Chemical Vapor Reaction Research Articles

Uniform, Dense Arrays of Vertically Aligned, Large-Diameter Single-Walled Carbon Nanotubes

Precisely controlled reactive chemical vapor synthesis of highly uniform, dense arrays of vertically aligned single-walled carbon nanotubes (SWCNTs) using tailored trilayered Fe/Al(2)O(3)/SiO(2) catalyst is demonstrated. More than 90% population of thick nanotubes (>3 nm in diameter) can be produced by tailoring the thickness and microstructure of the secondary catalyst supporting SiO(2) layer, which is commonly overlooked. The proposed model based on the atomic force microanalysis suggests that this tailoring leads to uniform and dense arrays of relatively large Fe catalyst nanoparticles on which the thick SWCNTs nucleate, while small nanotubes and amorphous carbon are effectively etched away. Our results resolve a persistent issue of selective (while avoiding multiwalled nanotubes and other carbon nanostructures) synthesis of thick vertically aligned SWCNTs whose easily switchable thickness-dependent electronic properties enable advanced applications in nanoelectronic, energy, drug delivery, and membrane technologies.

Synthesis and field emission properties of silicon carbide nanobelts with a median ridge

β-Silicon carbide (β-SiC) nanobelts with a median ridge have been synthesized for the first time via a template/catalyst-free chemical vapor reaction (CVR) method. The characterization results show that the β-SiC nanobelts with widths of 1.5 μm and thickness of about 10–20 nm consist of nanowires with rough surfaces in diameters of 100 nm (median ridge) and two symmetrical lateral flakes with uniform width. A reasonable two-step Vapor–Solid (TSVS) growth mechanism is proposed to explain the formation of the nanobelts on the basis of crystal growth theory. Using the β-SiC nanobelts as the cathode, the result of field-emission measurements shows that the turn-on field and threshold field of the β-SiC nanobelts are about 3.2 V μm−1 and 5.7 V μm−1, respectively. These features make the β-SiC nanobelts a promising candidate for field emission displays. The β-SiC nanobelts obtained in this work enrich the family of silicon carbides nanostructures; moreover, the method and growth mechanism could be an effective clue and example for the synthesis of belt-like nanostructures of other materials.

Reactive Chemical Vapor Deposition Method as New Approach for Obtaining Electroluminescent Thin Film Materials

The new reactive chemical vapor deposition (RCVD) method has been proposed for thin film deposition of luminescent nonvolatile lanthanide aromatic carboxylates. This method is based on metathesis reaction between the vapors of volatile lanthanide dipivaloylmethanate (Ln(dpm)3) and carboxylic acid (HCarb orH2Carb′) and was successfully used in case of HCarb. Advantages of the method were demonstrated on example of terbium benzoate (Tb(bz)3) ando-phenoxybenzoate thin films, and Tb(bz)3thin films were successfully examined in the OLED with the following structure glass/ITO/PEDOT:PSS/TPD/Tb(bz)3/Ca/Al. Electroluminescence spectra of Tb(bz)3showed only typical luminescent bands, originated from transitions of the terbium ion. Method peculiarities for deposition of compounds of dibasic acids H2Carb′ are established on example of terbium and europium terephtalates and europium 2,6-naphtalenedicarboxylate.

In situ chemical vapor reaction in molten salts for preparation of platinum nanosheets via bubble breakage

This paper presents an in situ chemical vapor reaction in a molten salt (KOH–NaOH, MS) for the synthesis of Pt nanosheets. Unlike conventional solution phase synthesis of nanoparticles in organic and aqueous solutions, no organic surfactant or capping agent was used. The surfactant-free Pt nanosheets were used directly to prepare oxygen reduction electrocatalysts without carbon supports and showed little loss of electrochemical surface area in the accelerated durability test. As the MS has a low vapor pressure under ambient pressure, this approach should also have environmental benefits because of reduced risk for releasing toxic gases from volatile organic solvents (VOCs).

Evolution of GaN nanoflowers from AlN–SiO 2 grains on a silicon substrate by chemical vapor reaction

Gallium nitride (GaN) nanoflowers were synthesized on a silicon (Si) substrate at growth temperatures of 650 and 600 °C and under HCl:NH 3 flow ratios of 1:20, 1:30, and 1:40 by hydride vapor phase epitaxy. Numerous nanorod and nanoneedle burs were formed within each nanoflower. The nanoflower size increased with increasing NH 3 gas flow rate. The nanoflower formation mechanism is proposed based on cross-sectional scanning electron microscopy images and bright field image of scanning transmission electron microscopy. Nanoflowers were evolved from irregular regions with AlN–SiO 2 grains on a Si substrate, i.e., the roughness of substrate affects nanoflower formation by causing nanoburs to protrude, exposing them to higher gas concentrations.

