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G. R. GO- Raju Departraent of Electrical Engineering

university of Windsor Windsor, Ontario, N ~ B 3P4, Canada

Measurement of thermally stimulated discharge currents 'yield considerable information with regard to the relaxation process that occur in polymers. Considerable volume of literature exists on the techniques of resolving the observed currents [l] and assigning the peaks to a specific mhanism of relaxation. As far as the present author is aware no data are available on the TSD currents in N o m and this paper presents the results of such an investigation.

THEoRETIcAt B2awomD

Considering the case in which TSD currents arise as a result of de-polarization, the rate at which polarization decays nay be expressed as [2]

P dP dt r

--

in which P is the polarization and T is the relaxation t k . For a linear rate of heating the solution of Eqn. (1) is [4].

dt P = P exp 0

Measurements of r at various temperatures shows that

in which ( is the characteristic energy of the dipoles, k the Boltzmann constant and T~ a constant.

If the initial polarization is due to an electric field

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NP2 EF P = o 3k TF ( 4 )

in which N is the number density and p the dipole m n t . The current density is

in which To is the initial temperature and B the heating rate. For linear rate of heating

T=a+/?t (6)

In the case of charge carriers being released from traps the current density is given by an equation similar to (5) except for the pre-exponential factor. The charge released to the external circuit is

Ely differentiating Eqn. (5) w.r.t. T one gets

in which Tm is the is observed.

temperature at which a current peak

The low temperature tail of j vs T curve m y be expressed as

h j(T) = C - k T L (9)

where C is a constant. be determined from a plot of h I vs 1/T.

The activation energy can thus

The aromatic polyamide (trade name NOMEX) used in this study was in the form of paper with a thickness of 127 p. The electrode-dielectric assembly was placed in an

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environmental chamber equipped with a micro-processor to give a desired thermal profile.

In order to obtain consistent and reproducible results the protocol to which the spechen was subjected to is shown in Fig. 1.

The TSD currents were measured by heating the sample at a uniform rate. Both the temperature and the current was measured using a strip chart recorder having a sensitivity of 2 pV/m and A / m respectively.

REs[JLTs AND DISCUSSION

Figure 2 shows the TSD currents of the material which is not subjected to an electrical field and is usually referred to as blank TSD. Curve 1 is obtained with a rate of heating of 0.45' C/min. and the current increases with increasing temperature even with zero bias voltage. With a higher rate of heating the currents observed are lower as shown by Curves 2 and 3 . Curve 4 is obtained with a different sample and a cooling rate of 2'/min. and must be conpared with Curve 3 because both have the same rate of change of temperature. The currents while cooling are higher than those with heating. Curve 5 is the current observed with a bias of 100 V and /3 = l'/min. It is important to note that the currents shown by Curves 1-4 are negative in polarity (the electrode connected to the electrometer has acquired a positive potential) and is opposite to those in Curve 5. Since the currents observed during heating are lower for higher values of /3 it cannot be considered a normal polarization current, because a normal polarization yields currents which are greater in magnitude for higher values of p [5]. The currents measured during cooling phase are indeed higher for higher p (not shown in Fig. 2) and indicated a depolarization. We therefore concluded that there is s m sort a b n o m l polarization within the materials, the origin of which was not studied further. Sacher [ 3 ] has also reported the existence of such currents in polyimide films and suggests that there may possibly be a them1 polarization within the material.

Figure 3 shows the TSD currents for two different conditions. curve 1 is obtained with the raw film and

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Curve 2 with the film thermally aged as described earlier. The current for the thermally aged sample is lower and the latter shows a peak at 152" C. By plotting I Vs 1000/T and employing a least square fit according to Fqn. (9) the activation energy is determined as 0.68 0.05 ev (Fig. 4). No data are available for comparison.

The charge released to the external circuit may be obtained by integrating the Curve I Vs t provided that the TSD current decreases to zero at the highest temperature investigated. This procedure may be adopted in a dielectric which has a low glass transition and low melting point such as linear polyimides. However and as we can see from Fig. 3, the film retains considerable charge even at the highest temperature used in this study. To obtain the total charge in the material the temperature was held constant at the end of the TSD run and the decay of current was measured as a function of time. The total charge released to the circuit for the conditions which apply to Curve 2 (Fig.4) is 6.1 x 10-6c.

Both the peak current and the charge released to the circuit are found to vary linearly with EF suggesting that the observed peak is possibly due to polarization within the volume of the material.

REFERENCES

[l] J. Van Turnhout, "Thermally Stimulated Discharge of Electrets in E ' L E ~ T S " , Ed: G.M. Sessler, Spring- Verlag, Berlin, pp. 81-215, 1980.

[2] M.M. Perlman, "Thermal Currents and the Internal Polarization in carnauba W a x Electrets", J. -1.

[3] E. Sacher, "Dielectric Properties of Polyimide Film II", IEEE Trans. Electr. Insul. Vol. EI-14, pp. 85- 93, 1979.

PhyS., Vol. 42, Pp. 2645-52, 1971.

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T' I

Fig. 1 - Stimulated

60 1 3 0 200 T ( O C 1

Curve 1, p = 0.45*/min; Curve 2, p = l'/min; Curve 3, p = 2*/min; curve 4, /3 = -2*/min; all with zero bias Curve 5, p = l*/min; Bias Field 7.87 kV an-1.

Fig. 2 - TSD Current with no electric field.

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/ /

/

Fig. 3

Fig. 4 -

Curve 1: Curve 2:

Raw film, TF = 125' C. Thermally aged film, TF = 180' C.

2 2.5 3 3.5 iooo/e ( 0 K

!&I I Vs 1000 /~ . Curve 2 in Fig. 3.

E x p r i m n t a l conditions as for