Pyroelectric properties and the nature of the polarization of polyacrylonitrile

~0

8. 9. 10. I1. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23.

V. S. L I K H O V I D O V et u 7,.

D. PAUL, D. LIPKIN and S. WEISSMAN, J. Amer. Chem. Soc. 78: 116, 1956 T. LIPATOVA, G. SHAPOVA and E. SHEVCHUK, J. Macromolee. Sei. A5: 345, 1971 E. SCHABEL, R. HOSEMANN and B. RODE, J. App[. Phys. 43: 3237, 1974 C. KRATKY, O. KRATKY and E. WRENTSCUR, Acta phys. austriaca 41: 105, 1975 C. VONK, FFSAXS, Program for the Processing of Small Angle X-ray Scattering Data, DSM, Geleen, Netherlands, 1975 C. VONK, Calculations with Absolute Intensities in the Programm FFSAXS 2, DSM, Geleen, Netherlands, 1975 L. I. MIRKIN, Spravoehnik po rentgenostrukturnomu ana]izu polikristallov (Handbook of X-ray Structure Analysis of Polycrystals). Fixmatgiz, 1961 A. CUINIER and G. FOURNET, Small-Angle of X-rays N.Y., 1955 G. POROD, Kolloid-Z. 124: 83, 1951 O. KRATKY, G. POROD and K. SKALA, Acta phys. austrim~.a 13: 76, 1960 I. H . D U R C H S C H L A G , G. P U C H W F J N , O. K R A T K Y , I. SCHUSTER and K. KIRSCHNER, Europ. J. Biochem. 19: 9, 1971 J. WENDORFF and E. FISCHER, Kolloid-Z. und Z. f'tir Polymere 251: 876, 1973 J. WENDORFF and E. FISCHER, Kolloid-Z. und Z. f'tir Polymere 251: 884, 1973 T. LIPATOVA and N. BASILEVSKAYA, J. Maeromolec. Sci. AIO: 1257, 1976 T. LIPATOVA, G. SHAPOVAL, N. BASILEVSKAYA, International Symposium of Macromolecules, v. 2, p. 572, Helsinki, 1972 T. LIPATOVA, G. SHAPOVAL, N. BASILEVSKAYA and E. SHEVCHUK, J. Polymer Sci. C 42: 1, 1973

PolymerScienceU.S.S.R.Vol. 20, pp. 80-87. ~) Pergamon Press Ltd. 1978. Printed in Poland

0032-3950/78/0101-0080507.50/0

PYROELECTRIC PROPERTIES AND THE NATURE OF THE POLARIZATION OF POLYACRYLONITRILE* V. S. LIKHOVIDOV, V. V. GOLOVAI~OV a n d A. V. VA~mKOV Electrochemistry Institute, U.S.S.R. Academy of Sciences

(Received 11 February 1977) The pyroelectric peroperties and the nature of the polarization of PAN have been investigated. The polarization that gives rise to the pyroelectric properties is caused by the "freezing in" of oriented dipole groups during cooling to room temperature in an electric field. The pyroelectric current is determined by the rotation of these dipoles around an equilibrium position as the temperature is changed. The maximum value of the pyroelectrie coefficient, P, is equal to 3-9 × 10-° coulomb/ /cm 2. deg K. The production of electret films of large area and having a high value of pyroelectric coefficient makes it possible to put forward the question of using them as element~ for the direct ~ansformation of solar energy into electrical energy. * Vysokomol. soyed..%20: No. 1, 71-76, 1978.

