Simulation of H---C---S containing gas mixtures relevant to diamond chemical vapour deposition

Mole fractions of 24 species present within x%H₂S/1%CH₄/H₂ (x=0–1%) and 1%CS₂/H₂ gas mixtures have been calculated using the CHEMKIN computer package in conjunction with a mechanism based on the composite conversion: CH₄+2H₂SCS₂+4H₂. Arrhenius parameters for each elementary reaction involving S-containing species are presented, along with associated thermodynamic properties for each species. Molecular beam mass spectrometric measurements of species mole fractions in microwave activated x%H₂S/1%CH₄/H₂ (x=0–1%) mixtures agree well with the model calculations, if we assume a (reasonable) gas temperature of 1630 K. The agreement between similar measurements of both 0.5%H₂S/1%CH₄/H₂ and 1%CS₂/H₂ hot filament activated gas mixtures is less good, but the calculations succeed in reproducing many of the observed trends in species mole fraction with change in filament temperature.     

Carbon/carbon composites prepared by chemical vapour deposition

This paper summarizes the results of the authors' work on composites prepared by chemical vapour deposition (CVD), using propylene as a source of carbon and various substrates (cellulose carbon, natural graphite, different grades of carbon fibres bonded by phenolic or CVD carbon). The equations relating

1. (i) open porosity P with infiltration rate (− dP/dt = kP).

2. (ii) apparent density of the composites d_app with P (d_app = nP + q).

3. (iii) composite properties Y with d_app (Y = ad_app^b) have been confirmed for the systems studied up to the highest composite densities attained. The constants k, n, q, a and b vary with infiltration conditions, nature of the substrate (involving bonding carbon in the skeleton), heat treatment (HT) conditions and geometrical factors.

For a given set of conditions, infiltration rate increases with temperature and the partial pressure of propylene, but decreases with residence time. Contrary to the composite properties, the kinetics of infiltration are not influenced by the nature of the substrate, except for the initial stage before the substrate is coated by the CVD carbon, but depends on the shape and size of open pores. The influence of the nature of the carbon used to bond the fibres may be of particular importance for composite properties, the latter being also influenced by the state of the surface of the fibres. Among the geometrical factors, the studied effect of fibre content has been found to strongly influence the infiltration rate, in agreement with predictions, as well as the composite properties. The influence of HT on composite properties can be explained by the resulting structural changes in the substrate and the matrix and by the effect of stress relaxation.

Surface acidic properties of alumina-supported niobia prepared by chemical vapour deposition and hydrolysis of niobium pentachloride

Niobia–aluminas were prepared by chemical vapour deposition at 150°C of niobium pentachloride on the surface of γ-aluminas calcined at different temperatures and with controlled degrees of hydration, followed by hydrolysis with water vapour at 150°C and a thermal treatment with steam at 440°C aimed at removing surface chloride contamination. The samples were characterised with respect to chemical composition, surface area, acidity by temperature-programmed desorption of ammonia, nature of acid sites by infrared spectroscopy of adsorbed pyridine and catalytic activity at 370°C in the dealkylation of cumene.

The results showed that, for each alumina calcination temperature, the catalysts with the lowest niobium content have a higher density of acid sites than the alumina support, but the acidity decreased, within each series with an increase in the niobium content. Comparatively to the TPD results, catalytic activity in cumene dealkylation was much more sensitive to the history and composition of the samples. The niobia-alumina samples were much less active than the alumina support, but this was most likely due to the severe hydrothermal treatment for chlorine removal, since their activity was close to that of an alumina submitted to the same treatment. A strong decrease in the acidic activity was observed with increase in the niobium content. A sample of pure niobium oxide had a much higher activity than the niobia-alumina samples. Brønsted acidic sites could only be observed by the IR spectra of adsorbed pyridine on the surface of the pure niobium oxide sample.     

Multiwalled carbon nanotubes synthesis with high yield utilizing Pt/Al-MCM-41 via catalytic chemical vapour deposition technique

Multiwalled carbon nanotubes (MWNTs) were synthesized over Pt impregnated Al-MCM-41 catalyst by decomposition of acetylene and characterized by XRD and nitrogen sorption isotherm to study the mesophase nature of the material. The optimum temperature and flow rate of the carbon source for CNTs synthesis are 800 °C and 60 mL/min, respectively, within a short reaction period, typically 10 min. Moreover, longer reaction time (i.e. 30 min) favours the formation of more amorphous carbon. When the reaction time is reduced to less than 10 min, formation of amorphous carbon is greatly suppressed by the high yield of MWNTs (85%). The products obtained from the decomposition of acetylene over these catalysts were characterized by TGA, SEM, TEM and Raman spectroscopy. The TEM analysis reveals that CNTs are free from amorphous carbon, whereas Raman spectrum shows two prominent peaks at 1,327 and 1,594 cm⁻¹ as the tangential modes of CNTs. As a conclusion, Pt/Al-MCM-41 is an effective template for MWNTs synthesis using acetylene as a carbon source.     

