Minoru Yaga

Nanoparticle formation in the expansion process of a laser ablated plume

In the present article, we describe the process of nanoparticle formation during pulsed laser ablation in an inert gas atmosphere. We investigated the interaction between laser ablated plumes and shock waves using one dimensional Eulerian fluid dynamics equations combined with a rate equation relating to a classical nucleation model of supersaturated vapors. The initial values for the plume immediately after laser irradiation onto a silicon target were calculated based on stochastic thermodynamics, which was first used by Houle et al. We found a certain case wherein the rate of nanoparticle formation becomes higher when a reflected shock wave passes through the plume. In that particular case, mono-dispersed nanoparticles can be generated by carrying out nucleation and nanoparticle growth as separate processes.

Impingement heat transfer by jet issuing from a cross-shaped nozzle

Heat transfer characteristics and flow patterns were measured over a plate for various separation distances between the nozzle exit and target plate when air issues from a sharp-edged cross-shaped nozzle and impinges on a plate. The local heat transfer coefficients in the radial direction for different circumferential positions were calculated using the wall temperatures measured by means of thermocouples, and flow patterns were observed using an oil-titanium IV oxide method. The isotherms of the infrared images were also measured using an infrared radiometer with a two-dimensional array of indium-antimony (InSb) sensors. The geometric axes were switched as a result of the self-induced velocity of a vortex filament; the convex corners became flat and the concave corners generated outward ejection. The distributions of the iso-heat transfer coefficient contours correspond well to the flow pattern and the isotherm contours. These contours extended diagonally and demonstrated the St. Andrews cross pattern for short separations, subsequently changing to an octagonal pattern, and then becoming circular at large separations. The correspondence of the heat transfer characteristics to the flow behavior, as well as the heat transfer mechanism are also described.

Study of Interaction between Unsteady Supersonic Jet and Vortex Rings Discharged from Elliptical Cell

Pulsed laser ablation with an elliptical cell gives well-defined thermodynamic conditions to the growth of high-quality thin films. The unsteady supersonic jet formed by the shock tube with small high-pressure chamber was used as a simple alternative model of pulsed laser ablation. The vortex ring formed by the shock wave is important to reveal behavior of unsteady supersonic jet discharged from elliptical cell. However, there has been little effort to investigate the interaction between the vortex ring and the jet. The purpose of the present study is to investigate the behavior of the vortex rings and the jet. The experiment and numerical calculation were carried out by schlieren method and by solving the axisymmetric two-dimensional compressible Navier-Stokes equations, respectively. The system of the calculation and the experiment is a model of laser ablation of a certain duration followed by a discharging process through the exit. Moreover, a parametric study was performed to demonstrate the effect of pressure ratio on the interaction among vortex rings and the supersonic jet. The interaction between the supersonic jet and the vortex rings increased the velocity of the supersonic jet up to the magnitude of the velocity at the center of the vortex rings. Closing a distance between the vortex ring and the jet is higher interaction between the vortex rings.

Experimental Study of Effect of Jets Injected into Supersonic Main Flow on Porous Cavity

In this study, the interaction between the supersonic main duct flow and jet surrounded by a porous cavity was experimentally investigated by means of schlieren flow visualization and measurements of flow direction in a cavity underneath of jets. The detection of flow directions was done with so called thermal tuft probe that has two heat sensors and one heat source middle of the two sensors. The parameters of the experiments are jet arrangements and pressure ratio defined by the ratio of total pressure in the settling chamber to atmospheric pressure. As a result, the backward flow in the cavity is confirmed in case of jet injections. Moreover, it is found that the change in the flow direction has dominant frequency between 300HZ and 400Hz only when the starting shock wave exists around the porous cavity

Study of Unsteady Supersonic Jet Using Shock Tube with Small High-Pressure Chamber with Elliptical Cell

The unsteady supersonic jet formed by the shock tube with small high-pressure chamber was used as a simple alternative model of Pulsed laser ablation. Since the pressure of the shock wave formed by Pulsed laser ablation is very high, the interaction between the shock wave and the plume is important for the formation of nanoparticles especially under high background gas pressure. An elliptical cell is used for controlling the shock wave. The shock wave discharged in one focal point converge another focal point. Numerical calculation was carried out by solving the axisymmetric two-dimensional compressible Navier-Stokes equations. The wall static pressure monitored at the center of substrate is used to evaluate the influence of the shape and the pressure ratio on the behavior of the shock wave, the plume, and their interaction. As a result, it was found that the vortex ring was generated downstream of the cell, which is followed by the discharging shock wave from the cell exit. It was also found that the influence of the vortex ring on the wall static pressure at the center of the substrate.

