Micro- and Nano-structure Characterization of Isotropic Pyrocarbon Obtained via Chemical Vapor Deposition in Hot Wall Reactor

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1 J. Mater. Sci. Technol., 2010, 26(12), Micro- and Nano-structure Characterization of Isotropic Pyrocarbon Obtained via Chemical Vapor Deposition in Hot Wall Reactor Kezhi Li, Dongsheng Zhang, Lingjun Guo and Hejun Li State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi an , China [Manuscript received May 21, 2009, in revised form January 28, 2010] The hot wall reactor was used to deposit isotropic pyrocarbon. The density of the product was measured by Archimedes method. The fracture morphology and texture of the product were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Raman spectroscopy was also applied to characterizing the structure of the product. The results show that the density of the product is about 1.31 g/cm 3. The pyrocarbon consists of granular particles with the size of about 1.5 µm under SEM images. Under TEM images, the pyrocarbon is composed of irregular grains and onion-like particles are also detected. The orientation angle (180 deg.) of pyrocarbon reveals its isotropic character. The I D /I G ratio in Raman spectrum is about 1.2, indicating the isotropic feature of the product qualitatively. KEY WORDS: Isotropic pyrocarbon; Chemical vapor deposition; Structure characterization; Onion-like pyrocarbon 1. Introduction Isotropic pyrocarbon, deposited onto a hot substrate via pyrolysis of hydrocarbon species in chemical vapor deposition (CVD) apparatus, has been widely applied in heart valves and nuclear fuel particles coating [1 3]. Some preparation methods, such as fluidized bed CVD and tumbling bed CVD, have been applied to manufacture isotropic pyrocarbon. Bard et al. [4] investigated the pyrolytic carbons deposited in fluidized bed at 1200 to 1400 C from various hydrocarbons. It was found that fragments dissociated by hydrocarbons took in polymerizationdehydrogenation reactions and gas-born droplets or soot particles were responsible for the various structures of pyrolytic carbon. Recently, the effect of deposition conditions on microstructure and anisotropy of pyrocarbon fabricated by fluidized bed CVD has been systematically researched [5,6]. But the substrate in fluidized bed CVD is constantly moving in the reaction tube, so the deposition Corresponding author. Prof., Ph.D.; Tel.: ; Fax: ; address: likezhi@nwpu.edu.cn (K.Z. Li). condition is totally different (close to the inlet, the residence time and temperature is short and low). Kim and Lee [3] researched the primary factors determining the microstructures of pyrolytic carbon deposited in a tumbling bed. They thought that the degree of supersaturation of pyrolysis product and the gas flow pattern of recirculation in deposition zone were two essential factors determining the microstructure of pyrolytic carbon. Je and Lee [7] studied the deposition rate of pyrolytic carbon in a tumbling bed. They found that the deposition rate increased with increasing number of gas-born nuclei, with increasing number of gas-born particles colliding with the substrate, and with increasing amount of carbon source in deposition zone. Different from fluidized bed CVD, the reaction tube in tumbling bed CVD was rotated by a variable motor and the substrate was moved and tumbled with the rotating of reaction tube, so the residence time was hard to determine and gas flow pattern was greatly influenced. Compared with fluidized bed and tumbling bed CVD, the substrate and reactor tube in our experiment are both stationary. The residence time, the

