Masato Ohkohchi

Production of petal-like graphite sheets by hydrogen arc discharge

By DC arc-discharge evaporation of graphite in the presence of rarefied hydrogen gas, not only carbon nanotubes but also petal-like graphite sheets were produced on the graphite cathode as a carbon deposit. A large number of interlaced petal-like graphite sheets were observed by scanning electron microscopy on the cathode surface which surrounded the region of carbon deposit including multiwalled carbon nanotubes. The crystal structure of these petal-like graphite sheets was confirmed to be stacking of graphene by using a selected area electron diffraction pattern. Here, we propose a mass production method of petal-like graphite sheets by hydrogen arc discharge evaporation.

Preparation of high-grade carbon nanotubes by hydrogen arc discharge

Fine and long multiwalled carbon nanotubes attached with less carbon nanoparticles were prepared by d.c. arc discharge plasma of graphite electrodes in hydrogen gas. These high-grade carbon nanotubes grew on the central part of the cathode as a carbon deposit like black soot. They were mainly observed by a scanning electron microscope and subsequently by a high resolution transmission electron microscope. By comparing with helium and methane gases, the predominance of the use of hydrogen gas as the environmental gas is discussed. A new type of carbon allotrope, namely petal-like graphite sheets consisting of a number of interlaced graphene sheets, was also found in the outside region surrounding the central cathode deposit.

Yield of fullerenes generated by contact arc method under He and Ar: dependence on gas pressure

The yields of toluene-soluble material from carbon soot depend on the buffer gas as well as the pressure. Helium was more effective for yielding fullerenes than argon, and the optimum pressure was 20 Torr, under which a maximum yield of about 13 wt% was obtained. C60 showed a maximum abundance at 20 Torr, while C70 and higher fullerenes (C76, C78 and C84) at a slightly higher pressure between 20 and 50 Torr. Raw soot was also studied by electron microscopy. The pressure dependence of the fullerene yield is discussed in terms of the cooling rate and diffusion of carbon vapor around the evaporation source.

Morphology of Carbon Nanotubes Prepared by Carbon Arc

Carbon nanotubes were obtained as deposits on the cathode of carbon arc in He and CH4 gases. The morphology of carbon nanotubes was observed by scanning electron microscopy of the top surface of the cathode deposit, and was found to change with evaporation conditions. The variation of carbon nanotube morphology was more marked for the case of evaporation in CH4 gas than that in He gas. Thick carbon nanotubes embellished with a number of nanoparticles were obtained on the cathode of carbon arc under the conditions of high CH4 gas pressure and high dc arc current.

SPONGE OF PURIFIED CARBON NANOTUBES

We first used an infrared radiation heating system in air to purify multiwalled carbon nanotubes (MWNTs) containing unnecessary carbon nanoparticles. The raw MWNTs were obtained in the cathode deposit when a graphite electrode was evaporated in hydrogen gas by DC arc discharge. The purification reached the depth of 0.1 mm from the deposit surface, which revealed highly oriented MWNTs vertical to the cathode surface. Using this methode, we can easily obtain a spongy bulk of highly oriented and purified MWNTs, with the size on the order of 10 mm2 in surface area and 0.1 mm in thickness.

Electric conductivity and band gap of solid C60 under high pressure

Abstract Measurement of the electric resistivity of compressed C60 powder up to 22 GPa at ambient temperature has revealed that the electric conductivity changes four orders of magnitude from 2 × 10−6 Ω−1 cm−1 (below 8 GPa) to 3 × 10−2 Ω−1 cm−1 (around 20 GPa). From the temperature dependence (ambient temperature to 400°C) of the conductivity, which behaves like a semiconductor, the band gap is derived as follows: 1.6 ± 0.1 eV at 3 and 6 GPa, and 1.2 ± 0.1 eV at 10 GPa.

SYNTHESIS OF SINGLE-WALLED CARBON NANOTUBES BY AC ARC DISCHARGE

We evaporated carbon bilateral rods containing different catalytic metals by AC arc discharge in He gas. No carbonaceous deposit was formed on either of the two electrodes, which meant that all of the evaporated carbon became soot in which single-walled nanotubes existed. It was clarified that the method is superior for high-efficiency synthesis of single-walled nanotubes.

Production of ultrafine powder of β-Sic by arc discharge

Abstract Silicon and carbon were simultaneously evaporated by an arc discharge in an atmosphere of rarefied argon gas. Evaporated silicon and carbon atoms react to form ultrafine powder of β -SiC in the gas. The mean diameter of its particles is 10–60 nm and larger in higher pressure. The β -SiC powder includes pure silicon particles, the amount of which can be reduced by evaporating in an atmosphere of methane gas. It also contains about 10% of amorphous carbon and a few percent of SiO 2 . The yield of the powder per unit input and time is 0.15 g/kW·min.

Morphology of carbon allotropes prepared by hydrogen arc discharge

Abstract Evaporation of graphite electrodes in H 2 gas by DC arc discharge was found to form not only multiwalled carbon nanotubes (MWNTs) but also graphene sheets in the cathode deposit. By irradiating the deposit surface using an infrared radiation heating system in air, the MWNTs could be easily purified. The morphology and structure of as-grown MWNTs, graphene sheets and purified MWNTs were investigated using scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. X-ray diffraction patterns of MWNTs and graphene sheets were similar to each other.

Carbon Nanotubes Grown on the Surface of Cathode Deposit by Arc Discharge

Abstract Carbon nanotubes prepared by de arc discharge of graphite electrodes in He and CH4 gas took markedly different morphology. Thick nanotubes embellished with many carbon nanoparticles were obtained by evaporation under high CH4 gas pressure and high arc current. Thin and long carbon nanotubes were obtained under a CH4 gas pressure of 50Torr and an arc current of 20A for the anode with a diameter of 6mm.