Thermodynamically stable amorphous phase formed in nanometer-sized particles and its background

为了弄清纳米合金颗粒中热力学稳定的非晶相的形成原因，利用具有几乎为零混合热的锡铋系颗粒，用原位电子显微镜研究了纳米合金颗粒中合金相的形成和相稳定性。采用200KV电子显微镜，样品室配有双源蒸发器，进行了现场观测。从第一种来源，锡原子被蒸发和气相沉积在非晶态碳衬底上(在石墨薄片上)，并在衬底上形成纳米尺寸的锡粒子。然后，在第二个源中，铋原子被蒸发并气相沉积到同一基片薄膜上。用原位透射电子显微镜研究了在纳米级锡颗粒上沉积铋所形成的合金相，以及合金相的热处理行为。主要内容如下：(1)沉积的铋原子…的溶解更多的S纳米级锡颗粒在固相中迅速发生，形成无定形的锡铋合金。(2)在纳米颗粒中出现的所谓的盐椒对比度呈现出随时间的波动：即明暗斑点在颗粒对比度中的位置和强度随时间的变化。颗粒对比度的这种波动反映了颗粒中原子的高迁移率。(3)流动非晶相具有很高的相稳定性，加热时熔化而不需要预先结晶，冷却时熔体直接凝固成非晶相。在纳米颗粒中，共晶温度(TEU)被大大抑制，从而实现了玻璃化转变温度(Tg)位于液态比相应固相更稳定的温度区域的情况。结果，在从TEU到Tg的温度范围内，在纳米合金颗粒中形成了热力学稳定的非晶相。较少

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 … More 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