| Research Abstract |
In an attempt to clarify the reasons for the formation of a thermodynamically stable amorphous phase in nanometer-sized alloy particles, alloy phase formation and phase stability in the particle has been studied by in-situ transmission electron microscopy (TEM), using particles in the Sn-Bi system with almost zero of heat of mixing. A 200kV TEM with the specimen chamber room equipped with a double-source evaporator, was employed for the in-situ observations. From the first source, Sn atoms were evaporated and vapor-deposited onto an amorphous carbon substrate film (on a graphite flake), and nm-sized Sn particles were formed on the substrate. From the second source, then, Bi atoms were evaporated and vapor-deposited onto the same substrate film. Alloy phase formation associated with Bi deposition onto nm-sized Sn particles, as well as the annealing behavior of the resultant alloy phase, were examined by in-situ TEM. The followings have been clarified.
(1) Dissolution of deposited Bi atom
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s into nm-sized Sn particles takes place quickly in a solid state, and an amorphous Sn-Bi alloy forms.
(2) The so-called salt-pepper contrast appearing in the nanoparticles exhibits a fluctuation with time: namely, bright and dark spots in the granular contrast change in position and intensity with time. This fluctuation in the granular contrast reflects a high atomic mobility in the particle.
(3) The fluid amorphous phase possesses a high phase stability so that upon heating it melts without prior crystallization and upon cooling the melt solidifies directly into the amorphous phase.
From these experimental results, the following conclusion was obtained. In nanometer-sized particles, the eutectic temperature (Teu) is greatly suppressed so that a situation where the glass transition temperature (Tg) lies in a temperature-region in which a liquid phase is more stable than the corresponding solid phase, is realized As a result of this situation, over a temperature range from Teu upto Tg, a thermodynamically stable amorphous phase forms in nanometer-sized alloy particles. Less
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