Polarization of polyacrylonitrilo

81

THE pyroelectrie properties observed in electret polymeric films have recently enabled rapidly acting detectors of electromagnetic radiation to be produced, which work over a wide range of wavelengths, the films being used in these detectors as registering elements [1, 2]. Such films are especially promising for working in the super-high frequency region when receivers with reception areas of large size are required that should be sensitive to electromagnetic waves of various modes being propagated in a wave guide. The possibility of using electret polymeric films as targets for "Vidicon" television transmission tubes has also been discussed [3]. The principal feature of such tubes is the almost complete absence of the red spectral boundary. Electrostatic colouring, based on the pyroelectric effect in an electret polymeric film (as distinct from that based on photo-conductivity in the electrographic method) does not depend on the wavelength used [4]. Pyroelectric properties are observed in polymers which have been subjected to the action of a strong electrical field at an elevated temperature and have then been cooled without removal of the field. It is accepted at the present time that the formation of an electret state during thermopolarization can occur by two mechanisms: through the injection of charges into a near-surface layer of the film either during the orientation of constant dipole groups, existing in the polymer, in an imposed film or through the migration of ions through minute distances in the volume. Charges are injected from electrodes or from the air through the local breakdown of the air and breakdown of the dielectric-electrode interface. The value of the homocharge injected, ahomo, depends on the following: the magnitude of the imposed voltage, structural heterogeneities both in the film itself and in its surface (that is, the number of traps), the depth of these traps, the electrical conductivity of the polymer and temperature. The case when charges of the same sign are injected into the volume of the polymer has been analysed [5]. The distribution of entrapped charges through the film thickness is set by the condition that the potential difference between the two surfaces of the film should be equal to zero. The creation of a pyroelectrie current under these conditions is possible only through non-uniform thermal expansion of the film whilst it is heated. However, the value of the pyroelectrie coefficient, 1o, calculated with such a mechanism for PVDPh, is four orders of magnitude less than that obtained experimentally. The polarization connected with the orientation of dipole groups in an electric field at an elevated temperature has volume characteristics. Its value P, that is, the value of the charge surface density, depends on the number of polar groups in unit volume and on the magnitude of the dipole moments of these groups. The sign of the polarization P corresponds to a heterocharge in this case. Since the polarization is volumetric, the formation of the pyroelectric current c a n be connected with the volumetric expansion during heating [6] and with the

89

V . S . LIK.HOVIDOV ¢4 al.

change in dipole characteristicsi

dP P = -d-~

=tiP+PfI~ d-fi) ' -d-T '

(*)

where P is the value of polarization for unit volume of the polymer, fl is the coefficient of volume expansion and f (~, d~/dT) is a function connecting the change in the mean dipole moment of the group ~ with change in temperature. If it is assumed that all the dipole groups in the polymer are oriented in the direction of the external applied field, E, the value of P for PVDPh is approximately 6 X 10-° coulomb/cm2 [7]. The value of fl for PVDPh is equal to 6 x 10 -4 (deg K) -x (fl=3a, where a--2 x 10-4 [8] is the coefficient of linear expansion). With these values of P and fl, the pyroelectric coefficient, p, is equal to 3.9 X 10-* coulomb/cm2.deg K. The value of p obtained experimentally for PVDPh is equal to 2.4x 10-9 coulomb/cm~.deg K [9]. It may be seen that these values agree well in magnitude. As a rule, however, such large values of P, caused only by the orientation of dipole groups, are n o t observed in polymer films. Such values of P are difficult to obtain even with the injection of charges into the polymer (for example, by use of a coronary discharge [10]). Thus the greatest value of ~o should have occurred in the case of PTFE, being the polymer having' the strongest electret properties, that is, the greatest Value of P connected with entrapped homocharges. This is not, in fact, observed. The value of p for P T F E is equal to 0:8X 10-9 coulomb/cm~.deg K [11]. The pyroelectric coefficient p is therefore clearly determined principally by the second term in equation (1). In the general case, the polarization of electret polymer films is the sum of two types of charges (hetero- and homocharges), each of which can make its own contribution to the pyroelectric current. We have discussed [11] the pyroelectric properties of electret polymeric films made from non-polar PTFE. It was shown that the polarization is principally formed through injecfed charges. In addition to the homocharges, heterocharges connected with the orientational polarization in P T F E of chemical impurity dipoles and structural defects were observed for the first time. The present work deals with the nature of the polarization and the pyroelectric properties of the polar polymer PAN having a dielectric permeability e = 6.5. Polymer films were obtained b y evaporation of a solution of P A N in DMF on to quartz substrate having a conducting layer of SnO, which served as the lower electrode. The upper semi-transparent aluminium electrode was deposited b y vacuum sputtering. The specimens obtained were held at 75-100°C (the region of the first glass point is 85-96°C) in an electric field ( E = 10~ V]em) for one hour, after which the specimens were cooled t o room temperature in two hours without removal of the field. To measure the polarization, the films were separated from the Quartz substrate a n d measurements were made from the side not having an electrode. Polarization was measured b y the lifting electrode method. The pyroelectric properties were investigated dynamically I l l ] using as radiation sources CO, and H e - N e lasers, whose power was constantly moni.

