Synthesis and Growth Mechanism of SiC/SiO2 Nanochains Heterostructure by Catalyst‐Free Chemical Vapor Deposition

The SiC/SiO₂ nanochains heterostructure with double amorphous layers was successfully synthesized by a catalyst‐free chemical vapor deposition process at 1280°C. The SiC/SiO₂ nanochains experience an apparent regular periodic structure, whose SiO₂ beads with diameters of about 160 nm are positioned on the SiC strings beside one another. Their growth mechanism could be mainly ascribed to Rayleigh instability and vapor‐solid mechanism. The double layers structure of amorphous silica on the surface of the strings results from the different silica deposition stages. SiC nanowires with diameters of 20–50 nm were also found accompanied with the SiC/SiO₂ nanochains and not changed into the chains‐shaped morphology because of their small diameters and much higher additive surface pressure of bending silica melt layer on the nanowires.     

Growth Mechanism of Silicon Carbide Films by Chemical Vapor Deposition below 1273

SiC films were prepared from the reaction of Si₂H₆ with C₂H₄ or C₂H₂ at relatively low temperatures ranging from 873 K to 1273 K by low-pressure chemical vapor deposition. The deposition rate profiles determined by gravimetry and the alloy composition (carbon content, x, for Si_1-xC_x) profiles determined by X-ray photoemission spectroscopy in the reactor were mainly investigated. The results revealed that the carbon content, x , was a function of the temperature, ratio of partial pressures of source gases, and source gas species (C₂H₄, C₂H₂). From these results we deduced that SiC formation occurred by two major competing reaction processes: (1) the silicon deposition processes, SiH₂ Si (wall) and Si₂H₆ Si (wall), and (2) the carbon deposition process, C₂H₄+ SiH₂ vinylsilane or C₂H₂+ SiH₂ ethynylsilane.     

On the Growth and Microstructure of Carbon Nanotubes Grown by Thermal Chemical Vapor Deposition

Carbon nanotubes (CNTs) were deposited on various substrates namely untreated silicon and quartz, Fe-deposited silicon and quartz, HF-treated silicon, silicon nitride-deposited silicon, copper foil, and stainless steel mesh using thermal chemical vapor deposition technique. The optimum parameters for the growth and the microstructure of the synthesized CNTs on these substrates are described. The results show that the growth of CNTs is strongly influenced by the substrate used. Vertically aligned multi-walled CNTs were found on quartz, Fe-deposited silicon and quartz, untreated silicon, and on silicon nitride-deposited silicon substrates. On the other hand, spaghetti-type growth was observed on stainless steel mesh, and no CNT growth was observed on HF-treated silicon and copper. Silicon nitride-deposited silicon substrate proved to be a promising substrate for long vertically aligned CNTs of length 110–130 μm. We present a possible growth mechanism for vertically aligned and spaghetti-type growth of CNTs based on these results.     

Deposition Mechanism for Chemical Vapor Deposition of Zirconium Carbide Coatings

Zirconium carbide (ZrC) coatings were fabricated by chemical vapor deposition (CVD) using ZrCl₄, CH₄/C₃H₆, and H₂ as precursors. Both thermodynamic calculation results and the film compositions at different temperatures indicated that zirconium and carbon deposited separately during the CVD process. The ZrC deposition rates were measured for CH₄ or C₃H₆ as carbon sources at different temperatures based on coating thickness. The activation energies for ZrC deposition demonstrated that the CVD ZrC process is controlled by the carbon deposition. This is also proven by the morphologies of ZrC coatings.     

Synthesis of One-Dimensional Nanostructured Silicon Carbide by Chemical Vapor Deposition

Nanorods of the wide-bandgap semiconductor silicon carbide belong to a promising group of one-dimensional materials with potential applications extending from reinforcement of composites to applications as building blocks that can be logically assembled into appropriate two- (and three-) dimensional architectures, permitting researchers to exploit their unusual electronic, optical, and other properties. Specific to the most common silicon carbide polytypes are a low intrinsic carrier concentration, an exceptionally high breakdown electric field, high thermal conductivity, high-temperature stability, and resistance to an aggressive environment. This should permit one to develop even submicron-level SiC-based devices operating under high-temperature, high-power, and/or high-radiation conditions, under which conventional semiconductors cannot function. Detailed control of the conditions favorable for the nucleation and growth processes of nanorods of a given SiC polytype is necessary because the electrical and optical properties of each SiC polytype are very different. Therefore, a systematic investigation of factors that primarily influence the morphology and polytype of a vapor-phase-grown SiC has been made in the present work. These factors were the temperature, the flow rates of the gaseous precursors, and the Si/C molar ratio in the gas phase. In order to investigate the role of these factors, the “cold gas-hot substrate” chemical vapor deposition (CVD) method has been applied, because it permits them to be closely controlled in a wide range. While in the overwhelming majority of previous investigations nanorods of the 3C SiC polytype have been grown, the present work delineates conditions that are favorable for the growth of single-phase 2H, 3C, 15R, and 6H SiC nanorods, respectively.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Pampuch, Gorny, Stobierski.This article was submitted by the authors in English.     

