CH4 Flow Rate Research Articles

Influences of deposition temperature, gas flow rate and ZrC content on the microstructure and anti-ablation performance of CVD-HfC-ZrC coating

To amend the oxidation and ablation resistance of C/C composites, HfC-ZrC biphase coating was synthesized by CVD. Influences of deposition temperature and CH4 flow rate on the deposition rate, phase constitution and microstructure of the HfC-ZrC coating were investigated. Ablation behavior and ablation mechanism of the HfC-ZrC coating with different ZrC contents were examined. With the deposition temperature rising, the deposition rate and grain size of the HfC-ZrC coating increased. High flow rate of CH4 was beneficial to improving the deposition rate and reducing the grain size of the HfC-ZrC coating. Moderate ZrC content in the HfC-ZrC coating was conducive to the process of solid solution sintering among HfO2 and ZrO2 grains, leading to generating a continuous and compact oxide layer. The coating with HfC/ZrC mole ratio of 1:1 exhibited superior anti-ablation performance, owing to its flat and compact structure and sufficient solid solution sintering among the oxide grains during ablation.

Optimizing the Microstructure, Mechanical, and Tribological Properties of Si-DLC Coatings on NBR Rubber for Its Potential Applications

Si doped diamond-like carbon (Si-DLC) films were deposited on nitrile-butadiene rubber (NBR), and the effects of deposition parameters on the mechanical and tribological properties of an Si-DLC top layer on NBR were investigated. Then, the sample with the best performance is selected to investigate its tribological behaviors and mechanism under different contact loads. The results show that the growth rate and the doped Si content are also decreased with increasing the CH4 flow rate. The Si atom exists in the form of Si-C bonds at low CH4 flow rate (≤40 sccm) and Si-C + Si-O-C bonds at high CH4 flow rate (≥60 sccm). Furthermore, the sp3 content increases monotonously, while the hardness and H3/E2 ratio firstly decreases and then increases. As a result, the friction and wear behaviors are in line with the change trend of the hardness. The lowest friction coefficient (~0.19) and a slight wear were achieved for the Si-DLC3 film under the relatively high load of 3 N. The tribological results indicate that the friction coefficient and wear increase monotonously with the increase of load, which is mainly attributed to the brittle fragmentation of films at a higher load, and thus a high strength and super toughness DLC films should be needed. Furthermore, the friction and wear behaviors of samples depend critically on its surface topography, and the wear is lower when the friction direction is parallel to the stripes.

Accurate estimation of DLC thin film hardness using genetic programming

Abstract In the current research, diamond-like carbon thin films are deposited on silicon substrates by plasma-enhanced chemical vapor deposition. The effect of argon-C2H2 flow rate, hydrogen flow rate and deposition temperature on the thin film hardness is investigated. Morphology of the DLC films is investigated by scanning electron microscopy and atomic force microscopy, while the nano-hardness is investigated using nanoindentation. Raman spectroscopy is used for the characterization of the structural properties of the film. A metamodel of the DLC deposition process with argon- C2H2 flow rate, H2 flow rate and deposition temperature as the regressor variables and coating hardness as the response is built by using a novel symbolic regression approach. A state-of-the-art machine learning approach - genetic programming (GP) - is used for the symbolic regression. By carefully evaluating the performance of the current GP metamodel against a classical RSM (response surface methodology) metamodel, it is seen that the GP significantly outperforms RSM.

Influence of RF power and CH4 flow rate on properties of diamond-like carbon films deposited by PECVD technique

In this work, Diamond like carbon (DLC) films were deposited on titanium substrate by plasma enhanced chemical vapor deposition (PECVD) technique. The influences of RF power and CH4 gas flow rate on the properties of the DLC films were studied using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and synchrotron-based near-edge X-ray absorption find structure (NEXAFS) spectroscopy. The results show that the sp3/sp2 ratio decreases, while the ID/IG increases when RF power increases. The sp3/sp2 ratio increases with CH4 flow rate.

