IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 4; August 2009

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Study on Charge Transport Mechanism and Space Charge Characteristics of Polyimide Films

L. R. Zhou, G. N. Wu, B. Gao, K. Zhou, J. Liu, K. J. Cao and L. J. Zhou

School of Electrical Engineering Southwest Jiaotong University

Chengdu, Sichuan, 610031, China

ABSTRACT As a core component of ac drive electric locomotive, performance of inverter-fed motor directly influences the stability and reliability of locomotive running. However, insulation early failures occur frequently. Presently, polyimide film, as a special type of engineering plastic film, widely applied in turn to turn insulation and turn to ground insulation of inverter-fed motors is a kind of basic insulating material. This paper investigates charge transport mechanism and space charge characteristics of polyimide films based on measuring and comparing space charge accumulation threshold field between corona-resistant and common polyimide films and analyzing the effect of temperature on space charge distribution. The results reveal that space charge accumulation threshold field of corona-resistant polyimide film is higher than that of common polyimide film; the adding of nano-particles effectively increases dielectric properties of corona-resistant film. Additionally, raising temperature promotes the electrode injecting charge and increases the charge energy and conductivity which gradually enhances space charge number and extends trapping position into the bulk, closely related to polyimide film insulation aging and breakdown which is an important characteristic.

Index Terms — Inverter-fed motor, polyimide, charge transport, space charge, PEA, pulse voltage, aging

1 INTRODUCTION

AS a core component of AC drive electric locomotive, performance of inverter-fed motor directly influences the stability and reliability of locomotive running. Inverter-fed motor, in this paper, is referred as the asynchronous traction motor driven by inverter. However, insulation early failures occur frequently [1-3]. Consequently, research on the insulation aging and failure mechanism can provide theoretical foundation for the design of insulation structure of inverter-fed motor so that its service life can be prolonged.

In contrast to an ideal crystal, polymer does not, in general, invariably retain the nature that it possesses on first being manufactured. Over a period of time, both its chemical composition [4] and physical morphology [5] may change. Thereby, some of its properties may alter. For example its conductivity and dielectric loss may increase and mechanical and electrical strength reduce. Then the material will be aged gradually and eventually may not be able to perform its insulating function to the required standard [6]. The major stresses leading to the aging and deterioration of

insulation materials are as follows: electrical stress caused by the voltage gradient in the material; thermal stress caused by a combination of losses generated in the motor and the ambient; mechanical stress caused by manufacture and assembly process; environmental stress caused by oxidation, radiation, and moisture [7]. Presently, polyimide film, as a special type of engineering plastic film, characterized by high temperature resistance (400 °C), low temperature tolerance (-269 °C), radiation resistance and excellent dielectric properties widely applied in turn to turn insulation and turn to ground insulation of inverter-fed motors is a kind of basic insulating material. However, chemical impurities and structural defects are produced inevitably during the polyimide film manufacture process, resulting in the deviation from its intrinsic property. Injected carriers (electrons or holes), during charge transport process, will be captured by these impurities and defects under the electric field and become space charge. The existence of space charge has a great effect on dielectric property and is the main reason leading to dielectric breakdown. Hence, the study on the charge transport mechanism and space charge characteristic in polyimide film is valuable and meaningful.

This paper investigates charge transport mechanism and space charge characteristics of polyimide film based on Manuscript received on 3 October 2008, in final form 21 February 2009.

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L. R. Zhou et al.: Study on Charge Transport Mechanism and Space Charge Characteristics of Polyimide Films 1144

measuring and comparing space charge accumulation threshold field between corona-resistant and common polyimide film and analyzing the effect of temperature on space charge distribution.

