CH4 Gas Mixture Research Articles

Growth of Vertically Aligned Carbon Nanotube Bundles using Atmospheric-Pressure Microplasma

Vertically aligned bundles of multiwalled carbon nanotubes (MWNTs) were grown on the tip of a φ0.5 mm nickel wire using an atmospheric-pressure microplasma (APµP) generated by a RF-powered pipe electrode. A CH4 and H2 gas mixture was used for the source gas, and the nickel wire used as the substrate was negatively biased to heat it by ion bombardment. At a ratio of CH4:H2=1:35, a RF power of 6 W, a DC bias voltage of -100 V and a growth time of 2 min, the tip of the nickel wire was covered by bamboo-like MWNT bundles, whose diameter and length were about 600 nm and 2–3 µm, respectively. Each bundle consisted of about 1500 tubes with diameters of 10–25 nm. The growth temperature of the MWNT bundles was approximately 500 °C.

Effect of N 2/CH 4 flow ratio on microstructure and composition of hydrogenated carbon nitride films prepared by a dual DC-RF plasma system

Hydrogenated carbon nitride (a-CN:H) films were deposited on n-type (1 0 0) silicon substrates making use of direct current radio frequency plasma enhanced chemical vapor deposition (DC-RF-PECVD), using a gas mixture of CH 4 and N 2 as the source gas in range of N 2/CH 4 flow ratio from 1/3 to 3/1 (sccm). The deposition rate, composition and bonding structure of the a-CN:H films were characterized by means of X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectrometry (FTIR). The mechanical properties of the deposited films were evaluated using nano-indentation test. It was found that the parameter for the DC-RF-PECVD process had significant effects on the growth rate, structure and properties of the deposited films. The deposition rate of the films decreased clearly, while the N/C ratio in the films increased with increasing N 2/CH 4 flow ratio. CN radicals were remarkably formed in the deposited films at different N 2/CH 4 flow ratio, and their contents are related to the nitrogen concentrations in the deposited films. Moreover, the hardness and Young’s modulus of the a-CN:H films sharply increased at first with increasing N 2/CH 4 flow ratio, then dramatically decreased with further increase of the N 2/CH 4 flow ratio, and the a-CN:H film deposited at 1/1 had the maximum hardness and Young’s modulus. In addition, the structural transformation from sp 3-like to sp 2-like carbon–nitrogen network in the deposited films also was revealed.

Kinetic investigation of chemical vapor deposition of B4C on tungsten substrate

AbstractProduction of β‐rhombohedral boron carbide (B4C) on a tungsten substrate by the chemical vapor deposition from a BCl3–H2–CH4 gas mixture was achieved. An impinging‐jet reactor was used to minimize the mass‐transfer limitations on the reaction kinetics, which made a detailed kinetic investigation possible. Results of the XRD and XPS analyses showed that the solid product formed on the substrate is a rhombohedral B4C phase. Both dichloroborane and boron carbide formation rates were found to increase with an increase in the inlet molar fraction of BCl3. The formation rate of boron carbide increased with an increase in the inlet molar fraction of CH4. However, no effect on the formation rate of dichloroborane was observed with an increase in the inlet molar fraction of methane. The activation energy of the boron carbide formation reaction was ascertained to be 56.1 ± 4.0 kJ/mol. The boron carbide formation reaction was proportional with the 0.34 ± 0.055 power of the initial boron trichloride concentration and 0.64 ± 0.084 power of the initial methane concentration. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Residual stress minimum in nanocrystalline diamond films

Nanocrystalline diamond films have been deposited on silicon nitride substrates by hot filament chemical vapor deposition. Gas mixtures of CH4–H2–Ar were used with variation of the Ar∕H2 ratio in order to study the influence of the Ar content on the formation of nondiamond phases at the grain boundaries and thus in the film residual stress assessed by x-ray diffraction techniques. By varying this ratio it is possible to optimize conditions, decreasing the film’s residual stress to a minimum of 0.09GPa.

