Cheng-I Weng

Three-dimensional molecular dynamics analysis of processing using a pin tool on the atomic scale

A three-dimensional model of molecular dynamics (MD) is proposed to study the effects of tool geometry and processing resistance on the atomic-scale cutting mechanism. The model includes the utilization of the Morse potential function to simulate the interatomic force between the workpiece and a tool. The results show that the cutting resistance increases with the angle of the pin tool and the depth of cut, and the cutting force is essentially constant over the range of velocities simulated. In addition, the obtained cutting resistance of present MD simulation exhibits an evident relationship to the ratio of the vertical and the horizontal contact area between the tool and the workpiece within the range of a pin angle of 90-150°. Finally, work hardening and stick-slip phenomena during the process are also observed.

The effect of an external magnetic field on the structure of liquid water using molecular dynamics simulation

Through a series of molecular dynamics simulations based on the flexible three-centered water model, this study analyzes the structural changes induced in liquid water by the application of a magnetic field with a strength ranging from 1to10T. It is found that the number of hydrogen bonds increases slightly as the strength of the magnetic field is increased. This implies that the size of a water cluster can be controlled by the application of an external magnetic field. The structure of the water is analyzed by calculating the radial distribution function of the water molecules. The results reveal that the structure of the water is more stable and the ability of the water molecules to form hydrogen bonds is enhanced when a magnetic field is applied. In addition, the behavior of the water molecules changes under the influence of a magnetic field; for example, the self-diffusion coefficient of the water molecules decreases.

Molecular dynamics analysis of temperature effects on nanoindentation measurement

A three-dimensional molecular dynamics (MD) model is carried out to study the effects of temperature on the atomic-scale nanoindentation process. The model utilizes the Morse potential function to simulate interatomic forces between the sample and tool. The results show that both Youngs modulus and hardness become smaller as temperature increases. The results also indicate that elastic recovery is smaller at higher temperatures. The softening behavior is similar to the prior experiment and the estimated elastic moduli are much higher than the prior experiment. The discrepancy may be due to simulations performed on defect-free single crystals. In addition, some defects of vacancies, atomic steps and plastic indent are observed on the surface region.

Molecular dynamics simulation of nano-lithography process using atomic force microscopy

A three-dimensional molecular dynamics (MD) model is utilized to study the effects of the scribing feed on the atomic-scale lithography process. The model utilizes the Morse potential function to simulate interatomic forces between the atoms of the workpiece and the tool, and also between the atoms of the workpiece themselves. MD simulation results are compared to atomic force microscopy (AFM) experimental results. Results show that both resultant force and surface roughness have a positive correlation with rate of feed when the feed is smaller than a critical value, after which they remain constant. Comparison of the feed effect behavior of the MD theoretical analysis and the AFM experiments shows good qualitative agreement.

An analytical solution to coupled heat and moisture diffusion transfer in porous materials

An analytical method is proposed for solving the problem of coupled heat and moisture transfer in porous materials. The coupled partial differential equations and boundary conditions are first subjected to Laplace transformation, the equations are reduced to ordinary differential equations, then the equations are converted into a single fourth-order ordinary differential equation by introducing a transformation function. The solution of the equation can be easily obtained, and thus, the temperature and moisture distributions in the transform domain can be determined. Finally, the transformed values are analytically or numerically inverted to obtain the time domain results. Therefore, the transient solution at any given time can be evaluated. The results are identical with published analytical solutions for a special case using decoupling technique, and they agree with a published analytical solution for wood slab. The method is compact enough to be generally applied to problems of heat and mass transfer in porous media.

A new modified Reynolds equation for ultrathin film gas lubrication

A new modified Reynolds equation for solving the ultrathin film gas lubrication is proposed to overcome the complicated and time-consuming difficulties of solving the linearized Boltzmann equation. The model equation is based on modified high-order slip-flow velocity distribution with three adjustable coefficients, which are corrected according to the Boltzmann model. The results are compared to those obtained using other kinds of currently employed modified Reynolds equations. It shows that the present model produces a closer approximation to that of the exact Boltzmann model than do other models, in a wider range of inverse Knudsen number. In addition, the newly derived equation is widely applicable to practical use.

Inverse determination of the cutting force on nanoscale processing using atomic force microscopy

This study presents a means of calculating the cutting force during the nanomachining process using an atomic force microscope (AFM) cantilever. The determination of the cutting force in the machining system is regarded as an inverse vibration problem. The conjugate gradient method is applied to treat the inverse problem using available displacement measurements. Numerical results show that the method can accurately estimate the cutting force even for problems with error of displacement measurement. Furthermore, the initial guesses for the cutting force can be arbitrarily chosen and the computing time required for the inverse calculations takes less than one second using a Pentium III-450 MHz PC.

An investigation into the structural features and thermal conductivity of silicon nanoparticles using molecular dynamics simulations

Th es tructural features and thermal conductivity of silicon nanoparticles of diameter 2–12 nm are studied in a series of molecular dynamics simulations based on the Stilling–Weber (SW) potential model. The results show that the cohesive energy of the particles increases monotonically with an increasing particle size and is independent of the temperature. It is found that particles with a diameter of 2 nm have a heavily reconstructed geometry which generates lattice imperfections. The thermal conductivity of the nanoscale silicon particles increases linearly with their diameter and is two orders of magnitude lower than that of bulk silicon. The low thermal conductivity of the smallest nanoparticles is thought to be the result of particle boundary and lattice imperfections produced during fabrication, which reduce the phonon mean free path (MFP). Finally, it is found that the influence of the temperature on the thermal conductivity decreases significantly as the temperature increases. Again, this is thought to be the result of a reduced phonon MFP at elevated temperatures. (Some figures in this article are in colour only in the electronic version)

Analysis of finite width journal bearings with micropolar fluids

Abstract The performance of finite width journal bearings lubricated with micropolar fluids is analysed by using, directly, the finite difference method to solve the generalized three-dimensional Reynolds equation. In this paper, the characteristics of finite journal bearings with micropolar and newtonian fluids in the cases of B/D = 1.0 and B/D = π 10 are obtained and presented graphically. The analysis reveals that the prominent feature of increasing load capacity and decreasing friction coefficient for micropolar fluids is more pronounced at higher eccentricity ratio and lower width-to-diameter ratio, and the volumetric side flow rate is almost the same for both micropolar and newtonian fluids.

Inverse problem of coupled heat and moisture transport for prediction of moisture distributions in anannular cylinder

Abstract This study presents a means of solving the inverse boundary value problem of coupled heat and moisture transport in an annular cylinder. While knowing the temperature history at any point of the body, the boundary time-varying moisture flux can be computed and subsequently the moisture distribution in the body can be determined as well. The surface moisture flux is then determined to minimize the sum of squares of the deviation between the calculated and measured temperatures in the body. The accuracy of the inverse analysis is examined by using simulated exact and inexact measurements obtained on the surface and in an interior location of the cylinder. Numerical results demonstrate that excellent estimations on the moisture distributions can be obtained for all the test cases considered here. The proposed method is highly promising for designing a moisture sensor for some solids.