2 EXPERIMENTAL DETAILS

2.1 MEASUREMENT SYSTEM Space charge measurement techniques are the basis of the

research about space charge and have been developed rapidly in the recent decades [8]. Pulsed Electro-Acoustic (PEA) Method [9] is a worldwide popular measurement technique. As shown in Figure 1, the testing device used in this investigation mainly consisted of five parts: 1) aging voltage was output from adjustable bipolar high voltage pulse source [10], used to simulate the output voltage of inverter, of which peak to peak value was 0-10 kV, frequency range was 500 Hz-20 kHz, and duty cycle was 50%. 2) pulse power [11] of which pulse width was 8 ns and amplitude was 0-600 V. 3) PVDF piezoelectric sensor of which thickness was 28 μm. 4) amplifier of which band width was 0.1-500 MHz. 5) Tektronix digital oscilloscope and personal computer.

Figure 1. Experimental setup for the PEA method.

2.2 SAMPLE DETAILS The samples, supplied by DUPONT Co., Ltd., were:

1) Kapton® common polyimide film-100HN, having size 50 mm×50 mm and thickness 0.025 mm;

2) corona-resistant polyimide film-100CR, having size 50 mm×50 mm and thickness was 0.025 mm;

3) Kapton® 500HN, having size 50 mm×50 mm, too, but thickness was 0.125 mm.

All the samples were cleaned in absolute alcohol with 99.99% purity and thermally treated at temperature 100 oC for 5 hours. Each specimen was placed between two aluminum electrodes and was coated with silicon oil as an acoustic coupling medium to eliminate air gap.

2.3 EXPERIMENT PROTOCOLS The experiment consisted of two parts: threshold field test

and space charge distribution test.

2.3.1 THRESHOLD FIELD TEST The samples were common polyimide film-100HN and

corona-resistant polyimide film-100CR. The aging voltage was bipolar pulse voltage and step-rising voltage was adopted in this test. Voltage peak to peak value initiated at 250 V (10 kV/mm) and terminated at 2000 V (80 kV/mm); voltage frequency was 1 kHz; duty cycle was 50%. The samples were aged for 30 minutes and the space charge profiles were measured immediately after short-circuiting. Based on the measured data, space charge density can be plotted as a function of electric field and then space charge accumulation threshold field can be obtained.

2.3.2 SPACE CHARGE DISTRIBUTION TEST The samples were polyimide film-500HN. The aging

voltage was bipolar pulse voltage and its peak to peak value was 4kV; voltage frequency was 1 kHz; duty cycle was 50%. Space charge distribution was measured at five temperatures, i.e. 25 °C, 50 °C, 80 °C, 110 °C and 140 °C, immediately after short-circuiting. When the temperature was 140 °C and other conditions kept constant, the test was prolonged until all specimens had failed; then, sample service life data were analyzed statistically under the Weibull hypothesis and the statistical results showed that samples service life at 63.2% probability was 2 h 19 minutes and 26 s under such conditions. So the aging time in this test was designed to be 2 h.

3 RESULTS AND DISCUSSION

3.1 THRESHOLD FIELD TEST The concept of “threshold” must be clarified first. It has been

shown that, below a certain electric field (the threshold for space charge accumulation), the same number of injected charge from one electrode is extracted by the other electrode, and no space charge remains trapped in the insulation bulk [12, 13]. When the electric field was below threshold field, insulating material could be used for a relatively long time before electrical degradation took place [14]. If the electric field was above that threshold field, space charge would start to accumulate in the insulation bulk [15]. On the premise of the absence of other major stresses acting on the dielectric, if the electric field was below the threshold field, long service life and high reliability of insulating material could be ensured. Accordingly, study on the space charge accumulation threshold field is of significant theoretical guidance and practical meaning.

As far as the formation of space charge is concerned, e.g. via the bipolar injection mechanism, electrons and holes are injected into the dielectric by the Fowler-Nordheim or the Richardson-Schottky emission according to the intensity of electric field [6]. Small part of charges are captured by deep traps and most are captured by shallow traps. After voltage polarity reverses, charges trapped in the shallow traps will be extracted and recombination of charges with opposite sign will take place. Due to the different dynamics involved in trapping and extraction, a few charges will remain in the traps

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 4; August 2009 1145

and become space charge. Over a period of time, maybe these charges will be captured by higher energy level traps [16].