2 1134 K.Z. Li et al.: J. Mater. Sci. Technol., 2010, 26(12), Density / (g/cm 3 ) Position Fig. 1 Diagram of the deposition apparatus and reactor chamber: 1 gas inlet, 2 thermocouple, 3 water wall, 4 graphite electrode, 5 cover plate, 6 gas outlet, 7 reactor wall, 8 deposition substrate, 9 support pillar gas flow pattern, the intermediate species could be determined easily. As long as proper gas inlet pipe, gas distributor and reactor tube are schemed out, pyrolytic carbon could deposited steadily. In this paper, we investigated the deposition of pyrolytic carbon at 1250 C in hot wall reactor. 2. Experimental 2.1 Sample preparation The hot wall CVD reactor was made from a 180 mm long graphite tube with 100 mm outer diameter and 80 mm inner diameter. The diagram of the reactor is shown in Fig. 1. The reactor is heated by the Joule heat generated due to the resistance of graphite when low voltage, high current pass through it. The deposition substrate, a 2 mm thick graphite plate (60 mm 40 mm), was cleaned by water, acetone in ultrasonic cleaning machine and was placed on a support pillar fixed in the reactor. The deposition temperature was measured by a Pt/Rh thermocouple fastened at the external of the reactor. The deposition temperature was 1250 C. The natural gas acting as precursor gas was pumped in and outflow from the bottom and top of the reactor and its volume flow rate was 1.2 m 3 /h. 2.2 Sample characterization The density of the pyrocarbon was measured using the Archimedes methods in ethanol. And then, the SANS universal mechanical machine (CMT5304, Shenzhen, China) was employed to test the mechanical property of samples. The morphology and microstructure of pyrocarbon was observed by scanning electron microscopy (SEM, JOEL JSM-6460, Japan) and transmission electron microscopy (TEM, JEOL, JEM-3010, Japan). Morphology observ- Fig. 2 Density distribution along the axis of deposition substrate Stress / MPa Strain / % Fig. 3 Stress vs strain curve of the pyrocarbon ation was performed on fracture surface and TEM samples obtained from the transverse section vertical to the deposition direction was thinned by focused ion beam milling. Texture was examined quantitatively by measuring the orientation angle resulted from azimuthal intensity scans of selected area electron diffraction patterns (SAED) via the method proposed by Bourrat et al. [8]. Raman analysis was carried out by single spot (about 1 µm) measurements. 3. Results and Discussion 3.1 Density and mechanical property The density distribution of the product along the axis of the deposition substrate is illustrated in Fig. 2 and the resultant density is averaged from three samples at five different points. The average density is about 1.31 g/cm 3 and the variation among five different points is not more than 0.1 g/cm 3. This indicates that pyrocarbon deposits homogeneously onto the substrate. The size of the samples for flexural strength testing is 35 mm 8 mm 2 mm. The span is 20 mm and the decreasing rate of indenter is 0.5 mm/min. The flexural strength is averaged from three samples. One

3 K.Z. Li et al.: J. Mater. Sci. Technol., 2010, 26(12), Fig. 4 Fracture surface morphology of the pyrocarbon: (a) image of graphite substrate and pyrocarbon, (b) intermediate magnification image of the pyrocarbon, (c) high magnification image of pyrocarbon, (d) fracture surface image of the pyrocarbon after flexural strength test typical stress vs strain curve is shown in Fig. 3. The average flexural strength is MPa. The curve of stress dependence on strain is interesting for its four apparent increasing first and decreasing afterwards features. This is perhaps ascribed to the layered defects in the process of pyrocarbon deposition, which is observed in its fracture morphology. 3.2 SEM The fracture surface morphology of the pyrocarbon is shown in Fig. 4. The pyrocarbon deposits on graphite substrate homogeneously and transition layer between pyrocarbon and substrate is hardly observed (Fig. 4(a)). Figure 4(b) is the intermediate magnification image of pyrocarbon. It can be found that pyrocarbon is composed of granular particles and its dimension is uniform. The granular particles contacting each other with point to point induce plenty of pores, which is why the density of the pyrocarbon is so low (about 1.31 g/cm 3 ). The size of the granular pyrocarbon particles is about 1.5 µm. In Fig. 4(c), there are also some small granular particles except large ones and the round pits positioned at the large granular particles are the remains of pyrocarbon particles when sample is fractured. In Fig. 4(d), three different layers (marked by double white arrows) exist and their structures are distinguished from each other. The first layer consists of small granular particles and its aggregate is more like coral reef. The second layer is formed by pyrocarbon flake and the third layer consists of granular particles. The transition zone of the three layers is apparent and loose. This could explain the phenomenon of four increasing first and decreasing afterward features on stress-strain curve. 