Fabrication of porous SiC nanostructured coatings on C/C composite by laser chemical vapor deposition for improving the thermal shock resistance

Recently, fabricating one-dimensional (1D) nanomaterials on C/C composite has been recognized effective to improve the thermal shock resistance of the coated composites. However, the remaining metal catalyst in CVD process and the week bond of 1D nanomaterials with substrate limit the strengthening effect. Herein, laser chemical vapor deposition (LCVD) was proposed for fabricating porous SiC nanostructured coating on C/C composite without metal catalyst. The laser heating resulted in a temperature gradient between the top and bottom of the coating, providing an external driving force for the vertical growth of whiskers with side-branches, forming a porous network nanostructure. The porous nanostructure was beneficial to reduce CTE and effectively relieve thermal stress. After 10 times of thermal shock test from RT to 1723 K, the porous SiC nanostructured coating remained intact. This work provides a novel methodology to produce functional coating on C/C composite with outstanding thermal shock resistance.     

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.     

Al2O3 hollow nanospheres prepared by fluidized bed chemical vapor deposition and further heat treatment

Hollow-structured Al₂O₃ nanospheres were prepared by chemical vapor deposition (CVD) and further heat-treatment process. For the CVD process, a unique cooling gas loop was designed in the precursor delivery system and amorphous Al₂O₃ nanospheres with incomplete pyrolysis intermediated products were obtained in a vertical fluidized bed at 500℃. The microstructure of the nanospheres tended to hollow after subsequent heat treatment at 1300℃ in air. It was found that the obtained Al₂O₃ hollow nanospheres showed a size of 100-450 nm in diameter and 20-80 nm in shell thickness. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis confirmed that the Al₂O₃ hollow nanospheres were α-Al₂O₃. The possible formation mechanism was proposed based on the results of the thermal gravimetric (TG) measurement connected with a Fourier transform infrared spectrometry (FTIR). The novel method to prepare Al₂O₃ hollow nanospheres can inspire ideas for the design and synthesis of other hollow-structured nanospheres.     

Fabrication of SiC/diamond composite coatings by electrophoretic deposition and chemical vapor deposition

In this research, SiC/diamond composite coatings were fabricated by a novel procedure that consisted of the electrophoretic deposition (EPD) of diamond particles onto graphite substrates followed by chemical vapor deposition (CVD) of SiC. Various concentrations of MgCl₂ were employed to increase the deposition rate and uniformity of the deposits during the EPD process by giving a positive charge to diamond particles. The CVD of SiC was found to have a tightly connected diamond‐graphite interface and spherical texture. With higher weight fraction of diamond particles deposits, the wear of steel ball increased, while the wear of SiC coating decreased.     

A novel core-shell structured ultra-coarse WC-Co composite powders prepared by fluidized bed chemical vapor deposition

The ultra-coarse WC-Co composite powders with a core-shell structure were effectively prepared by fluidized bed chemical vapor deposition (FBCVD) using CoCl₂ precursor. An excellent interfacial bonding was formed between WC and the deposited Co. Defluidization was the major barrier to depositing high-Co-content composite powders, which was caused by the adhesion of the deposited Co particles. Decreasing the deposition temperature reduced the cohesion force of the deposited Co particles, which was thus beneficial to preventing the defluidization. Increasing the WC particle size and the gas velocity increased the collision force and benefited the fluidization. The final Co contents were largely dependent on the deposition and fluidization behaviors. For the conditions tested, the optimal deposition temperature was 800°C while the minimum WC particle size suitable for FBCVD was 25 μm.     

Structural study of β‐SiC(001) films on Si(001) by laser chemical vapor deposition

β‐SiC thin films have been epitaxially grown on Si(001) substrates by laser chemical vapor deposition. The epitaxial relationship was β‐SiC(001){111}//Si(001){111}, and multiple twins {111} planes were identified. The maximum deposition rate was 23.6 μm/h, which is 5‐200 times higher than that of conventional chemical vapor deposition methods. The density of twins increased with increasing β‐SiC thickness. The cross section of the films exhibited a columnar structure, containing twins at {111} planes that were tilted 15.8° to the surface of substrate. The growth mechanism of the films was discussed.     

