Yong W. Kim

Thermophysical property measurement at high temperatures by laser-produced plasmas

Excitation by a high-power laser pulse of a material surface generates a sequence of plasma, fluid flow, and acoustic events. These are well separated in time, and their detection and analysis can lead to determination of material properties of the condensed phase target. We have developed a new methodology for real-time determination of molten metal composition by time-resolved spectroscopy of laser-produced plasmas (LPP). If the laser pulse is shaped in such a way that the movement of the bulk surface due to evaporation is kept in pace with the thermal diffusion front advancing into the interior of the target, the LPP plume becomes representative of the bulk in elemental composition. In addition, the mass loss due to LPP ablation is very well correlated with the thermal diffusivity of the target matter. For several elemental solid specimens, we show that the product of the ablation thickness and heat of formation is proportional to the thermal diffusivity per unit molecular weight. Such measurements can be extended to molten metal specimens if the mass loss by ablation, density, heat of formation, and molecular weight can be determined simultaneously. The results from the solid specimen study and the progress with a levitation-assisted molten metal experiment are presented.

Effects of Surface Morphology and Composition Modification on Spectral Emissivity and Thermal Diffusivity Profile at High Temperatures

A new all-spectroscopic method for depth-resolved thermal diffusivity measurement of metallic specimens has been demonstrated. The method entails measurement of the mass entrained into a laser-produced plasma (LPP) plume in such a manner that the plume is representative of the specimen in elemental composition. Both the abundance of matter and its elemental composition are measured by time-resolved spectroscopy for each LPP plume. In order to delineate the morphology versus composition basis of the depth dependence, a new study on a Nichrome ribbon specimen heated by ohmic heating in a vacuum is presented. A set of depth-resolved thermal diffusivity measurements is carried out, while noting the attendant changes in the spectral emissivity and elemental composition at succeeding ablation layers. Additional measurements are carried out after the specimen has been treated under varying heating conditions. Preferential diffusion of chromium at high temperatures has been found to contribute to the dynamics of surface thermophysical properties at high temperatures. Representative LPP ablation is well suited for removal of surface impurities prior to thermophysical property measurements by the pulse heating technique.

Metallic nanocluster formation in neutral gas-confined laser produced plasma afterglow

Dense atom vapors are created as remnants of a three-dimensional (3D) laser-produced plasma (LPP) plume driven from a solid aluminum target by a gigawatt-class Nd:glass laser pulse. Formation of nanoclusters as the terminal state of aluminum vapors has been investigated. By plasma structure diagnosis, we have fully characterized the LPP plume in 3D as a function of time and from its evolution modeled its cooling to the critical point in more than 110 000 vapor cells. Clustering of atoms is calculated by numerical simulation of interparticle collisions. When the cluster size histogram is rescaled relative to the maximum population and the maximal radius, the rescaled histograms all collapse into a single functional form regardless of the time or the initial atom vapor density. The cluster size distribution for the entire plasma plume is obtained by summing over all plasma cells. Independently, the clusters are captured onto electron microscope grids and size analyzed by means of transmission electron micro...

Polarization spectroscopic imaging of charge separation in neutral gas-confined laser-produced plasmas

Weakly nonideal plasmas can be produced by a high power-density laser incident on a metallic target. We have shown that plasma nonideality of the laser-produced plasma (LPP) plume may be further increased when its free expansion is suppressed by immersing the target in a dense neutral gas. As the gas density is further increased, the LPP plume develops interfacial instability, which turns out to be of Rayleigh–Taylor nature. We have developed a diagnostic method to help visualize development of local electric fields due to charge separation in the regions of significant density and temperature gradients. A bandpassed view of the LPP plume is split into two mutually exclusively polarized images, which are then captured simultaneously by a single, gated, intensified CCD detector. Representative results are presented of the two-dimensional distribution of the degree of polarization for a LPP plume of aluminum confined by low-density argon.

Evolution of near-surface elemental composition profile and its effect on thermal diffusivity

In a series of recent experiments, utilizing the method of time resolved spectroscopy of laser-produced plasma (LPP) plumes from specimen surfaces, the near-surface elemental composition profiles were observed to be nonuniform and significantly different from the respective bulk composition. A new study of three alloy systems is reported, with a view toward establishing the causal relationship between the near-surface elemental composition profile of a specimen and its thermophysical properties in general and thermal diffusivity in particular. The systems in question are as follows: two-element Nichrome ribbon; four-element magnetic Mumetal foil; and four-element Woods alloy. The method of LPP plume spectroscopy has been used throughout to successively expose new surface layers and measure the composition and thermal diffusivity. With two of the systems, modification of the near-surface elemental composition profiles has been forced. Sustained electrical heating of a Nichrome ribbon specimenrevealed preferential diffusion of chromium to the surface, affecting the spectral emissivity and thermal diffusivity as well as depth-dependent local heating rates. In the case of Woods alloy a sample is melted and re-solidified under a protocol that highlights gravitational forcing. The noncontact spectroscopic method has been used to discover that the top and bottom surfaces acquire two different composition profiles and exhibit commensurate disparity in the measured thermal diffusivity profiles.

