Mechanism studies on CVD of boron carbide from a gas mixture of BCL3, CH4, and H2 in a dual impinging‐jet reactor

Nearly pure boron carbide free from impurities was produced on a tungsten substrate in a dual impinging‐jet chemical vapor deposition reactor from a BCl₃, CH₄, and H₂ mixture. The Fourier Transform Infrared (FTIR) analysis proved the formation of reaction intermediate BHCl₂, which is proposed to occur mainly in the gaseous boundary layer next to the substrate surface. Among a large number of reaction mechanisms proposed only the ones considering the molecular adsorption of boron carbide on the substrate surface gave reasonable fits. In the proposed mechanism dichloroborane is formed in the gas phase only as a by‐product. Boron carbide, on the other hand, is formed through a series of surface reactions involving adsorbed boron trichloride, adsorbed methane and gas phase hydrogen. The simultaneous fit of the experimental rate data to the model expressions gave correlation coefficient values of 0.977 and 0.948, in predicting the B₄C and BHCl₂ formation rates, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Gas-phase kinetic of boron carbide chemical vapor deposition using BCl3+CH4+H2 mixture

A detailed chemical kinetic mechanism consisting of 472 reactions and 61 species for the gas-phase thermal decomposition of BCl₃+CH₄+H₂ mixtures is used to model the chemical vapor deposition (CVD) of boron carbide. The mechanism is constructed using a reaction mechanism generator (RMG), which is automatic and requires no or less human intervention. New functionality, considerable thermodynamic and kinetic data have been added to RMG to account for boron chemistry. The model considers all necessary reactions in the reaction pathways and reasonably predicts the experimental data available from the literature. The sensitivity analysis identifies crucial species responsible for boron carbide formation. Besides, a reduced mechanism (15 species and 26 reactions) is derived from a detailed mechanism, which could be used to expedite an effective CVD reactor design through numerical simulations. The performance of the reduced model is also evaluated concerning the complete mechanism, which shows that the reduced model can predict the reactor behavior reasonably well.     

Production of B4C coatings by CVD method in a dual impinging‐jet reactor: Chemical yield,morphology, and hardness analysis

β‐rhombohedral boron carbide (B₄C) was deposited on a tungsten substrate from a BCl₃? H₂? CH₄ gas mixture in a dual impinging‐jet chemical vapor deposition reactor. On‐line FTIR analysis of the product stream proved the formation of BHCl₂ and HCl as by products, in a competing parallel reaction. A maximum of 13% chemical yield of boron carbide was observed, and the yield was found to have increasing trend with an increase in temperature. XRD analysis proved the existence of rhombohedral B₄C phase at 1300°C without any other B₄C phases or impurities. At this temperature, the formation of 5‐fold icosahedral boron carbide crystals up to 30 micron sizes was observed. Such highly symmetric crystalline regions were observed to have a very high hardness value of 4750 kg/mm² as revealed from the microhardness analysis. The change in product morphology at low substrate temperatures resulted in a decrease in the hardness values. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

CVD of boron and dichloroborane formation in a hot-wire fiber growth reactor

Chemical vapor deposition (CVD) of boron by hydrogen reduction of BCl₃ on a hot tungsten substrate was investigated in a parallel flow reactor. Effect of substrate temperature (1100–1250°C) on the relative rates of formation of BHCl₂ and boron was observed by the on-line analysis of the reactor effluent stream composition using an FT-IR spectrophotometer. It was concluded that BHCl₂ was majorly formed in the gas phase within the thermal boundary layer adjacent to the substrate with possible contribution of surface reactions at higher temperatures. Comparison of results obtained in the impinging jet and parallel flow reactors indicated the significance of diffusion resistance in the parallel flow system. Tubular flow reactor experiments indicated that BHCl₂ formation reaction started at temperatures as low as 350°C and reached equilibrium in less than a second at temperatures over 420°C.     

Crystallization of hexagonal boron nitride exhibiting excitonic luminescence in the deep ultraviolet region at room temperature via thermal chemical vapor phase deposition

Boron nitride exhibiting intense exciton-related luminescence at 216–227 nm in the ultraviolet (UV) region was synthesized on nickel substrates by chemical vapor phase deposition using the BCl₃-NH₃ system. We investigated the effects of the deposition temperature and flow rate ratio of source gases on the cathodoluminescence property in the deep ultraviolet (DUV) light region measured at room temperature. UV-luminous hBN can form at temperatures of 1170 °C and above with a ratio of NH₃ gas flow to BCl₃ flow of 2 or below. Drastic surface roughening accompanies the formation of UV-luminous hBN with high crystallinity, showing that the etching process of nickel substrates by BCl₃ at high temperatures is related to the formation of UV-luminous hBN.     

