350 Nuclear Instruments and Methods m Physics Research 227 (1984) 350-353 North-Holland, Amsterdam

E L E C T R E T C H A R G I N G U T I L I Z I N G T H E E L E C T R E T I O N I Z A T I O N C H A M B E R

G u n t e r P R E T Z S C H

Techmsche Unlversttat Dresden, Sektlon Physlk, 8027 Dresden, Mommsenstrasse 13, DDR

Received 1 December 1983 and m rewsed form 1 March 1984

Isothermal charge deposition on polymers to form stable electrets by using the two-electret lomzation chamber with parallel-plate geometry is reported One of the electrets exhibits a high starting voltage compared with the second electret to be charged and hence an electric field exists in the chamber volume Charge carriers produced by lrra&atlon of the sensitive chamber air volume drift in the electric field and cause a charge enhancement of the second electret due to compensauon of surface charges of the first electret This process is finished when both electrets reach the same value and polarity of the surface potential The method reqmres no external power supply and allows two electrets of the same surface potential to be produced

I. I n t r o d u c t i o n

The product ion of wax electrets using a thermal forming procedure was first reported by Eguchi [1]. However, it was only in the last decade that electrets based on f luorcarbon polymers with excellent charge storage propert ies have gained prormnence because of their numerous practical applicat ions m science and industry. Beside thermal electret formation, i.e. heat ing and coohng of a dielectric m the presence of an exter- nally applied electric field, Isothermal charging methods such as corona charging or the l iqmd contact technique are favored because of the ease and short process t ime in which they allow polymer films to be charged A survey has been given by Sessler [2].

Recently Fal lone and Podgorsak proposed a new isothermal technique for charging of one sided metal- lized foil electrets [3]. It is based on axr iomzat ion produced by X-rays in a chamber r e sembhng a paral lel-plate ionizat ion chamber . High voltage is ap- plied between the opposi te metal electrode and the metalhzed back of the electret. Charge ca reers pro- duced by i r radiat ion of the sensitive chamber mr volume drift in the externally apphed electric field and get t r apped on the polymer surface to form electrets. S~mi- lar investigations to form electrets were also made in our laboratory.

In this paper a new charging method is presented. It operates on the same p n n o p l e as descnbed in ref. [3] However, the externally applied electric field is replaced by the electric field o f ' ano the r electret with high start ing voltage (electret 1) and therefore an external power supply IS not necessary. Dur ing i r radiat ion the metal- hzed backs of the two electrets are short-circuited (see fig. l a). In the a]r volume between the two electret

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surfaces an electric field exists due to the different charge states. Charge carriers created by iomzing radia- t ion drift in the electric field in dependence of their s]gn to the corresponding electret surface and cause a charge compensat ion, 1 e. voltage decrease of electret 1 as well as a charge deposi t ion of the non-charged electret 2. Thereby the i r radiat ion doses are less than some Gy so that radia t ion reduced polarizat ion effects m the poly- mer can be neglected. Hence the charge states of the electrets are altered only by surface charges from the chamber air volume This charging method was investi- gated for different s tart ing voltages and polari ty of the electrets 1 and 2, different thacknesses of the electrets and the air gap between them.

2. B a s i c r e l a t i o n s

Fig 1 shows (a) the schematic representat ion of the two-electret iomzat ton chamber dur ing irra&at~on and

a n

$1 ~ ] _ _ _ ! . . . . I 1

I, 2 s2 L _ _

b

1 ~

! ~E%- c c G ~,

U0

Fig 1 (a) Schematic representation of the electret charging process 1 - Electret 1 with tugh starting voltage, 2 - electret 2 to be charged, 3 - sensitive chamber air volume (b) Schematic representation of the electret voltage measurement by the dy- namic capacitor method 1 - Measunng electrode, 2 - electret, 3 - vane with rotation axis

G Pretzsch / Electret chargmg 351

(b) the c o n & t m n of electret voltage measurement on the basis of the dynamic capaci tor method. Consider ing fig. la , the electric field in the air gap results to be [2]

Sl°I - s2°2 (1) E c = %(S l + s 2 + S c ~ ) '

Where s l, s 2 and o l, 0 2 describe the thicknesses and surface charge densities of electrets 1 and 2, respec- tively, s c the thickness of the mr gap in the chamber , c the dielectric cons tan t of the electret material (for Teflon it is taken to be 2) and c 0 the permlt t ivi ty of free space. The iomzat lon chamber used operates in the saturat ion region, i.e the electric field is s t rong enough to move all charge carriers produced by Ionizing radiat ion to the electret surfaces wi thout recombina t ion loss. As men- taoned, the start ing condi t ion is

ISl°o II > [S2°o,2l, (2)

where o01 and %z are the values before ~rradiatlon. Dur ing irradiation, charge carriers move to electret surface 1 and compensa te o 1 part ly by a value a o , so that o 1 decreases The charge carriers of opposite sign move to electret surface 2 independen t of its polari ty Therefore the charge densities can be described by

o l = o 0 1 - A o , ( 3 )

