Catalytic chemical vapour deposition synthesis of carbon spheres

Carbon spheres (CSs) were successfully grown in the presence of cobalt-silicon-mesoporous aluminum silicate (Co-Si-MAS) by chemical vapour deposition (CVD) using C₂H₂ as the source of carbon at 850 °C. The Co-Si-MAS catalysts with Si/Al molar ratios of 25, 50, 100 and 150 were synthesized by sol-gel preparation and followed by anchoring process. The scanning electron microscopy (SEM), transmission electron microscopy (TEM) images and Raman spectroscopy experiments showed that the obtained CSs possess nearly perfect spheres with diameters range from 650 to 1000 nm. It was found that CSs were fabricated under a thin film that was deposited during the catalytic reaction. On the basis of qualitative analysis it was revealed that the film included cobalt, silicon and aluminum elements. This thin film was applied as a catalyst for the synthesis of CSs.     

Thermogravimetric analysis of cobalt-filled carbon nanotubes deposited by chemical vapour deposition

In this paper, we report results from an investigation studying the purification of Co-filled carbon nanotubes (CNTs) using Thermogravimetric analysis (TGA). The as-grown CNTs were prepared using Microwave Plasma Chemical Vapour Deposition (MPCVD). Transmission electron microscopy (TEM), Fourier Transform Infrared (FTIR) and Raman spectroscopy were used to characterise the CNT samples. The CNTs produced by MPCVD were filled with cobalt and consisted of thick multi-walls. After TGA purification at 900 °C, 30 wt.% Co-filled CNTs remained in the TGA pan. However, while investigating the un-filled commercial CNTs (thin multiwalled), the sample completely burnt out at around 650 °C in the TGA furnace. The high thermal stability and the ability of thick-walled CNTs to act as an effective protective shield which prevents the oxidation of encapsulated cobalt have been demonstrated.     

Crystalline carbon nitride films prepared by microwave plasma chemical vapour deposition

Crystalline carbon nitride films have been synthesized on Si (100) substrates by a microwave plasma chemical vapour deposition technique, using mixture of N₂, CH₄ and H₂ as precursor. Scanning electron microscopy shows that the films consisted of hexagonal bars, tetragonal bars, rhombohedral bars, in which the bigger bar is about 20 μm long and 6 μm wide. The X-ray photoelectron spectroscopy suggests that nitrogen and carbon in the films are bonded through hybridized sp² and sp³ configurations. The x-ray diffraction pattern indicates that the films are composed of α-, β-, pseudocubic and cubic C₃N₄ phase and an unidentified phase. Raman spectra also support the existence of α- and β-C₃N₄ phases. Vickers microhardness of about 41.9 GPa measured for the films.     

Synthesis and characterization of carbon spheres prepared by chemical vapour deposition

Carbon spheres, with uniform diameters of about 1 μm, have been achieved via Chemical vapour deposition (CVD). The fabricated materials have been fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX). The results show that the spheres are 95% carbon. The formation mechanism of carbon spheres has also been discussed.     

Synthesis of carbon nanotubes by swirled floating catalyst chemical vapour deposition method

A series of developments have been made in synthesizing Carbon Nanotubes (CNTs) by Catalytic Vapour Deposition (CVD) methods since its discovery as a possible route to the large scale and high quality production of CNTs. In this study, CNTs were synthesized continuously in a swirled floating catalytic chemical vapour deposition reactor using acetylene as carbon source, ferrocene as catalyst, with argon and hydrogen as carrier gases within the temperature range of 900-1050 degrees C. The effects of pyrolysis temperature, acetylene flow rate, hydrogen flow rate, and ratio of flow of acetylene to hydrogen on the rate of production of CNTs were investigated. The CNTs produced were purified with dilute nitric acid and the nature and quality of the CNTs were analysed by TEM, Raman spectrometer, EDX, and TGA. Results obtained revealed that a mixture of single and multi wall carbon nanotubes were produced continuously with a maximum yield rate of 0.31 g/min at 1000 degrees C and a flow ratio of acetylene to hydrogen of one to five.     

