Prasad, R S

Prasad, S N

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Thermally stimulated discharge currents in bees* wax electretsS. N . P r a sa d* a n d  R . S. P rasadDepartment of Physics  ^ Patna University  ^ Patna 800005 (Received 10 December 1974, revised 14 April 1975)Thermally stimulated disohaige currents have been obtained from Bees’ wax eloctrots prepared at various polarising voltages in the temperatures range 52°C to 62‘^ C. Total betorooharge released on depolarisation is not linear with applied polarising field indicating non-uniform volume polarisation Honco besides dipolo orientation, the charge injection from the cloctiodos and migration of iAns with microscopic displacement with trapping with subsequent spacflj charge build up is the proposed modeltndiam J» Phya. 4^ * 590-60^ (197^)1. I ntroductionAn oloctrot is a dielectric slab exhibiting a i>ermanont electric field. Heaviside (1919) was the first who got the idea of electrot and Eguchi (1925) made the first eloctrot by using oarnauba wax The most of the expeiim ental work on cleetret as reported in the litreatuie so far are confined to the study of the nature of surface charges, their magnitude and period of retention and effects o f polarising field and temper atuie on the nature of surface charges. But very little work has been mentioned about the factors responsible for the formation of electrets Investiga­tions of the electret state opens a new chapter foi studying the mechanism of polarisation of dielectrics. Theoretical explanations regarding formation of electret are still inconclusive. Gemant (1935) proposed that the heterocharge on the electrot is due to displacement of ions and dipole orientation inside a d i­electric. Important contributions were made by Gross (1949) and confirmed by  Wiseman & Feaster (1957). According to them heterocharge is formed by the process of charge absorption in dielectrics, while homochaige is due to break down at the diclectric-oleotrode interface. The lieterooharge is an internal volume effect, the nature of which remains in doubt. A non-uniform volume polarisation con be produced by space charge build up associated with the migra­tion of ions within the dielectric over microscopic distances or charge injection from the electrodes. A uniform volume polarisation is produced by dipole orien­tations or migration of charge over microscopic distances with trapping or both* Permanent address ; Department of Physics, Bhagalpur CoUege of Engmeering, Bhagalpur, Bihar.696Discharge ckrremits in eledrets 697-Baoently Perlman (1971) and Campoa (1971) have used the technique o f  iheim ally stimulated currents for analysing the olectiet effect and for studying of trapping parameters in electret. They hold the view that polarisation is uni­form in oarnauba wax eleotieta and, charges suffer microscopic displacement during polarisation and are trapped. This paper reports on our work on the depolarisation o f the thormoeloctret state o f bees’ wax. It is mainly concerned with the study o f the thermal release o f charges believed to arise from the trapping of electrons and holes injected by electrodes along with migration of charges over microscopic distances and dipole reorientation. The results throw more light on the main points of dispute in electret studies, that is, whetlier a uniform volume polarisation or a space charge mechanism or a rombination of two is responsible for the electret state formation.2. E xperimbjntalThermoolectrets were prepared from commercially obtained bees’ wax (imported). The bees’ wax sample contained a number of in gradients and hence ]t did not have a sharp melting point, Howciver, the sample melted completely between 60°C to 61 °C. The principle of measurement employed in the present woik was that of two parallel plate electrodes capacitor. Both the electrodes wore made o f aluminium of size three cm in diameter. Lower electrode was fixed in small copper basin while upper electrode was made to move vertically on screw arrangement. The separation between the olootrodes was kept one mm The copper basin was placed on insulated base made of asbestos. This assembly was kept in thermostat (model Hopplor N B E ) and an electric field was applied b y  high voltage power supply (model HV211, Electronic Corporation of India). A positive polarising voltage was applied to the upper electrode while the lower electrode was grounded. The temperature was raised and at the fixed temperature the sample was left for throe hours and it was then cooled to room temperature in the presence of electric field. I t  took nearly three hours to reach the room temperature. After taking out the sample from thermostat, it was kept in desiccator in short circuit condition to remove all the stray charges deve­loped due to the close contact between the metallic electrodes and the dielectric. After 15 minutes the sample was again placed in thermostat which could be heated from room temperature to 70°C. The samples were depolatieed at a linear heating rate of 1 °C per five minutes. The depolarisation current was recorded with upper electrode connected to  an electrometer amplifier (model EA810, Electronic Corporation of India) with lower electrode grounded. The thermal current spectrum was recorded from the room temperature to above the polaris­ing temperature th at is nearly 70°*C.Different sets of readings were taken on