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Catalytic and enh

aHarbin Institute of Technology, Center for C

China. E-mail: niuyongan@gmail.com; yao

Tel: +86 451 86402345bHarbin Institute of Technology, School of

Harbin, 150040, P. R. China. E-mail: jiupen

Cite this: RSC Adv., 2014, 4, 42569

Received 10th May 2014Accepted 20th August 2014

DOI: 10.1039/c4ra04371e

www.rsc.org/advances

This journal is © The Royal Society of C

anced effects of silicon carbidenanoparticles on carbonization and graphitizationof polyimide films

Yongan Niu,a Xin Zhang,b Jie Wu,b Jiupeng Zhao,*b Xiangqiao Yana and Yao Li*a

Silicon carbide nanoparticle (SiC NPs)/polyimide (PI) composite films were prepared by a solution blending

process. To obtain carbon films, pure PI and SiC/PI composite films were carbonized at 1000 �C under a N2

atmosphere and graphitized at 2300 �C for 2 h under an Ar atmosphere. The catalytic behaviour of SiC NPs

in the carbonization and graphitization processes was investigated by Fourier transform infrared

spectrometry (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission

electron microscopy (TEM). In these processes, the presence of SiC NPs obviously promoted the

carbonization and graphitization yields of the SiC/PI composite films and enhanced the fracture

toughness of the resulting carbon films. Because of the catalytic effects of SiC NPs, the graphitization

temperatures decreased to 2300 �C and the degree of graphitization exceeded 31.40% for the 3 wt% SiC

NPs sample. Furthermore, the electrical conductivity of the graphitized films was estimated by the four-

point probe method.

1 Introduction

Carbon lms, including carbonized lms and graphitized lms,have attracted signicant attention due to their extensiveapplications in catalysts,1 high thermal conductivity materials,2

fuel cells,3 lithium ion batteries4 and supercapacitors.5 Recently,the pyrolysis of aromatic PI lms at a high temperature wasconsidered to be the most promising approach for producinghigh performance carbon lms.6 Carbonized and graphitizedmechanisms of various commercial PI lms, including Kapton,Upilex and Novax lms, have been investigated, and themechanical properties of carbon lms were also studied inpervious works.7–10 However, in the graphitization of these PIlms, a critical disadvantage is that they are non-graphitizable.The graphitization temperature usually exceeded 3050 �C andthe starting crystalline temperature of graphitization alsoreached up to 3200 �C.11

To overcome this disadvantage, two main routes have beensuggested for increasing the graphitization yield and degree ofgraphitization.12 One route involves improving the degree oforientation of the PI molecules because random orientationscan easily produce glassy carbon and other non-graphitizedstructures.13 T. Takeichi et al. reported the inuence of theorientation of PI lms graphitized at 2800 �C on the

omposite Materials, Harbin 150001, P. R.

li@hit.edu.cn; Fax: +86 451 8640 3767;

Chemical Engineering and Technology,

gzhao@126.com

hemistry 2014

graphitization process.14 The other route involved utilizing thecatalytic effects of several catalysts, i.e. transition metal andmetal oxide particles. Recently, the catalytic effects of nickel (Ni)particles15 and Fe2O3 NPs16 were investigated in PI and chlori-nated poly(vinyl chloride) (CPVC) resins. The results demon-strated some excellent advantages, which could shorten thecarbonized process at 1600 �C and improve the degree ofgraphitization at 2600 �C. Furthermore, PI lms containinggraphene,17 boron,18 nitrogen19 and sulfur20 atoms were alsoused to accelerate the carbonization process and enhance themechanical properties of the carbonized lms. However, it wasdifficult to homogeneously disperse thesemetallic particles intothe majority of PI lms because of the differences in polarityand compatibility.21

This work was motivated by the catalytic effects of graphenegrown on the Ni,22 copper (Cu)23 or SiC24 substrates by thechemical vapor deposition (CVD) method, which possessedsimilar structures as those of the graphite-like (honeycomb)carbon materials. In particular, the SiC NPs, which exhibitgreater compatibility than the metallic (Ni and Cu) NPs, couldproduce surface activities and various intrinsic defects at hightemperatures, i.e., six-fold scattering centers and sub-surfacestructures.25,26 Furthermore, SiC NPs could also be well-dispersed in the PI matrix aer modication.27

In this work, the well-dispersed SiC/PI composite lms wereprepared by a solution blending process from polyamic acid(PAA) and SiC NPs. The catalytic effects of SiC NPs were inves-tigated by FTIR, XRD, SEM and TEM. The turbostratic structuresand degree of graphitization were estimated by Bragg diffrac-tion and Mering–Maire formulas. The enhanced effects of SiC

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NPs on the carbonization and graphitization processes werefurther conrmed by Raman spectroscopy and XRD analysis.Finally, the electrical conductivities of graphitized lms weremeasured by the four-point probe method.

