Comparison of a-C:H films deposited from methane–argon and acetylene–argon mixtures by electron cyclotron resonance–chemical vapor deposition discharges

Hydrogenated amorphous carbon (a-C:H) films are deposited from methane–argon and acetylene–argon gas mixtures in a microwave electron cyclotron resonance plasma reactor. The films deposited with the two different gas mixtures under similar input parameter conditions have substantially different properties, including deposition rate, mass density, optical absorption coefficient, refractive index, optical bandgap and hydrogen content. The deposition parameters varied include rf-induced dc substrate bias voltage (0 to −60 V), pressure (1–5 mTorr) and argon/hydrocarbon gas flow ratio (0–1.0). The discharge properties of the two different gas mixtures, including electron temperature, ion saturation current, and residual gas composition of the exit gas flow, are measured to help explain the different deposition results from the two different gas mixtures. The use of lower pressures is found to be critical for obtaining denser, lower hydrogen content films from acetylene. For the methane-deposited films the addition of argon to the discharge increased the film's mass density and lowered the hydrogen content. In both methane- and acetylene-based deposition processes the rf-induced bias is also a critical determining factor of film properties.     

Photocatalytic activity of thin TiO2 films deposited using sol–gel and plasma enhanced chemical vapor deposition methods

Thin titanium dioxide films, deposited using RF PECVD and sol–gel techniques, were studied comparatively with respect to their bactericidal as well as self-cleaning properties. The effect of the deposition process on film morphology, chemical and crystalline structure, bactericidal activity and hydrophilic properties was investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), surface profilometry, optical microscopy and contact angle measurements. It was found that the bactericidal activity of amorphous TiO₂ films, produced using the RF PECVD method, as either comparable to or better than those of crystalline (anatase) films deposited by means of the sol–gel technique. One reason for such advantageous behavior of plasma deposited materials is thought to be their substantially higher surface roughness, as revealed by AFM measurements. The hydrophilic effect, induced with UV irradiation, was strongest in the case of sol–gel films, but the RF PECVD synthesized coatings were found to be only slightly less hydrophilic. The conclusion follows that both sol–gel and RF PECVD techniques are equally capable of producing titanium dioxide films of high photocatalytic quality.     

Structural and chemical analysis of amorphous B–N–C thin films deposited by RF sputtering

Ternary Boron–Nitrogen–Carbon (B–N–C) thin films were deposited, onto silicon substrates, by reactive radio frequency (RF) sputtering from a boron carbide (B₄C) target in a gas mixture of nitrogen and argon. The influence of the RF power (P_RF) on the structure and the chemical composition of these films are studied by Fourier transform Infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements. The two techniques reveal the presence of B, C and N atoms in the deposited films. The presence of nitrogen in the atmosphere of the deposition chamber produces ternary B–N–C films composed mainly with a mixture of B–N and CN bonds as revealed by these techniques. The boron content increases while carbon and nitrogen contents decrease with P_RF. The higher proportion of boron atoms produced a strong contribution of the boron nitride in the final compound B–N–C films.     

Growth mechanism of porous 3C–SiC films prepared via laser chemical vapor deposition

Porous SiC film is an excellent electrode material for robust micro-supercapacitors used in extremely harsh environments. In one of our previous studies, porous 3C–SiC film with high areal capacitance and high deposition rate was prepared via laser chemical vapor deposition (LCVD). However, it is still urgent to explore the formation mechanism of porous structures using LCVD. In this study, the microstructure of the porous cubic SiC film prepared via LCVD was analyzed in detail using scanning and transmission electron microscopy techniques. The growth mechanism of the deposits has been proposed according to the “growth competing theory” and “shadow effect theory.”     

Selective synthesis of single-walled carbon nanotubes on Fe–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality and diameter distribution by methane decomposition over Fe–MgO catalyst is reported. The catalyst was examined by nitrogen physisorption, X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. High resolution electron microscopy, Raman and optical absorption spectroscopy, temperature programmed oxidation, energy dispersive X-ray spectroscopy and nitrogen physisorption were used to probe reaction selectivity, SWCNT chirality and diameter distribution, carbon yield and effectiveness of purification protocols. The yield of carbon increased with an increase in temperature, although SWCNTs selectivity decreased above the optimum synthesis temperature. Results established a clear link between the degree of dispersion of iron oxide species inside the MgO lattice and the catalyst selectivity for SWCNT growth.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Prospective growth region for chemical vapor deposition synthesis of carbon nanotube on C–H–O ternary diagram

Chemical vapor deposition has become a standard process for synthesizing carbon nanotubes. Since the successful use of chemical vapor deposition for the first time, much effort has been expended into exploring various carbon sources that can be used to synthesize carbon nanotubes, such as methane, ethane, and ethanol. However, whole perspectives for suitable carbon sources have not been clear. In this study, we performed experiments in order to determine that the appropriate C–H–O components ratio in raw materials can be used to synthesize carbon nanotubes. We also examined a variety of raw materials in our newly developed round-trip-type vacuum furnace in order to determine whether they could be used to synthesize a carbon nanotube. We used Raman spectroscopy to identify the developed carbon nanotube, and we plotted the component ratios of effective and ineffective materials on a C–H–O ternary diagram; in this diagram, the growth region became highly apparent. It should be noted that for the growth of the carbon nanotube, this region should satisfy the equation O < C < (H + O) in molar ratio. Furthermore, it was observed that adjusting the component ratios by mixing raw materials did not cause an inconsistency in the growth region.     

