IEEE Transactions on Electrical Insulation Vol. EI-21 NO.3q June 1986

INDUCED CONDUCTIVITY OF MYLAR AND KAPTONIRRADIATED BY X-RAYS

R. Gregorio Filho

Departamento de Engenharia de MateriaisUniversidade Federal de Sao Carlos

Sao Carlos, Brasil

B. Gross and R. M. Faria

Instituto de Flsica e Qufmica de Sao CarlosUniversidade de Sao Paulo

Sao Carlos, Brasil

ABSTRACT

This paper presents a wide range of experimental results ofprompt and delayed components of the conductivity induced byx-rays in Mylar (PET) and Kapton (Polyimide). Measurementswere carried out under a vacuum at room temperature. Theywere made for a large range of parameters: electric fieldE (104 to 4. 8X105 V/cm); exposure rate X (85 to 550 R/s);and thickness d of the sample (6 to 75 pm). Electrodes weremade of evaporated aluminum, but in some measurements goldwas used. Gold increased the exposure rate, due to its higher atomic number. Finally, a study of the recovery of theirradiated samples was made.

I NTRODUCT I ON

The generalized use of polymeric materials in highradiation environments has increased the initerest inprocesses of interaction between ionizing radiation andinsulating materials. The electrical properties ofthose materials, in particular the conductivity, changestrongly under radiation. Radiation-induced conductiv-ity (RIC) is today an important aspect of the electricalproperties of polymers. Several papers dealing with RICin Mylar® have been published [1-5], but in all studiesthe irradiation duration was insufficient to reach asteady-state. On the other hand, no study of RIC inKapton® has been presented until now, as far as weknow.

In this work we present RIC measurements in Mylar upto 7 h of irradiation, a time sufficient to reach thesteady-state. The steady-state value was independent ofthe applied electric field for values below 8X104 V/cm;above this field the RIC increased with the field, obey-ing the initial recombination theory of Onsager [6]. InKapton the RIC was found to increase fast after 2000 sfrom the beginning of the irradiation, after having re-mained almost constant for 103 s up this time. Even af-ter 7 h of irradiation the RIC continues to increase.

EXPERIMENTAL PROCEDURE

The samples of Mylar and Kapton were made of 80 mm di-ameter circular foils, thickness varying from 6 to 75 pm.

The vacuum-evaporated electrodes of Al or Au on bothsides were concentric with the sample having a diameterof 40 mm. Guard-rings (60 mm inner diameter, 66 mmouter diameter), were evaporated on the samples toavoid leakage in the electrical current measurements.

Fig. 1 shows schematically a cross section of themeasuring apparatus used. The samples were pressed be-tween two foils: a 75 pm thick Kapton foil was placedat the top, facing the x-ray beam, and another foil madeof the same material as the sample, was placed at thebottom. To eliminate charge build up at interfaces andsurface charge-up due to Compton currents [7], thefirst foil had its top face grounded, while the secondone had its bottom face grounded, and its top face con-nected to the electrometer.

Measurements were made under a vacuum of 10-2 Pa.The samples remained in vacuum for ten minutes beforethe application of the voltage, to avoide vacuum-induceddepolarization [8]. All measurements were carried outwith virgin samples, except in recovery measurements.The electrometer used was a 610 C Keithley, and the ra-diation unit was a x-ray Muller MG-150 with a Philips2184/00 tungsten tube.

Exposure rate was monitored by a "free air" ionizationchamber and with CaF2 thermoluminescent crystals. Inall measurements, the dark current was recorded for 15minutes before the irradiation was turned on. Afterthis time the dark current was very small compared tothe induced current.

0018-9367/86/060)O-0431$l .0 cg, 1986 IEEE

431

IEEE Transactions on Electrical Insulation Vol. EI-21 No.3, June 1986

CROSS SECTION OF THE APPARATUV

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Fig. 1: Cross section of the measuring apparatus.

RESULTS

Measurements with Mylar

Figs. 2-5 show values of the RIC against irradiationtime t (logarithmic scale) for different values of theparameters E, X, d and with different values of the ef-fective radiation energy. After the onset of irradia-tion the RIC increases to a value which remains approxi-mately constant for a time t'. Subsequently it starts

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1 10 10llog t(s)

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° 140 R/s4 280 R/so 330 R/sa 420 R/s

E 8ox104V/cm23 pum

0 A0

0 AD0. .

0 00 A

o0 0~Oo o0 o o ° ° ° ° °o 0 000

410 02 3 10

log t (s)

Fig. 3: RIC as a function of time for PET withdifferent exposure rates.

