open- and short-circuit thermally stimulated currents in ethyl cellulose

Polymer International 39 (1996) 303-307

Open- and Short-circuit Thermally Stimulated Currents in Ethyl Cellulose

P. K. Khare,* J. M. Keller, M. S. Gaur, Ranjeet Singh & S. C. Datt

Department of Postgraduate Studies and Research in Physics, Rani Durgavati Vishwavidyalaya, Jabalpur-482 001 (MP), India

(Received 12 September 1995; accepted 14 October 1995)

Abstract: Thermally stimulated currents (TSCs) in short-circuit and open-circuit configuration for ethyl cellulose samples (w40pm thick) have been studied as a function of polarizing field (25-100 kV/cm) and polarizing temperature (323- 353 K). The thermograms have been found to be characterized by two prominent peaks located around 70 & 10°C and 140 10°C and the appearance of anomalous current in the high temperature region of the short-circuit TSC thermograms. The observed behaviour has been explained in terms of the existence of heterocharge due to dipole orientation and ionic homocharge drift, together with the injection of charge carriers from electrodes and their subsequent localization in surface and bulk traps.

K e y words: open-circuit and short-circuit thermally stimulated currents, ethyl cellulose, dipole orientation, space charge.

I NT R 0 D U CTI 0 N

The use of thermally stimulated current (TSC) has been widely employed to study the carrier trap nature in dielectric materials. The technique has shown that for improving the charge storage properties of polymers and for obtaining strong and stable electrets, a better understanding of the structural or morphological details and dynamic properties of polymers is required on both the molecular and super-molecular level. Several reports on TSC behaviour of ethyl cellulose (EC) thermoelectrets and the different relaxation processes contributing to the observed peaks in the corresponding thermograms are However, the nature of the various polarization processes and their relative contribution to the electret state of the polymer are not yet fully understood. This is particularly true of the space charge relaxation mechanism and the detaiIs of trap structure (including the trap distribution in energy levels and also over the volume of the polymer). Such information can best be obtained by a combined study of open-circuit and short-circuit TSCs. Synthetic high

* To whom correspondence should be addressed at: 5, Shakun Sadan, New Anand Nagar, Adhartal, Jabalpur- 482 004 (MP), India.

polymers of cellulosic materials are excellent electrical insulators when dry,7 contain large numbers of traps and exhibit structural deformations at certain transition temperatures. The knowledge of trap parameters and phase transition mechanisms is of the utmost impor- tance in characterizing them for a specific use. The use of short-circuit TSC in combination with open-circuit TSC helps in the reliable investigation of many aspects of charge storage and its slow change with time, and also the effect of various material parameters. The com- parison of open-circuit TSCs with short-circuit TSCs facilitates a quantitative picture of the geometric and energetic trap structures of charged polymeric samples.

Ethyl cellulose (EC) is an atactic weakly polar polymer well known for its valuable physical and chemical properties.' The slight polar nature of the polymer is due to the difference in electronegativity of the main chain and the side groups. The reported glass transition temperature (T,) for atactic EC lies between 60 and 75"C, and the softening and melting temperatures are 135-153°C and 175-190"C, respectively.' Its structure is as shown in Scheme 1 (where R =

In the present paper we report the results of open- circuit and short-circuit TSC measurements carried out on EC samples.

C A ) .

303 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

304 P. K. Kkare et al.

CHpOR

Scheme 1

EXPERIMENTAL

The ethyl cellulose (EC) films used in the present investigation were prepared using the isothermal immersion technique. The solution of a particular concentration was prepared in a glass beaker by dissolving EC (50g) in 100ml chloroform at room temperature (30°C). The mixture was kept for 24h to give a homogeneous and transparent solution. The solution thus prepared was poured onto an optically clean glass plate and the solvent was then allowed to evaporate inside an oven at 40°C for 12h to yield the desired sample. The dried sample was subjected to room temperature outgassing at lod5 torr for a further period of 12 h to remove any residual solvent. Such preconditioned samples were uni- laterally and bilaterally vacuum aluminized over a central circular area of 36 mm diameter. The thickness of the samples was of the order of 40pm, which was estimated by measuring the capacitance of the fabri- cated sandwiches taking the value of the dielectric constant E of ethyl cellulose as 3.00.

