m thickness was vacuum deposited on one surface of each sample that was earthed during corona charging at potentials of -7.5 kV or +7.5 kV in air at room temperature for a charging time of 10 min. For corona charging a micromesh control grid was introduced between the sample surface and the corona point that was connected to an adjustable and stabilized EHT supply. The control grid was connected to a similar but separate EHT supply which was maintained at a potential of 2 kV below that of the corona point. The surface potential of the sample was monitored continuously with a meld-mill-tie electrostatic voltmeter. In all cases the surface potential was observed to reach the desired values 10 s after the application of the high-voltage supplies to the corona point and the grid. At the end of poling time the nonmetallized surface of the sample was earthed temporarily. Following the corona charging, an identical second aluminium electrode was vacuum deposited on the other surface. The sample was then short circuited over a range of time from 0.5 min to 288 h to allow the sample to discharge isothermally at room temperature. The TSDC the~o~ams were subsequently obtained at a linear temperature increase of l.S”C/min in the temperature range from 20 to 22O”C, discharge current being measured by an electrometer and recorded appropriately.
3. Results and discussion The TSDC spectra of negatively as we11as of positively charged samples possess only relatively broad peaks. Fig. 1 shows four typical TSDC the~o~ms for negatively corona-charged Tefzel at - 7.5 kV. This series of the~o~ams (Fig. 1) presents the effect of increasing annealing time from 0.5 min to 24 h subsequently to corona charging of polymer. As seen in
zE -75 -60
-+ 24 -I3.5 hours - 43
Fig. I. TSDC the~o~ms of negatively corona-charged Tefzel for short annealing times: f I ) 0.5 min; f2) 20 min; (3) 3.5 h; (4) 24 h.
Fig. 1, the TSDC spectrum changes very much as the annealing time increases. Every thermogram is complex, being formed from two or three maxima. For sample 1, the annealing time during which the sample was allowed to discharge isothermally before TSDC run was 0.5 min. The TSDC is small and three peaks appear at 54,108 and 180°C. The peaks are of negative polarity, the current flow being in the opposite direction as the charging current. Thermally stimulated currents have been measured under opencircuit conditions for Teflon FEP (tetrafluoroethylene-hexafluoroethylene copolymer) by Sessler and West [ 5 ] who have observed multiple peaks of complex nature at 145, 190 and 220°C after a conventional thermal poling. Such studies with electron- and gamma-irradiated Teflon [ 61 have shown a low and a high temperature peak, which are compatible with the present results. From Fig. 1, curve 2, it is observed that if the annealing time is increased from 0.5 to 20 min, a broad peak is obtained, the main peak intensity increases and it is centered at 8 lo C. The total charge Q liberated during a TSDC run may be obtained using the following expression: Q=gr(t, 11
dt,
(1)
where 1(t) is the current released from the sample during heating over a time period from 1, to tz. When the annealing time is increased to 3.5 h a single peak is obtained (curve 3 in Fig. 1). The peak intensity increases very much and the location of the peak moves to lower tempe~tures. The results obtained for the total charge Q liberated during a TSDC run are given in Table 1, where it can be seen that the
E. Neagu1MaterialsLetters21(1994) I I9- 125
121
Table 1 Summa~ of the results of TSDC anaiysis for negatively corona-charge charged samples Sample
Annealing time (h)
T%zl a t”cf
Q (IO-BC)
1
l/120
1.48
2
1/3
3 4 5 6 7
3.5 24 48 96 288
54 108 180 81 155 6% 7s gt 108 108
5.16 20.7 l1.i 10.7 IO.4 7.5
& (ev)
% (t@* m-‘f
0.56
1.6
0.73 0.8 0.86 0.97 I.1
1.47 1.1 0.9 0.6 0.4
* ?‘, is the peak temperature for a current maximum.
charge released increases as the annealing time increases from 0.5 min to 3.5 h. When the annealing time increases further, the charge released starts to decrease (curve 4 in Fig. 1) and the peak position moves to higher temperatures. Fig. 2 shows the complex behavior of the TSDC thermograms as a function of the annealing time, during which the sample was allowed to discharge isothermally prior to a TSDC run for longer period of time. This series of tbe~o~~s~ curves 5,6 and 7, represent the effect of increasing annealing time from 4&h, curve 5, to 96 h, curve 6, and to 288 h, curve 7. As it is seen from Fig. 2, if the annealing time is greater than 96 h, the peak position remains almost the same and the peak intensity decreases. It is established that a persistent polarization can be induced in a polymer by corona charging. It has been shown [7f that with positive corona charging in air at atmospheric pressure, hydrated versions of II+, NO” and NO: ions are produced whereas for negative corona charging CO, ions are generated. In
-
2 88 hours-7
Fig, 2. TSDC thermogmms of R~~tiy~i~ corona-charged Tefzel for long annealing times: (5 f 4g h; (6 f 96 h; ( 7 ) 288 h.
