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Surface morphology and Raman analysis of the polyimide film aged under bipolar pulse voltage.

INTRODUCTION

The pulse-width modulated (PWM) variable speed drive (inverter) is one of the newest and fastest evolving technologies in nonlinear devices used in motor drive systems. With increasing emphasis of energy conservation and low cost, the use of higher performance PWM drives has grown at an exponential rate (1). However, the PWM drives have the disadvantage of overstressing electrical insulation with respect to the bipolar pulse supply, which demands higher level of reliability for the high-voltage insulation materials.

As essential parts of modern generation and transmission systems, the reliability of high-voltage insulation materials is critical to system performance. In recent years, polymers have largely replaced traditional materials used as high-voltage insulation due to their easy processability and low manufacturing cost. Among various polymers, polyimide possesses unique properties such as excellent dielectric properties, thermal stability, and mechanical properties, which makes it most suitable as polymeric insulation (2), (3). However, the electrical properties always deteriorate overtime and then result in the degradation when a polymeric insulation is subjected to high electric stresses (4-9). It is reported that the electrical stress caused by the voltage gradient in the material, the thermal stress caused by a combination of losses generated in the motor and the ambient, as well as the environmental stress caused by oxidation will lead to degradation of polymeric insulation (10). The degradation in time of the electric properties up to the breakdown of the polymeric insulation is called "aging phenomena" and is characterized by irreversible deteriorations affecting its performance and lifetime.

As for the polyimide, the weakness of low corona resistance restrains its further development (11-13). Researches on the insulation aging and failure mechanism provide us information for the design of insulation structure to prolong service life. Therefore, the study of degradation of polyimide (especially the initial changes) under electric stress and high temperature is very important and attracts much attention (14-18). Some of the initial changes cannot be observed by conventional method, but the ignorance will lead to much disaster.

In this article, our work is focused on the survey of initial surface morphology changes as well as reorientation of the chemical bonds of polyimide influenced by corona aging under bipolar pulse voltage at different frequencies using the atomic force microscope (AFM) and Raman spectra. The purpose is providing a way to improve the properties of polyimide film and prolong service life.

EXPERIMENTAL

Kapton 100 HN polyimide films with a thickness of 50 pm were purchased from Dupont Company. The corona aging of the film was carried out for 4 h by needle-plane electrode with 2-mm air gap and 25-[micro]m radius of curvature. The value of the bipolar pulse voltage was 4 kV, and the frequencies were 300Hz, 500Hz, 600Hz, 800Hz, and 900Hz.

AFM measurements were done using a SPM-9500J3 (SHIMADZU) AFM in contact mode under atmospheric condition. Its horizontal resolution is 0.1 nm, and the perpendicular resolution is 0.02 nm. Both of planar and three-dimensional graphs were taken with the sizes of 5 x 5 [micro]m.

Raman measurements were carried out using a Laser Confocal Raman Micro-spectroscopy (LabRAM HR800, HPRIBA JOBIN YVON) equipped with an objective (50xmagnification). Spectra were acquired in the range of 100-1800 [cm.sup.-1], and the applied laser wavelength during the experiments was 785 nm. All spectra were recorded at a resolution of 1 [cm.sup.-1] using a focused laser beam with a power of 17 mW. The diameter of the facula was 1-2 [micro]m, and the exposal time of the CCD camera was 10 s.

RESULTS AND DISCUSSION

AFM Results

Figure 1 shows the AFM results of the pristine polyimide, and the parameters obtained from the AFM are present in Table 1. The surface morphology of the pristine polyimide is smooth except for few streak-like bumps. Also, the parameters present in Table 1 reveal that the surface is flat, for example, the arithmetic mean roughness ([R.sub.a]) is only 1.897 nm, the average height ([R.sub.v]) is 10.468 nm, and the square average roughness ([R.sub.ms]) is 2.799 nm. While for the polyimide film aged under bipolar pulse voltage at 900 Hz for 4 h, as shown in Fig. 2, the morphology is very rough with many gulfs and swells on the surface, and the parameters have sharp increases (Table 1), especially the arithmetic mean roughness (Ra) increases from 1.897 to 43.806 nm and the square average roughness ([R.sub.ms]) increases from 2.799 to 60.106 nm. The AFM results demonstrate that the surface of polyimide has been damaged by the corona aging, which can be attributed to charges' bombardment, increase of temperature on the surface, space charge accumulation, as well as the surface partial discharge (PID).