The structure and properties of amorphous diamond-like carbon films deposited by helicon wave plasma chemical vapor deposition

The diamond-like carbon (DLC) films have been synthesized on silicon substrates by using chemical vapor deposition assisted with a high-density m=+1 mode helicon wave-excited plasma in Ar/CH4 gas mixtures. The structure and morphology of the DLC films are characterized by scanning electron microscopy (SEM), Raman spectroscopy and atomic force microscopy (AFM). The surface energies of the DLC films are calculated by the the Owens–Wendt–Rabel–Kaelble (OWRK) method. SEM results show that the surfaces of DLC films are found to be very uniform and dense, and the deposition rate of the film can reach up to 45 μm/h. The Young's modulus of the DLC films reaches the maximum (11 GPa) value at 25 sccm CH4 flow rate. With the increase of CH4 flow rate from 5 sccm to 35 sccm, the surface energy and friction coefficient of the DLC films decrease from 73.5 to 37.6 mJ m − 2 and from 0.56 to 0.25, respectively. It indicates that the deposited DLC films have good friction properties.The Young's modulus value is obtained here using AFM [48]. The results show that the Young's modulus increases from 4.1 to 10. 77 GPa, as the CH4 flow rate increases from 5 to 25 sccm. However, with further increasing the CH4 flow rate, the Young's modulus decreases gradually. This is consistent with the trend of sp3 bond content in Raman results. According to literature [49], CC sp3 bond of DLC films has a certain influence on Young's modulus. The result shows that the CC sp3 content reaches its maximum when the CH4 flow rate reaches 25 sccm. Moreover, the RMS results of DLC films show that the smoother the film is, the higher the Young's modulus is, likewise the rougher the DLC film is, the lower the Young's modulus is. It can be inferred that when DLC film is rough, the accumulated carbon polymer sp3 content in DLC film is low.

Accurate estimation of DLC thin film hardness using genetic programming

Abstract In the current research, diamond-like carbon thin films are deposited on silicon substrates by plasma-enhanced chemical vapor deposition. The effect of argon-C2H2 flow rate, hydrogen flow rate and deposition temperature on the thin film hardness is investigated. Morphology of the DLC films is investigated by scanning electron microscopy and atomic force microscopy, while the nano-hardness is investigated using nanoindentation. Raman spectroscopy is used for the characterization of the structural properties of the film. A metamodel of the DLC deposition process with argon- C2H2 flow rate, H2 flow rate and deposition temperature as the regressor variables and coating hardness as the response is built by using a novel symbolic regression approach. A state-of-the-art machine learning approach - genetic programming (GP) - is used for the symbolic regression. By carefully evaluating the performance of the current GP metamodel against a classical RSM (response surface methodology) metamodel, it is seen that the GP significantly outperforms RSM.

Syngas evolution and energy efficiency in CO2-assisted gasification of pine bark

Forestry residues, one of the main sources of carbon–neutral lignocellulosic biomass, are abundant to support energy sustainability and reduce carbon emissions into the atmosphere. In this paper, the characteristics of syngas from the gasification of pine bark in CO2 atmosphere are presented using a fixed-bed reactor at temperatures of 700, 800, 900 and 1000 °C. Gasification behavior at these temperatures was investigated in terms of evolved flow rate of CO, H2, CH4, total hydrocarbons (CmHn) and total syngas yield. Solid residues were analyzed for morphological characteristics. Total accumulative syngas rate and overall energy efficiency at different temperatures were also calculated and compared. Results showed that gas yields of H2, CO, and total syngas increased with increase in temperature. However, the yield of CmHn was maximum at 800 °C. Syngas with heating value of 21.0 to 23.3 MJ/kg was obtained from CO2 gasification. CO mole fraction accounted for approximately 66–80% (vol.) of the total syngas from char and CO2 gasification via Boudouard reaction and hydrocarbon reforming with CO2 at high temperatures. Gasification of each gram of pine-bark provided with the capability of converting ~0.25–1.74 g of CO2 into valuable products examined at 800 to 1000 °C. The porosity and thus specific surface area of solid char residue increased with increase in temperature. Overall energy efficiency increased with increase in temperature to values as high as 56.6% at 1000 °C, while maintaining this high efficiency over long reaction times that revealed promising pathway for harnessing of energy production and CO2 utilization.