addition, corona discharges can produce radiation damage of the polymer surface when molecular scission may occur, thus creating localized states. The ions produced during corona discharges may dissociate on arrival at the polymer surface and inject electrons that may subsequently be trapped. The ionized gas molecules may also attach themselves to the energetic hydroperoxide scission products with different binding energies in the polymer. There would be very few electrons in the corona itself at any potential that may reach the polymer as their range in air is considerably smaller than the air gap generally used in a corona-charging experiment. Thus, the depolarization behavior of corona-charged polymers may be expected to be quite complex and dependent on polymer morphology and surface kinetics in addition to the environment. In some polymers, space charge may exist for a very long time. As a result, the nature of the TSDC thermograms of corona-charged polymers may be expected to be dependent on the annealing time, pa~icularly if the time is short. In a previous paper [ 41, we have observed that the TSDC thermograms are strongly dependent on charging voltage and annealing time. More than this, we have observed that for negatively corona-charged Tefzel at - 5 kV, the discharge current changes its sign for an annealing time of 96 h. In the present work, no change of sign was observed even for a longer annealing time. To explain this fact, we must take into consideration that with corona charging the depth at which the charges penetrate into the polymer depends on the charging voltage. The charge decay can
be explained by a simple theory, assuming two processes [8,9]: the charge drift to the rear electrode in its own electric field and, in the same time, the gradual compensation by the ohmic conduction of the dielectric. To understand which way the charge drift occurs, we should consider the fact that the electric field E(x) generated by a static charge dist~bution in a short circuited dielectric, changes the sign at a given depth X@determined by the condition 19, IO]: E&)=0.
(2)
The depth at which the field changes the sign depends on the corona charging voltage. Fig. 3 shows a possible charge distribution. The charge carriers trapped among the zero-field plane and the front electrode may reach this electrode during a TSDC run, without signi~cant detrapping, while the carriers trapped among the zero-field plane and rear electrode are detrapped before they can reach the efeo trade. Thus the predominant current flow is the I, component (Fig. 3 ) which provides a current peak of negative polarity. The fact that for a short annealing time three peaks are obtained may suggest that there are three different trapping levels. As the annealing time increases the injected charges move isothermally and are recaptured in the traps. We consider that the traps are di~~buted in energy and the number of shallow traps is higher than that of deep traps. After charge injection the sample was introduced in the vacuum vessel for vacuum evaporation of second aluminium electrode. During the evacuation of the vessel it is possible that the electric field intensity at the sample surface exceeds the dielectric breakdown strength of the air (Paschen b~akdown ). In this situation, as a consequence of dielectric breakdown of the air nearby the sample, a positive electric
Fig? 3. A schematic ~~~~a~t~~a of charge d~st~b~tj~R, zeroEetd plane and discharge current Bow through negatively coronacharged sample.
charge is deposited on the non-metallized surface of the sample [ 11,12,14]. This positive charge can neutralize the negative charge existing on the surface of the sample. If the negative charge is localized deep in the polymer, a positive charge can appear on the sample surface. The existence of this positive charge can explain the behavior observed in Fig. 1, For a short annealing time, the negative thermally activated charge of the sample recombines with the positive charge and the external current is very small. If the annealing time increases, during the isothe~al discharge of the sample, this positive charge vanishes and the released current in the external circuit during TSDC experiment increases. So, as the annealing time further increases, the charge is liberated from the shallow traps in the external circuit and the TSDC peak is located at lower temperatures. If the annealing time is long enough, during the isothe~al discharge of the sampie, the charge captured in the shallow traps is liberated and the TSDC peak starts to move to higher temperatures, corresponding to the charge liberated from deeper and deeper traps. Charge dist~bution studies [ 14- 17 ] performed with one of the pressure-pulse techniques, namely the laser-induced pulse method, show that for a relatively low deposited charge density, two charge layers develop; one at the surface of the sample and another at the depth corresponding approximately to the electron range for electron beam charged films. For higher deposited charge density, the space charge layer is broadened and is located deeper in the material. We can conclude that for the samples charged at - 7.5 kV for 10 min, the zero-field plane is initially located near the center of the sample and during annealing it moves towards the latter electrode. What happens when the leading edge of the space-charge cloud arrives at the electrode? We make the assumption that the metal-polymer contact is partially blocking [ 14,16,19 1. We can say that even for a long annealing time, the current I, is the main current and no change of current sign will be observed. If annealing time increases from 24 to 96 h, the peak position moves to higher temperatures. Thus during annealing, a shift of population may occur from the shallower traps into the deeper trans. As will be seen later, this shift of population is accompanied by an increase in activation energy. If the annealing time is sufficiently long, in our ex-
E. Neagu /Materials Letters21(1994) 119-125
96 h, as is seen from Fig. 2, the peak temperature does not increase as the annealing time increases. The peak appears at the same temperature for longer and longer annealing times, From these we can conclude that the peak at 108’ C corresponds to the deeper energy level for traps in the forbidden band of the material. Fig. 4 shows three typical TSDC thermograms for positively corona-charged Tefzel at +7.5 kV. The peaks are of negative polarity and located at 73 and 45’ C. It is known that the positive ions penetrate less into the sample. The positive charge at the surface of the sample generates a high electric field which breaks down the surrounding air when the Paschen limit of the air is exceeded. Then the breakdown deposits a negative charge which overcompensates the positive surface charge. So, the only possible effect of the annealing time is to diminish this negative charge. No charge movement inside the sample is expected to occur. As seen in Fig. 4, the two peaks remain in the same position and the released charge decreases as the annealing time increases. The total liberated charge Q during a TSDC run is calculated using Eq. ( 1) and the obtained results for the Q values are given in Table 2. It may be observed from Table 2 that the periment
-=- 1 0.3 hours 15 hours
-.2
--a- 3 26 hours
123
total charge Q has a smaller value than that for negatively charged samples. In the same time, for positively corona-charged samples the charge decays faster. It is generally accepted that the charge stability is poor for positively charged Teflon foils. Von Seggern [ 2 1 ] has shown that for such a case there are also two current peaks at 50°C and at 140-200” C, the amplitude of the current peak at lower temperature being greater than the high-temperature current peak. These results are in agreement with the present observations. Gunther [ 221 showed that positively charged SiOZ samples have less stability than negatively charged SiOZ samples. The initial rise method was applied to TSDC peaks in order to estimate activation energies. The activation energies E,for negatively corona-charged Tefzel are given in Table 1. It may be observed from Table 1 that the value of E,for negatively charged samples increases as the annealing time increases. In Fig. 5 a semi-logarithmic plot of activation energy E,versus annealing time t for negatively corona-charged sample is presented. The magnitude of E,for positively corona-charged sample is greater than that for negatively corona-charged specimen. It is likely that the positive charges are of ionic nature as the activation energies associated with ionic traps in amo~hous materials are usually higher than those associated with electronic traps [ 2 1,23 1. The activation energy value of 1.25 eV for the low-temperature TSDC peak for positively corona-charged Teflon [ 2 1 ] is close to the corresponding value, i.e. 1.12 eV, obtained in the present work. The concentration of charge carriers n, captured by traps may be found from graphical integration of the following equation [ 24 ] :
Fig. 4. TSDC thermograms of positively corona-charged Tefzel. Annealingtime: 1/3h;(2) 15h;(3)26h.