TABLE 1. Parameters of the AFM results.

Parameters                  Pristine polyimide     Aged polyimide
                                                   (Bipolar pulse
                                                voltage 4 kV. 900
                                                              Hz)

[R.sub.a] (arithmetic mean               1,897              43.80
roughness, nm)

[R.sub.y] (maximum height,               31.23              453.2
nm)

[R.sub.z] (10 point                      13.92              223.3
roughness, nm)

[R.sub.ms] (square average               2.799              60.10
roughness, nm)

[R.sub.p] (average depth,                20,74              251.9
nm)

[R.sub.v] (average height,               10.46              201.3
nm)


During the aging process under bipolar pulse voltage, the charges emitted from the needle point will bombard the film surface, and the fast rise time and high frequency of the bipolar pulse voltage will enable to generate local dielectric heating in the film, as a result of which local temperature increases (19). Meanwhile, the space charges captured by impurities and defects in polyimide also have a great effect on dielectric property and are an important reason, leading to dielectric breakdown (17). The degradation of polyimide is significantly influenced by space charge accumulation; charges stored in polyimide bulk and on the surface can produce electric field perturbations inside the insulation and in the air gap between the needle-plane electrode, respectively (20). Also, surface morphology of polyimide film has been greatly influenced by the surface PD under the bipolar pulse voltage. The surface PD is generated between the turns of the twist when there are voltage overshoots and is the main factor accelerating insulation degradation (20). Moreover, the energy released by the PD is another reason that makes the surface uneven and rough.

Raman Analysis

Raman vibration spectroscopies were taken to assess the nature of chemical bonding, interactions, conformations, and even orientations of molecules in polyimide film. An overview of the Raman spectra of the film surfaces aged under bipolar pulse voltage at different frequencies is shown in Fig. 3. The characteristic absorption bands and the corresponding chemical bonds are present in Table 2 (21-24). Raman absorption bands can be assigned to fully cured PI (polyimide) and PAA (polyamic acid) as precursor or end-groups (25). Characteristic bands for polyimide are imide I(C=0 stretch at 1786 [cm.sup.-1]), imide II(C-N-C axial vibration stretch at 1395 [cm.sup.-1]), imide III(C-N-C transverse vibration stretch at 1124 [cm.sup.-1]) (26), (27). Typical D (1340 [cm.sup.-1]) and G (1580 [cm.sup.-1]) bands, characteristic for crystalline graphitic carbon, are not observed as was done by, for example, Raimondi et al. (28) after ion radiation. It means that no carbonization happen to the polyimide after corona aging for 4 h under 4 kV bipolar pulse voltage. There are no significant changes in Raman spectra of the aged polyimide compared with that of the pristine one because the voltage of 4 kV is not high and the aging time of 4 h is not long enough to induce the breakdown of the polyimide film (29). Besides, the rapid changes in polarity of the electric field do not allow the charges injected by electrodes to penetrate inside the insulation bulk under bipolar waveforms (20). As a result, charges are mainly at the interfaces between electrodes and insulation, thereby causing the initial degradation of the surface. But further investigations show that both peaks positions and the relative band intensities change after the corona aging.

TABLE 2. Raman characteristic absorption bands and
the corresponding chemical bonds of polyimide (21-24).

Raman absorption hands  Chemical bonds
([cm.sup.-1])

