Catalytic and enhanced effects of silicon carbide nanoparticles on carbonization and graphitization of polyimide films

Abstract

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.

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1 Introduction

Carbon films, including carbonized films and graphitized films, have attracted significant attention due to their extensive applications in catalysts,¹ high thermal conductivity materials,² fuel cells,³ lithium ion batteries⁴ and supercapacitors.⁵ Recently, the pyrolysis of aromatic PI films at a high temperature was considered to be the most promising approach for producing high performance carbon films.⁶ Carbonized and graphitized mechanisms of various commercial PI films, including Kapton, Upilex and Novax films, have been investigated, and the mechanical properties of carbon films were also studied in pervious works.^7–10 However, in the graphitization of these PI films, a critical disadvantage is that they are non-graphitizable. The graphitization temperature usually exceeded 3050 °C and the starting crystalline temperature of graphitization also reached up to 3200 °C.¹¹

To overcome this disadvantage, two main routes have been suggested for increasing the graphitization yield and degree of graphitization.¹² One route involves improving the degree of orientation of the PI molecules because random orientations can easily produce glassy carbon and other non-graphitized structures.¹³ T. Takeichi et al. reported the influence of the orientation of PI films graphitized at 2800 °C on the graphitization process.¹⁴ The other route involved utilizing the catalytic effects of several catalysts, i.e. transition metal and metal oxide particles. Recently, the catalytic effects of nickel (Ni) particles¹⁵ and Fe₂O₃ NPs¹⁶ were investigated in PI and chlorinated poly(vinyl chloride) (CPVC) resins. The results demonstrated some excellent advantages, which could shorten the carbonized process at 1600 °C and improve the degree of graphitization at 2600 °C. Furthermore, PI films containing graphene,¹⁷ boron,¹⁸ nitrogen¹⁹ and sulfur²⁰ atoms were also used to accelerate the carbonization process and enhance the mechanical properties of the carbonized films. However, it was difficult to homogeneously disperse these metallic particles into the majority of PI films because of the differences in polarity and compatibility.²¹

This work was motivated by the catalytic effects of graphene grown on the Ni,²² copper (Cu)²³ or SiC²⁴ substrates by the chemical vapor deposition (CVD) method, which possessed similar structures as those of the graphite-like (honeycomb) carbon materials. In particular, the SiC NPs, which exhibit greater compatibility than the metallic (Ni and Cu) NPs, could produce surface activities and various intrinsic defects at high temperatures, i.e., six-fold scattering centers and sub-surface structures.^25,26 Furthermore, SiC NPs could also be well-dispersed in the PI matrix after modification.²⁷

In this work, the well-dispersed SiC/PI composite films were prepared by a solution blending process from polyamic acid (PAA) and SiC NPs. The catalytic effects of SiC NPs were investigated by FTIR, XRD, SEM and TEM. The turbostratic structures and degree of graphitization were estimated by Bragg diffraction and Mering–Maire formulas. The enhanced effects of SiC NPs on the carbonization and graphitization processes were further confirmed by Raman spectroscopy and XRD analysis. Finally, the electrical conductivities of graphitized films were measured by the four-point probe method.

2 Experimental

2.1 Preparation of SiC/PI composite films

3,3′,4,4′-Benzo-phenonetetracarboxylic dianhydride (BTDA), 4,4′-oxydianiline (ODA) and SiC NPs were purchased from Sinopharm Chemical Reagent Co., Ltd. A schematic representation for the preparation of SiC/PI composite films is shown in Fig. 1(a). PAA was synthesized by polycondensation between ODA and BTDA monomers at an equimolar ratio (1 : 1.02) at room temperature under a nitrogen (N₂) atmosphere.²⁸ SiC NPs were added to the PAA solutions at different dosages, i.e., 1 wt%, 2 wt% and 3 wt%. After stirring for 1 h, mixtures of PAA and SiC NPs were obtained, which were then cast on clean glass plates. Solvents were removed in vacuum by maintaining the temperature at 70 °C for 5 h. The curing process was carried out by heating the films at 100 °C for 2 h, 200 °C for 2 h and 300 °C for 1 h. Finally, the SiC/PI composite films were obtained. For comparison, a pure PI film was also prepared using the same process.