Table 2 Summary of the results of TSDC analysis for positively corona-charge charged samples Sample 1
Annealing time(h) i/3
2
15
3
26
Till (“C)
Q
-5%
4
( Io-8c)
(W
(1018m-3)
73 145 74 145 145
3.82
1.12
0.36
2.5
1.03
0.21
0.8
0.95
0.06
decreases as the annealing time is increased, Fig. 5 repre~nts a semi-logarithmic plot ofcharge released versus time, As is seen, a straight line was obtained, which suggests for Q the following equation: QU)=!& expf -0)
Fig. S. urns-loga~thmi~ plots of activation energy versus annealing time for negatkely ccm3na-charged samples.
n, =2.~J~~T~/q~~~~
f
(4)
where J, is the maximum value of current density, q the electronic charge, k the Boltzmann constant, T,,, the temperature at which the maximum current occurs, b the heating rate and E, the activation energy. Eq. (4) has been obtained f 2 I ] from a simultaneous sohuion ofanalytical equations of both the TSDC and TSC current densities; n, vahtes given by Eq. f 4 $ can be dire&y obtained from the ex~~rne~ta~ly determined parameters J,, T,, E, and b and do not require a curve integration. In the case of bimolecular recombination, the expression for n, differs from Eq, (4 ) only by a factor not exceeding three, The calculated IZ$vaiues, using Eq. (4) are given in Table 1 for negatively corona-charged Tefzel and in Table 2 for positively corona-charged Tefzd. As tbe annealing time increases, n, decreases for both cases, and the trapped charge density for positively corona-charging case is signi~ca~t~y fess than that for negatively corona-charging case. From Table 1, it. is observed that the released charge during TSI3C n~easurements
(5)
where t is the time and T the retaxation time for the charge. If Eq, (5) is fitted to the experimental data from Fig. 6, the obtained value for T is 673 h. On the other hand, if we consider the Maxweli relaxation time, T&j= f/G3
where I( 1) is the thermally stimulate discharge current, L the sample thickness, i the time, Vthe sample volume, E the electric field, q the electronic charge, fi the electron mobility and Tthe free carriers’ life time. However, Eq. ( 3 ), which is applicable to a TSC spectrum, may not be applicable under mono-molecular r~ombination process, i.e. T is constant, and for a thermal& stimulated discharge current TSDC, spectrum. The ~ncentration of charge carriers n, captured by traps for the case of mono-molecular recombination and weak detrapping is however given by [ZS];
J
(6)
where f:is the dielectric ~~iti~ity and Dthe electrical conductivity, for Tefzel we obtain: T, ~0.63 h . Because the experimental value is much higher, we can conclude that the electrodes are partially blocking, as we already have supposed. The drift is characterized by a transit time: t, =~2f#&n~X 3
(71
where L is the sample thic~~s, # is the carrier trapmodulated mobility and vm, is the maximum applied voltage, If we consider that it = TM [ 18 ] I ,a can be calculated from Eq. (7): /t=3x IO-‘5 cm’/V s. This value is ixl good agreement with the value obtained by van Turnhout [ f ] for the steady-state trapmodulated mobility in Teflon for electrons. The mobihty at the TSC peak temperature can also be evaluated from the condition for current maximum: p=bEa Ve/%T:Q ,
ISI
where b is the heating rate, E the diefectric permitiv-
E. Neagu /Materials Letters21(1994) 119-125
tJ L_.l_~.~~0.11
05 0.8 1 12 I.4 CHARGE CONCENTRATION r$1d8/m3)
125
peak is at 108 ” C and the activation energy is 1.10 eV. The stored charge decays exponentially in time and the relaxation time is longer than the Maxwell relaxation time. A semilogarithmic plot of the activation energy versus trapped charge concentration is a straight line. For positively corona-charged Tefzel, the peaks’ positions do not depend on annealing time. The stored charge decays fast. The activation energy for positively corona-charged samples is higher than that for negatively corona-charged samples.
