CH4 Flow Rate Research Articles

Raman imaging on high‐quality graphene grown by hot‐filament chemical vapor deposition

We report the synthesis of high‐quality graphene on Cu foils using hot‐filament chemical vapor deposition technique and demonstrate that by suitably varying the CH4 and H2 flow rates, one can also obtain hydrogenated graphene. Micro‐Raman spectroscopy studies confirm the growth of monolayer graphene as inferred from the intensity ratio of 2D to G peak which is nearly four in unhydrogenated samples. Detailed Raman area mapping confirms the uniform coverage of monolayer graphene. The grown layer is also transferred onto a Si substrate over ~10 × 10 mm sq. area. The present results provide a leap in synthesis technology of high‐quality graphene and pave way for scaling up the process. Copyright © 2012 John Wiley & Sons, Ltd.

Process Optimization for Synthesis of High-Quality Graphene Films by Low-Pressure Chemical Vapor Deposition

Low-pressure chemical vapor deposition (LPCVD) is a simple and useful method for the large-area synthesis of graphene films. Here, we have investigated how to adjust and optimize process conditions for the synthesis of single-layer graphene films by LPCVD. Through our experimental procedure, uniform and high-quality graphene films could be grown on Cu foil at 1000 °C for 20 min with an H2 flow rate of 20 sccm, CH4 flow rate of 40 sccm, total pressure of 1.7 Torr, and a fast cooling rate (>10 °C/s). In a Raman spectrum measured from synthesized graphene film, we found that the full width at half-maximum (FWHM) of a symmetric 2D peak centered at 2682.5 cm-1 was 34 cm-1 and the 2D-to-G intensity ratio was 1.35.

Growth of Horizontally-Aligned Single-Walled Carbon Nanotubes on Sapphire Surface by Needle-Scratching Method

We report the growth of single-walled carbon nanotubes (SWNTs) over metal nanoparticles which are formed by scratching sapphire surface with metal wires. The chemical vapor deposition over sapphire substrate scratched with Fe and Co metal wires gives horizontally aligned SWNTs, while no nanotube growth is observed for Au, Mo, and Ni wires. This result suggests that the nanoparticles scattered from Fe and Co wires act as the catalyst for SWNT growth, being different from the previously proposed substrate-catalyzed reaction mechanism. Further, we study the effects of the flow rates of CH4–H2 gases during the SWNT growth on the nanotube density and diameter.

Growth of Horizontally-Aligned Single-Walled Carbon Nanotubes on Sapphire Surface by Needle-Scratching Method

We report the growth of single-walled carbon nanotubes (SWNTs) over metal nanoparticles which are formed by scratching sapphire surface with metal wires. The chemical vapor deposition over sapphire substrate scratched with Fe and Co metal wires gives horizontally aligned SWNTs, while no nanotube growth is observed for Au, Mo, and Ni wires. This result suggests that the nanoparticles scattered from Fe and Co wires act as the catalyst for SWNT growth, being different from the previously proposed substrate-catalyzed reaction mechanism. Further, we study the effects of the flow rates of CH4–H2 gases during the SWNT growth on the nanotube density and diameter.

Effect of N2 flow rate on the properties of CNx thin films prepared by radio frequency plasma enhanced chemical vapor deposition from ethane and nitrogen

Carbon nitride (CNx) thin films were deposited using radio frequency plasma enhanced chemical vapor deposition (rf PECVD) from a mixture of nitrogen (N2) gas and either methane (CH4) or ethane (C2H6) gases. The CH4 and C2H6 flow rates were kept constant, while the N2 flow rate was varied. The effects of nitrogen incorporation on the growth rate and structural properties of the films were studied. The use of these two hydrocarbon precursors was also compared. It was found that the effects of N incorporation are significant for films deposited from the CH4 mixture and it greatly affects the bonding and optical properties of the films. In contrast, the effects of N incorporation on the films produced from C2H6 are not as significant, though these films appear to be more uniform and show lower film porosity. Generally, the photoluminescence (PL) intensities increase with the increase in N incorporation for film deposited from both hydrocarbon mixtures. However, the PL properties of these CNx films are enhanced by the use of C2H6 as compared to CH4 since the films produced show lower defects.

