CH4 Flow Research Articles

Structural and optical characterization of carbon nitride layers deposited by plasma assisted chemical vapor deposition at various conditions

Amorphous hydrogenated carbon nitride layers, a-C:N:H, were fabricated by radio-frequency, 13.56MHz, plasma assisted chemical vapor deposition (RF-PACVD) from gaseous CH4 and N2 diluted in argon. Five series of layers were deposited at various processing conditions. In the respective experiments one of technological parameters (pressure, radio frequency power, temperature, CH4 or N2 flow) varied while the other were kept on constant level. The optical and structural properties of the layers were studied with use of spectroscopic ellipsometry and Fourier transform infrared spectroscopy (FTIR). It is shown that the layers deposited at various technological conditions, but for the same time duration, have different thicknesses. An influence of the processing parameters on the layers growth rate is discussed. The differences in atomic structure of the layers, as revealed by the FTIR spectra and optical constants (refractive and extinction indices, band gap) are shown to be insignificant.

Nickel-doped sodium zirconate catalysts for carbon dioxide storage and hydrogen production through dry methane reforming process

NiO-doped sodium zirconate ceramics with different amounts of NiO (between 0 and 10wt%) were synthetized and characterized by powder XRD, SEM-EDS and N2 physisorption. Structural and microstructural characteristics of a Na2ZrO3 based-ceramic were maintained in all NiO-containing samples. These materials were tested for CO2 capture (TGA), desorption processes (TPD) and dry CH4 reforming (DMR, catalytic tests). Initially, samples were dynamically tested for CO2 chemisorption; these tests showed a slight inhibition for CO2 capture due to the presence of NiO, which partially blocked Na2ZrO3 surface sites where CO2 can be chemisorbed. Then, NiO-doped samples were carbonated and exposed to a CH4 flow in order to perform DMR reaction, using carbonate samples as CO2 source. In all cases, NiO addition resulted in greater production of H2 than that of pure Na2ZrO3. Additionally, a drastic reduction in the reaction temperature was observed, especially for NiO-doped Na2ZrO3 containing 10wt% of NiO. Additionally, regeneration and cyclic behavior showed that it is possible to accomplish consecutive cycles of CO2 capture-DMR with considerable Na2ZrO3 regeneration. On the other hand, cyclability was affected due to a partial NiO reduction after DMR steps. However, if a pre-oxidation step was performed, the catalytic activity and H2 production were recovered. Hence, it was established that NiO-doped Na2ZrO3 materials can be used as bifunctional materials as (i) CO2 captors and then as (ii) catalytic materials during DMR reaction.

Optical and mass spectroscopic properties of microwave CH4/H2/Ar plasma for diamond deposition in a resonance cavity

The properties of microwave CH4/H2/Ar plasma for diamond deposition in a resonant cavity have been investigated by optical emission spectroscopy (OES) and quadrupole mass spectroscopy (QMS). Ar emission intensity decreases monotonically with gas pressure, where electron impact excitation plays an important role. Hα. CH and C2 intensities exhibit the different pressure dependences, which decrease at first and then rise with the further increasing of gas pressure, suggesting the change of electron activation mechanism to thermally driven chemistry for the formation of excited reactive hydrocarbon species at high gas pressures. The addition of small amounts of CH4 to the H2/Ar plasma causes increases in emissions of Hα and CH generated from the CH4 dissociation. With further increase of CH4 flow rate, both emissions and abundances of 2-carbon species enhance, while a reduced intensity of CH emission is observed, reflecting the complex dissociation and dimerization reactions of hydrocarbon species. The dissociation of CH4 is enhanced in plasma under higher power condition. A slight change can be observed in both emission and abundance of hydrocarbon species with increasing Ar flow rate.

Combined Ceria Reduction and Methane Reforming in a Solar-Driven Particle-Transport Reactor.