3.3 TEM The morphology and texture of pyrocarbon is also investigated by TEM on submicron- and nano-meter scale. The terminology assisting in describing pyrocarbon structure is the one proposed by Reznik and Hüttinger [9] on the basis of preferred orientation. According to the difference of orientation angle obtained from azimuthal intensity scans of SAED via the method proposed by Bourrat et al. [8], four kinds of pyrocarbon structures are classified as: high textured, medium textured, low textured and isotropic. Figure 5(a) is the low magnification TEM image of pyrocarbon which reveals some round components (marked by white arrows). Figure 5(b) is the intermediate image of pyrocarbon. It can be seen that pyrocarbon is formed by irregular components defined as grains. Beside these grains, both onion-like pyrocarbon and carbon black are observed in the meantime. The onion-like pyrocarbon with a concentric point orientation is composed of short, curved graphene, shown in Fig. 5(c). In Fig. 5(d), the fibrous materials orientating randomly and surrounding some particles form the grains. The SAED pattern of the pyrocarbon is inserted in Fig. 5(c) and a homogeneous intensity distribution of (002) ring indicates an isotropic feature.

4 1136 K.Z. Li et al.: J. Mater. Sci. Technol., 2010, 26(12), Fig. 5 (a) Low magnification TEM image of the pyrocarbon, (b) intermediate magnification TEM image of the pyrocarbon, (c) fringe lattice image of the onion like pyrocarbon, (d) fringe lattice of the pyrocarbon, where the SAED patter of the pyrocarbon is inserted Intensity / a.u Raman shift / cm -1 Fig. 6 Raman spectrum of the pyrocarbon 3.4 Raman spectroscopy Raman spectroscopy is a powerful technique to investigate carbon materials, because Raman signal is very sensitive to the disorders contained in materials. The Raman spectrum of the pyrocarbon is shown in Fig. 6. Except two first order Raman spectra at around 1347 cm 1 (D band) and 1586 cm 1 (G band), there are four second order Raman spectra at about 2490, 2692, 2937 and 3238 cm 1, respectively. The G band is assigned to the in-plane E 2g zone center vibration mode and the D band induced by disorder is recently ascribed to the double resonant Raman scattering [10 12]. The intensity ratio of D band and G band has been related empirically to the domain size La with the following equation [13,14] : La(nm) = 4.4/(I D /I G ) The La value is about 1.2 nm and the interlayer distance determined by X-ray diffraction (XRD) is about nm. Therefore, the pyrocarbon could be regarded as high disorder graphitic lattice. 3.5 Discussion Before our discussion, it is necessary to distinguish

5 K.Z. Li et al.: J. Mater. Sci. Technol., 2010, 26(12), both pyrocarbon formation and deposition mechanism. Pyrocarbon deposition mechanism is a process from small linear or aromatic hydrocarbon species to pyrocarbon, which does not directly correlate with the texture of pyrocarbon. Pyrocarbon formation mechanism is a process from precursor source to polyaromatic hydrocarbon. Formation mechanism determines the pyrocarbon textures [15 17]. The mechanisms used in this paper to describe the pyrocarbon deposition are nucleation and growth mechanism [17]. Short residence time and large As/V R ratio favor growth mechanism chemisorbing hydrocarbon species on active sites at the edge of the graphene layers, but long residence time and small As/V R ratio favor nucleation mechanism physically adsorbing large hydrocarbon species at the top of the graphene. Pyrocarbon formation mechanisms include both homogeneous gas-phase and heterogeneous surface reactions. In homogeneous gas phase reactions, the precursor gas undergoes complicated pathway to polyaromatic hydrocarbons. First, free radicals are produced by cleavage of C-H bond and they recombined to form nonsaturated species. Then, aliphatic species are formed via different