Elucidation of hydrogen transfer behavior of coal with tritiated gaseous hydrogen in the absence and the presence of a catalyst using a fixed-bed reactor

Hydrogen transfer behaviors of four Argonne coals with gas phase tritiated H₂ in thermal (non-catalytic) and catalytic reactions (in the presence of 3 wt% Pt/Al₂O₃) in temperature range from 250 to 400 °C have been investigated using a fixed-bed reactor. The result showed that the efficiency of hydrogen transfer reaction of coal with the gas phase in the thermal run increased with increasing oxygen functionalities, providing the indication that some oxygen functional groups have a role for effective promotion of hydrogen transfer reaction with the gas phase H₂ under mild condition. When the reaction was run in the presence of the catalyst, the efficiencies of hydrogen transfer reaction for the all coals, except for the POC coal, were significantly enhanced even at temperature as low as 250 °C. The results for the catalytic reaction have provided indications into the reactive sites (formation of free-radicals) in coal and patterns for coal matrix degradation reactions.     

Effect of temperature on the growth of boron nitride interfacial coatings on SiC fibers by chemical vapor infiltration

In order to improve bonding property between SiC fibers and matrix of SiC_f/SiC composites, boron nitride (BN) interfacial coatings were synthesized by chemical vapor infiltration. BN coatings were fabricated from BCl₃–NH₃ gaseous mixtures at four different temperatures (843 °C, 900 °C, 950 °C and 1050 °C) with different deposition times. Growth kinetics, nucleation and growth processes, microstructure and chemical composition of boron nitride coatings were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectrometry. Results showed that deposition rate increased as the temperature increased from 843 °C to 950 °C. However, deposition rate decreased slightly from 23.10 ± 0.85 nm/min (950 °C) to 21.39 ± 0.67 nm/min when the temperature was increased further to 1050 °C. It could be due to the nucleation occurring in the gas and the consumption of a large amount of BCl₃ and NH₃. When deposition temperature was 843 °C, BN grains deposited on top layer of the coating could not completely cross Ehrlich-Schwoebel barrier and grew in island growth mode. On the other hand, the deposition pattern followed a layer-by-layer growth mode when deposition temperature was 1050 °C. Deposition temperature significantly affected the microstructure of as-deposited BN coatings. At 843 °C, 950 °C and 1050 °C, the coatings presented amorphous, polycrystalline and hexagonal structures, respectively.     

Gas-to-Particle Conversion in the Particle Precipitation–Aided Chemical Vapor Deposition Process I. Synthesis of the Binary Compound Titanium Nitride

Particle precipitation-aided chemical vapor deposition (PP-CVD) is a modification of the conventional CVD process, where an aerosol is formed in the gas phase at an elevated temperature, and particles are deposited on a cooled substrate. The synthesis of titanium nitride (TiN), using titanium tetrachloride vapor (TiCl₄), nitrogen (N₂), ammonia (NH₃), and hydrogen (H₂), by the PP-CVD process is studied. TiN is formed by a heterogeneous reaction, using TiCl₄, N₂, H₂, whereas simultaneously TiCl₄ and NH₃ react to form an aerosol. The activation energy of this homogeneous reaction is on the order of 100 kJ/mol. The powder formation process is determined by the dissociation of a titanium containing intermediate species. At low temperature differences between substrate and gas phase (i.e., < 2 K), only dense columnar microstructures, with growth rates of around 20 μm/h, are observed. At these temperature differences no particle deposition is observed. The layers are formed by a molecular diffusion controlled CVD growth mechanism. Porous coherent layers are found in experiments, where intermediate temperature differences are applied (i.e., approximately 2–10 K). The observed interconnection of the particles has to originate from a heterogeneous reaction. Apparently, under these conditions the heterogeneous reaction is fast enough, with respect to the particle precipitation rate, to interconnect the precipitated particles. A further increase in temperature difference between the susceptor and the gas phase only leads to loose powder deposits. In principle, the PPCVD process is a suitable method for the synthesis of thin porous layers of ceramics. To obtain uniform coherent porous layers two separate reaction mechanisms are required under the same experimental conditions. There should be a homogeneous reaction in the gas phase as well as a heterogeneous reaction, which is controlled by surface kinetics, in order to interconnect precipitated particles to obtain a coherent porous layer. Porous ceramic layers can be formed as long as the particle precipitation rate is slow enough with respect to the heterogeneous reaction rate.     