Polarization of polyacrylonitrile

88:

t o r e d during measurement of the pyroelectrie signal b y reflecting some of the energy of the laser radiation on to the receiving element of an IMO-2 power meter. The pyroelectric coefficient was calculated from the oscillograph record (Fig. la) of t h e pyroelectric voltage Up [11]:

Up

~PovezT [Ae_~/,T+

Be_t/,.],

(2)

TT-- re

where 2'0 is the power absorbed per unit area, r e = R C , where R and C are the total resistance and capacity of the specimen, leads a n d entry resistance and capacity of the amplitier, vT= C~,/G~, where CT is the thermal capacity of the specimen and GT is the speeimen's thermal conductivity, ~=p/ec,~, where cv is the specific volumetric heat capacity and A and B are constants depending on the time of action of the radiation and the time of cooling of the specimen. Up,V 1 al ~ 2

~

[/N

.°

ii i JA'

2

~

p,arb,L un.

r

J

,z L/381 l"-" I\/ I : \ / [ I\/ _1 I " t ~" l IV a

-1

CL =I I Z !AI Y ~2#

J#8

l~r//seC l p~ orb.un.

b

1

I

4 ~_;_~.o " o - ~ ~°-

~

%=°0.0.0.0-

2 t

~

tO

i

I

20 Fro. 1

~

200~ o

o

~5ol-

o~

z

I

30 l , lO -z, prn

!

2

22

Dose, Mt,ad

Fie. 2

FIG. 1. a - - T h e pyroeleetric signal from PAN: the regions 1 described b y equation (2)) correspond to the illumination being on, and regions 2 to the illumination off; b - - t h e change in the pyroelectric coefficient as the laser beam is scanned over the specimen surface. Fzo. 2. The change in the pyroelectric coefficient with dosage of 7-radiation absorbed: 1--thermopolarized specimens; 2--unpolarized specimens. Since pyroelectric characteristics depend considerably on the conditions of polarization, the molecular weight of the monomer and the film thickness, the values of pyroelectrie coefficient for the films investigated v a r y from specimen to specimen. The m a x i m u m value of pyroelectric coefficient for the specimens investigated, calculated from equation (2), was equal to 3.9× 10 -s coulomb/cm2-deg K, the value of the quality index ~ (~----p/~cr) being 1000 cruZ/coulomb. The minimum values of p and ~ were equal to 1.2 × 10 -9 coulomb/era l• deg K and 314 crag/coulomb respectively. I t should be noted that, when the laser beam was scanned over the surface of the P A N specimen (the cross-section of the beam was reduced to less t h a n 1 ram* with a diaphragm), the value of the pyroeleetric coefficient remained constant for the different points on t h e surface (Fig. lb) whereas, in the case of P T F E specimens, the pyroelectric coefficient was substantialJy different at the different points [11].