2 Experimental2.1 Preparation of SiC/PI composite lms

3,30,4,40-Benzo-phenonetetracarboxylic dianhydride (BTDA),4,40-oxydianiline (ODA) and SiC NPs were purchased fromSinopharm Chemical Reagent Co., Ltd. A schematic represen-tation for the preparation of SiC/PI composite lms is shown inFig. 1(a). PAA was synthesized by polycondensation betweenODA and BTDA monomers at an equimolar ratio (1 : 1.02) atroom temperature under a nitrogen (N2) atmosphere.28 SiC NPswere added to the PAA solutions at different dosages, i.e., 1 wt%,2 wt% and 3 wt%. Aer stirring for 1 h, mixtures of PAA and SiCNPs were obtained, which were then cast on clean glass plates.Solvents were removed in vacuum by maintaining the temper-ature at 70 �C for 5 h. The curing process was carried out byheating the lms at 100 �C for 2 h, 200 �C for 2 h and 300 �C for1 h. Finally, the SiC/PI composite lms were obtained. Forcomparison, a pure PI lm was also prepared using the sameprocess.

2.2 Carbonization and graphitization of SiC/PI compositelms

The carbonization of SiC/PI composite lms was executed in atube electric furnace (GSL-1500X) under N2 atmosphere.Fig. 1(b) shows a schematic representation of the carbonizationand graphitization of SiC/PI composite lms. The PI lms werecut into 10 mm � 10 mm pieces and were sandwiched betweentwo Al2O3 ceramic plates. The composite lm samples wereannealed at a heating rate of 2 �C min�1 up to 600, 800 and1000 �C. Aer maintaining the temperature for 2 h, thecarbonized lms were cooled to room temperature at a coolingrate of 10 �C min�1. During the graphitization process, samplesof carbonized lms were sandwiched between two polishedgraphite plates. The carbonized lms were graphitized at 2300�C for 2 h at a heating rate of 2 �C min�1 under an Ar (>99.99%)atmosphere. Finally, the graphitized lms were prepared aercooling to room temperature at a cooling rate of 10 �C min�1.

Fig. 1 Schematic representations of the preparation process of (a)SiC/PI composite films and (b) carbon films.

42570 | RSC Adv., 2014, 4, 42569–42576

Carbonization (graphitization) yields were calculated by themass ratios between the carbonized (graphitized) lms and theoriginal SiC/PI composite lms.

Fig. 2 shows the digital photographs of the SiC/PI compositelms, carbonized lms and graphitized lms. The results revealthat a carbonized lm of pure PI lm was easily smashed.However, all the carbonized SiC/PI composite lms remainedwhole without additional cracks.

2.3 Measurements and characterization

The chemical composition and structure of the samples werecharacterized by FTIR spectroscopy (Avatar 360, Nicolet). SEMtechnique (Quanta 200FEG, FEI) was employed to determine themorphologies and fracture behaviours of the carbon lms.Elemental compositions were estimated using energy-disper-sive X-ray spectroscopy (EDX). The crystalline structures andsizes of the carbonized and graphitized lms were characterizedusing the XRD technique (D/max-rb, 12 kW, Rigaku). Moreover,the degree of graphitization was calculated using the Braggdiffraction and Mering–Maire formulas. The internalmorphologies of the graphitized lms were observed by theTEM technique (Tecnai G2 F30, FEI). To estimate crystallinesizes, Raman spectra were measured using a Raman spec-trometer (HR-800, Olympus) using a laser operating at a wave-length of 532 nm as the excitation source. Electricalconductivity was estimated using the four-point probe method(Keithley 2100).

3 Results and discussion3.1 Structural evolution during the carbonization process

To investigate the structural evolution of the carbonizationprocess, FTIR was used to characterize carbonized lms atdifferent carbonization temperatures. Fig. 3 shows the FTIRspectra of the carbonized lms. The results demonstrate that

Fig. 2 Digital photographs of (a) the original SiC/PI composite films,(b) films carbonized at 1000 �C and (c) films graphitized at 2300 �C forpure PI and SiC/PI composite films with 1 wt%, 2 wt%, 3 wt% SiC NPsincorporation.