Effect of WC grain growth inhibitors on the adhesion of chemical vapor deposition diamond films on WC–Co cemented carbide

Two sets of Co-cemented tungsten carbide (WC–Co) cutting inserts were sintered using WC powders having different average sized particles (1 and 6 μm). Fine grained WC–Co inserts contained 5.8 wt.% Co and were doped by 0.2 wt.% VC and 0.2 wt.% TaC, which acted as grain growth inhibitors in the liquid-phase sintering. Coarse grained substrates contained 6 wt.% Co and no dopants. Prior to deposition, the inserts were etched using Murakami reagent and then with an acid solution of hydrogen peroxide. The substrates were coated by 31–33-μm diamond films using hot filament chemical vapor deposition (HFCVD) in an atmosphere of 1.5% methane in hydrogen for 14 h, at a substrate temperature of 950 °C. Upon cooling from CVD temperature, only films deposited onto coarse grained inserts were adherent, while films grown on fine grained substrates underwent spontaneous delamination. This fact was due to the presence of a layer of graphitic carbon at the interface between the diamond film and fine grained substrates only. The formation of this sp²-carbon layer correlated well with the observed huge segregation of grain growth inhibitors at the interface between diamond and fine grained substrates.     

Intrinsic stress evolution in diamond films prepared in a CH4H2NH3 hot filament chemical vapor deposition system

The intrinsic stress in diamond films prepared in a CH₄H₂NH₃ hot filament chemical vapor deposition system has been investigated by the substrate curvature technique as a function of film thickness (2.2–50 μm) and ammonia concentration (0–1.4%). Our results indicated that the film stress changed from compressive to tensile with the increase of film thickness and diamond quality at a constant ammonia concentration of 0.5%. The existence of a non-diamond phase was found to be beneficial to the relaxation of intrinsic tensile stress in the films. The intrinsic stress in diamond films was tensile at an ammonia concentration from 0 to 1.4%, while the maximum tensile stress existed at 0.75% NH₃. The possible origin of intrinsic tensile stress was discussed.     

Effect of co-feeding carbon dioxide on Fischer–Tropsch synthesis over an iron–manganese catalyst in a spinning basket reactor

The effect of co-feeding CO₂ on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO₂ partial pressure in the feed gas. It was found that co-feeding CO₂ to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO₂ from the feed gas. The addition of CO₂ led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C₁₉⁺ selectivity. It also slightly decreased CO₂ formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO₂ can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions.     

Simulation of nanoparticle synthesis in an aerosol flame reactor using a coupled flame dynamics–monodisperse population balance model

Flame aerosol synthesis is one of the commonly employed techniques for producing ultra fine particles of commodity chemicals such as titanium dioxide, silicon dioxide and carbon black. Large volumes of these materials are produced in industrial flame reactors. Particle size distribution of product powder is the most important variable and it depends strongly on flame dynamics inside the reactor, which in turn is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel and the carrier gas and the burner geometry. A coupled flame dynamics–monodisperse population balance model for nanoparticle synthesis in an aerosol flame reactor is presented here. The flame dynamics was simulated using the commercial computational fluid dynamics software CFX and the particle population dynamics was represented using a monodisperse population balance model for continuous processes that predicts the evolution of particle number concentration, particle volume and surface area. The model was tested with published experimental data for synthesis of silica nanoparticles using different burner configurations and with different reactor operating conditions. The model predictions for radial flame temperature profiles and for the effects of process variables like precursor concentration and oxygen flow rate on particle specific surface area and mean diameter are in close agreement with published experimental data.     

The structure of 1D and 3D CuI nanocrystals grown within 1.5–2.5 nm single wall carbon nanotubes obtained by catalyzed chemical vapor deposition

Catalyzed chemical vapor deposition (CCVD) grown single wall carbon nanotubes (SWCNT) with diameter of D_m = 1.5–2.5 nm were used as templates to host one-dimensional nanocrystals of CuI. The CuI@SWCNT nanocomposite was obtained using capillary filling of preopened SWCNTs by CuI melt at 650 °C. Nanocomposite structural studies were performed on a FEI Titan 60–300 at 80 kV. According to the model and image simulation CuI crystallizes within 1.5–2.0 nm SWCNTs in the form of one-dimensional crystals with zinc blende or rock salt type unit cell connected by [0 0 1] edges and translated along 〈1 1 0〉. Copper cations occupy tetrahedral or octahedral sites in the lattice. In SWCNTs with D_m > 2.0 nm 3DCuI@SWCNTs were generated. The crystals of copper halides exhibit acceptor behavior as supported by Raman spectroscopy.     