Continuum-based diagnostics of weakly nonideal laser-produced plasmas

We have developed a novel diagnostic technique for weakly nonideal laser-produced plasma (LPP) plumes, based entirely on the continuum, and present the result for aluminum plasma. Due to significant self-absorption, the analysis is carried out in close coupling with plasma equilibrium calculations. We first invoke scaling relations linking the time- and position-resolved specific continuum intensity I, integrated over the spectral range of a streak camera, to the local temperature T and pressure p: T=STIα and p=SpIα+β. Inversion of the two-dimensional luminosity profile into the specific continuum intensity profile (three-dimensional) is carried out self-consistently, subject to the 16 independent measurements: target mass loss, energy deposited into the LPP plume and laser beam attenuation at fundamental and its harmonic frequencies, all at four different laser energies. An agreement to within 6.1% of the measurements overall has been achieved with α=0.45±0.03, β=1.0±0.03, ST=0.190±0.003 and Sp=340±5.

Reconstruction of time-resolved continuum intensity profile of nonaxisymmetric laser-produced plasma

A single laser pulse is used to produce weakly nonideal plasma from a metallic aluminum target immersed in a dense neutral gas. The attendant increase in plasma density due to neutral gas confinement precipitates interfacial instability when the gas density exceeds a threshold value. This is accompanied by large fluctuations in the total attenuation of the laser beam by the laser-produced plasma plume. We have developed a new diagnostic method utilizing two mutually orthogonal side-view streak photographs of plasma continuum luminosity at a fixed distance from the target surface. The lack of axial symmetry is overcome by using a front-view luminosity image of the plasma at time zero as a two-dimensional weighting factor. The resulting profile at one time is used as the weighting factor for the next time segment. The time-resolved reconstructed plasma profiles clearly exhibit the near-threshold behavior of Rayleigh–Taylor type instability.

Three-dimensional plasma structure reconstruction from mutually orthogonal streaks of nonaxisymmetric laser-produced plasma plumes

A method is developed for structure reconstruction of an arbitrary plasma from two luminosity streaks and a front-view snapshot as a weighting function (WF). Two scaling relations link the pressure and temperature to the local specific emission intensity. A trial plasma structure is proposed in terms of specific intensity, and the luminosity streaks are calculated according to the Saha equilibrium. Plasma absorption is included. Error signals between the calculated and measured luminosity are allocated according to the WF to find corrections to local specific intensities. Minimization of the errors completes the reconstruction at a given time, which in turn updates the WF for the next time step. Summing over the full plasma over time facilitates calculation of the total plasma mass, energy and beam attenuation through the plasma. When agreement with their measured counterparts is maximized, the scaling relations are calibrated.

Depth-Resolved Surface Thermophysical Property Measurement by Laser-Produced Plasmas

A new laser-based method for real-time in situ measurement of thermophysical properties of materials has been developed. It entails production by a high-power laser pulse of a plasma plume from the surface of a condensed-phase specimen and simultaneous measurement of a materials response to the excitation. The specimen may be a solid or in a molten state at high temperatures. It has been shown that the thermal diffusivity can be determined, for instance, from the mass loss due to laser excitation. In one implementation the mass loss is determined from the impulse imparted on the surface by the ablated matter which is measured by an impulse transducer. In this paper, we present a new spectroscopic method for measurement of the mass loss, facilitating in situ non-contact measurement of the thermal diffusivity for the first time. An implementation of this method is described, whereby the thermal diffusivity of a complex layered surface is determined as a function of depth with resolutions as small as 13 nm.

Direct sampling of gas and particulates from electric arc furnaces

During a given arc furnace heat for melting of scrap steel charges and subsequent steel making, considerable amounts of particulates are produced, varying in composition, size distribution and production rates corresponding to several distinct stages of the heat. In an effort to develop a detailed model for the particle production mechanisms, a new program for direct sampling of the furnace gas and particulates from the interior of the furnace has been devised and successfully implemented. It consists of a new high temperature sampling tube, capable of withstanding temperatures up to 1900 C for an indefinite period of time, and an experimental protocol designed to extract certain specific information necessary for development of a theoretical model. The results from two complete runs are described in detail. A theoretical model has been formulated, as guided by the measurements, which facilitates realistic predictions of the growth rate and elemental compositon of the particulates.