cyclization process. At last, monoaromatic and polyaromatic species are formed with increasing residence time [18,19]. Heterogeneous reactions are the decisive step of pyrocarbon deposition and determine the deposition rate. At atmospheric pressure, the deposition was carried out at 1250 C. The residence time and As/V R ratio is 2.7 s and 0.05 cm 1, respectively. The rate of C-H bond scission and methyl radical formation was promoted at high temperature, so they were not the restrictive step any more in methane pyrolysis. After a series of radical reaction, C2 and C3 species were produced. For small As/V R ratio, homogeneous gas phase reactions were enhanced. C2 and C3 species were dominantly consumed to form five-membered and six-membered aromatic rings. Relative long residence time not only enables five-membered and sixmembered aromatic rings to mature and to form polyaromatic hydrocarbons but also increases the collision probability between the reactants and deposition substrate. According to the theory proposed by Hu and Hüttinger [17], high temperature, long residence time and small As/V R ratio favor nucleation mechanism. Aromatic and polyaromatic hydrocarbons, as the main pyrocarbon formation species, were the predominant productions of homogeneous gas phase reaction. The aromatic hydrocarbons would gradually chemisorb C2 and C3 species at their zig-zag active sites to increase the number of five-membered and six-membered rings further to form polyaromatic hydrocarbon. Meanwhile, polyaromatic hydrocarbons would attract each other and form oligomers, which played a role of nuclei around which aromatic hydrocarbons were attracted to form aggregates [20], which was collided with heat substrate and absorbed onto it. To minimize their energy, the aggregates preserved their non-planarity. Due to the dehydrocyclizaion and intramolecular forces, the conformation of aggregates was changed and many parallel aromatic units were formed. Thus the aggregates were mostly transferred to isotropic pyrocarbon. In some aggregates, because of the curvatures induced by the five-membered rings in hexagonal graphene layers [21], the orientation of aromatic hydrocarbon stack was changed and onionlike particles were formed. In addition, the residual gases flow out from the holes drilled at the up wall, but some gaseous species with heavy molecular mass back mix because of the hinder of the cover plate. This causes the balance between the aromatic and C2 or C3 to shift the aromatic sides. The larger aromatic species nucleate and solidify in gas phase to form soot particles (Fig. 5(a) and (b)). This is another reason responsible for the low density of the pyrocarbon. The back mixing gas also affects the homogeneous deposition and leads to the layered pyrocarbon shown in SEM image (Fig. 4(d)) and on the stress-strain curve (Fig. 3). 4. Conclusion Isotropic pyrocarbon was obtained via CVD with the hot wall reactor. The density of the pyrocarbon is about 1.31 g/cm 3 measured by Archimedes method. The Raman spectrum (I D /I G =1.2) indicates the isotropic feature. The pyrocarbon consists of granular particles with the dimension about 1.5 µm observed by SEM. The TEM images show that the pyrocarbon is formed by irregular domains. The interior wall of the domain is resulted from curved hexagonal graphene layers and the exterior wall is formed by linear hydrocarbon species with good orientation. The orientation angle (nearly 180 deg.) obtained from the SAED pattern reveals the isotropic characteristic. Acknowledgements This research was financially supported by the National Natural Science Foundation of China under Grant Nos and , and the Program of Introducing Talents of Discipline to University under Grant No. D REFERENCES [1 ] L. Ma and G. Sines: Carbon, 2002, 40, 451. [2 ] E.L. Honorato, P.J. Meadows, P. Xiao, G. Marsh and T.J. Abram: Nucl. Eng. Des., 2008, 238, [3 ] M.H. Kim and J.Y. Lee: J. Mater. Sci., 1987, 22, [4 ] R.J. Bard, H.R. Baxman and J.P. Bertino: Carbon, 1968, 6, 603. [5 ] E.L. Honorato, P.J. Meadows and P. Xiao: Carbon, 2009, 47, 396. [6 ] P.J. Meadows, E.L. Honorato and P. Xiao: Carbon, 2009, 47, 251.

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