化学气相催化热解法制备螺旋纳米碳纤维

采用酒石酸铜前驱体热分解得到纳米铜粒子作为催化剂,分别对250℃、280℃、310℃分解产生的纳米铜粒子进行测试分析,在3个温度下用化学气相沉积法生长螺旋纳米碳纤维并进行综合热分析。采用X-射线衍射(XRD)分析其物相组成,晶粒大小;用扫描电子显微镜(SEM)观察螺旋纳米纤维的外观形貌。结果表明,310℃生长出的螺旋纳米碳纤维纯度高、外观形貌清晰,热分析质量损失少。     

Textures of pyrolytic carbon formed in the chemical vapor infiltration of capillaries

Capillaries, 1.1 mm in diameter and 17.0 or 32.5 mm in length, were infiltrated at a temperature of 1100 °C and methane pressures from 5 to 30 kPa. Layer thickness and carbon texture were determined at cross-sections of 2, 16 and 32 mm from the open end of the capillaries using polarized light microscopy. Average deposition rates, determined from layer thickness and infiltration time, as a function of methane pressure indicate a rate increase up to a saturation adsorption at pressures between 10 and 15 kPa (range 1) and a strong rate increase above these pressures (range 2). This result implies carbon formations based on the growth mechanism in range 1 and the nucleation mechanism in range 2. The carbon texture shows a maximum in range 1 and a minimum in the transition from range 1 to range 2 followed by a clear increase in range 2. The maximum in range 1 corresponds to the particle-filler model describing formation of various textures of carbon by the ratio of aromatic species to C₂ species. Increasing texture degrees in range 2 suggest that the nucleation mechanism may lead to high textured carbon provided that the residence time for intramolecular rearrangments of polycyclic aromatic hydrocarbons is sufficient.     

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.     

Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition

An ultrathin sheet-like carbon nanostructure, carbon nanosheet, has been effectively synthesized with CH₄ diluted in H₂ by an inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Nanosheets were obtained without catalyst over a wide range of deposition conditions and on a variety of substrates, including metals, semiconductors and insulators. Scanning electron microscopy shows that the sheet-like structures stand on edge on the substrate and have corrugated surfaces. The sheets are 1 nm or less in thickness and have a defective graphite structure. Raman spectra show typical carbon features with D and G peaks at 1350 and 1580 cm⁻¹, respectively. The intensity ratio of these two peaks, I(D)/I(G), increases with methane concentration or substrate temperature, indicating that the crystallinity of the nanosheets decreases. Infrared and thermal desorption spectroscopies reveal hydrogen incorporation into the carbon nanosheets.     

Chemistry and kinetics of chemical vapor deposition of pyrocarbon: VI. influence of temperature using methane as a carbon source

The chemical vapor deposition of carbon from methane was investigated at an ambient pressure of about 100 kPa, a methane partial pressure of 10 kPa and temperatures ranging from 1050–1125°C. Carbon deposition rates and compositions of the gas phase as a function of residence time have been determined using a substrate with a surface area/reactor volume ratio of 40 cm⁻¹. Increasing temperatures lead to strongly increasing deposition rates, decreasing partial pressures of ethane and increasing partial pressures of ethene, ethine and benzene. The overall activation energy of carbon deposition, determined from the initial deposition rates at a residence time versus zero amounts to 446 kJ/mol as compared to 431.5, 448 and 452.5 kJ/mol reported in earlier papers. Two possible rate-limiting steps are discussed, namely dissociation of methane, which is favored in the earlier papers, and dissociation of carbon–hydrogen surface complexes.     

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.     

化学气相沉积工艺制备绳状纳米碳管的研究

用硝酸铁作催化剂,乙炔作碳源气体,高纯氮气作稀释气体,在750℃下化学气相沉积生长了绳状纳米碳管,用高分辨扫描电镜观察了所得绳状纳米碳管的形貌.纳米碳管的直径为100～200nm,长度为10～20 μm.文中还提出了绳状纳米碳管的生长机理.     

