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.

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.     

Synthesis of SiC nanowires by a simple chemical vapour deposition route in the presence of ZrB2

The SiC nanowires (NWs) were fabricated by a simple chemical vapour deposition (CVD) method at high temperature using Si, phenolic resin, and ZrB₂ powder. The morphologies of the fabricated SiC NWs included SiC/SiO₂ chain-beads and straight wires with core-shell structures. The fabricated SiC NWs were micrometre-to-millimetre in length, with chains 100–300 nm in diameter and beads with diameters of less than 1 μm. The core-shell-structured SiC NWs consisted of crystalline SiC cores and thin amorphous SiO₂ shells. SiC crystals grew in the [111] direction governed by a vapour-solid (VS) mechanism. The added ZrB₂ promotes the generation of gaseous species at higher gas pressures, which contributes to the formation of SiC NWs by CVD. The fabricated SiC NWs exhibited good photoluminescence properties due to many stacking faults and the presence of amorphous SiO₂.     

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.     

The fracture stress of chemical vapour deposited diamond

The factors which control the fracture stress of chemical vapour deposited diamond have been studied using the 3-point bend geometry. Fracture stress values of 300–800 MPa for the growth side and 600–1200 MPa for the nucleation side were recorded for samples of thickness 0.4–2.4 mm. A Weibull modulus of 23 was calculated for the growth surface data, showing that the fracture stress variability was low for a brittle material. A theory based on these results demonstrates that the material behaviour is remarkably simple, depending only on the grain size and the sample thickness, regardless of wide variations in other parameters such as optical transmission and stress state. The paper also contains a possible explanation for this well-defined behaviour based on microstress variations resulting from differences in defect density in different growth sectors within a grain.     

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.     

Liquid polycarbosilane as a potential chemical liquid vapour deposition precursor for SiC

A low-viscous liquid polycarbosilane (LPCS) is being proposed as a silicon carbide (SiC) precursor for chemical liquid vapour deposition (CLVD) process. The LPCS was characterized by Gel permeation chromatography (GPC), Fourier transformation infrared (FT-IR) spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, GC-MS and Thermogravimetric analysis (TGA). Spectroscopic investigations together with Gas chromatography Mass spectrum (GC-MS) indicated that the synthesized LPCS is a mixture of cyclic/linear silanes and carbosilanes. TGA showed the complete evaporation of the LPCS below 250 °C, a suitable property for application as a CLVD precursor. Ceramic conversion of LPCS at different temperatures (900 °C, 1100 °C and 1300 °C) under argon, indicated the formation of nano-sized crystallites of β-SiC at 1300 °C.     

Covalent functionalization of boron nitride nanotubes through photo-initiated chemical vapour deposition

Boron nitride nanotubes (BNNTs) are analogous nanostructures to carbon nanotubes (CNTs), possessing similar properties such as Young's modulus and thermal conductivity, but superior resistance to oxidation and thermal stability. In addition, BNNTs are insulating materials, whereas CNTs are electrically conductive. They could be used as reinforcements in polymeric matrices as heat dissipators or as protective coatings in harsh environments. However, when incorporating them into polymers, one main drawback is their tendency to agglomerate. To improve their dispersion, covalent surface modification can be applied, with solvent-free approaches being preferred. Herein, we used syngas photo-initiated chemical vapour deposition (PICVD) to incorporate oxygen functionalities on the surface of BNNT. X-ray photoelectron spectroscopy analysis showed a highly oxidized BNNT surface after treatment. In addition, a decrease in water contact angle and an increase in surface energy were observed for the treated material. These results open new possibilities to incorporate hydrophilic BNNTs surfaces into polar polymers or other matrices of interest.     

Microstructure and mechanical properties of Si3N4f/BN composites with BN interphase prepared by chemical vapor deposition of borazine

The boron nitride (BN) interphase of silicon nitride (Si₃N₄) fiber-reinforced BN matrix (Si₃N_4f/BN) composites was prepared by chemical vapor deposition (CVD) of liquid borazine, and the microstructure, growth kinetics and crystallinity of the BN coating were examined. The effects of coating thickness on the mechanical strength and fiber/matrix interfacial bonding strength of the composites were then investigated. The CVD BN coating plays a key role in weakening the interfacial bonding condition that improves the mechanical properties of the composites. The layering structure of the BN coating promotes crack propagation within the coating, which leads to a variety of toughening mechanisms including crack deflection, fiber bridging and fiber pull out. Single-fiber push-out experiments were performed to quantify the fiber/matrix bonding strength with different coating thicknesses. The physical bonding strength due to thermal mismatch was discussed.     

