The first specimen, which was used by Iijima to discover carbon nanotubes (CNTs), was prepared by arc discharge evaporation of graphite by the present author. After then, the arc discharge evaporation is one of the powerful methods to produce CNTs. Here, the growth condition of CNTs in arc discharge method is reviewed, especially focused on the kind and the pressure of environmental gas for multiwalled carbon nanotubes (MWNTs) growth. Pure H_2 gas atmosphere for arc discharge evaporation is effective to prepare MWNTs having highly crystallized and extremely thin innermost tubes, In the case of single wall carbon nanotubes (SWNTs) growth by arc discharge evaporation, the kind of catalyst also plays an important role. Arc evaporation of graphite rod including iron catalyst in H_2 and Ar mixed gas produces macroscopic SWNTs Web.
Various metal catalysts are used to synthesize single-wall and double-wall carbon nanotubes by arc discharge method. Metals used commonly as catalysts are classified into the iron-group, the platinum-group and the rare-earth elements. The catalytic activities are greatly enhanced by combining two (or more) metals for certain combinations. Effects of catalysts on the structure and the yield of carbon nanotubes are first reviewed. Then, recent findings on field emission of electrons from carbon nanotubes are presented. Field emission microscopy revealed the preferential emission of electrons from the pentagons on a nanotube tip and the adsorbed molecules. We succeeded in observing atomically resolved five carbon atoms forming a pentagon by field ion microscopy.
Single-wall carbon nanohorns (SWNHs) are a type of single-wall carbon nanotubes (SWNTs). In forming SWNTs, metal catalysts are necessary, while SWNHs are formed without metal catalysts. We compared the formation process of these two materials, and discussed the methods of growing SWNTs without metal catalysts.
The d-CNTs were produced by direct-vaporization of graphite without metal catalyst, using radio-frequency (RF) plasma. They were quite high in purity. The d-CNTs are multi-wall CNTs packed densely toward the center, and the innermost tubes have a diameter of 0.4nm, the smallest diameter possible for a single-wall carbon nanotube. All of them had acute tips with a cone angle of 19.2°. We observed that most of these d-CNTs became hollow after heat treatment at more than 2200℃ in vacuum. The inner diameter of the hollow multi-wall CNTs formed by heat treatment of the d-CNTs was about 3nm. We deduce that the inner nanotubes narrower than about 3 nm decomposed faster than the outer nanotubes did because the inner tubes had more structural stress due to their small diameters. This enables us to strictly control the outer and inner diameters of CNTs.
By using alcohol as carbon source molecules, high-purity single-walled carbon nanotubes are grown by the catalytic CVD process at low-temperature. The proposed alcohol CCVD method demonstrates its excellence in high yield production of SWNTs when Fe/Co supporting USY-zeolite powder is employed as a catalyst. At an optimum CVD condition, the SWNT yield of more than 800 wt % is achieved over the weight of the catalytic metal. In addition to the advantages in mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with a dip-coat technique to mount catalytic metals on the surface of substrates. This method allows an easy and costless loading of catalytic metals without the need of any support or under-layer materials that are required in previous studies. Finally, the result of molecular dynamics simulation for the SWNT growth process is demonstrated to obtain a fundamental insight of initial growth mechanism on the catalytic particles.
Well-aligned carbon nanotube (CNT) films were synthesized by surface decomposition of silicon carbide (SiC) (0001). In the initial stage of the decomposition, at 1200-1250℃, the generation of semispherical carbon caps of several nanometers all over the surface of SiC was found by cross-sectional transmission electron microscopy (TEM) and atomic force microscopy (AFM) . The diameter of the grown CNTs is determined initially by that of the nanocaps. Moreover, zigzag-type CNTs are selectively produced by surface decomposition of a well-polished SiC single crystal. The SiC wafer was heated to 1500℃ at a very small heating rate under vacuum. TEM and electron diffraction patterns revealed that almost all the well-aligned CNTs formed perpendicularly to the SiC (0001) surface are double-walled and of zigzag type. In addition, the results of high-resolution electron microscopy (HREM) indicate that the zigzag-type structure evolves from Si-C hexagonal networks in the SiC crystal by the collapse of carbon layers remaining after the process of decomposition.
Studies of the deposit concentration and crystal shape of Monoammonium Glycyrrhizinate (MG) were made. For the purpose of observation of basic data for batch crystallization experiment, the deposit concentration was measured at different mixtures (75-25, 80-20 and 85-15) vol % of ethanol-water. To obtain the relation between MG purity, AT (difference of the temperature of a fixed deposit concentration solution and crystallization temperature) and crystal shape, the shape of produced crystals was observed at different MG purity and different ΔT (ΔT=1, 5, 10, 20, 30 and 40℃) by a controlled batch crystallization. In the case of MG purity 68.8 wt %, rectangle crystal shape was observed. In the case of MG purity 50 wt %, the crystal shape changed from rectangle to octagon and hexagon-like. In the case of MG purity 35.2 wt %, lozenge crystal shape was observed. It was found that the degree of purity and ΔT influenced markedly the shape of produced crystals.