Effect of microwave power on diamond-like carbon films deposited using electron cyclotron resonance chemical vapor deposition

Diamond-like carbon (DLC) films have been deposited using electron cyclotron resonance chemical vapor deposition (ECR-CVD) under various microwave power conditions. Langmuir probe measurement and optical emission spectroscopy (OES) were used to characterize the ECR plasma, while the films were characterized using Raman and infrared (IR) spectroscopies, hardness and optical gap measurements. It was found that the ion density and all signal peaks in the optical emission (OE) spectra increased monotonously following the increase in microwave power. Raman spectra and optical gap measurements indicate that the films become more graphitic with lower content of sp³-hybridized carbon atoms as the microwave power was increased. IR and hardness measurements indicate a reduction in hydrogen content and decrease in hardness for the film produced at relatively high microwave powers. A deposition mechanism is described which involved the ion bombardment of film surfaces and hydrogen–surface interactions. The deposition rate of DLC film is correlated to the ion density and CH₃ density.     

Chemical vapor deposition and infiltration processes of carbon materials

The chemical vapor deposition (CVD) and the chemical vapor infiltration (CVI) processes of carbon materials are reviewed starting from the historical aspects and including the latest developments in the preparation of C/C composites. Our presentation is based on an analysis of the different types of reactors, of the composite materials with different types of pyrocarbon as matrices and a comparison between the different processes. In particular, the classical isothermal-isobaric technique and temperature or pressure gradient reactors, which lead to a higher deposition efficiency, are compared. A complementary aspect is the structural and physical analysis of the deposited pyrocarbons: they are considered as reproducible metastable phases which are obtained under non-equilibrium thermodynamic conditions. The final relevant point concerns the relationship between the process parameters and the type of pyrocarbon. In particular, the so-called rough laminar microstructure, useful for most composite applications, is described.     

A time‐dependent multiphysics,multiphase modeling framework for carbon nanotube synthesis using chemical vapor deposition

A time‐dependent multiphysics, multiphase model is proposed and fully developed here to describe carbon nanotubes (CNTs) fabrication using chemical vapor deposition (CVD). The fully integrated model accounts for chemical reaction as well as fluid, heat, and mass transport phenomena. The feed components for the CVD process are methane (CH₄), as the primary carbon source, and hydrogen (H₂). Numerous simulations are performed for a wide range of fabrication temperatures (973.15–1273.15 K) as well as different CH₄ (500–1000 sccm) and H₂ (250–750 sccm) flow rates. The effect of temperature, total flow rate, and feed mixture ratio on CNTs growth rate as well as the effect of amorphous carbon formation on the final product are calculated and compared with experimental results. The outcomes from this study provide a fundamental understanding and basis for the design of an efficient CNT fabrication process that is capable of producing a high yield of CNTs, with a minimum amount of amorphous carbon. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Electrochemical properties of N-doped hydrogenated amorphous carbon films fabricated by plasma-enhanced chemical vapor deposition methods

Nitrogen-doped hydrogenated amorphous carbon thin films (a-C:N:H, N-doped DLC) were synthesized with microwave-assisted plasma-enhanced chemical vapor deposition widely used for DLC coating such as the inner surface of PET bottles. The electrochemical properties of N-doped DLC surfaces that can be useful in the application as an electrochemical sensor were investigated. N-doped DLC was easily fabricated using the vapor of nitrogen contained hydrocarbon as carbon and nitrogen source. A N/C ratio of resulting N-doped DLC films was 0.08 and atomic ratio of sp³/sp²-bonded carbons was 25/75. The electrical resistivity and optical gap were 0.695 Ω cm and 0.38 eV, respectively. N-doped DLC thin film was found to be an ideal polarizable electrode material with physical stability and chemical inertness. The film has a wide working potential range over 3 V, low double-layer capacitance, and high resistance to electrochemically induced corrosion in strong acid media, which were the same level as those for boron-doped diamond (BDD). The charge transfer rates for the inorganic redox species, Fe^2+/3+ and Fe(CN)₆^4−/3− at N-doped DLC were sufficiently high. The redox reaction of Ce^2+/3+ with standard potential higher than H₂O/O₂ were observed due to the wider potential window. At N-doped DLC, the change of the kinetics of Fe(CN)₆^3−/4− by surface oxidation is different from that at BDD. The rate of Fe(CN)₆^3−/4− was not varied before and after oxidative treatment on N-doped DLC includes sp² carbons, which indicates high durability of the electrochemical activity against surface oxidation.     

