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.     

Enabling the low-cost preparation of core-shell WC–Ni powder by developing a non-noble metal-based catalytic

A novel method of incorporating fluidized bed chemical vapor deposition (FBCVD) with electroless plating was developed to effectively prepare the core-shell structured high-Ni-content WC–Ni composite powders. FBCVD offered a unique approach to decorating Ni particles on WC particles. The small Ni particle acted as an active catalyst to effectively produce high-Ni-content composite powders in the subsequent electroless plating process. The particle size and quantity of decorated Ni particle determined the electroless plating rate. FBCVD pretreatment temperature and time had a conflicting influence on obtaining both the fine size and large quantity of deposited Ni particles. For the conditions tested, the maximum electroless plating rate of 2.93 mg / g/min was obtained using an optimal FBCVD pretreatment at 750°C for 5 minutes, corresponding to a 0.11 wt. % Ni deposition and 50 nm particle size.     

Fabrication of core-shell structured TiC–Fe composite powders by fluidized bed chemical vapor deposition

Fluidized bed chemical vapor deposition (FBCVD) was an effective way of preparing the core-shell structured TiC–Fe composite powders by employing FeCl₃ as a precursor. Fully covered TiC–Fe composite powders with the controllable Fe contents were readily achievable. An excellent interfacial bonding was formed between the TiC and the deposited Fe coating. The defluidization caused by the directional growth and self-nucleation-aggregation of the deposited Fe particles was the major barrier to depositing high-Fe-content composite powders. But it could be prevented by controlling the gas partial pressure of precursor and further eliminating the directional growth mode and self-nucleation behavior of Fe atoms. The optimal deposition temperature was 600°C and the partial pressure was about 20 kPa, corresponding to the gasification temperature of 275°C.     

Elimination of ƞ phase in WC–Co cemented carbides during laser powder bed fusion by powder coating compensation strategy

The formation of ƞ phase induced by the C-loss for the laser powder bed fusion (LPBF) of WC–Co cemented carbides largely deteriorates the fracture toughness. The current approach of mixing C additive into powder cannot mitigate the ƞ phase formation. This study proposed a new carbon compensation strategy of coating carbon resource on powder surface by fluidized bed chemical vapor deposition to address this issue. C nanoparticles and carbon nanotubes (CNTs) were selectively deposited on WC–Co powder to make uniform C- and CNTs-coated powders by tuning the deposition temperature. Compared with CNTs-coated powder and C–WC–Co powder mixtures, the C-coated powder was more effective in impeding the ƞ phase formation because it had higher reactivity and stronger dissolution ability to compensate the C-loss in the Co–W–C liquid. However, the single-carbon compensation was not enough to eliminate the ƞ phase due to the extreme nonequilibrium characteristics of LPBF, which required secondary heat treatment. The conventional heat treatment procedure of 1000°C for 3 h eliminated the ƞ phase for the C-coated powder but failed for the C–WC–12Co powder mixtures. Because of the absence of ƞ phase, the heat-treated sample made from C-coated powder exhibited the highest transverse rupture strength.     

Demonstration of SiO2/SiC-based protective coating for dental ceramic prostheses

SiO₂/SiC coatings were deposited onto ceramics disks using plasma-enhanced chemical vapor deposition. The effects of deposition pressure and gas-flow ratio on the refractive index, extinction coefficient, and SiC composition were studied. For the highest studied SiH₄ to CH₄ gas-flow ratio of 1.5, the refractive index increased by 17% from 2.53 (at the wavelength of 845 nm) to 2.96 (at the wavelength of 400 nm). For the lowest studied SiH₄ to CH₄ gas-flow ratio of 0.5, the refractive index only increased by 4% from 2.11 (at the wavelength of 845 nm) to 2.20 (at the wavelength of 400 nm). At higher deposition pressures, the variation in refractive index of the SiC coatings was significantly lower showing a slight increase from 1.93 (at a wavelength of 845 nm) to 1.96 at a wavelength of 400 nm. Except for the case of a low SiH₄ to CH₄ gas-flow ratio of 0.5, for light with wavelengths ≤650 nm, the extinction coefficient of the SiC coatings increased significantly. Light with a wavelength >650 nm had an extinction coefficient near 0 in all cases. After annealing the sample at 400°C for 4 hours, hydrogen-related bonds broke and the stress of the film was reduced from −245 to −71 MPa. By utilizing different thicknesses of SiC, the full standard dental shade guide was matched with the ΔE of each coated disk being less than 3.3 compared to the shade guide.     