A rapid hard template method for the synthesis of N-doped mesoporous carbon replicated from TUD-1

A porous silica material, TUD-1, was used as the hard template to synthesize nitrogen-doped mesoporous carbon (NMC) using a modified hard template method with fewer synthesis steps, compared to the conventional method. This method starts with the formation of a triethanolamine (TEA)/TUD-1 composite, followed by direct carbonization in CH4, where TEA and CH4 are used as nitrogen (N) and carbon precursors, respectively. The direct use of the templating agent TEA as the N precursor skips the calcination step and the infiltration of another N precursor, simplifying the conventional hard template method. The obtained NMC material possesses a similar three dimensional sponge-like mesostructure to its parental TUD-1. Several characterization techniques were used to characterize textural structure, carbon yield, N content, graphitization and surface chemical composition of the resulting NMC materials. These properties can be controlled by manipulating synthesis parameters, such as carbonization temperatures, CH4 flow rate, CH4 partial pressure, heating ramp rate and the structural characteristic of the TUD-1 template. In addition, NMC with high N content (9.6at.%) was also synthesized using acetonitrile as an alternative N-containing precursor.

Nanocrystalline diamond coating on carbon fibers produced at different temperatures: Morphological, structural and electrochemical study

In this paper, the morphological, structural and electrochemical properties of nanocrystalline diamond (NCD) films grown on carbon fibers (CF) were investigated. The CF substrates were produced at three different heat treatment temperatures (HTT):1000, 1500 and 2000°C. The HTT variation promoted different organization indexes on the CF structures. Consequently, the NCD coating formation was strongly affected by the substrate HTT. The changes in the properties of the diamond films were discussed as a function of the film morphology evolution using CH4 flow rate of 0.25, 0.5 and 1.0sccm in the feed gas. The X-ray diffraction measurements for the CF and NCD/CF composites were determinant to characterize the crystallinity of the NCD films as a function of the CF HTT and of the CH4 addition. Based on the diffractograms, the Scherrer's equation was applied to the (111) NCD peak, resulting in grain size values varying from 11.0 to 5.0nm depending on the CH4 flow rate and on the CF HTT. The scanning electron microscopy images confirmed the deposition of a continuous NCD coating with high nucleation rate covering the whole CF, while their quality was analyzed by Raman spectroscopy measurements. The NCD grain agglomerates increased as a function of the increase in the CH4 flow rate from 0.25 to 1.0sccm, showing similar film morphology to that of the unfaceted diamond balls obtained by chemical vapor deposition. This behavior confirmed the expected tendency by decreasing the diamond quality with the CH4 addition, especially for the films grown on CF treated at 1500 and 2000°C. This performance was also corroborated by the cyclic voltammetry measurements concerning the electrode potential window and their responses in a redox couple.

Experimental evaluation of strategies to increase the operating range of a biogas-fueled HCCI engine for power generation

In this research oxygen enrichment, gasoline pilot port injection, and delayed time of 50% cumulative heat release (CA50) are evaluated to expand the range for stable and safe combustion of a lean-burning biogas-fueled HCCI engine. A 4-cylinder 1.9L Volkswagen TDI engine was modified to run in HCCI mode at 1800rpm, and boost pressures and charge heating are used to promote autoignition of the biogas-in-air mixture at desired combustion timings. A typical biogas composition of 60% CH4 and 40% CO2 in a volumetric basis was simulated by controlling the CH4 and CO2 flow rates. A range of 2–2.2bar absolute intake pressure and 473–483K initial charge temperatures allowed HCCI operation. At lowest equivalence ratio (0.25) excessive cycle-to-cycle variations were observed and at highest equivalence ratio (0.4) unacceptable ringing intensities were observed. To reduce cycle-to-cycle variability at low equivalence ratios, two strategies were used in enhancing the autoignition behavior of biogas: (1) oxygen enrichment of the inducted charge, and (2) gasoline pilot port injection. To increase gross Indicated Mean Effective Pressure (IMEPg) without excessive ringing intensities at high equivalence ratios, delayed CA50 was used. Oxygen enrichment increased cycle-to-cycle variability and total hydrocarbon emissions because of decreased burning rates and delayed CA50. Gasoline pilot port injection lowered cycle-to-cycle variability, CO and THC emissions, and increased IMEPg at low loads. Higher IMEPg was achieved with high equivalence ratios (above 0.4) and ringing intensities were kept within acceptable limits using delayed CA50, however NOx emission was increased also.HCCI combustion of biogas enables high overall efficiency, and the strategies explored in this research allow higher power output, and more stabilized combustion.