Fig, 7. Semi-logarithmic plots of activation energy versus charge concentration for negatively corona-charged Tefzel.
ity, & the activation energy, V the sample volume, k the Boltzmann constant and Q the total charge. if we use the experimentally determined parameters, we obtain for fi values around 1O-3 cm’/V s, which are different from the values given by Eq. (7). We conclude that the carriers are subject to fast detrapping that has a pronounced effect on mobility. Fig. 7 presents a semilogarithmic plot of activation energy E, versus charge concentration n, for negatively corona-charged Tefzel. A straight line was obtained. We think that this straight line is an evidence of the fact that the traps are logarithmically distributed in energy: lnE(1)=ln
&-n(t)/no.
(9)
This can explain why the low-temperature peak is higher in Fig. 1.
4. Conclusions The thermally stimulated discharge current in corona-charged Tefzel is strongly dependent on the annealing time period. For negatively corona-charged samples, the peak temperature moves to higher temperatures as the annealing time increases and the peak intensity decreases. The traps are exponentially distributed in energy, the number of shallow traps being higher than that of deep traps. As the annealing time is longer than 96 h, the peak remains in the same position if the annealing time increases. From this we can conclude that this peak corresponds to the deeper energy level for traps in the forbidden band of the material. For negatively corona-charged Tefzel, this
References J. van Turnhout, in: Electrets, ed. G.M. Sessler (Springer, Berlin, 1980) pp. 81-215. J. Vanderschueren and J. Casio& Topics Appl. Phys. 37 (1979) 135. S.H. Carr, Electrical properties of polymers, ed. D.A. Scanor (Academic Press, New York, 1982) pp. 215-239. E. Neagu and D.K. Das-Gupta, IEEE Trans. Electrical Insul. 24 (1989) 489. [ 51 G.M. Sessler and J. West, J. Appl. Phys. 8 (1976) 3480. [6] B. Gross, G.M. Sessler and J.E. West, J. Appl. Phys. 47 (1976) 968. [7] M.M. Shalin, J. Chem. Phys. 45 (1969) 2600. [8] K.K. Kanazawa, I.P. Batra and H.J. Wintle, J. Appl. Phys. 43 (1977) 719. [ 9 ] E. Neagu, in: 7th Intern. Symp. on Electrets, Berlin, 199 I, pp. 147-152. [lo] D.J. Gibbons, J. Phys. D, 7 (1974) 433. [ 111E. Neagu, Bul. Inst. Polit. Iasi. XXI (XXV) (1975) 107. [ 121 E. Neaguand R. Neagu, Bul. Inst. Polit. Iasi. XXII (XXVI) (1976) 107. [ 131 B. Gross, J. Phys. D 7 (1974) L103. [ 141 B. Gross and M.M. Perlman, J. Appl. Phys. 43 (1972) 853. [ 1S] G.M. Sessler, J.E. West, R. Gerhard-Multhaupt and H. von Seggem, IEEE Trans. Nucl. Sci. 29 (1982) 1644. [ 161 R. Gerha~-Muithaupt, M. Haordt, W. Eisenmenger and G.M. Sessler, J. Phys. D 16 (1983) 2247. [ 171 J.E. West, H.J. Wintle, A. Berraissoul and G.M. Sessler, IEEE Trans. Insul. 24 ( 1989) 533. [ 181 J. van Tumhout, Thermally stimulated discharge of polymer electrets (Elsevier, Amsterdam, 1975). [ 191 E. Neagu, R. Neagu, W. Dally and 1.1. Negulescu, IEEE Trans. Electrical Insul. 28 (1993) 122. [20] G.F. Garlick and A.F. Gibson, Proc. Phys. Sot. A 60 ( 1948) 547. [2l]H.vonSeggem,J.Appl.Phys.52(1981)4081. [22] P. Gunther, IEEE Trans. Electric. Insul. 24 (1989) 439. [23] J. van Tumhout, Polym. J. 2 ( 1971) 173. 1241 R.H. Bule, J. Chem. Phys. 23 (1955) 18. 1251 G.A. Bordowsky, Phys. Stat. Sol. 29a (1975) K183.
MaterialsLetters
(1994) 119-125
The influence of annealing time on thermally stimulated discharge current of corona-charged polymers Eugen Neagu Technical University Iasi, lasi 6600, Romania
Received 26 April 1994; in final form 29 June 1994; accepted 30 June 1994
Abstract The influence of annealing time on thermally stimulated discharge current of negatively and positively corona-charged Tefzel has been studied. For negatively charged samples the peak position and temperature are strongly dependent on annealing time. The TSDC spectrum for short annealing time is more complex than that for long annealing time. If the annealing time is long enough, the peak remains in the same position and it corresponds to the deeper energy level for traps, the traps being distributed exponentially in energy. The stored charge decays exponentially in time and the relaxation time is longer than the Maxwell relaxation time. The activation energy increases as the annealing time increases. For positively corona-charged Tefzel the peak positions do not depend on the annealing time. The stored charge decays fast. The activation energy for positively coronacharged samples is higher than that for negatively corona-charged samples.