This journal is © The Royal Society of Chemistry 2014

Fig. 3 FTIR spectra of the SiC/PI composite films with 1 wt% SiC NPs:(a) SiC/PI composite film, (b) carbonized film at 600 �C for 2 h, (c)carbonized film at 800 �C for 2 h, (d) carbonized film at 1000 �C for 2 h.

Fig. 4 XRD patterns of (a) pure PI film at room temperature, (b) SiC/PIcomposite films, carbonized films (c) at 600 �C for 2 h; (c) at 800 �C for2 h; (d) at 1000 �C for 2 h. All films were contained 1 wt% SiC NPs.

Fig. 5 Elemental contents of SiC/PI composite films for films con-taining 1 wt% SiC carbonized at 600, 800 and 1000 �C for 2 h.

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the characteristic absorption peaks of the PI lm disappearedwhen the carbonization temperature exceeded 600 �C. Theintensity of the broad absorption peak around 2930 cm�1,assigned to the stretching vibration peak of the C–H residue,gradually decreased on increasing carbonization temperaturefrom 600 to 1000 �C. Similarly, the absorption peaks at 1623 and1400 cm�1, which correspond to the stretching vibration ofC]O and C–N–C bonds of an aromatic heterocyclic structure,were also weakened. This was caused by the decomposition andreorganization of the main chains of PI molecules during thecarbonization process.29 Furthermore, as shown in Fig. 3(a) and(b), the intensity of the vibration absorption peaks between 867and 801 cm�1, related to the C–H bending of aromatic rings,reduced in intensity with increasing carbonization temperature,and then disappeared at 1000 �C. However, a peak at 1580 cm�1,indicative of C]C bonds in the hexagonal rings of carbonmaterials, was evidently observed in these carbonized lms.These results indicated that the carbonization of PI/SiCcomposite lms must be accomplished at 1000 �C.30

The structural evolution during the carbonization processwas also conrmed by XRD results, as shown in Fig. 4. The peakat 2q ¼ 18.74� in Fig. 4(a), assigned to the layered structure ofthe PI lm, disappeared in Fig. 4(b). This indicated that thedecomposition of chain segments in the PI molecules occurredduring this process. Meanwhile, the appearance of a clearerpeak at 2q ¼ 22� and a weaker peak at 2q ¼ 42� suggested thatthe intrinsic molecular structures of the PI lm were damagedand amorphous structures were formed.31 As shown in Fig. 4(c)and (d), the main (002) peak shied to a higher angle than thatof Fig. 4(b). For the lm carbonized at 1000 �C, the diffractionpeak at 2q ¼ 24.46� corresponds to the (002) peak, and thediffraction peak at 2q ¼ 43.77� is assigned to the (100) facet. Inaddition, an obtuse peak appeared at 2q ¼ 80.11�, which wasdifferent from that of 600 and 800 �C, is considered to be thediffraction peak of the (110) facet. The observed phenomenacould be adequately explained by the fact that the layeredstructures of SiC/PI composite lms were transformed into

This journal is © The Royal Society of Chemistry 2014

hexagonal structures of the carbonized lms. As suggested byliterature reports,32 these results indicate that SiC/PI compositelms are easily-graphitized precursors.

Using the EDX spectra of the carbonized lms, theelemental contents of SiC/PI composite lms with 1 wt% SiCNPs incorporation carbonized at different temperatures for2 h are shown in Fig. 5. The results demonstrate that thecarbon (C) and silicon (Si) elemental content increased withincreasing carbonization temperature, while nitrogen (N) oroxygen (O) content gradually decreased. Aer carbonizationat 1000 �C, the carbon content increased up to 14%,compared to that of the sample carbonized at 600 �C. Simi-larly, the oxygen content decreased by 46% in the sameprocess. These results illustrate the tremendous structuralchanges that were observed in the carbonized lms due to thedeoxidization and denitrogenization reactions that occurinside the SiC/PI composite lms,33 which is also consistentwith the XRD results.

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Fig. 7 SEM photographs of the cross-section of films carbonized at1000 �C for 2 h. (a) film carbonized without SiC NPs, carbonized filmswith (b) 1.0 wt%, (c) 2.0 wt% and (d) 3.0 wt% SiC NPs incorporation.