Integrated coal pyrolysis with methane aromatization over Mo/HZSM-5 for improving tar yield

In this paper, a new process to integrate methane aromatization over Mo/HZSM-5 catalyst with coal pyrolysis was put forward for improving tar yield. Shenmu coal was used to confirm the validity of the process. The effects of pyrolysis temperature, CH4 flow rate, Mo loading on the tar, water and char yields were investigated in an atmospheric fixed-bed reactor containing upper catalyst layer and lower coal layer. The results show that the tar yield can be improved by integrating the coal pyrolysis with methane aromatization. The tar yield is 21.5wt.% (daf) under the optimum conditions of 700°C pyrolysis temperature, 25ml/min CH4 flow rate, 4wt.% Mo loading and 30min holding time, obviously higher than 14.6% in N2 atmosphere and 15.3% in H2 atmosphere. Further studies confirm the synergistic effect between the coal pyrolysis and methane aromatization.

Design and Synthesis of DLC Films with Different sp<sup>3</sup>/sp<sup>2</sup> Ratios by Middle Frequency Magnetron Sputtering

Diamond-like carbon (DLC) films with different sp3/sp2ratios were deposited using middle frequency magnetron sputtering. The sp3/sp2ratio of the films can be tailored by codoping of Ti and Si elements under different CH4flow rate. The films were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The results show that the sp3/sp2ratio ranges from 0.18 to 0.63 with increasing of CH4flow rate. Hydrogen atoms in the films are mainly bonded to sp3carbon atoms. The film deposited at 60 sccm CH4flow rate exhibits the highest bonded hydrogen content.

Microstructure and properties of nc-TiC/a-C∶H films deposited by radio frequency reactive sputtering

Amorphous carbon films containing titanium carbide (nc-TiC/a-C∶H) were deposited onto n-type silicon (100) by radio frequency reactive sputtering titanium target in an Ar–CH4 mixed atmosphere. The composition and microstructure of the films were characterised by means of X-ray photoelectron spectroscopy, field emitted SEM, XRD and Raman spectra. The mechanical and tribological properties of the films were measured by a nanoindentation tester and a ball-on-disc UMT–2MT tribometer. By adjusting the CH4 flowrate, Ti content in the films could be controlled, and a transition in structures of the films from loose polymer-like to glassy and dense nanostructure was observed. The density of coatings was improved by the introduction of TiC nanocrystalline particles. The mechanical and lubricious properties were different accordingly.

Microstructure and mechanical properties of reactive magnetron sputtered Ti–B–C–N nanocomposite coatings

Ti–B–C–N nanocomposite coatings with different C contents were deposited on Si (100) and high speed steel (W18Cr4V) substrates by closed-field unbalanced reactive magnetron sputtering in the mixture of argon, nitrogen and acetylene gases. These films were subsequently characterized ex situ in terms of their microstructures by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), their nanohardness/elastic modulus and facture toughness by nano-indention and Vickers indentation methods, and their surface morphology using atomic force microscopy (AFM). The results indicated that, in the studied composition range, the deposited Ti–B–C–N coatings exhibit nanocomposite based on TiN nanocrystallites. When the C2H2 flow rate is small, incorporation of small amount of C promoted crystallization of Ti–B–C–N nanocomposite coatings, which resulted in increase of nano-grain size and mechanical properties of coatings. A maximum grain size of about 8nm was found at a C2H2 flux rate of 1sccm. However, the hardness, elastic modulus and fracture toughness values were not consistent with the grain size. They got to their maximum of 35.7GPa, 363.1GPa and 2.46MPam1/2, respectively, at a C2H2 flow rate of 2sccm (corresponding to about 6nm in nano-grain size). Further increase of C content dramatically decreased not only grain size but also the mechanical properties of coatings. The presently deposited Ti–B–C–N coatings had a smooth surface. The roughness value was consistent with that of grain size.

