Chemical Vapor Reaction Process Research Articles

A novel approach for preparing a SiC coating on a C/C-SiC composite by slurry painting and chemical vapor reaction

In order to improve the oxidation resistance of C/C-SiC composites, a SiC coating was prepared on a C/C-SiC composite by slurry painting combined with a chemical vapor reaction process. The oxidation resistance and microstructural evolution of the coated samples were investigated. The results show that the as-prepared SiC coating contained a large amount of residual silicon, and the presence of these Si promoted the formation of a complete SiO2 glass layer in the initial stage of oxidation. However, the evaporation of the residual Si also accelerated the failure of the SiC coating, which caused the weight loss of the sample to be about 2.2% after oxidation in static air at 1500 °C for 300 h. Attributed to a large number of SiC ceramics in the C/C-SiC composite, the oxidation weight loss rate of the coating sample after coating failure was reduced.

Selenium-doped carbon nanotubes/nickel selenide coaxial nanocables for energy storage

Coaxial nanocable heterostructures consisting of highly conductive carbon nanomaterial core and active transition-metal compound sheath are promising potential candidates for advanced energy storage materials. Herein, the multi-functional selenium‐doped carbon nanotubes/nickel selenide coaxial nanocables with chemical bonding interface are prepared by using an electroless deposition combined with the following chemical vapor reaction process, in which the nickel layer is first deposited onto the carbon nanotube core and subsequently transformed in-situ into the nickel selenide sheath by the controlled selenization reaction at elevated temperature. Such coaxial structure provides the large active material/electrolyte contact area, and the fast diffusion and transfer of mass and charge in the electrochemical process, thus which endows superior supercapacitive and good lithium storage performances. The coaxial nanocables exhibit a high specific capacity of 340.0 mAh g −1 at 1 A g −1 as an electrode for supercapacitor, and a lithium storage capacity of 600.0 mAh g −1 at 0.1 A g −1 as an anode for lithium-ion batteries. A supercapacitor based on the nanocables delivers a high energy density of 70.0 Wh kg −1 at 803.0 W kg −1 and long cycling stability with 65.0% capacitance retention after 8000 cycles at 10 A g −1 . • Preparing multi-functional Se-CNTs@NiSe nanocables with chemical bonding interface. • Graphene/NiSe nanoribbons are derived due to splitting of coaxial nanocables. • Nanocables show the excellent supercapacitive and lithium storage performances.

Tunable Synthesis of SiC/SiO₂ Heterojunctions via Temperature Modulation.

A large-scale production of necklace-like SiC/SiO2 heterojunctions was obtained by a molten salt-mediated chemical vapor reaction technique without a metallic catalyst or flowing gas. The effect of the firing temperature on the evolution of the phase composition, microstructure, and morphology of the SiC/SiO2 heterojunctions was studied. The necklace-like SiC/SiO2 nanochains, several centimeters in length, were composed of SiC/SiO2 core-shell chains and amorphous SiO2 beans. The morphologies of the as-prepared products could be tuned by adjusting the firing temperature. In fact, the diameter of the SiO2 beans decreased, whereas the diameter of the SiC fibers and the thickness of the SiO2 shell increased as the temperature increased. The growth mechanism of the necklace-like structure was controlled by the vapor-solid growth procedure and the modulation procedure via a molten salt-mediated chemical vapor reaction process.

Fabrication and characterization of carbon-bonded carbon fiber composites with in-situ grown SiC nanowires

In order to improve the mechanical properties of CBCFCs, the CBCFCs with in-situ grown SiC nanowires (SiCNW) were fabricated based on the chemical vapor reaction process and precursor impregnation and pyrolysis (PIP) process. The effect of multi-scaled reinforcements (micro-scaled carbon fibers and nano-scaled SiCNW) on the mechanical properties was investigated. The phase, microstructure and fracture surface of the composites were characterized by XRD, SEM and TEM. Microstructure characterization found that the in-situ grown SiCNW randomly distributed in the matrix and formed a network structure in the CBCFCs. The carbon fibers and the SiCNW formed the multi-scaled reinforcements can significantly improve the mechanical properties of CBCFCs. More importantly, the orientation anisotropy on mechanical properties of the composites is also obviously deceased due to the in-situ grown SiCNW. The improved mechanical properties and decreased orientation anisotropy could be attributed to the pull-out and bridging of carbon fibers and SiCNW, which formed multi-scaled reinforcements.