化学气相沉积法合成纳米碳管阵列研究

研究以环己烷为前驱体采用化学气相沉积法制备纳米碳管阵列。将催化剂二茂铁定量溶解在环己烷中，通过载气夹带进入反应器中，采用化学气相沉积方法定向生长出炭纳米管阵列，此法有效地控制反应体系中的催化剂含量，使生产稳定性及重现性较好。并通过透射电子显微镜、扫描电子显微镜、拉曼光谱及X射线衍射对产品形态和结构进行分析和表征，所制备出的纳米碳管阵列形态比较规整，纯度较高，具有较好的石墨微晶结构；并对纳米碳管的生长机理进行了详细讨论。     

化学气相沉积法合成碳纳米管研究进展

碳纳米管由于其独特的结构和优异的电学、光学、力学、热学等物理化学性能,在材料、电子器件、传感器、催化剂和能源等领域具有广泛的应用前景,但是如何低成本批量制备高品质的碳纳米管是实现碳纳米管大规模应用的关键。本文综述了近年来化学气相沉积法合成碳纳米管的研究进展,表明化学气相沉积法是大规模可控制备碳纳米管最有效的方法,并对其未来的发展方向进行了分析和展望。     

Continuous Fabrication of Silicon Carbide Fiber Tows by Chemical Vapor Deposition

The feasibility of preparing small-diameter, high-strength, thermally stable silicon carbide fiber tows by the continuous chemical vapor deposition (CVD) of SiC onto carbon fiber tows was experimentally evaluated. Calculations of bending stresses and stresses caused by thermal expansion mismatch between the substrate and coating were used to evaluate the influence of coating thickness and substrate fiber diameter and type. Statistically designed and analyzed processing studies quantitatively showed the influence of key CVD process variables (temperature, pressure, and flow rates of CH₃SiCl₃ and H₂) on fiber attributes such as coating thickness and uniformity, surface roughness, percent agglomeration, and strength. Emphasis was given to conceiving and evaluating various fiber spreading devices in order to enhance coating uniformity and to minimize filament agglomeration within a tow. Uniform coatings and fiber tensile strengths as high as 4 GPa were achieved.     

Initiated‐Chemical Vapor Deposition of Polymer Thin Films: Unexpected Two‐Regime Growth

Initiated‐chemical vapor deposition (iCVD) is a very promising technique which has demonstrated the ability to deposit a large variety of polymers that can be integrated in micro‐nanotechnology applications. However, studies on the underlying growth mechanisms responsible for the formation of these thin films remain scarce in the literature. This work shows that the iCVD growth follows surprisingly two regimes: in the first stage of the growth, the deposition rate is relatively slow then increases with the deposition time until a linear growth is reached. The presence of these two growth regimes can be interpreted by taking into account, as the iCVD growth progresses, that the synthesized polymer chains help the monomer adsorption on the substrate which locally increases the concentration of monomers available for the polymerization and thus the growth rate. This increase of the local concentration of monomer consistently correlates with the formation of polymer chains with higher molar mass.     

Jet Assisted Aerosol Chemical Vapor Deposition for Optical Fiber Synthesis

Various kinds of high quality optical fibers are routinely fabricated by the modified chemical vapor deposition (MCVD), in which fine particles are generated through the oxidation of chemical precursor and deposited in a silica tube reactor. Efficiency, rate, and uniformity of particle deposition determine the quality and cost of optical fibers; therefore efforts to enhance aerosol deposition performance should be important for further improving both quality and cost. Here we propose a jet assisted aerosol chemical vapor deposition method utilizing gas jets in the conventional MCVD silica tube reactor for the purpose of enhancing the efficiency, rate, and uniformity of particle deposition. High temperature helium gas is injected radially through an electrically heated thin tube inserted inside the silica tube. High temperature gas jets push particles generated in a tube toward the tube wall and therefore shorten the axial length of particle trajectories before deposition and cause particles to experience higher thermophoretic force. As a result, deposition efficiency (and rate) was found to considerably increase compared to the conventional method, and the uniformity was also significantly improved.     