V. S. LIKHOVIDOV et ~[.

The value of p did not change over a period of several months, although the polarization changed very considerably with time, tl~e polarization even changing to the opposite ~ign, PAN films that have not been thermally polarized beforehand also have pyroeleetric properties but the value of the pyroelectric coefficient varies considerably from specimen to specimen and iS an order of magnitude less than the values of p for thermally polarized ~peeimens. During the T-radiation of polymeric films, conditions are known to be created ~for elimination of trapped charges of one sign through recombination processes ~vith charges generated by the radiation. I t m a y be seen from Fig. 2, which ~hows the change in pyroelectric coefficient as a function of the dose of T-radiation for thermally polarized and unpolarized specimens, that the value of p for t h e thermally polarized specimens is not altered. Thus, on the one hand we have, in the case of thermally polarized specimens, a v a l u e of the pyroelectric coefficient that is constant with time and is constant e v e r the surface, and on the other hand we have a value of P that varies strongly ~vith time. This can be explained if we assume that the total polarization meas~1 by the lifting electrode method, is the sum of two polarizations: P~--Pvol-~-Psurface, w h e r e Pro1 is the volume polarization connected with the "freezing in" of dipoles ~luring cooling in the electric field and Psurtace is a surface polarization connected ~vith the trapping of charges in a layer of the film adjacent to the surface; Peel
Polarization of polyacrylonitrile

85

rization that we measured (Pvol. ~ 10 -9 c o u l o m b / c m z) is small and the value o f the coefficient of volume expansion ( p ~ 1 0 - 4 T -1 gives a value of p four orders of magnitude less than that obtained experimentally. The second term in e q u a • p, orb.un. 0

25-

0

Q

o

!

15-

5-

2 I¢0

80

120

160 7;,°0

Fro. 3

Fro. 4

FIa. 3. Temperature dependence of the pyroelectric coefficient: /--heating; 2--subsequen~ cooling. FIe. 4. Formation of Pro1 in an external field. 1 is the film thickness, Pvol = P + - I - P - {without~ the field, P, ol= 0, in the electric field, Pro1 # 0); E is the strength of the applied field and r~ the normal to the film surface. tion (1) is connected with the change in the mean dipole moment of unit v o l u m e , caused b y thermal vibrations of the dipoles about an equilibrium position. Since~ the total dipole moment of unit volume in the unpolarized polymer is equal to, zero, it m a y be considered that the number of dipoles N+, having a p r o ~ c t i o n in the direction of the normal to the film surface, will be eqgal to the number of: dipoles N_, having the opposite direction, that is, Pvol=N+/~ 0 cos ~-~N-po cos ( ~ + ~ ) = 0 ,

(3)~

where #0 is the dipole moment of a group and ~0 is the mean angle between th~ normal to the film surface and the direction of the overall dipole moment N+g~ of unit volume (Fig. 4). The angle q depends on the internal structure of the polymer chain, the degree, of crystallinity and its temperature. • During thermal polarization, the dipoles are turned through an anglo whose~ average value ~ depends on the conditions of polarization, that is, on the temperature of polarization and the magnitude of the imposed field E. In the general case, Pro1 is thus different from zero and is given by:

Pvol=N+~0 cos ( ~ - - ~ ) + N - ~ 0 cos [ ~ + ( ~ + ~ ) ] = N ~0 sin ~ sin ~,

(4)

where N is the number of dipoles in unit volume. The pyroelectric coetiicient~

: 86

V . S . LIKHOVIDOV e$ al.

is determined from the equation: dPvol

/

--[T- P,o, COt

a~

+cot

_ d~\

(5)

I t may be seen from equation (5) that the change in polarization is connected with a change in the angles ~ and -~. The change in the angle ~ occurs irreversibly, that is, as the temperature increases, the polarization Pvol decreases and does not reach its initial value upon cooling. Thus for each fixed temperature, the pyroelectrie coefficient has its own constant value, since the change in the angle is reversible. Pyroelectric properties will be observed in the polymer so long as the angle ~ is different from zero. In order to find the explicit form for the equations for ~ (T) and-~ (T), detailed information about the molecular supermolecular structure of the polymer is required. 5-

3-

[\

I

3g

50

7or,~

~ G . 5. Change in the pyroelectric coefficient as a result of heating; time of holding a t each temperature, 10 min; values of Up recorded at room temperature.