Growth and Field Emission of Single‐Crystalline Hafnium Carbide Nanowires

<100>‐oriented hafnium carbide (HfC) nanowires are synthesized on the graphite substrate by chemical vapor deposition. The dependence of temperature, HfCl4content, and flow rate of CH4gas on the morphology of nanowires is investigated herein. The results indicate that the HfC nanowire often kinks to another growth direction at a high HfCl4content or a low temperature due to the destabilization of the catalyst droplet induced by either a faster growth rate or slower thermal activation. Reduced nanowire density and length are observed at the sample synthesized at a lower CH4flow rate. Based on the experimental results, high‐crystallinity nanowires with high aspect ratios and high densities are prepared at a lower HfCl4content, higher temperature, and high CH4flow rate. The field‐emission properties of a single HfC nanowire are also characterized by assembling a single HfC nanowire emitter. An extremely high field enhancement factor of 5.57 × 106 m−1is obtained herein.

The Structure, Morphology, and Mechanical Properties of Ta-Hf-C Coatings Deposited by Pulsed Direct Current Reactive Magnetron Sputtering

Ta, Hf, TaCx, HfCx, and TaxHf1-xCy coatings were deposited by reactive pulsed Direct Current (DC) magnetron sputtering of Ta or Hf pure metallic targets in Ar plus CH4 gas mixtures. The properties have been investigated as a function of the carbon content, which is tuned via the CH4 flow rate. The discharge was characterized by means of Optical Emission Spectroscopy and, in our conditions, both Ta-C and Hf-C systems seem to be weakly reactive. The structure of the as-deposited pure tantalum film is metastable tetragonal β-Ta. The fcc-MeCx carbide phases (Me = Ta or Hf) are {111} textured at low carbon concentrations and then lose their preferred orientation for higher carbon concentrations. Transmission Electron Microscopy (TEM) analysis has highlighted the presence of an amorphous phase at higher carbon concentrations. When the carbon content increases, the coating’s morphology is first compact-columnar and becomes glassy because of the nano-sized grains and then returns to an open columnar morphology for the higher carbon concentrations. The hardness and Young’s modulus of TaCx coatings reach 36 and 405 GPa, respectively. For HfCx coatings, these values are 29 and 318 GPa. The MeCx coating residual stresses increase with the addition of carbon (from one-hundredth of 1 MPa to 1.5 GPa approximately). Nevertheless, the columnar morphology at a high carbon content allows the residual stresses to decrease. Concerning TaxHf1-xCy coatings, the structure and the microstructure analyses have revealed the creation of a nanostructured coating, with the formation of an fcc superlattice. The hardness is relatively constant independently of the chemical composition (22 GPa). The residual stress was strongly reduced compared to that of binary carbides coatings, due to the rotation of substrates.

Comparative study on the performance of microwave-assisted plasma DRM in nitrogen and argon atmospheres at a low microwave power

Dry reforming of methane (DRM) is an attractive route to convert CH4 and CO2 into syngas (a mixture of CO and H2). In this work, the performance of microwave-assisted DRM at atmospheric pressure (in terms of CH4 and CO2 conversions, H2 and CO selectivities, H2 and CO yields, and H2 to CO ratio) is studied as functions of additive gas flow rate, microwave power, CO2 to CH4 inlet supply ratio, and reaction time. Two additive gases were used, i.e., nitrogen and argon. Microwave-assisted DRM experiments in both gases show identical trend of conversions, selectivities, yields, and product ratio. DRM performances in both additive gases atmosphere were stable for up to 8 h. Under the same operating conditions, using Ar as an additive gas, however, led to higher H2 and CO selectivities and yields and thus, higher H2 to CO ratio relative to using N2 as an additive gas. Maximum CH4 and CO2 conversions of 79.35% and 44.82%, H2 and CO selectivities of 50.12% and 58.42%, H2 and CO yields of 39.77% and 32.89%, and H2 to CO ratio of 0.86 were obtained at 700 W and N2, CO2, and CH4 flow rates of 1.5, 0.4, and 0.2 L min−1, respectively.