Microstructure of TaC coatings on carbon fibers

This paper examines the possibility of modifying the surface of carbon fibers with tantalum carbide via reactive chemical vapor deposition at temperatures from 800 to 1000°C. We have studied the surface morphology and topography of the coatings on carbon fibers and determined their phase composition by a variety of physicochemical characterization techniques, including X-ray diffraction, high-resolution scanning electron microscopy, and atomic force microscopy. The strength of the modified fibers has been measured at room temperature. The results demonstrate that the coatings are continuous and uniform in thick-ness along and across the monofilaments, with good surface adhesion. The microstructure and surface topography of the coatings depend on the deposition time and temperature.

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.

Influence of Chemical Vapor Reaction SiC Coating on Mechanical Properties of C/C Composites

SiC coatings were prepared on carbon/carbon(C/C) composites by chemical vapor reaction(CVR) method.The structure and morphology of the coating were characterized by XRD,SEM and EDS analysis,and the influence of SiC coating and preparation technology on the fracture behavior of the composites were investigated by three point bending tests.The results show that the flexural strength and maximum fracture displacement of the composites are obvious improved after the coating process.The average flexural strength of carbon/carbon composites is 172.4MPa,while the average flexural strength of the coated composites is 239.8MPa.The improvement of the flexural strength is mainly contributed to the improved interface bonding caused by the vapor diffusion and reaction in the preparation process and the SiC particle reinforcement.Furthermore,the fracture mode of the coated composites is not changed and the coated samples also exhibit pseudo-plastic fracture and ductile frature character.

Electron emission from diamond films seeded using kitchen-wrap polyethylene

Diamond has many potential electronic applications, but the diamond seeding methods are generally harsh on the substrates rendering them unsuitable for integration in electronics. We report a non-abrasive, scalable and economic process of diamond film seeding using kitchen-wrap polyethylene employing hot filament chemical vapour reaction of H2S/CH4/H2 gas mixtures on Cu substrates. The fabricated diamond films were characterized with scanning electron microscopy, transmission electron microscopy and Raman spectroscopy, which confirm that the deposited film consists of a microcrystalline diamond of size in the range 0.5–1.0 µm. The synthesized diamond films exhibit a turn-on field of about 8.5 V µm−1 and long-term stability. Diamond film synthesis using polyethylene will enable the integration of diamond heat sinks into high-power and high-temperature electronic devices.

A Novel Route to the Synthesis of Silica Nanowires without a Metal Catalyst at Room Temperature by Chemical Vapor Deposition

Silica nanowires were synthesized by employing inherent directionality of chemical vapor reaction between bis(ethylmethylamino)silane (H(2)Si[N(C(2)H(5))(CH(3))](2)) precursor and water without a metal catalyst at room temperature. The difference in the oxidation reactivity between Si-H and Si-N bonds with water leads to the formation of silica nanowires. The mean diameter and length of the silica nanowires grown for 10 min were 60-80 nm and 1.9 μm, respectively. Transmission electron microscopy revealed that the obtained nanowires had the concave tip, differing from other silica nanowires produced by a conventional vapor-liquid-solid method, and were amorphous. Energy dispersive X-ray spectroscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy results also proved that the nanowires have a close composition to stoichiometric SiO(2). Silica nanowires were successfully synthesized on a poly(ethylene terephthalate) film. The nanowires can emit strong blue light and ultraviolet light under excitation at 266 nm.

Synthesis and mechanism of single-crystalline β-SiC nanowire arrays on a 6H-SiC substrate

The single-crystalline cubic-silicon carbide (β-SiC) nanowire (NW) arrays on a 6H-SiC substrate (0001) were synthesized via a simple chemical vapor reaction (CVR) approach at 1250 °C by using Ni as the catalyst. The obtained nanostructures were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). The results indicate that the obtained arrays are high purity β-SiC single crystals aligned along the [111] direction. In order to explain the growth process of the β-SiC nanowire arrays on the 6H-SiC substrate, a reasonable quasi-homogeneous substrate vapor–liquid–solid (VLS) growth mechanism is proposed.

Direct growth of β-FeSi2 nanowires with infrared emission, ferromagnetism at room temperature and high magnetoresistance via a spontaneous chemical reaction method

Self-catalyzed β-FeSi2 nanowires with a high aspect ratio have been synthesized by a spontaneous chemical vapor transport and reaction method. The as-synthesized single-crystalline β-FeSi2 nanowires exhibit photoluminescence at a wavelength of 1.54 μm, which is suitable for Si-based optical communication, at room temperature. The conductivity of a single-stem β-FeSi2 nanowire was found to increase with decreasing diameter. The room temperature ferromagnetism and high magnetoresistance performance of 6% at 200 K indicates that β-FeSi2 nanowires are potentially applicable for spintronic nanodevices.