610                     Monosubstituted benzene
                        deformation

645                     C-O-C bonding

730                     C-H vibration

753                     Aromatic imide ring in
                        dianhydride part

820                     C-H vibration

852                     Diamine nut; breathing

1124                    C-N-C transverse vibration
                        (imide III)

1272                    C-O-C backbone

1395                    C-N-C axial vibration (imide
                        II)

1513                    C=C bonding in the aromatic
                        phenylene ring

1601                    Ring vibration of carboxylic
                        acid

1612                    Aromatic imide ring in
                        dianhydride part

1786                    C-O asymmetric stretch (imide
                        I)


Figure 4a-f shows that the peak shifts of the bands at 1612 [cm.sup.-1], 1124 [cm.sup.-1], and 1395 [cm.sup.-1] are 2.3 [cm.sup.-1], 2 [cm.sup.-1], and 2.8 [cm.sup.-1], respectively, much more than other bands' shifts for about 1[cm.sup.-1] (such as the band at 1601 [cm.sup.-1], 1786 [cm.sup.-1], and 1513 [cm.sup.-1]). The difference of peak shifts indicates that aromatic imide ring and C-N-C bonds are much influenced by corona aging under bipolar pulse voltage (30), (31). because the reason is that the dipole moment of C=0 groups in aromatic imide ring tends to vibrate in the direction of the electric field under corona aging, which allows the movement of molecular segments and orientation of the dipole in the direction of the electric vector (27). When the electric field of bipolar pulse voltage changes its polar, the dipole moment in aromatic imide ring will vibrate in the opposite direction. Through the repeated vibration in the two opposite directions, the chemical bonds in aromatic imide ring will be much affected, and the peak shifts would be larger than other bands. In addition, we notice that the same trend in peak shifts of C=0 stretching mode has been reported in the thermal aged polyimide (32), which confirms that the increase of temperature on polyimide film's surface during corona aging may be another reason for the peak shifts.

Figure 5 shows the correlation between the relative band intensities and the aging frequency. It helps us to monitor the decomposition (relationship between amide/imide bonds) and the orientation (relationship between imide bonds) after corona aging. With the 1612 [cm.sup.-1] band being chosen as a reference band correlated to aromatic imide ring structure, it permits demonstrating the orientations of different functional groups relative to the imide phenyl ring (33).

Figure 5a shows the orientation of C-O-C backbone structure relative to the aromatic imide ring [I(1272)/I(1612)] and C=0 bond [I(1272)/I(1786)]. With the increase of aging frequency, both of the ratios of the relative band intensity decrease and reach the minimum value at 800 Hz; the same trend is attributed to C=O bond being part of the aromatic imide ring. It demonstrates that the intensity of the band at 1272 [cm.sup.-1] has a trend of diminution--C-O-C stretch becomes relaxed, and the C=O bond becomes more stretched during corona aging. The stretch of the C=O bond is attributed to the influence of the alternating electric field. The relaxation of C-O-C can be ascribed to the influence of the radiation and hot electron. Energy will be released when the injected carriers are trapped or recombined with carriers of the opposite type. The remnant energy will be transferred through radiation or nonradiation mode to another electron and make it to be a hot electron, resulting in the bond-breaking, free radicals formation, and degradation of polyimide macromolecules (34). Unlike thermal decomposition resulting in the reversion of polyimide into its monomers (33), corona aging leads to reorientations and breakages of the chemical bonds. This can be demonstrated by the ratio of the intensities I(1612)/I(1601) shown in Fig. 5b; the increase of value with frequency is not attributed to the imidization process but the breakage of CONH (1601 [cm.sup.-1]) under electric stress. Also, the stretched C=0 bond and breakage of CONH lead to the increase of the ratio of intensities I(1786)/I(1601) as shown in Fig. 5b.

Figure 5c,d shows the orientation of C-N-C backbone structure and the C=0 bond relative to the aromatic imide ring, respectively. The influences under bipolar pulse voltage for imide II(1395 [cm.sup.-1]) and imide III(1124 [cm.sup.-1]) bands are the same. The ratio rises with the increase of aging frequency, which indicates that C-N-C vibrations become progressively stretched. The sharp drop at 800 Hz is attributed to influence by the orientation of C=0 side groups relative to the aromatic imide ring as shown in Fig. 5d. The orientation of the C=0 bond (1786 [cm.sup.-1]) relative to the aromatic imide ring (1612 [cm.sup.-1]) doesn't show the linear variation with the increase of aging frequency, but there is a sharp drop at 800 Hz. The progressive orientation of C=0 under corona aging may induce additional stress into polymer chains, leading to a reorientation of neighboring C-N-C bonds (33).

