Preparation of carbon micro-coils involving the decomposition of hydrocarbons using PACT (plasma and catalyst technology) reactor

Carbon micro-coils as well as carbon fibers with various morphologies were prepared by the decomposition of hydrocarbons, such as acetylene, methane, propane, ethylene, etc., at 770°C using a PACT (plasma and catalyst technology) reactor. The preparation conditions, growth mechanism and morphology of the carbon micro-coils were examined. The Ni electrode of the PACT reactor was used as the catalyst as well as a plasma source electrode. It was found that hydrocarbons, such as methane, propane and ethylene, decomposed under the plasma and catalyst atmosphere to form acetylene as the main decomposition product, and then this acetylene was further decomposed to form carbon micro-coils. Using a Ni powder catalyst dispersed on the substrate, the carbon micro-coils with a double helix structure, in which two pieces of carbon coils entwine each other in the same coiling direction, grew among the single straight carbon fibers and paired straight fibers. On the other hand, the carbon micro-coils with a single helix structure and wide coil pitch were obtained by the indirect decomposition of acetylene using the N₂ plasma formed by the PACT reactor.     

Chemical vapour deposition of tantalum diboride

Tantalum diboride (TaB₂) was deposited on a quartz substrate from a gas mixture of TaCl₅, BCl₃, H₂, and Ar at a temperature between 900 and 1300° C. When the atomic ratio (B/Ta) in source gas was held above 1.0 at 1000° C, TaB₂ with a composition of between TaB_1.90 and TaB_1.95 was obtained in a single phase. The deposits grew to grain crystals with an increase in temperature and with an increase in the atomic ratio (B/Ta) in the source gas. The mass transfer of TaCl₅ was supposed to be the rate-determining step. The Vickers microhardness values for the coating deposited at 1100° C from a source gas with atomic ratio (B/Ta) above 1.0 were 3500 to 4100 kg mm^–2. Dispersing Ni or Pd on the substrate as an impurity, woolly crystals of up to 100m in length were grown in 30 min at 1050° C, and the growth mechanism was thought to be that of tip-VLS.     

Oxidation characteristics of the graphite micro-coils, and growth mechanism of the carbon coils

The oxidation characteristics of the graphite coils obtained by high-temperature heat treatment of the vapor grown carbon micro-coils were examined, and the growth mechanism of the carbon micro-coils is discussed. The ruptured cross section of the graphite coils with a circular cross section (circular graphite coils) exposed in air or an Ar atmosphere at 800–1400°C have generally negative or positive trigonal cone-forms. On the other hand, that of the graphite coils with flat or rectangular cross sections (flat graphite coils) have negative or positive rectangular cone or roof-like forms. The edge between two graphite layers was preferentially oxidized to form three deep striations that extended in the direction of the fiber axis, and then formed six coils from the double circular graphite coils. It is reasonably considered that eight thin coils are formed from the double flat graphite coils. These observations strongly supported the growth mechanism based on the catalytic anisotropy between the catalyst crystal faces.     

Crystal morphology of ternary compound (Cr,Fe)5Si3 obtained by in situ chemical vapour deposition

Ternary compound crystals of (Cr, Fe)₅Si₃ were obtained on a quartz substrate by the in situ CVD process using in situ reaction of the stainless steel 410 powder, Si₂Cl₆ and hydrogen, and its crystal morphology was examined in some detail. Crystals with various interesting morphologies, such as spiral, conical, rose-like, seaweed-like, globefish-like, etc., were obtained. The most commonly observed growth habits of the crystals were spiral, conical and seaweed-like crystals.     

Technique for calculation of the propagation constant in an optical planar waveguide with a gaussian profile

We performed analysis of a planar waveguide with arbitrary index variations. We obtained numerical results for the propagation coefficient by using first-order Langer and Liouville transformations. The accuracy of the numerical results is confirmed by a comparison with those obtained by other methods.     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Preparation and properties of TaC/C/TaC∼TaC composite micro-tubes by vapor phase tantalizing of the regular carbon micro-coils/micro-tubes

TaC/C/TaCTaC composite micro-tubes were prepared by the vapor phase tantalizing of the regular carbon micro-coils/micro-tubes, and the preparation conditions and some properties were examined. The carbon micro-coils with a tube-like morphology were tantalized from the surface to the core of the carbon fibers with full preservation of the tube-like morphology to form TaC/C/TaCTaC composite micro-tubes. The bulk electrical resistivity and specific surface area of the TaC/C/TaCTaC composite micro-tubes were 4 × 10^–3 to 5 × 10^–4 ·m and 5 × 10³ to 2 × 10⁴ m²/kg, respectively, depending on the tantalized ratio and the bulk density.     

A combination of hard and soft templating for the fabrication of silica hollow microcoils with nanostructured walls

Hollow silica microcoils have been prepared by using functionalized carbon microcoils as hard templates and surfactant or amphiphilic dye aggregates as soft templates. The obtained materials have been characterized by electron and optical microscopy, nitrogen sorption and small angle X-ray scattering. The obtained hollow microcoils resemble the original hard templates in shape and size. Moreover, they have mesoporous walls (pore size ≈ 3 nm) with some domains where pores are ordered in a hexagonal array, originated from surfactant micelles. The obtained silica microcoils also show preferential adsorption of cationic fluorescent dyes. A mechanism for the formation of silica microcoils is proposed.