We report on the experimental performance of a solar aerosol reactor for carrying out the combined thermochemical reduction of CeO2 and reforming of CH4 using concentrated radiation as the source of process heat. The 2 kWth solar reactor prototype utilizes a cavity receiver enclosing a vertical Al2O3 tube which contains a downward gravity-driven particle flow of ceria particles, either co-current or counter-current to a CH4 flow. Experimentation under a peak radiative flux of 2264 suns yielded methane conversions up to 89% at 1300 °C for residence times under 1 s. The maximum extent of ceria reduction, given by the nonstoichiometry δ (CeO2−δ), was 0.25. The solar-to-fuel energy conversion efficiency reached 12%. The syngas produced had a H2:CO molar ratio of 2, and its calorific value was solar-upgraded by 24% over that of the CH4 reformed.

Effects of CH4 flow ratio on the structure and properties of reactively sputtered (CrNbSiTiZr)Cx coatings

High-entropy (CrNbSiTiZr)Cx coatings were successfully fabricated by reactive radio-frequency magnetron sputtering using an equimolar CrNbSiTiZr alloy target under CH4-containing Ar flow. The effect of the CH4 flow ratio on the chemical composition, microstructure, mechanical and tribological properties of (CrNbSiTiZr)Cx coatings was investigated. The carbon content in the coatings varied from 36.7 at% to 87.8 at% with an increasing CH4 flow ratio from 3% to 20%. The (CrNbSiTiZr)Cx coatings revealed a simple NaCl-type FCC (face-centered cubic) structure. The maximum hardness of 32 GPa was obtained at the carbon content of 36.7 at% by applying substrate bias of −100 V. On the other hand, the coating with high carbon content of 87.8 at% possessed superior wear resistance, which friction coefficient and wear rate were, respectively, 0.07 and 2.0 × 10−7 mm3/N·m.

Fast deposition of thick diamond-like carbon films by ion-beam technique

A diamond-like carbon film doped with TiC nanocrystallites (TiC–DLC) with a thickness of 35.8 μm was successfully prepared on a stainless steel substrate by employing a combination of metal vapor vacuum arc and filtered cathode vacuum arc techniques. A maximum deposition rate of 0.25 μm/min was achieved for TiC–DLC films. The structure and properties of the TiC–DLC films were systematically analyzed using different methods such as transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, X-ray diffraction, and friction and wear tests. The results indicated that typical a-C:H films containing nano-sized TiC grains were deposited which exhibit improved mechanical properties such as high cohesive strength, Vickers hardness, and capacity against high temperature. Parameter windows for C2H2 flow rate and solenoid current were also provided for the deposition of TiC–DLC films to meet the requirements for using the material for specific commercial applications.

Defect Emission and Optical Gain in SiCxOy:H Films.

Luminescent SiCxOy:H films, which are fabricated at different CH4 flow rates using the plasma-enhanced chemical vapor deposition (PECVD) technique, exhibit strong photoluminescence (PL) with tuning from the near-infrared to orange regions. The PL features an excitation-wavelength-independent recombination dynamics. The silicon dangling bond (DB) defects identified by electron paramagnetic resonance spectra are found to play a key role in the PL behavior. The first-principles calculation shows that the Si DB defects introduce a midgap state in the band gap, which is in good agreement with the PL energy. Moreover, the band gap of a-SiCxOy:H is found to be mainly determined by Si and C atoms. Thus, the strong light emission is believed to result from the recombination of excited electrons and holes in Si DB defects, while the tunable light emission of the films is attributed to the substitution of stronger Si-C bonds for weak Si-Si bonds. It is also found that the light emission intensity shows a superlinear dependence on the pump intensity. Interestingly, the film exhibits a net optical gain under ultraviolet excitation. The gain coefficient is 53.5 cm-1 under a pumping power density of 553 mW cm-2. The present results demonstrate that the SiCxOy system can be a very competitive candidate in the applications of photonics and optoelectronics.

