Zheng H. Zhu

Tunneling resistance and its effect on the electrical conductivity of carbon nanotube nanocomposites

In this paper, we examined the effect of electron tunneling upon the electrical conductivity of carbon nanotube (CNT) polymer nanocomposites. A CNT percolating network model was developed to account for the random distribution of the CNT network using Monte Carlo simulations, where the tunneling resistance between CNTs was established based on the electron transport theory. Our work shows several novel features that result from this tunneling resistance: (i) direct contact resistance is the result of one-dimensional electron ballistic tunneling between two adjacent CNTs, (ii) the nanoscale CNT-CNT contact resistance should be represented by the Landauer-Buttiker (L-B) formula, which accounts for both tunneling and direct contact resistances, and (iii) the difference in contact resistance between single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) can be modeled by the channel number in the L-B model. The model predictions reveal that the contact resistance due to electron tunneling effects in nanoc...

Modeling electrical conductivities of nanocomposites with aligned carbon nanotubes

We have developed an improved three-dimensional (3D) percolation model to investigate the effect of the alignment of carbon nanotubes (CNTs) on the electrical conductivity of nanocomposites. In this model, both intrinsic and contact resistances are considered, and a new method of resistor network recognition that employs periodically connective paths is developed. This method leads to a reduction in the size effect of the representative cuboid in our Monte Carlo simulations. With this new technique, we were able to effectively analyze the effects of the CNT alignment upon the electrical conductivity of nanocomposites. Our model predicted that the peak value of the conductivity occurs for partially aligned rather than perfectly aligned CNTs. It has also identified the value of the peak and the corresponding alignment for different volume fractions of CNTs. Our model works well for both multi-wall CNTs (MWCNTs) and single-wall CNTs (SWCNTs), and the numerical results show a quantitative agreement with existing experimental observations.

Analysis of cracks perpendicular to bimaterial interfaces using a novel finite element

Inverse square root, 1/√γ, singularity characterizes the stress field at the crack tip of homogeneous isotropic elastic media. This 1/√γ singularity does not, however, hold for cracks present in inhomogeneous solids; such as, a crack terminating at a right angle to bimaterial interface, which is the subject of the current paper. A few attempts have been made to treat this problem analytically. However, in view of the complexity of the resulting equations and the numerical difficulties associated with these attempts, only a very limited number of approximate solutions exist. It is therefore the objective of this study to: (i) provide a comprehensive theoretical treatment of the current boundary value problem using the eigenfunction expansion method, and (ii) to utilize the results of the eigenfunction method to develop a novel singular finite element which is capable of treating cracks terminating perpendicularly to interfaces accurately and efficiently. To establish the validity of the method, a number of test cases are examined and compared with existing simplified solutions. Furthermore, numerical examples are provided to show the effect of the elastic mismatch and crack length upon the resulting stress intensity factors and the associated stress fields.

Percolation threshold and electrical conductivity of a two-phase composite containing randomly oriented ellipsoidal inclusions

An explicit, analytical theory for the percolation threshold, percolation saturation, and effective conductivity of a two-component system involving randomly oriented ellipsoidal inclusions is proposed. The ellipsoids may take the shape of a needle, prolate or oblate spheroid, sphere, or disk. This theory is based upon consideration of Ponte Castaneda--Willis [P. Ponte Castaneda and J. R. Willis, J. Mech. Phys. Solids 43, 1919 (1995)] microstructure in conjunction with Hashin--Shtrikman [Z. Hashin and S. Shtrikman, J. Appl. Phys. 33, 3125 (1962)] upper bound. Two critical volume concentrations, c* and c**, that represent the respective percolation threshold at which the conductive network begins to develop, and the percolation saturation, are identified. During this very short range of concentration, the electrical conductivity of the composite is found to exhibit a very sharp increase, while over the entire range, the calcutilated conductivity exhibits the widely reported sigmoidal shape. Comparison wi...

Elastodynamic Analysis of Aerial Refueling Hose Using Curved Beam Element

The elastodynamic analysis of an aerial refueling hose by classic cable theory suffers the singularity problem when the hose slackens under dynamic loadings. The difficulty is addressed and overcome by modeling the refueling hose with a new three-noded locking-free curved beam element. The large deformations and rotations of curved beams are formulated in terms of an updated Lagrangian framework with consistently coupled quintic polynomial displacement fields to satisfy the membrane locking-free condition. The stability and accuracy of the new element is validated by experiments involving an instrumented free-swinging steel cable. Good agreement is observed between the experimental results and the predictions of the new element. The numerical capability of modeling a refueling hose and drogue system has been demonstrated by simulating 1) the oscillation of hose due to the disturbance from the tanker and the vortex-induced velocity and 2) a receiver coupling with a hose reel malfunction. The analysis results show clearly the formation and propagation of oscillations along the hose, the consequent whipping near the drogue, and the associated variation of hose tension. The results of new element agree well with field observations and existing analysis results.

