ABSORPTION AND POLARIZATION CURRENTS IN AROMATIC POLYAMIDES

M.A. Sussi and G.R. Govinda Raju Department of Electrical Engineering

University of Windsor Windsor, Ontario, N9B 3P4, CANADA

ABSTRACT

Aromatic Polyamide (Trade Name Nomex type 410) is temperature dielectric which has many potential engii applications. In this study charging, discharging and th stimulated polarization (TSP) currents are measured ovei range of electric field strengths and temperatures. The infli electrode materials and rate of heating on the TSP curre investigated for 127 Mm and 76 um thick paper. The resul that the observed current during charging and discharg dissimilar and both decay according to I(t) = At-" where t is and n is the decay constant. The absorption currents are derive the low frequency dielectric properties of the materi TSP current is observed to be dependent on electric f i heating rate. Possible mechanisms are discussed.

INTRODUCTION

Aromatic polyamide is a polymer commercially known as manufactured by E.I. Dupont De Nemours & Co. Inc. Thi reports for the first time the results of an investigation absorption currents in Nomex 410 type paper at varioui electric fields and temperatures up to 200' C. The materia Crystalline and 50% non-Crystalline. The discharging curl dielectric for a voltage step applied isothermally obeys a pc according to

I(t) = A (T) t-"

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From the absorption currents one can calculate the low frequency behaviour of the polymer.

Experimental Procedure

Measurements of charging and discharging currents were made on paper thicknesses of 76 and 127 bm having a radius of 44 mm. Before the samples were used they were heat treated for 5 hours at 260' C to remove any moisture and to improve the reproducibility of results.

Two types of measuring electrodes, aluminum and silver of 50 nm thickness, were vacuum deposited on the polymer at a pressure of 10-6 torr.

Figure 1 shows the schematic arrangement of the experimental setup. The currents were measured using a Keithley 610 C electrometer which was connected to an IBM PC through an amplifier with variable gain and A/D converter. The data acquisition system is capable of handling 16 channels with individual control of each channel. The frequency of sampling can be adjusted up to 4000 Hz, though most of the data reported in this study was acquired at 1 Hz.

RESULTS AND DISCUSSION

Chargin4 and Absorption Currents

Figures 2-4 show typical charging and discharging currents in Nomex paper at various poling fields at 200' C for time duration of 1 O4 s. Figure 2 shows that the charging current decays according to Eqn. (1) with a small value of n. This is true for both thicknesses.

Figures 3 and 4 show the absorption currents which decay very fast for times up to 1 O3 s and for longer time periods the decay is less pronounced. Table 1 shows the values of n for charging and absorption currents at 200' C. There is no significant dependence of the decay constant on the electrode material, polarity or material thickness. Jonscher [2] suggests that a decrease in the value of n at longer periods is probably due to interfacial polarization where as an increase is due to diDolar mechanism. The data shown in Fia. 4

a dipolar mechanism at all temeratures except at 200' mechanism gradually transforms to interfacial polarization at

Low Frequency Dielectric Loss

The absorption currents which flow may be used tc information on the frequency dependence of its loss factor Hamon approximation [3], which is valid for 0.3 < n expressed as

1 (t) I ( O . U f ) - e" ( f ) = -

2uf c v 0

2uf c v 0

where I is the current magnitude of the discharging current t, CO is the geometric capacitance of the electrode assembb the material, V is the magnitude of the voltage applied known as the Hamon frequency = O.l/t. Figure 5 sh calculated values of e" at different temperatures. Two re frequencies are observed at temperatures lower than 200' increase in the magnitude of E " at much lower freque attributed to the conductivity of the material. e " incre frequency decreases (for frequencies much less than 1 Debye Peak) when conduction is dominated by electronic conduction [2]. In this case the low frequency is domina second power law of the type given in Eqn. (1) with a smal of the exponent n. Since the material abounds in crysta non-crystalline regions the observed behaviour can ju explain on the basis of the Jonscher theory. This observatic consistent with the results of Govinda Raju [4] who has I adequate evidence for the ionic nature of the conduction cur

Thermally Stimulated Polarization Current

In this study a sample with a thickness of 76 Mm is chargt voltage of 500 V while the temperature was raised line: room temperature to 200' C at a constant rate 2 K/min.

. . -.

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temperature range of 40' C - 200' C, with the currents approaching equal values in the high temperature region. This supports the earlier findings [4], that polarization occurs within the bulk, the current being smaller than the ionic current. At higher temperatures the ionic current is the predominant mechanism.

REFERENCES

1.

2.

3.

4.

D.K. Das Gupta and K. Joyner, "On the Nature of Absorption Currents in Polyethylene Terephthalate (PET)", J. Phys. D; Appl. Phys. Vol. 9, pp. 829-840, 1976.

A.K. Jonscher, "The Universal Dielectric Response", CElDP Annual Report, pp. 23-40, 1990.

B.V. Hamon, "An Approximate Method for Deducing Dielectric Loss Factor from Direct-Current Measurements", Proc. IEE (London), Vol. 99, pp. 151-5, 1952.

G.R. Govinda Raju, "Conduction and Thermally Stimulated Discharge Currents in Armid Paper", IEEE Transactions on Electrical Insulation, 1991. (To be published)

TABLE 1

Pollng Voltagnm

Tm. Eladrode Materid h

(a) -75 -250 .IOW +IOW Thidcneu

in Valubs of 'n' During Charging

M (127 rm) 10 - 1 0 4 0.03 0.04 0.01 . 0.02 AL(76 m) - 0.01 -

(11) Valubs of h' During Dhchrghg

10 - 1 0 3 . 0.88 0.99 - Ae

Id - 1 0 4 - 0.22 0.36 . Aa

10 -3.5~102 0.85 . Aa 3.5x1$-104 0.18 - &

- 0.77 - 0.67 AL

- 0.38 - 0.38 AL

Table 1 Valuos of n during charping a d discharging currenls rnndlions cbtained at a wnstanl t O W l a t U r b of c.

R

O v e n

he r m o c oup 1 c J

I BE

E l e c t r m e ter - - & Amp. A/D

Fig. 1 Schematic arrangement of the experimental setup

t i m e ( s ) t i m e ( s )

a: v

H 1

Fig. 2 Fig. 3

Fig. 2 Charging currents at a poling voltage of 500 vc various poling temperatures for 76 m.

Fig. 3 Absorption currents at a poling voltage of 500 vo various poling temperatures for 76 m.

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5 . -

Fig. 4

Fig. 5

Fig. 6

4

3

P U,

2

1

C

Fig. 5

v = s o 0 V o l t s 61

0 y 1 0 -

11 - 12.- '

2 . 4 2 . 8 3 . 2 3 . 6 4 . 0

l o g t (SI

Fig. 6

Absorption currents at a poling temperature of 200 C and various poling voltages for 127 pm.

Calculated values of the loss factor at different temperatures for 76 pm aromatic polyamides film.

TSP current for 127 pm film at p = 2K/,in and poling . . _ I I - - - - I cnn . . - I & -