Deposition and Characterization of Reactive Magnetron Sputtered Tungsten Carbide Films

Tungsten carbide thin films were deposited on silicon (100) substrates by DC reactive magnetron sputtering using CH4 as a carbon source. The microstructure, compressive stress, hardness and tribological behaviors showed great dependences on the rates of CH4 flow (FCH4). Increasing the FCH4 from 2 to 5 sccm, the film exhibited a phase transition from hexagonal-W2C to cubic-WC1-x. Further increasing the FCH4 larger than 10sccm, the film presented amorphous state. As the FCH4 increased, the Raman revealed that the films showed a graphitization trend, meanwhile, the surface of the films became smoother and smoother. The hardness of tungsten carbide films first increased, and then decreased after reaching the maximum 38.5GPa (FCH4=10 sccm). While the sample deposited at 15 sccm obtained the lowest wear rate (2.17×10-6 mm3/Nm) and low coefficient of friction (CoF, 0.24) and still maintained a high hardness of 32.1 GPa. The lowest wear rate could be ascribed to the highest ratio of H3/E2.

Magnetic Fenton and Photo-Fenton-Like Catalysts Supported on Carbon Nanotubes for Wastewater Treatment

Carbon nanotubes (CNTs) synthesized by the catalytic decomposition of methane were used as the support for magnetic Fenton and photo-Fenton catalysts to treat real wastewater contaminated with dyes and Escherichia coli. The effect of methane flow, the use of diluent (N2), and the reaction time in the production of CNTs were studied. An increase in the production of CNTs with increased CH4 flow and a decrease over the reaction time were recorded. Catalysts with 1, 3, and 5% w/w Fe were obtained and characterized by several spectroscopic and microscopic techniques. Multi-walled CNTs and bamboo-like carbon nanofibers with average diameters of 44.0 nm and average lengths of 237.0 nm were obtained. The catalysts had Fe x O y (oxide species) crystallite sizes between 10 and 18 nm and soft ferromagnetic properties. A factorial 33 design was used for selecting variables for the catalytic tests, wherein the concentration of H2O2, the catalyst mass, and the percentage of iron were evaluated. Subsequently, kinetic experiments were performed. The photo-Fenton process (5% Fe, 200 mg, and 0.4 M H2O2) showed the best results in terms of total organic carbon (TOC) abatement, discoloration, and E. coli inactivation without leaching of Fe.

In situ-grown silicon quantum dots in SiCx:H/a-C:H hetero-multilayer films prepared by plasma enhanced chemical vapor deposition method

Silicon quantum dots (Si QDs) were grown in situ in SiCx:H/a-C:H hetero-multilayer films at a low substrate temperature using a radio frequency (13.56 MHz) plasma enhanced chemical vapor deposition (PECVD) method. The component and bonding configuration of the SiCx:H/a-C:H hetero-multilayer films have been systematically investigated by Raman, FTIR and HRTEM testing techniques. The results show that there are amorphous Si QDs in SiCx:H/a-C:H hetero-multilayer films. Without high temperature post-deposition annealing treatments, as increasing CH4 flow rate, the coexistence of amorphous Si QDs and silicon nanocrystals in SiCx:H matrix layers has been confirmed by Raman, HRTEM and FTIR measurements. X-ray photoelectron spectroscopy and UV–Vis spectrometry measurements show that the a-C:H layer is mainly consisted of the sp2-bonded and sp3-bonded carbon atoms and its optical band gap is 1.63 eV. The smaller optical band gap suggested that SiCx:H/a-C:H hetero-multilayer films can reduce the barrier height between Si QDs and barrier layer in vertical direction as compared with SiCx/SiC multilayer films.

