Growth rate improvements in the hot-filament CVD deposition of nanocrystalline diamond

The hot-filament CVD, a less used technique for NCD films growth using Ar/H₂/CH₄ gas mixtures, is optimized for the coating of silicon nitride ceramics. Parameters such as gas composition (Ar/H₂ and CH₄/H₂ ratios), total gas pressure, total mass flow and substrate and filament temperatures, are studied to assess their effect on NCD growth kinetics as well as on film quality and morphology. The smallest diamond crystallite sizes (8 nm) were recorded for the slowest growth rate of 0.1 μm h^− 1. A remarkable result is the very high growth rate of 1.6 μm h^− 1 of continuous NCD coatings with 28 nm of crystallite size, obtained in selected deposition conditions.     

Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation

We discuss growth of high-quality carbon nanotube (CNT) films on bare and microstructured silicon substrates by atmospheric pressure thermal chemical vapor deposition (CVD), from a Mo/Fe/Al₂O₃ catalyst film deposited by entirely electron beam evaporation. High-density films having a tangled morphology and a Raman G/D ratio of at least 20 are grown over a temperature range of 750-900 °C. H₂ is necessary for CNT growth from this catalyst in a CH₄ environment, and at 875 °C the highest yield is obtained from a mixture of 10%/90% H₂/CH₄. We demonstrate for the first time that physical deposition of the catalyst film enables growth of uniform and conformal CNT films on a variety of silicon microstructures, including vertical sidewalls fabricated by reactive ion etching and angled surfaces fabricated by anisotropic wet etching. Our results confirm that adding Mo to Fe promotes high-yield SWNT growth in H₂/CH₄; however, Mo/Fe/Al₂O₃ gives poor-quality multi-walled CNTs (MWNTs) in H₂/C₂H₄. An exceptional yield of vertically-aligned MWNTs grows from only Fe/Al₂O₃ in H₂/C₂H₄. These results emphasize the synergy between the catalyst and gas activity in determining the morphology, yield, and quality of CNTs grown by CVD, and enable direct growth of CNT films in micromachined systems for a variety of applications.     

Vertically-aligned carbon nanotubes prepared by water-assisted chemical vapor deposition

Vertically-aligned single walled carbon nanotubes (V-SWNTs) were prepared by water-assisted and ethylene-pyrolyzing chemical vapor deposition. Water vapor was introduced into the growth environment by passing a small fraction of Ar carrier gas through a water bubbler. The effects of processing gas flow rate, growth time, and water vapor content on the growth of SWNTs using Mo/Fe as the catalyst were systematically studied. For the catalyst of SWNTs growth, multi-metal layers consisting of an Al underlayer with 10 nm in thickness and Mo/Fe layers on top were deposited onto the silica film. Evidences of V-SWNTs with high-purity were found by Raman spectroscopy and TEM morphology. The trace amount of water vapor enhanced the activity of metal catalysts to further assist the growth of SWNTs. For the Mo (0.5 nm)/Fe (1 nm) structure, V-SWNTs grew from surface when the growth time lasted 10 min shown in SEM diagram meanwhile the portion of SWNTs increased until 30 min and then a layer of SWNTs developed shown in the RBM peak in the Raman spectra.     

A Temperature Window for the Synthesis of Single-Walled Carbon Nanotubes by Catalytic Chemical Vapor Deposition of CH4 over Mo2-Fe10/MgO Catalyst

A temperature window for the synthesis of single-walled carbon nanotubes by catalytic chemical vapor deposition of CH₄ over Mo₂-Fe₁₀/MgO catalyst has been studied by Raman spectroscopy. The results showed that when the temperature is lower than 750 °C, there were few SWCNTs formed, and when the temperature is higher than 950 °C, mass amorphous carbons were formed in the SWCNTs bundles due to the self-decomposition of CH₄. The temperature window of SWCNTs efficient growth is between 800 and 950 °C, and the optimum growth temperature is about 900 °C. These results were supported by transmission electron microscope images of samples formed under different temperatures. The temperature window is important for large-scale production of SWCNTs by catalytic chemical vapor deposition method.     

Parametric study of synthesis conditions in plasma-enhanced CVD of high-quality single-walled carbon nanotubes

High-quality single-walled carbon nanotubes (SWCNTs) have been synthesized from H₂-CH₄ mixtures on a MgO-supported bimetallic Mo/Co catalyst using microwave plasma-enhanced chemical vapor deposition (PECVD). Reaction parameters including temperature, H₂:CH₄ ratio, plasma power, and synthesis time have been examined to assess their influence on SWCNT synthesis. Raman spectroscopy and high-resolution field emission scanning electron microscopy reveal that the quality, selectivity, density and predominant diameter of SWCNTs depend on the varied synthesis parameters. Results of this study can be used to optimize SWCNT synthesis conditions and products and to improve understanding of the growth of SWCNTs by PECVD.     

