直接观测胶体晶体中固固相变的成核过程

A solid can transform to another type of solid with a different crystalline structure by changing temperature or pressure. Such solid-solid (s-s) phase transitions constitute one of the most fundamental and widespread types of structural transitions in nature, including a large majority of the transitions found in elemental crystals, alloys, minerals and soft matter systems [1]. They have broad implications in earth science [2], steel production, man-made diamonds [3,4] and advanced ceramic materials [5]. Despite the considerable fundamental and practical importance to science and technology, their microscopic mechanisms are poorly understood [1, 2]. In particular, the initial nucleation process of the new phase formation is central to the phase transition and is a hotly debated subject. These processes are at the molecular level and hence difficult to measure, owing to both the spatial and time resolutions required. Colloidal particles are particularly well-suited for investigating phase transitions [6] because the thermal motions of micrometer-sized colloidal particles can be directly visualized and measured by video microscopy. In the past decade, significant experimental efforts have been made to study colloidal crystal melting [7-9], freezing [10,11], sublimation [12] and glass/jamming transitions [13,14], but few to solid-solid transitions. Here we propose the first colloidal experiment on s-s transitions using novel, thermal-sensitive, diameter-tunable, colloidal microgel spheres which will enable us to drive the phase transitions easily in a single sample [4, 12]. This breakthrough will allow us to study phase transitions at the single-particle level which is difficult to do in atomic systems. In contrast to conventional atomic systems where only heterogeneous nucleation can be studied, our system allows both heterogeneous nucleation and homogenous nucleation to be studied by local heating and cooling. We propose to study the transitions between square and triangular lattices in colloidal thin films. Many important questions can be systematically studied: What are the precursor defects in homogenous nucleation? How do the nonzero strains in the solid phases give rise to new phenomena beyond the classical nucleation theory (CNT)? How do nucleus shape, dynamics and energy barrier change with different types of single defect in heterogeneous nucleation? In our preliminary tests, a few surprising phenomena, such as two-step nucleation, were observed. Further research is needed to understand their mechanisms.

固固相变广泛存在于自然界与工业中，但人们对其微观过程和机制还非常缺乏了解。特别是相变初始阶段的成核过程是一个重要而又颇有争议的课题。成核过程发生在分子尺度上，难以直接观测。而胶体系统特别适合研究这类相变过程，因为微米大小胶体粒子的布朗运动可以直接在光学显微镜下观测。近十余年，利用胶体研究晶体熔化，结晶以及玻璃态转变已取得很多成果，但还没有固固相变成核方面的实验。 我们将直径可调的热敏微胶小球夹在两平板间自组装成四方薄膜晶格，通过改变体积分数使之变为三角晶格。利用局部变温技术我们可以研究均匀成核和单缺陷影响下的非均匀成核。在这个简洁的准二维模型系统中，一系列基本问题可以被系统地研究。在初步实验中，我们已经观察到一些从未料到的一步与两步成核的新现象。本实验为首次在单粒子尺度上观测固固相变的成核过程，对完善固体中的成核理论有重要意义。另外我们将对以上实验做计算机模拟，来更准确地揭示其微观机制。

悬浮在溶液中的微米胶体粒子可看作是‘大原子’，能自组装成各种晶体，液体，气体，为研究相变提供一个有力的平台。 尤其是对从一种晶体转变为另一种晶体的固固相变，在原子晶体中很难在观测到单粒子尺度上的相变过程。而我们利用新颖胶体系统，可以准静态地或加外场地驱动胶体晶体的固固相变，并在光学显微镜下观察并测量胶体粒子的运动轨迹，在单粒子尺度上观测固固相变成核过程等结构变化。..我们主要研究了均匀成核和非均匀成核固固相变在有外界应力和无应力平衡态下组合出的四种情况，并进一步研究各类缺陷对成核的影响。对核心结果还做了相应的模拟证实。..我们实验发现四方薄膜晶格转变为三角晶格为两步成核过程，伴随着液体中间态，并普遍存在于均匀成核和各种缺陷附近的非均匀成核，我们还给出这个新奇结果的普遍机制。这些结果作为封面长文发表在《自然。材料学》，被国际科学网站广泛报道，之后其他组在各种原子晶体相变中也发现同样行为，成为胶体系统可预言原子系统行为的一个例证。我们还发现当样品受到一个方向性压力时，固固相变过程有很多不同。包括一种新奇的初期马氏体转变，后期为扩散式的固固转变。这表明实际应用中关于马氏体转变的一个判据需要修正。这些结果发表在《自然。通讯》。另外，我们对子相核后期形状演化，两个核相互融合等难以在原子系统中观察的现象都做了研究。..这些结果不仅有重要基础意义，对冶金，地质，材料制备中固固相变的控制有很多实际意义，比如可以通过掺杂帮助形成液体核从而加速固固转变，通过增加母相晶体中的晶界造成固固相变中更多的低能量共格界面以及子相晶体中特殊取向的晶畴，这将有效控制相变后子相多晶的性质。