H. Iwanaga

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.

Anisotropic thermal expansion in wurtzite-type crystals

The temperature dependence of the lattice parameters of six wurtzite-type crystals, BeO, AlN, GaN, ZnO, CdSe, and CdS, has been measured by X-ray powder diffractometry. The thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion along the a-axis is larger than that along the c-axis (more than 50% larger in ZnO, CdS and CdSe), resulting in a decrease of the c/a ratio with increasing temperature. The calculated Grüneisen parameters range between 0.27 and 1.45 and have a positive correlation with the bulk moduli. The anisotropic thermal expansion is associated with the anisotropies of both elasticity and anharmonicity.

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.

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.

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.

Stacking-fault energy of II–VI compounds

Abstract Stacking-fault energies on the {111} plane in ZnTe (zinc blende structure) and on the (0001) plane in ZnO (wurtzite structure) have been estimated to be 16 ± 4 and greater than 43 mJ m−2, respectively, by weak-beam electron microscopy of dislocations. The stacking-fault energies of the above crystals and previously obtained ones of other II-VI compounds, CdTe, CdS, CdSe, ZnSe and ZnS, are correlated with material parameters such as the ionicity of the crystal. It is found that the stacking-fault energies of II-VI compounds, together with those of six III-V compounds so far reported, show a strong correlation with the charge redistribution indices found by Phillips end Van Vechten (1969). The physical implication of the correlation is discussed.

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.

Preparation of micro-coiled carbon fibres by metal powder-activated pyrolysis of acetylene containing a small amount of sulphur compounds

Micro-coiled carbon fibres were prepared by the transition metal-activated pyrolysis of acetylene containing a small amount of sulphur compounds, and the preparation conditions were examined in detail. The coiled carbon fibres grew at the reaction temperatures of 700–850 °C and thiophene gas flow rates of 0.14–0.45 standard cm3min−1 (0.10–0.35 vol % reaction atmosphere). The optimum values depended on the type of metal catalysts used. Among the metal catalysts used, nickel, titanium and tungsten were the most effective for the growth of the coiled carbon fibres and a maximum yield of about 50%–55% was obtained. The bulk resistivity of the coiled carbon fibres decreased with increasing bulk density and was 100 S−1 cm at a bulk density of 1.

Carbon coatings on carbon micro-coils by pyrolysis of methane and their properties

Abstract Carbon micro-coils were coated with pyrolytic carbon films by the decomposition of methane in an argon atmosphere, and the coating conditions and some of their properties were examined. Carbon micro-tubes or micro-solenoids were obtained using very regular and densely-coiled carbon micro-coils as the source under the optimum CH4 flow rate of 10 sccm and Ar 50 sccm at 1000–1200°C. The bulk electrical resistivity, density and surface area of the carbon-coated coils were 1–0.1 Ω cm, 1.7090 g/cm3 and 5 m2/g, respectively. The oxidation onset temperature of the carbon micro-tubes obtained at 1200°C for 1h was 660°C, which is comparable to that of the vapor grown carbon fibers. The carbon micro-pipes or micro-solenoids obtained using the long carbon micro-coils of 1–10 mm long as the source could absorb a wide range of electromagnetic waves of 250–950 MHz, and the reflection loss (absorption) reached 90–95%.

Inter-leg angles in tetrapod ZnO particles

Abstract Measured sets of angles of legs in 28 tetrapod ZnO particles produced by oxidizing zinc in air have been analyzed in terms of the octahedral multiple twin (octa-twin) nucleus model proposed previously (J. Crystal Growth 134 (1993) 275; Phil Mag. A 69 (1994) 1125). Nineteen sets of them coincide with the angle relation for the octa-twin nucleus completely relaxed by cracking along twin boundaries; the initial octa-twin nucleus is accompanied by a large elastic strain of 5%. The other sets, except two, can also be well interpreted by the growth process from some types of incompletely relaxed nucleus, where legs grow with some nucleus twins unrelaxed without forming cracks.