Solid-state reactions of silicon carbide and chemical vapor deposited niobium

Niobium films were deposited on silicon carbide by chemical vapor deposition using niobium chloride and hydrogen at a temperature range of 900–1300°C. The solid-state reactions between the deposited niobium and silicon carbide matrix were studied by examining the obtained films using X-ray diffraction and energy dispersion spectroscopy. The results indicated that niobium silicides could be formed at the beginning, which blocked further reactions between carbon and niobium to form niobium carbides. When the deposition temperature was increased, silicon would diffuse outward, which allowed the formation of niobium carbides. The reaction process and mechanism are discussed based on the thermodynamics and kinetics.     

High quality double wall carbon nanotubes with a defined diameter distribution by chemical vapor deposition from alcohol

We have synthesized double wall carbon nanotubes (DWNTs) with few defects and little amorphous carbon by hot wall chemical vapor deposition (CVD) of alcohol. Catalysts for the DWNT growth were made from cobalt and molybdenum acetates. Scanning electron microscopy, transmission electron microscopy, multi frequency resonance Raman spectroscopy and optical absorption spectroscopy were used for characterization of the product with regard to DWNT yield, the nanotube diameter distribution, defect concentration and amorphous carbon content. Base pressures lower than 1 × 10⁻⁵ mbar in the CVD reactor considerably suppress defects in the DWNTs. Optimized growth conditions for DWNT formation are presented.     

Microstructure response of Amosic-3 SiC/SiC composites under self-ion irradiation

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

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.     

Diamond nucleation suppression in chemical vapor deposition process

A systematic study of the effect of different pre-treatments of the Si substrate surface in suppressing diamond nucleation was performed to investigate the nature of the nucleation centers in chemical vapor deposition (CVD) of diamond. The Si substrates were initially scratched with diamond powder and then submitted to one of the following pre-treatments: thermal annealing in high vacuum and in air, deposition of an amorphous silicon film, and ⁸⁴Kr⁺ ion implantation. The pre-treated substrates were used in a hot filament CVD diamond process, and the diamond films obtained were analyzed by different techniques. The results suggest that the observed nucleation reduction under certain pre-treatment conditions is related to modifications induced on the original topographical features of the scratched substrate surface, which would be responsible for the CVD diamond nucleation. The dimensions of these surface features are estimated to be of the order of 5 nm.     

Synthesis and properties of macroporous SiC ceramics synthesized by 3D printing and chemical vapor infiltration/deposition

Open porosity cellular SiC-based ceramics have a great potential for energy conversion, e.g. as solar receivers. In spite of their tolerance to damage, structural applications at high temperature remain limited due to high production costs or inappropriate properties. The objective of this work was to investigate an original route for the manufacturing of porous SiC ceramics based on 3D printing and chemical vapor infiltration/deposition (CVI/CVD). After binder jetting 3D-printing, the green α-SiC porous structures were reinforced by CVI/CVD of SiC using CH₃SiCl₃/H₂. The multiscale structure of the SiC porous specimens was carefully examined as well as the elemental and phase content at the microscale. The oxidation and thermal shock resistance of the porous SiC structures and model specimens were also studied, as well as the thermal and mechanical properties. The pure and dense CVI/CVD-SiC coating considerably improves the mechanical strength, oxidation resistance and thermal diffusivity of the material.     

Influence of Si doping on the microstructure and hardness of an AlTiSiN coating deposited by low pressure chemical vapor deposition