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

2.2 Carbonization and graphitization of SiC/PI composite films

The carbonization of SiC/PI composite films was executed in a tube electric furnace (GSL-1500X) under N₂ atmosphere. Fig. 1(b) shows a schematic representation of the carbonization and graphitization of SiC/PI composite films. The PI films were cut into 10 mm × 10 mm pieces and were sandwiched between two Al₂O₃ ceramic plates. The composite film samples were annealed at a heating rate of 2 °C min⁻¹ up to 600, 800 and 1000 °C. After maintaining the temperature for 2 h, the carbonized films were cooled to room temperature at a cooling rate of 10 °C min⁻¹. During the graphitization process, samples of carbonized films were sandwiched between two polished graphite plates. The carbonized films 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 films were prepared after cooling to room temperature at a cooling rate of 10 °C min⁻¹. Carbonization (graphitization) yields were calculated by the mass ratios between the carbonized (graphitized) films and the original SiC/PI composite films.

Fig. 2 shows the digital photographs of the SiC/PI composite films, carbonized films and graphitized films. The results reveal that a carbonized film of pure PI film was easily smashed. However, all the carbonized SiC/PI composite films remained whole without additional cracks.

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 for pure PI and SiC/PI composite films with 1 wt%, 2 wt%, 3 wt% SiC NPs incorporation.

2.3 Measurements and characterization

The chemical composition and structure of the samples were characterized by FTIR spectroscopy (Avatar 360, Nicolet). SEM technique (Quanta 200FEG, FEI) was employed to determine the morphologies and fracture behaviours of the carbon films. Elemental compositions were estimated using energy-dispersive X-ray spectroscopy (EDX). The crystalline structures and sizes of the carbonized and graphitized films were characterized using the XRD technique (D/max-rb, 12 kW, Rigaku). Moreover, the degree of graphitization was calculated using the Bragg diffraction and Mering–Maire formulas. The internal morphologies of the graphitized films were observed by the TEM technique (Tecnai G² F30, FEI). To estimate crystalline sizes, Raman spectra were measured using a Raman spectrometer (HR-800, Olympus) using a laser operating at a wavelength of 532 nm as the excitation source. Electrical conductivity was estimated using the four-point probe method (Keithley 2100).

3 Results and discussion

3.1 Structural evolution during the carbonization process

To investigate the structural evolution of the carbonization process, FTIR was used to characterize carbonized films at different carbonization temperatures. Fig. 3 shows the FTIR spectra of the carbonized films. The results demonstrate that the characteristic absorption peaks of the PI film disappeared when the carbonization temperature exceeded 600 °C. The intensity 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 temperature from 600 to 1000 °C. Similarly, the absorption peaks at 1623 and 1400 cm⁻¹, which correspond to the stretching vibration of C=O and C–N–C bonds of an aromatic heterocyclic structure, were also weakened. This was caused by the decomposition and reorganization of the main chains of PI molecules during the carbonization process.²⁹ Furthermore, as shown in Fig. 3(a) and (b), the intensity of the vibration absorption peaks between 867 and 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 carbon materials, was evidently observed in these carbonized films. These results indicated that the carbonization of PI/SiC composite films must be accomplished at 1000 °C.³⁰

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.

