Research on the origin of the thermodynamically stable amorphous phase in nm-sized particles

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In this research, the origin of the thermodynamically stable amorphous phase formed in nanometer-sized alloy particles was studied by both experiment and thermodynamic calculation.First, the effect of surface and interface on the stability of nanometer-sized particles was examined using particles of the Sn-Bi system. An approximately 8-nm-sized particle of the terminal tin solid solution directly changed into a particle of the liquid phase with increasing concentration of bismuth. On the other hand, an approximately 8-nm-sized particle of the terminal bismuth solid solution changed first to a particle with a crystal/liquid two-phase microstructure and eventually to a particle of the liquid phase, with increasing concentration of tin. Even in the bismuth-rich side, an approximately 5-nm-sized particle of the terminal bismuth solid solution, however, directly changed into the liquid phase, without taking a stage of crystal/liquid coexistence. These experimental results were discussed fro … More m a thermodynamic viewpoint. As a result, it was confirmed that not only surface but also interface is a governing factor of the thermodynamical stability of nanometer-sized alloy particles. Namely, the existence of the heterointerface between two crystalline solids stabilizes the liquid phase.Next, the relative stability of solid solution and liquid in nanometer-sized particles was examined using particles of the Pb-Sn system. Approximately 15-nm-sized particles of a lead-56at.%Sn solid solution were formed at 110℃ by tin deposition onto nanometer-sized pure lead particles. This fact indicates that the solubility limit of tin in 15-nm-sized lead particle is almost five times higher than the solubility limit of tin in bulk lead (i.e., 10at% Sn). It was confirmed that the relative stability between the highly concentrated solid solution and the amorphous phase governs the structure of nanometer-sized alloy particles.Conclusions obtained are given below.(1)In nanometer-sized alloy particles, the liquid phase becomes relatively stable as compared with the crystalline phase due to both surface and interface.(2)As a result, in nanometer-sized alloy particles the eutectic point becomes to be suppressed down to a temperature even below the glass transition temperature. This results in the formation of the thermodynamically stable amorphous phase over a temperature range between these two temperatures. Less