Mao-Yu Wen

Potential and electron density calculated for freely expanding plasma by an electron beam

This paper investigates the radial distributions of potential and electron density in free expansion plasma induced by an electron beam irradiating on the plate. The region of plasma production is assumed to be cylindrical, and the plasma expansion is assumed to be from a cylindrical source. Therefore, the one-dimensional model in cylindrical coordinates is employed in order to analyze the radial distributions of the potential and electron density. The Runge–Kutta method and the perturbation method are utilized in order to obtain the numerical and approximate solutions, respectively. The results reveal that the decrease in the initial ion energy makes most of the ions gather near the plasma production region and reduces the distribution of the average positive potential, electron, and ion density along the radial direction. The oscillation of steady-state plasma along the radial direction is also presented in this paper. The ions induce a larger amplitude of oscillation along the radial direction than do ...

Thermal analysis of femtosecond laser processing for metal thin films

The temperature distributions in a metal film heated by femtosecond lasers are investigated in this paper. The time scale in which energy transfers from the electrons to the lattice is of the order of a picosecond for metals. Therefore, when the duration of femtosecond laser heating of metal films is of the order of or shorter than a picosecond, substantial nonequilibrium can occur between the electron and lattice temperatures and the metal lattices stay almost thermally undisturbed in this highly nonequilibrium regime. Assuming the system to be one dimensional and insulated at the front and rear surfaces, a parabolic two-temperature model is employed to investigate temperature distributions in metal films irradiated by a femtosecond laser. Using the Laplace transformation technique and the iterative method to solve the nonlinear model, the results reveal that the electron temperature distribution along x decreases with increasing absorption depth, coupling factor and electron specific heat. The high thermal conductivity leads to a decrease in electron temperature with time near the front surface of the film but an increase in electron temperature with time near the rear surface of the film.

PARAMETER EFFECTS ON TEMPERATURES IN THE THIN FILM IRRADIATED BY ULTRAFAST-PULSE LASER

The effects of parameters on temperatures in thin metal films heated by ultrafast-pulse laser are investigated in this paper. The time scale in which energy transfers from the electrons to lattice is on the order of a picosecond for metals. Therefore when the duration of ultrafast-pulse laser heating metal films is on the order of or shorter than a picosecond, a substantial nonequilibrium can occur between the electron and lattice temperature and the metal lattices stay almost thermally undisturbed in this highly nonequilibrium regime. A parabolic two-temperature heating model is employed to investigate thermal transport in thin metal films irradiated by ultrafast-pulse laser. Using the Laplace transform and assuming the thermal properties to be independent of temperature, a closed form solution of temperature in the thin film is provided. The effects of laser parameters and material properties on temperatures are also discussed.

An optimal parametric design to improve pool boiling heat transfer of sintered surfaces

W/m .In the experimentation, the effects of the sintering pressure, sintering time, sintering temperature, heating rate, and particle size on the boiling heat-transfer coefficient of the sintered surface were investigated using the Taguchi method, and an 5 15 (3 ) L orthogonal array table was selected as an experimental plan for some parameters earlier mentioned. Based on the results of SN (signal/noise) ratio and ANOVA (analysis of variance), the optimal conditions of specifications of parameters will be provided. It was found that all the chosen sintering factors have significant effects on the pool boiling heat transfer coefficient. Flow visualization is also used to investigate the behavior of the pool boiling phenomena of the present sintered surfaces. Optimum pool boiling heat transfer coefficient of 5.29 2 kW/m K was achieved with a sintering pressure of 2 atmospheres, a sintering time of 2 h, a sintering temperature of 900°C, a heating rate of 5°C/min and a particle size of 0.35 mm in a nitrogen container.

Nanoscale Removal of Picosecond Laser Ablation for Polymer.