Study of the electrophoretic deposition copper–carbon nanotubes composite coatings in deep eutectic solvent using a Taguchi experimental design approach

ABSTRACT

The present work discusses the electrophoretic deposition (EPD) of copper–carbon nanotubes (Cu–CNTs) composite coatings in a Deep Eutectic Solvent (DES) media, using a non-symmetric deposition process. A Taguchi experimental design is implemented in order to assess the effect of the different parameters on the microstructural characteristics of the coatings. The analysis of the design of experiments (DOE) is performed with the signal to noise (S/N) ratio and the analysis of variance. The results clearly reveal that the time of deposition is the most influential parameter on crystallite size, whereas the asymmetric factor has the highest effect on the preferential deposition of Cu or C and thus on the chemical composition. It is therefore concluded that by changing some of the parameters, EPD can be implemented to develop nanostructured composite coating having a desired crystallite size and morphology.     

Etching process of hydrogenated amorphous carbon (a-C:H) thin films in a dual ECR–r.f. nitrogen plasma

Hydrogenated amorphous carbon (a-C:H) films deposited from CH₄ in a dual electron cyclotron resonance (ECR)–r.f. plasma were treated in N₂ plasma at different r.f. substrate bias voltages after deposition. The etching process of a-C:H films in N₂ plasma was observed by in situ kinetic ellipsometry, mass spectroscopy (MS), and optical emission spectroscopy (OES). Ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the etched film surface. XPS analysis proves that the nitrogen treatment on the a-C:H film, induced by r.f. substrate bias, causes a direct nitrogen incorporation in the film surface up to 15–17 at.% to a depth of about 20–40 Å depending on the r.f. bias. Various bonding states between carbon and nitrogen, such as tetrahedral sp³ C–N, and trigonal sp² C–N were confirmed by the deconvolution analysis of C 1s and N 1s core level spectra. The evolution of etching rate and the surface roughness in the film measured by AFM exhibit a clear dependence on the applied r.f. bias. MS and OES show the various neutral species in the N₂ plasma such as HCN, CN, and C₂N₂, which may be considered as the chemical etching products during the N₂ plasma treatment of a-C:H film.     

Thermodynamic analysis on the codeposition of ZrC–SiC by chemical vapor deposition using the ZrCl4–C3H6–MTS–H2–Ar system

A thermodynamic calculation on co-deposition of ZrC–SiC from the ZrCl₄–C₃H₆–MTS–H₂–Ar system was performed using the FactSage thermochemical software and verification experiments were performed. The surface diagrams of condensed-phases in this system were expressed as functions of the deposition temperature, total pressure, reactant ratio of MTS/(MTS+C₃H₆) and ratio of H₂/(ZrCl₄+MTS+C₃H₆), and the composition of the products was determined by the diagram. The calculation results indicate that their yields strongly depend on the molar ratio of the injected reactants and temperature, and ZrC–SiC can be co-deposited under a proper condition. The experimental results show that ZrC–SiC coating was successfully co-deposited on graphite substrates and carbon fibers according to the thermodynamic calculation.     

Effect of the design of a feedstock injection device in a fluidized-bed reactor on the efficiency of the reaction using the dehydrogenation of <Emphasis Type="Italic">iso</Emphasis>-paraffins in a fluidized chromia–alumina catalyst bed as an example

Mathematical modeling is performed for the operation of two units of industrial chemical fluidized-bed reactors with different gas feedstock injection devices, i.e, three toroidal rings with nozzles in unit 1 and a false bottom with nozzles distributed over it in unit 2. Efficiency is analyzed (using the target product (iso-butylene) yield) for the operation of the two units over 4 months under industrial conditions and revealed the higher efficiency of unit 2. To dedetrmine the reasons for different product yields in the two units, a numerical solution is found by mathematical modeling to obtain characteristic pictures of catalyst particle concentrations and temperature fields in these units. It is concluded that unit 2 is characterized by a more uniform and dense distribution of the catalyst along with more uniform heating of the reactor. Pictures of the principal catalyst circulation flows are plotted to explain the considerable difference between the catalyst concentrations and gas temperature fields. Based on the numerical solution, the operational efficiency of the two units is subjected to comparative analysis, which showed good agreement with the results from an analysis of industrial reactors. The approach used in this work could be used in designing new units and optimizing existing units.