Growth and structural properties of gallium oxide nanowires prepared by chemical vapour deposition

Abstract

Gallium oxide nanowires have been prepared on Si(100) substrates using metallo-organic chemical vapour deposition. Growth behaviour has been investigated with various deposition times and the structural morphologies of the nanowires have been studied. The thickness and surface coverage ratio of the deposits increased with increasing deposition time. The gallium oxide nanowires were amorphous phase, having a circular cross-section with diameter of about 50–250 nm.     

Ferroelectric properties of pure ZrO2 thin films by chemical solution deposition

Ferroelectricity in pure zirconia (ZrO₂) thin films, manufactured on Si (100) substrates via the chemical solution deposition method using all-inorganic aqueous salt precursor, has been demonstrated for the first time. The influence of thickness on the crystalline structure and ferroelectric properties of the thin films were measured and showed that they were strongly affected by the film thickness. The structural data indicated that as the film thickness increased from 30 nm to 50 nm, the m-phase fraction increased, and a phase transition from orthorhombic to cubic and then tetragonal occurred near the main diffraction peak of 30.7°. The lowest m-phase fraction of 15.4% was obtained in the pure ZrO₂ film with a thickness of 30 nm, and after 10³ field cycling, it exhibited the highest relative permittivity of 39.6 as well as the highest residual polarization of 8.5 μC/cm².     

Preparation by chemical solution deposition and transparent conducting properties of LaRh1-xNixO3 thin films

Transparent conducting thin films have been widely used in lots of fields. The absence of high-performance hole-type transparent conducting thin films, however, seriously limits the wider applications. LaRhO₃ as a type of perovskite material shows hole-type conduction with semiconductor-like properties and no investigations have been carried out about transparent conducting properties on LaRhO₃ thin films. Here, LaRh_1-xNi_xO₃ (x = 0, 0.05, 0.1) thin films were firstly deposited by chemical solution deposition, showing epitaxial growth on single crystal SrTiO₃ (001) substrates with the epitaxial relationship of LaRhO₃(001)[110]||SrTiO₃(001)[110]. With the doping of Ni element, the surface morphology became denser. Hall measurements confirmed that the hole concentration was enhanced with Ni doping, resulting in the decreased resistivity. Low resistivity of 17.3 mΩ cm at 300K was obtained for the LaRh_0.9Ni_0.1O₃ thin films. The electrical transport mechanisms were investigated, showing thermal activation at high temperatures and variable range hopping model for the doped thin films at low temperatures. The transmittance within the visible range for all thin films was higher than 50%. The results will provide a feasible route to deposit hole-type transparent conducting LaRhO₃-based thin films.     

Electrochromic properties of polycrystalline thin films of tungsten trioxide prepared by chemical vapour deposition

The electrochromic properties of polycrystalline thin films of tungsten trioxide prepared by chemical vapour deposition were studied using cyclic voltametry and chronoamperometry measurements. Two kinds of films were investigated depending on the conditions of preparation. Although the composition of the layers obtained after pyrolysis of W(CO)₆ is influenced by the presence of oxygen flow, the final annealing of these products leads to the same polycrystalline structure. Electrochromic properties have been investigated in acid and hydro-organic electrolytes. Cyclic voltametry shows that both colouration and bleaching of the films are associated with electrochemical reactions. The optical efficiencies and the response times were studied in both media and compared with amorphous WO₃ thin films prepared by vacuum evaporation. Best results have been obtained in acid electrolyte for films prepared by pyrolysis of W(CO)₆ in the presence of oxygen flow. We also observed that cycling greatly enhanced the response time. Current injection during colouration was found to depend strongly on time and to be mainly controlled by the resistance of the electrolyte at short times (f<200 ms).     

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.     

CVD preparation of alumina-supported niobium nitride and its activity for thiophene hydrodesulfurization

Niobium nitride was synthesized on a Si(400) substrate and a γ-alumina pellet using a CVD method with a stream of NbCl₅/Ar, NH₃, and H₂ gases at 723–973 K under reduced pressure. The composition and surface properties of the deposited niobium nitride were analyzed using XRD and XPS measurements. The activity of alumina-supported niobium nitrides for the hydrodesulfurization (HDS) of thiophene at 673 K and atmospheric pressure was determined. The alumina had a surface area of 177 m² g⁻¹ and the alumina-supported niobium nitride catalyst had surface areas of 179–190 m² g⁻¹. Although the catalysts had low activity in the initial stages, the activity increased after 200–300 min started to about three times the initial activities. XPS analysis indicated that the activity of the niobium nitride catalysts was decreased by sulfur accumulation on the surface and nitrogen released from niobium nitride. The relationship between the surface properties of the niobium nitride catalysts and the activities for thiophene HDS is discussed.     