Improved oxidation resistance of expanded graphite through nano SiC coating

Expanded graphite with nano SiC and amorphous SiC_xO_y coating was successfully prepared through pyrolysing silane coupling agent (SCA), where the grafting of SCA dominated the final products. The results show that mainly amorphous SiC_xO_y coating covers expanded graphite at 1000 °C, regardless of the SCA concentration. In comparison, nano SiC coating can be synthesized at 1200 °C depending on the good dispersion of SCA (with a SCA concentration of 50 vol%). The formed SiC coating contributes to much higher peak oxidation temperature (812.1 °C) than 678.0 °C of the pure expanded graphite. Meanwhile, the oxidation activation energies of expanded graphite are remarkably improved from 149.15 kJ/mol to 176.16 kJ/mol (based on Kissinger method), attributing to the derived nano SiC and SiC_xO_y coating.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

化学气相沉积PPy/UHMWPE纤维的研究

采用化学气相沉积制备了聚吡咯/超高相对分子质量聚乙烯(PPy/UHMWPE)纤维,测试了不同氧化剂浓度、不同沉积时间和温度下PPy/UHMWPE纤维的表面剪切强度,用扫描电镜、动态热机械分析仪、傅立叶变换红外光谱仪分析了PPy/UHMWPE纤维的表面形态、热机械性能和复合材料官能团的变化。结果表明:PPy均匀分布在UHMWPE纤维表面,UHMWPE纤维与PPy之间无化学键作用而是分子间作用力;随着氧化剂三氯化铁浓度的增加和吡咯沉积时间的延长,PPy/UHMWPE纤维表面剪切强度先增大后减小;随着处理温度的升高,PPy/UHMWPE纤维表面剪切强度先增大,当处理温度超过85℃时,其剪切强度则减小。     

Protecting nuclear graphite from liquid fluoride salt and oxidation by SiC coating derived from polycarbosilane

Modification process has been conducted on commercial nuclear graphite IG-110 (Toyo Tanso Co., Ltd., Japan) by impregnation and pyrolysis of polycarbosilane (PCS) solution for getting the modified IG-110 (M-IG-110) coated by dense SiC coating for molten salt reactor. The microstructure and properties of graphite were systematically investigated and compared before and after the modification process. Results indicated that the M-IG-110 possessed of more excellent integrated properties including molten salt barrier property and oxidation resistance than bare IG-110 due to the filling effect of SiC particles in the pores of M-IG-110 and dense SiC coating adhering to the surface of M-IG-110. The fluoride salt infiltration amount of M-IG-110 under 5 atm was only 1.1 wt%, which was much less than 14.9 wt% for bare IG-110. The SiC coating derived from PCS exhibited remarkable compatibility with graphite substrate under high temperature and gave rise to excellent oxidation resistance of M-IG-110.     

Micro‐nanostructured silicone‐carbon composite coatings with superhydrophobicity and photoluminescence prepared by oxidative chemical vapor deposition

Transparent, superhydrophobic, and colored silicone–carbon composite coatings were prepared by oxidative chemical vapor deposition (oCVD) of bulk silicone at ambient pressure. The colors, wettability, morphologies, and transparency of the coatings can be easily varied via changing both the concentration of gaseous oxygen and the deposition temperature. Typically, the black, brown, and yellow silicone–carbon composite coatings with different superhydrophobicity and transparency were achieved under oxygen‐deficient atmospheres. Furthermore, the colored samples showed photoluminescence when they were excited by ultraviolet (UV) light, which is due to the fluorescence of carbons embedded inside the as‐prepared coatings. In addition, more regular papillae and nanofibers with excellent superhydrophobicity were obtained at higher deposition temperatures. Our method was believed to develop a new strategy for fabricating multifunctional silicone–carbon composite coatings. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40400.     

Chemical vapor deposition derived ultralong SiC nanofibers: structural characterization and growth mechanism

In this study, the ultralong SiC nanofibers (SiC NFs) were synthesized through the sol–gel method assisted the chemical vapor deposition technique. The scanning electron microscope, transmission electron microscopy, X-ray diffraction, Raman, Fourier transform infrared, and X-ray photoelectron spectroscopy techniques were systematically employed to investigate the microstructure, morphology, and phase composition of the as-prepared products. The results demonstrated that the as-obtained products were β-SiC nanofibers with face-centered cubic crystal structure. Meanwhile, the ultralong SiC NFs present an average diameter of about 18 nm and a length up to several hundreds of micrometers and grew along the [1 1 1] direction with a planar stacking faults. In addition, we also investigated the formation mechanism and growth process of the ultralong SiC NFs. The successful preparation of such ultralong SiC NFs provides new idea for fabricating of other silicon-based ultralong nanofibers.     

Synthesis and visible‐light‐derived photocatalysis of titania nanosphere stacking layers prepared by chemical vapor deposition

BACKGROUND: In this study, visible‐light‐derived photocatalytic activity of metal‐doped titanium dioxide nanosphere (TS) stacking layers, prepared by chemical vapor deposition (CVD), was investigated. The as‐grown TS spheres, having an average diameter of 100–300 nm, formed a layer‐by‐layer stacking layer on a glass substrate. The crystalline structures of the TS samples were of anatase‐type. RESULTS: Ultraviolet (UV) absorption confirmed that metallic doping (i.e. Co and Ni) shifted the light absorption of the spheres to the visible‐light region. With increasing dopant density, the optical band gap of the nanospheres became narrower, e.g. the smallest band gap of Co‐doped TS was 2.61 eV. Both Ni‐ and Co‐doped TS catalysts showed a photocatalytic capability in decomposing organic dyes under visible irradiation. In comparison, Co‐doped TiO₂ catalyst not only displays the adsorption capacity, but also the photocatalytic activity higher than the N‐doped TiO₂ catalyst. CONCLUSION: This result can be attributed to the fact that the narrower band gap easily generates electron–hole pairs over the TS catalysts under visible irradiation, thus, leading to the higher photocatalytic activity. Accordingly, this study shed some light on the one‐step efficient CVD approach to synthesize metal‐doped TS catalysts for decomposing dye compounds in aqueous solution. Copyright © 2010 Society of Chemical Industry     

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge-discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.     