1. Introduction
Thermally stimulated discharge current (TSDC) analysis of non-metallic solids was firmly established as an alternative method of studying molecular motion, charge trapping and transport and other physical and chemical effects. Before a TDSC analysis, a dielectric may be charged positively or negatively by: (i) corona discharge; (ii) an electron beam; (iii) liquid contact; (iiii) the conventional thermal poling. Following the charging process, the sample electrodes are short circuited to allow isothermal charge discharge for a suitable period of time (i.e. annealing period) before it is heated at a low and uniform rate (typically lo C/min ). As the temperature raises, the detrapping and movement of the charge injected into the sample during formation or the induced polarization decay produce a current flowing in the external circuit. A plot of this current versus temperature is a thermogram that may contain current peaks re-
lated to the molecular relaxation processes and delocalization of charges injected in the polymer during the polarization process [ 1,3]. Although the TSDC technique has been employed extensively in the characterization of the macromolecule states there is little information available on the effect of annealing time on the nature of the thermograms. In a previous paper [4], we studied the TSDC spectrum of coronacharged Tefzel that is a high temperature thermoplastic, produced by Du Pont. Chemically, Tefzel is a copolymer of ethylene and tetrafluorethylene. The volume resistivity of Tefzel is higher than 1016Q cm and its relative dielectric permitivity E= 2.6 and tan 6= 10V4. Tefzel has a chemical inertness approaching that of Teflon. The melting point of Tefzel is 27 1‘C and it can be used for continuous operation at 150°C. The present work provides the results of the influence of annealing time period on the TSDC spectrum of negatively and positively corona-charged Tefzel.
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SsDIO167-577x(94)00151-0
120
E. Nea~/~ateri~s~tters21(1994)
The initial rise method is applied to some characteristic TSDC peaks in order to estimate the activation energy.
119-125
20
60
100
140
lt3OTK)
L
-15 Z-30 g
1z-45
2. Experimental procedures The short-circuit TSDC measurements were performed with Tefzel samples of 25 pm thickness. An aluminium electrode of 5 cm in diameter and 7 X 1O-’ m thickness was vacuum deposited on one surface of each sample that was earthed during corona charging at potentials of -7.5 kV or +7.5 kV in air at room temperature for a charging time of 10 min. For corona charging a micromesh control grid was introduced between the sample surface and the corona point that was connected to an adjustable and stabilized EHT supply. The control grid was connected to a similar but separate EHT supply which was maintained at a potential of 2 kV below that of the corona point. The surface potential of the sample was monitored continuously with a meld-mill-tie electrostatic voltmeter. In all cases the surface potential was observed to reach the desired values 10 s after the application of the high-voltage supplies to the corona point and the grid. At the end of poling time the nonmetallized surface of the sample was earthed temporarily. Following the corona charging, an identical second aluminium electrode was vacuum deposited on the other surface. The sample was then short circuited over a range of time from 0.5 min to 288 h to allow the sample to discharge isothermally at room temperature. The TSDC the~o~ams were subsequently obtained at a linear temperature increase of l.S”C/min in the temperature range from 20 to 22O”C, discharge current being measured by an electrometer and recorded appropriately.
3. Results and discussion The TSDC spectra of negatively as we11as of positively charged samples possess only relatively broad peaks. Fig. 1 shows four typical TSDC the~o~ms for negatively corona-charged Tefzel at - 7.5 kV. This series of the~o~ams (Fig. 1) presents the effect of increasing annealing time from 0.5 min to 24 h subsequently to corona charging of polymer. As seen in
zE -75 -60
-+ 24 -I3.5 hours - 43
Fig. I. TSDC the~o~ms of negatively corona-charged Tefzel for short annealing times: f I ) 0.5 min; f2) 20 min; (3) 3.5 h; (4) 24 h.
Fig. 1, the TSDC spectrum changes very much as the annealing time increases. Every thermogram is complex, being formed from two or three maxima. For sample 1, the annealing time during which the sample was allowed to discharge isothermally before TSDC run was 0.5 min. The TSDC is small and three peaks appear at 54,108 and 180°C. The peaks are of negative polarity, the current flow being in the opposite direction as the charging current. Thermally stimulated currents have been measured under opencircuit conditions for Teflon FEP (tetrafluoroethylene-hexafluoroethylene copolymer) by Sessler and West [ 5 ] who have observed multiple peaks of complex nature at 145, 190 and 220°C after a conventional thermal poling. Such studies with electron- and gamma-irradiated Teflon [ 61 have shown a low and a high temperature peak, which are compatible with the present results. From Fig. 1, curve 2, it is observed that if the annealing time is increased from 0.5 to 20 min, a broad peak is obtained, the main peak intensity increases and it is centered at 8 lo C. The total charge Q liberated during a TSDC run may be obtained using the following expression: Q=gr(t, 11
dt,
(1)
where 1(t) is the current released from the sample during heating over a time period from 1, to tz. When the annealing time is increased to 3.5 h a single peak is obtained (curve 3 in Fig. 1). The peak intensity increases very much and the location of the peak moves to lower tempe~tures. The results obtained for the total charge Q liberated during a TSDC run are given in Table 1, where it can be seen that the
E. Neagu1MaterialsLetters21(1994) I I9- 125
121
Table 1 Summa~ of the results of TSDC anaiysis for negatively corona-charge charged samples Sample
Annealing time (h)
T%zl a t”cf
Q (IO-BC)
1
l/120
1.48
2
1/3
3 4 5 6 7
3.5 24 48 96 288
54 108 180 81 155 6% 7s gt 108 108
5.16 20.7 l1.i 10.7 IO.4 7.5
& (ev)
% (t@* m-‘f
0.56
1.6
0.73 0.8 0.86 0.97 I.1
1.47 1.1 0.9 0.6 0.4
* ?‘, is the peak temperature for a current maximum.