Laser chemical vapour deposition

The special role of lasers in material processing is outlined in this article. In the background of the various chemical vapour deposition processes, the laser-induced chemical vapour deposition processes have been described. The unique aspects of pyrolytic and photolytic laser chemical vapour deposition have been stressed. Some of the recent experimental results on thin film deposition by laser have been reviewed. The problems and future of laser deposition processes have also been mentioned. Based on the talk given at the Winter School of Laser Material Processing, November 16–21, 1987, Pune.     

Monolithic material fabrication by chemical vapour deposition

The subatmospheric pressure chemical vapour deposition (CVD) process has been used to fabricate theoretically dense, highly pure, void free and large area infrared-optical and ceramic materials such as ZnS, ZnSe, CdS, CdZnTe, Si and SiC. In this paper, an overview of a large scale CVD process is presented emphasizing the important technical and engineering issues such as control of material properties, injector heating and its effect on growth, selection of an appropriate mandrel material, grain growth, material bowing, nodular growth, and storage, transport and scrubbing of a large quantity of hazardous chemicals. Further, the flow pattern in our CVD reactors is described and its importance in achieving good control over thickness and composition uniformity over large areas is discussed.     

Multi-phase structured silicon carbon nitride thin films prepared by hot-wire chemical vapour deposition

In this work, Silicon Carbon Nitride (Si-C-N) thin films were deposited by Hot Wire Chemical Vapour Deposition (HWCVD) technique from a gas mixture of silane (SiH₄), methane (CH₄) and nitrogen (N₂). Six sets of Si-C-N thin films were produced and studied. The component gas flow rate ratio (SiH₄:CH₄:N₂) was kept constant for all film samples. The total gas flow-rate (SiH₄ + CH₄ + N₂) was changed for each set of films resulting in different total gas pressure which represented the deposition pressure for each of these films ranging from 40 to 100 Pa. The effects of deposition pressure on the chemical bonding, elemental composition and optical properties of the Si-C-N were studied using Fourier transform infrared (FTIR) spectroscopy, Auger Electron Spectroscopy (AES) and optical transmission spectroscopy respectively. This work shows that the films are silicon rich and multi-phase in structure showing significant presence of hydrogenated amorphous silicon (a-Si:H) phase, amorphous silicon carbide (a-SiC), and amorphous silicon nitride (a-SiN) phases with Si-C being the most dominant. Below 85 Pa, carbon content is low, and the films are more a-Si:H like. At 85 Pa and above, the films become more Si-C like as carbon content is much higher and carbon incorporation influences the optical properties of the films. The properties clearly indicated that the films underwent a transition between two dominant phases and were dependent on pressure.     

Influence of deposition temperature on the hardness and dimensions of steels with hard coatings produced by chemical vapour deposition

CVD hard coatings are applied to steel parts at high and at medium reaction temperatures. The paper presents an introduction to the effect of CVD temperature on hardness and on the dimensions of the CVD-coated steel parts.In combined high temperature CVD and hardening processes, the coatings are deposited at temperatures which are equal to the austenitizing temperature of the steel base; the coated pieces are then quenched in the CVD reactor. Hence the hardness and dimensions of the steel bases depend on the CVD reaction temperature. The required hardness values of the base may be adjusted by tempering according to the hardness-austenitizing temperature-tempering temperature diagram. If the dimensions of the steel parts exceed about 20 mm, the dimensional changes due to heat treatment are larger than those involved in coatings of the usual thickness of 5–10 μm. Dimensional corrections may be made by means of cold treatment and tempering.In medium temperature CVD processes good dimensional stability of the treated steel parts may be achieved by pretempering the prehardened steel parts at temperatures that are equal to or very near to those which are applied in CVD, and by working the pretempered parts to dimensions which will change to within the allowed tolerances during deposition of the coatings. In these processes, high hardness values in D-type steel bases may be reached by the application of hardening temperatures higher than the usual ones.The anisotropic behavior of D-type tool steels may cause some problems in dimensional stability and in the distribution of internal stresses in the hard coatings; the stresses will be compressive in most kinds of CVD hard coatings.     