Chemistry and kinetics of chemical vapour deposition of pyrocarbon: VII. Confirmation of the influence of the substrate surface area/reactor volume ratio

Carbon deposition from a methane–hydrogen mixture (p_CH4=17.5 kPa, p_H2=2.5 kPa) was studied at an ambient pressure of about 100 kPa and a temperature of 1100°C, using deposition arrangements with surface area/reactor volume ratios, [A_S/V_R], of 10, 20, 40 and 80 cm⁻¹. Steady-state deposition rates and corresponding compositions of the gas phase as a function of residence were determined. The deposition rates in mol/h increase with increasing [A_S/V_R] ratio at all investigated residence times up to 1 s. However, surface-related deposition rates in mol/m²h decreased. As the same results have been obtained in a preceding study using pure methane at a partial pressure of 10 kPa, it has been confirmed that all the kinetics can be determined by changing the [A_S/V_R] ratio.     

Thermoluminescence properties of nitrogen containing chemical vapour deposited diamond films

The behaviour of centres due to nitrogen impurities introduced during the growth process into the chemical vapour deposited (CVD) diamond lattice is reported for the first time by thermoluminescence (TL) studies between 300 and 670 K after 200–400 nm ultraviolet (deuterium lamp) illumination at 300 K (RT) in air. TL curves exhibit some glow peaks at 490, 520 and 620 K, characterized by activation energies of approximately 1.2, 1.4 and 1.9 eV, respectively. Spectral analysis of these peaks which reveals some differences due to nitrogen content in the films shows well defined emission bands and interesting features leading to a better knowledge of the broad red photoluminescence (PL) band observed in our films. Nitrogen addition during the growth of the CVD material leads to the quenching of both of the 1.68 eV line related to Si centre and the broad green band centred at 2.25 eV which were observed for the high quality film.     

Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method

High purity, aligned multi-wall carbon nanotube films were grown on quartz substrates by injecting a solution of ferrocene in toluene into a suitable reaction furnace. The injection CVD method allows excellent control of the catalyst to carbon ratio. The detailed study presented here demonstrates how such a system can be used to control the nanotube diameter, length, alignment and yield by manipulating the experimental parameters. Primary growth was found to occur via a base growth mechanism, although overgrowths of single wall carbon nanotubes were obtained under certain conditions. Such a method also allows nanotubes of various packing densities to be produced which may be useful for specific applications such as electrodes.     

Effect of Y2O3 interlayer on the electric properties of Y-doped HfO2 film deposited by chemical solution deposition

Ferroelectric oxide becomes the focus of memory industry again after the discovery of ferroelectricity in doped-HfO₂ polycrystalline films. Thermal stress is an important factor for the variation of ferroelectric phase content. In this paper, the effect of film stress induced by Y₂O₃ interlayer in the ferroelectric properties of Y-doped HfO₂ (Y: HfO₂) ferroelectric films, which is deposited by chemical solution deposition (CSD), is investigated systematically by polarization-voltage measurement. Compared with Y-doped HfO₂ film without interlayer, 1 nm Y₂O₃ interlayer enhances the remanent polarization of Y: HfO₂ film due to the effects of film stress and surface energy. And thick Y₂O₃ interlayer benefits to reduce leakage current density. But polarization switching of HfO₂ film is restrained due to the capacitor voltage divider caused by thick Y₂O₃ interlayer. The obvious enhance effect of Y₂O₃ interlayer still exists in Y: HfO₂ film at high voltage due to the breakdown of Y₂O₃ interlayer, realizing a huge remanent polarization (P_r) of 22.8 μC/cm² in Y: HfO₂ film (doping content: 4 at %). It is 3.6 times than that of ferroelectric doped-HfO₂ film without Y₂O₃ interlayer.     