流化床化学气相沉积多晶硅的技术进步与挑战

Various methods for production of polysilicon have been proposed for lowering the production cost and energy consumption, and enhancing productivity, which are critical for industrial applications. The fluidized bed chemical vapor deposition (FBCVD) method is a most promising alternative to conventional ones, but the homogeneous reaction of silane in FBCVD results in unwanted formation of fines, which will affect the product quality and output. There are some other problems, such as heating degeneration due to undesired polysilicon deposition on the walls of the reactor and the heater. This article mainly reviews the technological develop-ment on FBCVD of polycrystalline silicon and the research status for solving the above problems. It also identifies a number of chal-lenges to tackle and principles should be followed in the design of a FBCVD reactor.     

Fast preparation of (111)‐oriented β‐SiC films without carbon formation by laser chemical vapor deposition from hexamethyldisilane without H2

(111)‐oriented β‐SiC films were prepared by laser chemical vapor deposition using a diode laser (wavelength: 808 nm) from a single liquid precursor of hexamethyldisilane (Si(CH₃)₃–Si(CH₃)₃, HMDS) without H₂. The effects of laser power (P_L), total pressure (P_tot) and deposition temperature (T_dep) on the microstructure, carbon formation and deposition rate (R_dep) were investigated. β‐SiC films with carbon formation and graphite films were prepared at P_L ≥ 170 W and P_to ≥ 1000 Pa, respectively. Carbon formation strongly inhibited the film growth. β‐SiC films without carbon formation were obtained at P_tot = 400‐800 Pa and P_L = 130‐170 W. The maximum R_dep was about 50 μm·h^?1 at P_L = 170 W, P_tot = 600 Pa and T_dep = 1510 K. The investigation of growth mechanism shows that the photolytic of laser played an important role during the depositions.     

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.     

Controllable synthesis of SiC@Graphene core-shell nanoparticles via fluidized bed chemical vapor deposition

SiC@Graphene (SiC@G) core-shell nanoparticles were successfully prepared by a facile fluidized bed (FB) chemical vapor deposition (CVD) method. SiC@G core-shell nanoparticles with an average size of 10 nm and graphene from 1 to 5 layers with a controllable thickness were obtained by finely adjusting the experimental temperatures. The formation of SiC nanoparticles and graphene layers was confirmed by the results of X-ray diffraction (XRD) and Raman Spectroscopy. The graphene content in SiC@G core-shell nanoparticles prepared at different temperatures was measured from thermogravimetric analysis (TG), which varied from 5.89% to 11.88 mass%. From X-ray photoelectron spectroscopy (XPS) results, no absorption assigned to Si-O band was detected, indicating the effective protection of the SiC nanoparticles against oxidation by the graphene shell to resist oxidation of SiC nanoparticles. This novel method of preparation of SiC@G core-shell nanoparticles could be applied to large-scale production and find diverse applications in related fields.     

The effect of substrate temperature on microstructural evolution and hardenability of tungsten carbide coating in hot filament chemical vapor deposition

Effect of substrate temperature on microstructural evolution and hardenability of tungsten carbide coating produced by hot filament chemical vapor deposition (HFCVD) process was studied. Annealed sheets of 316L stainless steels were used as the substrate. HFCVD technique, with substrate temperatures of 400 and 500°C, was used to deposit tungsten carbide coating on these sheets. Field Emission Scanning Electron Microscope (FE‐SEM) was used to study the evolution of microstructure. X‐Ray Diffraction spectroscopy was used to analyze the phases formed and Raman spectroscopy was employed to differentiate molecular composition of the coatings. The amount of the porosity of the coatings was measured and Vickers hardness measurement was used for hardness assessment. Results show that the tungsten carbide coatings have a honeycomb structure and increasing the temperature of the substrate increases the amount of porosity of the coating. XRD results showed that 3 different crystalline structures containing W, WC, and W₂C were formed in the coating deposited on the 316L stainless steel. Increasing the temperature of the deposition has increased the intensity of the peaks in the XRD results. Raman spectroscopy results indicated the presence of a carbon in the tungsten carbide coatings. Finally, microhardness of the tungsten carbide coating increases with increasing the temperature of the substrate.     

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.