Ferrocene‐Catalyzed Growth of Single‐Crystalline 6H‐SiC Nanoribbons

Single crystal 6H–SiC nanoribbons were in situ synthesized in large scale on the as‐prepared SiC–Si ceramic substrates via a facile heat‐treatment approach using ferrocene as catalyst. The as‐synthesized nanoribbons were up to several millimeters long, with widths of 200–300 nm and thicknesses of 20–80 nm. A novel combination growth mechanism of vapor–liquid–solid‐based and vapor–solid‐tip was proposed for the growth mode of the as‐synthesized nanoribbons. This study provided not only a new method for synthesizing SiC nanoribbons but also a new insight into the growth mode for SiC nanoribbons.     

催化化学气相沉积法合成单壁纳米碳管的研究进展

介绍了合成单壁纳米碳管的三种主要方法,总结了国内外催化化学气相沉积法合成单壁纳米碳管的研究现状,着重介绍了催化剂对合成单壁纳米碳管影响的研究情况,并分析了反应工艺条件对合成单壁纳米碳管的影响.     

Modeling of Laser-Induced Chemical Vapor Deposition of Silicon Carbide Rods from Tetramethylsilane

Finite-difference fluid-dynamics modeling has been used to predict deposition rates, fractional amounts of phases, and deposition morphology for the codeposition of silicon carbide and pyrolitic carbon from tetramethylsilane via laser-induced chemical vapor deposition (LCVD). Calculated results agree fairly well with rod deposition experiments. The morphologic features of rods that have been grown using LCVD are examined and explained using the results of the finite-difference calculations.     

Chemical Vapor Deposition Growth of Composite Silicon-Silica Nanowires from Silicon Monoxide Vapor

Si/SiO₂ nanowires were synthesized directly by and on silicon substrate surface without the use of a metal catalyst. Since these nanowires grow directly from the silicon substrate, they do not need to be manipulated or aligned for subsequent applications. The obtained nanowires are amorphous with diameters ranging between 50 to 200 nm and few micrometers in length. Parameters like heating temperature, deposition time, and carrier gas flow-rate were found critical in determining the size, structure, growth yield and morphology of the obtained nanowires. FTIR absorption spectra showed high intensity Si–O asymmetric stretching mode and no absorption for Si-Si backbone vibration mode at 620 cm^?1which indicates the non-crystalline nature of grown wires.     

Low-Temperature Chemical Vapor Deposition Tungsten Carbide Coatings for Wear/Erosion Resistance

A new chemical vapor deposition (CVD) process has been developed to deposit hard coatings, containing tungsten carbide, at temperatures below 500°C. These coatings, which have been applied to both ferrous and nonferrous alloys, exhibit excellent resistance to wear and erosion. The coatings comprise a mixture of tungsten and the tungsten carbide, the latter being present as W₂C, W₂C + W₃C, or W₃C. The coatings' composition and properties can be controlled by varying the CVD process parameters. The unique lamellar, fine-grained microstructures of these coatings contribute to their good tribological properties.     

Co／Mo催化剂制备碳纳米管的研究

以氧化铝凝胶负载Co／Mo合金为催化剂，C2H2为碳源，用CVD法合成了纯度较商的多壁碳纳米管（MWCNT）。通过甩TEM和XRD等方法，对碳纳米管（CNTs）进行了表征，表明制备的碳纳米管具有较高的石墨化程度。研究得出当催化剂的沉淀pH=7．5，H2为还原气和载气，升温速率为50℃／min时，碳纳米管的纯化后的产率高达85％，为下一步的大规模化生产打下了良好构基础。     

The Influences of H2 Plasma Pretreatment on the Growth of Vertically Aligned Carbon Nanotubes by Microwave Plasma Chemical Vapor Deposition

The effects of H₂ flow rate during plasma pretreatment on synthesizing the multiwalled carbon nanotubes (MWCNTs) by using the microwave plasma chemical vapor deposition are investigated in this study. A H₂ and CH₄ gas mixture with a 9:1 ratio was used as a precursor for the synthesis of MWCNT on Ni-coated TaN/Si(100) substrates. The structure and composition of Ni catalyst nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present findings showed that denser Ni catalyst nanoparticles and more vertically aligned MWCNTs could be effectively achieved at higher flow rates. From Raman results, we found that the intensity ratio of G and D bands (I _D/I _G) decreases with an increasing flow rate. In addition, TEM results suggest that H₂ plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles. As a result, the pretreatment plays a crucial role in modifying the obtained MWCNTs structures.