Transmissive diamond photocathodes

The first studies are reported on the characteristics of ultra-violet transmissive mode, CVD diamond photocathodes, in which photoelectrons are emitted from a diamond membrane via the opposite interface from that of the incident radiation. For membranes in the thickness range 5-40 μm, the transmissive photoyield at threshold wavelengths is shown to be less but comparable to that measured for reflective, thick film diamond photocathodes. The characteristics are found to be insensitive to the phase purity of differing diamond membranes used, but are very dependent on whether the growth or substrate interfaces of the diamond membrane face the incident UV radiation. The factors influencing the characteristics observed are discussed.     

Modelling chemical vapour infiltration of pyrolytic carbon as moving boundary problem

Chemical vapour infiltration (CVI) of pyrolytic carbon is described as a moving boundary problem to determine the evolution of the pyrolytic carbon layer in space and time. Derived from real geometries, a one-dimensional single pore model is developed yielding a nonlinear coupled system of partial differential equations for the concentrations of the gas phase species and the height of the carbon layer within cylindrical pores. The evolution of the moving boundary of the gas phase domain is governed by a non-differentiable minimisation condition. Additionally, a CVI reactor model to describe the infiltration of several cylindrical pores within a porous substrate is presented on the basis of the single pore model. Both models are new in that they combine the following features: (i) derivation of the equations rigorously taking into account the temporal change of the gas phase, (ii) the explicit construction of the position of the gas-solid interface, (iii) the influence of the local curvature of the carbon layer on its growth velocity, and (iv) modelling of chemical kinetics using a reduced reaction scheme with intermediate gas phase species and several surface reactions. The models are solved numerically using a staggered strongly decoupled scheme with implicit Euler time integration. The results allow the identification of process conditions and geometries for which a complete infiltration of the pores or the whole substrate is achieved. For low pressures, the predictions of the CVI reactor model are in agreement with the available experimental data.     

Zero-bias operation of polycrystalline chemically vapour deposited diamond films for Intensity Modulated Radiation Therapy

A detailed investigation of the performance as a dosimeter of state-of-art polycrystalline CVD (pCVD) diamond detectors operated in photovoltaic regime for applications in clinical radiotherapy has been carried out with conventional 6-10-18 MV X-photons, as well as with a 10 MV photon Intensity Modulated Radiation Therapy (IMRT) beam from a linear accelerator. Our results show that the performances of a pCVD diamond dosimeter improves dramatically when operated in null-bias conditions. Main improvements with respect to operation with an external voltage applied are: a reduced pre-irradiation dose; an excellent time stability, characterised by standard deviations less than 0.5%; a rise-time comparable to that of commercial reference dosimeters; a linearity with dose proven over three decades; a reduced deviation from linearity of the current vs. dose-rate curve, output factors comparable to that of commercial reference dosimeters. These results represent a significant step towards clinical applications as IMRT with synthetic polycrystalline CVD diamond films.     

Microwave heated chemical vapour infiltration of SiC powder impregnated SiC fibre preforms

Abstract

Abstract

A microwave heated methyl trichlorosilane based chemical vapour infiltration technique has been used to form SiC_f/SiC composites from SiC fibre preforms preimpregnated with SiC powder using two different fabrication techniques. While infiltration rates obtained from samples loaded with powder were generally higher than for preforms without powder, preferential infiltration occurred in regions where the SiC powder was most concentrated as a result of the initial non-uniform distribution of the powder across the preforms. The consequence was density gradients in the final composites. Nevertheless, average densities as high as 75% could be achieved in a 10 h process.     

Properties of Si/SiC ceramic composite subjected to chemical vapour infiltration

Si/SiC ceramic composite was prepared by infiltration of liquid silicon into carbon preforms that was made from cotton fabric and phenolic resin. This composite was subjected to the chemical vapour infiltration (CVI), using methyltrichlorosilane as a precursor gas. The effect of infiltration time on densification and mechanical properties was studied. Results show a significant improvement in density by pore closure. Flexural strength increases with increasing infiltration time. However, beyond 60 h of infiltration, the strength improvement was insignificant. The high temperature oxidation resistance of the above ceramics was also studied. The CVI treated samples show considerable resistance to oxidation compared to untreated samples. Thermogravimetric analysis also confirmed the better oxidation resistance of the CVI treated samples.     