The effect of Si doping on the microstructure and hardness of an AlTiSiN coating deposited by the low pressure chemical vapor deposition (LPCVD) method was studied in detail using grazing incidence X-ray diffraction (GIXRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and a nanoindenter instrument. The results show that Al_0.82Ti_0.18N coating exhibits a typical columnar crystal structure, while Al_0.88Ti_0.09Si_0.03N and Al_0.82Ti_0.08Si_0.10N coatings show a fine equiaxed crystal structure. The number of internal substructures (dislocation, stacking fault, etc.) decreased, while the volume fraction of the amorphous structure increased with the increase of Si content. The results of GIXRD and HRTEM show that all AlTiSiN samples mainly consist of fcc AlN phase with a small amount of hcp AlN phase. Furthermore, a small amount of fcc TiN phase can only be observed in Al_0.82Ti_0.08Si_0.10N coating. The hardness of Al_0.82Ti_0.18N, Al_0.88Ti_0.09Si_0.03N, and Al_0.82Ti_0.08Si_0.10N coatings is 33.0 ± 0.6 GPa, 38.3 ± 1.2 GPa and 27.0 ± 0.8 GPa, respectively. Al_0.88Ti_0.09Si_0.03N coating has obvious potential value for industrial applications.     

Complex geometry macroporous SiC ceramics obtained by 3D-printing,polymer impregnation and pyrolysis (PIP) and chemical vapor deposition (CVD)

We present here an original route for the manufacturing of SiC ceramics based on 3D printing, polymer impregnation and pyrolysis and chemical vapor deposition (CVD). The green porous elastomer structures were first prepared by fused deposition modeling (FDM) 3D-printing with a composite polyvinyl alcohol/elastomer wire and soaking in water, then impregnated with an allylhydridopolycarbosilane preceramic polymer. After crosslinking and pyrolysis, the polymer-derived ceramics were reinforced by CVD of SiC using CH₃SiCl₃/H₂ as precursor. The multiscale structure of the SiC porous specimens was examined by X-ray tomography and scanning electron microscopy analyses. Their oxidation resistance was also studied. The pure and dense CVD-SiC coating considerably improves the oxidation resistance.     

Influence of SiAlON addition on the microstructure development of hot-pressed ZrB2–SiC composites

The impact of SiAlON on densification behavior and microstructure of the ZrB₂-SiC composite was investigated. ZrB₂, SiC, and SiAlON were used as the initial materials to produce ZrB₂-SiC composite by hot pressing at 1900 °C. A fully dense composite was obtained having ~99.9% relative density. High-resolution X-ray diffraction (HRXRD) assessment verified the in-situ formation of ZrC, and the presence of residual carbon, SiAlON, and ZrB₂ and SiC phases in the as-sintered ceramic. Furthermore, the thermodynamic calculations confirmed the results attained by HRXRD. In addition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized for the microstructural investigation. SEM fractographs indicated the impact of SiAlON on the hindering of grain growth and the formation of flaky phases (graphitized carbon or solidified liquid phase) at the grain boundaries. TEM studies revealed the presence of a transparent glassy phase at the particle interfaces. A significant impact of liquid phase sintering was also affirmed in the clean interfaces.     

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.     

Micromechanical testing of carbon fibers deposited by low-pressure laser-assisted chemical vapor deposition

Carbon fibers were deposited directly from ethylene by laser-assisted chemical vapor deposition. The precursor gas pressures and the incident laser powers were varied. Micro-mechanical testing was carried out using a high-precision micro-manipulator. During three-point bend testing the fibers showed an elastic response, with no residual strain upon unloading, until fracture. The fibers’ strength and Young’s modulus are reported. A model for fiber fracture is proposed based on fiber cross-section analysis. Scanning electron microscopy was used to study the fiber cross-sections and the fiber surface morphology. The mechanical properties are related to the characteristic fiber microstructure investigated by Raman spectroscopy.     

