Chemical vapor deposition of pyrolytic boron nitride ceramics from single source precursor

Pyrolytic boron nitride ceramics were prepared on graphite substrates from borazine as the single source precursor by hot-wall chemical vapor deposition in deposition temperature range from 1300 °C to 1600 °C with a total pressure of 200 Pa. The chemical composition and the effect of deposition temperature on the morphology, phase, and structure of the pyrolytic boron nitride were investigated. A high purity product with stoichiometric B/N ratio is obtained. The deposition surface of the product exhibited a pebble-like structure, and the fracture surface showed an apparent laminar structure having a preferential (002) orientation parallel to the surface of the substrate at temperatures above 1400 °C. The product contained some turbostratic and amorphous boron nitride as evidenced from XRD and FTIR examinations. With the increase of deposition temperature, the crystallinity of the pyrolytic boron nitride increased with the turbostratic and amorphous boron nitride turned into hexagonal structure, and the crystallinity of the product became higher.     

Micromorphology and structure of pyrolytic boron nitride synthesized by chemical vapor deposition from borazine

Pyrolytic boron nitride (PBN) plates were synthesized by chemical vapor deposition (CVD) with temperatures of 900–1900?°C and total pressures of 50–1000?Pa on graphite by using borazine as the precursor. The effects of temperature and pressure on the micromorphology and crystal structure of the PBN were investigated. The as-deposited PBN possessed three typical types of micromorphologies depending on the deposition condition. PBN with dense and laminated structure (Type A) were deposited at temperatures of 1150–1900?°C with relative low pressures of 50–200?Pa, and PBN with porous and isotropic structure (Type C) was deposited at temperatures above 1100?°C with higher pressures above 250?Pa. PBN with dense and glass-like fracture structure (Type B) was obtained at the other range of the deposition condition. The interlayer spacing (d₍₀₀₂₎) and the preferred orientation (PO) of the crystallite were calculated by using XRD data of the PBN plates. The degree of the preferred orientation tended to be higher with the increase of temperature and decrease of pressure, and higher temperature led to smaller value of d_(002). The crystal growth mechanism of the three types of PBN was discussed.     

High-speed deposition of silicon nitride thick films via halide laser chemical vapor deposition

Silicon nitride shows significant potential in the field of surface protection for electronic devices owing to its excellent insulation performance and mechanical properties. In this study, silicon nitride films were fabricated via halide laser chemical vapor deposition (LCVD). The effects of deposition parameters on the crystallinity, microstructure, deposition rate (R_dep), Vickers microhardness, nano-hardness and electrical resistivity were investigated. The maximum R_dep of the silicon nitride thick films was 972 µm/h at T_dep of 1573 K and P_tot of 10 kPa, which is the highest value compared with those obtained via conventional CVD. As T_dep increased, the Vickers microhardness and nano-hardness of the films increased to the highest value of 25.1 GPa and 34.8 GPa at 1573 K, respectively. The electrical resistivity of the films decreased with increasing T_dep and showed a maximum value of 1.49 × 10¹⁴ Ω·cm at T_dep of 1273 K.     

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.     

Synthesis of boron nitride nanotubes by combining citrate-nitrate combustion reaction and catalytic chemical vapor deposition

High-quality boron nitride nanotubes were successfully synthesized via a novel two-step method, including citrate-nitrate combustion reaction and catalytic chemical vapor deposition. The composition, bonding features and microstructures of as-synthesized sample were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, transmission electron microscopy and selected area electron diffraction techniques. The results show that the as-synthesized boron nitride nanotubes with smooth surface are relatively pure. The diameter ranges between 20 and 80?nm, while the length is about dozens of micrometers. During the synthesis process of boron nitride nanotubes, citric acid chelates the cobalt ions and reacts with nitrate to form the cobalt oxide, depositing on the surface of boron powder homogeneously. The catalyst content and annealing temperature have a significant impact on the composition and microstructures of the final products. Based on the experimental results and thermodynamic analysis, the possible chemical reactions are listed, and vapor-liquid-solid mechanism is proposed to be dominant for the formation of boron nitride nanotubes.     

