Wei-Qiang Han

Rotational actuators based on carbon nanotubes.

Nanostructures are of great interest not only for their basic scientific richness, but also because they have the potential to revolutionize critical technologies. The miniaturization of electronic devices over the past century has profoundly affected human communication, computation, manufacturing and transportation systems. True molecular-scale electronic devices are now emerging that set the stage for future integrated nanoelectronics. Recently, there have been dramatic parallel advances in the miniaturization of mechanical and electromechanical devices. Commercial microelectromechanical systems now reach the submillimetre to micrometre size scale, and there is intense interest in the creation of next-generation synthetic nanometre-scale electromechanical systems. We report on the construction and successful operation of a fully synthetic nanoscale electromechanical actuator incorporating a rotatable metal plate, with a multi-walled carbon nanotube serving as the key motion-enabling element.

Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes

Abstract We have conducted pulling and bending tests on individual carbon nanotubes in-situ in a transition electron microscope. Based on our observation of the force required to break the tube, a tensile strength of 0.15 TPa was computed. From corresponding bending studies on such nanotubes, the Youngs modulus was estimated to be 0.9 TPa (0.8 TPa after ‘sub continuum’ corrections). These results suggest a strength that is a large fraction of the elastic modulus, although previous measurements of their elastic stiffness have yielded higher modulus values, by as much as a factor of 2. The result does indicate that individual nanotubes can fail as essentially defect-free materials. Furthermore, we observed no obvious reduction in cross-sectional area prior to the failure. In addition, the bending experiments revealed a remarkable flexibility in these tubes. These unique properties support the potential of nanotubes as reinforcement fibers in structural materials.

Synthesis of boron nitride nanotubes from carbon nanotubes by a substitution reaction

A method involving carbon nanotubes substituted reaction was developed for the synthesis of mass quantities of boron nitride nanotubes. Boron oxide vapor was reacted with nitrogen gas in the presence of carbon nanotubes to form boron nitride nanotubes, whose diameters and lengths are similar to those of the starting carbon nanotubes. It is proposed that carbon atoms of carbon nanotubes can be fully substituted by boron and nitrogen atoms through a general chemical reaction. The results suggest that the synthesis methodology developed here may also be extended to form nanotubes from other novel materials.

Structure of chemically derived mono- and few-atomic-layer boron nitride sheets

We prepared mono- and few-layer hexagonal boron nitride sheets by a chemical-solution-derived method starting from single-crystalline hexagonal boron nitride. Using high-resolution transmission electron microscopy and electron-energy-loss spectrometry, we characterized the microstructure, composition, and near-edge fine structure of the boron nitride sheets. We conclude that the fringe contrast in the edge and the moire patterns are feasible criteria for determining the number of layers and their stacking orientation in the sheets. These criteria are also useful for other mono- and few-layer materials, such as graphene sheets.

Amorphous Hierarchical Porous GeOx as High-Capacity Anodes for Li Ion Batteries with Very Long Cycling Life

Many researchers have focused in recent years on resolving the crucial problem of capacity fading in Li ion batteries when carbon anodes are replaced by other group-IV elements (Si, Ge, Sn) with much higher capacities. Some progress was achieved by using different nanostructures (mainly carbon coatings), with which the cycle numbers reached 100-200. However, obtaining longer stability via a simple process remains challenging. Here we demonstrate that a nanostructure of amorphous hierarchical porous GeO(x) whose primary particles are ~3.7 nm diameter has a very stable capacity of ~1250 mA h g(-1) for 600 cycles. Furthermore, we show that a full cell coupled with a Li(NiCoMn)(1/3)O(2) cathode exhibits high performance.

Continuous synthesis and characterization of silicon carbide nanorods

Abstract A two-step reaction scheme has been employed for the synthesis of SiC nanorods at 1400°C. SiO vapour was generated via the silicon reduction of silica, and then this SiO vapor reacted with carbon nanotubes to form SiC nanorods. The morphology and structure of the nanorods were characterized by XRD, TEM, IR and Raman spectroscopy. The nanorods are single crystalline β-SiC with the diameters ranging from 3 to 40 nm. A broad photoluminescence peak located around 430 nm under 260 nm UV fluorescent light excitation at room temperature is observed. A growth model of SiC nanorods is proposed.

Synthesis of silicon nitride nanorods using carbon nanotube as a template

A method to prepare silicon nitride nanoscale rods using carbon nanotube as a template has been presented in this letter. The products of the reaction of carbon nanotubes with a mixture of Si and SiO2 powder in nitrogen atmosphere are β-Si3N4, α-Si3N4, and Si2N2O nanorods. The sizes of the nanorods are 4–40 nm in diameter and up to several microns in length. The formation mechanism of the nanorods has also been discussed.

Synthesis of GaN–carbon composite nanotubes and GaN nanorods by arc discharge in nitrogen atmosphere

A method using an arc discharge in a nitrogen atmosphere for synthesizing large quantities of gallium nitride (GaN)–Carbon composite nanotubes and GaN nanorods is reported. The reaction is achieved by a dc arc discharge between a graphite anode filled with a mixture of GaN, graphite, and nickel powders and a graphite cathode in a nitrogen atmosphere. The GaN are presented as rodlike fillings in the composite tubes and the isolated GaN nanorods have diameters in the range of 7–45 nanometers and a length of up to 40 μm. The outer graphitic shells of the composite carbon nanotubes have thicknesses ranging from 1 to 8 nm. It was found that the use of a nitrogen atmosphere plays a crucial role for the growth of the GaN nanorods fillings and the individual GaN nanorods.

Boron-doped carbon nanotubes prepared through a substitution reaction

Abstract Boron-doped carbon nanotubes (CNTs) have been prepared through a partial substitution reaction, where some carbon atoms of CNTs are substituted by boron atoms. Boron oxide vapor reacts with CNTs to form B x C ( x ≤0.10) nanotubes at 1373 K in 4 h under an argon atmosphere. The B x C nanotubes have diameters and lengths similar to those of the starting CNTs. Boron is seen to enhance the graphitization of CNTs. B 4 C and B 13 C 2 crystalline nanorods are also formed, with typical diameters between 6 and 30 nm. It is suggested the synthetic method described here might be used to produce a large class of new doping CNTs.

Transformation of BxCyNz nanotubes to pure BN nanotubes

We demonstrate that multiwalled BxCyNz nanotubes can be efficiently converted to BN multiwalled nanotubes via an oxidation treatment. The microstructure and composition of the precursors and final products have been characterized by high-resolution transmission electron microscopy, electron energy-loss spectroscopy, and energy dispersive x-ray spectroscopy. The conversion process is monitored by thermogravimetric analysis. Carbon layers of BxCyNz nanotubes start to oxidize at 550 °C, thereby transforming BxCyNz nanotubes into pure BN nanotubes. The remarkable thermal stability of pure BN nanotubes in an oxidizing environment is also established.