As to the essence of the trap, actually trap is the charge attraction center [17]. In the molecular structure, any place where an action center of positive and negative charge is detached can form trap. For instance, impurity decomposed, chain scissioned, and bond composed of various atoms can result in an action center of positive and negative charge detached. As a result of electrostatic interaction of heterocharge, trapped charge which is attracted and can not move in the dielectric forms space charge effect.

The space charge density within the bulk sample region, excluding induced or image charges on the electrodes, was integrated for each space charge distribution measured to obtain average bulk density:

( ) ( )1

01 0

1, ,x

xE t x t dx

x xρ ρ=

− ∫ (1)

where x0 and x1 are the electrode positions, t is the time at which the measurement is done and E is the applied field. Space charge average bulk density of unaged 100HN and 100CR film versus electric field is shown in Figure 2. If the slope changed at certain point, the corresponding electric field was the space charge accumulation threshold field [12]. Thereby, as seen in Figure 2, the threshold fields at which space charge accumulates are 31.5 kV/mm for unaged 100HN sample and 35 kV/mm for unaged 100CR sample. Slope k in the trap acting region reflects space charge accumulation rate. The larger k value, the faster accumulation rate of space charge in the dielectric. As shown in Figure 2, k value of 100CR film is larger than that of 100HN film; therefore charge accumulation rate in 100CR film is faster which illustrates that 100CR film contains more traps.

Figure 2. Threshold field of space charge accumulation for 100HN and 100CR.

When two kinds of material with different Fermi level contact, free carriers will migrate from one material to the other one until equilibrium condition is established, that is, the Fermi level of these two materials become equal at the contact position [18]. The barrier between metal and dielectric hinders charge injection into the dielectric. Consequently, injection difficulty degree depends on work function difference of contact interface. Figure 3 shows the barrier of charge injection into the dielectric under the Schottky effect [18].

From Figure 3, it can be deduced that barrier height measured from metal Fermi level is:

2

( )16m

qx qExx

ψ φ χπε

= − − − (2)

Where mφ is the metal work function, χ is the affinity of dielectric surface, q is charge quantity, ε is dielectric constant and E is the applied electric field. The dielectric constant of a corona-resistant film is larger than that of a common film. The reason is that the dielectric constant of inorganic composition added is relatively larger than that of the original film; according to the property of multiphase heterogeneous material, in the case of a two-phase material with first phase featuring minε and second phase featuring maxε it holds min maxε ε ε< < , thereby suggesting the increase of dielectric constant. Through the measurement, dielectric frequency spectrum of ε is shown in Figure 4, which is in agreement with the theory. From equation (2), it can be seen that, keeping constant the other parameters, the barrier height will increase with the increase of dielectric constant illustrating that it is harder to inject charge into a dielectric with higher dielectric constant, as the corona resistant polyimide, and the number of injected charge decreases. That’s why space charge accumulation threshold field of 100CR is larger than that of 100HN.

Figure 3. Barrier of charge injection into dielectric under Schottky effect.

Figure 4. Dielectric frequency spectrum of ε.

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L. R. Zhou et al.: Study on Charge Transport Mechanism and Space Charge Characteristics of Polyimide Films 1146

The introduction of nano-particles raises the number of traps in the dielectric. Because a large number of defects exist on the surface of nano-particles, during the process of recombination with polyimide, numerous traps will be created at the surface of nano-particles and interface of polyimide resulting in the enhancement of trap number. Figure 5 shows Scanning Electrical Microscope (SEM) images of film surface with 10000 times magnification. Surface of common polyimide film is smooth and uniform without any adulterant as shown in Figure 5a; however, surface of corona-resistant polyimide film partially presents protuberant clusters. As shown in Figure 5b, successive deep color part is organic phase and spherical granules are inorganic nanometer particles and organic phase coats inorganic nanometer particles. Figure 6 shows SEM images of film section with 2000 times magnification. Three-layer structure appears in the unaged corona-resistant film. Upper and lower layer are doped with nano-particles and section morphology of interlayer is the same as that of common polyimide film.