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3.2 Inuence of SiC NPs on the carbonization of PI lms

To illustrate the inuence of SiC NPs on the crystalline structureand carbonization yields, different SiC/PI composite lms werecarbonized at 1000 �C for 2 h. Fig. 6 shows the XRD patterns ofcarbonized lms with different SiC NPs contents. In Fig. 6(a),the SiC NPs diffraction peaks at 2q ¼ 35.6�, 41.3�, 59.6� and71.76� were consistent with the standard b-SiC phase (JCPDScard no. 29-1129). These diffraction peaks are also displayed inFig. 6(c)–(e).34 In all the cases, XRD patterns demonstrated thatthe crystalline structure of SiC NPs in carbonized lms had noobvious changes at 1000 �C for 2 h. The peak at 23.8�, which isconsistent with the (002) diffraction peak of carbon materials,35

was strengthened following an increase in the SiC NPs incor-poration. Furthermore, for the carbonized lms without SiCNPs, the full width at half maximum (FWHM) was larger thanthat of the carbonized lms with different SiC NPs incorpora-tions. The intensity of the (002) diffraction peak in Fig. 6(a) waslower than that of Fig. 6(b)–(d). This nding indicates that SiCNPs could promote the carbonization of PI to form thecarbonized structure. Therefore, the catalytic effects of SiC NPson carbonized lms are shown for a carbonization process at1000 �C for 2 h.

The brittleness of the carbonized lms was investigatedqualitatively using the SEM photographs of the fracturesections as a measure of fracture toughness.36 Fig. 7 showsthe SEM photographs of the cross-sections of carbonizedlms under the bending stress fracture. The comparison ofmorphologies demonstrates that the micro-cracks located inthe carbonized lm were extended directly to a smoothfracture, which corresponds to the brittle fracture mode.37

With increasing SiC NPs incorporation, micro-cracks gradu-ally emerged and magnied in the carbonized lms. Thisphenomenon demonstrates that in the fracture process,more rupture work is required to make up the energyconsumed by crack propagation, leading to enhanced frac-ture toughness of the carbonized lms.

Fig. 6 XRD patterns of films carbonized at 1000 �C for 2 h. (a) filmscarbonized without SiC NPs, carbonized films with (b) 1.0 wt%, (c) 2.0wt%, and (d) 3.0 wt% SiC NPs incorporation.

42572 | RSC Adv., 2014, 4, 42569–42576

3.3 Catalytic effects of SiC NPs on the graphitization

Fig. 8 provides a comparison among the XRD patterns observedfor graphitized lms with different SiC NPs incorporations,which helps in estimating the crystalline size and degree ofgraphitization. The disappearance of the b-SiC characteristicsdiffraction peaks indicates that the SiC NPs have completelydecomposed into C and Si elements, which have overown fromthe graphitized lms because the boiling point of elementalsilicon is below 2300 �C. Under graphitization conditions at2300 �C, crystalline silicon becomes molten and is transformedinto an amorphous structure, making its characteristicsdiffraction peak disappear in the XRD patterns.38 As it is known,the b-SiC undergoes a phase transformation from b-SiC to a-SiC,which could occur during pyrolysis to produce the honeycombcarbon structure.39,40 Furthermore, the characteristic diffraction

Fig. 8 XRD patterns of films graphitized at 2300 �C for 2 h, (a) puregraphitized films without SiC nanoparticles, graphitized films with (b)1.0 wt%, (c) 2.0 wt%, (d) 3.0 wt% SiC NPs incorporation.

This journal is © The Royal Society of Chemistry 2014

Fig. 9 Raman spectra of films graphitized at 2300 �C for 2 h, (a) purefilms graphitized without SiC NPs, films graphitized with (b) 1.0 wt%, (c)2.0 wt%, (d) 3.0 wt% SiC NPs incorporation.

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peaks associated with the (002), (100), (004) and (110) facetscould be easily observed, which are related to the graphite-likestructures.

Moreover, the FWHM and intensity of the (002) facetexhibited an enhanced effect with increasing SiC NPs incor-poration. For the quantitative analysis of catalytic effects inthe graphitization, the degree of graphitization was esti-mated by the Bragg diffraction formula (1) and Mering–Maireformula (2).41

l ¼ 2d sin q (1)

where l ¼ 0.154178 nm is the wavelength of Cu Ka radiation, dis the interplanar spacing (nm), and q is the Bragg diffractionangle.

G ¼ (0.3440 � d002)/(0.3440 � 0.3354) � 100% (2)

where d002 is the interplanar spacing of (002) of the carbon lms(nm) and G is the degree of graphitization of the carbon lms.