The structural evolution during the carbonization process was also confirmed by XRD results, as shown in Fig. 4. The peak at 2θ = 18.74° in Fig. 4(a), assigned to the layered structure of the PI film, disappeared in Fig. 4(b). This indicated that the decomposition of chain segments in the PI molecules occurred during this process. Meanwhile, the appearance of a clearer peak at 2θ = 22° and a weaker peak at 2θ = 42° suggested that the intrinsic molecular structures of the PI film were damaged and amorphous structures were formed.³¹ As shown in Fig. 4(c) and (d), the main (002) peak shifted to a higher angle than that of Fig. 4(b). For the film carbonized at 1000 °C, the diffraction peak at 2θ = 24.46° corresponds to the (002) peak, and the diffraction peak at 2θ = 43.77° is assigned to the (100) facet. In addition, an obtuse peak appeared at 2θ = 80.11°, which was different from that of 600 and 800 °C, is considered to be the diffraction peak of the (110) facet. The observed phenomena could be adequately explained by the fact that the layered structures of SiC/PI composite films were transformed into hexagonal structures of the carbonized films. As suggested by literature reports,³² these results indicate that SiC/PI composite films are easily-graphitized precursors.

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

Using the EDX spectra of the carbonized films, the elemental contents of SiC/PI composite films with 1 wt% SiC NPs incorporation carbonized at different temperatures for 2 h are shown in Fig. 5. The results demonstrate that the carbon (C) and silicon (Si) elemental content increased with increasing carbonization temperature, while nitrogen (N) or oxygen (O) content gradually decreased. After carbonization at 1000 °C, the carbon content increased up to 14%, compared to that of the sample carbonized at 600 °C. Similarly, the oxygen content decreased by 46% in the same process. These results illustrate the tremendous structural changes that were observed in the carbonized films due to the deoxidization and denitrogenization reactions that occur inside the SiC/PI composite films,³³ which is also consistent with the XRD results.

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

3.2 Influence of SiC NPs on the carbonization of PI films

To illustrate the influence of SiC NPs on the crystalline structure and carbonization yields, different SiC/PI composite films were carbonized at 1000 °C for 2 h. Fig. 6 shows the XRD patterns of carbonized films with different SiC NPs contents. In Fig. 6(a), the SiC NPs diffraction peaks at 2θ = 35.6°, 41.3°, 59.6° and 71.76° were consistent with the standard β-SiC phase (JCPDS card no. 29-1129). These diffraction peaks are also displayed in Fig. 6(c)–(e).³⁴ In all the cases, XRD patterns demonstrated that the crystalline structure of SiC NPs in carbonized films had no obvious changes at 1000 °C for 2 h. The peak at 23.8°, which is consistent with the (002) diffraction peak of carbon materials,³⁵ was strengthened following an increase in the SiC NPs incorporation. Furthermore, for the carbonized films without SiC NPs, the full width at half maximum (FWHM) was larger than that of the carbonized films with different SiC NPs incorporations. The intensity of the (002) diffraction peak in Fig. 6(a) was lower than that of Fig. 6(b)–(d). This finding indicates that SiC NPs could promote the carbonization of PI to form the carbonized structure. Therefore, the catalytic effects of SiC NPs on carbonized films are shown for a carbonization process at 1000 °C for 2 h.

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

The brittleness of the carbonized films was investigated qualitatively using the SEM photographs of the fracture sections as a measure of fracture toughness.³⁶ Fig. 7 shows the SEM photographs of the cross-sections of carbonized films under the bending stress fracture. The comparison of morphologies demonstrates that the micro-cracks located in the carbonized film were extended directly to a smooth fracture, which corresponds to the brittle fracture mode.³⁷ With increasing SiC NPs incorporation, micro-cracks gradually emerged and magnified in the carbonized films. This phenomenon demonstrates that in the fracture process, more rupture work is required to make up the energy consumed by crack propagation, leading to enhanced fracture toughness of the carbonized films.

Fig. 7 SEM photographs of the cross-section of films carbonized at 1000 °C for 2 h. (a) film carbonized without SiC NPs, carbonized films with (b) 1.0 wt%, (c) 2.0 wt% and (d) 3.0 wt% SiC NPs incorporation.

