Abstract
The level of liquid undercooling below the equilibrium melting point experienced typically in a bulk material is limited by the presence of potent heterogeneous nucleation sites. By subdividing the material into a collection of fine (10 to 150μm) droplets the most potent nucleants are isolated into a small fraction of the droplet population, allowing the remainder to reach deep levels of undercooling prior to the onset of solidification. Droplet surface coating characteristics influence both the level and uniformity of the undercooling and, in addition, may alter the nucleation kinetics thereby encouraging the formation of a metastable crystalline phase from the undercooled liquid. An effective means of both creating a stable dispersion of fine droplets and influencing the surface coating is through the droplet emulsion technique. This approach has allowed the level of undercooling for pure antimony to be extended from 0.08 to 0.237T m through proper control of droplet size, surface coating and applied cooling rate. The formation of a metastable simple cubic phase from the melt has been confirmed using both differential thermal analysis and X-ray diffraction techniques. The simple cubic phase is retained in droplets to temperatures approaching the melting point at a heating rate of 20° C min−1. Drop tube treatment of emulsified samples has also been successful at producing the metastable simple cubic structure by containerless processing.
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Graves, J.A., Perepezko, J.H. Undercooling and crystallization behaviour of antimony droplets. J Mater Sci 21, 4215–4220 (1986). https://doi.org/10.1007/BF01106533
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DOI: https://doi.org/10.1007/BF01106533