The calculated results obtained from all the graphitizedsamples are included in Table 1. The pure PI lm without SiCNPs was non-graphitizable at 2300 �C. Meanwhile, for SiC NPsincorporation with 1.0 wt%, 2.0 wt%, 3 wt%, the degrees ofgraphitization reached up to 3.49%, 9.30% and 31.40%,respectively. This indicates that the catalytic effects of the SiCNPs could lead to an enhancement in the graphitization ofcarbonized lms and an increase in the size of the crystallites.As previously reported, the SiC crystalline structure was similarto that of carbon materials due to defects on the surface and thesix-fold scattering centers of SiC NPs.42 In the carbonization andgraphitization processes, the carbon atoms on the surface of SiCNPs were arranged to follow a graphite-like layered structure,which makes the carbonized lms more easily graphitizedmaterials.43 In contrast, the SiC NPs catalyst decomposed into Cand Si elements, and evaporated from the bulk lms during thegraphitization process at 2300 �C. The other regions in thegraphitized lms would form an amorphous structure.44

Raman spectroscopy is a highly sensitive and non-destruc-tive technique for identifying carbon materials.45 Fig. 9 providesthe typical Raman spectra of graphitized samples investigatedin our experiments. All the samples show a weak D band at 1370cm�1 and an intense G band at 1590 cm�1, which was attributedto the rst order Raman scattering. Furthermore, the intensityratio of ID/IG is an important parameter for graphite-likematerials. The crystalline size (La) of graphitized lms deter-mined by rst order Raman scattering could be estimated byformula (3):46

Table 1 Graphitization parameters of graphitized films at 2300 �Cfor 2 h

Graphitized lms 2q (�) d002 (nm) Degree of graphitization (G)

Without SiC 25.49 0.3490 —1 wt% SiC 25.89 0.3437 3.5 � 0.1%2 wt% SiC 25.93 0.3432 9.3 � 0.1%3 wt% SiC 26.08 0.3413 31.4 � 0.2%

This journal is © The Royal Society of Chemistry 2014

La ¼ C(l)/(ID/IG) (3)

where La is the in-plane crystal size, C(l) ¼ 4.4 nm is a ttingconstant, ID/IG is the intensity ratio between the D and G bands.

As observed, the crystalline size of the graphitized lmsincreased from 8.324 nm to 16.48 nm upon increasing the SiCNPs incorporation from 0 wt% (without) to 3 wt%, which isconsistent with the results estimated by the XRD patterns.Second order Raman scatting is an alternative approach for theexact quantication of differences in crystalline sizes, which iseffective for the determination of crystalline parameters ofcarbonized and graphitized lms. Herein, the materialsproduced in this study exhibit obvious second order Ramanscatterings, G0 band at 2735 cm�1, D + G band at 2960 cm�1 andD00 band at 3248 cm�1, which also become weaker and sharperwith increasing SiC NPs content. These results are importantevidence for conrming that the degree of graphitizationincreased due to the catalytic effects of SiC NPs.47

3.4 Fracture behaviours of cross-sections of graphitizedlms

Fig. 10 presents the SEM photographs of the cross-sections ofgraphitized lms. The cross-sections exhibited brighter regionsin the graphitized lms with various SiC NPs loadings, whichcorrespond to the turbostratic structure. However, the turbos-tratic structure could not be observed in the pure graphitizedlms in Fig. 10(a). Moreover, Fig. 10(b)–(d) also demonstratethat the bright regions were augmented with increasing SiC NPsincorporation in the graphitized lms. These results also illus-trated that the catalytic effects of SiC NPs play an important rolein the graphitized process at 2300 �C, which suggests that theSiC NPs are very effective catalysts for the enhanced graphiti-zation of PI lms.48

Furthermore, TEM provides insights into the innermorphologies and structures of carbon materials.49 Fig. 11shows the TEM photographs of the cross-sections of lmsgraphitized at 2300 �C. The uniform ake domains in Fig. 11(a)were consistent with the glassy carbon texture.50 For the other

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Fig. 10 SEM photographs of cross-sections of graphitized films at2300 �C for 2 h: (a) pure graphitized films without SiC NPs, graphitizedfilms with (b) 1.0 wt%, (c) 2.0 wt%, (d) 3.0 wt% SiC NPs incorporation.

Fig. 11 TEM photographs of graphitized films with various SiC NPsincorporations. (a) Pure graphitized films without SiC NPs, graphitizedfilms with (b) 1.0 wt%, (c) 2.0 wt% and (d) 3.0 wt% SiC NPsincorporation.

