The melting point depression of tin in mechanically milled tin and germanium powder mixtures
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S.S. Grossa) Corning Glass Works, Corning, New York 14831 (Received 25 August 1988; accepted 27 December 1988) A melting point depression, ATM, has been observed for Sn in Ge/Sn (50 at.% Sn) dispersions which were prepared by mechanical milling of Ge and Sn powders. Sn and Ge are immiscible and form a fine dispersion of the pure components when milled in a high energy ball mill. The magnitude of ATM, as measured by DSC, increases with milling time, i.e., with refinement of the dispersion. Melting is observed to begin as low as 36 °C below the equilibrium bulk melting temperature. The magnitude of ATM is reduced by about 25% after melting the Sn. Subsequent remelts do not change ATM further. Impurities cannot account for ATM. While stored energy of cold work may contribute to ^ 2 5 % of ATM before Sn is melted, it is concluded that the major contribution to ATM comes from the nucleation of disorder/melting at the Ge/Sn interfaces. I. INTRODUCTION
evidenced by a "particle" size dependence of ATM. Tagaki6 saw a TM depression in thin films of Pb, Sn, and Bi with A liquid metal can often have its freezing point deAT M increasing as film thickness decreased. Blackman and pressed well below the equilibrium melting temperature, Curzon 7 and Wronski8 studied tin particles evaporated in TM, if heterogeneous nucleation is discouraged by, for a chamber attached to an electron diffraction camera. example, division of the melt into small discrete volumes.1 Electron diffraction was used to determine when melting A large body of evidence exists for this phenomenon of had occurred. Substantial depressions of TM (~80 K) were supercooling (e.g., Ref. 2). Most bulk metals, however, seen for the smallest particle sizes (~4 nm). Buffat and are found to melt on heating at the equilibrium TM. Only Borel9 showed that small gold particles could exhibit a recently have experiments been performed which clearly ATM of over 600 K for the smallest diameter particles show superheating of a metal above its equilibrium TM.3 (2-3 nm). While the above experiments involve metals The conventional explanation4 for this asymmetry of meltwith a solid/vapor interface, Willens et al.10 observed ing vs freezing behavior (a common thermodynamic feature ATM of lead in modulated Pb-Ge foils. As the Pb layer of many phase transitions) is that most liquid metals efthickness decreased, ATM increased and broadened. Subsefectively wet their own solid, i.e.,
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