Inductively coupled plasma etching of graded-refractive-index layers of TiO2 and SiO2 using an ITO hard mask

Transparent dielectric layers with varying compositions of TiO2 and SiO2, and ITO are deposited on sapphire and Si substrates by using an RF sputter system. Inductively coupled plasma (ICP) reactive ion etching (RIE) of the ITO hard mask is examined under H2, CH4, and Cl2 chemical environments. The slope of the sidewall and the etch residue on the sidewall of the ITO hard mask are controlled by the flow rates of H2, CH4, and Cl2. ICP-RIE dry etch of TiO2 and SiO2 is investigated under fluorinated environments. Comparable etch rates of TiO2 and SiO2 (ratio ≈ 2:1) and high selectivity ≫ 1 over ITO are found. Graded-refractive-index (GRIN) layers, made up of multiple dielectric layers of TiO2 and SiO2, are patterned to form cylindrical pillars by ICP etching using the ITO hard mask. Fluorine containing residues are identified on the TiO2 and SiO2 surfaces. Various etch chemistries are investigated to obtain smooth, vertical, and residue-free sidewalls of the GRIN pillars.

Methane coupling in microwave plasma under atmospheric pressure

Methane coupling in microwave plasma under atmospheric pressure has been investigated. The effects of molar ratio n(CH 4)/ n(H 2), flow rate and microwave power on the reaction have been studied. (1) With the decrease of n(CH 4)/ n(H 2) ratio, methane conversion, C 2 hydrocarbon yield, energy yield and space-time yield of acetylene increased, but the yield of carbon deposit decreased. (2) With the increase of microwave power, energy yield of acetylene decreased, but space-time yield of acetylene increased. (3) With the increase of flow rate, energy yield and space-time yield of acetylene increased first and then decreased. Finally, under the reaction conditions of CH4 flow rate of 700 mL/min, n(CH 4)/ n(H 2) ratio of 1/4 and microwave power of 400 W, the energy yield and space-time yield of acetylene could reach 0.337 mmol/kJ and 12.3 mol/(s·m 3), respectively. The reaction mechanism of methane coupling in microwave plasma has been investigated based on the thermodynamics of chemical reaction. Interestingly, the acetylene yield of methane coupling in microwave plasma was much higher than the maximum thermodynamic yield of acetylene. This phenomenon was tentatively explained from non-expansion work in the microwave plasma system.

Hydrogenated Amorphous Carbon Films Prepared by Filtered Vacuum Arc Method with Various C2H2 Pressures

Hydrogenated amorphous carbon films (a-C:H) were deposited on silicon (100) substrates using a filtered vacuum arc (FVA) method. A graphite cathode and acetylene (C2H2) at various flow rates were used to synthesize the carbon films. The deposition rate of the carbon films without C2H2 addition was 5 nm/min, whereas the deposition rate increased from 23 to 82 nm/min with increasing C2H2 flow rate from 2.5 to 20 sccm. The supply of C2H2 gas induces an increase in CH radical density near the substrate, resulting in a high deposition rate. The plasma diagnostics using optical emission spectroscopy showed that the emission peak intensity of the CH radicals (A 3Πg–X 3Πu, 431.26 nm) increased with increasing C2H2 flow rate. Raman spectroscopy revealed a change in the deposited films from nano-crystalline graphite to a-C:H as the C2H2 flow rate was increased.

Production of carbon molecular sieves from palm shell through carbon deposition from methane

The possibility of production of carbon molecular sieve (CMS) from palm shell as a waste lignocellulosic biomass was investigated. CMS samples were prepared through heat treatment processes including carbonization, physiochemical activation and chemical vapor deposition (CVD) from methane. Methane was pyrolyzed to deposit fine carbon on the pore mouth of palm shell-based activated carbon to yield CMS. All the deposition experiments were performed at 800 ?C, while the methane flow rate (100, 200, 300 mL min-1 CH4 diluted in 500 mL min-1 N2) and deposition time (30 to 60 min) were the investigated parameters. The textural characteristics of the CMSs were assessed by N2 adsorption. The largest BET surface area (752 m2 g-1), micropore surface area (902.2 m2 g-1) and micropore volume (0.3466 cm3 g-1) was obtained at the CH4 flow rate of 200 mL min-1 and deposition time of 30 min. However, prolonging the deposition time to 45 min yielded in a micropouros CMS with a narrow pore size distribution.

Structural and optical properties of the SiCN thin films prepared by reactive magnetron sputtering

Silicon carbonitride (SiCN) thin films were deposited on n-type Si (100) and glass substrates by reactive magnetron sputtering of a polycrystalline silicon target in a mixture of argon (Ar), nitrogen (N2) and acetylene (C2H2). The properties of the films were characterized by scanning electron microscope with an energy dispersive spectrometer, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectrometry and ultraviolet-visible spectrophotometer. The results show that the C2H2 flow rate plays an important role in the composition, structural and optical properties of the films. The films have an even surface and an amorphous structure. With the increase of C2H2 flow rate, the C content gradually increases while Si and N contents have a tendency to decrease in the SiCN films, and the optical band gap of the films monotonically decreases. The main bonds are Si–O, N–Hn, C–C, C–N, Si–N, Si–C and Si–H in the SiCN films while the chemical bonding network of Si–O, C–C, C–O, C–N, N–Si and CN is formed in the surface of the SiCN films.