2.0

rE-C

0 1

_ 1.C

1 10 1f0log t(s)

10o 10"

Fig. 4: RIC as a function of time PET withdifferent scanpZe thicknesses.

2.0

1`44.I

co.0

Fig. 2: RIC as a function of time for PET withdifferent values of the appZied field.

to increase again, goes through a maximum, at time t",

and drops slowly to a constant value. Subsequently itremains constant even if irradiation is continued formany hours. The total time to reach the steady-state isof the order of 7 h. The value of t' depends on E, dand, more strongly, on X. As an example, for a samplewith d=23 ,um, E-8x104 V/cm and X=280 R/s, one findst '=250 s and tl=l1400 s. Fig. 6 shows the behavior ofthe induced current when the applied voltage V is in-verted during irradiation. If the effect of the renewedabsorption current is substracted, the polarity reversalis found to change the polarity of the current, but notits absolute value. This seems to contradict the viewthat the increase of current is due to electrode switch-ing [2]. Fig. 7 shows the value of the steady-statecurrent (after 7 h of irradiation) as a function of X,

for different values of E and d. The relation between

0-0 Ix= 10mA0 [a 55 KV

0 vx * 75 KV0 0 0 0 0 o 95 KV0 9K

0 AA E=3.2 xl05Vi/cA A 23pm

2A AAAA AA

1 10 i0. 10. 10.I 10 102 103 1C04

log tf(s)

Fig. 5: RIC as a function of time for PET withdifferent vaZues of the effective radiationenergy. Vx and Ix are respectiveZy the voZtageappZied to the x-ray tube and the current throughthe tube.

the current I and X follows a power law I%A withA=0.93+0.01, for all values of E and d covered by thepresent measurements. The steady state current as a

function of E is shown in Fig. 8 for different valuesof X. The increase of the current slope for fieldsabove 8x104 V/cm can be quantitatively interpreted interms of Onsager's theory of initial recombination [6].Due to this effect, the generation rate of free carrierpairs increases with increasing field.

Successive irradiations of the same sample give re-

sults which depend on the time intervale between

a 6jimA aA A o 12.5 pm

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Filho et al.: Induced conductivity of Mylar and Kapton irradiated by x-rays

Fig. 6: PET: Effect of voltage reversals duringirradiation on the induced current.

/>0 n~~~~l O

a E'

1010

a E

2 3 4 6 8 10 20 30 40 60 80log E (104 V/cm)

Fig. 8: PET: Steady-state current, as a functionof appZied fieZd, in Zogarithmic scaZe, fordifferent vaZues of exposure rate. Before eachmeasurement the sampZe was irradiated with thecorrespondings field and exposure rate for 7 h.

2.C

1.5

'01.0

0.5

Fig. 7: PET: Steady-state current, as a functionof exposure rates for various applied fields.Before each measurement the sample was irradiatedwith the correspondings field and exposure ratefor 7 h.

measurements, as shown in Fig. 9. Gradual recovery ofthe sample is observed with increasing radiationlessstorage time at room temperature. Practically completerecovery occurs after a storage of 2 h at 800C.

Fig. 10 presents a measurement in which the irradia-tion was interrupted twice while the field remained ap-plied. The dots show that after each interruption theinduced current starts with the same value which it hadwhen the irradiation was cut off. This proves that theinduced conductivity depends exclusively on the interaction between radiation and material, and is not af-fected by polarization.

When the irradiation was discontinued, the value ofthe RIC did not disappear immediately. Initially itpresented an abrupt drop, followed by a slow decay.This part of the induced conductivity is called "delayed

1 10 lo0log t (s)

103 104

Fig. 9: PET: Recovery curves. Induced current asa function of time for successive irradiations ofthe same sampZe. Between irradiation the sampZewas stored in short circuit at 260C and atmosphericpressure (humidity 60%).

og Cs)

Fig. 10: RIC for PET as a function of time with anintermittent appZication of the radiation.

* VIRGIN SAMPLEa REIRRADIATED AFTER 43 HOURS AT 26°Co REIRRADIATED AFTER 112 HOURS AT 26°Co REIRRADIATED AFTER 240 HOURS AT 26°C

E - 8.7 x 1O4V/cm- 280R/s

T-26Clim 0 00A

23jim 0t-0 0 "I

4; Iz

IEEE Transactions on Electrical Insulation Vol. EI-21 No.3. June 1986

.-

9k G

* E 3.2.10 0,X-280,° E:~2 - tO cm,

0 280 R

L Ez-3.2.x10 cy - nz R

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° o

10log t(s )

Fig. 11: DRIC for PET as a function of time, in aZogarithmic scale, for different fields and ex-posure rates (slow decay).

radiation-induced conductivity" (DRIC). In Fig. 11,the DRIC is shown for different fields and exposurerates.