Role of leaves in methane emission from sacred lotus (nelumbo nucifera)

Sacred lotus (Nelumbo nucifera) is known to vent pressurized air in the leaf blade through stomata on the central plate of the leaf via the rhizome. This study measured diurnal CH4 emissions from lotus-cultivated in tanks during July and August 2016. CH4 emissions from the central plate of leaves of different age were also evaluated. Average daily CH4 fluxes of lotus-cultivated tanks were 23.1 ± 5.6 and 11.4 ± 6.0 mg C m−2 h−1 during July and August, respectively. Both the diurnal pattern of flow rate and the CH4 concentration of gas emitted from the central plate were similar to those of lotus and other wetland plants. However, the diurnal pattern of CH4 flux of younger leaves was dissimilar to that of tanks with cultivated lotus. Younger leaves exhibited a higher flow rate and greater concentration of CH4 than older leaves. This implies that pressurized air in older leaves is emitted through younger leaves, which contrasts with other plants that emit gas from older leaves or columns via pressurization.

Study of cracking of methane for hydrogen production using concentrated solar energy

In this study, the cracking phenomenon of methane taking place in a cylindrical cavity of 16 cm in diameter and 40 cm in length under the heat of concentrated solar radiation without any catalyst is analysed. Three cases have been chosen; in all cases the primary phase contains methane and hydrogen gases. In the first case, we consider two phases; the secondary phase is a homogeneous carbon black powder with 50 nm of diameter; in the second case we have three phases where the two secondary phases are a particles powder with two diameters 20 and 80 nm and finally, a third case of five phases with a powder of four different diameters 20, 40, 60 and 80 nm. The low Reynolds K-ε turbulence model was applied. A calculation code "ANSYS FLUENT" is used to simulate the cracking phenomena where an Eulerian – Eulerian model is applied. The choice of several diameters greatly increases the calculation time but it approaches more of the physical reality of the radiation by these particles during the cracking. Results have shown that increasing the number of diameters gives higher cracking rates; the case of the powder of 4 different diameters gives the highest cracking rate. A parametric study as a function of the inlet velocity, carbon particle diameters and the intensity of solar radiation is realized. For the cracking heat, provided by the choice of the two concentrators of 5 and 16 MW/m2 used in this simulation, the CH4 inlet velocity is a decisive parameter for the cracking rate. Any increase in the inlet velocity requires more heat and this leads to a decrease in the cracking rate. For a velocity not exceeding 0.177 m/s (i.e. 0.3 L/min), both solar concentrations give the same amount of hydrogen produced. These quantities of hydrogen obtained reach maximum values for an inlet flow rate of CH4 between 0.58 L/min (i.e. 0.34 m/s) and 0.62 L/min (i.e. 0.3655 m/s) for both reactors. The results are interpreted and compared with experimental work.

Effects of acetylene flow rate and bias voltage on the structural and tribo-mechanical properties of sputtered a-C:H films

The properties of sputtered a-C:H films are significantly influenced by the C2H2 flow rate and bias voltage. A suitable Design of Experiments allows to consider their effects on the mechanical and tribological properties. The a-C:H films are deposited by varying the C2H2 flow rate from 5.9 to 34.1 sccm and the bias voltage from −83 to −197 V, following the Central Composite Design. In Raman scattering studies, the presence of CH bands with increasing C2H2 flow rate is identified. Additionally, a decrease of the I(D)/I(G) ratio is observed with increasing C2H2 flow rate. Both observations indicate the formation of sp³-hybridized CH bonds. In contrast, a low C2H2 flow rate and a high bias voltage result in a higher I(D)/I(G) ratio and a lower intensity of the CH stretching bands, indicating a lower amount of CH bonds. The mechanical properties are also considerably influenced by these parameters. A higher C2H2 proportion results in a lower hardness and elastic modulus, which are related to a higher H content. However, a higher bias voltage increases the hardness and elastic modulus due to densification mechanisms, which increase the degree of distortion of the a-C:H films. Consequently, a low C2H2 flow rate and a high bias voltage ensure a high hardness of up to ~24 GPa due to a lower amount of CH bonds and a higher degree of distortion. In tribometer tests, most a-C:H films exhibit a low coefficient of friction against steel, ranging from 0.23 to 0.25. All a-C:H films are marked by a deformative wear, indicating a high resistance against abrasive wear when sliding against steel.

