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Applied Surface Science 263 (2012) 302–306

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Applied Surface Science

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icrostructure changes of polyimide/MMT-AlN composite hybrid films underorona aging

.H. Chena, J.H. Yina,b,∗, W.B. Bua, X.X. Liua, B. Sua, Q.Q. Leia,b

School of Applied Science, Harbin University of Science and Technology, Harbin 150080, PR ChinaKey Laboratory of Engineering Dielectric and Applications (Ministry of Education), Harbin University of Science and Technology, Harbin 150080, PR China

r t i c l e i n f o

rticle history:eceived 12 July 2012eceived in revised form0 September 2012ccepted 10 September 2012vailable online 16 September 2012

a b s t r a c t

A novel method using montmorillonite-AlN (MMT-AlN) composite particles as dopants in polyimide (PI)has been proposed and tested. We choose MMT-AlN as dopants in PI for lamellar structure of MMT andglobular structure of AlN and hope such mixed micro- and nano-particles can enhance PI’s resistance tocorona aging. Our corona aging test result has confirmed our method and a 7% doped PI/MMT-AlN film hascorona aging time extended more than 40 times compared with pure PI film. The microstructure changes

eywords:olymericrostructureanocompositesorona aging

of PI/MMT-AlN films due to corona aging have been analyzed by multiple surface analysis techniques. Theanalysis result shows coexistence of mass fractal and surface fractal in the sample films of PI/MMT-AlN.A dense inorganic block layer of MMT-AlN formed on the surface of doped PI films during corona agingis believed to be the mechanism of providing the protection against further corona aging.

© 2012 Published by Elsevier B.V.

ractal characteristics

. Introduction

Polymer-matrix materials, such as polyimide have wide applica-ions in multiple industries for their outstanding chemical, physical,hermal, and mechanical properties even at high temperatures1–3]. For electrical insulation purpose, however, polymer-matrix

aterials suffer premature failure caused by corona aging, and its difficult to overcome such failure by improving polymer itself.t is generally believed that the damage to polymer-matrix insu-ating materials is a slow erosion process, which eventually leadso a completely failure [4,5]. Recent researches revealed that themprovement in the voltage endurance and corona resistance ofhese polymers is accomplished by dispersing a certain amount ofnorganic nanoparticles into the polymer [6–9]. Zha et al. reportedI/nano-TiO2 hybrids with excellent corona resistance lifetime6]. PI/Al2O3 hybrid films prepared via an in situ process, whichhow similar corona resistance, have also been studied extensively7–10].

In this paper, we prepared the pure PI and PI/MMT-AlN hybrid

lm sample using in situ polymerization process and the filmhickness is about 25 �m. The purpose of this novel film designf PI/MMT-AlN is to produce effective corona resistant, owing to

∗ Corresponding author at: Harbin University of Science and Technology, Harbin50080, PR China. Tel.: +86 451 86390777; fax: +86 451 86390777.

E-mail address: yinjinghua1@126.com (J.H. Yin).

169-4332/$ – see front matter © 2012 Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.apsusc.2012.09.048

the lamellar structure of montmorillonite (MMT) and the globu-lar structure of aluminum nitride (AlN) [11]. Synchrotron radiationSmall Angle X-ray Scattering (SAXS) technology was used for thehybrid films to detect microstructure changes before and aftercorona aging. By combining with other analysis methods, suchas atomic force microscope (AFM), scanning electron microscope(SEM) and Fourier Transform Infrared spectroscopy (FTIR), the anal-ysis results reveal structural clues for further enhancing the coronaresistance of similar materials.

2. Experimental procedure

PI/MMT-AlN hybrid films were prepared by in situ polymer-ization method. In this study, we selected 4 films with MMT-AlNdoping concentrations of 1%, 3%, 5% and 7%. MMT and AlN (in 1:1ratio) inorganic composite particles were modified by couplingagent processing.