Dynamics of Nanosatellite Deorbit by Bare Electrodynamic Tether in Low Earth Orbit

This paper studies the dynamics of nanosatellite deorbit by a bare electrodynamic tether. The orbital dynamics of the tethered nanosatellite is modeled in Gaussian perturbation equations and the motion-induced voltage-current relationship along the electrodynamic tether is analyzed by using the 2000 International Geomagnetic Reference Field model including up to seventh-order terms and the International Reference Ionosphere 2007 model. The analysis reveals that the high-order magnetic model of Earth affects the dynamic characteristics of the tethered nanosatellite, especially in orbits with high inclination angles, by changing its orbit from circular to elliptical forms. This is beneficial for deorbiting the nanosatellite in near-polar orbits where the electrodynamic force is not as effective as in the equatorial orbit because the denser atmosphere at a lower perigee will provide a larger atmospheric drag. Moreover, the analysis shows that the electrodynamic force is always against the satellite motion in...

Modeling electrical conductivity of nanocomposites by considering carbon nanotube deformation at nanotube junctions

This paper investigates the effect of carbon nanotube (CNT) deformation on the electrical conductivity of CNT polymer composites at crossed nanotube junctions using a revised 3-dimensional CNT percolating network model. Two aspects of the work are considered. The first is concerned with the effect of CNT deformation on its intrinsic and contact resistances at CNT-CNT junctions. An analytical model based on electron ballistic tunneling theory and Landauer-Buttiker formula is proposed to describe the variation of CNT-CNT contact resistance at the CNT-CNT junction in terms of local deformation of CNT walls and CNT-CNT distance. In addition, a model exclusively based on experimental data to describe the change of CNT intrinsic resistance in terms of its cross-section deformation is adopted. The second is concerned with the relationship among the CNT-CNT distance, the angle between two adjacent CNTs, and the dimensions of local deformation of CNT walls and its impact on the corresponding intrinsic and contact ...

Effect of carbon nanotube geometry upon tunneling assisted electrical network in nanocomposites

This paper examines the effect of carbon nanotube (CNT) geometry upon the electrical properties of the corresponding functionalized nanocomposites. Specifically, Monte Carlo (MC) simulations are conducted to evaluate the effect of CNT length non-uniformity and waviness upon tunneling. Three aspects of the work are considered. The first is concerned with the application of periodic boundary condition that ensures periodic connectivity of the percolating paths via the use of an improved connective percolating network recognition scheme. The second is concerned with the determination of the electrical conductivity of the percolated system rather than the critical percolation threshold for varied CNT geometries using Weibull distribution to statistically account for the geometry variations. The third is concerned with the validation of our MC simulations. Our results reveal that (i) the CNT geometry, as determined by CNT length variability and waviness, plays a more dominant role in percolation threshold rath...

Autonomous robotic capture of non-cooperative target using visual servoing and motion predictive control

This paper presents a framework for autonomous capture operation of a non-cooperative mobile target in a 3-dimensional workspace using a robotic manipulator with visual servoing. The visual servoing with an eye-in-hand configuration is based on motion predictive control using Kalman filter for the on-line state and parameter estimation of the target. A transitional decision making process is developed to guide the robotic manipulator between the different phases of the capture operation by employing a custom metric that translates visual misalignments between the end-effector and the target into a guidance measurement. These phases include the target acquisition and approach stage and the alignment and capture phase. Experiments have been carried out on a custom designed and built robotic manipulator with 6 degrees of freedom. The objective is to evaluate the performance of the proposed motion predictive control scheme for the autonomous capturing task and to demonstrate the robustness of the proposed control scheme in the presence of noise and unexpected disturbances in vision system, sensory-motor coordination and constraints for the execution in real environments. Experimental results of the visual servoing control scheme integrated with the motion predictive Kalman filter indicate the feasibility and applicability of the proposed control scheme. It shows that when the target motion is properly predicted, the tracking and capture performance has been improved significantly.

Optimal Control of Nanosatellite Fast Deorbit Using Electrodynamic Tether

This paper proposes a piecewise two-phased optimal control scheme for fast nanosatellite deorbit by a short electrodynamic tether. The first phase concerns the open-loop control trajectory optimization, where the optimal control problem is formulated only for the tether libration motion by assuming the slow-varying orbital elements of the electrodynamic tether system as constant within a discretized interval. The second phase deals with the closed-loop optimal control for tracking the derived optimal reference trajectory subject to multiple major orbital perturbations. The finite receding horizon control method is used in the optimal trajectory tracking. Both optimal control problems are solved by a direct collocation method based on the Hermite–Simpson method using discretization schemes with coincident nodes. The resulting nonlinear programming problem significantly reduces the problem size and improves the computational efficiency. Numerical results for fast nanosatellite deorbit by an electrodynamic t...