Synthesis,characterization and field emission of single wall carbon nanotubes

In this work, high-quality SWCNTs were synthesized by pyrolysis of ethanol at 900 °C in a tubular furnace. The catalyst was manufactured by depositing Fe and Mo metal on the surface of sol–gel MgO nanopowder. The SWCNTs produced by sol–gel MgO nanopowder have better yield, higher purity, and smaller diameters than those produced by commercial milling MgO powder. The lengths of SWCNTs are up to several microns with diameters in the range of 0.73–1.40 nm. The extremely small turn-on field E_to (0.01 V/μm) and threshold field E_th (0.07 V/μm) show that the SWCNTs have potential applications in field emission displays (FEDs).     

Nucleation of nanocrystalline diamond on masked/unmasked Si3N4 ceramics with different mechanical pretreatments

Results are presented concerning different mechanical pretreatments performed on silicon nitride substrates and their influence on the nucleation and growth of nanocrystalline diamond (NCD). All substrates were equally sintered and finished, but differently pretreated. Then, they were diamond coated in a microwave chemical vapor deposition system (MPCVD) for relatively short periods, using Ar/H₂/CH₄ gas mixtures. The main objective was to identify the best pretreatment among those proposed, while verifying how it correlates with film uniformity and surface roughness after post-growth. The effect of a molybdenum mask during growth is investigated.The top surface analysis revealed major differences in the nucleation morphology of diamond nuclei on the pretreated samples, two different nucleation types having been identified. For all pretreatments, samples exhibited a very smooth and uniform underlayer of very fine grain particles before the formation of larger aggregates, suggesting a bi-phase nucleation mechanism. When no mask is used considerable changes in the nucleation concentration are found, the resulting films showing grain enlargement near the edges, where the morphology assumes microcrystalline nature. This effect is suppressed by the use of a mask that allowed obtaining very uniform smooth films (R_rms∼ 30 nm, thickness ∼ 1.3 μm, MUS pretreatment), indicating a strong edge effect for the unmasked case. This fact can be attributed both to increased local temperature, plasma density and gas turbulence.     

Oxy-CO2 reforming of methane to syngas over CoOx/MgO/SA-5205 catalyst

The oxy-CO₂ methane reforming reaction (OCRM) has been investigated over CoO_x supported on a MgO precoated highly macroporous silica–alumina catalyst carrier (SA-5205) at different reaction temperatures (700–900 °C), O₂/CH₄ ratios (0.3–0.45) and space velocites (20,000–100,000 cc/g/h). The reaction temperature had a profound influence on the OCRM performance over the CoO/MgO/SA-5205 catalyst; the methane conversion, CO₂ conversion and H₂ selectivity increased while the H₂/CO ratio decreased markedly with increasing reaction temperature. While the O₂/CH₄ ratio did not strongly affect the CH₄ and CO₂ conversion and H₂ selectivity, it had an intense influence on the H₂/CO ratio. The CH₄ and CO₂ conversion and the H₂ selectivity decreased while the H₂/CO increased with increasing space velocity. The O₂/CH₄ ratio and the reaction temperature could be used to manipulate the heat of the reaction for the OCRM process. Depending on the O₂/CH₄ ratio and temperature the OCRM process could be operated in a mildly exothermic, thermal neutral or mildly endothermic mode. The OCRM reaction became almost thermoneutral at an OCRM reaction temperature of 850 °C, O₂/CH₄ ratio of 0.45 and space velocity of 46,000 cc/g/h. The CH₄ conversion and H₂ selectivity over the CoO/MgO/SA-5205 catalyst corresponding to thermoneutral conditions were excellent: 95% and 97%, respectively with a H₂/CO ratio of 1.8.     