Cutting performance of time-modulated chemical vapour deposited diamond coated tool inserts during machining graphite

In this investigation, two types of diamond coatings were deposited onto commercially available cemented tungsten carbide (WC-Co) tool inserts using two distinctive processes: i) conventional hot filament chemical vapour deposition (HFCVD) and ii) our recently developed time-modulated chemical vapour deposition process (TMCVD). The TMCVD process enabled the production of smaller sized diamond grains by the promotion of secondary nucleation processes occurring during the larger flow methane modulations. With the conventional HFCVD process, the methane concentration in the gas phase was kept constant during the entire diamond chemical vapour deposition (CVD) process and we labelled the resulting coating as “conventional diamond”, whereas the coating was referred to as “TMCVD diamond” when the methane content in the gas phase was time-modulated by changing its flow rate during CVD. Inserts coated employing both conventional HFCVD and TMCVD deposition process were characterized by scanning electron microscopy (SEM) and Raman spectroscopy and then were tested for turning performance using graphite as working material. For sake of comparison, polycrystalline diamond (PCD) and bare WC-Co inserts were also used for graphite turning. The different wear mechanisms have been compared and discussed in terms of diamond coating microstructure. Repeated turning tests showed that the TMCVD diamond coated inserts exhibit a better wear resistance with respect to inserts coated with conventional diamond coatings, PCD and bare WC-Co inserts.     

Field emission of polycrystalline diamond films grown by microwave-plasma chemical vapor deposition. I. Effects of surface morphology of diamond

The effects of surface morphology on the field emission of non-doped polycrystalline diamond films with thicknesses ranging from 5 to 55 μm were studied. Diamond films grown by a microwave-plasma chemical vapor deposition technique had both the diamond and non-diamond components with pyramidal and angular crystalline structures. Although the average crystallite size increased with increasing the film thickness (d), the volume fraction of the non-diamond components in the films was insensitive to the film thickness. However, the turn-on electric field, F_T, (defined as the low-end electric field to emit electrons) showed a U-shape dependence on the film thickness. This U-shape dependence was explained by a model in which the emission current was controlled by Fowler–Norheim tunneling of electrons at surface of the pyramids when d was thinner than 20 μm and by carrier transport in the polycrystalline diamond film when d was thicker than 20 μm. The lowest field of 4 V/μm was obtained in the film with 20 μm thick.     

Solid-state reactions of silicon carbide and chemical vapor deposited niobium

Niobium films were deposited on silicon carbide by chemical vapor deposition using niobium chloride and hydrogen at a temperature range of 900–1300°C. The solid-state reactions between the deposited niobium and silicon carbide matrix were studied by examining the obtained films using X-ray diffraction and energy dispersion spectroscopy. The results indicated that niobium silicides could be formed at the beginning, which blocked further reactions between carbon and niobium to form niobium carbides. When the deposition temperature was increased, silicon would diffuse outward, which allowed the formation of niobium carbides. The reaction process and mechanism are discussed based on the thermodynamics and kinetics.     

Chemical vapour infiltration of pyrocarbon: I. Some kinetic considerations

Chemical vapour infiltration of pyrocarbon is analysed with respect to (i) the nature of the hydrocarbon (carbon source), (ii) the pyrolysis chemistry and kinetics of homogeneous gas phase reactions, (iii) the kinetics of heterogeneous pyrocarbon deposition reactions, (iv) pore diffusion in the Fick and Knudsen regime, (v) total pressure, (vi) initial partial pressure of the hydrocarbon, (vii) inert diluent gases and (viii) hydrogen, both as reaction product and non-inert diluent gas. The analysis shows aspects for the optimization of chemical vapour infiltration, and it clearly shows methane to be the ideal hydrocarbon for various reasons.     

Chemical vapour deposition of pyrolytic carbon on carbon nanotubes: Part 3: Growth mechanisms

As a first step to identify the growth mechanism of various pyrolytic carbon deposit morphologies onto multiwall carbon nanotubes (MWNTs) presented in earlier papers, we determined their growth chronology by carrying-out synthesis experiments involving a large time range. We propose that the formation of any of the deposit morphologies is the consequence of the primary formation of hydrocarbon liquid droplets in the gas phase and their subsequent deposition onto the MWNTs. This makes the formation mechanisms of the various deposit morphologies depend on physical phenomena related to the wetting of nanotube surfaces by the droplets, where the [droplet diameter]/[nanotube diameter] ratio plays an important role. The droplets are the result of the recombination of species issued from the cracking of the gaseous precursor (methane), and their characteristics (number, size, and aromaticity) depend on experimental parameters such as temperature, time of flight, and gas phase composition. The results bring a new light to the currently admitted hypotheses for the mechanisms of pyrolytic carbon deposition, and revitalise the liquid droplet theory formerly proposed by Grisdale in the 1950s, at least in the range of conditions investigated.