Selective etching of boron nitride phases

Cubic boron nitride (c-BN) films can be used as hard coatings and for electronic devices due to their outstanding material properties, but the gas phase deposition of c-BN is still a challenging task. Until now it has only been possible to achieve nanocrystalline c-BN layers via physical vapor deposition (PVD) methods with rather weak film qualities. Only a chemical vapor deposition (CVD) process for c-BN can produce high quality films with material properties similar to those of the product achieved by high pressure, high temperature processes (HPHT) conventional routes. Therefore it is essential to tune the individual steps in the CVD process (nucleation, growth and selective etching) in a similar manner to that for diamond CVD to enable continuous growth of c-BN.Since selective etching of hexagonal boron nitride (h-BN) and sp² phases is still a major problem, we investigated the interaction of h-BN and c-BN with different reactive gases — ammonia (NH₃), chlorine (Cl₂), hydrogen chloride (HCl) and boron trifluoride (BF₃) — regarding their etching behaviour and surface stabilisation properties. Etching ratios from ≈10:1 up to 450:1 were found in the temperature range 600–1300°C for the h-BN/c-BN system, clearly indicating a high selectivity due to kinetic effects.The reaction mechanisms will be discussed with respect to the kinetic differentiation of the degradation of c-BN and h-BN (selective etching). The morphological changes and the quality of the remaining BN phases was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared and Raman spectroscopy and these indicated a homogeneous decay of the individual phases. Since a homogeneous decay of c-BN resembles the reversed growth, the study of the interaction of both BN phases with reactive gases allowed us to collect more detailed information of the molecular mechanisms involved in the formation of the individual phases. These results will provide new routes for growing c-BN in a CVD process.     

Synthesis of c-BN films by using a low-pressure inductively coupled BF3–He–N2–H2 plasma

Cubic boron nitride (c-BN) films were synthesized by low-pressure inductively coupled radio-frequency plasma (ICP) chemical vapor deposition (CVD) from a gas mixture of borontrifluoride (BF₃), nitrogen, hydrogen and helium. BN films containing 50–80% cubic phase were obtained under 100 mTorr and at 750–1050 °C of substrate temperature. Substrate bias voltage required to obtain c-BN decreased down to − 20 V with increasing substrate temperature. The adhesion was also improved at high substrate temperatures as compared with those obtained in the B₂H₆–Ar–N₂–H₂ gas system, probably because of the decrease of bombarding energy and chemical effects of fluorine for selective deposition of c-BN.     

Effect of fluorine chemistry in the remote plasma enhanced chemical vapor deposition of silicon films from Si2H6-SiF4-H2

SiF₄ was added into Si₂H₆-H₂ to deposit polycrystalline silicon films at low temperatures around 400°C in a remote plasma enhanced chemical vapor deposition reactor. It was found out that the fluorine chemistry obtained from SiF₄ addition had an influence on the chemical composition, crystallinity, and silicon dangling bond density of the film. The fluorine chemistry reduced the amount of hydrogen and oxygen incorporated into the film and also suppressed the formation of powders in the gas phase, which helped the crystallization at low temperatures. Effect of SiF₄ concentration as well as the deposition temperature was also significant.     

Synthesis of poly(borazinylamine)

1,3,5-Trichloroborazine (TCB) was prepared from the reaction of ammonia chloride with boron trichloride. TCB along with hexamethyldisilazane and boron trichloride were used to synthesize boron nitride (BN) preceramic polymer poly(borazinylamine). This study showed that, the lower the reaction temperature, the higher the synthetic yield. Poly(borazinylamine)'s solubility mainly depended on the ratio of TCB, (Me₃Si)₂NH, and BCl₃. The solvent used in the reaction had a large effect on the ceramic yield of poly(borazinylamine). A soluble poly(borazinylamine) with good synthesis and ceramic yields was obtained when the reaction temperature was −15°C, cyclohexane was the solvent, and the ratio of TCB : (Me₃Si)₂NH : BCl₃ was 1 : 6 : 1. By means of infrared and mass spectroscopy analyses, the structure of the poly(borazinylamine) was identified. Thermal decomposition of the poly(borazinylamine) precursor to hydrolyzed BN was also examined. Hydrolyzed BN was obtained at 1000°C, where the ceramic yield was 35–45%. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 863–868, 1999     