Fig. 12 Effect of SiC content on the (a) carbonization and (b) graphi-tization yields of SiC/PI composite films.

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graphitized samples, the irregular domains, which were attrib-uted to the turbostratic structures,51 appeared and graduallyincreased with increasing SiC NPs incorporation into thegraphitized lms, as shown in Fig. 11(b)–(d). These ndingsdemonstrate that the carbonized lms containing SiC NPstransformed into well-graphitized materials due to the catalyticeffects of SiC NPs. Consequently, the areas of turbostraticstructure were still lower than 10 mm for the graphitized lmswith 3 wt% SiC NPs incorporation. It can be concluded thatthe SiC NPs are highly effective catalysts for the graphitizationof PI lms.

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3.5 Carbonization and graphitization yields

Although the structures and mechanisms of carbonization andgraphitization have been investigated, it wasmore interesting toincrease the carbonization and graphitization yields of poly-imide materials.52 However, this problem has not been paidmuch attention in previous studies. Herein, we studied theinuence of SiC NPs on the carbonization and graphitizationyields. Fig. 12 shows the relationships between the incorpora-tion of SiC NPs and the carbonization and graphitization yields.The results demonstrate that the SiC NPs are highly effectiveadditive agents for increasing the output of carbonization andgraphitization processes. The ndings could also be explainedby the catalytic effects of the SiC NPs, which improved thestability of PI lms and prevented the pyrolysis of the mainchains of PI lms and the generation of small molecules, i.e.,CO, N2, and H2.53 Moreover, these effects could also improve thewholeness and compactness of the as-produced carbonized andgraphitized lms.

3.6 Electrical conductivities of the graphitized lms

Previous studies demonstrated that the turbostratic (graphi-tized) phase exhibited better conductive properties than theglassy carbon phase.54 To understand this phenomenon,current–voltage (I–V) curves of graphitized lms with variousSiC NPs incorporation were measured using the four-pointprobe method, as shown in Fig. 13. Sheet resistance could beestimated using eqn (4).

s

�p

ln 2

��V

I

�(4)

where s is the sheet resistance, V is the potential differencebetween two probes, and I is the current between twoprobes.55

Because of an increase in the turbostratic regions in thegraphitized lms, the conductive properties were signicantlyenhanced with SiC NPs incorporation. The lm graphitized with3 wt% SiC NPs displayed the highest electrical conductivity,with a sheet resistance reaching up to 0.96 U,�1. This ndingwas consistent with the results of the structural analysis.

This journal is © The Royal Society of Chemistry 2014

Fig. 13 Current–voltage curves of graphitized films at various SiC NPsincorporations.

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Therefore, the enhanced effects of SiC NPs also may beconsiderably valuable for improving the physical properties ofcarbon materials.

4 Conclusions

SiC/PI composite lms were successfully prepared using asolution blending process. To obtain carbonized lms,carbonization was carried out at 600, 800 and 1000 �C for 2 h.According to the FTIR spectra and XRD patterns, carbonizedlms were gradually formed with increasing carbonizationtemperature. The EDX spectra also conrmed that the Celemental content increased during the carbonized process. Atthe same time, the O and N elemental contents decreased,which indicates that the carbonization of the PI lms mainlyoccurred via deoxidization and denitrogenization reactions.Aer graphitization at 2300 �C for 2 h, the degree of graphiti-zation and crystalline size were augmented with increasing SiCNPs incorporation. These results illustrate that the SiC NPsexhibit catalytic effects on the carbonization and graphitizationof polyimide lms. The SEM and TEM photographs of cross-sections of graphitized lms demonstrated that the turbostraticphases were present in the graphitized lms at various SiC NPsincorporations. The carbonized and graphitized yields were alsoclearly increased. The electrical conductivities exhibited asignicant enhancement, especially for the 3 wt% SiC NPssample, whose resulting graphitized lm exhibited a sheetresistance of up to 0.96 U ,�1. These results conrmed thecatalytic and enhanced effects of SiC NPs on the carbonizationand graphitization of PI lms, which would be valuable forapplications in fuel cells, supercapacitors and carbon matrixcomposites.

Acknowledgements

We thank the National Natural Science Foundation of China(no. 51010005, 91216123, 51174063), the program for NewCentury Excellent Talents in University (NCET-08-0168). Natural

This journal is © The Royal Society of Chemistry 2014

Science Funds for Distinguished Young Scholar of HeilongjiangProvince. The project of International Cooperation supportedby Ministry of Science and Technology of China(2010DFR50160).