High yield of aromatics from CH4 in a plasma‐followed‐by‐catalyst (PFC) reactor

The oxygen-free dehydroaromatization of methane is of great scientific importance and industrial benefit for the effective utilization of natural gas and biogas to provide alternative petrochemical feed stocks and COx-free hydrogen production for fuel cells.1 Methane oxygen-free conversion into aromatics and hydrogen is not thermodynamically favorable at low temperatures ( 773 K); the equilibrium CH4 conversion at 973 K is 12%. At this temperature, benzene and naphthalene, in approximately equimolar amounts, are the thermodynamically preferred hydrocarbon products.2 The thermodynamic limitation is the main barrier for this process. To enhance the conversion of methane into aromatics beyond thermodynamic equilibrium, studies using H2-permeation membrane catalytic reactors are already underway by several research groups.3,4 However, preliminary reports on this approach are not very optimistic, and catalyst deactivation was more pronounced in a membrane catalytic reactor than in a fixed-bed reactor because of continuous withdrawal of the coproduced H2. Atmospheric nonthermal equilibrium plasma has been used as an efficient method for oxygen-free conversion of methane into C2 hydrocarbons (mainly acetylene) and H2 using pulsed corona/streamer discharges, or pulsed spark discharges.5-10 Only small amounts of C3-C5 hydrocarbons and trace amounts of aromatics were usually coproduced. Recently, Heintze and Magureanu reported the conversion of CH4 into aromatics in pulsed microwave plasma at atmospheric pressure in the presence of a heterogeneous catalyst.11 In their work, the authors suggested that methane aromatization is very likely catalyzed on coke surfaces and highly unlikely by the trimerization of acetylene. The most abundant aromatic formed was benzene, and 5% of the maximum benzene yield was obtained. Herein, we show a two-stage plasma-followed-bycatalyst (PFC) reactor for the oxygen-free conversion of methane into aromatic-rich hydrocarbons and coproduced H2. Methane is converted to acetylene by plasma in the first stage, with the second stage for acetylene trimerization on nickel-loaded HZSM-5 catalyst. Oxygen-free conversion of methane was performed at atmospheric pressure in a two-stage PFC reactor, as shown in Figure 1. The first stage of the PFC reactor, for generating pulsed spark discharge, was located in the upper part of a quartz tube (i.d. 10 mm) and consisted of a stainless steel wire (2 mm diameter) as the high-voltage electrode and a perforated iron plate (9 mm diameter) as the ground electrode. The distance between the two electrodes was 10 mm. The high-voltage power supply source for generating pulsed spark discharge was described before.10 The second stage of the PFC reactor was filled with 0.5 g of HZSM-5 or Ni/HZSM-5 pellets of 20-40 mesh. The Ni/HZSM-5 catalysts were prepared by the conventional impregnation method using HZSM-5 (SiO2/Al2O3 25, Nankai University, China) and aqueous solution of nickel nitrate. The impregnated samples were dried at 393 K for 4 h, calcined at 723 K for 4 h in air, and reduced at 673K for 2 h in hydrogen. The flow rates for CH4, H2, and N2 were controlled through a mass flow controller. The feed gas mixture of CH4 (99.99%) and H2 (99.99%) with a molar ratio of CH4/H2 1 was introduced into the PFC reactor at a flow rate of 9.2 cm/min. 2.2 cm/min of N2 (99.99%) was used as an internal standard for analysis and introduced into the effluent gas from the PFC reactor to avoid conversion of N2 in the plasma. Hydrocarbon products were sampled by a six-way valve heated to 473 K and analyzed on-line by a gas chromatograph (Agilent 1790F) with a flame ionization detector (FID) and using a 2 mm (i.d.) 3m Porapak-P column with N2 as the carrier gas. N2 and CH4 were analyzed on-line by another gas chromatograph (Agilent 1790T) with a thermoconductive deY. Xu and A.-M. Zhu are also affiliated with State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Dalian 116024, China. Correspondence concerning this article should be addressed to A.-M. Zhu at amzhu@dlut.edu.cn.

Absolute electron-impact ionization cross sections of free radicals from threshold to 30eV: CHx=2,3, Si(CH3)x=1,2,3, and H2Si(CH3) radicals

We report absolute electron-impact ionization cross sections from threshold to 30eV for the formation of the parent ions from free radicals. A microwave discharge is used as a source to produce a large amount of these free radicals, and measurements are mainly performed by means of mass spectrometry. We detail the method used to measure electron-impact ionization cross sections of the main radicals produced. First, this method is tested on well known radicals such as CH3 and CH2, produced in a discharge sustained in an Ar–CH4 gas mixture; then it is applied to the free radicals Si(CH3)3,2,1 and H2Si(CH3), which are produced in a discharge sustained in Ar–Si(CH3)4. The experimental results are compared with the calculations performed using the binary-encounter-Bethe model and using a simplified form of this theory, the total ionization cross section model. Absolute electron-impact ionization cross sections have been measured at 30eV for Si(CH3) and CH3 by means of the selective energy transfer reaction process between Ar(P23) metastable species and the tetramethylsilane molecule [Si(CH3)4]. The value measured for Si(CH3) is used as the reference to calculate absolute values from relative ionization cross sections measured for the other radicals, Si(CH3)3,2,1 and H2Si(CH3), from threshold to 30eV. For example, we find at 30eV the absolute cross section values, 13.9×10−20±1.8×10−20, 2.0×10−20±0.3×10−20, 1.5×10−20±0.1×10−20, and 12.8×10−20±1.7×10−20m2 for m∕z=73, 58, 43, and 45, respectively.