Isothermal Reaction of NiO Powder with Undiluted CH4 at 1000 K to 1300 K (727 °C to 1027 °C)

In this study, isothermal reaction behavior of loose NiO powder in a flowing undiluted CH4 atmosphere at the temperature range 1000 K to 1300 K (727 °C to 1027 °C) is investigated. Thermodynamic analyses at this temperature range revealed that single phase Ni forms at the input \( {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } \mathord{\left/ {\vphantom {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}}} \right. \kern-0pt} {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}} \) mole fractions (\( X_{{{\text{CH}}_{ 4} }} \)) between ~0.2 and 0.5. It was also predicted that free C co-exists with Ni at \( X_{{{\text{CH}}_{ 4} }} \) values higher than ~0.5. The experiments were carried out as a function of temperature, time, and CH4 flow rate. Mass measurement, XRD and SEM-EDX were used to characterize the products at various stages of the reaction. At 1200 K and 1300 K (927 °C and 1027 °C), the reaction of NiO with undiluted CH4 essentially consisted of two successive distinct stages: NiO reduction and pyrolytic C deposition on pre-reduced Ni particles. At 1200 K (927 °C), 1100 K (827 °C), and 1000 K (727 °C), complete oxide reduction was observed within ~7.5, ~17.5, and ~45 minutes, respectively. It was suggested that NiO was essentially reduced to Ni by a CH4 decomposition product, H2. Possible reactions leading to NiO reduction were suggested. An attempt was made to describe the NiO reduction kinetics using nucleation-growth and geometrical contraction models. It was observed that the extent of NiO reduction and free C deposition increased with the square root of CH4 flow rate as predicted by a mass transport theory. A mixed controlling mechanism, partly chemical kinetics and partly external gaseous mass transfer, was responsible for the overall reaction rate. The present study demonstrated that the extent of the reduction can be determined quantitatively using the XRD patterns and also using a formula theoretically derived from the basic XRD data.

Self‐modification of Ni Metal Surfaces with CeO2 to Suppress Carbon Deposition at Solid Oxide Fuel Cell Anodes

AbstractHigh durability against carbon deposition on Ni anodes of internal‐reforming solid oxide fuel cells (SOFCs) is one of the most significant challenges in the utilization of SOFCs. Carbon deposition is strongly affected by the type of solid oxide electrolyte, although it occurs on Ni surfaces which are a few hundred nanometers away from the solid oxide electrolyte. Carbon deposition under CH4 flow at 1,073 K on Ni particles on yttria‐stabilized ZrO2 was determined to occur more readily than on Ni particles on Gd‐doped CeO2. We have examined how the solid oxides interact with the Ni surfaces in anodes using Auger electron microanalysis and infrared spectroscopy with adsorbed molecules. The results suggest that the CeO2 species migrate on the Ni surfaces under reducing conditions. The chemical influence of CeO2 migration on the Ni surface was confirmed by infrared spectra of adsorbed CO on Ni sites.

Synthesis of Fine Mo2C Powder from Prereduced Mo in Undiluted CH4 Flow

The carburization behavior of prereduced Mo was investigated in undiluted CH4 flow at 900–1000 K. Prior to the experiments, equilibrium thermodynamic analysis was carried out in the Mo-C-H system. The products were characterized by mass measurement, x-ray diffraction and scanning electron microscopy techniques. A single Mo2C phase was obtained within ~45 min, ~5 min, and ~2.5 min at 900 K, 950 K, and 1000 K, respectively, at CH4 contents higher than the predicted ones. The reasons for this behavior were discussed in terms of CH4 stability, open tube flow, and self-created atmosphere in the powder bed. The fractional conversion-time curves indicated that the carburization kinetics followed a linear rate law. The Mo2C crystallite size (26–37 nm) and platelet thickness (50–100 nm) were found to be smaller than those of the parent Mo phase. These findings were attributable to the defects formed as a result of stresses associated with the reduction and the carburization.

