The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters We report a parameter-free test of the theory predicting the critical solute concentration that destabilizes a planar solid-liquid interface in the high-velocity regime where nonequilibrium interface kinetics are important. Rapid solidification following pulsed laser melting was used to make metastable solid solutions of silicon-tin. Rutherford backscattering spectrometry and transmission electron microscopy were used to measure the breakdown concentration. Samples remained microsegregation free with near perfect crystallinity at tin concentrations up to 10 times the maximum equilibrium solubility and 100 times that predicted by linear stability theory with local interfacial equilibrium. These measurements, covering velocities from 1 to 10 m/s, agree with the predictions of linear stability theory when the latter incorporates a velocity-dependent partition coefficient and a thermodynamically consistent kinetic liquidus, and contains no adjustable parameters. We also report a systematic increase of the breakdown concentration with increasing deviation from steady-state conditions, which is not addressed by current stability theories, parametrized by the concentration gradient just prior to breakdown. ͓S0163-1829͑98͒07117-3͔

Pulsed laser melting experiments were performed on GexSi1−x alloys (x≤0.10) with regrowth velocities ranging from 0.25 to 3.9 m/s. Analysis of post-solidification Ge concentration profiles, along with time-resolved melt depth measurements, allowed determination of the liquid-phase diffusivity Dl for Ge in Si and the dependence of the Ge partition coefficient k on interface velocity v. A Dl of 2.5×10−4 cm2/s was measured. The k vs v data were analyzed using various models for partitioning, including both the dilute and nondilute Continuous Growth Models (CGM). Extrapolating to zero velocity using the partitioning models, an equilibrium partition coefficient of approximately 0.45 was obtained. Best fitting of partitioning data to the nondilute CGM yields a diffusive speed of 2.5 m/s. These measurements quantify previous indications of partitioning observed in other studies of pulsed laser processed GexSi1−x alloys.

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At the solidification velocities observed during pulsed laser annealing, the planar interface between solid and liquid is stabilized by capillarity and nonequilibrium effects such as solute trapping. We used Rutherford backscattering and electron microscopy to determine the nonequilibrium partition coefficient and critical concentration for breakdown of the planar interface as a function of interface velocity for Sn-implanted silicon. This allows us to test the applicability of the Mullins- Sekerka stability theory to interfaces not in local equilibrium and to test the Coriell-Sekerka and other theories for oscillatory instabilities.

ABSTRACrA review of data on narcotics and explosives particulates is presented. Methods to generate particles of narcotics and explosives will be discussed with respect to resulting particle size distribution and mass output. The application of these standards to the testing of narcotic and explosive particle detection systems will be addressed 1 . INTRODUCTIONThere are a number of commercial instruments that claim to detect concealed contraband via the collection and analysis of particles of either the drug/explosive itself or a particle coated with the target molecule. The US Customs Service (USCS) and the United States Federal Aviation Administration (FAA) are in the process of testing these systems for their performance in the collection and detection of narcotic and explosive respectively. Standards are needed to perform these tests. One type of standard that has been proposed is a simulated rprit.2 This standard is ideal from an operational standpoint since it realistically duplicates the sample that the detection device will encounter in the "real world." The generation of fingerprints from explosives have been referred to being more of an art than a science1 due to the number of variables that as yet are not controlled. The variables include, age of explosives, type of surface the explosive is transferred to, size of finger, pressure applied to the explosive, and personal variations such as skin texture and oiliness. The FAA is currently involved in an international study to characterize the residue resulting from fingerprints deposited after someone handles explosives.3 The residue resulting from contact with C-4, P-4 and Semtex will be characterized with respect to total mass of RDX present and particle size distribution. The U. S. Customs service and Revenue Canada Customs and Excise are also involved in similar work with cocaine and heroin residues. Another type of standard proposed is one that can deposit particles on a surface as well as simulate the transport and migration of explosives/narcotics through space. Because most explosives and narcotics present negligible vapor pressure, (RDX and PETN are 0.0005 ppb and .0015 ppb respectively) it is believed that transport of the material is via micron sized particulates. The narcotics or explosives are present either as particulates or adsorbed onto airborne carriers such as dust. Thus, the need exists to develop particulates for airborne applications as well. Aerosol generation techniques commonly used in air quality research4 will be applied to this problem. These techniques can be exploited as a baseline standard for the assessment and development of vapor/particle contraband detection systems This paper will describe two methods of generating aerosols of contraband materials and the application of these devices to the preparation of standards. The estimated perfonnance of the equipment is based on manufacturers specifications and theory. Experiments are currently being performed at the authors laboratory to evaluate the performance of this equipmen...