Seiji Motojima

Growth of regularly coiled carbon filaments by Ni catalyzed pyrolysis of acetylene, and their morphology and extension characteristics

Regularly coiled carbon filaments have been obtained by the catalytic pyrolysis of acetylene at 350–750 °C using Ni plate and powder as a catalyst. Morphology and extension characteristics of the obtained coiled filaments were examined in some detail. The regularly coiled filaments have generally a 0.1–0.3 μm thickness, a 2–8 μm coil diameter, and a 0.1–5 mm coil length. The coiled filaments were always formed by the entwistness of two pair coils which grew in the same direction simultaneously from a diamond‐shaped Ni seed. We have found that the coiled filaments could be elastically extended up to about three times versus the original coil length.

Low temperature deposition of metal nitrides by thermal decomposition of organometallic compounds

Decomposition reaction of dialkylamides of boron, silicon, tin, titanium, zirconium, niobium, and tantalum was investigated. The amides of transition metals decomposed to the corresponding nitrides at 300~176 whereas those of boron, silicon, and tin yielded elemental deposits at higher temperatures. In the deposition of titanium nitride from titanium tetrakis(dimethylamide), two optimum temperatures at 400 ~ and at 800~ in nitrogen or hydrogen atmosphere were found, but in argon only low temperature deposition was possible. The low and high temperature processes are discussed in relation to mass spectral analysis of the exhaust gases formed at various decomposition temperatures of titanium amide. Metal nitrides are well known to have outstanding physical and chemical properties and have been prepared by the reactions of metal halides or hydrides with nitrogen + hydrogen or ammonia, or of metals with nitrogen (1). These reactions usually require temperatures higher than 1000~ which have limited the application of the nitrides as the useful coatings on many materials. Therefore, their preparation at lower temperature, where deformation of substrate materials can be avoided, should be important.

Preparation of coiled carbon fibers by catalytic pyrolysis of acetylene, and its morphology and extension characteristics

Abstract Regularly coiled carbon fibers have been obtained by the catalytic pyrolysis of acetylene at 330 to 750°C using a Ni plate and powders as a catalyst. We have examined the morphology and extension characteristics of the obtained coiled fibers in some detail and discussed the growth mechanism. It has been observed that the coiled fibers were almost formed by the crossing or entwisting of two primary coils which grew in the same (coiling) direction simultaneously from a diamond-shaped nickel seed (combination coils). Other type of the coiled fibers, such as a single coils, double coils, and flat coils were also sometimes observed. The coiled fibers generally have a 0.1 to 0.3 μm thickness, 2 to 8 μm coil diameter, and 0.1 to 5 mm coil length. We have found that the coiled fibers could be extended elastically up to about three times versus the original coil length and semielastically up to about 4.5 times.

Three-dimensional growth mechanism of cosmo-mimetic carbon microcoils obtained by chemical vapor deposition

Cosmo-mimetic carbon microcoils with a three-dimensional helical/spiral structure similar to deoxynucleic acid, having coil diameters of 3–6 μm and coil lengths of 5–8 mm could be obtained by the catalytic pyrolysis of acetylene. A three-dimensional growth model of the carbon coils, based on the anisotropy for the carbon deposition among three crystal faces, is proposed. The microcoiling morphology is formed by the rotation of a Ni catalyst grain, from which six fibers grow and coalesce with each other, followed by the formation of two fibers, and these two fibers entwine in the same direction and at the same speed of about one cycle per second around the coil axis.

A growth mechanism of regularly coiled carbon fibers through acetylene pyrolysis

Abstract Regularly coiled carbon fibers were prepared by Ni catalytic pyrolysis of acetylene. A small amount of H 2 S was indispensable for the growth of the coiled carbon fibers. A Ni compound seed observed on the tip of the pair-coiled carbon fibers is a single crystal. It is suggested that each crystal plane of the Ni compound seed has a different catalytic ability for the growth of the coiled carbon fibers. A growth mechanism for the coiled carbon fibers called the “quasi-VLS mechanism” is proposed, which involves the surface diffusion of carbon species on the Ni compound seed.