Inductively coupled plasma reactive ion etching of ZnO using C2F6 and NF3-based gas mixtures

Inductively coupled plasma reactive ion etching (ICP RIE) of ZnO film using C2F6 and NF3-based gas mixtures was investigated as a function of ICP power, bias power, pressure, and plasma chemistry. An etch rate of about 410 nm min−1 in the case of C2F6 plasma and about 380 nm min−1 in the case of NF3 plasma was obtained at the optimum condition, with vertical sidewalls and smooth surfaces, which is about 30% higher than the etch rates obtained in the previous reports on ICP RIE of ZnO using CH4 or chlorine-related gas mixtures. Basically no chamber contamination and corrosion was observed, indicating that fluorine plasmas are more suitable for ZnO dry etching than previously studied CH4 or BCl3 plasma. The addition of 40% or more CH4 to the C2F6 plasma improved the etch rate and sidewall verticality of the etched mesas, but resulted in rough surface morphology.

Low-Temperature Growth of Carbon Nanofiber by Thermal Chemical Vapor Deposition Using CuNi Catalyst

Thermal-CVD was carried out for the low-temperature growth of carbon nanofibers (CNFs) using a CuNi alloy catalyst film with a thickness of 5 nm on Si in a gas mixture of C2H2 and He (C2H2/He=3/12 sccm). The experimental results obtained using the CuNi alloy catalyst film were compared with those obtained using the Fe, Ni, and FeNi catalyst films with the same thickness of 5 nm. It was shown that an amorphous CNF with a diameter of 20 nm can be grown even at 400 °C using the CuNi catalyst film, but not using the Fe, Ni and FeNi catalysts. A reduction in the growth temperature of CNFs was considered to be achieved using small CuNi catalyst particles with a comparatively smaller surface energy than FeNi catalyst particles.

Hydrogenated amorphous silicon–carbon alloys obtained from Ar–SiH4–CH4 gas mixtures: Structural and transport properties

We study some transport and structural properties of a-SiC:H films deposited by glow discharge with silane/methane/argon mixtures in the so-called low power regime. We observe a degradation of the transport properties with increasing rf power density. This behavior is explained both by a disorder increase and by the incorporation of graphitic-like clusters. The photoluminescence efficiency at room temperature agrees with this conclusion. Such clusters are created by surface bombardment of Ar∗ or Ar+ species which, in a first stage, allow to increase the deposition rate.

Electrical properties of nitrogen incorporated nanocrystalline diamond films

A series of nitrogen incorporated nanocrystalline diamond (ND) films were synthesized by microwave plasma-enhanced chemical vapor deposition (MP CVD) in a gas mixture of CH 4/H 2/N 2 with varying nitrogen concentration in a wide range (0–75%). The phase composition, structure, and morphology of these ND films were systematically studied by using UV and visible Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). It was found that the sp 2/sp 3 ratio of carbon bonds increased, and the grain size decreased with the increase of nitrogen concentration in plasma. A peak located at ∼ 1190 cm − 1 was observed in visible Raman spectra. Based on the correlation between its intensity and the nitrogen concentration in plasma, the origin of this peak was discussed. The current–voltage ( I– V) measurements showed a dramatic decrease of the resistivity of nanocrystalline diamonds due to the nitrogen incorporation, and the dependence of the resistivity on the nitrogen concentration was revealed.

A novel approach to mass synthesis of raw CNTs for printed field emission cathodes by chemical vapour deposition

Using nickel foil as both substrate and catalyst, we report a simple, cheap and easily controllable thermal chemical vapor deposition (CVD) for mass synthesis of carbon nanotubes (CNTs). Grams of CNTs with high purity and good uniformity have been obtained on the nickel foil at 550 °C in a gas mixture of C2H2 and H2. The as-prepared CNTs were screen-printed on Ag electrodes patterned on glass to form field emission cathodes. The cathodes exhibit excellent field emission properties, with a turn-on field of 1.0 V/μm, the emission current density of 1 mA/cm2 at 2.5 V/μm, and uniform emission luminescence. This CVD method shows great promise in large-scale production of raw materials of CNTs for application of field emission display.