SAXS experiments were carried out on the SAXS station at beam-line 4B9A of Beijing Synchrotron Radiation Facility. The storagering was operated at 2.2 GeV with beam current about 80 mA.The incident X-ray wavelength � was selected to be 0.154 nm bydouble-crystal Si (1 1 1) monochromator. Corona aging was per-formed under 3 kV AC voltage with a needle-plate electrode aging

system, as illustrated in Fig. 1. The specimens used for corona agingwere prepared with single-side evaporation on Al electrodes withdiameter of 20 mm (d = 20 mm). The air gap between the specimensurface and the electrode was set at 2 mm. The specimen for post

M.H. Chen et al. / Applied Surface S

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Fig. 1. The illustration of the corona aging system used in this study.

orona aging analysis was continuously eroded in the aging systemntil breakdown.

. Results and discussion

Fig. 2 shows the AFM images of pure PI and 7% doped PI/MMT-lN before and after corona aging in 5 �m × 5 �m area. The pureI film surface looks quite smooth before corona aging as shownn Fig. 2(a); while 7% doped PI/MMT-AlN film AFM image showslear clusters of doped composite particles as in Fig. 2(b), which isonsistent with MMT particle size of 1 �m. The damages of coronaging are evident in both films as shown in Fig. 2(c) and (d).

After corona aging, the smooth surface of pure PI film changed

rastically with appearance of little bumps—spike like features

n Fig. 2(c). The bumps are about 200 nm in diameter at theottom and 20–30 nm in height. The surface of the hybrid film,n the other hand, is quite rough to start with due to mostly

Fig. 2. AFM images of the sample films: (a) pure PI, unaged; (b) 7% doped PI/MMT

cience 263 (2012) 302–306 303

micro-particles of MMT. After corona aging, the surface of thehybrid film changed drastically, as well. Fig. 2(d) shows 2–3 �mflat regions connected to each other, like loosely connected tiles.The larger field of view (FOV) of SEM image confirms dense andenlarged micro-particles on a 7% doped hybrid film after coronaaging [see Fig. 3(d)]. It appears that MMT and AlN composite parti-cles migrated and deposited on the surface of the hybrid films aftercorona aging and collectively formed a dense, net-like layer. Webelieve that the formation of inorganic layer on top of the hybridfilms prevents further damage during corona aging thus providesenhanced corona resistance. Further investigation will be given onmechanisms the formation of inorganic layer of hybrid films andsmall bumps on pure PI film.

Fig. 3 shows SEM images of a pure PI film and a 7% dopedPI/MMT-AlN hybrid film before and after corona aging with largerFOV than AFM images. The surface morphology changes indicatethat significant damage occurs after corona aging for both pure PIand hybrid films. Before corona aging, the surface of the pure PIis very smooth and the 7% doped PI/MMT-AlN looks rough clus-tered with composite particles. After corona aging, some large size(∼1 �m) of particle-like clusters appear [mostly in the left side ofSEM image in Fig. 3(c)], as well as small bumps, which is consis-tent with AFM images in Fig. 2. 5 �m AFM scan area does not catchparticle-like clusters due to their relatively low count density com-pared with small bumps. We find that the 7% doped PI/MMT-AlNfilm surface change is quite interesting as shown in Fig. 3(d). Thefloccus like features with size in the range of 2–3 �m appear onthe surface of the 7% doped PI/MMT-AlN film after corona aging,

which are the lamellar structure of MMT and globular structureof AlN composite particles. It is well known that the erosion ofcorona aging is mainly damage to the organic molecular chain andhas much less effect on inorganic elements. However, it seems that

-AlN hybrid film, unaged; (c) pure PI, aged; (d) 7% doped PI/MMT-AlN, aged.