Tribological and protective coatings by chemical vapour deposition

When steel and nickel-based alloys are to be used as substrate materials for the chemical vapour deposition of hard wear-resistant and corrosion-protective coatings because of their high mechanical strength, the criteria for choosing the correct grade have to be established. In this paper we contribute towards this goal. We also discuss the friction and wear behaviour of a number of refractory coatings, such as TiC, TiN, SiC, Cr₇C₃, Fe_xB and Al₂O₃, under different environmental conditions.Some industrial tribological applications at moderately high temperatures (350°C in a helium atmosphere) or where the environment precludes the use of fluid lubricants are presented to illustrate the technical and economic feasibility and the usefulness of such coated machine elements and wear parts.Finally we deal with a very special application, the industrial exploitation of which is only just beginning: a multiple refractory compound coating to improve the protection of niobium tubes in the core of a nuclear reactor from damage due to metallurgical corrosion by liquid steel at high temperature (1600°C) which would result from accidental failure of a fuel pin.     

Carburization of Si microwires by chemical vapour deposition

We report the elaboration of silicon carbide (SiC) nanostructures thanks to the carburization of silicon microwires (MWs) under methane at high temperature. The produced SiC nanostructures display a tubular shape and are polycrystalline. The as-prepared silicon carbide microtubes (MTs) were characterized and studied by scanning electron microscopy (SEM), dual focused ion beam-scanning electron microscope (FIB-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The formation of microtubes can be explained by the out-diffusion of Si through the SiC during the carburization process.     

Electrochemical utilisation of chemical vapour deposition grown carbon nanotubes as sensors

We overview recent developments in the fabrication of the world’s smallest electrode, the carbon nanotube via chemical vapour deposition (CVD) and demonstrate how these electrodes are beneficially utilised and tailored towards the electrochemical sensing of target analytes. The use of carbon nanotubes arrays grown via CVD to beneficially tailor the arrays to their intended electro-analytical application is also highlighted.     

Preparation of alumina coatings by chemical vapour deposition

A short survey is given of improvements in the performance of cemented carbide tools for steel cutting as a result of a coating. The preparation of Al₂O₃ coatings by chemical vapour deposition is discussed in terms of nucleation, growth, preferred orientations and equilibrium growth shapes of the Al₂O₃ crystals as well as the influence of impurities on the layer formation. It is also pointed out that, although impurities can in principal have beneficial effects, they usually seem to be detrimental.     

Thermal analysis of hydrogenated amorphous carbon films prepared by plasma enhanced chemical vapour deposition

Hydrogenated amorphous carbon (a-C∶H) films were produced from propane and argon by an inductively coupled radio frequency (r.f.) glow discharge process under a particular deposition condition. Thermal analysis for the deposit by GC, DSC, DTA, and TG gave information for the structural changes upon heating. Most C-H vibration spectra disappeared by heating up to 600 °C. The gas desorption began above 300 °C and reached maxima above 650 °C with several peaks. The desorption reaction was endothermic. Up to 600 °C the desorbed gases was not the hydrogen. The large weight change was observed without the thickness reduction. The weight change rate was maximum at 480 °C. Hydrocarbons are believed to desorb below 600 °C. A hydrocarbon desorption model is suggested. Hydrocarbons are formed in the inner surface of a microvoid and effuse out through the interconnected microvoids in the column boundaries. The proposed desorption reaction is also endothermic.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

化学气相沉积法制备定向碳纳米管及其生长机理的研究

以环己烷为碳源,二茂铁作催化剂,采用浮动催化化学气相沉积法制备了定向碳纳米管,并用SEM、TEM及Raman光谱对样品进行了鉴定和表征.并从不同的角度,提出了定向碳纳米管遵循底部生长的机理.