R. Greif

Explosive change in crater properties during high power nanosecond laser ablation of silicon

Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreemen...

Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.

Natural Convection in Undivided and Partially Divided Rectangular Enclosures

/ Heat transfer by natural convection in a two-dimensional rectangular enclosure fitted with partial vertical divisions is investigated experimentally. The horizontal walls of the enclosure are adiabatic while the vertical walls are maintained at different temperatures. The experiments are carrir8 out with water, n 3.5, for Rayleigh numbers in the range, 2.3 X 10 < RaL < 1.1 x 10 , and an aspect ratio, A = H/L = 1/2. The effect of the partial-vertical divisions on the fluid flow and temperature fields is investigated by dye-injection flow visualization and by thermocouple probes, respectively. The effect of the partitions on the heat transfer across the enclosure is also studied and correlations for the Nusselt number as a function of RaL and partition length are generated for both conducting and non-conducting partition materials. Partial divisions are found to have a significant effect on the heat transfer, especially when the divisions are adiabatic. The results also indicate that the partial divisions may have a stabilizing effect on the laminar-transitional flow on the heated vertical walls of the enclosure.

Initiation of an early-stage plasma during picosecond laser ablation of solids

Picosecond time-resolved images of plasma initiation were recorded during pulsed-laser ablation of metal targets in an air atmosphere. An early-stage plasma was observed to form before the release of a material vapor plume. Close to the target surface, interferometry measurements indicate that the early-stage plasma has an electron number density on the order of 1020 cm−3. The longitudinal expansion of the ionization front for this plasma has a velocity 109 cm/s, during the laser pulse. In contrast, a material–vapor plume forms approximately 200 ps after the laser pulse, and it moves away from the target at 106 cm/s. The experimental observations of the early-stage plasma were simulated by using a theoretical model based on a two-fluids description of laser plasmas. The results indicate that the initiation of the plasma is due to air breakdown assisted by electron emission from the target.

Geometric Structure and Thermal Conductivity of Porous Medium Silica Aerogel

In this paper, three periodic structures are used to derive the apparent conductivity of a porous medium and a specific application is made to aerogel. The characteristics of the structure are determined using results for the porosity and specific surface area obtained from nitrogen adsorption-desorption measurement. 10 refs., 2 figs., 2 tabs.

Laser ablation induced vapor plume expansion into a background gas. II. Experimental analysis

Laser ablation of copper with a 4ns laser pulse at 1064nm was studied with a series of synchronized shadowgraph (100fs laser pulses at 400nm) and emission images (spectral line at 515nm). Data were obtained at two laser pulse energies (10 and 30mJ) and in three background gases (He, Ne, and Ar) at atmospheric pressure. The laser energy conversion ratio and the amount of sample vaporized for ablation in each condition were obtained by the theoretical analysis reported in paper I from trajectories of the external shock wave, internal shock wave, and contact surface between the Cu vapor and the background gas. All three quantities were measured from shadowgraph and emission images. The results showed that E, the amount of energy that is absorbed by the copper vapor, decreases as the atomic mass of the background gas increases; and M, the mass of the sample converted into vapor, is almost independent of the background gas [Horn et al., Appl. Surf. Sci. 182, 91 (2001)]. A physical interpretation is given based...

An investigation of natural convection in enclosures with two- and three-dimensional partitions

Abstract The heat transfer and fluid flow in a rectangular enclosure fitted with a vertical adiabatic partition is investigated experimentally. The partition is oriented parallel to the two vertical isothermal walls, one of which is heated and the other cooled while all other surfaces of the enclosure are insulated. The experiments are carried out with water for Rayleigh numbers over the range 10 10 − 10 11 and an aspect ratio (height : width ratio) of one-half. Fluid temperatures are obtained with thermocouple probes and the cross-cavity heat transfer is obtained as a function of the Rayleigh number and partition geometry. The flow is visualized with dye injection. Two cases have been studied. In the first case, the partition is of constant height over the entire breadth of the enclosure resulting in a two-dimensional geometry. The effect of the transverse location and the upward or downward extension (orientation) of the division is examined. In the second case the partition completely divides the enclosure except for a rectangular opening which allows convection to occur across the enclosure. The dependence of the flow and the cross-cavity heat transfer on the Rayleigh number and on the size of the opening in the partition is studied.

Transport properties of gas in silica aerogel

Abstract The motion of gas molecules in silica aerogel is restricted by the solid silica matrix. As a result, the mean free path, velocity distribution function, diffusivity, viscosity and thermal conductivity are changed. In this work, relations for these quantities are derived for a gas in silica aerogel using an approach similar to that used for a gas in free space. Results for the mean free path predict that, for p ≥ 10 bar, the mean free path of the gas molecules in aerogel will be almost the same as in free space. However, as the pressure is reduced, the mean free path reaches a constant finite value instead of increasing as in free space. The thermal conductivity of a gas in aerogel starts to decrease at p = 10 bar and is almost negligible at 0.01 bar, while the thermal conductivity of a gas between parallel walls 1 cm apart starts to decrease at p = 10 −4 bar and is almost negligible at p = 10 −7 bar. The predicted thermal conductivity of a gas in aerogel is in good agreement with experimental results.

A study of traveling wave instabilities in a horizontal channel flow with applications to chemical vapor deposition

Abstract The flow and heat transfer of helium in a horizontal channel of height H and length L with a heated bottom surface and a cooled top surface are studied. Numerical solutions of the transient, two-dimensional Navier-Stokes and energy equations reveal that for conditions of interest in chemical vapor deposition (CVD), a thermal instability in the fluid (the Rayleigh-Benard instability), produced by the temperature difference between the horizontal surfaces of the channel, can result in traveling, transverse waves. The results show the effects of these waves on the flow and the heat transfer over a range of Reynolds numbers, Re = u H v o (10 −1 2 ) , Grashof numbers, Gr = gϵH 3 v o 2 (2.5 × 10 3 5 ) , aspect ratios, L H (4 L H , for two temperature ratios, ϵ = (T s − T o ) T o , 0.0333 and 2.333, corresponding to constant and variable property flow, respectively. The existence of transverse, traveling waves is shown to enhance the heat transfer from 50 to more than 300% over the condition without traveling waves. The important effect of the aspect ratio on the results is also emphasized.

Laminar convection of a radiating gas in a vertical channel

An exact solution is obtained for the problem of fully-developed, radiating, laminar convective flow in a vertical heated channel. The effect of radiation is to decrease the temperature difference between the gas and the wall, thereby reducing the influence of natural convection. Thus, the reduction in velocity occurring in a heated upflow is less for a radiating gas. Graphs are presented for the dimensionless velocity and temperature profiles and for the volume and heat fluxes.