Silicon (2009) 1:125–129 DOI 10.1007/s12633-009-9016-0
A New Technique for Coating Silicon Carbide Onto Carbon Nanotubes Using a Polycarbosilane Precursor Rakesh Kumar Gupta & Raghwesh Mishra & Kingsuk Mukhopadhyay & Rajesh Kumar Tiwari & Ashok Ranjan & Arvind Kumar Saxena
Received: 30 March 2009 / Accepted: 13 July 2009 / Published online: 26 August 2009 # Springer Science & Business Media B.V. 2009
Abstract Multiwalled carbon nanotubes (MWCNTs) have been coated with silicon carbide (SiC) using polycarbosilane as precursor in order to improve their thermo oxidative stability. The polycarbosilane coated MWCNTs were heated to ~1300°C under an inert atmosphere to generate the SiC coating. X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy have confirmed the formation of SiC on the MWCNTs. The retention of the tubular structure of the MWCNTs has been confirmed by transmission electron microscopy. Thermogravimetric analysis has been performed to evaluate the thermo oxidative stabilities of coated and virgin MWCNTs. Sonication studies have shown that the mechanical strength of the MWCNTs was increased after coating with SiC. Keywords Polycarbosilane . SiC . MWCNTs
1 Introduction Carbon nanotubes (CNTs) have generated a great deal of scientific and commercial interest due to their extraordinary properties like high mechanical strength [1, 2], high thermal conductivity [3] and electrical properties [4– 6]. CNTs have been used for a number of high tech applications such as making conductive and high strength
R. K. Gupta : R. Mishra : K. Mukhopadhyay : R. K. Tiwari (*) : A. Ranjan : A. K. Saxena Defence Materials and Stores Research and Development Establishment, G T Road, Kanpur 208013, India e-mail:
[email protected]
composites, energy storage and energy conversion devices, chemical sensors, field emission displays and radiation sources, hydrogen storage media, nanometer sized semiconductor devices, probes and interconnects [7]. It has also been reported that the physico-mechanical properties of some composites are enhanced by the reinforcement of CNTs [8, 9]. However such applications are restricted due to the limited thermo-oxidative stability (~530°C) of the CNTs. To improve this, very recently a SiC coating has been formed on multi-walled carbon nanotubes (MWCNTs) using SiO vapour using pulsed electric current sintering at 1150–1550°C [10]. As such studies are very scarce, it is considered worthwhile to develop a simple process to generate a SiC coating on MWCNTs. In the present investigation we describe the development of a simple method to generate a SiC coating on MWCNTs using a –Si-H group bearing polycarbosilane (PCS). The morphological and thermal studies of coated and uncoated MWCNTs have also been studied in detail.
2 Experimental 2.1 Materials All solvents were dried by distillation over a sodium benzophenone system. Sodium metal was used as received from SD Fine-Chem. Ltd. and other chemicals were used as received from Aldrich Chemical Co. Polydimethylsilane (PDMS) and PCS (Mn ¼ 1800, Mw ¼ 7159, Dp ¼ 3:97) were synthesized as reported [11]. MWCNTs of diameter of OD~15 nm and ID~4 nm were prepared by catalytic chemical vapour deposition (CCVD) in our lab as reported in the literature [12, 13]. Petroleum ether (60–80°C, AR grade) was used as received.
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Fig. 1 FTIR spectra of PCS and PCS coated CNTs
PCS
2367.67 3449.49
1407.97 1356.35 2897.90 2954.88 2103.52
%T
1254.14 1023.49 823.29
PCS coated CNTs 1654.53 1408.00 1354.83
2111.94 3448.74 2957.97 4000.0
3000
1023.52 828.08
2900.36 1258.51 2000
1500
1000
400.0
cm-1
2.2 Instrumentation Infrared (IR) spectra were recorded on a FTIR Perkin Elmer RX-1 spectrophotometer using a KBr discs in the range 4000–400 cm−1. Thermogravimetric analysis (TGA) was performed on a Setsys evolution 24 instrument. X-ray diffraction (XRD) data were taken on a Philips PW1320 refractometer using CuKa radiation with a nickel filter. A Carl Zeiss, Evo 50 instrument was used for scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) studies. A Tecnai instrument was used for transmission electron microscopy (TEM) studies.
Afterwards the temperature was further raised up to 1300°C at the rate of 200°C/h and held at the final temperature for 0.25 h. Thereafter the furnace was allowed to cool under argon flow for 24 h. 2.4 Sonication of Coated and Uncoated MWCNTs 1.0 mg of coated and non coated MWCNTs were suspended separately in 10 ml of ethanol and ultrasonicated for 0.5 h using an ultrasonicator with a 2 watt power delivery. Afterwards the residue was filtered and examined by SEM.
