Chengxiang Li

Interfacial shear strengths between carbon nanotubes

Interfacial shear strengths or static frictions between carbon nanotubes (CNT) in contact at different cross angles are studied by using atomic mechanics. It is shown that the axial interfacial shear strengths between parallel CNTs in commensurate are two orders of magnitude greater than those in incommensurate. This strong chiral dependence is not surprising and is similar to that of the friction between two graphite basal planes. In contrast, we find that the interfacial shear strengths of crossly contacted CNT pairs are much less dependent upon chirality. The estimated values of interfacial shear strengths, ranging from 0.05 to 0.35 GPa, agree very well with experimentally measured results available in the literature. These results may thus be used as a basis for explaining the observed tension strengths of CNT bundles and films that are mainly bonded by van der Waals interactions and the mechanical behaviors of composite materials with highly concentrated CNTs.

Reversibly Switching Silver Hierarchical Structures via Reaction Kinetics

Here we report a study on controllable synthesis of hierarchical silver structures via regulating reaction kinetics. Silver particles with various morphologies are synthesized by a solution-based reduction approach at the addition of amino acids. The amino acid is used to coordinate with silver ions to slow down the reduction of silver ions. With the increase of glycine concentration, the morphologies of silver particles switch from dendrites, to flowers and to compacted spheres, which is attributed to the decrease of reaction rate as a result of the coordination. Three more amino acids are examined and confirms the role of reaction kinetic in shaping silver particles. Furthermore, by increasing the concentration of the reductant, the silver morphologies change from compact spheres to loose flowers as a result of the increase of reaction rate. Therefore the silver hierarchical structure can be reversibly switched by reaction kinetics. The silver particles synthesized are tested for surface enhanced Raman scattering (SERS) property and the dendritic particles present a remarkable SERS activity. This study shows that reaction kinetics is a powerful tool to tune hierarchical structures of silver particles, which is expected to be transferable to other material systems.

MULTI-SCALE SIMULATION OF GRAIN BOUNDARY STRUCTURE EFFECTS IN POLY-Si THIN FILM SOLAR CELL

The atomic structures of grain boundary (GB) and their effect on the performance of poly-Si thin film solar cell are studied by multi-scale simulations. First, the atomic structures of various GBs are calculated using molecular dynamics. Subsequently, the energy band diagram are obtained by ab-initio calculations. Then, finite difference method is performed to obtain solar cell performance. The results show that the Σ5 (twist) GB can greatly enhance the carriers recombination and results in small short-circuit current density (JSC) and open-circuit voltage (VOC). However, the Σ17 (twist and tilt) GBs have little influence on the cell performance. Also revealed in the simulations is that the GB near the p–n junction leads to very small JSC and VOC. When the distance between GB and p–n junction increases from about 1.10 μm to 3.65 μm, the conversion efficiency increases by about 29%. The thickness effect of solar cell containing the Σ5 (twist) GB on the cell performance is also studied. The results show that the conversion efficiency and JSC increase rapidly as the thickness increases from about 5.2 μm to 40 μm. When the thickness ranges from about 40 μm to 70 μm, the efficiency and the JSC both increase gradually and reach their own peak values at about 70 μm. When the thickness exceeds 70 μm, the efficiency and JSC both decrease gradually. However, the VOC keeps increasing with increase in thickness. The effects of GB on the carrier transport and recombination processes are discussed to understand the above results.

ATOMISTIC STUDY OF THE STRENGTH AND ELASTIC CONSTANTS OF PERFECT AND DEFECTED SILICON

The effects of vacancies on the strength and elastic constants of silicon, such as Youngs modulus and Poissons ratio are investigated using the molecular dynamics simulations with the Stillinger–Weber potential. The defected crystalline cells contain randomly generated defect distributions in the simulation models. The ideal strength is found to be 33.6 GPa at the strain 0.26. The Youngs modulus and Poissons ratio is 148 GPa and 0.252, respectively. It is found that the strength decreases as the point defect fraction increases, and the variation of the strength versus the point defect fraction coincides with a decaying exponential function. In addition, vacancies are shown to reduce the elastic constants. In general, the elastic constants of silicon vary linearly versus the defect fraction.

Hard-sphere/pseudo-particle modelling (HS-PPM) for efficient and scalable molecular simulation of dilute gaseous flow and transport

Abstract Continuum methods are not accurate enough for flows at high Knudsen numbers, whereas rigorous molecular dynamics (MD) methods are too costly for simulations at practical dimensions. Hard-sphere (HS) model is a simplified MD method efficient for dilute gaseous flow but is of poor parallelism due to its event-driven nature, which sets a strong limitation to its large-scale applications. In this work, pseudo-particle modelling, a time-driven modelling approach is coupled with HS model to construct a scalable parallel method capable of simulating flows and transport processes at high Knudsen numbers without losing necessary molecular details in describing their macro-scale behaviours. The method is validated in several classical simulation cases and its performance is evaluated to be favourable. To demonstrate the potential applications of this method, we also simulate the diffusion of small molecules in multi-scale porous media which is related to catalysis, material preparation and micro chemical engineering in the long term.

Simulation Study on the Reaction-Diffusion Coupling in Simple Pore Structures

Most porous media (just like catalyst pellets) have complicated pore structures, and understanding the coupling of the diffusion and reaction processes in these pores is very important for improving their performance. In this work, a diffusion factor (D) and a reaction factor (R) are proposed to quantitatively describe the diffusion and reaction performance in these pores respectively at molecular level. The yield in unit time is used to quantify their productivity and is expressed as the product of D and R. Molecular dynamic simulations with the hard-sphere algorithm are carried out to study the reaction-diffusion coupling in several simple pore structures with the same volume, such as straight, T-shaped, and cross-shaped pores. The reaction formula based on activation energy is given for a simple irreversible reaction process from A to B. In terms of the proposed factors, D and R, analysis on the simulation results shows clearly that the overall productivity of these pore structures depends on the competition of D and R, which are both determined by the size and shape of the pore structures. The results demonstrate the effectiveness of the simulation approach used for evaluating the performance of the simple pore structures for simple reactions and the potential of its application in more complicated and practical cases. It also suggests the effectiveness of the proposed factors, D and R, for charactering the diffusion and reaction processes at molecular level.