Different growth mechanisms of vertical carbon nanotubes by rf- or dc-plasma enhanced chemical vapor deposition at low temperature

Vertically aligned carbon nanotubes (CNTs) were synthesized using FeNi or Fe sputtered catalyst layers on glass substrates by radio frequency or direct current plasma enhanced chemical vapor deposition (rf- or dc-PECVD). This article compared the growth mechanisms of CNTs synthesized by rf- and dc-PECVD, based on gas flow rate, plasma power, and catalysts. Tip growth CNTs were produced at 180 °C, 10 SCCM (SCCM denotes cubic centimeter per minute at STP) CH4, and 30 W by rf-PECVD using 8 or 4 nm FeNi or 4 nm Fe island films sputtered onto glass substrates. CNTs could not grow using dc-PECVD under the same deposition conditions; tip growth of CNTs occurred at 180 °C, 15 SCCM CH4, and 50 W using dc-PECVD with sputtered FeNi island catalysts on increasing the plasma power and CH4 flow rate. This article explained why rf-PECVD provided more efficient decomposition of gas molecules than dc-PECVD by plasma theory. The major difference between rf- and dc-PECVD was the higher concentration of reactive radicals in the former. However, in dc-PECVD, the CNT growth was well aligned vertically. FeNi thin film catalysts exhibited higher activity and better wetting ability than the Fe island thin film catalysts.

Effect of process parameters on mechanical and tribological performance of pulsed-DC sputtered TiC/a-C:H nanocomposite films

Mechanical, structural, chemical bonding (sp3/sp2), and tribological properties of films deposited by pulsed-DC sputtering of Ti targets in Ar/C2H2 plasma were studied as a function of the substrate bias voltage, Ti-target current, C2H2 flow rate and pulse frequency by nanoindentation, Raman spectroscopy and ball-on-disc tribometry. The new findings in this study comprise: dense, column-free, smooth, and ultra-low friction TiC/a-C:H films are obtained at a lower substrate bias voltage by pulsed-DC sputtering at 200 and 350kHz frequency. The change in chemical and phase composition influences the tribological performance where the TiC/a-C:H films perform better than the pure a-C:H films. In the case of TiC/a-C:H nanocomposite films, a higher sp2 content and the presence of TiC nanocrystallites at the sliding surface promote formation of a transfer layer and yield lower friction. In the case of a-C:H films, a higher sp3 content and higher stress promote formation of hard wear debris during sliding, which cause abrasive wear of the ball counterpart and yield higher friction.

Chemical Structure and Growth Mechanism of a-Si<sub>x</sub>C<sub>1-x</sub>:H Films Prepared by Plasma Enhanced Magnetron Sputtering

Hydrogenated amorphous silicon carbide (a-Si1-xCx:H) films were prepared by microwave electron cyclotron resonance (MW-ECR) plasma enhanced unbalance magnetron sputtering with a silicon target and CH4 as Si and C sources, respectively. The influence of CH4 flow rate and the deposition temperature on the chemical structure, stoichiometry, and hardness were investigated by Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and nano-indentation. The results indicated that, as the CH4 flow rate increased from 5 to 45 cm3·min-1 (standard state), the amount of Si—CH2 groups and C—H groups increased constantly, but the number of Si—H groups did not change. The atomic concentration of C increases from 28% to 76% while Si decreases from 62% to 19%. The amount of Si—H and C—H groups in the deposited films decreases dramatically while the Si—C bonds and the hardness of the resultant films increase with an increase in deposition temperature at a constant CH4 flow rate. The atomic concentrations of Si and C remain almost constant at about 52% and 43%, respectively. The hardness of the deposited films with a constant CH4 flow rate of 15 cm3·min-1 increases to 29.7 GPa at a deposition temperature of 600 ℃. We propose a growth mechanism for the a-Si1-xCx:H films at room temperature (25 ℃) and at high temperature based on the characterization results.