Synthesis and field emission property of SiCN cone arrays

Silicon carbon nitride (SiCN) cone arrays were synthesized on Si wafers using a microwave plasma chemical vapor deposition reactor with gas mixtures of CH 4, SiH 4, Ar, H 2 and N 2 as precursors. The SiCN cones have nanometer-sized tips and their roots vary from nanometers to micrometers in sizes. A lowest turn-on field of 0.6 V μm −1 as well as field emission current densities of 4.7 mA cm −2 at an applied field of 2.8 V μm −1 was obtained from these SiCN cones. Moreover, the SiCN cone arrays exhibited rather stable emission current under constant applied voltage.

PECVD of Carbon Nanostructures in Hydrocarbon‐Based RF Plasmas

AbstractDifferent aspects of the plasma‐enhanced chemical vapor deposition of various carbon nanostructures in the ionized gas phase of high‐density, low‐temperature reactive plasmas of Ar+H2+CH4 gas mixtures are studied. The growth techniques, surface morphologies, densities and fluxes of major reactive species in the discharge, and effects of the transport of the plasma‐grown nanoparticles through the near‐substrate plasma sheath are examined. Possible growth precursors of the carbon nanostructures are also discussed. In particular, the experimental and numerical results indicate that it is likely that the aligned carbon nanotip structures are predominantly grown by the molecular and radical units, whereas the plasma‐grown nanoparticles are crucial components of polymorphous carbon films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nitrogen-incorporated multiwalled carbon nanotubes grown by direct current plasma-enhanced chemical vapor deposition

The nitrogen-incorporated multiwalled carbon nanotubes (N-MWCNTs) were synthesized by dc plasma-enhanced chemical vapor deposition with a gas mixture of C2H2, NH3, and N2. Nitrogens in the N-MWCNTs were pyridinic nitrogen and graphitic nitrogen. With increase in the flow rate of N2 gas during the synthesis of MWCNTs, the pyridinic nitrogen increased much more than graphitic nitrogen. The near-edge x-ray absorption fine structure spectra revealed that the density of states such as π*, σ*, and π*+σ* bands of the N-MWCNTs decreased with increase of concentration of pyridinic nitrogen incorporated in the MWCNTs. The intensity ratio of the D band to the G band of Raman spectrum increased with the incorporation of nitrogen into MWCNTs.

Composition of Ti-C:H films obtained by pulsed and continuous magnetron sputtering

The Ti-C:H films have been prepared by reactive pulsed and continuous DC magnetron sputtering of the Ti target in a gas mixture of Ar and CH4. The influence of pulsed and continuous discharge power on the film composition has been studied. For continuous sputtering, the discharge power was 300, 950 and 2850 W. At pulsed sputtering, the average discharge power was about 950 W in most depositions, while pulse power was varied from 2850 to 13000 W.The composition of the obtained films depended very strongly on the power for continuous discharge and only slightly on the pulse power at constant average power. In the latter case the film composition depended on pulse duration, CH4/Ar ratio and, in general, did not coincide with that obtained by continuous sputtering at the same power. The increase of discharge power at continuous sputtering led to reactive gas pressure decrease and to enrichment of the films with metal component. In contrast, the high level of pulsed discharge power cannot decrease considerably the reactive gas pressure during short pulse-on time followed by relatively long pulse-off time so that the film composition cannot be changed significantly as in the case of continuous reactive sputtering. It was supported by optical emission measurements.

Field Emission Properties of Nanocrystalline and Amorphous Silicon Carbon Nitride Prepared from Microwave Plasma Chemical Vapor Deposition

Silicon carbon nitride (SiCN) films have been deposited on silicon wafers by microwave plasma chemical vapor deposition (MPCVD). Gas mixture of H2, CH4, N2, and SiH4 was used as precursors, in which the flow rate of N2 was changed. X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy were employed to characterize the composition and bonding structures, while field-emission scanning electron microscopy were used to investigate the microstructure of the films. With increasing the flow rate of N2 from 50 sccm to 300 sccm, the SiCN films changed from amorphous to nanocrystalline. Characteristic current-voltage measurements indicate a low turn-on field of 10.8 V/µm. Field emission current density of 4.5 mA/cm2 has been observed at 20 V/µm.