Dual layer SiC coating on matrix graphite sphere prepared by pack cementation and fluidized‐bed chemical vapor deposition

A dual layer silicon carbide (SiC) coating including inner porous SiC (p‐SiC) layer and outer dense SiC (d‐SiC) layer was fabricated on the matrix graphite (MG) spheres of high‐temperature gas‐cooled reactor fuel elements by pack cementation and fluidized‐bed chemical vapor deposition process to improve the oxidation‐resistant property. Microstructure of the coating demonstrates different density and structure of the two SiC layers with no obvious boundaries between them. Weight gain curves of oxidation tests at 1773 K for 200 hours show that the coating could effectively protected the MG sphere by isolating the air infiltration with p‐SiC layer as the main functional layer and d‐SiC layer as the transition layer to improve the bond strength. Due to the transition function of p‐SiC layer, the coated spheres could understand more than 50 times thermal shocking tests from 1773 K to room temperature with no stress cracking.     

化学气相沉积致密针刺C/C复合材料的结构及力学性能研究

以T300炭纤维无纬布、网胎为原材料,层叠针刺成型炭纤维预制体,并采用化学气相沉积工艺对预制体进行致密,制成密度为1.55 g/cm3的针刺C/C复合材料。对针刺C/C复合材料的微观结构进行了观察分析,并对材料力学性能进行了测试。结果表明:化学气相沉积致密的针刺C/C复合材料呈现出以层间大量垂直纤维束为节点的类钉板状网状结构,这种特殊结构使材料层间结合更好,材料整个结构更加紧密;针刺C/C复合材料内部纤维被沉积形成的热解炭所包裹,热解炭的织构类型为光滑层(SL)和粗糙层(RL)并存;针刺C/C复合材料的各项力学性能均达到了较高水平,并且高温力学性能比常温力学性能有了很大幅度的提高。     

Effect of catalyst on carbon nanotubes synthesis on titanium diboride via chemical vapor deposition

Carbon nanotubes (CNTs) were in situ synthesized on titanium diboride (TiB₂) substrates using different catalysts (Fe, Co, and Ni) via ethylene chemical vapor deposition (CVD). The effects of various catalysts on the quality and quantity of grown CNTs were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman characterization techniques. Next, as-received powders were sintered by spark plasma sintering to obtain compact bulk composites. The effects of the metallic catalysts as sintering aids on the microstructural and mechanical properties of TiB₂-based composites were also studied. Results show that metal Ni not only plays a significant role in the catalyst to produce larger numbers of CNTs but also promotes the densification of TiB₂ ceramics in sintering and has been shown to provide good reinforcement. Maximum values of flexural strength (1093 ± 19 MPa) and toughness (13.9 ± 0.3 MPa ·m^1/2) were obtained using the Ni catalyst, which were significantly higher than the same values for Fe and Co catalysts.     

Fabrication of an ultra-thick-oriented 3C-SiC coating on the inner surface of a graphite tube by high-frequency induction-heated halide chemical vapor deposition

Cubic SiC (3C-SiC) is a promising material for nuclear industry applications due to its excellent properties. In this report, a highly oriented thick 3C-SiC coating with good crystallinity was prepared on the inner surface of a monolithic graphite tube via high-frequency induction-heated halide chemical vapor deposition using SiCl₄, CH₄, and H₂ as precursors. The texture coefficient (T_C(hkl)), microstructure, and deposition rate along the tube axis was studied. 3C-SiC coating with a high (111) orientation and crystallinity was obtained. Along the tube axis, T_C(111) was consistent with the temperature distribution. The surficial morphology of the 3C-SiC coating changed from pebble-like to hexangular facet and then to hemispherical. The deposition rate and coating thickness were 300 μm/h and 615 μm, respectively, which is sufficiently rapid and thick enough to obtain free-standing SiC tubes for nuclear reactors.     