0" 2 = 0"0. 2 + A o (4)

Thus, charging of electret 2 is possible if %2 and Ao have the same sign. The var ia t ion A o of the charge densi ty can be expressed as a funct ion of the dose D in air by the relat ion

ep ,ao = ~ s c , a D, (5)

where e designates the electron charge, p the air densi ty and W the energy expendi ture to produce an ion pair Assuming a cons tan t dose rate b the var ia t ion of the dose is given by

A D = b A t , (6)

where A t s tands for the t ime interval of i rradiat ion. Fng. l b shows the configurat ion dur ing measurement

of the electret voltage. The electret voltage is defined as the potent ia l of the electret surface relative to the mea- suring electrode under short-circuit c o n d m o n s [2] Usu- ally, the relat ion ~s M >> s I is satisfied, where s M denotes the thickness of the air gap. In this case the electret voltage is given by [2]

U 1 = °lSl (7) C0~

The electret voltage was measured using a special mea- suring device based on the dynamic capaci tor method [4]. Thereby the electric field E M in the air gap IS compensa ted by an externally applied dc-voltage U 0 which in the case of E M = 0 is equal to the electret voltage, Relat ion (7) is also true for electret 2 Taking

into considerat ion eqs. (3)-(7), the following relations for the var ia t ion of the electret voltages are obta ined:

ep s c s i b A t , (8) ,aUl W CoC

ep ScS2 b A t . (9) ,aU2 = + - ~ ~oC

Thus the variat ions of the electret voltages for a given time interval or dose are connected by

A U 2 = - Sz AU1, (10) s~

and hence charging of electret 2 is possible due to discharging of electret 1. Relat ion (10) was derived for the saturat ion region of the chamber . For the recombn- na t ion region, 1 e. at lower field s t rength E c, the dif- ferential form of eq (10) can be derived [2].

dU2 s2 dUI (11) d D s 1 d D '

analogous to eq. (10) The process is finished when

E c = 0 , i.e U I = U 2 - (12)

3. Experimental

Thermoelectrets were made from Teflon PTFE foils with thicknesses from 0 1 to 1 6 m m and d iameter 30 ram. One surface of each foil was a t tached to an a lumlnlum disc of thickness 1 mm and diameter 34 mm.

The thermal forming procedure and the forming fixture were described elsewhere [5] After format ion the electrets were annealed for one hour at 160°C to achieve good charge stability The forming and anneal ing parameters were chosen in such a way that electret voltages of positive as well as negative polari ty in the range from 0 3 kV to 1 5 kV resulted [6]. The voltage decay over a period of 200 days was less than 3%. Fig. 2 schematically shows the cross section of the two-electret ionizat ion chamber . It consists of the metalhzed elec- trets and the capsule with the air filled chamber volume. Dur ing irradiat ion the backs of the two electrets are electrically connected through a metal pin. I r radiat ions were performed in the gamma radia t ion field of a 6°Co source of dose rate from 10 to 100 m G y / h in air. The electret voltage was measured immediately after nrradia- t ion by a measuring device based on the principle of the dynamic capaci tor [4,5]. The electret is therefore placed opposi te to a measur ing electrode. In the air gap be- tween the electret and the electrode a rota t ing metal propeller periodically screens the measuring electrode from the electric field of the electret and thus causes an ac-voltage on the electrode, which serves as the measur- ing signal. The electric field in the air gap can be compensa ted by an externally applied dc-voltage U o so

352 G Pretzsch / Electret charging

~ / / / / / / / / / / / / : / / ~ l~

rl i I 1

S

r 1 2 L J

Fig 2 Cross section of the electret lomzatlon chamber 1 - Electret 1, 2 - electret 2, 3 - capsule (Teflon), 4 - electret holder, 5 - metal pin, 6 - sensmve chamber air volume.

tha t the g e n e r a t e d ac -vo l t age van i shes . In th is case the

e lec t re t vo l tage is equa l to U 0 (see fig. lb ) . T h e u n c e r -

t a i n t y in the r ep roduc i b i l i t y of the m e a s u r e d e lect re t

vo l t age was less t h a n 1%.

4. Resu l t s and d i scuss ion

Elect re t c h a r g i n g e x p e r i m e n t s wi th c h a m b e r s o f dif-

f e r en t t h i c k n e s s e s [see eq. (5)], d i f f e r en t e lec t re t th ick-

n e s s e s a n d s t a r t i n g va lue s o f e lectret vo l t ages were m a d e .