Open-air carbon coatings on fused quartz by laser-induced chemical vapor deposition

Carbon films are deposited on fused quartz substrates by CO₂ laser-induced chemical vapor deposition (LCVD) using a novel open-air coating system. The hydrocarbon precursor gases are methane, propane and butane. The deposition rates of the three hydrocarbon gases are determined by measuring the mass of the carbon film deposited at constant temperature, and validated by film thickness measurements obtained using an environmental scanning electron microscope. The results indicate that butane and propane have significant deposition at 1375-1500 K, while deposition starts at 1550 K for methane. All three hydrocarbons have an exponential increase in deposition rate. Deposition rates obtained with butane and propane show a much stronger influence of temperature compared to methane. Raman spectra of deposited carbon films indicate that the surface consists of glass-like or nanocrystalline carbon. The ratio of D-peak to G-peak Raman band intensities decreases as the deposition temperature is increased, which is possibly due to self-annealing and additional surface reaction during high temperature deposition. This finding indicates that the carbon film exhibits higher structural order at high deposition temperatures, which can significantly enhance the film’s hermeticity and mechanical properties for use as an optical fiber coating.     

Growth mechanism of vapor phase CVD-grown multi-walled carbon nanotubes

The formation mechanisms involved in the growth of carbon nanotubes (CNTs) by spray pyrolysis was studied. Both iron and nickel were used as catalysts for growth, and nanotubes were also produced using thermal chemical vapor deposition for comparison. Transmission electron microscopy was used to analyze the encapsulated metal catalyst particles found within the tubes, and the dimensions and location of these particles was recorded. CNTs grown by spray pyrolysis were found to have encapsulated particles in both the middle and end of tubes, with large length to diameter ratios. As a result of these observations, it is concluded that nanotubes grown using spray pyrolysis are formed via an open-ended, root growth mechanism. Additionally, the presence of multiple, high aspect ratio particles within single tubes is explained by an additional growth theory. During the continued growth of these CNTs, metal atoms or nanoscale metal catalyst particles deposit in the open ends of growing tubes, forming new particles and helping to prevent tube closure. CNTs grown with thermal CVD did not contain similar elongated particles or particles along the middle of the tubes, indicating that this new growth mechanism is only applicable in the case of tubes grown via spray pyrolysis or other vapor phase CVD growth methods.     

Edge defect-assisted synthesis of chemical vapor deposited bilayer molybdenum disulfide

The synthesis of two-dimensional molybdenum disulfide (MoS₂) has invoked much research interest in recent years. In this work, chemical vapor deposition (CVD) was employed to grow a novel molybdenum disulfide. The synthesis reaction was operated in an inert gas environment, with the sulfur precursor placed in the upstream zone of a double zone tube furnace, and the molybdenum trioxide (MoO₃) precursor placed in a small tube in the downstream zone. A piece of glass was used as the substrate, which was placed beside the small tube. We intentionally built a high concentration of molybdenum precursor above the substrate. The specific morphology hinged highly on the concentration of molybdenum. Abundant molybdenum atoms adsorbed on the edges of the bottom MoS₂ and formed defects, which promoted the nucleation of the top layer MoS₂ and preferentially grew from the edges. This result provides an innovative method to controllably synthesize novel bilayer molybdenum disulfide by a one-step method of chemical vapor deposition.     

Growth of carbon nanotubes by open-air laser-induced chemical vapor deposition

Carbon nanotubes have remarkable mechanical, electronic and electrochemical properties, but the full potential for application will be realized only if the growth of high quantity and quality carbon nanotubes can be optimized and well controlled. In this study, carbon nanotubes have been successfully grown on fused quartz rods by a novel open-air laser-induced chemical vapor deposition (LCVD) technique with gold palladium nanoparticles as catalyst material. In this LCVD technique, a curtain of inert nitrogen gas was used to shield the deposition zone from the surrounding environment and allows the growth of the nanotubes to occur under open-air conditions. A 35-W continuous CO₂ laser was used as a heat source to induce a local temperature rise on the substrate surface covered with metal nanoparticles, subsequently resulting in deposition of multi-wall carbon nanotubes. The carbon nanotubes deposited in this study are derived from a precursor mixture that consists of propane and hydrogen, and are in tangled form with different diameters (10-250 nm) and structures. Raman spectroscopy, transmission and scanning electron microscopy are used to investigate the microstructure and composition of the carbon nanotubes.