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

为了阐明纳米合金颗粒中热力学稳定的非晶相形成的原因，采用原位透射电镜（TEM）研究了Sn-Bi体系中几乎为零混合热的颗粒中合金的相形成和相稳定性。现场观测采用200kV瞬变电磁炉，样品室配有双源蒸发器。从第一个源开始，Sn原子蒸发并气相沉积到非晶碳衬底膜上（在石墨片上），并在衬底上形成纳米尺寸的Sn颗粒。然后，从第二个源，Bi原子蒸发并气相沉积到相同的衬底膜上。利用原位透射电镜（TEM）研究了Bi沉积在纳米级Sn颗粒上形成的合金相及其退火行为。澄清如下：(1)沉积的Bi原子…More s在固态状态下快速溶解到纳米级Sn颗粒中，形成非晶态Sn-Bi合金。(2)纳米颗粒中出现的所谓盐-胡椒对比呈现出随时间的波动，即颗粒对比中亮斑和暗斑的位置和强度随时间而变化。颗粒对比度的波动反映了粒子中的高原子迁移率。(3)流体非晶相具有高的相稳定性，因此在加热时，它没有预先结晶而熔化，在冷却时，熔体直接凝固成非晶相。从这些实验结果中，得到以下结论：在纳米尺寸的合金颗粒中，共晶温度（Teu）被大大抑制，从而实现了玻璃化转变温度（Tg）位于液相比相应固相更稳定的温度区域的情况。这种情况的结果是，在从Teu到Tg的温度范围内，纳米尺寸的合金颗粒形成了热力学稳定的非晶相。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