Xiuqin Chen

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.

Three-dimensional vapor growth mechanism of carbon microcoils

Carbon microcoils were grown by the Ni-catalyzed pyrolysis of acetylene. The growth patterns and the tip morphologies of the carbon coils are examined in detail, and a growth mechanism is proposed. Basically, six thin fibers grew from a Ni catalyst grain during the initial growth stage immediately followed by the coalescence of the four fibers to form two fibers and then forming double-helixed carbon coils. A small amount of S and O, as well as C and Ni, was observed on the periphery of the cross section of the Ni catalyst grain. On the other hand, S and O were not observed in the central part. The driving force of the coiling of the straight fibers to form carbon coils is considered to be the strong anisotropy of the carbon deposition between different crystal faces.

Morphologies of carbon micro-coils grown by chemical vapor deposition

The carbon micro-coils were obtained by the Ni-catalyzed pyrolysis of acetylene. The carbon micro-coils with various coiling morphology: regular double coils, coils built up by circular or flat fibers, super helix coils, single coils, etc. can be observed. The carbon coils with various coil diameters and coil pitches were obtained by controlling reaction conditions, such as reaction temperature, source gas flow rate of sulfur-impurity, acetylene or hydrogen.

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%.

Morphology and growth models of circular and flat carbon coils obtained by the catalytic pyrolysis of acetylene

Carbon micro-coils, which is a kind of novel carbon material with a 3D double-helix/spiral structure similar to DNA, were obtained by the Ni-catalyzed pyrolysis of acetylene at 750 °C. The morphology and microstructure of these carbon coils were examined in detail. The growth mechanisms of the flat coils with slender-shaped cross-sections as well as circular coils with circular-shaped cross-sections are discussed. It is considered that the circular coils are predominantly formed during the initial growth stage and then change to flat coils with increasing reaction time because of the change in the catalyst shape from a cubic-form to a slender-form. A change in the catalyst form may be effected by the electromagnetic (EM) field (force) emitted from the outer electric heater.

Tactile microsensor elements prepared from arrayed superelastic carbon microcoils

Superelastic carbon microcoils (SCMCs) with high elasticity and coiling chirality were prepared by the Ni-catalyzed pyrolysis of acetylene, and novel tactile microsensor elements using the SCMCs as the sensor material were prepared. The sensor elements with a very small size of 80×80×80μm3 showed a very high sensitivity of 0.3mgf (1Pa). It was found that the SCMC arraying in the matrix and the placement method on the electrodes dramatically affected the sensing properties.

Coiling-chirality changes in carbon microcoils obtained by catalyzed pyrolysis of acetylene and its mechanism

As can be seen in the double helix of DNA, the single helix of proteins, etc., the three-dimensional (3D) helical/spiral structure is a fundamental structure of living things, and affords them critical functionalities. Helically coiled carbon fibers, which usually take the peculiar form of either a microcoil or a helix or twisted form, referred to as “carbon microcoils hereafter,” are of great interest due to their novel functionality and various potential applications. They can potentially be used in electromagnetic absorbers and/or filters, 3D composites, smart tunable electrical devices, microsensors, chiral catalysts, etc.

Observation and analysis of percolation behavior in carbon microcoils/silicone-rubber composite sheets

The electrical properties of carbon microcoils (CMCs)/ silicone-rubber composites were studied on the changes in the values of the electrical parameters (impedance, phase angle, resistance, and capacitance) as a function of the CMC content in the matrix, using an impedance analyzer in the frequency range from 40to200kHz. Percolation paths were observed at a 3wt% CMC content in the matrix. The properties of the composites were separated at the percolation threshold. The capacitance with a small value was dominant at CMC content less than 3wt%, and the resistance was dominant at CMC content higher than 3wt%.

High-temperature heat treatment of carbon microcoils obtained by chemical vapor deposition process and their properties

Carbon microcoils obtained by the catalytic pyrolysis of acetylene at 770 °C were heat treated at 3000 °C for 6 h in a CO + CO 2 atmosphere. The effect of the heat treatment on the morphology, microstructure, and properties was examined. The coiling morphology of the carbon coils was almost preserved even after the heat treatment, though it became brittle. The ruptured cross section of the two fibers, which form the coils, generally has either a trigonal pyramidlike form or negative pyramidal hollow. These characteristic ruptured patterns demonstrate the growth mechanism of the carbon coils. Distinct graphite layers ( d = 0.339 nm) were developed by the heat treatment with an inclination of 10–40° versus the fiber axis to form a “herringbone” structure. The bulk electrical resistivity, density, and specific surface area were 10–0.1 Ωcm, 2.2077–2.087 g/cm 3 , and 6–8 m 2 /g, respectively.

Effect of external electromagnetic field and bias voltage on the vapor growth, morphology and properties of carbon micro coils

Abstract Carbon microcoils were obtained by the Ni-catalyzed pyrolysis of acetylene at 770°C using thiophene as an impurity with and without an external electromagnetic (EM) field and by applying or not applying a bias voltage to the substrate. The effects of the external EM field and bias voltage on the growth rate, morphology and some properties of the carbon microcoils were examined. It was observed that the external EM field accelerated the decomposition of acetylene resulting in a coil yield of 1.3–1.7 times higher than that without the external EM field. The application of the bias voltage also resulted in an increase of the coil yield; the yield linearly increased with increasing bias voltage. The coil yield obtained with a DC bias of 600 V and an AC bias of 2000 V with an applied external EM field was 1.9 and 2.1 times higher than that without the EM field and bias voltage, respectively. The very regularly-coiled carbon fibers with flat fiber cross-sections and irregularly-coiled carbon fibers with circular fiber cross-sections were obtained with the EM field and without the EM field irrespective of the bias voltage, respectively.