Influence of total pressure on the microstructures and growth mechanism of ZrC coatings prepared by chemical vapor deposition from the Zr-Br2-C3H6-H2-Ar system

Zirconium carbide (ZrC) coatings were deposited on graphite substrates by chemical vapor deposition from the Zr-Br₂-C₃H₆-H₂-Ar system. The influence of total pressure on the growth of ZrC was investigated in the range of 5–60 kPa. As the total pressure increased, the deposition rate increased evidently, and the preferential orientation of ZrC coatings changed from the (200) plane to the (220) plane. The growth mechanism changed from a mass transport reaction to a surface reaction at the total pressure of 20–40 kPa. At the total pressure below 20 kPa, the deposition was dominated by crystal growth, so the coatings were composed of well-faceted pyramidal-shaped crystals growing along the <001> direction. At the total pressure above 60 kPa, the growth of ZrC coatings was controlled by the nucleation mechanism, so the coatings were cluster-like crystals rapidly growing along the <110> direction. In addition, low pressure was conducive to the formation of near-stoichiometric ZrC without free carbon. These variations of ZrC coatings can mainly be attributed to gas supersaturation and remarkably changed transport diffusion coefficients with increasing total pressure.     

Local chemical vapor deposition of carbon nanofibers from photoresist

The addition of nanofeatures to carbon microelectromechanical system (C-MEMS) structures would greatly increase surface area and enhance their performance in miniature batteries, super-capacitors, electrochemical and biological sensors. Negative photoresist posts were patterned on a Au/Ti contact layer by photolithography. After pyrolyzing the photoresist patterns to carbon patterns, graphitic nanofibers were observed near the contact layer. The incorporation of carbon nanofibers in C-MEMS structures via a simple pyrolysis of modified photoresist was investigated. Both experimental results considered to consist of a local chemical vapor deposition mechanism. The method represents a novel, elegant and inexpensive way to equip carbon microfeatures with nanostructures, in a process that could possibly be scaled up to the mass production of many electronic and biological devices.     

Influence of Si doping on the microstructure and hardness of an AlTiSiN coating deposited by low pressure chemical vapor deposition

The effect of Si doping on the microstructure and hardness of an AlTiSiN coating deposited by the low pressure chemical vapor deposition (LPCVD) method was studied in detail using grazing incidence X-ray diffraction (GIXRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and a nanoindenter instrument. The results show that Al_0.82Ti_0.18N coating exhibits a typical columnar crystal structure, while Al_0.88Ti_0.09Si_0.03N and Al_0.82Ti_0.08Si_0.10N coatings show a fine equiaxed crystal structure. The number of internal substructures (dislocation, stacking fault, etc.) decreased, while the volume fraction of the amorphous structure increased with the increase of Si content. The results of GIXRD and HRTEM show that all AlTiSiN samples mainly consist of fcc AlN phase with a small amount of hcp AlN phase. Furthermore, a small amount of fcc TiN phase can only be observed in Al_0.82Ti_0.08Si_0.10N coating. The hardness of Al_0.82Ti_0.18N, Al_0.88Ti_0.09Si_0.03N, and Al_0.82Ti_0.08Si_0.10N coatings is 33.0 ± 0.6 GPa, 38.3 ± 1.2 GPa and 27.0 ± 0.8 GPa, respectively. Al_0.88Ti_0.09Si_0.03N coating has obvious potential value for industrial applications.     

Room temperature synthesis of boron nitride thin films by dual-ion beam sputtering deposition

Boron nitride (BN) films are prepared by dual-ion beam sputtering deposition at room temperature (~25 °C). An assisting argon/nitrogen ion beam (ion energy E_i=0–300 eV) directly bombards the substrate surface to modify the properties of the BN films. The effects of assisting ion beam energy on the characteristics of BN films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectra, atomic force microscopy, and optical transmittance. The density of the B–N bond in the film increased with the increase in assisting ion beam energy. The highest transmittance of more than 95% in the visible region was obtained under the assisting ion beam energy of 300 eV. The band gap of BN films increased from 5.54 eV to 6.13 eV when the assisted ion-beam energy increased from 0 eV to 300 eV.     