304 M.H. Chen et al. / Applied Surface Science 263 (2012) 302–306

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[15].Fig. 5 shows FTIR spectra of a pure PI film (a) and a 7% doped

PI/MMT-AlN film (b) before and after corona aging. The chemicalchanges of both films caused by corona aging are quite obvious in

ig. 3. SEM images of surface topography of the sample films: (a) pure PI unaged; (

orona aging has changed both pure PI film and 7% doped PI/MMT-lN hybrid films as evidenced in Fig. 3(c) and (d). It will be beneficial

o understand microstructure changes the hybrid films after coronaging.

SAXS is an effective technique for investigating microstructuresf polymer materials [12–14]. In general, SAXS intensity can beescribed as follows:

(q) ∝ Cq−∂ (1)

here, I(q) is the scattering intensity, q is the amplitude of scat-ering vector, and C is a constant. The coefficient can be obtainedrom the plot of ln I(q) vs. ln(q). The fractal dimensions of Dmnd Ds are used to quantify the mass and the surface changes ofhe scatters. Fig. 4 shows the relationship of ln(I(q)) with ln(q)or 7% doped of PI/MMT-AlN before and after corona aging. Themplitude of the scattering vector q is related to scattering anglethrough 4� sin(�)/�. The curves in Fig. 4 have the typical half

ell shape with 3 segments—the top [−4 < ln(q) < −2.5], the mid-le [−2.5 < ln(q) < −0.5] and the tail [−0.5 < ln(q) < 0.5]. After coronaging, the SAXS curve of the hybrid film shows relatively largehange in the top and the tail segments and two curves cross atbout ln(q) = −1.5 (which corresponds to � = 0.156◦). The scatter-ng intensity decrease in the top segment (relative small angle)ndicates the decrease of nano-scale particles distribution; whilehe scattering intensity increase in the tail segment indicates thencrease of micro-scale particles, especially near the surface. SAXSesult is quite consistent with AFM and SEM observations. It is quiteseful to analyze self-similar fractal characteristics of polymersith SAXS [9]. The fractal dimensions of Dm and Ds are used to

uantify the mass and the surface changes of the scatters. The cal-ulated fractal dimensions are obtained by curve-fitting in the top

nd the middle segments of Fig. 4, where Dm is 2.65 and Ds is 1.81efore corona aging, while Dm is 2.72 and Ds is 1.38 after coronaging. The mass fractal and surface fractal coexist in the speci-ens tested. Ds decreases more than 23% after corona aging, which

doped PI/MMT-AlN unaged; (c) pure PI aged; (d) 7% doped PI/MMT-AlN aged.

indicates the weakening of chemical coupling between the com-posite nanoparticles and organic matrix.

Chemical changes of polyimide films caused by corona agingcan be further analyzed by FTIR spectroscopy. The typicalmolecular vibrational modes of polyimide films and their cor-responding Infrared absorption peaks are listed in Table 1.Similar FTIR analysis of chemical changes of polyimide hasbeen performed by other researchers with the similar results

Fig. 4. SAXS plot of ln(I(q)) as a function of ln(q) for 7% doped PI/MMT-AlN film before(black squares) and after corona aging (red circles) and the curve-fitting results. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of the article.)

M.H. Chen et al. / Applied Surface Science 263 (2012) 302–306 305

Table 1Infrared absorption spectra of typical organic molecules vibrational modes.

v (cm−1) 710 1077 1170 1220 1360

Type C O rocking C O C stretching C C rocking C O C asymmetrical stretching C N stretching

v (cm−1) 1495 1616 1708 2931 3745

Type Aromatic C C ring stretch C O symmetrical stretching COOH N H stretching

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effects and improve electrical properties of polyimide. This studyhas shown that is the case.