Synthesis and mechanical wear studies of ultra smooth nanostructured diamond (USND) coatings deposited by microwave plasma chemical vapor deposition with He/H2/CH4/N2 mixtures

Ultra smooth nanostructured diamond (USND) coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H₂/CH₄/N₂ gas mixture. The RMS surface roughness as low as 4 nm (2 micron square area) and grain size of 5–6 nm diamond coatings were achieved on medical grade titanium alloy. Previously it was demonstrated that the C₂ species in the plasma is responsible for the production of nanocrystalline diamond coatings in the Ar/H₂/CH₄ gas mixture. In this work we have found that CN species is responsible for the production of USND coatings in He/H₂/CH₄/N₂ plasma. It was found that diamond coatings deposited with higher CN species concentration (normalized by Balmer H_α line) in the plasma produced smoother and highly nanostructured diamond coatings. The correlation between CN/H_α ratios with the coating roughness and grain size were also confirmed with different set of gas flows/ plasma parameters. It is suggested that the presence of CN species could be responsible for producing nanocrystallinity in the growth of USND coatings using He/H₂/CH₄/N₂ gas mixture. The RMS roughness of 4 nm and grain size of 5–6 nm were calculated from the deposited diamond coatings using the gas mixture which produced the highest CN/H_α species in the plasma. Wear tests were performed on the OrthoPOD®, a six station pin-on-disk apparatus with ultra-high molecular weight polyethylene (UHMWPE) pins articulating on USND disks and CoCrMo alloy disk. Wear of the UHMWPE was found to be lower for the polyethylene on USND than that of polyethylene on CoCrMo alloy.     

Nano-stuctured Pt-Fe/C as cathode catalyst in direct methanol fuel cell

Pt-Fe/C catalysts were prepared by a modified polyol synthesis method in an ethylene glycol (EG) solution, and then were heat-treated under H₂/Ar (10 vol.%) at moderate temperature (300 °C, Pt-Fe/C300) or high temperature (900 °C, Pt-Fe/C900). As comparison, Pt-Fe/C alloy catalyst was prepared by a two-step method (Pt-Fe/C900B). X-ray diffraction (XRD) and transmission electron microscopy (TEM) images show that particles size of the catalyst increases with the increase of treatment temperatures. Pt-Fe/C300 catalyst has a mean particle size of 2.8 nm (XRD), 3.6 nm (TEM) and some Pt-Fe alloy was partly formed in this sample. Pt-Fe/C900B catalyst has the biggest particle size of 6.2 nm (XRD) and the best Pt-Fe alloy form. Cyclicvoltammetry (CV) shows that Pt-Fe/C300 has larger electrochemical surface area than other Pt-Fe/C and the highest utilization ratio of 76% among these Pt-based catalysts. Rotating disk electrode (RDE) cathodic curves show that Pt-Fe/C300 has the highest oxygen reduction reaction (ORR) mass activity (MA) and specific activity (SA), as compared with Pt/C catalyst in 1.0 M HClO₄. However, Pt-Fe/C catalyst does not appears to be a more active catalyst than Pt/C for ORR in 1.0 M HClO₄ + 0.1 M CH₃OH. Pt-Fe/C300 exhibits higher ORR activity and better performance than other Pt-Fe/C or Pt/C catalysts when employed for cathode in direct methanol single cell test, the enhancement of the cell performance is logically attributed to its higher ORR activity, which is probably attributed to more Pt⁰ species existing and Fe ion corrosion from the catalyst.     

Spray coating as a simple method to prepare catalyst for growth of diameter-tunable single-walled carbon nanotubes

Spray coating is proposed as an optional wet method for preparing nano-sized particles suitable for the growth of single-walled carbon nanotubes (SWCNTs). The obtained SWCNT films are characterized by Raman spectroscopy and electron microscopy, and are confirmed to be comparable to SWCNTs produced by the conventional dip-coating process in terms of crystallinity, tube diameter and carbon yield. The mean diameter of SWCNTs can be effectively reduced from 1.85 to 1.35 nm by prolonging the deposition of Mo. In addition, spray coating allows catalyst preparation on supports other than flat wafers, as demonstrated by the synthesis of high-quality SWCNTs on Al₂O₃ fiber and quartz wool supports.     

Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH₄ plasma. When grown in H₂-rich plasma (H₂ = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H₂-plasma. In the Ar/CH₄ plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5-10 nm. The H₂-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H₂-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.     

Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.     