Template-directed synthesis of boron nitride nanotube arrays by microwave plasma chemical reaction

Highly-ordered boron nitride (BN) nanotube arrays have been synthesized by microwave plasma-enhanced chemical vapor deposition (MW-PECVD) below 520 °C under the confinement of anodic aluminum oxide (AAO) template with borane/argon and ammonia/nitrogen as precursors. The low growth temperature and aligned arrangement of the BN nanotubes are beneficial to practical applications despite of the amorphous nature of the product. Novel morphology of Y-branching and dendriform BN nanotubes were also observed when the branching AAO template was used.     

Growth of vertically aligned BN nanosheets via Fe-catalyzed thermal chemical vapor deposition

Hexagonal boron nitride nanosheets (BNNSs) demonstrated potential applications for ultraviolet (UV) nanoelectronics, catalytic support, electron field emission, and many other areas. However, their extensive industrial application is limited by the complicated conventional procedures for scalable preparations. In this study, vertically aligned BNNSs were directly synthesized without any substrates using Fe-catalyzed thermal chemical vapor deposition (CVD) method under a NH₃ gas flow at 1200 °C via the catalytic pyrolysis of the inorganic hybrid precursor. BNNSs grew via an Oswald ripening process assisted by oriented attachment mechanism under Fe catalyzed by analyzing the evolution process. Most nanosheets are <10 nm thick, and the number of constituent layers is typically between 10 and 20 nm. BNNSs improved crystallinity, had high UV light emission, and had excellent anti-oxidation behavior because of their unique structures. The development of a simple, high yield and environmentally friendly synthesis method for BNNSs allows for the exploration of industrial upscaling applications.     

Degradation of boron nitride interfacial coatings fabricated by chemical vapor infiltration on SiC fibers under ambient air/room temperature conditions

Hexagonal boron nitride (h-BN) interfacial coatings were deposited on SiC fibers by chemical vapor infiltration (CVI) and their degradation behavior under ambient air/room temperature conditions was studied with time. Degradation of the interfacial coatings with time was investigated by characterizing the morphology and microstructure of these materials with scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Thermogravimetry coupled with differential thermal analysis (TG-DTA) and X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical reactions and phase transitions taking place in the BN coatings. The results showed that the as-deposited BN interfacial coatings fabricated by CVI were compact and well bonded to the SiC fibers. BN coatings remained relatively stable under ambient air/room temperature conditions for 50?h, while severe degradation was observed after 500?h of exposure. The degradation of BN interfacial coatings was mainly caused by two factors, namely, reaction with atmospheric air to produce boric oxide and amorphization of the hexagonal structure. The degradation observed under ambient air/room temperature might be due to incomplete crystallinity of BN interfacial coatings. Presence of water vapor may accelerate degradation of the coatings. The results of this degradation test can be used as a reference for the storage of BN coatings fabricated by CVI.     

Analysis of gas phase compounds in chemical vapor deposition of carbon from light hydrocarbons

Product distributions in the pyrolysis of ethylene, acetylene, and propylene are studied to obtain an experimental database for a detailed kinetic modeling of gas phase reactions in chemical vapor deposition of carbon from these light hydrocarbons. Experiments were performed with a vertical flow reactor at 900 °C and pressures from 2 to 15 kPa. Gas phase components were analyzed by both on-line and off-line gas chromatography. More than 40 compounds from hydrogen to coronene were identified and quantitatively determined as a function of the residence time varied up to 1.6 s. Product recoveries were generally more than 90%. Analysis of the kinetics of the conversion of the hydrocarbons resulted in global reaction orders of 1.2 (ethylene), 2.7 (acetylene), and 1.5 (propylene). First order dehydrogenation reactions and third order trimerization reactions leading to benzene are decisive reactions for ethylene and acetylene, respectively. Conversion of propylene should also be based on two simultaneous reactions, a first order dissociation reaction, and second order reactions such as bimolecular reaction of propylene resulting an allyl and a propyl radical. These insights should be useful to develop a reaction mechanism based on elementary reactions in forthcoming studies.     