As a result, once space charge begins to accumulate in the dielectric, the accumulation speed in 100CR film is faster than that in 100HN film, as shown in Figure 2, the former k value being larger than the latter one. As the added transition metal has strong affinity for charges and can form dispersive system in polyimide film, charges are firmly captured by these strong affinity structures under the electric field and become trapped charges. Cooperative action of these trapped charges forms stable space charge electric field, which effectively increases dielectric properties of corona-resistant film.

(a) SEM image of 100HN film surface

(b) SEM image of 100CR film surface Figure 5. SEM images of two kinds of unaged polyimide film surface

（a）SEM image of 100HN film section

（b）SEM images of 100CR film section

Figure 6. SEM images of two kinds of unaged polyimide film section.

3.2 SPACE CHARGE DISTRIBUTION TEST During the practical running of inverter-fed motor, the

produced heat makes the insulating material operating temperature reach nearly 140°C [19]. Consequently, study on space charge characteristic in polyimide film under various temperatures, especially under high temperature, is remarkably important.

Space charge distribution within the thickness of specimens of material Kapton® 500HN under various aging temperatures, measured immediately after short-circuiting, are shown in Figure 7. As seen in the space charge profiles, heterocharges are trapped in the vicinity of both electrodes. Heterocharge increases the electric field near the electrodes, which is helpful to the charge injection. Therefore, space charge density continuously increases with the increase of aging time under the same temperature. From Figure 7, it can be seen that trapping position gradually moves into the bulk, as temperature rise leads to the increase of charge conductivity which makes charge move longer distance before being captured. With the increase of aging time, material internal structure will be continuously damaged until dielectric fails. Figure 7e shows space charge distribution just before the dielectric insulation life ends. Moreover, space charge density also continually increases with the increase of temperature under the same aging time which indicates that raising temperature promotes the electrode injecting charge.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 4; August 2009 1147

(a) temperature 25 oC

(b) temperature 50 oC

(c) temperature 80 oC

(d) temperature 110 oC

(e) temperature 140 oC

Figure 7. Space charge distribution within the thickness of specimens of material Kapton® 500HN under various aging temperatures, measured immediately after short-circuiting.

Focusing on electrons for example, in practice the aging process can be described as follows. Part of electrons in the conduction band captured by the traps in the dielectric will be the recombination center of holes. During the process of capturing electrons and recombination with holes, electrons will transfer from high energy state to low energy state. Remnant energy will be transferred through radiation and/or non-radiation mode to another electron which is made to be a hot electron [20]. When holes injection takes place, released energy by capturing holes will also be transferred through radiation and/or non-radiation mode to the electron being a hot electron. The interaction between hot electron and polyimide macromolecule makes the latter one become free radical. Free radical chain reaction will be induced subsequently which results in the polymer degradation, generating lower molecule products and forming partial low density region. The whole process can be illustrated by the following reaction equation:

AB+e (hot) → (decomposition) A+B+e (cold)

→ A+B+e (trap) + energy Formation of lower molecule and low density region increase polymer inhomogeneity and trap density [21]. With the rising of trap density, the probability of electron being trapped increases. So does hot electron generation. Then the above process will be accelerated. In this experiment, besides the function of electric field, there also exists thermal effect which will accelerate the charge injection and free radical reaction.

Obviously, the rate of generation and energy of hot electron depend on the trap density and depth. That is to say, the generation probability and energy of hot electron can be altered through changing the trap density and depth. When the deep trap density decreases and shallow trap density increases, the probability of electrons being captured by deep trap will decrease and by shallow trap will increase, so that the number of hot electrons with high energy will decrease. Consequently, dielectric insulation life will be effectively extended if deep trap density decreases and shallow trap density increases.