T y p i c a l resu l t s are p r e s e n t e d m figs. 3 a n d 4

~ffi Irnm

kV I ,'-'

l,J

1,1

1,o

o,g

o,~

07

O,6

0,5

OA

0,3

O,2

0,1

-0 / /

/ o ~1 mO,15mm

A S 2 - 1,5 r n m

Gy O

Fig. 3. Charging of electret 2 wroth thickness s 2 = 1 5 mm due to discharging of electret 1 with thickness s 1 = 0 15 mm m a chamber of thickness s c = 1 mm as a funchon of the dose D m a i r

1'5k' ' ' ' I ' ' ' ~ J ' ' ' ' I . . . . I ' '

- U 0,5 ~ _

-0,5

• S~ : alrnm AS2 = 1,0rrv~

-I,00 , , , I .... I .... I , , , , I , 5 10 15 rr~ 20

O

Fig. 4 Charging and discharging of electrets with different thicknesses as mdmcated in a chamber of thickness s c = 20 mm as a function of the dose m air Corresponding curves are open orcles and triangles and full orcles and triangles, respectively

Fig. 3 s h o w s c h a r g i n g o f a n o n - c h a r g e d electret 2

(%.2 = 0) of t h i cknes s s 2 = 1.5 m m by an electret 1 of

t h i c k n e s s s 1 = 0.15 m m a n d a s t a r t i ng vo l tage o f U t =

- 1 2 kV u s i n g a c h a m b e r t h i cknes s o f s c = l m m

O w i n g to the s t r o n g electr ic field E c at the b e g i n n i n g

t he c h a m b e r o p e r a t e s m the reg ion o f p r o p o r t i o n a l i t y

a n d the dec rease o f e lectret vo l t age U 1 is no t l inear up

to 1 15 kV whe re the s a t u r a t i o n reg ions begins . If the

vo l t age d i f f e rence ]U1-U2I dec rease s be low 0 4 kV

r e c o m b i n a t i o n ef fec ts c o m e in to the play. In the s a tu r a -

t ion region, I.e. the l inear pa r t of the dose cha rac t e r i s -

tics, re la t ion (10) is e x p e r i m e n t a l l y c o n f i r m e d

In fig. 4 resu l t s for a c h a m b e r o f t h i c k n e s s 20 m m

are p r e sen t ed . T h e e n h a n c e d c h a r g i n g eff ic iency, ]e .

ra t io o f p r o d u c e d ion pa i r s a n d dose , c o m p a r e d wi th the

1 m m thick c h a m b e r is In a g r e e m e n t wi th eq. (5). T h e

e lect re t 2 c h a n g e s tts po la r i ty a f te r c o m p e n s a U o n of all

p r i m a r y ex i s t ing su r f ace cha rge s 002. Re l a t i on (10) as well as c o n d l h o n (12) are c o n f i r m e d .

5. C o n c l u s i o n s

F r o m the e x p e r i m e n t a l resu l t s the fo l lowing conc lu -

s ions can be made . C h a r g i n g of an electret of low

elect re t vo l tage is poss ib le , due to d i s cha rge o f an

e lect re t of h igh e lect re t vo l t age in the two-e lec t re t mom-

za t l on c h a m b e r w i t h o u t ex t e rna l p o w e r supp ly .

T h e rat io of the e lectret vo l tage va r i a t i ons b e h a v e s as

G Pretzsch / Electret charging 353

the ratio of the corresponding electret thicknesses The charging efficiency depends m a hnea r m a n n e r upon the chamber thickness. The dose characterist ics of the elec- tret voltages are hnea r in the sa tura t ion region of the chamber Electret charging and discharging is f imshed when the voltages of electrets 1 and 2 reach the same value and polari ty Hence, using this method two equally charged electrets can be produced The charge deposi- t ion on the electret surface ~s very uniform Charging and discharging can be described theoretically

Al though electret 1 has to be charged by a common method as described above, this charging techmque has some advantages Once electret 1 is formed a lot of electrets 2 can be charged without external power supply if a metal sheet is fixed on the top centre of electret 1 as described in ref. [7] This method of using the charge of the electret without d~rectly using the electret has the advantage that the electret remains intact and can be reused

Fur thermore , a two-electret ionizat ion chamber real- izes a dosimeter of two different sensitivities (see e.g

fig. 1). The thin electret 1 can be used for long term cumulat ive dose measurement with low sensitivity and the thick electret 2 for short term moni tor ing with high sensitivity. To enlarge the operat ion t~me electret 2 can be exchanged by another electret with zero potent ia l after exposition.

References

[1] M Egucht, Phd Mag 49 (1925) 178 [2] G M Sessler (ed.), m Topics m apphed physics, vol. 33

(Springer, Berhn, Heidelberg, New York, 1980) [3] B G Fallone and E B Podgorsak, Phys Rev B 27 (1983)

2615 [4] G Pretzsch and A Leuschner, Exper Techn Phys 31

(1983) 265 [5] G Pretzsch, B Dorschel and A Leuschner, Rad Prot Dos

4 (1983) 79 [61 G Pretzsch, Phys. Stat Sol (a) 79 (1983) K 139. [7] P Kotrappa, S K Dua, N S Plmpale, PC Gupta, K S.V

Nambl, A M Bhagwat and S.D Soman, Health Phys. 43 (1982) 399