Mechanism of carbon nanotube growth from camphor and camphor analogs by chemical vapor deposition

Single walled nanotubes have been synthesized by chemical vapor deposition from camphor, camphor analogs (camphorquinone, norcamphor, norbornane, camphene, fenchone), and various other precursors (menthone, 2-decanone, benzene, methane). The high temperature conditions (865 °C) and Fe/Mo alumina catalyst used in the syntheses are archetypal conditions for the production of single walled carbon nanotubes. It has been shown that the mechanism of tube growth is unlikely to depend upon the production of reactive five- and six-member rings, as has been previously suggested. The results suggest that the presence of oxygen in the precursor does not significantly improve the quality of tubes by etching amorphous carbon: it is suggested that the control of the flux of the precursor to the catalyst is more important in the production of high quality tubes. There is, however, evidence for different distributions of tube diameter being produced from different precursors.     

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.     

Influence of hexagonal boron nitride nanosheets on phase transformation,microstructure evolution and mechanical properties of Si3N4 ceramics

Fully dense Si₃N₄ materials with 1 wt.% (～ 1.5 vol.%) and 2 wt.% (～ 3.0 vol.%) h-BN nanosheets were prepared by spark plasma sintering at 1750 °C with the dwell of 7 min under a pressure of 50 MPa in a vacuum. BN nanosheets with different dimensions were prepared by ultrasound-assisted liquid phase exfoliation of h-BN powder, followed by centrifugation at different speeds (1000 rpm and 3000 rpm). The addition of BN nanosheets hindered the particle rearrangement stage of sintering, which resulted in the delayed α→β phase transformation of Si₃N₄. To study a direct effect of BN nanosheets on the mechanical properties of Si₃N₄, the results were compared to the monolithic Si₃N₄ with similar grain size and α/β-Si₃N₄ ratio. The addition of 2 wt.% h-BN nanosheets (1000 rpm) increased both the fracture toughness (～ 26 %) and the strength (～ 45 %) of Si₃N₄, when compared to the monolithic Si₃N₄ with similar microstructure.     

Initiated chemical vapor deposition of polyvinylpyrrolidone-based thin films

Initiated chemical vapor deposition (iCVD) is used for the first time to deposit a non-acrylic carbon-based polymer, polyvinylpyrrolidone (PVP). PVP is known for its hydrophilicity and biocompatibility, and its thin films have found many applications in the biomedical community, one of which is as antibiofouling surfaces. From vapors of 1-vinyl-2-pyrrolidone (VP) and tert-butyl peroxide (TBPO), iCVD produces PVP thin films that are spectroscopically identical to bulk PVP without using any solvents. iCVD works by selectively fragmenting gaseous TBPO with heat to create radicals for initiation of polymerization. This selectivity ensures that the monomer VP does not disintegrate to form species that do not conform to the structure of PVP. Fourier-transform infrared (FTIR), nuclear magnetic resonance, and X-ray photoelectron spectroscopy (XPS) show full retention of the hydrophilic pyrrolidone functional group. Number-average molecular weights range between 6570 and 10,200 g/mol. The addition of ethylene glycol diacrylate (EGDA) vapor to the reaction mixture creates a cross-linked copolymer between VP and EGDA. Films with different degrees of cross-linking can be made depending on the partial pressures of the species. Methods for quantifying the relative incorporation of VP and EGDA using FTIR and XPS are introduced. The film with the lowest degree of cross-linking has a wetting angle of 11°, affirming its high hydrophilicity and iCVD's ability to retain functionality.     

Effect of the reaction atmosphere on the diameter of single-walled carbon nanotubes produced by chemical vapor deposition

The influence of the reaction atmosphere on the type of single-walled carbon nanotubes (SWNT) grown during chemical vapor deposition (CVD) was investigated. Methane decomposition was catalyzed by Fe/MgO and Fe-Mo/MgO catalysts in argon, nitrogen and their mixtures. Nitrogen influences the carbon species significantly. The aggregation of iron nanoparticles in nitrogen results in the growth of N-doped carbon nanofibers on the Fe/MgO catalyst. A limited iron nanoparticle aggregation in nitrogen occurred on a Fe-Mo/MgO catalyst, on which there was an increase in the diameter of the SWNTs as the reaction atmosphere was more enriched in nitrogen, which was characterized by Raman spectroscopy. These results provide an experimental basis for the rational selection of the reaction atmosphere, and suggest an approach to control the size of the SWNTs in a CVD method.