charge released increases as the annealing time increases from 0.5 min to 3.5 h. When the annealing time increases further, the charge released starts to decrease (curve 4 in Fig. 1) and the peak position moves to higher temperatures. Fig. 2 shows the complex behavior of the TSDC thermograms as a function of the annealing time, during which the sample was allowed to discharge isothermally prior to a TSDC run for longer period of time. This series of tbe~o~~s~ curves 5,6 and 7, represent the effect of increasing annealing time from 4&h, curve 5, to 96 h, curve 6, and to 288 h, curve 7. As it is seen from Fig. 2, if the annealing time is greater than 96 h, the peak position remains almost the same and the peak intensity decreases. It is established that a persistent polarization can be induced in a polymer by corona charging. It has been shown [7f that with positive corona charging in air at atmospheric pressure, hydrated versions of II+, NO” and NO: ions are produced whereas for negative corona charging CO, ions are generated. In
-
2 88 hours-7
Fig, 2. TSDC thermogmms of R~~tiy~i~ corona-charged Tefzel for long annealing times: (5 f 4g h; (6 f 96 h; ( 7 ) 288 h.
addition, corona discharges can produce radiation damage of the polymer surface when molecular scission may occur, thus creating localized states. The ions produced during corona discharges may dissociate on arrival at the polymer surface and inject electrons that may subsequently be trapped. The ionized gas molecules may also attach themselves to the energetic hydroperoxide scission products with different binding energies in the polymer. There would be very few electrons in the corona itself at any potential that may reach the polymer as their range in air is considerably smaller than the air gap generally used in a corona-charging experiment. Thus, the depolarization behavior of corona-charged polymers may be expected to be quite complex and dependent on polymer morphology and surface kinetics in addition to the environment. In some polymers, space charge may exist for a very long time. As a result, the nature of the TSDC thermograms of corona-charged polymers may be expected to be dependent on the annealing time, pa~icularly if the time is short. In a previous paper [ 41, we have observed that the TSDC thermograms are strongly dependent on charging voltage and annealing time. More than this, we have observed that for negatively corona-charged Tefzel at - 5 kV, the discharge current changes its sign for an annealing time of 96 h. In the present work, no change of sign was observed even for a longer annealing time. To explain this fact, we must take into consideration that with corona charging the depth at which the charges penetrate into the polymer depends on the charging voltage. The charge decay can
be explained by a simple theory, assuming two processes [8,9]: the charge drift to the rear electrode in its own electric field and, in the same time, the gradual compensation by the ohmic conduction of the dielectric. To understand which way the charge drift occurs, we should consider the fact that the electric field E(x) generated by a static charge dist~bution in a short circuited dielectric, changes the sign at a given depth X@determined by the condition 19, IO]: E&)=0.
(2)
The depth at which the field changes the sign depends on the corona charging voltage. Fig. 3 shows a possible charge distribution. The charge carriers trapped among the zero-field plane and the front electrode may reach this electrode during a TSDC run, without signi~cant detrapping, while the carriers trapped among the zero-field plane and rear electrode are detrapped before they can reach the efeo trade. Thus the predominant current flow is the I, component (Fig. 3 ) which provides a current peak of negative polarity. The fact that for a short annealing time three peaks are obtained may suggest that there are three different trapping levels. As the annealing time increases the injected charges move isothermally and are recaptured in the traps. We consider that the traps are di~~buted in energy and the number of shallow traps is higher than that of deep traps. After charge injection the sample was introduced in the vacuum vessel for vacuum evaporation of second aluminium electrode. During the evacuation of the vessel it is possible that the electric field intensity at the sample surface exceeds the dielectric breakdown strength of the air (Paschen b~akdown ). In this situation, as a consequence of dielectric breakdown of the air nearby the sample, a positive electric
Fig? 3. A schematic ~~~~a~t~~a of charge d~st~b~tj~R, zeroEetd plane and discharge current Bow through negatively coronacharged sample.
charge is deposited on the non-metallized surface of the sample [ 11,12,14]. This positive charge can neutralize the negative charge existing on the surface of the sample. If the negative charge is localized deep in the polymer, a positive charge can appear on the sample surface. The existence of this positive charge can explain the behavior observed in Fig. 1, For a short annealing time, the negative thermally activated charge of the sample recombines with the positive charge and the external current is very small. If the annealing time increases, during the isothe~al discharge of the sample, this positive charge vanishes and the released current in the external circuit during TSDC experiment increases. So, as the annealing time further increases, the charge is liberated from the shallow traps in the external circuit and the TSDC peak is located at lower temperatures. If the annealing time is long enough, during the isothe~al discharge of the sampie, the charge captured in the shallow traps is liberated and the TSDC peak starts to move to higher temperatures, corresponding to the charge liberated from deeper and deeper traps. Charge dist~bution studies [ 14- 17 ] performed with one of the pressure-pulse techniques, namely the laser-induced pulse method, show that for a relatively low deposited charge density, two charge layers develop; one at the surface of the sample and another at the depth corresponding approximately to the electron range for electron beam charged films. For higher deposited charge density, the space charge layer is broadened and is located deeper in the material. We can conclude that for the samples charged at - 7.5 kV for 10 min, the zero-field plane is initially located near the center of the sample and during annealing it moves towards the latter electrode. What happens when the leading edge of the space-charge cloud arrives at the electrode? We make the assumption that the metal-polymer contact is partially blocking [ 14,16,19 1. We can say that even for a long annealing time, the current I, is the main current and no change of current sign will be observed. If annealing time increases from 24 to 96 h, the peak position moves to higher temperatures. Thus during annealing, a shift of population may occur from the shallower traps into the deeper trans. As will be seen later, this shift of population is accompanied by an increase in activation energy. If the annealing time is sufficiently long, in our ex-