Study on CH4 dry reforming process by high power inductively coupled plasma torch at atmospheric pressure

CO2 dissociation and CH4 dry reforming, by high power inductively coupled plasma (ICP) torch at atmospheric pressure, have been studied. At a frequency of 400 kHz and power of 30 kW, the ICP torch source dissociates CO2 gas directly, causing CH4 dry reforming. The resulting products are composed of syngas, C2H6, C2H4, and C2H2. The results show conversion efficiencies (CE) for both CO2 and CH4 of 95%. At an input power of 22 kW, a CO2 flow rate of 30 SLM, and a CH4 flow rate of 36 SLM, the energy conversion efficiency (ECE) is 57%. As input CH4 flow rate increases, the selectivity of CO and H2 decreases and that of C2 hydrocarbons increases. In this condition, ECE increases. As a result, the high CE of CO2 and CH4, the large amount of products, and the high selectivity of C2 hydrocarbons can be seen as important factors for achieving higher energy conversion efficiency in the CH4 dry reforming process. The associated chemical reactions are simulated using CHEMKIN-PRO tool, and the results illustrate the tendency of CE to vary with variations in selected parameters, and syngas and C2 hydrocarbons production trends achieved in the simulation agree with experimental results.

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH4 flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.

Silicon Heterojunction Solar Cells with p-Type Silicon Carbon Window Layer

Boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) thin films are deposited using high frequency 27.12 MHz plasma enhanced chemical vapor deposition system as a window layer of silicon heterojunction (SHJ) solar cells. The CH4 gas flow rate is varied to deposit various a-SiC:H films, and the optical and electrical properties are investigated. The experimental results show that at the CH4 flow rate of 40 sccm the a-SiC:H has a high band gap of 2.1 eV and reduced absorption coefficients in the whole wavelength region, but the electrical conductivity deteriorates. The technology computer aided design simulation for SHJ devices reveal the band discontinuity at i/p interface when the a-SiC:H films are used. For fabricated SHJ solar cell performance, the highest conversion efficiency of 22.14%, which is 0.33% abs higher than that of conventional hydrogenated amorphous silicon window layer, can be obtained when the intermediate band gap (2 eV) a-SiC:H window layer is used.

Effect of MoS2 on tribological properties and corrosion resistance of MoS2/a-C:H films fabricated via reactive magnetron sputtering technology

The single-function protective coating does not meet the mechanical requirements. Herein,we have designed a film with excellent anti-wear and anti-corrosion properties to provide adequate protection for the machine. The MoS2/a-C:H films with low friction coefficient and anti-corrosion were successfully prepared by sputtering MoS2 target under different CH4 flow rates via the reactive direct-current magnetron sputtering. The composition, microstructure, tribological properties and corrosion resistance of as-prepared films were systemically studied using various analytical techniques. The results showed that the MoS2/a-C:H films could have typical nanocrystalline/amorphous characteristics. As the CH4 flow rate increased, the C content increased while MoS2 content exhibited an opposite law for the MoS2/a-C:H films. When the CH4 flow rate was 12 sccm, the as-prepared MoS2/a-C:H films could exhibit superior tribological performances with the friction coefficient of 0.02 and wear rate of 0.34 × 10−7 mm3 Nm−1 under atmospheric environment. Also, the film deposited at CH4 flow rate of 12 sccm presented superior corrosion resistance with the corrosion current density of 8.62 × 10−8 A · cm−2 after immersed in 3.5 wt% NaCl solution for 24 h. These results indicated that the as-prepared MoS2/a-C:H films might have broad prospects for expanding the applications as protection materials due to their low friction coefficient and excellent corrosion resistance.