Microstructure and integrity of multilayer ceramic coating with alumina top coat on silicon carbide by laser chemical vapor deposition

Thin double-layer coatings comprising an alumina top coat and mullite bond coat were deposited on SiC substrates by laser chemical vapor deposition (CVD). The effect of the presence of mullite bond coat and the phases of alumina top coat on the structural integrity against the thermal residual stress loaded by the high-temperature CVD process was examined by microstructural characterization and simulative consideration. A laminated layered structure having a dense γ-alumina top surface with a cone-like morphology are grown at a deposition temperature of 1323 K, whereas an α-alumina top layer comprising densely packed faceted grains was grown at 1473 K. The interfaces between the SiC and mullite layers were coherent owing to the formation of a thin transition layer. The γ-alumina layer formed an adhesive interface bordering the mullite layer, whereas small residual defects were formed in the α-alumina layer bordering the mullite layer. Spacings of surface cracking induced by the high-temperature deposition process in the double-layer coatings were approximately half of those in the coatings without mullite layers. As simulative results by finite element method suggested, the double-layer coatings were experimentally verified to be more tolerant to the formation of surface cracks and interfacial delamination, compared to single-layer coatings without the mullite bond layers.     

The properties of fluorine-containing diamond-like carbon films prepared by pulsed DC plasma-activated chemical vapour deposition

Diamond-like carbon films containing up to 23.1 at. % of fluorine (F-DLC), were deposited onto silicon substrates by low-frequency, pulsed DC, plasma-activated, chemical vapour deposition (PACVD). The influence of fluorine on plasma current density, deposition rate, composition, bonding structure, surface energy, hardness, stress and biocompatibility was investigated and correlated with the fluorine content. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence C–C, C–CF and C–F for F-DLC films with a low fluorine concentration (1.5–12.1 at. %), however for films with a higher fluorine content (23.0 at. %) an additional peak due to CF₂ bonding was detected. The addition of fluorine into the DLC film resulted in lower stress and hardness values. The reduction in these values was attributed to the substitution of strong C=C by weaker C–F bonds which induces a decrease in hardness. Ion scattering spectrometery (ISS) measurements revealed the presence of fluorine atoms in the outmost layer of the F-DLC films and there was no evidence of surface oxygen contamination. The water contact angle was found to increase with increasing fluorine content and has been attributed to the change of the bonding nature in the films, in particularly increasing CF and CF₂ bonds. Biocompatibility tests performed using MG-63 osteoblast-like cell cultures indicated homogeneous and optimal tissue integration for both the DLC and the F-DLC surfaces. This pulsed-PACVD technique has been shown to produce biocompatible DLC and F-DLC coatings with a potential for large area applications.     

Propylene epoxidation over Ti/MCM-41 catalysts prepared by chemical vapor deposition

Ti-containing mesoporous catalysts were prepared by chemical vapor deposition (CVD) of TiCl₄ on silica MCM-41 in the 700–900 °C temperature range. These samples were characterized (with XRD, ICP, nitrogen adsorption, FT-IR, ESCA, and TEM) and evaluated for the epoxidation of propylene with two alkyl hydroperoxides. The increase of CVD temperature resulted in the decrease of titanium content, catalyst hydroxyl population, crystallinity, and surface area. Catalyst selectivity to the desired product – propylene oxide – was highly sensitive to the deposition temperature. The best Ti/MCM-41 catalyst was prepared at the temperature of 800 °C, which had the maximum propylene oxide yield of 94.3%.     

HfB2 coating on C/C composites prepared by chemical vapor deposition: Thermodynamics and experimental investigation

A thermodynamic calculation on the HfB₂ coating prepared by chemical vapor deposition (CVD) through HfCl₄-BCl₃-H₂-Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB₂ coating could be obtained above 900 °C with the ratios of BCl₃/HfCl₄ and H₂/HfCl₄ higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H₂ and BCl₃ flow rates had significant effects on the grain size, growth rate and phase composition of HfB₂ coatings. A dense and uniform HfB₂ coating was prepared at 1150 °C with a BCl₃/HfCl₄ ratio of 3 and a H₂/HfCl₄ ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 μm/s under oxyacetylene flame.