3.3 Catalytic effects of SiC NPs on the graphitization

Fig. 8 provides a comparison among the XRD patterns observed for graphitized films with different SiC NPs incorporations, which helps in estimating the crystalline size and degree of graphitization. The disappearance of the β-SiC characteristics diffraction peaks indicates that the SiC NPs have completely decomposed into C and Si elements, which have overflown from the graphitized films because the boiling point of elemental silicon is below 2300 °C. Under graphitization conditions at 2300 °C, crystalline silicon becomes molten and is transformed into an amorphous structure, making its characteristics diffraction peak disappear in the XRD patterns.³⁸ As it is known, the β-SiC undergoes a phase transformation from β-SiC to α-SiC, which could occur during pyrolysis to produce the honeycomb carbon structure.^39,40 Furthermore, the characteristic diffraction peaks associated with the (002), (100), (004) and (110) facets could be easily observed, which are related to the graphite-like structures.

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

Moreover, the FWHM and intensity of the (002) facet exhibited an enhanced effect with increasing SiC NPs incorporation. For the quantitative analysis of catalytic effects in the graphitization, the degree of graphitization was estimated by the Bragg diffraction formula (1) and Mering–Maire formula (2).⁴¹

λ = 2d sin θ (1)

where λ = 0.154178 nm is the wavelength of Cu Kα radiation, d is the interplanar spacing (nm), and θ is the Bragg diffraction angle.

G = (0.3440 − d₀₀₂)/(0.3440 − 0.3354) × 100% (2)

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

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

Table 1 Graphitization parameters of graphitized films at 2300 °C for 2 h

Graphitized films	2θ (°)	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%

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Raman spectroscopy is a highly sensitive and non-destructive technique for identifying carbon materials.⁴⁵ Fig. 9 provides the typical Raman spectra of graphitized samples investigated in our experiments. All the samples show a weak D band at 1370 cm⁻¹ and an intense G band at 1590 cm⁻¹, which was attributed to the first order Raman scattering. Furthermore, the intensity ratio of I_D/I_G is an important parameter for graphite-like materials. The crystalline size (L_a) of graphitized films determined by first order Raman scattering could be estimated by formula (3):⁴⁶

L_a = C(λ)/(I_D/I_G) (3)

where L_a is the in-plane crystal size, C(λ) = 4.4 nm is a fitting constant, I_D/I_G is the intensity ratio between the D and G bands.

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

As observed, the crystalline size of the graphitized films increased from 8.324 nm to 16.48 nm upon increasing the SiC NPs incorporation from 0 wt% (without) to 3 wt%, which is consistent with the results estimated by the XRD patterns. Second order Raman scatting is an alternative approach for the exact quantification of differences in crystalline sizes, which is effective for the determination of crystalline parameters of carbonized and graphitized films. Herein, the materials produced in this study exhibit obvious second order Raman scatterings, G′ band at 2735 cm⁻¹, D + G band at 2960 cm⁻¹ and D′′ band at 3248 cm⁻¹, which also become weaker and sharper with increasing SiC NPs content. These results are important evidence for confirming that the degree of graphitization increased due to the catalytic effects of SiC NPs.⁴⁷

3.4 Fracture behaviours of cross-sections of graphitized films

Fig. 10 presents the SEM photographs of the cross-sections of graphitized films. The cross-sections exhibited brighter regions in the graphitized films with various SiC NPs loadings, which correspond to the turbostratic structure. However, the turbostratic structure could not be observed in the pure graphitized films in Fig. 10(a). Moreover, Fig. 10(b)–(d) also demonstrate that the bright regions were augmented with increasing SiC NPs incorporation in the graphitized films. These results also illustrated that the catalytic effects of SiC NPs play an important role in the graphitized process at 2300 °C, which suggests that the SiC NPs are very effective catalysts for the enhanced graphitization of PI films.⁴⁸

Fig. 10 SEM photographs of cross-sections of graphitized films at 2300 °C for 2 h: (a) pure graphitized films without SiC NPs, graphitized films with (b) 1.0 wt%, (c) 2.0 wt%, (d) 3.0 wt% SiC NPs incorporation.