4 CONCLUSION In this paper, space charge accumulation threshold field and

space charge distribution are measured and analyzed. The conclusions are as follows.

(1) The dielectric constant of corona-resistant polyimide film is larger than that of common polyimide film, which enhances the barrier height for injecting charge into the dielectric, thereby increasing space charge accumulation threshold field.

(2) The introduction of nano-particles raises the number of traps in the dielectric, thereby forming stable space charge electric field, which effectively increases dielectric properties of corona-resistant film.

(3) Raising temperature promotes the charge injection from the electrodes and increases the charge energy and conductivity, which gradually enhances space charge number and extends trapping position into the bulk, closely related to polyimide film insulation aging and breakdown, which is an important characteristic.

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L. R. Zhou et al.: Study on Charge Transport Mechanism and Space Charge Characteristics of Polyimide Films 1148

ACKNOWLEDGMENT The authors would like to acknowledge National Science

Fund of China (NSFC project number 50377035), Doctoral Subject Fund of China (project number 20050613008), and Key Project of Education Ministry for their financial support.

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L.R. Zhou was born in Shanghai, China, on 31 January 1985. He received the B.Sc. degree in electrical engineering from Southwest Jiaotong University, China, in 2007. He is now pursuing the M.Sc. degree in the School of Electrical Engineering in Southwest Jiaotong University, Chengdu. Currently, his research interests are in failure mechanism for electrical insulation under PWM pulse voltage.

G.N. Wu （M’97） was born in Nanjing, China, on 26 July, 1969. He received the B.Sc., M.Sc. and Ph.D. degrees in electrical engineering, from Xi’an Jiaotong University, respectively in 1991, 1994 and 1997. Currently, he is a Professor in the School of Electrical Engineering, Southwest Jiaotong University. His research interests include condition monitoring, fault diagnosis and insulation life-span evaluation for electric power equipment.

B. Gao was born in Hebei, China, on 11 November 1976. He received the B.Sc. degree in electrical engineering in 1999, and the M.Sc. degree in 2003, from Southwest Jiaotong University, Chengdu. He is currently pursuing the Ph.D. degree in the School of Electrical Engineering at Southwest Jiaotong University. Currently, he is doing research work on the condition monitoring and fault diagnosis of the power equipment.

K. Zhou was born in Yibin, Sichuan, China, on 21 August 1975. He received the B.Sc. and M.Sc. degrees in electrical engineering, from Chongqing University, respectively in 1998 and 2003; the Ph.D. degree, from Southwest Jiaotong University, in 2008. Currently, he is a Lecturer in the School of Electrical Engineering at Southwest Jiaotong University. His research interests are in failure mechanism for electrical insulation under PWM pulse voltage.

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J. Liu was born in Chengdu, China, on 17 September 1984. He received the B.Sc. degree from Southwest Jiaotong University, China, in 2006. He is now pursuing the Ph.D. degree in the School of Electrical Engineering in Southwest Jiaotong University, Chengdu. Currently, he is engaged in research on the on-line monitoring and fault diagnosing for electric power equipment.

K.J. Cao was born in Luzhou, Sichuan, China, on 6 June 1985. He received the B.Sc. degree in electrical engineering in 2007 from Southwest Jiaotong University, Chengdu. He is currently pursuing the M.Sc. degree in the School of Electrical Engineering at Southwest Jiaotong University. Currently, his research interests are in failure mechanism for electrical insulation under PWM pulse voltage.

L.J. Zhou was born in Hangzhou, China, on 8 May, 1978. He received the B.Sc., M.Sc. and Ph.D. degrees in electrical engineering, respectively in 2001, 2004 and 2007 from Southwest Jiaotong University, Chengdu, where he is an associate professor of electrical engineering. His fields of interest are condition monitoring, fault diagnosis and insulation life-span evaluation for electric power equipment.

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