Electromagnetic wave absorption property of carbon microcoils in 12–110 GHz region

The electromagnetic (EM) wave absorption properties in the gigahertz region (12–110 GHz) of carbon microcoils (CMCs) with a three-dimensional-helical/spiral chiral structure (1–10 μm coil diameter and 0.1–10 mm coil length) were examined using the open space method. It was found that the target value of reflection loss, over −20 dB (above 99% absorptivity) necessitated for commercial applications, could be obtained for EM absorption composites of only 1–2 wt % addition of CMCs in a polyurethane matrix for the 30–35, 50–55, 75–80, and 95–100 GHz bands. A CMCs addition of more than 3 wt % resulted in a decrease of the EM wave absorptivity, because of the increase in the reflection of the EM waves. Multilayer absorption composites showed a higher EM absorptivity than that of single-layer composites. The longer the coil length, the higher the absorptivity could be obtained. The absorption mechanism of the EM waves by CMCs is discussed.

Catalytic effects of metal carbides, oxides and Ni single crystal on the vapor growth of micro-coiled carbon fibers

Abstract Micro-coiled carbon fibers were prepared by the impurity-activated chemical vapor deposition using the powders of metal carbides, metal oxides, and a Ni single crystal. The catalytic effect of these impurities on the coil yield and morphology were examined, and the growth mechanism is discussed. The pre-oxidized Ti plate, Ti 2 O 3 and TiC powders, and a Ni single crystal as well as various metal powders provided the micro-coiled carbon fibers. For the Ni single crystal, the Ni(100) crystal gave the highest coil yield followed by Ni(111)and then Ni(110). It is considered that the anisotropic deposition rate of a carbon among the Ni single crystal planes is the driving force of the coiling of carbon fibers to form the coiled fibers.

The growth patterns and morphologies of carbon micro-coils produced by chemical vapor deposition

Abstract Carbon micro-coils with coil diameters of micron orders were obtained by the Ni-catalyzed pyrolysis of acetylene containing a small amount of thiophene impurity. The effect of reaction conditions on the growth patterns and morphologies of the carbon coils were examined in detail. The carbon coils with very regular and small coils diameter of about 5 μm, regular coil pitches, and a coil length of >8 mm, could be obtained with the maximum coil yield of about 20 mg/cm 2 -substrate, and the maximum thickness of coil layers of about 8 mm under optimum reaction conditions. The coil pitch and coil diameter could be controlled by controlling the gas flow rate and ratio, and the reaction temperature. The carbon coils prepared by different reaction times from the initial to 2 h or more can be vividly described as ‘embryo coils’, sprout coils, short coils and long coils with increasing reaction time.

Preparation of coiled carbon fibers by pyrolysis of acetylene using a Ni catalyst and sulfur or phosphorus compound impurity

Coiled carbon fibers were prepared by pyrolysis of acetylene activated using a Ni catalyst and a small amount of impurities at 600–800 °C. It was found that the addition of a small amount of sulfur or phosphorus compound impurities in acetylene was indispensable for the growth of coiled carbon fibers. Among the sulfur compounds used, thiophene was the most effective for growing coiled carbon fibers with uniform coil diameter and producing a high yield (about 50% coils). Similar results were obtained with phosphorus trichloride.

Vapor deposition of thin cadmium sulfide layers using thermal decomposition of dithiolatocadmium complexes

Deposition process of CdS thin layers by pyrolysis of dimethyldithiophosphinatocadmium complex, [(CH3)2PS2]2Cd, is reported. The deposit obtained by the process is optically, electrically and chemically pure wurtzite-type cadmium sulfide. The substrate effect on the deposition rates and crystal orientation is observed and discussed.