Furthermore, TEM provides insights into the inner morphologies and structures of carbon materials.⁴⁹ Fig. 11 shows the TEM photographs of the cross-sections of films graphitized at 2300 °C. The uniform flake domains in Fig. 11(a) were consistent with the glassy carbon texture.⁵⁰ For the other graphitized samples, the irregular domains, which were attributed to the turbostratic structures,⁵¹ appeared and gradually increased with increasing SiC NPs incorporation into the graphitized films, as shown in Fig. 11(b)–(d). These findings demonstrate that the carbonized films containing SiC NPs transformed into well-graphitized materials due to the catalytic effects of SiC NPs. Consequently, the areas of turbostratic structure were still lower than 10 μm for the graphitized films with 3 wt% SiC NPs incorporation. It can be concluded that the SiC NPs are highly effective catalysts for the graphitization of PI films.

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

3.5 Carbonization and graphitization yields

Although the structures and mechanisms of carbonization and graphitization have been investigated, it was more interesting to increase the carbonization and graphitization yields of polyimide materials.⁵² However, this problem has not been paid much attention in previous studies. Herein, we studied the influence of SiC NPs on the carbonization and graphitization yields. Fig. 12 shows the relationships between the incorporation of SiC NPs and the carbonization and graphitization yields. The results demonstrate that the SiC NPs are highly effective additive agents for increasing the output of carbonization and graphitization processes. The findings could also be explained by the catalytic effects of the SiC NPs, which improved the stability of PI films and prevented the pyrolysis of the main chains of PI films and the generation of small molecules, i.e., CO, N₂, and H₂.⁵³ Moreover, these effects could also improve the wholeness and compactness of the as-produced carbonized and graphitized films.

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

3.6 Electrical conductivities of the graphitized films

Previous studies demonstrated that the turbostratic (graphitized) phase exhibited better conductive properties than the glassy carbon phase.⁵⁴ To understand this phenomenon, current–voltage (I–V) curves of graphitized films with various SiC NPs incorporation were measured using the four-point probe method, as shown in Fig. 13. Sheet resistance could be estimated using eqn (4).where σ is the sheet resistance, V is the potential difference between two probes, and I is the current between two probes.⁵⁵

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

Because of an increase in the turbostratic regions in the graphitized films, the conductive properties were significantly enhanced with SiC NPs incorporation. The film graphitized with 3 wt% SiC NPs displayed the highest electrical conductivity, with a sheet resistance reaching up to 0.96 Ω □⁻¹. This finding was consistent with the results of the structural analysis. Therefore, the enhanced effects of SiC NPs also may be considerably valuable for improving the physical properties of carbon materials.

4 Conclusions

SiC/PI composite films were successfully prepared using a solution blending process. To obtain carbonized films, carbonization was carried out at 600, 800 and 1000 °C for 2 h. According to the FTIR spectra and XRD patterns, carbonized films were gradually formed with increasing carbonization temperature. The EDX spectra also confirmed that the C elemental content increased during the carbonized process. At the same time, the O and N elemental contents decreased, which indicates that the carbonization of the PI films mainly occurred via deoxidization and denitrogenization reactions. After graphitization at 2300 °C for 2 h, the degree of graphitization and crystalline size were augmented with increasing SiC NPs incorporation. These results illustrate that the SiC NPs exhibit catalytic effects on the carbonization and graphitization of polyimide films. The SEM and TEM photographs of cross-sections of graphitized films demonstrated that the turbostratic phases were present in the graphitized films at various SiC NPs incorporations. The carbonized and graphitized yields were also clearly increased. The electrical conductivities exhibited a significant enhancement, especially for the 3 wt% SiC NPs sample, whose resulting graphitized film exhibited a sheet resistance of up to 0.96 Ω □⁻¹. These results confirmed the catalytic and enhanced effects of SiC NPs on the carbonization and graphitization of PI films, which would be valuable for applications in fuel cells, supercapacitors and carbon matrix composites.

Acknowledgements

We thank the National Natural Science Foundation of China (no. 51010005, 91216123, 51174063), the program for New Century Excellent Talents in University (NCET-08-0168). Natural Science Funds for Distinguished Young Scholar of Heilongjiang Province. The project of International Cooperation supported by Ministry of Science and Technology of China (2010DFR50160).

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