The samples were thermally polarized with fields, E, , of 25, 50, 75 and 100kV/cm at temperatures, Tp, of 50, 60, 70 and 80°C. After polarizing for 1 h at the desired temperature, the samples were cooled to room tem-

perature under the application of the field. Total time of polarization was adjusted to be 2.5 h in each case. The polarized samples were subsequently short-circuited for an arbitrary time of 10min so as to remove any fric- tional and stray charges present. The short-circuit TSCs were then recorded by reheating the samples at a linear rate of 4 K/min.

For registering TSC in open-circuit, the polarized samples were mounted in an electrode assembly with the non-metallized surface parallel to the sensing electrode at a distance of 3 mm, while the metallized surface rested on the other metal electrode.

A high voltage power supply, EC-4800D, provided stabilized DC voltages for polarization, while TSCs were measured using a Keithley 610 C electrometer.

RESULTS AND DISCUSSION

The characteristic short-circuit TSC curves for the EC thermoelectrets poled with various polarization fields at polarization temperature T p = 70°C and at different Tps with E, = 75 kV/cm are shown in Figs 1 and 2, respectively. The thermograms are characterized by two major peaks, located around 70 f 10°C and 140 f 10°C, respectively. It is evident from Fig. 1 that the current of the first peak increases with increasing value of E , and the peak shifts towards higher temperatures. The activation energy values calculated by the initial rise method range from 0.28-0.32 eV and the relaxation time is approximately 7.793 x lo-" s. The current of the

Temperature ("C)

Field (kV I cm) 1 -25 2 -50 3 -75 4 -100

3 1

Fig. 1. Short-circuit TSC spectra for ethyl cellulose samples poled with different fields (25, 50, 75 and 100 kV/cm) at 70°C.

POLYMER INTERNATIONAL VOL. 39, NO. 4, 1996

Thermally stimulated currents in ethyl cellulose

75 kV I cm 25

20

15

2z 10

:

P 5 5 0

Y I c

10 -.

305

Fig. 2. Short-circuit TSC spectra for ethyl cellulose samples poled at different temperatures (50, 60, 70 and 80°C) with a field of 75 kV/cm.

second peak also increases with increasing E,, but the position of this peak tends to shift towards lower temperatures. Activation energy values for this peak range from 0.49-0.53 eV.

It was found that for samples poled with 75 kV/cm at T, = 70°C (Fig. 2) the current is anomalous over a certain temperature range in the later part of the ther- mogram. However, in all other cases the observed current is normal for the whole temperature range over which the sample was heated. The magnitude of the first peak increases with increasing T, . The peak changes its position over the range 60-70°C; however, no corre- lation between T, and shift in peak position could be observed. So far as the effect of T, on peak characteristics of the second peak is concerned, it is observed that as with the low temperature peak, the current increases and its position shifts towards higher temperatures with increasing T, .

In some cases the current on the high temperature side of the thermograms is observed to be of the same polarity as that of the charging current; this is known as anomalous current. It may be due to a high return rate of the charge carriers released from traps. This high return rate may exceed the charge exchange rate of the electrode resulting in its blocking. This blocking is assumed to cause movement of the zero field plane away from the charge injecting electrode, resulting in separation of carrier flow towards the electrode and a net carrier flow away from it, leading to anomalous behaviour. At higher T,s the carriers are assumed to move easily in the bulk of the sample leaving less space

charge on the injecting side. On the other hand, when the samples are polarized at lower Tps, large numbers of carriers are considered to be trapped on the sample surface, or in the near surface region, resulting in the blocking of the electrode. Such a situation is possible if the various energetically distributed traps are located on the sample surface, or in the near surface region, which seems to be true in the present case.

Characteristic open-circuit TSCs are shown in Figs 3 and 4. Figure 3 depicts thermograms for samples poled with different E,s at T, = 70°C. Figure 4 shows the thermograms for samples polarized by 75 kV/cm at different T,s. Generally the currents observed in the open- circuit case have been found to be of the same polarity as that of the charging current. However, for the sample poled with 25dV/cm at 70"C, a normal current has been observed up to 115°C. The thermograms are also characterized by two sharp peaks as found for short- circuit TSCs. Generally, the current of both peaks increases and their position shifts towards higher temperatures with increasing T,.