Hollow BN microspheres constructed by nanoplates: synthesis, growth mechanism and cathodoluminescence property

Hollow microspheres constructed by single-crystalline hexagonal boron nitride (h-BN) nanoplates have been synthesized via a facile chemical vapour reaction route employing ammonia borane as a starting material. The as-synthesized products are extensively characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). The hollow microspheres are in nest-like morphology with diameters of ∼3 μm and are composed of numerous single-crystalline h-BN nanoplates of 200–500 nm in diameter and 10–30 nm in thickness. The dependence of the morphology of the BN hollow microspheres on experimental parameters, such as reaction temperature, holding time and gas pressure, is systematically investigated. The reaction temperature and holding time are proved to be key parameters to tailor the morphology of the final products. Controlled experiments indicate that the growth process of the BN hollow microspheres involves the formation of smooth bowl-shaped hollow microspheres and their subsequent growth into the hierarchical ones. The BN hollow microspheres exhibit intense cathodoluminescence emissions in the region of 200 to 400 nm, indicating that they could be potentially used as compact ultraviolet (UV) laser emitters.

Morphology-dependent field emission characteristics of SiC nanowires

Felted, curly, and straight β-SiC nanowires (SiCNWs) had been synthesized by chemical vapor reaction, and morphology-dependent field emission (FE) characteristics were chiefly investigated. The turn-on and threshold fields were 2.0 and 5.3 V/μm for felted nanowires, 1.5 and 3.25 V/μm for curly nanowires, and 1.0 and 2.05 V/μm for straight nanowires, respectively, suggesting that morphology played an important role in FE properties of the SiCNWs. Magnetic field shielding effect is proposed to explain the morphology-dependent FE characteristics, and it may stand as a nice referential work for researching FE properties of other wirelike materials.

Preparation and Ultraviolet−Visible Luminescence Property of Novel BN Whiskers with a Cap-Stacked Structure

A new kind of boron nitride (BN) whisker with a novel cap-stacked structure and excellent ultraviolet−visible luminescence properties has been successfully prepared via a facile chemical vapor reaction route. SEM analysis shows that the BN whiskers are uniform with a mean diameter of 1 μm and a length up to at least 1 mm. TEM images and the electron diffraction patterns indicate that the BN whiskers are constructed by stacking numerous cap-shaped BN sheets along the axial direction. The BN sheets are almost perpendicular to the whisker axis. The chemical compositions and the bonding characteristics of the BN whiskers are confirmed by EELS, XPS, and FTIR results. The diameters of the BN whiskers could be controlled by tuning the reaction pressure and temperature. The growth mechanism and morphology evolution of the BN whiskers are explained on the basis of the vapor−liquid−solid mechanism. Both photoluminescence and cathodoluminescence spectra of the BN whiskers exhibit broad emission bands from 200 to 700 nm, indicating that they could be used as compact light emitters at both ultraviolet and visible wavelengths.

Novel BN Hollow Microspheres with Open Mouths – Facile Synthesis, Growth Mechanism, Resonant Raman Scattering Effect, and Cathodoluminescence Performance

AbstractNovel BN hollow microspheres have been successfully fabricated by a facile chemical vapour reaction approach using ammonia–borane as a precursor. The morphology and structure of the as‐synthesized products are characterized by scanning electron microscopy (SEM), high‐resolution transmission electron microscopy (HRTEM), selected‐area electron diffraction (SAED), as well as electron energy loss spectroscopy (EELS). The hollow microspheres have open mouths and are 0.5–6 μm in diameter. The walls of the hollow spheres with thicknesses ranging from 200 to 800 nm are composed of poorly crystallized BN. The growth mechanism of the hollow microspheres is investigated and a growth model is proposed. The BN hollow spheres show pronounced resonant Raman scattering character under illumination of a 457.9 nm laser line, which implies a potential photothermal effect. The hollow microspheres also exhibit intense cathodoluminescence emissions in the region of 200–400 nm, indicating that they could be potentially used as compact ultraviolet laser emitters. Moreover, the unique open‐mouth feature, in combination with the superior thermal and chemical stability of BN, makes the BN hollow microspheres potential candidates as microreactors for investigations of high‐temperature space‐confined chemical reactions.

Large-Scale Fabrication of Boron Nitride Nanotubes via a Facile Chemical Vapor Reaction Route and Their Cathodoluminescence Properties.