Notes and references

1 L. Kesavan, R. Tiruvalam, M. H. A. Rahim, M. I. bin Saiman,D. I. Enache, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, D. W. Knight, C. J. Kiely andG. J. Hutchings, Science, 2011, 331, 195–199.

2 Y. Wang, J. Q. Liu, S. Christiansen, D. H. Kim, U. Gosele andM. Steinhart, Nano Lett., 2008, 8, 3993–3997.

3 M. Inagaki, Carbon, 2012, 50, 3247–3266.4 H. C. Tao, L. Z. Fan and X. H. Qu, Electrochim. Acta, 2012, 71,194–200.

5 B. Qiu, C. T. Pan, W. J. Qian, Y. J. Peng, L. H. Qiu and F. Yan,J. Mater. Chem. A, 2013, 1, 6373–6378.

6 M. Inagaki, N. Ohta and Y. Hishiyama, Carbon, 2013, 61, 1–21.

7 M. Inagaki, H. Tachikawa, T. Nakahashi, H. Konno andY. Hishiyama, Carbon, 1998, 36, 1021–1025.

8 K. C. Yung, D. W. Zeng and T. M. Yue, Appl. Surf. Sci., 2001,173, 193–202.

9 M. Inagaki, L. J. Meng, T. Ibuki, M. Sakai and Y. Hishiyama,Carbon, 1991, 29, 1239–1243.

10 Y. Hishiyama, A. Yoshida and Y. Kaburagi, Carbon, 1992, 30,333–337.

11 T. Takeichi, Y. Eguchi, Y. Kaburagi, Y. Hishiyama andM. Inagaki, Carbon, 1999, 37, 569–575.

12 T. Maitra, S. Sharma, A. Srivastava, Y. K. Cho, M. Madou andA. Sharma, Carbon, 2012, 50, 1753–1760.

13 V. E. Smirnova, I. V. Gofman, T. A. Maritcheva, V. E. Yudin,K. Eto, T. Takeichi, Y. Kaburagi and Y. Hishiyama, Carbon,2007, 45, 839–846.

14 T. Takeichi, Y. Endo, Y. Kaburagi, Y. Hishiyama andM. Inagaki, J. Appl. Polym. Sci., 1998, 68, 1613–1620.

15 Y. Bin, K. Oishi, A. Koganemaru, D. Zhu and M. Matsuo,Carbon, 2005, 43, 1617–1627.

16 J. Gong, K. Yao, J. Liu, Z. W. Jiang, X. C. Chen, X. Wen,E. Mijowsk, N. N. Tian and T. Tang, J. Mater. Chem. A,2013, 1, 5247–5255.

17 Y. A. Niu, X. Zhang, J. P. Zhao, Y. Q. Tian, X. Q. Yan and Y. Li,J. Mater. Chem. A, 2014, 2, 10502–10507.

18 M. Kaarik, M. Arulepp, M. Karelson and J. Leis, Carbon, 2008,46, 1579–1587.

19 D. Y. Zhang, Y. Hao, L. W. Zheng, Y. Ma, H. X. Feng andH. M. Luo, J. Mater. Chem. A, 2013, 1, 7584–7591.

20 Y. Yan, Y. X. Yin, S. Xin, Y. G. Guo and L. J. Wan, Chem.Commun., 2012, 48, 10663–10665.

21 C. M. Thompson, H. M. Herring, T. S. Gates andJ. W. Connell, Compos. Sci. Technol., 2003, 63, 1591–1598.

22 Z. Z. Sun, Z. Yan, J. Yao, E. Beitler, Y. Zhu and J. M. Tour,Nature, 2010, 468, 549–552.

23 J. D. Wood, W. Scott, S. W. Schmucker, A. S. Lyons, E. Popand J. W. Lyding, Nano Lett., 2011, 11, 4547–4554.

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24 S. H. Ji, J. B. Hannon, R. M. Tromp, V. Perebeinos, J. Tersoffand F. M. Ross, Nat. Mater., 2012, 11, 114–119.

25 M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy,B. Zhang, X. Wu, C. Berger and W. A. Heer, Nat.Nanotechnol., 2010, 5, 727–731.