Fig. 5. FTIR spectra of a pure PI film (a) and a 7% do

TIR spectra. The C N stretching peak (at 1360 cm−1) disappearedfter corona ageing for pure PI, while it becomes weaker for 7%oped PI/MMT-AlN. The C C bending changed from two peaks at170 cm−1 into a single peak. Meanwhile, the C O C asymmetri-al stretching peaks changed from two peaks at 1220 cm−1 into aingle peak too. The C O symmetrical stretching and C O asym-etrical stretching peaks at 1708 cm−1 and 1770 cm−1 becomeseaker after corona ageing for both pure PI and 7% of PI/MMT-AlN.

he variation tendency is a great proof on the AFM analysis. TheH stretching peak at 3745 cm−1 appeared for both pure PI and

% of PI/MMT-AlN after corona ageing, and it is same as 100HN and00CR of Kapton-H polyimide done by the other researchers [8]. It

ndicates that chemical structure of the materials changes existeduring corona ageing for all of variation. The changes of chemi-al structure are caused by the decomposition of C O C bond iniamine and C N bond in imide ring possibly. It has been wellroved by the variation of the C and O element of 7% of PI/MMT-lN before and after corona ageing collected by the surface energypectra of SEM. There is the reason that the polyimide molecularhain such as the C C bond, C N bond and C O C bond are theasiest broken down by electron hit, ultraviolet radiation and heat,hich can be introduced during corona ageing.

The corona time and electrical breakdown strength, which arehe key parameters of electrical properties of insulation materials,ere tested for PI/MMT-AlN films with different doping concen-

rations. As shown in Fig. 6, the breakdown strength decreasesith the increase of the doping concentration of MMT-AlN, as

xpected, since polarized doping composite particles, together withheir interfaces with surrounding organic polyimide enhance localeld and breakdown happens at relatively lower field strengthompared to no doping polyimide. However, in comparisonith previous nanoparticle doping studies [9,10], the breakdown

trength decrease rate of MMT-AlN composite doped films (aboutkV/mm per 1% doping) is about a half the rate of Al2O3 nano-article doped films (about 12 kV/mm per 1% decrease). The corona

ging time, on the other hand increases with the increase of dopingoncentration of MMT-AlN. The corona aging time of a 7% dopedI/MMT-AlN film is 40 times more than that of pure PI (120 h vs.h). These could be attributed to the particular structure of com-

I/MMT-AlN film (b) before and after corona aging.

posites which can form a dense block layer by the distribution of thelamellar structure of MMT and globular structure of AlN for coronaaging.

The electrical test result is consistent with the analysis resultsby AFM, SEM, FTIR and SAXS. When the organic molecular chain ofpolyimide was gradually eroded, it would lead to the accumulationof inorganic composite particles on the surface of the hybrid films.With the growth of sizes of those composite particles, they form anet-like block layer to reduce the further erosion of the polymer.MMT has similar layered structures as mica which exhibits strongcorona resistance. It has the tendency to form an effective layer ofconnected micro-particles as shown in SEM image in Fig. 3(d). Onthe other hand, MMT can induce large micro defects due to its largelateral size. AlN nanoparticles have high thermal conductivity. Themixture of MMT and AlN can help to compensate those negative

Fig. 6. The corona time to breakdown and breakdown strength as the function ofMMT-AlN doping concentration.

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. Conclusion

In conclusion, MMT-AlN composite particles have been usedo dope polyimide films by in situ polymerization method. Theurface structure of PI/MMT-AlN undergo drastic change afterorona ageing, and it appears that a dense layer of mixed inor-anic nanoparticles of lamellar structure of MMT and globulartructure of AlN emerges, which prevents further corona aging.eanwhile, the changes of chemical structure caused by corona

ging are mainly the decomposition of C O C bond, C C bondnd C N bond in imide rings, as evidenced by FTIR spectrum. Fur-her investigation will be conducted in improving corona resistancey increase composite doping concentration and by adding otheromposite materials in polyimide.

cknowledgments

The work has being supported in part by the Natural Sci-nce Foundation (NSF) of China (51077028), NSF of Heilongjiang

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cience 263 (2012) 302–306

Province of China (A201006 and QC2011C106), and State Key Lab-oratory Breeding Base of Dielectrics Engineering (DE2011A04).

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