A New Vapor Deposition Method to Form Composite Anodes for Solid Oxide Fuel Cells

A solid oxide fuel cell (SOFC) is a complete solid-state energy conversion device with the potential advantages of high efficiency, silent operation, and low emissions. However, the current performance of SOFCs is still limited by a number of problems. Investigations in this field have indicated that it is imperative to fabricate efficient and compatible anodes for SOFCs to minimize polarization loss and to concurrently achieve long-term stability. In this paper, a critical review of previous studies is given and several criteria for the theoretically ideal anode are summarized. Accordingly, a newly developed vapor deposition technique, polarized electrochemical vapor deposition (PEVD), is applied to fabricate composite anodes to meet these criteria. The initial experimental results in the present study show that PEVD is capable of depositing a thin layer of yttria-stabilized zirconia on a porous metallic electrode to form a composite anode. This will not only provide continuous ionic and electronic conducting paths in the anode to reduce the overpotential loss and resistance, but also protect the metallic electrode from further sintering, vapor loss, and poisoning in the harsh SOFC operating conditions.     

Experimental Whisker Growth and Thermodynamic Study of the Hafnium-Carbon System for Chemical Vapor Deposition Applications

Chemical vapor deposition of nonstoichiometric hafnium carbide is studied by computer minimization of the Gibbs free energy of the system. Isostoichiometric curves in the HfC_{1- x} region of the deposition diagram are calculated as a function of HfCI₄, CH₄, and H₂ concentration and reaction temperature. Possible experimental reactions are investigated using thermodynamic efficiency and partial pressure diagrams. A comparison between the calculated solid-phase regions and the HfC phase diagram is given, and experimental results for HfC whiskers are also compared with the calculations.     

General method for highly controlled preparation of ceramic hollow spheres from core-shell structures

The spouted bed chemical vapor deposition (CVD) technique have been successfully applied to prepare hollow spheres from core-shell structures. The prepared SiC hollow spheres (SiC-HS) showed the desired characteristic, such as pure phase (β-SiC), uniform and concentrated distribution of diameter and sphericity. The flexibility of this method allows for independent control of the diameter, shell thickness and microstructures of the SiC hollow spheres. In particular, it is demonstrated that the precursor of SiC was carried by different carrier gas (hydrogen or argon), SiC-HS with porous or dense shells can be obtained. The porous SiC-HS have high density of stacking faults, which indicates that it can be used as photocatalytic materials. The dense SiC-HS with high density (99%TD) exhibits excellent mechanical properties (average Young's modulus of 410 GPa and hardness of 42 GPa), and it can be used as fusion ignition target and encapsulation vessels as well. Through using different precursor, the spouted bed CVD technique used here could be generally adopted to synthesize other types (eg other carbides or complex oxides) of hollow spheres.     

Vapor-phase polymerization of high-performance thin-film composite membranes for nanofiltration

Thin-film composite (TFC) membranes are commendable semipermeable barriers for water treatment. Although conventionally immiscible interfaces between aqueous and organic solutions are widely utilized for obtaining TFC membranes, interfacial polymerization still suffers from the issues of harmful solvents, complex diffusion/reaction of the reactants, and thermodynamic and kinetic instability of interfaces. In this study, vapor-phase polymerization with no requirements for organic solvent and immiscible interface is utilized for processing TFC nanofiltration membranes. Through cross-linking of β-cyclodextrin and piperazine layers by trimesoyl chloride vapor, polyester and polyamide TFC membranes with high cross-linking degree are simply prepared in a scalable and reproducible manner. The prepared TFC membranes exhibit stable nanofiltration and desalination performance for all water, organic solvent, and water–organic mixture systems, with permeance up to an order of magnitude higher than that of commercial membranes.     