depolarising the electret made at the temperatures 52°C to 62°C at different polarising voltages (800 volts, 1000 volts, 1200 volts and 1400 volts per mm thickness of the sample).6m K. Prasad and P>. S. PrasQ<d3 . T h b o b yTl^e potential developed on the electrodes of a shorted capacitor are due to induction by holds which originate from charges residing in the dielectric after polarisation. Observation of the induced charge of the electrodes therefoio gives information about the nature of the charges present in the dielectric at different temperatures.The volume polarisation leads to the appearance of the surface charges, S  =  —PAj where 8  =  surface charge, P  =  polarisation and A ^  area of the electrodes. The polarity of S is contrary to that of the ooiiesponding electrode during polarisation, thus accordingly 8  ie heterochargo.When the applied voltage is sufficiently high and the temperature well above room tomporatuie or even when very high voltage is applied at room temperature, the transfer of charge takes place from electrodes to the dielectric. Thuf  ^ the electrode which was connected with the positive pole of the liigh voltage unit 0allies a positive induced charge and the other electrode negativi induced charge hence known as homooharges. Thus it is attributed that at the moment of short circuit the dielectric caiiios two types of surface charges : (i) hiterocharges 8  ^and (ii) homoohargos/. Thus the total induced charge on the olectcQde is given by. . .  ( 1)whore the sign of f  is contrary to that of 8A quantitative idea about the charges is obtained by measurement of total oxteinal ouiront J(^) which is given by equation- m+% (2)where i{t) is conduction omrent across the dielectric electrode interface wlii'h  measures the decay of homochargo and dqldt is the disi>lacement current which measures the decay of heterocharge Heterooharge 8 (i) may bo expressed as8 {t) =  iJ(t)dt, ... (3)where external current J(t) mainly originates due to decay oflietcrocharge.The dischaige current J(t) in the external circuit is due to the decay of volume polarisation. Bueci (1964) on dipolar theory for ionic thermocuireut has given tlie expression for the glow peak of the discharge current J(t) during the heating of the sample at constant ratewhore (i =  dipole moment, N  =  dipole concentration, K  — Boltzmann’s cons­tant Tp and Ep =  polarising temperature and field, fi ~  rate of heating and U =  activation energy and Tq =  a constant given by formula t =  &xp(UjKT)tDischarge current in eUctrets 599where t is th e lelaxation time at the trmperature T. Perlman (1971) and Pillai (1973) used tlris technique and found that cq. (4) for the diBcharge current alBO holde true in the case o f microscopic charge displacement with trapping mechanism which is responsible for the heterooharge From eq. (4) it is seen that Q which is total charge released to the external circuit should be independent of the thick­ness of the sample provided that the polarising field Ep and temperature Tp are kept constant.The total charge released is given byQ =  i  J{t)dt. ... (5)On differentiating the eq. (4), the temperature Tm at which maximum emrent occurs (dJldt =  0) can be written asy f f C 7 e x p (^ y )Thus Tni is independent of both Ep and Tp but it is dependent on(6)4. R esultsAfter preparation of elec tret of bees' wax sample under different polarising helda (800 volts, 1000 volts, 1200 volts and 1400 volts) at different tom peratoes 52"“C, 56°C, 58°C, 60°C and 62°C), the theim al emrenta spectra wore rcccrdod. The curves were diawn for diffoient polatiaing voltages keeping the polarising t;(5mperature constant. Figure 1 is one of the representative curves of thermal curicnt spectra for different polarising voltages at the polarising temperature 58°C.Fig. 1, Current v b  temperature curves. Polaiieing temperature 6 8  °C. The curves (a), (b), (c) and (d) corresponding to Ep =  800, volts, 1000 volts, 1200 volts and 1400 volts per mm thiolmeBS of the sample respectively.Similazly to study the effect o f polarising temperature, the curve for thermal current spectra for different temperatures at oonstant field were drawn. Figure 2 is one of the representative curves of thermal current spectra for different polaris­ing temperatures at 1400 volts per xum thickness of the sample,600 S. N . Prasad and B . S. PrasadFig. 2. Current vs temperature curves. Polarising field 1400 volts/mn The curves (a), (b), (o), (d) and (o) oorrospond to Tp =  52 '’C, 66 *C, 68 00 °C an [62 ®C respectivelyI t  is aeon that the peak poaition in both figures 1 and 2 ia at al\jciost a constant tomporaturo with a little variation The magnitudes of peaks in both the above cases gradually increases with the increase of polaiising voltages end temper atiiros.In both figures J  and 2, it  is seen that thermal currents in the begiiming gives the negative value up to temporaburos nearly 30°C to 32'’C then positive value, showing pealcs at nearly 52°C to 55^0 Currents sharpljy dooreaso to zero at nearly 61 °0 to 65®C and then after pass to  negative value.The aroos of the positive portions of the ourvos of thermal current spectra for different polarising voltages and temperatures wore calculated. Figure 3 is a plot o f area of the positive portion of the curve versus polarising voltages. I t  is seen that the relationship is not Linear indicating non-uniformity in volume polarisation.6 . DisousaioNWhen a capacitor containing bees’ wax is charged at high voltage and tem­perature and then cooled to  room temperature in the presence of the electric field, homo and hetero charges are frozen in and retained in the dielectric as long as temperature is low. After short circuiting the sample for some tiine and then on reheating without any external i? o^ltage, the frozen in charges are released and discharge current J{t) as given by eq. (2) is obtained. In  the lower temperature range up to  nearly 32°0, the displacement current dqjdt is  negligible and the discharge current is m ostly due to the decay of homoohargo in the form of conduc­tion currant between the dielectric— electrode interface, In  the temperatureDischarge cvarents in electrets 601range isay from 32” 0  to 60” C or 65” C, the discharge current is positive and passes through a peak. This current seems entirely duo to the decay of the hotero- oJiargo originating from volume polarisation. Beyond 60”0  or 65”C the current again changes the direction and small conduction current due to homoohorge persists. So it  is concluded that first there is homocharge then hoterocharge and again homoohaige on the surface of the dielectric in contact with upper electrode which was connected with positive voltage. The heterocharge 8 {r) as the integral over J(t) was calculated from the area of the positive portion of the curve.Eq, (4) is true either for dipole orientation mechanism involved or micro­scopic charge displacement with trapping responsible for hetcrochargo in the eleotret state in the material 'Eq. (6) suggests that peak temperature should be independent o f polarising field Ep and polarising temperatuie I t  is seen from figures 1 and 2 that there is little variation in peak temperature which may be due to slight change in heating rate /? So eqs (4) axid (6) are in complete agreement with results. B ut Grerson (1955) ruled out a pure dipolo orientation inechaausm in electrets because even complete dipole alignment is insufficient to produce the observed chaige and Perlman (1971) calculated the number of dipoles and mentioned that too many dipoles had to be aligned to produce the observed effects. Moreover, hoterocharge has been also observed in non-polar substances. Thus it is felt that the polarisation may partly result from the orientation of the dipoles and partly from microscopic displacement of charge carriers trapped in bulk of the material.Fig. 3. Total heterooharge released to external oircuit vb polarising field b.602 S. N- Prasad and B. S. PrasadFurther, from the plot of figure 3, i t  is seen th a t to tal heterocharge releasedis noi linear with the applied polarising field hence a non-nniform volume polaiisa- tion may be predicted which suggests that along with migration of charge over microscopic distances with trapping and dipole orientation, there is space chaige build up associatcHi wjth the migration of ions within the material over micro­scopic distances or charge injection from the electrodes with trapping at different sites. As pointed out by the authors (1973, 1974) in  the study of the conduction*' mechanism in bees’ wax, the boundaries between crystalline and amorphous regions provide additional trapping sites for charge carriers.6 . ConclusionsThe study of depolarisation of bees’ wax thormoelectret has helped in under­standing the mechanism responsible for charge carriers frozen in in the eloctret From the above discussion it is concluded that ,(1 ) Some time after short circuit and on reheating there is homewhargo then heterocharge and lastly homochargo which persists \(2) Decay of li.omocharge is in the form of conduction current betweendielectric— electrode interface and decay of heterocharge is m the form of displacement current. \(3) Hetorocharge originates from the volume polarisation. This polarisa­tion is partly due to orientation of dipoles and partly from microscopic displaoeipent of charge carrieis trapped in the volume of the material.(4) There is non-uniform volume polarisation honco space charge build up is expected.A oknowlbdgmbnt 'The authors are thankful to the Department of Industries and Technical Education, Bihar Government for providing financial support to one of the authors (S.N.P.). W g are specially indebted to Dr. L. S Singh, Hoad of the Department of Physics, Patna University for providing all facilities and kind encouragement in the work.Bucci C. & Fioachi B. 1904 Pht/s. Jtev. Lett. 12, 10.Compos M., Masoarenhas S. & Ferreira G. L. 1971 Phye. Rev. Lett. 27, 1432.Caserta G. & Serra A. 1971 Appl. Phy$. 42, 3778.Eguchi M. 1925 Phil. Mag. 49, 178.Gemant A. 1935 Ph%l. Mag. 20, 929.Gerson K. & Kohrbaugh J. H. 1966 J. Chem. Phya. 23, 23&1.Gross B. 1949 J. Chem. Phya. 17, 866.Heaviside O. 1919 Electrical papera {U.S.A.) 1, 488,Perlman M. M. 1971 J. Appl. Phya. 42, 2646.Perlman M. M. & Creswoll R. A. 1971 J. Appl. Phya. 42, 631.Pillai V. K C., Jain Kamlesh & Jam V. K. 1973 Indian J. pure Appl. Phya. 11, 597, Prasad S. N. & Prasad R. S. 1973 Indian J. pure Appl, Phya. 11, 700,Prasad S. N. & Prasad R. S. 1974 Indian J. Plvya. 48, 252.Wiseman G. G. & Feaster Q. R. 1967 J. Chem. Phya. 26, 621.

Thermally Stimulated Discharge Currents in Bees Wax Electrets

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Thermally Stimulated Discharge Currents in Bees Wax Electrets

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