Synthesis and reaction sintering of YB4 ceramics

Within this work, the preparation of yttrium tetraboride (YB₄) in the form of powder as well as bulk material was investigated.Powders were synthesized via four different reaction methods, including direct synthesis from elemental powders, reduction of yttrium oxide with boron, boron carbide reduction, and combined boron carbide/carbothermal reduction at 1500 °C, 1700 °C and 1900 °C. Pure YB₄ powder was successfully synthesized using the combined boron carbide/carbothermal reduction method. Secondary phases, especially Y₂O₃, YB₂ or YBO₃, were found in powders prepared using the other three methods.Bulk material was prepared using direct synthesis from elements by reactive hot-pressing. Influence of temperature and boron content on densification and phase evolution of samples was studied. In situ reaction sintering was performed using conventional hot-pressing at temperatures from 1100 °C to 1800 °C in vacuum. The amount of boron varied from the stoichiometric content to 5 and 10 wt% excess (with respect to the reaction from elemental powders). Stoichiometric reactions led primarily to the formation of YB₂ and YB₄ and several secondary phases such as Y₂O₃, YBO₃ and Y_16.86B₈O₃₈. YB₄ as a main phase was formed only at elevated temperatures (1700 °C and 1800 °C) but certain content of impurities was still present. Excess of B resulted in the formation of YB₄ as a primary phase in all prepared samples with a small content of YBO₃ and/or Y_16.86B₈O₃₈. Moreover, SEM analysis revealed the presence of unreacted boron.     

Formation and growth mechanism of porous,amorphous, and fine particles prepared by chemical vapor deposition. Titania from titanium tetraisopropoxide

The formation and growth mechanism of porous, amorphous, and fine particles were investigated. TiO₂ particles were produced in a tubular flow reactor by a chemical vapor deposition technique using titanium tetraisopropoxide as a starting material at low temperatures (573-973 K) and atmospheric pressure. Prepared particles were of submicron size and had large surface area (as large as 270 × 10³ m²/kg). According to the proposed mechanism, reactions begin on the reactor wall and then the primary particles form in the gas phase by chemical reactions. The primary particles collide, coalesce with each other and grow. However, significant experimental deviations from the Brownian collision and coalescence theory imply that other processes, such as the surface reactions on the particle, play an important role in the growth, in addition to coalescence. Intraparticle reactions decreased the surface area by filling the pores.     

Manufacture of Granular Polysilicon from Trichlorosilane in a Fluidized‐Bed Reactor

Manufacturing of polysilicon by chemical vapor deposition from SiHCl₃ in a fluidized‐bed reactor was studied. The effects of reaction temperature, H₂/SiHCl₃ ratio, gas velocity, and seed particle loading were evaluated. The outlet gas composition was analyzed by gas chromatography. The physical features of the product particles were determined by scanning electron microscopy and laser particle size analyzer. Well‐grown product particles were obtained. The temperature and H₂/SiHCl₃ ratio significantly affected conversion, yield, and selectivity, which were less affected by gas velocity and seed particle loading at higher temperatures. The surface reaction kinetics determined the product yield only at lower temperatures, and thermodynamic equilibrium was approached at temperatures above 900 °C.     

Coke formation on the surface of α-Al2O3 in the catalytic pyrolysis of naphtha

The catalytic pyrolysis of naphtha has been carried out in a quartz reactor loaded with 5 mm α-A1₂0₃ spheres. The yields of ethylene and propylene exhibit about 10% and 5% higher values compared to the thermal pyrolysis in the absence of α-A1₂0₂ spheres at the same reaction conditions. The coke formation on α-A1₂0₃ spheres increased continuously along with the axial length of the reactor as well as with reaction time. Results suggest that the concentration of the coke precursors in the gas phase may increase along with the axial length of the reactor. Coke filled up completely the internal pore of the sphere near the exit of the reactor after reaction for 4 hr. The coke film on the external surface of the sphere grew continuously thicker. The coke formation was influenced strongly by the physical properties of the α-Al₂O₃ spheres. Coke deposition was the least on the α-A1₂0₃ sphere with the lowest surface area and pore volume among the tested α-A1₂0₃ spheres.     