Rational design of vapor-deposited self-crosslinking polymer for transparent flexible oxygen barrier coatings

Transparent flexible barrier coatings consisting of a copolymer of 2-isocyanatoethyl methacrylate and 4-aminostyrene were fabricated using a facile vapor-based initiated chemical vapor deposition (iCVD) method. The isocyanate and amino groups were well reserved during deposition and reacted on the substrate surface, inducing self-crosslinking. The resultant coating showed a fairly smooth pinhole-free surface with more than 99% light transmittance across the visible range. With a proper compositional ratio, the synthesized coating demonstrated oxygen permeability 26 and 8 times lower than commercial PET and PVDF barrier films, respectively. The achieved barrier performance is attributed to the designed molecular structure, where interchain crosslinks are produced between a combination of both rigid and flexible side chains. Such combination enabled efficient packing of space while avoiding loose structures with low permeation resistance. The crosslinked network with rather flexible urea linkages also imparted excellent mechanical strength and high flexibility to the coating. Furthermore, the reported method is compatible with current industrial manufacturing process, thus expecting great potential in flexible electronics encapsulation.     

Improved thermal shock resistance of SiCnw/PyC core-shell structure-toughened CVD-SiC coating

In-situ SiC nanowire (SiCnw)/pyrolytic carbon (PyC) core-shell structures were introduced to mainly improve the thermal shock performance of chemical vapor deposition (CVD)-SiC coating on carbon/carbon (C/C) composites. The microstructure, phase composition, and mechanical properties of the CVD-SiC coating toughened by SiCnw/PyC core-shell structures were studied as well. The results show that the introduction of SiCnw/PyC core-shell structures can effectively alleviate the mismatch of coefficient of thermal expansion (CTE) between SiC coating and C/C substrate, thus enhancing the thermal shock resistance of the coating. Furthermore, the increased numbers of interfaces in the SiC coating owing to the addition of core-shell structures are beneficial to the mechanical properties of the coating after thermal shock test.     

Optimization of Chemical Vapor Deposition Parameters for Fabrication of Oxidation-Resistant Mullite Coatings on Silicon Nitride

Fabrication of mullite (3Al₂O₃·2SiO₂) coatings by chemical vapor deposition (CVD) using AlCl₃–SiCl₄–H₂–CO₂ gas mixtures was studied. The resultant CVD mullite coating microstructures were sensitive to gas-phase composition and deposition temperature. Chemical thermodynamic calculations performed on the AlCl₃–SiCl₄–H₂–CO₂ system were used to predict an equilibrium CVD phase diagram. Results from the thermodynamic analysis, process optimization, and effects of various process parameters on coating morphology are discussed. Dense, adherent crystalline CVD mullite coatings ∼2 μm thick were successfully grown on Si₃N₄ substrates at 1000°C and 10.7 kPa total pressure. The resultant coatings were 001 textured and contained well-faceted grains ∼0.3–0.5 μm in size.     

(Mg,Co)O Solid-Solution Precursors for the Large-Scale Synthesis of Carbon Nanotubes by Catalytic Chemical Vapor Deposition

Single- and double-walled carbon nanotubes were produced in high yield using the selective reduction of solid solutions of Mg_{1– x} Co _x O in a methane and hydrogen atmosphere at 1000°C. The solid solutions were prepared using combustion synthesis with urea as the fuel. The BET surface areas ranged from 10 to 65 m²/g depending on the fuel content. A single crystalline phase was obtained only for fuel-rich compositions. Increased fuel content increased the surface area by a factor of 6. However, very high fuel contents (>4 times the stoichiometric amount) caused a demixed solid solution. Surface-area measurements and Raman spectra showed that the quantity of nanotubes formed depended on the surface area and composition of the precursor oxide.     

Role of Interfacial Debonding and Matrix Cracking in the Effective Thermal Diffusivity of Alumina-Fiber-Reinforced Chemical-Vapor-Infiltrated Silicon Carbide Matrix Composites

The thermal diffusivity of a biaxial weave alumina-fiberreinforced chemical-vapor-deposited (CVD) SiC composite heated to 1500°C, which is above the manufacturing temperature, was found to exhibit an increase for heat flow parallel to the fiber plane, whereas a decrease was observed perpendicular to the fiber plane. The increase parallel to the fiber plane was thought to be due to the annealing of the fibers and matrix. The decrease perpendicular to the fiber plane was found to be the result of interfacial debonding and matrix cracking within the plane of the fibers.     