A Single-Step Process for Direct Reduction of Iron Oxide to Sponge Iron by Undiluted Methane

A single-step pyrometallurgical process for the synthesis of sponge iron is presented in this study. It aimed to investigate the reduction behavior of Fe2O3 in undiluted CH4 flow to establish the process parameters for sponge Fe formation. Thermodynamic analysis predicted the reduction of Fe2O3 to Fe by CH4 at 1000–1200 K. The experiments were carried out at 800–1200 K for 60 min and at 1200 K for 0–60 min. Mass measurement and x-ray diffraction (XRD), and scanning electron microscope techniques were used to characterize the products. The extent of the oxide reduction was found to increase with the temperature and time. XRD showed that single phase Fe was obtained at 1100 K for 60 min and at 1200 K within 10 min. The products synthesized at 1200 K within 15 min had spongy morphology. C deposition on the pre-reduced Fe particles resulted in the disappearance of spongy character.

Hydrogen production by methane decomposition over Co-Al mixed oxides derived from hydrotalcites: Effect of the catalyst activation with H2 or CH4

This study examines the influence of catalyst activation for methane decomposition over Co-Al mixed oxides derived from hydrotalcites. Samples were prepared by coprecipitation and characterized by surface area measurements and temperature-programmed reduction. Spent catalysts and carbon produced in the reaction were characterized by X-ray diffractometry, temperature-programmed oxidation and scanning electron microscopy. Activity runs using previously reduced samples with H2 or activated under CH4 flow were carried out in a fixed-bed reactor between 500 and 750 °C using in-line gas chromatography analysis. The specific surface area decreases as the Co/Al ratio increases, which is related to the increased Co3O4 phase rather than Co-Al mixed oxides. The TPR results indicate the reduction of four types of Co species: Co3+ and Co2+ species from Co3O4, and Co from inverse spinel (Co2AlO4) and normal spinel (CoAl2O4). Reduction with hydrogen at 750 °C was very severe. Samples reduced with H2 showed large Co° crystallites, which increased with the Co/Al ratio. Co-Al catalyst activation under methane flow leads to lower crystallite size and higher thermal stability for hydrogen production by methane decomposition.

Optical emission spectroscopic study of Ar/H2/CH4 plasma during the production of graphene nano-flakes by induction plasma synthesis

Graphene nano-flakes using CH4 precursor were synthesized in a radio frequency inductively coupled plasma reactor with in-situ investigation of Ar/H2/CH4 plasma by optical emission spectroscopy at fixed H2 and Ar flow rates of 4 and 75 slpm, respectively, and at different plate powers (12 to 18 kW), pressures (400 to 700 mbar) and CH4 flow rates (0.3 to 2 slpm). Emissions from C2 Swan band, C3, CH and H2 are observed in the optical emission spectra of Ar/H2/CH4 plasma. Plasma temperature estimated analyzing the C2 Swan band emission intensities is found to be decreased with increasing pressure and decreasing plate power. The decreasing plasma temperature gives rise to increase in production rate due to increase in condensation process. The production rate is observed to be increased from 0 to 0.3 g/h at 18 kW and from 0 to 1 g/h at 15 kW with increase in pressure from 400 to 700 mbar at fixed CH4 flow rate of 0.7 slpm. Broad band continuum emission appears in the emission spectra at specific growth conditions in which the formation of vapor phase nanoparticles due to condensation of supersaturated vapor is facilitated. The production rate at 12 kW, 700 mbar, and 0.7 slpm of CH4 flow rate is found to be 1.7 g/h which is more than that at 15 and 18 kW. Thus, the broadband continuum emission dominates the optical emission spectra at 12 kW due to lower temperature and higher production rate, and is attributed to the emission from suspended nanoparticles formed in vapor phase. The synthesized nanoparticles exhibit flake like structures having average length and width about 200 and 100 nm, respectively, irrespective of the growth conditions. Nano-flakes have thickness between 3.7 to 7.5 nm and are composed of 11 to 22 graphene layers depending on the growth conditions. The intensity ratio (ID/IG) of D and G band observed in the Raman spectra is less than 0.33 which indicates good quality of the synthesized graphene nano-flakes.

