Temperature-Dependent Morphology Evolution of Boron Nitride and Boron Carbonitride Nanostructures

Boron nitride (BN) and boron carbonitride (BCN) nanostructures with versatile morphology were synthesized at different temperatures. The morphologies such as smooth microspheres, nanoflake-decorated microspheres, solid nanowires, hollow nanotubes (bamboo-like nanotubes, quasi-cylindrical nanotubes, and cylindrical nanotubes), and nanosheet-assembled microwires have been observed. Systematic investigation showed that the reaction temperature was responsible for the versatile morphologies through influencing the guiding effect of catalyst alloy droplet and the diffusion rates of growth species. The diffusion rate differences between surface diffusion (along the surface of the droplet) and bulk diffusion (through the bulk of the droplet) at different reaction temperatures were suggested to affect the final structure of the BN and BCN nanostructures.

Hexagonal Representative Volume Element for Modeling and Analysis of Mechanical Properties of Carbon Nanotube Reinforced Composites

Carbon nanotubes (CNTs) possess extremely high stiffness, strength and resilience, and may provide the ultimate reinforcing materials for the development of nanocomposites. In this paper, nanostructure is modeled as a linearly elastic composite medium, which consists of a homogeneous matrix having hexagonal representative volume elements (RVEs) and homogeneous cylindrical nanotubes. Formulas to extract the effective material constants from solutions for the RVE under axial as well as lateral loading conditions are derived based on the continuum mechanics approach. Numerical examples using the FEM are presented, which demonstrate that the load carrying capacities of the CNTs in a matrix are significant. For the RVEs having long carbon nanotube, better values of stiffness in axial direction are found as compared to stiffness in the lateral direction. Also, It is found that the square RVEs tend to overestimate the effective Youngs moduli of the CNT-based composites, and the hexagonal RVEs may be the preferred models for obtaining more accurate results.

Quantitative structural analysis of individual nanotubes by electron diffraction

A general method for quantitative structure analysis of individual, cylindrical, carbon nanotubes is described here. The method is based on electron diffraction of individual nanotubes and analysis using a combination of helical diffraction theory and diffraction geometry of the underlying lattice. Experimental recording of nanotube diffraction is achieved using a nanometer-sized electron beam. Procedures are developed for 1) the measurement of chiral angles in both single- and multi-wall nanotubes and 2) structure determination based on Bessel function fitting of layer line intensity oscillations. The accuracy of the method is demonstrated for the structure determination of a single- and double-wall carbon nanotubes and partial structural analysis of a multiwall carbon nanotube. The results show that the single-, double- and incommensurate multi-wall tubes are well described by the cylindrical tube model. However, a large Debye-Waller factor in the radial direction is obtained. The method developed here is general and can be applied to other cylindrical nanotubes.