Papers by Author: Ching Yen Ho

Abstract: This paper investigates effects of particle size on heat transfer in copper powders. The understanding for thermal properties of the powder is advantageous to the advancement of the processing technologies such as laser cladding, laser sintering, powder metallurgy and its other applications. Nanosized particles possess characteristic physical and chemical properties different from those of bulk materials due to the confinement of electrons, excitons, and photons into small volumes. Therefore it is valuable to discuss the thermal behaviours of powders constituted by nanometer-sized particles. The powder is wrapped up in the slender tube. One end of the slender tube filled with powder is connected to the low constant-temperature reservoir and the other end is kept at room temperature. The temperature histories at the 1cm location of the slender tube from the low constant-temperature reservoir are recorded using thermal couples. Powders of particles with the sizes 50nm and 5000nm are employed in this experiment. The results show that the thermal diffusion in the 50nm Cu powder is faster than that in the 5000nm Cu powder.

Abstract: Traditional materials processing in the nanometer range using laser technology is very difficult with conventional optics due to the diffraction limit of the beam wavelength, a near-field technology has been developed to circumvent the diffraction limit, permitting the spot size to be reduced down to 20 nm. In most near-field techniques, this technology is achieved by placing a small aperture or microparticle between the sample and the light source. Therefore this paper will analytically investigate the profile of the intensity for diffraction of laser irradiating an aperture or microparticle in nanostructure processing. Classical electromagnetic wave theory is employed to calculate the intensity for diffraction of laser irradiating a microparticle or aperture. The results will reveal the differences between an aperture and micoparticle for diffraction of laser. The effect of laser parameters on the intensity and distribution of diffraction will be also discussed.

Abstract: Transport variables in plasma column are analytically investigated in this paper. Low-energy electrons and ions are produced from electron beam evaporation of a metal target in the technological vacuum chamber of an electron beam welding machine. Assuming collisionless plasma motion in the radial direction, the electrostatic potential is obtained from model of plasma expansion. Transport variables such as ion density, electron density, conduction heat of the ions and electrons are calculated using the electrostatic potential and are compared with the available experimental data.

Abstract: Nanometer-sized particles possess characteristic physical and chemical properties different from those of bulk materials due to an increase in surface-to-volume ratios as well as of confinement of electrons, excitons, and photons into small volumes. Therefore it is worthwhile to discuss the thermal behaviours of powders constituted by nanometer-sized particles. The heat transfer in the powder composed of nanoparticles is experimentally investigated in this paper. The understanding for thermal properties of the 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 room temperature. The temperature histories at two different locations in the slender tube are recorded using thermal couples. The results show that the thermal diffusivity in the powder composed of nanoparticles is larger than that in bulk material. The pressure on the Al powders enhances the rate of heat transport due to the increase of contact area for thermal conduction.

Abstract: Dendrite needles grow from an undercooled melt and their shapes depend on the temperature distribution on the solidification front, which are specified by some parameters such as undercooling, capillary length, diffusivity, convection and kinetic effects. Neglecting the convection and kinetic effects, this study numerically computes the quasi-steady-state integral-differential equation to obtain the shape of a dendrite using solvability condition and investigates the effect of parameters changing the temperature field on the shape of a dendrite. The results reveal that the tip shape enlarges with the decreasing undercooling and increasing capillary length. On the other hand, the increase of thermal diffusivity only slightly reduces the tip radius and shape of a dendrite.

Abstract: This study has developed a thermal model for laser-assisted cutting of zirconia ceramics. Laser-assisted cutting can increase ceramics removal rates by utilizing a localized heat source to elevate the workpiece temperature prior to material removal with traditional cutting tool. At high temperatures the yield strength of ceramics can decrease below the fracture strength, changing the material deformation behavior from brittle to ductile and enabling the use of a single point cutting tool to remove material at rates approaching those of metal cutting. A thermal model has been developed for the workpiece of ceramics cylinder rotating with a constant speed, which was preheated by a laser and cut by cutting tool. Since the cutting tool followed the moving laser with a fixed distance in the axial direction, i.e. the feed rate of the cutting tool was the same as the moving velocity of the laser, this thermal model has been formulated in a cylindrical coordinate system that moved with the laser beam or cutting tool and therefore, this problem was a quasi-steady-state problem. An analytical solution for this thermal model has been obtained. The results calculated by this model agree with the available experimental data. The temperature field is presented during laser-assisted cutting of ceramics. The effects of feed rate and laser power on temperature field are also discussed in this study.

Abstract: This paper investigates that polarizations of a laser vary in the process of reflections after a linearly or circularly polarized beam is incident on a paraboloid of revolution-shaped cavity. This analysis is based on the viewpoint of geometry optics because the opening radius is about 100μm, which is ten times the size of the wavelength 10.6μm of an incident CO2 laser beam. Considering the profile of laser beam to be Gaussian distribution, the variation of polarization in the process of reflections is mathematically modeled. The result reveals that the state of polarization for an incoming ray with circular polarization is almost totally transformed into linear polarization perpendicular to the plane of incidence after the ray intervenes multiple reflections. The effects of cavity depth and absorptive index of material on the polarization variation are also discussed. The circularly polarized ray entering the deeper cavity is finally changed into the higher degree of linear polarization. The increase in absorptive index reduces the speed approaching the linear polarization.

Abstract: This paper utilized a wireless temperature sensing system to measure the temperature histories of tool and workpiece during cutting. It is essential to use a wireless sensing system to measure the temperature in cutting and monitor the cutting process due to tool movement, workpiece turning, network monitoring and safety consideration. The temperature sensing system includes the wireless transmitter and receiver, A/D and D/A converter, data processing software, and computer. The temperature histories for different feed rates and depths of cut were measured in real time during cutting. The data measured by using wireless temperature sensing system were also compared with these obtained from an infrared thermometer.

Abstract: The incident energy flux impinged on the free surface of liquid layer was considered to be balanced with the latent heat in evaporation and the heat in directional conduction but neglecting convective heat transfer due to a small Peclet number at the cavity base. The quasi-steady state model was developed in this study to analyze the effect of the energy density during the penetration process and an exponential expression for penetration velocity as a function of liquid-thickness and temperature was also derived. The penetration velocity versus energy density calculated by the present model showed good agreements with the experimental data for drilling copper, which the relative errors between the calculated and the experimental data are less than 15%. By the setup of non-uniform grids distribution in numerical method, this work had successfully predicted the variation of the penetration velocity with energy density distribution. The effects of the energy density on flow rate, thickness of liquid layer, base temperature of fusion zone had also been discussed in this study.

Abstract: This paper investigated the absorption of a micro-particle irradiated by laser. Micro-particles usually appear within the plasma induced by a laser or powder in the process of laser cladding. These particles are assumed to be spherical and neutral (no surface charge). Laser-particle interactions involve scattering, refraction, and diffraction phenomena. Refraction and diffraction can enhance radiation absorption. The complex optical indexes of material and size parameters of micro-particles characterize the absorption of particles in these materials processing. The electromagnetic wave theory and geometrical optics approach were utilized to analyze the absorption in the particle. The errors between these two methods were discussed for different indexes of absorption and size parameters. The compatibility of geometrical optics approach for a small particle is also presented.