B. H. Chang

Tensile tests of ropes of very long aligned multiwall carbon nanotubes

We have directly measured the Young’s modulus and tensile strength of multiwall carbon nanotubes by pulling very long (∼2 mm) aligned nanotube ropes with a specially designed stress-strain puller. This puller can apply an axial force to the rope and simultaneously measure the corresponding rope elongation and the change in rope resistance. The average Young’s modulus and tensile strength obtained were 0.45±0.23 TPa and 1.72±0.64 GPa, respectively, which are lower than those calculated and measured previously. The factors that affect the mechanical strengths of nanotubes are discussed.

Direct growth of aligned open carbon nanotubes by chemical vapor deposition

Abstract Open carbon nanotubes were directly obtained at a very high yield by pyrolysis of acetylene over iron/silica substrates. Scanning electron microscope images show that the nanotubes grow outwards perpendicularly and separately from the surface of the substrate and form an aligned nanotube array. The bottom ends of the tubes can be easily stripped off from the substrate and are found to be opened while the top ends are closed, as verified by transmission electron microscope. The tip growth of tubes might be responsible for the formation of open carbon nanotubes.

Synthesis, characterizations, and physical properties of carbon nanotubes coated by conducting polypyrrole

A new type of carbon nanotube (CNT) (diameter of <100 nm) coated by conducting polypyrrole (PPY) was synthesized by in situ polymerization on CNTs. The structure of the resulting complex nanotubes (CNT-PPY) was characterized by elemental analysis, X-ray photoelectron spectroscopy, Raman spectra, and X-ray diffraction. These indicated no significant chemical interaction between PPY and the CNT. The electrical, magnetic, and thermal properties of the complex nanotubes were measured and showed the physical properties of the CNTs were modified by conducting PPY.

Third-order optical nonlinearity of the carbon nanotubes

Third-order optical nonlinearity of the carbon nanotubes was investigated with both 30-ps-wide and 8-ns-wide Nd:YAG laser beams. Both 1064 and 532 nm were used in each case. The nonlinear optical temporal response was obtained both on the picosecond scale and on the nanosecond scale, and the corresponding mechanism is discussed. Finally, we have compared the average contribution of one carbon atom to the third-order optical nonlinearity in the carbon nanotubes with that in C60, finding some enhancement of the third-order optical nonlinearity in the carbon nanotubes.

Synthesis and properties of carbon nanotube-polypyrrole composites

Polypyrrole (PPY) was synthesized on carbon nanotubes (CNT) using in-situ polymerization method. The resulting CNT-PPY composites show a uniform tubular morphology with diameter less than 100 nm. The molecular structure and physical properties of the CNT-PPY nanotubes were discussed.

Growth of straight nanotubes with a cobalt-nickel catalyst by chemical vapor deposition

In this letter, we report the catalytic synthesis of a large amount of straight carbon nanotubes using a transition-metal cobalt–nickel/zeolite catalyst. High-resolution transmission electron microscopy images show that they are well graphitized. Raman spectrum shows its peak at 1349 cm−1 (D band) is much weaker than that at 1582 cm−1 (G band). We believe that straight carbon nanotubes contain much less defects than curved nanotubes and might have potential applications in the future.

Growth of carbon nanotubes on cobalt disilicide precipitates by chemical vapor deposition

We have successfully grown carbon nanotubes on cobalt-implanted silicon with various doses. The morphology of such tubes has been examined by scanning electron microscopy, transmission electron microscopy, and Raman scattering. On contrary to the commonly used transition-metal nanoparticle catalysts, nanometer-sized CoSi2 precipitates produced in the as-implanted substrates are believed to act as nucleation centers for the formation of carbon nanotubes.

Conductivity and magnetic susceptibility of nanotube/polypyrrole nanocomposites

A method has been developed to produce a carbon nanotube/conducting polymer nano-composite through in-situ polymerization of pyrrole in the carbon nanotube template. The nano-composites of carbon nanotube and polypyrrole have been characterized by SEM, TEM, XRD, Raman Scattering. The thermal stability was studied by TGA (Thermal Gravity Analysis). The measurements of conductivity and magnetic susceptibility of the composites have been studied.

Effects of temperature oscillations on the growth of carbon nanotubes by chemical vapor deposition

We have grown carbon nanotubes at oscillatory growth temperature by chemical vapor deposition. The high-resolution transmission electron microscopy (HRTEM) images show that they contain a number of incomplete carbon layers both on the outer and inner periphery and dislocations in the middle part of the carbon nanotubes walls. The defect structure of these nanotubes depends on the period of the T oscillation. In addition, HRTEM examinations of these carbon nanotubes from end-on reveal that some have polyhedral sections and contain dislocations. These features make them more active chemically and might have significant influence on the properties and applications of the carbon nanotubes.

Fabrication, characterization and property of aligned multi-walled carbon nanotubes

In this paper, we described the recent progress about fabrication and characterization of aligned and very long multi-walled carbon nanotubes (MWNTs) done in our group. Typically, MWNTs arrays were prepared by chemical vapor deposition (CVD) of hydrocarbon gas, for example acetylene, on porous substrates. Aligned MWNTs array with high density and purity were achieved by CVD on both bulk silica substrate and the film-like substrates. Basically, the carbon nanotubes have the uniform diameter of 10-20nm and the spacing among the tubes is about lOOnm. There are no catalysis metal particles detected by EDX and TEM in the arrays. Furthermore, very long carbon nanotubes of length ~ 2 mm were produced, which is an order of magnitude longer (1 mm vs. 100 μηι) than that described in most previous reports. Based on the aligned, discrete and long tubes rope, various physical properties have been investigated. At first, efforts have been made to characterize the mechanical properties of aligned MWNTs arrays such as tensile strength, Youngs modulus. We have directly measured the Youngs modulus and tensile strength of MWNTs arrays by pulling very long (~2mm) aligned carbon nanotubes ropes with a specially designed stress-strain puller. The average Youngs modulus and tensile strength obtained were 0.45Tpa and 1.72Gpa respectively, which were lower than those measured by AFM previously. Besides, we have also used a scanning probe microscope with an indentation/scratch function to investigate the radial compression of an individual multi-walled carbon nanotube under an asymmetric stress and to determine the radial compression elastic modulus at different compression levels. Using this kind of samples, we were able to measure the specific heat and the thermal conductivity of MWNTs arrays in the range from room temperature up to 20mk. We found that the specific heat of MWNTs follows a strikingly linear temperature dependence over the entire temperature range measured (10-300K), and the thermal conductivity takes a quadratic T-dependence below -120 Κ and turns to be linear above that temperature. The results indicate that the inter-wall coupling in MWNTs is extremely weak.