2.3 Coating of MWCNTs with SiC 3 Results and Discussion MWCNTs (1.0 g) were placed in a three neck quartz flask (250 ml), fitted with a condenser and magnetic stirrer, containing PCS (0.1 g) in 100 ml petroleum ether (60–80°C) under strict dry nitrogen atmosphere. The solution was stirred for 1 h and afterwards the solvent was removed under a current of nitrogen at ~80°C within 0.5 h. The residue was heated in a furnace at the rate of 20°C/h up to 700°C under argon and was held for 0.5 h.
MWCNTs have been tested to reinforce various matrices because they have many unique mechanical and physical properties. To improve their thermo-oxidative stability and mechanical strength we have developed a new and simple
TG % 100
SiC coated CNTs
95
CNTs
90 85 80 75 70 65
200
400
600
1200 800 1000 Furnace Temperature ˚C
1400
1600
Fig. 2 TGA thermogram of SiC coated and uncoated CNTs
Fig. 3 XRD pattern of SiC from polycarbosilane
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Fig. 4 XRD pattern of SiC coated MWCNTs
process for coating MWCNTs with SiC using PCS as a ceramic material precursor. The FTIR spectra of the polycarbosilane (Fig. 1), showed an absorption at v~1400, 2900 and 2950 cm−1 for C-H stretching whereas absorptions at v~600, 900 and 1250 cm−1 were indicative of the presence of Si-Me groups. The absorption bands that appeared at v~1020 and 1352 cm−1 were due to the Si-CH2-Si bond and the sharp absorption band at v2100 cm−1 was due to Si-H stretching. The FTIR spectra of CNTs coated with polycarbosilane (Fig. 1) showed a decrease in the intensity of the Si-H bond stretching band at 2100 cm−1. These observations
Fig. 5 EDX of SiC coated MWCNTs
Table 1 EDX data of SiC coated MWCNTs Element
Wt %
At %
k-ratio
Z
A
F
Ck Si k Total
98.87 1.13 100
99.52 0.48 100
0.8690 0.0101
1.0006 0.9441
0.8784 0.9487
1.0000 1.0000 Fig. 6 SEM Micrograph of SiC coated MWCNTs
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Fig. 9 SEM Micrograph of MWCNTs after sonication Fig. 7 TEM Micrograph of SiC coated MWCNTs
indicated that bonding of polycarbosilane with CNTs occurred through hydrosilylation of unsaturated surface moieties of the CNTs with Si-H moieties of PCS, which is more reactive than Si-Me bonds. The TGA analysis of the coated and uncoated MWCNTs has been carried out with a heating rate 10°C/min under flow of argon (20 ml/min) up to 1500°C to study thermo-oxidative stabilities. It is clearly evident from TGA analysis that the uncoated MWCNTs degraded at ~700°C whereas there is no degradation observed in the case of coated MWCNTs up to 1400°C (Fig. 2). Such increase in thermo-oxidative stability of MWCNTs may be attributed to the SiC coating. In the XRD pattern (Fig. 3) three diffraction peaks are observed at 36.33°, 60.40° & 72.15° which showed the formation of β-SiC when polycarbosilane was heated to 1300°C [14]. When the CNTs coated with the same polycarbosilane were heated up to 1300°C under argon, peaks for both carbon and SiC are observed at 26.3° and 42.3° for CNTs and at 63.8°, 77.4° and 81.4° for β-SiC (Fig. 4).
Fig. 8 HRTEM Micrograph of SiC coated MWCNTs
The presence of Si and C element peaks in the EDX analysis further confirmed the formation of a SiC coating on MWCNTs (Fig. 5 and Table 1). The SEM image of SiC coated MWCNTs showed single phase homogeneous morphology (Fig. 6). As MWCNTs have been heated up to 1300°C in the process of coating, it is essential to observe whether the tubular structure is retained or not. TEM micrographs (Figs. 7 and 8) clearly indicate that the tubular structure of MWCNTs has been retained up to 1300°C. Both coated and uncoated MWCNTs have been sonicated in an ultrasonicator with a 2 watt power delivery for 0.5 h. It has been observed that uncoated MWCNTs are damaged (Fig. 9) whereas coated MWCNTs structure remains unaltered, which clearly indicates that the mechanical strength of coated MWCNTs improved as compared to uncoated MWCNTs (Fig. 10).
Fig. 10 SEM Micrograph of SiC coated MWCNTs after sonication
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4 Conclusions PCS was found to be suitable as a coating material to generate SiC on MWCNTs. The thermo-oxidative stability and mechanical strength of MWCNTs improved after coating. Acknowledgements The authors are thankful to the Director of DMSRDE, Kanpur for providing the necessary lab facilities and encouragement. Thanks are also due to Sri N.C.Mehra, technical officer, USIC, Delhi University for the TEM results and Sri Mahender Prasad, Scientist, DMSRDE, Kanpur for the SEM results.
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