A New Experimental Method for Investigating the Nucleation Kinetics of Solids in Supercooled Liquid Si
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solid nucleation rates [10]. Here, we describe a new experimental method that is being developed in order to quantitatively characterize the rates of solid nucleation in supercooled liquid Si. The method is noteworthy in that it possesses a unique combination of technical and procedural elements that may enable the method to potentially provide accurately measured rates of solid nucleation over a substantially wider frequency range than has previously been attained. In this paper, we scrutinize those aspects of the method that must be validated and proved in order for the method to be properly implemented. We accomplish this by focusing on characterizing a set of experiments that are carried out at a single transformation temperature of 1175 'C. (A more detailed analysis as well as the characterization and implications of the nucleation rates over wide temperature and frequency ranges will be presented elsewhere.) In addition to the immediate and apparent benefits it would bestow on current excimerlaser-crystallization activities, the information that would result from the application of the current method may make possible a quantitative and critical test of classical nucleation theory (and various modifications to the theory that have been proposed over the years); this is the eventual and ultimate aim behind the development of the current experimental method. BACKGROUND Carrying out systematic experiments on the nucleation of solids in a supercooled elemental metallic liquid is recognized as being a rather challenging task [11, 12, 13]. The difficulties stem partly from the fact that a nucleation event is extremely sensitive to the presence of difficult-to-control-or-eliminate catalyzing particulates (i.e., nucleants), is hyper93 Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
sensitive to the temperature of the liquid, and is a kinetic process that is statistical in nature. Additionally, the high growth rates of crystals in a supercooled liquid leads to significant generation of heat, which in turn increases the temperature of the system (i.e., recalescence) and nonlinearly affects the ensuing transformation of the material. As far as the controlled supercooling experiments are concerned, the application of the so-called "droplet technique" has formed the basis of experimental investigations that have been conducted in the past [11, 12, 13]. The key concept behind the technique, which was originally developed by Turnbull and other investigators, is the principle of isolation [11] in which the probability of uncontrolled nucleant-initiated heterogeneous nucleation of solids is reduced as a result of the small volume of the isolated and finely divided material. The subsequent supercooling and solidification of the droplets that are now largely free of the nucleants (at least of the more effective kind) may be detected via various technical means [12]. Still, the typical outcome of most investigations, which nearly always are of the continuous cooling type, consists of a roughly estimated si
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