Activities of Novel Materials Synthesis TEchnology Group in frontier carbon technology Project

Yoshinori Koga   

Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology AIST Tsukuba Central 5, 1-1-1 Higashi Tsukuba, Ibaraki-ken, 305-8565,Japan

 

Keywords: Carbon nanotube, nanofibers, polymerization of nanotube.

 

In the group of the novel materials synthesis technology, R & D is carried out to synthesize novel carbon materials such as nanotube, pressure-induced polymer of single carbon nanotube, heterodiamond, carbon nitride etc. using thermal decomposition, MWCVD and high pressure techniques.

In this paper we report (1) the higly oriented carbon nanotubes by sublimation decomposition from single crystal of SiC, (2) the highly oriented carbon nanofibers and hollow nanofibers by MWCVD from methane and argon mixture gas and, (3) the polymerization of single carbon nanotubes by high pressure method.  　　

 

(1)  Highly Oriented Carbon Nanotubes by Sublimating Decomposition of SiC film: (Dr.M.Kusunoki et al.)

      Carbon nanotube(CNT) has novel electric, optical, chemical and mechanical properties according to its unique configuration. CNTs have been produced by carbon-arc discharge technique, catalytic pyrolysis of hydrocarbons and condensed phase electrolysis. In many cases CNTs are usually oriented randomly with a wide distribution of diameter and length. Kusunoki(JFCC) et al. have developed a very simple method of producing highly oriented CNTs. The alignment method is based on the self-organization by sublimation decomposition of single crystal SiC in a vacuum at the temperature between 1500 and 1700 °C. Fig.1 shows the TEM micrograph of CNT film on the surface of an α-SiC(0001) wafer heated at 1700 °C  for 30 min. The formation mechanism of CNT is proposed according to the results of the observations of high-resolution transmission electron microscopy.  In the initial process of the formation of CNT the graphite nanocaps of 2-5 nm in size were observed. The surface of the SiC is oxidized by residual oxygen in the chamber and SiO gas and solid carbon are generated. With the evaporation of SiO gas the graphite sheets are successively deformed to form nanocaps of graphite. And then cylindrical graphite sheets with the diameter of the nanocaps grow perpendiculaly on the SiC. The CNTs grow, eroding the SiC single crystal.

Figure 1.Higly oriented CNTs produced at 1700 °C for 30 min.

 

(2) Highly oriented carbon nanofibers and hollow nanofibers by MWCVD: (Mr.F.Hoshi et al.)

      In Figs. 2(a) and 2(b) highly oriented carbon nanofibers and hollow carbon nanofibers grown by MW ECR-CVD method using methane and argon mixture gas at a temperature of 550 °C are shown. The carbon nanofibers and the hollow carbon nanofibers were grown perpendicularly on Si substrates and on Si substrates coated with Ni catalyst, respectively. From TEM analysis the diameter and length of the nanofibers are about 60 nm and 15μm, respectively. Raman spectra of these highly oriented carbon nanofibers showed new bands of 1340 and 1612 cm-1 of the first-order Raman scattering and 2660, 2940, and 3220 cm-1 of the second-order Raman scattering. By the measurement of XPS C1s band energies of 284.6 for the carbon nanofiber and 284.7 eV for the hollow carbon nanofiber indicate mainly sp2 carbon component.  Field emission characteristics of the highly oriented carbon nanofibers and hollow carbon nanofibers were investigated and the current densities were 7.25 mA/cm2 and 0.69 mA/cm2 at 12.5 V/μm, respectively.
 

. Figure 2. Higly oriented carbon fibers and carbon hollow fibers produced at 550 °C using ECR-MWCVD method.

 

(3) Polymerization of single carbon nanotubes by high pressure method: (Dr. M.Popov et al.)

     The P-SWNTs were synthesized by applying a shear deformation under load in a diamond anvil cell; the procedure of stress tensor variation. Single wall carbon nanotubes(SWNT) were pressurized up to 55 GPa. Experimental evidence of pressure-induced polymerization of SWNT under the pressure were obtained. A new superhard composed of polymerized single wall carbon nanotubes (P-SWNT) has been synthesized which exhibits a bulk modulas exceeding or comparable with that of diamond.

Figure ３. Force-depth indentation curves of P-SWNT (marked by solid circles), cubic BN faces (100) (marked by hollow circles) and (111) (marked by crosses) and fused silica (marked by hollow triangle) are plotted.

 

The process of polymerization is accompanied by irreversible changes in the Raman specra. Bulk modulus of 462 to 546 Gpa was found out for P-SWNT from the Raman mode of 1590cm-1. Hardness measured by nanoindentation method was in between 62 to 150 Gpa from the comparative study of P-SWNT and the single crystals of diamond and cubic boron nitride.