Methane dissociation process in inductively coupled Ar/H2/CH4 plasma for graphene nano-flakes production

The role of hydrogen and methane dissociation process in induction plasma synthesis of graphene nano-flakes (GNF) is studied by the optical emission spectroscopy of Ar/H2/CH4 plasma. The condensation of C2 species formed due to methane decomposition produces GNF, which depends on pressure. Electron impact and dehydrogenation processes dissociate methane, which promotes and hinders the GNF production, respectively. The effect of hydrogen is insignificant on quality, size and morphology of the GNF. The CH4 flow rate has no influence on particle temperature but has effect on cooling rate at the point of nucleation and, therefore, on production rate and thickness of GNF.

Thick diamond like carbon coatings deposited by deep oscillation magnetron sputtering

Hydrogenated diamond like carbon (DLC) coatings with thicknesses up to 20μm have been deposited on metal substrates by sputtering a graphite target in argon (Ar) and acetylene (C2H2) mixture using deep oscillation magnetron sputtering (DOMS). The coatings were deposited under moderate peak target current densities (0.47 to 0.6Acm−2) and a low bias voltage of −60V, to achieve dense microstructure but avoid building up of high residual stresses. Higher peak substrate current densities and deposition rates were found when C2H2 was introduced. The effects of the C2H2 flow rate (fC2H2) on the deposition rate, microstructure, adhesion, mechanical and tribological properties of the DLC coatings were studied by means of scanning electron microscopy, Raman spectroscopy, HRC indentation, nanoindentation, dry ball-on-disk test, and block-on-ring test in SAE 10W-30 engine lubricant. The coatings showed low compressive stresses in the range of −0.5GPa to −1.8GPa. The coatings exhibited dense structure, high sp3/sp2 ratio, and excellent wear resistance when the C2H2 flow rate was in the range of 10–20sccm (4%–8% of the total gas flow). The friction and wear behavior of the DOMS DLC coatings both in the ambient air and SAE 10W-30 engine lubricant showed positive results in terms of friction and wear reduction.

The composition, microstructure and mechanical properties of Ni/DLC nanocomposite films by filtered cathodic vacuum arc deposition

In this paper, Ni/DLC nanocomposite films were deposited on the un-heated silicon (100) by the filtered cathodic vacuum arc deposition (FCVAD) under different CH4 and C2H2 flow rates. The composition, microstructure and mechanical properties of the Ni/DLC films were investigated by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and nanoindentation. The Ni/DLC films typically consisted of nickel nanograins around 11nm, which were embedded in amorphous carbon matrix with the thickness of 0.35–2.8nm. The film structure and the mechanical properties are strongly affected by the precursor gas type and flow rate. The porous structure, with the nanopore size equaling to the nickel grain size, can be seen after the etching process of the as-deposited film, providing a possible method for the preparation of nanoporous DLC films. A maximum hardness of 13.2GPa and 21.64GPa is achieved for the Ni/DLC films under the CH4 flow rate of 30sccm and C2H2 flow rate of 40sccm, respectively, under which the maximum thickness of the amorphous carbon phase is also obtained.

Impact of injection pressure on CO2–enhanced coalbed methane recovery considering mass transfer between coal fracture and matrix

Although the production mechanism of CO2-enhanced coalbed methane (ECBM) recovery has been previously reported and corresponding models have been proposed, investigations on the effect of the injection pressure on the production efficiency considering the mass transfer between the coal fracture and matrix are lacking. In this study, laboratory core flooding experiments were conducted on the coal sample from South Sichuan Basin, China. Experimental results indicate that earlier breakthroughs and a higher initial CH4 flow rate and cumulative CH4 flow rate occurred under a higher injection pressure in the CO2-ECBM recovery. However, as CO2 was continuously injected, the CH4 flow rate and cumulative CH4 flow rate under a lower injection pressure exceeded those at the higher injection pressure. Furthermore, a dual porosity model was used to perform numerical simulations. The simulation results under four injection pressures matched the experimental results well. The simulation results indicate that the dispersion in fractures and the mass transfer rate between matrices and fractures significantly affect the CH4 flow rate. The injection pressure leads to changes in the experimental observations by affecting the dispersion and the mass transfer rate.