26 S. W. Poon, W. Chen, A. T. S. Wee and E. S. Tok, Phys. Chem.Chem. Phys., 2010, 12, 13522–13533.

27 Y. A. Niu, X. Zhang, J. P. Zhao, Y. Q. Tian, Y. Li and X. Q. Yan,RSC Adv., 2014, 4, 28456–28462.

28 C. M. Leu, Z. W. Wu and K. H. Wei, Chem. Mater., 2002, 14,3016–3021.

29 H. Ishida and M. T. Huang, Spectrochim. Acta, Part A, 1995,51, 319–331.

30 W. J. Chou, C. C. Wang and C. Y. Chen, Polym. Degrad. Stab.,2008, 93, 745–752.

31 B. K. Zhu, S. H. Xie, Z. K. Xu and Y. Y. Xu, Compos. Sci.Technol., 2006, 66, 548–554.

32 X. Yan, H. H. Song and X. H. Chen, J. Mater. Chem., 2009, 19,4491–4494.

33 G. Z. Wu, Y. C. Tang and R. X. Weng, Polym. Degrad. Stab.,2010, 95, 1449–1455.

34 Y. H. Baek, Y. H. Ryu and K. J. Yong,Mater. Sci. Eng., C, 2006,26, 805–808.

35 Y. I. A. Yoshida, Y. Hishiyama and Y. Kaburagi, Carbon, 2004,42, 1799–1805.

36 H. Konno, H. Oka, K. Shiba, H. Tachikawa and M. Inagaki,Carbon, 1999, 37, 887–895.

37 A. Kawasaki and R. Watanabe, Eng. Fract. Mech., 2002, 69,1713–1728.

38 J. G. Zhao, L. X. Yang, F. Y. Li, R. C. Yu and C. Q. Jin, Carbon,2009, 47, 744–751.

39 C. Berger, Z. M. Song, X. B. Li, X. S. Wu, N. Brown, C. Naud,D. Mayo, T. B. Li, J. Hass, A. N. Marchenkov, E. H. Conrad,P. N. First and W. A. de Heer, Science, 2006, 312, 1191–1196.

42576 | RSC Adv., 2014, 4, 42569–42576

40 S. I. Vlaskina, Semicond. Phys., Quantum Electron.Optoelectron., 2002, 5, 152–155.

41 N. Iwashita, C. R. Park, H. Fujimoto, M. Shiraishi andM. Inagaki, Carbon, 2004, 42, 701–714.

42 C. J. Shih, A. Vijayaraghavan, R. Krishnan, R. Sharma,J. H. Han, M. H. Ham, Z. Jin, S. C. Lin, G. L. C. Paulus,N. F. Reuel, Q. H Wang, D. Blankschtein and M. S. Strano,Nat. Nanotechnol., 2011, 6, 439–445.

43 Z. P. Chen, W. C. Ren, L. B. Gao, B. Liu, L. S. F. Pei andH. M. Cheng, Nat. Mater., 2011, 10, 424–428.

44 Q. H. Wang and M. C. Hersam, Nat. Chem., 2009, 1, 206–211.45 A. C. Ferrari and J. Robertson, Phys. Rev. B: Condens. Matter

Mater. Phys., 2000, 61, 14098.46 F. C. Tai, C. Wei, S. H. Chang and W. S. Chen, J. Raman

Spectrosc., 2010, 41, 933–937.47 H. Konno, K. Shiba, Y. Kaburagi, Y. Hishiyama and

M. Inagaki, Carbon, 2001, 39, 1731–1740.48 B. L. Lei, L. L. He, M. Z. Yi, L. P. Ran, H. J. Xu, Y. C. Ge and

K. Peng, Carbon, 2011, 49, 4554–4562.49 G. X. Du, C. Song, J. H. Zhao, S. A. Feng and Z. P. Zhu,

Carbon, 2008, 46, 92–98.50 T. Petersen, I. Yarovsky, I. Snook, D. G. McCulloch and

G. Opletal, Carbon, 2004, 42, 2457–2469.51 S. Welz, M. J. McNallan and Y. Gogotsi, J. Mater. Process.

Technol., 2006, 179, 11–22.52 M. L. Greene, R. W. Schwartz and J. W. Treleaven, Carbon,

2002, 40, 1217–1226.53 T. Nakahashi, H. Konno and M. Inagaki, Solid State Ionics,

1998, 113, 73–77.54 G. Savini, Y. J. Dappe, S. Oberg, J. C. Charlier,

M. I. Katsnelson and A. Fasolino, Carbon, 2011, 49, 62–69.55 T. Maitra, S. Sharma, A. Srivastava, Y. K. Cho, M. Madou and

A. Sharma, Carbon, 2012, 50, 1753–1761.

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