Small High-Pressure Chamber Shock Tube

The unsteady supersonic jet formed by the shock tube with small high-pressure section was used as a simple alternative system of pulsed laser ablation. The dynamic of the supersonic jet impinging upon a flat plate are discussed by comparing experimental and calculated results. The experiment and numerical calculation were carried out by schlieren method and by solving the axisymmetric two-dimensional compressible Navier-Stokes equations, respectively. The main parameters are distance between the open end of the shock tube and the flat plate, L/D, and the pressure ratio of the shock tube, Ph/Pb. Where, L, D, Ph and Pb are the distance between the open end of the shock tube and the flat plate, the diameter of the shock tube, pressure of the high and low section of the shock tube, respectively. Collision between the shock wave reflected at the flat plate and the head of supersonic jet takes place. Computational results well predict the experimental dynamic behavior of the shock wave and the supersonic jet. Marked increase in the static pressure on the flat plate under high Ph/Pb and short L/D is observed due to interaction between the shock wave and the unsteady jet flow.Copyright

Thermodynamics of Nanoparticle Formation in Laser Ablation

Nanometer-sized particles, or nanoparticles, are smaller than conventional solid-state materials and possess great potential for new, useful properties due to peculiar quantum effects (Roco, M. C., 1998). Highly functional devices synthesized from nanoparticles have been studied for use in various fields, such as semiconductors (Liqiang, J., 2003; Lu, M., 2006), photocatalysis (Liqiang, J., 2004), secondary batteries (Ito, S., 2005; Kim, K., 2009, 2010), superconductors (Strickland, N. M., 2008), and bonding substances (Ide, E., 2005). In the present chapter, we discuss the thermodynamics related to nanoparticle formation. Cooling processes of expanding vapor evaporated from a solid surface, such as gas evaporation, arc discharge, sputtering, pulsed microplasma and pulsed laser ablation (PLA), have been applied as a method of nanoparticle formation in the gaseous phase (Wegner, K., 2006). The PLA method, under reduced atmospheric pressure, has been found to be especially promising since it provides the following capabilities (Chrisey, D. B., 1994): (i) ablation of target material regardless of melting point due to the high intensity and focused laser beam pulse, (ii) flexibility in choice of atmospheric gaseous species and pressure, (iii) ease of production of the non-equilibrium state of the highpressure field due to the formation of shock waves, (iv) ability to obtain many different structured materials, from thin films to micrometer-sized particles, by controlling vapor association and condensation, and (v) ease of synthesis of nano-compounds of nonstoichiometric composition by preparing target materials with desired compositional ratios. The PLA method has been widely used for nanoparticle formation because the formed nanoparticles have diameters smaller than 10 nm with low size dispersion and can be formed as basic materials for highly functional devices via effective utilization of these capabilities (Li, S., 1998; Li, Q., 1999; Patrone, L., 1999, 2000; Wu, H. P., 2000; Suzuki, N., 2001; Inada, M., 2003; Seto, T., 2006). To understand the process of nanoparticle formation by the PLA method, two perspectives are necessary: (i) the thermodynamics of the microscopic processes associated with the nucleation and growth of nanoparticles, and (ii) the thermodynamics of the macroscopic processes associated with the laser irradiated surface of the target supplying the raw

Study of Interaction between Supersonic Flow and Rods Surrounded by Porous Cavity

In this paper, some preliminary experiments and the calculations were performed to clarify the flow field, in which the rods surrounded by a porous cavity were normally inserted into the main supersonic flow. As a result, it is found that the starting shock wave severely interacts with the rods, the bow shock wave, its reflections, and the porous wall, which are numerically well predicted under some conditions. In case of the single rod, the main supersonic flow is sucked in front of the rod and blown downstream of the rod. Furthermore, the suction mass flux through the porous holes depends on the number and the locations of the rods. On the other hand, the three rods aligned parallel to the flow make favorable suction area downstream of the rods and also generate as less total pressure loss as that for a single rod. In addition, the spanwised three rods cause the strongest bow shock wave making the large mass flux through the porous wall and also large total pressure loss downstream of the rods. The calculation also suggests that interaction between the bow shock wave and boundary layer along the upper wall results in the boundary layer separation.

Heat Transfer Characteristics by Impingement Jets Issuing from Dual Elongated Slot Nozzles.

The effects of spacing between dual jets on the heat transfer characteristics and flow behaviors over the plate surface were investigated experimentally when the dual slot jets impinge on the target plate. The local heat transfer distributions were measured for various spacings at small nozzle-to-plate separation distances and the flow pattern also visualized by the oil-titanium IV oxide method. An infrared radiometer with a two-dimensional array of InSb sensor was employed to clarify space varying in the flow and heat transfer accompanying by the interferences between jets, and jet and spent flow. The characteristics of heat transfer, flow patterns and thermal distributions change with the spacing of the dual jets and the nozzle-to-plate separation. The phenomenon of axes switching, which is caused by the differences in the self-induced velocity in the non-circular vortices, was observed farther downstream compared with that of a single impingement jet. These phenomena consequently played an important role in the heat transfer enhancement.

Temperature Distributions on an lmpinged Plate with an Underexpanded Jet

The temperature distributions on a flat plate impinged by an underexpanded jet issued from a converging nozzle were measured using infrared camera for various pressure ratio with a fixed nozzle to plate separation L/D=5. As a result, it is found that the temperature near the stagnation point has a maximum value at the total pressure ratio of about 2.5. Moreover the contour maps of isotherms on the plate show that the temperature contours have azimuthal structures mainly with five apexes for relatively large pressure ratio, which means that even the underexpanded jet is issued from the circular nozzle, the temperature distributions are not axsymetric.