流态化气相沉积过程中稀释气体流量对颗粒表面SiO2沉积的影响

利用Fe(Si)合金球形粉末为沉积基底,正硅酸乙酯为SiO2气相介质前驱体,采用引入流化环节的化学气相沉积工艺合成了 Fe(Si)/SiO2复合粉末.考察了沉积过程中Ar稀释气体流量对Fe(Si)基底粉末表面SiO2绝缘介质沉积过程的影响规律及形成完整核壳异质结构的稀释气体流量范围.实验结果表明,随着流态化气相沉积过程...     

CuO/PSSF复合催化剂的制备及其在苯酚降解中的应用

以纸状不锈钢微纤材料（paper-like sintered stainless steel fibers,PSSF）为载体,采用化学气相沉积法（chemical vapor depositon,CVD）制备CuO/PSSF复合催化剂并在固定床反应器上进行苯酚湿式催化氧化降解研究。采用SEM、XRD、XPS等技术对催化剂的表面形态、物相结构、元素价态进行分析,改变流量及床层高度考察停留时间对湿式催化氧化降解苯酚过程中苯酚转化率、H₂O₂转化率、TOC转化率及Cu²⁺浸出浓度的影响规律。催化剂表征结果表明,活性组分CuO成功负载在PSSF微纤材料上;活性评价测试结果表明,低流量条件对各活性指标变化影响十分明显;随床层高度增加各活性指标均显著提高,4cm床层高度下达到最高苯酚转化率和TOC转化率,分别为96.5%和47.4%,同时没有高毒副产物产生。本文初步探究了复合催化剂与固定床反应器结合的工艺可行性,旨在为工业化含酚废水的降解提供一些思路。     

Chemical surface treatment of ultrahigh molecular weight polyethylene for improved adhesion to methacrylate resins

Ultrahigh molecular weight polyethylene (UHMWPE) has high yield strength and modulus, but is nonpolar and chemically inert. For it to be used as an effective reinforcing agent for composites, methods to make the UHMWPE wettable or capable of reaction with the matrix are critical. In the current work, Spectra 900? (UHMWPE) fibers were surface modified by swelling in p‐xylene with: (1) methylmethacrylate (MMA) monomer; (2) PMMA; (3) camphorquinone (CQ); (4) 3‐methacryloxypropyltrichlorosilane (Cl‐MPS); (5) trimethoxysilyl modified polyethylene, N‐(triethoxysilylpropyl)‐dansylamide (fluorescent silane), or octadecyltrimethoxy silane (OMS), followed by hydrolysis and reaction with Cl‐MPS; and (6) by coating with SiO₂ films followed by reaction with MPS. These modifiers were used to improve wettability and provide sites for chemical interactions with the resin matrix. Beads of resin [60/40 BisGMA‐TEGMA (bis‐phenol A bis‐(2‐hydroxypropyl) methacrylate and tri(ethylene glycol) dimethacrylate)] were light‐cured around the treated fibers and the improvement in adhesion was tested by microbond shear strength (τ) tests. The improvements were comparable to those reported by acid etching and plasma treatments. The OMS, fluorescent silane, and SiO₂/Cl‐MPS treatments yielded the best results, that is fourfold increases in τ compared with untreated fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1564–1572, 2005     

Design of a C/SiC functionally graded coating for the oxidation protection of C/C composites

To reduce the residual thermal stress between the carbon fiber-reinforced carbon (C/C) composites and the SiC coating layer, functionally graded materials (FGM) consisting of a C/SiC compositionally graded layer (C/SiC interlayer) were adopted. After designing the compositional distribution of the C/SiC interlayer which can relieve the thermal stress effectively, the deposition conditions of the entire compositional range of the C/SiC composites were determined using a thermodynamic calculation. According to the design and calculation the C/SiC interlayer and the SiC outer layer were deposited on the C/C composites by a low pressure chemical vapor deposition (LPCVD) method at deposition temperatures of 1100 and 1300 °C. The stress calculation and the experimental results suggested that the SiC-rich compositional profile in the FGM layer is the most effective for relieving the thermal stress and increasing the oxidation resistance.