Furthermore, a progressive orientation of C-N-C bonds with increasing frequency is found in polyimide. As shown in Fig. 5e, the difference between C-N-C transverse and axial vibration is given by the relative band intensity of 1124 [cm.sup.-1] and 1395 [cm.sup.-1]. The transverse vibration decreases at high frequency in favor of the axial vibration, changing from transverse towards axial orientation and skip to a maximum at 800 Hz. Also, the value of I(1124)/I(1395) is around 0.4, which indicates that the band intensity of C-N-C transverse vibration is larger than that of C-N-C axial vibration.

Figure 5f shows the ratio of I(1513)/I(1612), which corresponds to the relative band intensities of the C=C bond/aromatic imide ring. The trend demonstrates that the C=C bond is reoriented by corona aging under bipolar pulse voltage.

There is a turning point at 800 Hz for almost every figure. This may be ascribed to the space-charge life model of polymers (35). Charges emitted from the needle point will be trapped by the impurities and defects in polyimide due to the decrease of the kinetic energy. The fast rise and fall of pulses make it possible for space charges to accumulate on the surface of the film and in the bulk over a period of time (29). A few of the charges injected by the electrodes will be confined in deep traps but most of them in shallow traps (35). There will be a rapid accumulation with a high frequency until the charges on the surface become saturated, and then it reaches to deep traps. When all the traps are filled with space charges, PD happens and leads to the total breakdown of the sample. Thus, we consider that 800 Hz is close to the frequency at which space charges fill up the shallow traps.

Figure 5 demonstrates that the corona aging of polyimide under bipolar pulse voltage becomes severe at high frequency. The aging phenomenon is severe when the frequency increases, leading to chain scission and breakdown of polyimide film till the end (36). First, with the increase of aging frequency, the charge depleted at each PD does not have enough time to leave interface before polarity reversal, so the overall field after inversion is magnified and the amplitude of PD rises. The discharges happen frequently and bring much disaster to the sample. And then the intrinsic aging of the polyimide film due to electro-mechanical fatigue will increase with frequency. Finally, the increase of dielectric and polarization losses with frequency (37) and the presence of oxygen and water that arises from the atmospheric moisture will accelerate the degradation of polyimide (38). So the corona aging will be severe, and lifetime of the samples will decrease with the increase of aging frequency (39).

CONCLUSIONS

In this work, the corona aging of Kapton 100 HN polyimide films was performed by needle-plane electrode under 4 kV bipolar pulse voltages for 4 h at different frequencies in atmosphere. AFM results show that the surface morphology has been greatly changed by the charges' bombardment, the PD, as well as the dielectric heating. The large peak shifts of the bands in the Raman spectra confirm that aromatic imide rings are more influenced than other bands. The changes in relative band intensities demonstrate the breakage of the CONH bond--the C-O-C stretch becomes relaxed, and the reorientation of the C-N-C imide bond changes from transverse into axial orientation during corona aging. Moreover, the corona aging of the polyimide film is severe at high frequency, and the orientations of the chemical bonds are much influenced by the bipolar pulse voltage. The study of degradation in polyimide under the electric field shows us an effective way to utilize the material and improve the properties to prolong service life.

Correspondence to: D. Yin; e-mail: dyin@whu.edu.cn

Contract grant sponsor: Program of National Key Basis and Development Plan (973); contract grant number: 2009CB724505; contract grant sponsor: Chinese National of Natural Science; contract grant number: 51172166 and. 61106005; contract grant sponsor: National Science Fund for Talent Training in Basic Science; contract grant number: J0830310.

DOI 10.1002/pen.23384

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Yang Yang, (1), (2) Di Yin, (1), (2) Chuyu Zhong, (1), (2) Rui Xiong, (1), (2) Jing Shi, (1), (2) Zhengyou Liu, (1), (2) Xuan Wang, (2), (3) Qingquan Lei (2), (3)

(1.) School of Physics and Technology, Wuhan University, Wuhan 430072, People's Republic of China

(2.) Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, People's Republic of China

(3.) Key Laboratory of Engineering Dielectric and its Application, Ministry of Education, Harbin University of Science and Technology, Harbin 150040, People's Republic of China
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Author:Yang, Yang; Yin, Di; Zhong, Chuyu; Xiong, Rui; Shi, Jing; Liu, Zhengyou; Wang, Xuan; Lei, Qingquan
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:Jul 1, 2013
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