E. Neagu /Materials Letters21(1994) 119-125
96 h, as is seen from Fig. 2, the peak temperature does not increase as the annealing time increases. The peak appears at the same temperature for longer and longer annealing times, From these we can conclude that the peak at 108’ C corresponds to the deeper energy level for traps in the forbidden band of the material. Fig. 4 shows three typical TSDC thermograms for positively corona-charged Tefzel at +7.5 kV. The peaks are of negative polarity and located at 73 and 45’ C. It is known that the positive ions penetrate less into the sample. The positive charge at the surface of the sample generates a high electric field which breaks down the surrounding air when the Paschen limit of the air is exceeded. Then the breakdown deposits a negative charge which overcompensates the positive surface charge. So, the only possible effect of the annealing time is to diminish this negative charge. No charge movement inside the sample is expected to occur. As seen in Fig. 4, the two peaks remain in the same position and the released charge decreases as the annealing time increases. The total liberated charge Q during a TSDC run is calculated using Eq. ( 1) and the obtained results for the Q values are given in Table 2. It may be observed from Table 2 that the periment
-=- 1 0.3 hours 15 hours
-.2
--a- 3 26 hours
123
total charge Q has a smaller value than that for negatively charged samples. In the same time, for positively corona-charged samples the charge decays faster. It is generally accepted that the charge stability is poor for positively charged Teflon foils. Von Seggern [ 2 1 ] has shown that for such a case there are also two current peaks at 50°C and at 140-200” C, the amplitude of the current peak at lower temperature being greater than the high-temperature current peak. These results are in agreement with the present observations. Gunther [ 221 showed that positively charged SiOZ samples have less stability than negatively charged SiOZ samples. The initial rise method was applied to TSDC peaks in order to estimate activation energies. The activation energies E,for negatively corona-charged Tefzel are given in Table 1. It may be observed from Table 1 that the value of E,for negatively charged samples increases as the annealing time increases. In Fig. 5 a semi-logarithmic plot of activation energy E,versus annealing time t for negatively corona-charged sample is presented. The magnitude of E,for positively corona-charged sample is greater than that for negatively corona-charged specimen. It is likely that the positive charges are of ionic nature as the activation energies associated with ionic traps in amo~hous materials are usually higher than those associated with electronic traps [ 2 1,23 1. The activation energy value of 1.25 eV for the low-temperature TSDC peak for positively corona-charged Teflon [ 2 1 ] is close to the corresponding value, i.e. 1.12 eV, obtained in the present work. The concentration of charge carriers n, captured by traps may be found from graphical integration of the following equation [ 24 ] :
Fig. 4. TSDC thermograms of positively corona-charged Tefzel. Annealingtime: 1/3h;(2) 15h;(3)26h.
Table 2 Summary of the results of TSDC analysis for positively corona-charge charged samples Sample 1
Annealing time(h) i/3
2
15
3
26
Till (“C)
Q
-5%
4
( Io-8c)
(W
(1018m-3)
73 145 74 145 145
3.82
1.12
0.36
2.5
1.03
0.21
0.8
0.95
0.06
decreases as the annealing time is increased, Fig. 5 repre~nts a semi-logarithmic plot ofcharge released versus time, As is seen, a straight line was obtained, which suggests for Q the following equation: QU)=!& expf -0)
Fig. S. urns-loga~thmi~ plots of activation energy versus annealing time for negatkely ccm3na-charged samples.
n, =2.~J~~T~/q~~~~
f
(4)
where J, is the maximum value of current density, q the electronic charge, k the Boltzmann constant, T,,, the temperature at which the maximum current occurs, b the heating rate and E, the activation energy. Eq. (4) has been obtained f 2 I ] from a simultaneous sohuion ofanalytical equations of both the TSDC and TSC current densities; n, vahtes given by Eq. f 4 $ can be dire&y obtained from the ex~~rne~ta~ly determined parameters J,, T,, E, and b and do not require a curve integration. In the case of bimolecular recombination, the expression for n, differs from Eq, (4 ) only by a factor not exceeding three, The calculated IZ$vaiues, using Eq. (4) are given in Table 1 for negatively corona-charged Tefzel and in Table 2 for positively corona-charged Tefzd. As tbe annealing time increases, n, decreases for both cases, and the trapped charge density for positively corona-charging case is signi~ca~t~y fess than that for negatively corona-charging case. From Table 1, it. is observed that the released charge during TSI3C n~easurements
(5)
where t is the time and T the retaxation time for the charge. If Eq, (5) is fitted to the experimental data from Fig. 6, the obtained value for T is 673 h. On the other hand, if we consider the Maxweli relaxation time, T&j= f/G3
where I( 1) is the thermally stimulate discharge current, L the sample thickness, i the time, Vthe sample volume, E the electric field, q the electronic charge, fi the electron mobility and Tthe free carriers’ life time. However, Eq. ( 3 ), which is applicable to a TSC spectrum, may not be applicable under mono-molecular r~ombination process, i.e. T is constant, and for a thermal& stimulated discharge current TSDC, spectrum. The ~ncentration of charge carriers n, captured by traps for the case of mono-molecular recombination and weak detrapping is however given by [ZS];
J
(6)
where f:is the dielectric ~~iti~ity and Dthe electrical conductivity, for Tefzel we obtain: T, ~0.63 h . Because the experimental value is much higher, we can conclude that the electrodes are partially blocking, as we already have supposed. The drift is characterized by a transit time: t, =~2f#&n~X 3
(71
where L is the sample thic~~s, # is the carrier trapmodulated mobility and vm, is the maximum applied voltage, If we consider that it = TM [ 18 ] I ,a can be calculated from Eq. (7): /t=3x IO-‘5 cm’/V s. This value is ixl good agreement with the value obtained by van Turnhout [ f ] for the steady-state trapmodulated mobility in Teflon for electrons. The mobihty at the TSC peak temperature can also be evaluated from the condition for current maximum: p=bEa Ve/%T:Q ,
ISI
where b is the heating rate, E the diefectric permitiv-
E. Neagu /Materials Letters21(1994) 119-125
tJ L_.l_~.~~0.11
05 0.8 1 12 I.4 CHARGE CONCENTRATION r$1d8/m3)
125
peak is at 108 ” C and the activation energy is 1.10 eV. The stored charge decays exponentially in time and the relaxation time is longer than the Maxwell relaxation time. A semilogarithmic plot of the activation energy versus trapped charge concentration is a straight line. For positively corona-charged Tefzel, the peaks’ positions do not depend on annealing time. The stored charge decays fast. The activation energy for positively corona-charged samples is higher than that for negatively corona-charged samples.