Influence of hydrogen on growth and microstructure of boron nitride coatings obtained from BCl3–NH3–H2 system by chemical vapor infiltration

Hexagonal boron nitride (h-BN) interfacial coatings were prepared by chemical vapor infiltration (CVI) process from BCl₃–NH₃–H₂ system with different hydrogen contents for improving the toughness of ceramic matrix composites. In this study, the yield of BN was found to be 94.90% without hydrogen present in the reactant system as calculated via FactSage, while it reached 99.95% at the [H₂]/[BCl₃] ratio of 10 and the [NH₃]/[BCl₃] ratio of 1, when chemical equilibrium was reached. BN interfacial coatings containing mixture of hexagonal and turbostratic phases were obtained. The deposition rate of coating increased from 18.2 ± 0.4 nm min⁻¹ (β = 0) to 23.0 ± 0.4 nm min⁻¹ (β = 5) with the increase of hydrogen content in reactants, then it significantly decreased when β was 10. Owing to different nucleation amounts on the surface of fibers, samples S2 (β = 2) and S3 (β = 5) exhibited particles with circular shapes and smooth surfaces, while the other coatings presented particles with polygonal shapes and rough surfaces. Moreover, the onset temperature of weight gain of sample S2 was 102 °C higher than that of sample S4, thus indicating the enhancement of the high-temperature oxidation resistance of BN coating.     

Microstructure and deposition mechanism of CVD amorphous boron carbide coatings deposited on SiC substrates at low temperature

Amorphous boron carbide (α-B₄C) coatings were prepared on SiC substrates by chemical vapor deposition (CVD) from CH₄/BCl₃/H₂/Ar mixtures at low temperature (900–1050 °C) and reduced pressure (10 kPa). The deposited coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that two kinds of α-B₄C coatings were deposited with different microstructures and phase compositions, and the effect of deposition temperature was significant. When deposited at 1000 °C and 1050 °C, the coatings exhibited a nodular morphology and had a relatively low content of boron. The free carbon was distributed in them inhomogeneously; in contrast, when deposited at 900 °C and 950 °C, the coatings presented a comparatively flat morphology and had a uniform internal structure and high boron content. They did not contain free carbon. At the last of this paper, the pertinent mechanisms resulting in differences in microstructure and phase composition were discussed.     

Beneficial effects of the use of a nickel membrane reactor for the dry reforming of methane: Comparison with thermodynamic predictions

The development of a nickel composite membrane with acceptable hydrogen permselectivity at high temperature in a membrane reactor for the highly endothermic dry reforming of methane reaction was the purpose of this work. A thin, catalytically inactive nickel layer, deposited by electroless plating on asymmetric porous alumina, behaved simply as a selective hydrogen extractor, shifting the equilibrium in the direction of a higher hydrogen production and methane conversion. The main advantage of such a nickel/ceramic membrane reactor is the elimination or limitation of the side reverse water gas shift reaction. For a Ni/Al₂O₃ catalyst, containing free Ni particles, normally sensitive to coking, the use of the membrane reactor allowed an important reduction of carbon deposition (nanotubes) due to restriction of the Boudouard reaction. For a Ni–Co/Al₂O₃ catalyst, where the metallic nickel phase was stabilized by the alumina, the selective removal of the hydrogen significantly enhanced both methane conversion (+67% at 450 °C, +22% at 500 °C and +18% at 550 °C) and hydrogen production (+42% at 450 °C, +32% at 500 °C and +22% at 550 °C) compared to the results obtained for a packed-bed reactor. The hydrogen selectivity during the catalytic tests at 550 °C, maintained with constant separation factors (7 for H₂/CH₄, 8 for H₂/CO and 10 for H₂/CO₂), higher than Knudsen values, attested to the high thermal stability of the nickel composite membrane.     

Synthesis of single-crystalline lanthanum hexaboride nanowires by Au catalyst

Lanthanum hexaboride (LaB₆) nanowires have been successfully fabricated by the facile catalytic reaction of lanthanum (La) powders, and gas mixture of boron trichloride (BCl₃), hydrogen and argon, where Au was used as the catalyst. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) were used to characterize the composition, morphology and structure of the samples. Single crystal column-shape LaB₆ nanowires were obtained. It is expected that LaB₆ nanowires can provide thermionic emission, field-induced emission, and thermal field-induced emission of electrons for TEM, SEM, flat panel displays, as well as many electronic devices that require high-performance electron source.