Effect of thermal treatment on states of molybdenum in Mo/H–ZSM-5 catalyst for methane dehydroaromatization: ESR and UV–VIS study

Valence and coordination states of molybdenum ions formed upon thermal treatment of Mo/H–ZSM-5 catalyst for methane dehydroaromatization in Ar and Ar/CH₄ media at 573–973 K have been studied by ESR and UV–VIS spectroscopy. For comparison, the characteristic ESR spectra of thermolyzed bulk ammonium heptamolybdate have been studied and analyzed in detail. The nature of earlier observed Mo⁵⁺ ions has been verified, and new paramagnetic states of molybdenum in Mo/H–ZSM-5 catalysts have been detected: Mo³⁺ ions, and Mo⁵⁺ ions in tetrahedral coordination with delocalization of unpaired electron to Al and H or Al and N atoms.     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

Optical and mass spectroscopy measurements of Ar/CH4/H2 microwave plasma for nano-crystalline diamond film deposition

Nano-crystalline diamond (NCD) thin film with grains of about 5–100 nm in size attracts much attention as new functional materials in various industrial fields, due to its unique properties that are different from the conventional microcrystalline diamond (MCD) thin film. Most commonly, NCD film can be synthesized using CH₄/Ar plasma with/without a small amount of hydrogen gas added. In this study, we carried out the measurements of quadrupole mass spectroscopy (QMS) and optical emission spectroscopy (OES) to investigate CH₄/H₂/Ar mixture plasma in detail. Combined with our experimental results of NCD film synthesis, the mechanism of CH₄ dissociation and the precursors of NCD were further explored.     

Defect structure for the ultra-nanocrystalline diamond films synthesized in H2-containing Ar/CH4 plasma

The modification on the microstructure of diamond films due to the addition of H₂ species into the Ar/CH₄ plasma was investigated. While the Ar/CH₄ plasma produced UNCD films with equiaxed grains (about 5 nm in size), the (Ar-H₂)/CH₄ plasma produced acicular-shaped grains (about 5 × 20 nm in size). Transmission electron microscopy studies indicate that these acicular-shaped grains actually are agglomerates of diamond flakes, which contain stacking faults lying on the (111) lattice plane. Presumably, the incorporation of H₂ species in the plasma leads to partial etching of hydrocarbons adhered onto the diamond clusters, such that the C₂- (or active carbon) species contained in the plasma can attach to the diamond surface anisotropically, leading to diamond flakes. The incorporation of H₂ in Ar plasma can also suppress the formation of i-carbons, an allotropic phase of diamonds. The critical proportion of H₂ in Ar plasma for inducing the changes in the granular structure is around 0.03%. The proportion of grain boundaries was thus reduced and the electron field emission properties of the materials were thus degraded. However, the suppression of the film electrical conductivity without sacrificing the smooth surface characteristic has the applications as high-thermal-conductivity heat spreaders and substrates for surface-acoustic-wave devices.     

Moderating carbon supply and suppressing Ostwald ripening of catalyst particles to produce 4.5-mm-tall single-walled carbon nanotube forests

Millimeter-tall single-walled carbon nanotube (SWCNT) forests were grown by chemical vapor deposition (CVD) from C₂H₂/H₂O/Ar using Fe/Al–Si–O catalysts. Using combinatorial catalyst libraries coupled with real-time monitoring of SWCNT growth, the catalyst and CVD conditions were systematically studied. The keys for this growth are to maintain the C₂H₂ pressure below its upper limit to prevent the killing of the catalysts and to grow the SWCNTs before the catalyst particles lose their activity because of coarsening through Ostwald ripening. Lower temperatures lead to lower limits for the C₂H₂ pressure which result in lower growth rates but also lead to even lower coarsening rates which result in even longer growth lifetimes. Using these principles, we grew 4.5-mm-tall SWCNT forests in 2.5 h at 750 °C.     

Effects of hydrogen on carbon nanotube formation in CH4/H2 plasmas

Carbon nanotubes (CNTs) were synthesized using CH₄/H₂ plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH₄/H₂ = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H₂ supply to the CH₄/H₂ mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH₄/H₂ = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H₂ plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H₂ plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C-H bonds were produced in CNTs during their growth.     

Synthesis of SiC whiskers by VLS and VS process

This study investigates the mechanisms of SiC whisker formation in the carbothermal reduction of quartz to SiC in different gas atmospheres. Reduction of quartz by graphite was studied in Ar, H₂, and CH₄–H₂–Ar gas mixture in a laboratory fixed bed reactor. The reduction products were characterised by XRD, SEM and TEM. Whiskers were not formed in the carbothermal reduction of quartz in argon. Two types of SiC whiskers were observed in the carbothermal reduction of quartz in H₂ and CH₄–H₂–Ar gas mixture. In the process of reduction at 1400–1600 °C in H₂ and at 1200–1600 °C in CH₄–H₂–Ar gas mixture, whiskers with hexagonal shape with diameter 100–800 nm and length up to tens of microns were formed by the VLS mechanism under catalytic effect of iron. The whiskers with the characteristics of cylindrical shape and high aspect ratio were synthesized in CH₄–H₂–Ar gas mixture at 1400–1600 °C by VS mechanism.