单晶及孪晶铁纳米粒子的固-固相变与固-液相变研究

Iron nanoparticles have exhibited extensive applications in fields of information storages, energy resources and chemical industry, and biomedicines due to their superparamagnetism, high reactivity, and low cost. Compared with other metallic nanoparticles, iron ones possess abundant crystal structures which may be transformed among each other under certain conditions, and therefore display rich physical and chemical properties. To date, however, the phase transition mechanism of iron nanoparticles remains unclear because the dynamics process of phase transition is considerably difficult to be captured in experiments. In this project, in order to gain an in-depth understanding of phase transition behavior and mechanism, atomistic simulations and experimental characterizations will be employed to systematically study the solid-solid and solid-liquid phase transformations of polyhedral single-crystalline and twinned iron nanoparticles. Subsequently, the mechanism in the formation of single-crystalline and twinned iron nanoparticles will be elucidated by investigation on the solidification of liquid iron nanoparticles during cooling process. Especially, nucleation and growth of new phase in iron nanoparticles will be examined. By comparison of phase transition dynamics in the inverse processes, this project aims to discover the intrinsic relations between the crystal structures and phase transition mechanism of iron nanoparticles, thus provides a basis for their future design, synthesis, and practical applications.

铁纳米粒子因其超顺磁性、高反应性和低廉的价格而在信息存储、能源化工和生物医学等领域中具有广泛应用前景。与其他金属纳米粒子相比，铁纳米粒子具有更为丰富的晶体结构,而且这些不同的结构在一定的条件下可以相互转变，因而展现出丰富的物理与化学性质。然而，由于相变的动力学过程实验难以捕获，有关铁纳米粒子的相变行为和机制目前仍不清楚。本项目拟采用分子动力学方法与第一性原理计算并结合实验表征，对多面体单晶和孪晶铁纳米粒子在升温过程中的固-固相变和固-液相变展开系统的研究，以获得对相变行为与机制的深入理解；在此基础上，通过研究液态铁纳米粒子在冷却过程中的凝固行为，进一步探讨单晶和孪晶铁纳米粒子形成的原因，重点分析铁纳米粒子中新相的成核和生长机制；期望通过以上互逆过程中的相变动力学对比研究，从原子和电子层次上揭示铁纳米粒子的晶体结构与相变机制的内在关联，为铁纳米粒子的设计、合成和应用提供科学依据和指导。

铁纳米粒子在催化、存储和生物医学等领域具有广泛的应用前景。本项目采用第一性原理计算，分子动力学模拟以及多种结构优化技术，对铁及其合金纳米粒子的结构与性质进行了系统的研究。第一性原理计算用来研究铁的马氏体相变的路径选择、相变势垒与磁性转变；分子动力学模拟用来研究铁及其合金纳米粒子升温过程中固态相变和固液相变的动力学行为；多种智能算法用来对铁及其合金纳米粒子进行结构优化与预测。研究结果表明：铁的马氏体相变路径具有不可逆性，其相变路径与磁性有关；由数十个原子组成的铁团簇的结构明显不同于体相，随着原子数增加，它们的结构展现出一定的生长规律；铁纳米粒子的熔化行为与其表面结构密切相关，低能的表面在较高的温度下才出现表面预熔；铁合金纳米粒子的固态相变与固液相变的温度由其合金的形态，分布和组分比例所决定。本项目的研究从原子层次上揭示了铁及其合金纳米粒子的结构与其稳定性之间的内在联系，为制备高性能、低成本金属纳米粒子提供了理论指导和参考意义。