The production of electret polymer films of large area and with a high value of th$ pyroelectric coefficient makes it possible to pose the question of using them as elements for the direct transformation of solar energy into electrical energy. The use of the semiconductor solar transforming elements that exist at present, despite their high efficiency (of the order of 10% [13]), to obtain energy on a large scale is difficult because of the high cost of semiconductor materials and t h e necessity of using concentrators [13]. Such semiconductor converters are principally used for space equipment. The results.of the present work give evidence of the possibility of forming cheap direct converters to change solar energy into electrical energy by using the pyroelectric effect in, for example, PA_~ film. It is true that the efficiency of such a converter, obtained in the present work, is small (of the order of 5X 10-s%). The maximum value of efficiency is determined by the thermal efficiency of a reversible Carnot cycle efficiency=

TI--T, T

'

(6)

Polarization of polyacrylonitrile

87

where T1 (in our case) is the m a x i m u m t em perat ure to which the film can be heated without causing a change in the pyroelectrie coefficient after it has been cooled to a temperature T2. I t follows from Fig. 5 t h a t T1 is 323°K for P A N films and, consequently, the m a xi mum efficiency is equal to approximately 10% from equation (6). The low value of efficiency t h a t we obtained is caused b y the fact t h a t the changes in temperature in the experiments to measure the pyroelectric coefficient were small (of the order of 0.1 deg C). The question of efficiency is not a basic problem if the cost price of the electric energy obtained by such a method is less than t h a t of energy obtained b y using converters with a high efficiency. Thus P A N thermally polarized in an external electric field is characterized b y a substantial pyroelectric coefficient (p~-3.9 × 10 -9 coulomb/cm ~. deg K). The polarization Pvol, causing the pyroelectric properties, is connected with the "freezing in" of oriented dipole groups during cooling to room temperature in the electric field. The pyroelectric current is determined by the rotation of these dipoles around an equilibrium position as temperature is changed. The production of eleetret films of large area and having a high value of the pyroeleetric coefficient makes it possible to p u t forward the question of using t hem as elements for th e direct transformation of solar energy into electrical energy. Translated by G. F. ~¢~ODLEN REFERENCES 1. J. COHEN, S. EDELMAN and C. F. VEZZETTI, Nature 12: 233, 1971 2. R. J. RHELAN, Jr., R. J. MAHLOV an4 A. R. COOK, Appl. Phys. Letters 19: 337, 1971 3. A. W. STEPHENS, A. W. L E ~ an4 P. D. SOUTHAGE, Chem. Abstrs. 81: 42, 1974 4. G. BERGMA_N, G. R. GRANE, A. A. BALLMAN and H. M. O'BRYAN, Appl. Phys. Letters 21: 497, 1972 5. R. E. SALOMON, H. LEE, C. S. BAK and M. M. LABES, J. Appl. Phys. 47: 4206, 1976 6. L. J. YU, H. LEE, C. S. BAK an4 M. M. LABES, Phys. Rev. Letters 36: 388, 1976 7. E. W. ASLAKSEN, J. Chem. Phys. 57: 2358, 1972 8. H. SASABE, S. SAIT0, M. ASAHINA an4 H. KAKUTANI, J. Polymer Sei. 7 A-2: I405, 1969 9, A. 1~. GLASS, J. M. MeFEE and J. G. BERGlYIAN, J. Appl. Phys. 42: 5219, 1971 10. M. E. BORISOVA, S. N. KOI~OV, Yo. V. KIRILLOV, V. A. PARIBOK and V. A. FOMIN, Sb. Trudov M~EM, vyp. 27, p. 92, 1972 11. V. S. LIKHOVIDOV, V. V. GOLOVANOV ~n4 A. V. VANNIKOV, Vysokomol. soyed. A18: 2058, 1976 (Translated in Polymer Sei. U.S.S.r. 18: 9, 2352, 1976) i2. Entsiklopediya polimerov (Encyclopaedia of Polymers) Sovetskaya ontsiklopediya, vol. 1, p. 43, 1972 13. I. I. SOBEL'MAN, Uspekhi fiz. nauk 120: 85, 1976