This paper analytically investigates the picosecond laser ablation of polymer. Laser-pulsed ablation is a well-established tool for polymer. However the ablation mechanism of laser processing for polymer has not been thoroughly understood yet. This study utilized a thermal transport model to analyze the relationship between the ablation rate and laser fluences. This model considered the energy balance at the decomposition interface as the ablation mechanisms and is applied to predict the laser-ablated depth of Acrylonitrile Butadiene Styrene/PolyVinyl Chloride (ABS/PVC). The calculated variation of the ablation rate with the logarithm of the laser fluence agrees with the measured data. The effects of material properties and processing parameters on the ablation depth per pulse are discussed for picosecond laser processing of ABS/PVC.

Nonlinear Characteristics of Plasma Induced by an Electron Beam Irradiating the Target Material

This paper studies the nonlinear characteristics of plasma induced by an electron beam irradiating the target material. Poissons equation, the equations of continuity, and momentum equation are employed to investigate the nonlinearity of the electron beam-induced plasma. The steady-state three equations are combined into a nonlinear second-order ordinary differential equation. The physical phenomena resulted from the nonlinear second-order ordinary differential equation are analyzed and presented in this paper. The effects of parameters on the nonlinear characteristics are also discussed.

Deformation in Thin Metal Films Irradiated by Femtosecond Laser

This paper utilizes the parabolic two-step model and dynamical theory of thermoeslasticity to investigate the deformation in thin metal films irradiated by femtosecond laser. In the femtosecond-laser processing of thin metal films, the hot electrons blast exerting on the metal lattices occurs because the metal lattices stay and almost thermally undisturbed at the stage of nonequilibrium energy transfer. The blasting force could be so strong that the cold lattices are impelled into the domain for damage. This study directly adds the driving force derived from the thermal field of hot electrons blast into the equation of dynamical theory of thermoelasticity that describes the dynamic response of the metal lattices. The results from this analysis show that the deformation of thin metal films decreases with the increasing electron thermal conductivity, optical penetration depth and film thickness.

Scattering of laser irradiating on nanoscale rectangular groove

The scattering of laser irradiating on nanoscale rectangular groove is investigated in this paper. The laser incident on the groove is partially reflected and absorbed for an opaque material. This possibly leads to the absorption enhancement and redistribution of the laser in the groove. Micro- and nano-scale structures usually occur in MENS and nano-technology. Therefore it is important for the development of MENS and nano-technology to understand the scattering of the laser in these microstructures. Using the classical electromagnetic wave theory, the scattering of the laser in a nanoscale rectangular groove is analyzed and the scale effect on the scattering of the laser is also discussed.

Ultrashort-pulse laser microablation of aluminum oxide ceramics

The ablation of the aluminum oxide ceramics is theoretically investigated using the femtosecond laser in this paper. There is now a growing interest in laser micromachining using pulses of femtosecond duration. Among its advantages is the possibility machining hard and brittle materials that can be easily damaged by conventional drilling and cutting procedures. Femtosecond laser micromachining of ceramics can produce nanoscale patterns due to the laser pulse duration shorter than the relaxation time of heat transport among phonons. Laser ceramics ablation possesses another features, which include the phase transition from solid to vapor occurs without melt, the strong dependence of the thermophysical parameters and the volumetric absorption. The thermal process of femtosecond laser ceramics ablation is modeled by the heat conduction equation considering material evaporation and plasma plume absorption. The predicted ablation depth per pulse is compared with the available experimental data.

Heat Transport in the Al Nanopowder

The heat transport in the Al nanopowder is experimentally investigated in this paper. The understanding for thermal behavior of the Al powder is advantageous to the advancement of the processing technologies such as laser cladding, laser sintering, powder metallurgy and its other applications. The powder is wrapped up in the slender tube made of insulating material. One end of the slender tube filled with powder is maintained at temperature 0°C and the other end is kept at 24°C. The temperature histories at two different locations in the slender tube are recorded using thermal couples. The results show that the temperature in the powder composed of nanoparticles descends more quickly than that in bulk material. The increase of pressure on the powders enhances the heat transfer.Copyright