Effects of carbon film roughness on growth of carbon nanotip arrays by plasma-enhanced hot filament chemical vapor deposition

Carbon nanotip arrays were grown from carbon films deposited on silicon substrates by plasma-enhanced hot filament chemical vapor deposition. The carbon films were characterized by the atomic force microscopy and micro-Raman spectrometry and the carbon nanotip arrays were investigated by micro-Raman spectrometry, scanning electron microscopy and X-ray photon spectroscopy. The results indicate that both carbon films and carbon nanotips are made of polyaromatic carbon with turbostratic structure. Carbon films possess various roughnesses, which greatly influence the nanotip growth rate. The theory related to plasma and sputtering effect is used to discuss the results.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure

In a previous study we showed both the formation of genuine vapor grown carbon fibers (VGCFs) and of new and peculiar carbon nanotube-supported morphologies using a chemical vapor deposition process. Briefly, the latter are an association of beads (or fiber segments) with a more or less rough surface and more or less extended cone-based sub-morphologies with a smooth surface. The investigation of these materials regarding their texture, nanotexture and structure by transmission electron microscopy is reported, as a first step to understanding the formation mechanisms. It is shown that VGCFs exhibit a concentric texture, however with a variable microporous character and nanotexture quality. On the other hand, beads and related morphologies have a coarsely concentric microporous texture, as opposed to the cones and related morphologies, which exhibit a perfectly concentric and dense texture similar to that of perfect multiwall carbon nanotubes. Cross-sections performed with ultra-microtomy have revealed the spatial and textural relationship between cones and beads.     

Deposition rates during the early stages of pyrolytic carbon deposition in a hot-wall reactor and the development of texture

Pyrolytic carbon layers were deposited from methane/oxygen/argon mixtures on planar substrates (silicon wafers) at a total pressure of 100 kPa, a maximum gas residence time of 2 s and a temperature of 1100 °C. The depositions were performed in a hot-wall reactor with the substrate oriented parallel to the gas flow. Particular attention was paid to factors that influence the reproducibility of the deposited layers. Scanning and transmission electron microscopy were applied to study the thickness profiles and the texture of the carbon layers. The surface topography was investigated by atomic force microscopy. For pyrolytic carbon deposited without oxygen, an alteration from medium- to high-textured carbon is observed with increasing residence time. Islands are observed on the surface of the layer whose size increases with the texture. For pyrolytic carbon deposited with 3% oxygen, lower deposition rates were obtained and a strong modification of the texture is found compared to gas mixtures without oxygen.     

Structure of copolymer films created by plasma enhanced chemical vapor deposition

The interface structure in copolymer films made using plasma enhanced chemical vapor deposition (PECVD) has been probed for the first time using X-ray reflectivity. Copolymer films made from comonomers benzene (B), octafluorocyclobutane (OFCB), and hexamethyldisiloxane (HMDS) show extremely sharp interfaces and scattering length density depth profiles that are uniform with depth, making them useful for optical applications. The polymer/air interface has an rms roughness (∼5 Å) that is only slightly larger than that of the supporting substrate (∼3 Å). Addition of either benzene or HMDS as a comonomer in the deposition of OFCB alters a transient deposition behavior at the silicon oxide interface that occurs when using only OFCB. For the B-OFCB copolymer films, a facile control of refractive index with monomer feed composition is achieved. A nonlinear variation in the X-ray scattering length density with composition for the HMDS-OFCB copolymer films is consistent with the nonlinear visible light refractive index (632.8 nm) variation reported earlier.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes: Part 1. Synthesis and morphology

The chemical vapor deposition of the pyrocarbon from a CH₄+H₂ mixture is investigated using nanofilamentous substrates. The process consists of growing carbon nanotubes via a catalytic process, which then are thickened by pyrolytic carbon deposition to reach diameters in the nanometer to micrometer range. A key characteristic of the experimental reactor used was the long length of its isothermal zone, preceded (and followed) by a low thermal gradient zone. This allowed us to investigate the role of the variation of the local gas phase composition, which depends on the post-cracking secondary reactions, and on the quantity and quality of the deposited carbon. The ‘time of flight’ of the reactive species was found to be a leading parameter in the pyrolytic carbon deposition process. Various nanometric and micrometric morphologies, several of which are new, were synthesised and found constituted with an association of different sub-morphologies. The various morphologies, that can be sorted following a factor of morphological complexity, were investigated by scanning electron microscopy.