The classic theory of G ~ b k i n , ~ Perlmann" and others, for the decay of charge of an electret, assumes the superposition of homocharge and heterocharge. Homocharge is the space charge consisting of ions and electrons and is produced by the discharge in the air gap between poling electrode and sample. Hetero- charge, on the other hand, is volume charge produced by either the rotation of dipoles, or the separation of ions. Since the total charge is the sum of homocharge and heterocharge, then if the observed TSC is due to


306 P. K. Khare et al.

Tp = 70%

Field (kV I cm) 1 -25 2 -50 3A-75 4 -100

3

4

2

1

Temperature ("C)

Fig. 3. Open-circuit TSC spectra for ethyl cellulose samples poled with different fields (25, 50, 75 and 100 kV/cm) at 70°C.

dipole disorientation, or ion displacement, during the same as that of the charging current, that is positive heating processes, the sign of the discharge current is in polarity. expected to be of opposite polarity to that of the charg- The increasing value of the current of the low- ing current. On the other hand, if it is due to dissipation temperature peak with poling field and the observed of space charges, the sign of the TSC is expected to be activation energy values indicate that the peak may be

Field = 75 kV / crn

0

.- c 2 8- - Tp ("C) .- 1 - 5 0 2- 60

7 3 A 4 7 0 4 -80

a

I -

5 t

Fig. 4. Open-circuit TSC spectra for ethyl cellulose samples poled at different temperatures (50, 60, 70 and 80°C) with a field of 75 kV/cm.


Thermally stimulated currents in ethyl cellulose 307

contributed to significantly by dipolar and ionic polarizations. The dipolar relaxation is at a maximum around the glass transition temperature, which for ethyl cellulose' is reported to be 60-80°C. The shifting of this peak over a particular temperature range, the observed difference in activation energy values and the increase in the broadness of the peak with increasing value of poling field, indicate that at low field the polarization is not saturated and increasing the value of the poling field leads to the completion of dipolar polarization. The various characteristics of the open-circuit TSC thermograms show that the low temperature peak, which is considered to be contributed to by dipole polarizations on the basis of its characteristics in short- circuit thermograms, is also contributed to significantly by space charge polarization. Positive polarity of this peak in open-circuit TSC thermograms can be understood to be due to the superimposition of space charge polarization on dipolar polarization. Thus the two types of TSCs indicate that the 60°C peak is contributed to by dipolar and space charge polarization. The location of the 140°C peak, the increase in peak current maximum with increase in T, and the observed value of activation energy indicate that it is a space charge peak arising from the relaxation of various electronic charges trapped in energetically and spatially distributed trapping sites in the polymer. The shift of the second peak towards higher temperatures with increasing value of T, (Figs 3 and 4) further indicates the trapping of electronic charges in energetically distributed traps existing in the polymer. These characteristics can be explained as follows: assume that the trap structure is not influ- enced by heating, then energetically deeper traps will be filled at elevated temperature rather than at room temperature. Assume a discrete set of trapping levels, each represented by an activation energy U i and an escape frequency v i , then the detrapping time constant for each trapping level will be of the form"

zi = [vi]-' exp(UJkT) i = 1, . . . , N o (1) where N o is the total number of trapping levels, zi is the mean detrapping time for the ith trapping level, k the Boltzmann constant and T the absolute temperature.

For a trapping level to contribute to the charge motion in the sample during charging, z i has to be smaller than the charging time t , . This means that with increasing temperature, the number of 'active' (contributing to charge motion) trapping levels N( T )

increases, whereas the number of inactive (not contributing to the charge motion) levels [ N o - N(T)] decreases. These inactive levels store the charge delivered to the sample during the charging process. In view of the above, the current of the high temperature peak is expected to increase and shift towards higher temperatures with increase in poling temperature, which is experimentally observed. This also explains the early initiation of the peak when the sample is poled at lower temperatures. Further, if the polarization is solely due to trapping of charge carriers, then the low temperature peak should decrease and shift towards higher temperatures for samples polarized at higher temperatures.

However, such behaviour is not observed in the present case. This further confirms that the low temperature peak (located around 70°C) is due to dipole relaxation.

CONCLUSIONS

Thermally stimulated currents measured in short-circuit and open-circuit configurations for ethyl cellulose reveal the existence of two broad relaxations around 70 k 10°C and 140 rf: 10°C. The low temperature relaxation is contributed to jointly by dipolar and space charge processes, while the high temperature relaxation can be consistently explained in terms of space charges localized in energetically distributed traps.

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open- and short-circuit thermally stimulated currents in ethyl cellulose

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