Low friction coefficient of superhard nc-TiC/a-C:H nanocomposite coatings deposited by filtered cathodic vacuum arc

Nanocomposite coatings formed by metal (Ti, Al, Cr, etc)-doped diamond like carbon coatings can exhibit excellent tribological properties due to the combined improvement in hardness and toughness in optimized deposition conditions. In this work, Ti-doped a-C:H coatings are successfully deposited at C2H2 flow rate (fC2H2) of 50–250 sccm on Si (100) and AISI 304 L stainless steel substrates using filtered cathodic vacuum arc. Chemical bonding state, microstructure, and mechanical and tribological properties of Ti-doped a-C:H coatings were investigated using XPS, FESEM, nanoindentation and ball-on-disc tribometer, respectively. Nc-TiC/a-C:H nanocomposite coating with dense microstructure and smooth surface morphology are obtained at fC2H2 of 50–250 sccm thanks to high-efficiency magnetic filtered depositions. The sp2/sp3 ratio gradually decrease from 1.6 to 1.1, while (Ti–C)/(C–C) ratio shows a sharp decrease from 0.361 to 0.047. Hardness and H/E* and H3/E*2 ratios decrease from 41.2 GPa, 0.098 and 0.41 at 50 sccm to 22.9 GPa, 0.071 and 0.117 at 250 sccm, respectively. Residual stress ranges from −0.39 to −1.72 GPa. Superhard nc-TiC/a-C:H nanocomposite coating with the low residual stress, high H/E* and H3/E*2 exhibit low coefficient of friction of 0.07 and low wear rate of 5.4 × 10−9 mm3 N−1 m−1, which is ascribed to the high sp2/sp3 and (Ti–C)/(C–C) ratios, dense microstructure and smooth surface morphology.

Thermodynamic analysis of syngas production via chemical looping dry reforming of methane

Chemical looping dry reforming of methane (CL-DRM) for syngas production was studied based on thermodynamic equilibrium analysis under various fuel reactor temperatures and pressures. Fe2O3 was used as the oxygen carrier (OC). Four CL-DRM stages can be identified based on the ratio of circulated Fe2O3 flow rate to fed CH4 flow rate (OC/CH4): combined methane decomposition (MD) and partial oxidation of methane (POM), combined POM and complete oxidation of methane (COM) with POM dominance, combined POM and COM with COM dominance, and COM. For syngas production the OC/CH4 ratio must be small and its value depends on reaction temperature. Based on the results obtained, it was found that CH4 oxidation due to the presence of OC is the key reaction that affects CL-DRM performance. Compared with conventional DRM in which no OC was introduced, CL-DRM has advantages of producing higher CH4 conversion and lower carbon formation. However, lower CO2 conversion results. It was found that both conventional DRM and CL-DRM were not favourable as reaction pressure increases. The CH4 conversion in CL-DRM can be enhanced by increased CO2/CH4 ratio and water addition in the feedstock. However, CO2 conversion decreased due to increased CO2 amount or enhanced water gas shift reaction.

Effect of methane additive on GaN growth using the OVPE method

The oxide vapor phase epitaxy method is expected to be a useful technique for bulk GaN growth, because it allows long-term growth without producing a solid byproduct. However, thick GaN crystals have not been realized due to the growth inhibition caused by polycrystal formation resulting from high H2O partial pressure. In this study, we formed GaN crystals with CH4 gas to decrease the H2O partial pressure in a growth zone by the reaction of CH4 with H2O to produce CO and H2. As a result, H2O partial pressure decreased with increasing CH4 flow rate, and GaN layers could be grown without decrease of growth rate or degradation of the crystalline qualities at a flow rate of 50–100 sccm of CH4 gas. Furthermore, we obtained high crystalline 400-um thick GaN crystals after a growth period of 10 h.