Li Zhi-Hua

Characteristics of Plasma Shock Waves Generated in the Pulsed Laser Ablation Process

We modify the Sedov theory to describe plasma shock waves generated in a pulsed laser ablating process. We also study the propagation characteristics of plasma shock waves during the preparation process of functional thin films deposited by a pulsed laser. In particular, we discuss in detail the temporal behaviour of energy causing the difference of the propagation characteristics between the plasma shock wave and the ideal shock wave in the point explosion model. Under the same experimental conditions, the theoretical results calculated with our modified Sedov theory are in good agreement with the existing experimental data.

A Fractal Model for the Effective Thermal Conductivity of Granular Flow with Non-uniform Particles*

The equipartition of energy applied in binary mixture of granular flow is extended to granular flow with non-uniform particles. Based on the fractal characteristic of granular flow with non-uniform particles as well as energy equipartition, a fractal velocity distribution function and a fractal model of effective thermal conductivity are derived. Thermal conduction resulted from motions of particles in the granular flow, as well as the effect of fractal dimension on effective thermal conductivity, is discussed.

Effect of pulse width and fluence of femtosecond laser on electron-phonon relaxation time

The electron phonon relaxation time as functions of pulse width and fluence of femtosecond laser is studied based on the two-temperature model. The two-temperature model is solved using a finite difference method for copper target. The temperature distribution of the electron and the lattice along with space and time for a certain laser fluence is presented. The time-dependence of lattice and electron temperature of the surface for different pulse width and different laser fluence are also performed, respectively. Moreover, the variation of heat-affected zone per pulse with laser fluence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the electron phonon relaxation time is reasonably accurate with the influences of pulse width and fluence of femtosecond laser.

A New Dynamics Expansion Mechanism for Plasma during Pulsed Laser Deposition

The dynamics expansion mechanisms for plasma plume generated by pulsed laser radiation are studied in detail, taking account of plasma ionization effect. Based on the consideration of local conservations of mass, momentum, collected as the assumption that plasma can be viewed as compressible ideal fluid and high temperature-high pressure ideal gas, we develop a new dynamics expansion mechanism for plasma produced by pulsed laser radiation. Using the analytical method, the space number density and pressure evolvement of plasma in cylindrical coordinate are obtained, the dynamics evolvement equations are also derived. The results from the present model indicate that the plasma dynamic expansion behaviour can be evidently influenced by the ionization fraction η. Its effect is similar to a new dynamic source for plasma expansion and increases the expansion acceleration in all directions. The predictions of the expansion of the plasma is affected by the temperature, the average atoms mass and the ionization degree of the plasma are consistent with the experimental results.

Monte Carlo Simulation of Growth of Thin Film Prepared by Pulsed Laser

We use the Monte Carlo simulation method to perform the early growth stage of thin film prepared by pulsed laser deposition. We focus on the number of point defects on the substrate surface varying with the energy density of laser and substrate temperature. The results show that the laser energy and the substrate temperature strongly affect the morphology and size of the growth islands and the deposition rate of thin film. Our results are in good agreement with the related experimental results.

A New Synthetical Model of High-Power Pulsed Laser Ablation

We develop a new synthetical model of high-power pulsed laser ablation, which considers the dynamic absorptance, vaporization, and plasma shielding. And the corresponding heat conduction equations with the initial and boundary conditions are given. The numerical solutions are obtained under the reasonable technical parameter conditions by taking YBa2Cu3O7 target for example. The space-dependence and time-dependence of temperature in target at a certain laser fluence are presented, then, the transmitted intensity through plasma plume, space-dependence of temperature and ablation rate for different laser fluences are significantly analyzed. As a result, the satisfactorily good agreement between our numerical results and experimental results indicates that the influences of the dynamic absorptance, vaporization, and plasma shielding cannot be neglected. Taking all the three mechanisms above simultaneously into account for the first time, we cause the present model to be more practical.

Effect of Incident Intensity on Films Growth in Pulsed Laser Deposition

Incident intensity, defined by the amount of particles deposited per pulse, is an important parameter in the film growth process of pulsed laser deposition (PLD). Different from previous models, we investigate the irreversible and reversible growth processes by using a kinetic Monte Carlo method and find that island density and film morphology strongly depend on pulse intensity. At higher pulse intensities, lots of adatoms instantaneously diffuse on the substrate surface, and then nucleation easily occurs between the moving adatoms resulting in more smaller-size islands. In contrast, at the lower pulse intensities, nucleation event occurs preferentially between the single adatom and existing islands rather than forming new islands, and therefore the average island size becomes larger in this case. Additionally, our results show that substrate temperature plays an important role in film growth. In particular, it can determine the films shape and weaken the effect of pulse intensity on film growth at the lower temperatures by controlling the mobility rate of atoms. Our results can match the related theoretical and experimental results.

Virtual Facility Layout Design Using Virtual Reality Techniques

Manufacturing facility layout design has long been recognized as one of the most critical and difficult design tasks in manufacturing industries. Traditional layout methods can not fully solve this complex spatial layout problem. Virtual reality (VR) is a new form of human-computer interaction, which has the potential to support a range of engineering applications. This paper discusses the reasons why manufacturing facility layout design is considered to be an appropriate new area of VR utilization. A VR-based layout design framework is proposed. The virtual environment construction issues are discussed. An example of the immersive VR application of facility layout is examined.

Quasi-Random Resistor Network Model for Linear Magnetoresistance of Metal–Semiconductor Composite

A new model for the linear magentoresistance (MR) of the Ag2+δSe and Ag2+δTe thin films is proposed. The thin him is considered as a metal-semiconductor composite and dispersed into an N – N quasi-random resistor network. The network is constructed from four-terminal resistors and the mobility of carries μ within the network has a quasi-random distribution, i.e. a Gaussian distribution with two constraint conditions. The model predicts that the MR increases with the increasing magnetic fields, and increases linearly at high field. Moreover, the MR decreases with the increasing temperatures. A good agreement between the theoretical MR and the available experimental data is found.

Anomalous Scaling Behaviors in a Rice-Pile Model with Two Different Driving Mechanisms*

The moment analysis is applied to perform large scale simulations of the rice-pile model. We find that this model shows different scaling behavior depending on the driving mechanism used. With the noisy driving, the rice-pile model violates the finite-size scaling hypothesis, whereas, with fixed driving, it shows well defined avalanche exponents and displays good finite size scaling behavior for the avalanche size and time duration distributions.