Cylinder- and bamboo-shaped boron nitride nanotubes (BNNTs) have been synthesized in large scale via a facile chemical vapor reaction route using ammonia borane as a precursor. The structure and chemical composition of the as-synthesized BNNTs are extensively characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and selected-area electron diffraction. The cylinder-shaped BNNTs have an average diameter of about 100 nm and length of hundreds of microns, while the bamboo-shaped BNNTs are 100–500 nm in diameter with length up to tens of microns. The formation mechanism of the BNNTs has been explored on the basis of our experimental observations and a growth model has been proposed accordingly. Ultraviolet–visible and cathodoluminescence spectroscopic analyses are performed on the BNNTs. Strong ultraviolet emissions are detected on both morphologies of BNNTs. The band gap of the BNNTs are around 5.82 eV and nearly unaffected by tube morphology. There exist two intermediate bands in the band gap of BNNTs, which could be distinguishably assigned to structural defects and chemical impurities.

Cold plasmas in reactive physical and chemical vapour deposition processes

Progress in plasma chemistry and plasma physics set up possibilities to fabricate high quality compound films for various applications by low-temperatur PVD and CVD techniques. Industrial ion platting processes for the production of wear resistant coatings on tools and machine parts and of hard and stabile optical coatings as well as a plasma polymerization process for the production of protective coatings on plastics are described. Die Fortschritte in der Plasmachemie und Plasmaphysik eröffnen neue Möglichkeiten, hochqualitative chem. Verbindungsschichten für verschiedene Anwendungen durch Niedertemperatur PVD und CVD herzuustellen. Es werden industrielle Ionenplattier-Prozesse zur Erzeugung von Verschleiß-Schutz-Schichten auf Stahlwerkzeugen und Maschinenteilen sowie zur Herstellung von harten und stabilen optischen Schichten beschrieben. Weiter wird ein Plasmapolymerisationsprozess zur Erzeugung von harten Schutzschichten auf Plastik vorgestellt.

Synthesis of TiC from porous carbon coating on Si–C–O (Nicalon) fibres by reactive chemical vapour deposition in pressure-pulsed mode or at atmospheric pressure

TiC coatings were synthesised on NL202 Nicalon fibres through the treatment of a porous carbon coating at the surface of the fibres by thermal reactive chemical vapour deposition (RCVD) either in pressure-pulsed (P-RCVD) mode at low pressure or at atmospheric pressure (AP-RCVD) with a H2/TiCl4 gaseous mixture. The conversion rate of C into TiC was studied as a function of the H2/TiCl4 pulse number for the P-RCVD method and as a function of the treatment time for the AP-RCVD method. Chemical and micro-structural characterisations of the coatings were carried out by X-ray photoelectron spectroscopy, Auger electron spectroscopy and scanning and transmission electron microscopies. Mechanical assessment was achieved by tensile tests at room temperature. For the P-RCVD method, the conversion rate increased when the number of H2/TiCl4 pulses increased from 20 to 100. Once 20 H2/TiCl4 pulses were performed, an interlayer made of TiC was observed at the carbon coating/fibre interface indicating that the H2/TiCl4 gas has diffused inside the porous carbon and has reacted in depth and more particularly at the coating/fibre interface. With 100 pulses, the carbon coating was totally converted. For the AP-RCVD method, the conversion rate increased when the treatment time increased from 30 to 60 min. The reaction of conversion began preferentially at the outermost surface of the carbon coating but several isolated TiC grains were also observed at the coating/fibre interface. At atmospheric pressure, the gaseous mixture was more reactive than at low pressure whereas the diffusion of the gaseous species was limited inside the carbon coating. The mechanical properties of the fibre decreased when the conversion rate of the carbon into TiC increased both in P-RCVD and AP-RCVD.

Thermal conductivity of porous SiC composite ceramics derived from paper precursor

Biomorphic porous SiC composite ceramics were produced by chemical vapor infiltration and reaction (CVI-R) technique using paper precursor as template. The thermal conductivity of four samples with different composition and microstructure was investigated: (a) C-template, (b) C-SiC, (c) C-SiC–Si 3N 4 and (d) SiC coated with a thin layer of TiO 2. The SiC–Si 3N 4 composite ceramic showed enhanced oxidation resistance compared to single phase SiC. However, a key property for the application of these materials at high temperatures is their thermal conductivity. The later was determined experimentally at defined temperatures in the range 293–373 K with a laser flash apparatus. It was found that the thermal conductivity of the porous ceramic composites increases in the following order: C-template < C-SiC < C-SiC–Si 3N 4 < SiC–TiO 2. The results were interpreted in regard to the porosity and the microstructure of the ceramics.