Measurements with Kapton

Figs. 12-15 show the RIC astion time for different valuesmeasurements a strong increase

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log t (s)Fig. 12: RIC as a function of time for Kapton with

different values of applied field (Zow fields).* 8 x104V/cm 0o 1.6x105V/cm 02.4x105V/cm X280 R/s 0 03 A 3.2 x105V/cm 25om 0n o

o4 x 105V/cm 0 aO0 0. .0 0 0 0 * *.0E 0 o

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AA A A A A A A 40b) A A

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IN

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1 10 10- 8- 0log t(s)

Fig. 13: RIC as a function of time for Kapton withdifferent vaZues of appZied field.

O 140 R/s

* 280R/s E = 8X104 V/cmno 420R/s 25 Am 0c

12 o- 550 R/s ° do

8E OOo

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00 0 00 00 0 000 0 00 000

0 ~~~0000S.0 5~~~~~~~0I

e:) o o~~~~~~~~~~~~o o °-1

4 -

a0 A A A A3 000ezni c

1 10 10 i03 104log t (s)

Fig. 14: Kapton: RIC as a funotion of time fordifferent exposure rates.

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E = 8 x104 V/cm %

5-mX280 R/s .000A

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Fig. 15: Kapton: RIC as a function of time fordifferent sample thicknesses.

observed after an irradiation time of the order of2x103 s which subsequently continues over the whole ir-radiation time of 7 h. The increase of the conductiv-ity is preceeded by a quasi-steady state whose durationdepends on E, X, and d. The relation between the qua-si-steady current and X is shown in Fig. 16. It fol-lows the same power law as for Mylar. Measurementscarried out using the same sample are shown in Fig. 17.The fast recuperation of the sample after heating isnoted. The RIC in Kapton is independent of the polar-ity of the field, shown in Fig. 18. This result issimilar to that obtained in Mylar.

Fig. 19 presents the slow decay of the DRIC for dif-ferent applied fields.

DISCUSSIONS AND CONCLUSIONSThe increase observed in RIC in Mylar after the time

t' has been interpreted by a switching model [2]. Thismodel assumes that a concentration of one-sign carriersin traps near the interface metal-insulator increasesthe electric field in the region causing an injectionof carriers from the electrode. If this was true, theabsolute value of the inverted current after a voltagereversal would be smaller. But the experimental resultpresented in Fig. 6 shows that the absolute value ofthe current does not change. Furthermore, when thevoltage was applied intermittently on the sample, no

103 l04

4

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16o

Filho et al.: Induced conductivity of Mylar and Kapton irradiated by x-rays

8

6

A 6jum

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-11 { ^70 um100 200 300 500 900

log X(R/s)

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Fig. 18: Kapton: Effect of voZtage reversalsduring irradiation on induced current, X=280R/s and d=12.5 pm.

-7

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Fig. 16: Kapton: Induced current (quasi-steadystate), as a function of exposure rate for dif-ferent sample thicknesses.

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5......5a. **0

04,00,

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F~~~~~~'

o 00

Aiq

0o

Jo 00o0

10 102 103 104log t(s)

Fig. 19: DRIC for Kapton as a function of time, ina Zogarithmic scaZe, for different fields.

Fig. 17: Kapton: Recovery curves. Induced currentas a function of time for: Virgin sample (e), afterstorage fbr 15 h at room temperature (o), andafter a storage time of 3 h at 80° C(A).

polarization currents were observed. The result shownin Fig. 10 also confirms that the RIC is only due tothe carriers generated by the irradiation.

Fig. 20 shows that the RIC in Mylar in the steady-state regime is field-independent up to values of8X104 V/cm. For higher values, the RIC varies almostlinearly with E. This effect already mentioned above,could be due to recombination. From Onsager's theorythe relation between the slope of the linear part ofthe curve and the intercept of its extrapolation withthe conductivity axis is:

6

E

4

2

1 2

Ex l Vm

3 4

Fig. 20: RIC (steady-state) of PET as a functionof the applied field. X=280 R/s and d=23 pm.

