Study of Atomic and Electronic Structures of Nano-Particles in the Early Stages of Their Formation Using Self-Searching Positrons

在材料中嵌入（亚）纳米粒子对控制基体材料的物理力学性能起着重要作用。因此，阐明纳米粒子的演化过程及其原子和电子结构是十分必要的。本研究旨在利用最新的正电子湮灭实验和理论方法，如重合多普勒展宽（CDB）、湮灭辐射的二维角相关（2D-ACAR）和相应的第一性原理计算，建立一种新的方法来解决这些问题。首先，通过详细的时效实验揭示了热时效Fe-Cu合金中Cu （bcc）颗粒的形成过程；用CDB方法研究了不同时效条件下Cu聚集的演化过程。其次，我们获得了2D-ACAR动量分布的详细老化变化，特别是在费米截止动量区附近。对于亚纳米尺寸的小铜粒子，在费米动量周围观察到更大的涂抹。这一观察结果有力地提出了一种新的、独特的方法来检测铁中亚纳米尺寸的Cu颗粒。在这个阶段，我们已经成功地观察到大于0.5 nm的纳米颗粒。我们还测量了铝合金单晶中时效诱导颗粒Ag_2Al和fcc Zn颗粒的2D-ACAR谱。结合第一性原理计算，我们获得了嵌入粒子的详细费米面。

Embedded (sub-)nano particles in materials play important roles in controlling the physical and mechanical properties of the host materials. Thus it is very indispensable to clarify the evolution processes of the nano particles as well as their atomic and electronic structures. This study aims to establish a novel approach to these issues with use of the latest positron-annihilation experimental and theoretical methods, such as coincidence Doppler broadening (CDB), two-dimensional angular correlation of annihilation radiation (2D-ACAR) and corresponding first-principles calculations.First we have revealed the formation process of Cu (bcc) particles in thermally aged Fe-Cu alloys by detailed aging experiments; the evolution of Cu aggregation in different aging conditions has been investigated by the CDB method. Second we have obtained detailed aging-induced changes in the 2D-ACAR momentum distribution, especially around Fermi cut-off momentum region. Larger smearing around the Fermi momentum is observed for the small Cu particles with sub-nano sizes. This observation strongly suggests a new and unique method for detecting sub-nano size Cu particles in Fe. At this stage we have succeeded to observe the nano particles larger than 0.5 nm.We also have measured 2D-ACAR spectrum of aging-induced particles in the single crystals of Al alloys: Ag_2Al and fcc Zn particles. Combining the first-principles calculations, we obtained detailed Fermi surfaces of the embedded particles.