Microstructure of NbSi 2/SiC nanocomposite coating formed on Nb substrate

The microstructure of a NbSi 2/α-SiC nanocomposite coating formed by a pack siliconizing process of Nb-carbide layers at 1100 °C, which had already been formed on the surface of a Nb substrate by a carburizing process, was investigated. The Nb-carbide layers formed by the carburizing process at 1400 °C using a gas mixture of CH 4–H 2 consisted of two layers, i.e., an inner layer of Nb 2C and an outer layer of NbC. While the monolithic NbSi 2 coating showed a typical columnar microstructure perpendicular to the Nb substrate, the NbSi 2/α-SiC nanocomposite coating formed by the solid-state displacement reaction of the NbC layer by Si was composed of equiaxed NbSi 2 grains with an average size of 67–134 nm and the α-SiC particles with an average size of 45–60 nm. The morphology of the α-SiC particles exhibited an oblate-spheroidal shape and were mostly located at the grain boundaries of NbSi 2. The nanocomposite coating formed by the solid-state displacement reaction of the Nb 2C layer by Si showed a lamella microstructure. The average lamella diameters of the NbSi 2 grains and the α-SiC particles were about 250 and 60 nm, respectively. The volume percentage of the α-SiC particles ranged from 17.3 to 31.1% with respect to the carbon concentration in Nb-carbide layers. No cracks were observed in the NbSi 2/α-SiC nanocomposite coating, indicating that its thermal expansion coefficient was close to that of the Nb substrate.

Mechanism And Characterization Studies on Boron Carbides Deposited by Chemical Vapor Deposition Technique

ABSTRACTBoron carbide was produced in an impinging jet CVD reactor from a gas mixture of BCl3, CH4 and H2. The mass transfer limitations on the reaction kinetics were minimised by the jet impingement on the substrate surface. XPS characterization of the produced deposits revealed a nearly pure boron carbide phase containing small amounts of oxy-boron and oxy-carbon species. After a detailed kinetic study, a reaction model was proposed to predict the rates of boron carbide and dichloroborane formation reactions. In this model, boron trichloride is adsorbed on the surface non-dissociatively, whereas hydrogen and methane are adsorbed dissociatively. BC is formed on the solid surface through the reaction of adsorbed boron trichloride with adsorbed methane in the form of CH3(s) (adsorbed CH3 on the surface). Produced BC is reacted in successive series reactions including adsorbed boron trichloride and adsorbed hydrogen. In the proposed mechanism, dichloroborane is produced only through the gas phase reaction between boron trichloride and hydrogen. The simultaneous fit of the experimental data to the model expression gave good fits for the boron carbide and dichloroborane formation reactions with the high correlation coefficient values.

Powder Synthesis of Ca-α‘-SiAlON as a Host Material for Phosphors

Fine powders of Ca-α‘-SiAlON have been synthesized from the system CaO−Al2O3−SiO2, by using an NH3−CH4 gas mixture as a reduction−nitridation agent. Completion of the reduction−nitridation was achieved at low temperatures of 1400−1500 °C without unfavorable particle coarsening or carbon contamination, while the phase purity and crystallinity of the powders were markedly improved by a postsynthesis heat treatment. The resulting fine Ca-α‘-SiAlON powders possessed high purity and uniform discrete particle morphology, which meet the requirements for the new application of SiAlON ceramics as a host material for phosphors. The reduction−nitridation synthesis of the yellow-emitting Ca-α‘-SiAlON:Eu powder phosphor was also performed for the first time, and better photoluminescence property was demonstrated compared to that of the conventionally prepared powder.

Thermophoretic control of building units in the plasma-assisted deposition of nanostructured carbon films

The possibility of the thermophoretic control of the plasma-grown building units in the plasma-assisted deposition of various carbon-based nanostructures on Ni-based catalyzed Si substrates is reported. It is experimentally demonstrated that varying the near-substrate temperature gradient, one can selectively deposit or levitate the carbon-based nanoparticles grown in the low-temperature reactive plasmas of Ar+H2+CH4 gas mixtures. When the nanoparticles are levitated in the plasma presheath, the arrays of vertically aligned carbon nanotips are assembled, whereas the enhanced deposition of the building units from the gas phase favors the formation of polymorphous nanostructured carbon films. The experimental observations are supported by the one-dimensional model of the nanoparticle dynamics in the near-electrode area. It is shown that the thermophoretic force is indeed a crucial factor that controls the deposition of the plasma-grown fine particles. The experimental and computation results suggest that it is likely that the aligned carbon nanotip structures are predominantly grown by the molecular or radical units, whereas the plasma-grown nanoparticles are presumably the most important component of polymorphous carbon films.