435

5 *VRGW SAMPLEo REIRRAIXATED AFTER 15 HOURS AT 26°C o

4 A REIRRADATED AFTER O 0 0 0 03 HOURS AT 801C 0

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1 10 102log t(s)

a.. 2.37 x id" (INTERCEPT)CL, =643 x tO (SLOPE)

_*~~C

Lo I

---I

-11

cle

IEEE Transactions on Electrical Insulation Vol. EI-21 No.3. June 1986

s'ope e_ 3intercept 8TTrcok2T2 (1)

where £ is the relative dielectric permittivity of thematerial, k the Boltzmann constant, T the absolute tem-perature, and e the electronic charge. This equationgives a value c=3.2, in good agreement with the knownvalue of £ for Mylar.

Fig. 9 shows effects of RIC recovery in a sample atroom temperature. Similar RIC recovery effects werealso observed in Teflon [9]. In this case, the steady-state current value was attributed to deep trap satura-tion instead of detrapping [10] .

To adjust the experimental results, we propose a ki-netic model of monopolar conduction, assuming bimole-cular recombination and two levels of traps, one ofwhich becomes saturated. But some effects are not ex-plained by this theory, such as the behavior of the RICin samples with 6 pm thickness. This might be due todifferent properties of the material, possible doping.

The influence of the electrode material on the RICwas also verified. Fig. 21 shows a measurement of theRIC as a function of irradiation time, in which the RICvalue with Au electrodes and X=280 R/s is almost thesame as with Al electrodes and X=420 R/s. This resultshows that the electrode can increase the dose rate,this effect being higher for electrode material of highatomic number.

3C

2

TE 2JD

O ,~

* - a00a

o X=280 R/s(AI)* X-4Z0 R/s(AI)& X- 280 R/s (Aud

E2x104V/cm23yum

*L

00 L

oo0000000000000 0 0 OC000

10 102 103 104log t (s)

Fig. 21: RIC for PET as a funotion of time fordifferent materials of the evaporated eLectrodes.

ACKNOWLEDGMENTS

We wish to thank Mr. Dante Chinaglia and Mr. MarcosSemenzato for technical assistance. One of the authors(B.G.) acknowledges receipt of a research grant fromCNPq.

REFERENCES

[1] H. Maeda; M. Kurashige and T. Nakakita, "Gamma-Ray-induced conduction In polyethylene-tere-phthalate under high electric fields", J. Appl.Phys. Vol. 50 (2), p. 758-764, 1979.

[2] R. C. Hughes, "The electronic properties of themetal-insulator contact: space-charge inducedswitching", J. Appl. Phys. Vol. 51 (11), p. 5933-J-944, 1980.

[3] -S. J. Walzade; P. P. Jog; S. B. Dake and S. V.Bhoraskar, "Electron beam induced conductivity inPET and FEP", Solid State Commun. Vol. 46 (5), p.393 396, 1983.

[4] A. P. Tyutnev; G. S. Mingaleev; V. S. Saenko andA. V. Vannikov, "Radiation induced conductivityof polyethyleneterephthalate and polystyrene",Phys. Stat. Soli. (a), lVol. 79, p. 651-659, 1983.

[5] R. M. Faria; B. Gross and R. Gregoria F0,"Radiation-induced conductivity of polyvinylidenefluoride, Mylar and Kapton", 1984 Annual Report,Conf. Elec. Insul. Dielec. Phenom., p. 417-422,1984. IEEE rep. 84 CH1944-3.

[6] L. Onsager, "Initial recombination of ions", Phys.Ref., Vol. 54, p. 554-557, 1938.

[7] B. Gross, "Compton Current and polarization ingamma-irradiated dielectrics", J. Appl. Phys.,Vol. 36 (5), p. 1635-1641, 1965.

[8] R. M. Faria and G. F. L. Ferreira, "Vacuum induceddepolarization in Mylar under an electric field",Sth Intl. Sump. on Electrets, Heidelberg, 1985.G. M. Sessler and R. Gerhard Sessler, Edts. IEEErep. 85 CH2166-7.

[9] B. Gross; R. M. Faria and G. F. L. Ferreira,"Radiation-induced conductivity in Teflon irra-diated by X-rays", J. Appl. Phys. Vol. 52, p.571-577, 1 981.

[10] B. Gross, V. von Seggern and D. A. Berkley, "Ratetheory of radiation-induced conductivity of fluo-roethylene propylene (FEP)", 1982 Annual Report,Conf. Elec. Insul. Dielec. Phenom., p. 206-213, 1982.

(Registered Trademark

Manuscript-was received 9 Otober 1985.

This paper was presented at the 5th InternatioonaSywposium on Eteatrets, HeideZberg, Germany, 4-6September 1985.

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