Catalyst-assisted synthesis of core–shell SiC/SiO2 nanowires via a simple method

Large scale core–shell SiC/SiO2 nanowires with the lengths ranging from several tens to several hundreds of micrometers have been successfully synthesized by using silicon and carbon black as raw materials with a catalyst-assisted chemical vapor reaction process. The nanowires with diameters in the range of 40–120nm consist of β-SiC core coated with a thickness of 2–3nm amorphous SiO2 shell and the catalyst played an important role in the growth of the nanowires. The main component of the crucible, namely alumina may be as a novel effective mediator playing an important role in the content of SiO2 during the growth of the core–shell nanowires, which provides a promising method to fabricate nanowires in a large scale production, and an alumina-assisted vapor–liquid–solid (VLS) mechanism was proposed for the growth mode of the core–shell SiC/SiO2 nanowires.

Growth and Properties of Core‐Shelled SiC–SiO2 Nanowires Using Chemical Vapor Deposition

Core‐shelled SiC–SiO2 nanowires have been synthesized on mechanically scratched Si substrates using a catalyst‐assisted chemical vapor reaction process at 1100°C. The average diameter and length are 30 nm and in the order of 101 μm, respectively. The SiC core exhibits a single crystalline structure, while the core is amorphous. High‐resolution transmission electron microscopy images and selected area diffraction patterns show that the (111) planes of the SiC are at an angle of approximately 71° to the nanowires axis. The existence of the SiO2 alters the cathodoluminescence spectra of the SiC nanowires. A growth mechanism is also suggested.

Synthesis of dense polycrystalline GaN of high purity by the chemical vapor reaction process

A process for producing high-purity, dense polycrystalline gallium nitride is proposed. Dense polycrystalline gallium nitride was produced by the reaction of ammonia, gallium metal, and a halide source in a quartz boat containing metallic gallium. The process is called the chemical vapor reaction process. The hard crust-like pieces of polycrystalline GaN obtained are of high purity, can be used as source material for single-crystal growth by the ammonothermal technique, sublimation, sputtering, and pulse laser deposition.

Growth of AlN by the Chemical Vapor Reaction Process

AlN was grown by a chemical vapor reaction process with metallic Al and ammonium chloride as source precursors. A detailed parameter study was performed for direct growth on sapphire substrates. Specular films were obtained with optimum ammonium chloride supply for a given temperature. Due to lack of optimization of initial growth conditions, these specular films on sapphire contained polycrystalline inclusions. These inclusions were successfully avoided by regrowing on metalorganic chemical vapor deposition (MOCVD)-grown AlN templates. Inversion domains which existed in the MOCVD templates were eliminated at the interface of regrowth although the density of edge-type dislocations was not reduced, and remained at 9.3×109 cm-2. Growth on grooved sapphire resulted in AlN domains with their c-axis perpendicular to the sidewalls of the grooves. The top free surface which was inclined by about 20° to the basal plane of the sapphire substrate was m-plane. The top portion of this domain showed reduced dislocation density, as low as 8×108 cm-2.

Growth of A1N by the chemical vapor reaction process and its application to lateral overgrowth on patterned sapphire substrates

Growth of A1N on patterned sapphire substrates was studied. Deep grooves were made on sapphire substrates by a laser-induced backside wet etching technique. KrF excimer laser was irradiated at an interface between sapphire and phenolphthalein/ N-methyl-2-pyrrolidone solution, which acts as a super-heating liquid. An etch rate of sapphire was as high as 6 μm/min for laser power of 1.0 J/cm 2. The A1N layer was grown on the patterned sapphire substrate by chemical vapor reaction process at 1190 °C for 1 h. On the inclined sidewall of the etched grooves, A1N grew with its c-axis perpendicular to the sidewall. This produced m-plane surfaces which were inclined around 20° to the basal plane of sapphire. A threading dislocation density of m-plane A1N domains was 8×10 8 cm −2.

Synthesis and Growth of Gallium Nitride by the Chemical Vapor Reaction Process (CVRP)

A new process for synthesis and bulk crystal growth of GaN is described. GaN single crystal c-plane platelets up to 9mm by 2mm by 100μm thick have been grown by the Chemical Vapor Reaction Process (CVRP). The reaction between gallium and a nitrogen precursor is produced by sublimation of solid ammonium chloride in a carrier gas, which passes over gallium at a temperature of approximately 900°C at near atmospheric pressures. Growth rates for the platelets were 25-100 μm/hr in the hexagonal plane. Seeded growth in the c-direction was also accomplished by re-growth on previously grown c-plane platelets. The crystals were characterized by X-ray diffractometry, atomic force microscopy, secondary ion mass spectrometry, inert gas fusion, and room temperature Hall effect and resistivity measurements.