Computational Study of Grain Boundaries and Nano-Reactive Materials.

摘要

The thesis summarized my research efforts in two areas:;Chapters 1 to 4 devote to the computational study of grain boundary (GB) structure, energy and mobilities. Despite of the considerable effort that has been placed on those topics, computational study of GB is still a daunting task. Chapter 1 gives a small glimpse into the rich world of GB studies in the past and the difficulties they met. In chapter 2, we propose a new approach to predicting and organizing interface structures in alloys that takes advantage of a disclination structural units model developed previously for grain boundaries in pure systems. This method is demonstrated using symmetric tilt GB in multiple alloy systems. Chapter 3 studies the GB migration using molecular dynamics simulation (MDS) with two methods: (i) direct simulation of GB motion under synthetic driving force. (ii) Transition path sampling (TPS) method. The two methods predict GB mobility differently. But they are consistent with each other because two GB migration mechanisms present in the first approach but only one mechanism is modeled in the TPS method. A simple and clear way to separate two GB migration mechanisms during the direct simulation is also reported in the chapter 3. Of the two GB migration mechanism, the shear coupled mechanisms has been well studied. In chapter 4, the spotlight is focused on the other GB migration mechanism: GB sliding. Two methods are reported, one uses MDS with nudged elastic band techniques and the other is based on macro-scale linear elasticity theory.;Chapters 5 to 8 summarize the computational study of nano energetic materials (NEMs), or more specifically, nano thermites and nano reactive intermetallic compounds. NEMs have raised substantial scientific and technological interest in recent years for their high reactivity, high energy density, low cost, and non-toxic nature. Chapter 4 provides an overview and information useful for synthesis and characterizations of NEMs. Special concern is given to the thermo analysis (TA) methods of NEMs. Compare to the self-sustaining reaction in NEMs, reactions in TA methods such as differential thermal analysis (DTA) or differential scanning calorimetry (DSC) are much slower. This makes TA a good candidate to unveil the coupled non-equilibrium processes during reactions. However, conventional TA requires a prior knowledge of "model functions" which are empirical approximations (for example, the 1st order reaction) of the overall reactions. In chapter 7, we proposed a better analysis method to study DSC traces of NEMs. This analysis is able to: i) reproduce the DSC curves; ii) predict the peak temperature positions as a function of system sizes and DSC heating rates; iii) explore micro-mechanisms during controlled reactions and provide clean way of retrieving diffusivity parameters. Tested with several experimental datasets and numerical simulations, the new analysis method shows significant improvement over the conventional TA methods. Further, the new TA method predicts that nano energetic particles may work better than nano ferro oxide particles do in hyperthermia cancer treatment (HCT) because of NEMs' capability of releasing heat rapidly. Chapter 8 reviews the past researches of using nano particles in the HCT. In conventional HCT, energy is transferred to cancer cells through alternating magnetic field (AMF). In chapter 8, a nano pillar comprising both ferro oxides and NEMs is proposed. Upon taken the nano pillars will accumulate in cancer cells due to the well-known Enhanced Permeability and Retention (EPR) effect. Ferro oxides can first be heated up through AMF, and then they will initiate the reactions in NEMs which then will kill the cancer cell. Based on our calculation, this design is promising.