Fig, 7. Semi-logarithmic plots of activation energy versus charge concentration for negatively corona-charged Tefzel.
ity, & the activation energy, V the sample volume, k the Boltzmann constant and Q the total charge. if we use the experimentally determined parameters, we obtain for fi values around 1O-3 cm’/V s, which are different from the values given by Eq. (7). We conclude that the carriers are subject to fast detrapping that has a pronounced effect on mobility. Fig. 7 presents a semilogarithmic plot of activation energy E, versus charge concentration n, for negatively corona-charged Tefzel. A straight line was obtained. We think that this straight line is an evidence of the fact that the traps are logarithmically distributed in energy: lnE(1)=ln
&-n(t)/no.
(9)
This can explain why the low-temperature peak is higher in Fig. 1.
4. Conclusions The thermally stimulated discharge current in corona-charged Tefzel is strongly dependent on the annealing time period. For negatively corona-charged samples, the peak temperature moves to higher temperatures as the annealing time increases and the peak intensity decreases. The traps are exponentially distributed in energy, the number of shallow traps being higher than that of deep traps. As the annealing time is longer than 96 h, the peak remains in the same position if the annealing time increases. From this we can conclude that this peak corresponds to the deeper energy level for traps in the forbidden band of the material. For negatively corona-charged Tefzel, this
References J. van Turnhout, in: Electrets, ed. G.M. Sessler (Springer, Berlin, 1980) pp. 81-215. J. Vanderschueren and J. Casio& Topics Appl. Phys. 37 (1979) 135. S.H. Carr, Electrical properties of polymers, ed. D.A. Scanor (Academic Press, New York, 1982) pp. 215-239. E. Neagu and D.K. Das-Gupta, IEEE Trans. Electrical Insul. 24 (1989) 489. [ 51 G.M. Sessler and J. West, J. Appl. Phys. 8 (1976) 3480. [6] B. Gross, G.M. Sessler and J.E. West, J. Appl. Phys. 47 (1976) 968. [7] M.M. Shalin, J. Chem. Phys. 45 (1969) 2600. [8] K.K. Kanazawa, I.P. Batra and H.J. Wintle, J. Appl. Phys. 43 (1977) 719. [ 9 ] E. Neagu, in: 7th Intern. Symp. on Electrets, Berlin, 199 I, pp. 147-152. [lo] D.J. Gibbons, J. Phys. D, 7 (1974) 433. [ 111E. Neagu, Bul. Inst. Polit. Iasi. XXI (XXV) (1975) 107. [ 121 E. Neaguand R. Neagu, Bul. Inst. Polit. Iasi. XXII (XXVI) (1976) 107. [ 131 B. Gross, J. Phys. D 7 (1974) L103. [ 141 B. Gross and M.M. Perlman, J. Appl. Phys. 43 (1972) 853. [ 1S] G.M. Sessler, J.E. West, R. Gerhard-Multhaupt and H. von Seggem, IEEE Trans. Nucl. Sci. 29 (1982) 1644. [ 161 R. Gerha~-Muithaupt, M. Haordt, W. Eisenmenger and G.M. Sessler, J. Phys. D 16 (1983) 2247. [ 171 J.E. West, H.J. Wintle, A. Berraissoul and G.M. Sessler, IEEE Trans. Insul. 24 ( 1989) 533. [ 181 J. van Tumhout, Thermally stimulated discharge of polymer electrets (Elsevier, Amsterdam, 1975). [ 191 E. Neagu, R. Neagu, W. Dally and 1.1. Negulescu, IEEE Trans. Electrical Insul. 28 (1993) 122. [20] G.F. Garlick and A.F. Gibson, Proc. Phys. Sot. A 60 ( 1948) 547. [2l]H.vonSeggem,J.Appl.Phys.52(1981)4081. [22] P. Gunther, IEEE Trans. Electric. Insul. 24 (1989) 439. [23] J. van Tumhout, Polym. J. 2 ( 1971) 173. 1241 R.H. Bule, J. Chem. Phys. 23 (1955) 18. 1251 G.A. Bordowsky, Phys. Stat. Sol. 29a (1975) K183.