【作者】 翟琪；

【导师】 孙景；

【作者基本信息】 天津大学 ， 材料学， 2007， 博士

【摘要】 以激光为能量源的金刚石制备方法因可获得纯度高、粒径小、分布范围窄的纳米金刚石而表现出强大的生命力,但由于激光与材料的相互作用是一个极端非平衡过程,许多实验参数在瞬间发生急剧变化,很难进行实时分析与监控,因此,在激光作用下其它碳材料向金刚石转变的物理、化学机制目前尚不十分清楚。本文从实验及计算机模拟两方面对激光作用下碳材料相变机理进行了深入研究。首先从碳原料的结构、尺寸、激光功率密度和工作介质四个方面对激光作用下碳材料相变的关键影响因素进行了实验和分析。研究结果表明:(1)在较低功率密度激光作用下,石墨是炭黑向金刚石转变的中间相,即石墨比炭黑更易于转变为金刚石;(2)随着晶体尺寸的减小,石墨的形成热增大,稳定性降低,导致微晶石墨比片状石墨更易于发生向金刚石的转变;(3)激光功率密度越高,加热效率越高,热损失越小,石墨表面温度越高,随后的冷却速率越高,越有利于发生碳的相变;(4)由于空蚀现象的存在,水中激光利用效率要高于空气中的激光利用效率,同时,在激光作用下,水介质分解、离化形成的H+、OH-等活性粒子对金刚石的形核、长大过程具有促进作用。为了能够对碳材料相变的动力学过程进行模拟,在对现有分子动力学软件进行系统分析的基础上,按照面向对象软件工程规范,设计开发了适合于模拟碳材料原子间相互作用的分子动力学模拟软件CMMD(carbon material molecular dynamics)。该软件的特色在于提供了适合于描述碳材料原子之间相互作用的势函数,从而能够模拟共价键的形成和断裂,反映碳材料相变的微观过程。在该软件的设计中采用了MVC架构模式,将体系结构设计为三层,即表现层、控制与转换层和模型层;并使用面向对象的标准C++语言、GCC编译器、MPI、Qt和GLUT在RedHat Linux操作系统上编程,完成了CMMD。分析了脉冲激光与石墨相互作用的物理机制,提出石墨对激光的吸收具有非金属的特征,而对激光的能量传递则具有金属的特征,用自由电子的经典模型来描述激光与石墨的能量耦合过程,用分子动力学方法对脉冲激光作用下石墨的升温和降温过程进行模拟。结果表明,在功率密度为109W/cm2的纳秒脉冲激光作用下,石墨晶格在极短的时间内快速升温(晶格温度可达5100K)以致熔化形成液态碳。在随后的降温过程中,sp2、sp3杂化的碳原子共存,即石墨与金刚石同时形核、长大,但sp2杂化的碳原子在系统中占有主导地位。由于系统的冷却速率高达1011K/s,在降温结束后,石墨结构与金刚石结构得以共存。该模拟结果揭示了激光诱导的碳材料相变遵循熔化—结晶机制,而非固态相变机制。更多还原

【Abstract】 Diamond synthesis methods by laser irradiation are promising because they can obtain nanodiamond with high purity, fine granularity and uniformity. However, the interaction between laser and carbon materials is an extreme non-equilibrium process in which many experimental parameters vary too rapidly to be monitored in real time. Until now, the transformation mechanisms from the other carbon materials to diamond are still unclear. Study on the phase transformation mechanisms of carbon materials under laser irradiation were carried out by using both experiments and computer simulations in this paper.First, four critical factors (the structure, size of original carbon materials, laser power density and liquid medium) were studied which play important roles in the laser-induced phase transformation process of carbon materials. Experimental results demonstrated that: (1) Under laser with low power density, graphite is the intermediate phase in the transformation from carbon black to diamond, and graphite is easier to transform into diamond by laser irradiation than carbon black. (2) As the size of crystals decreases, the formation heat of graphite increases and its stability descends. As a result, microcrystalline graphite is more advantageous in the synthesis of nanodiamond than crystalline flake graphite. (3)As the power density increases, the heat efficiency of laser increases and the heat loss decreases. This leads to higher graphite surface temperature and faster cooling velocity which benefit to the carbon phase transformation. (4)Due to the cavitation damage, the laser efficiency in water is higher than that of in air. Besides, the H+、OH- produced from water decomposition promote the forming and growth of diamond nucleus.Second, in order to simulate the dynamics process of carbon phase transformation, we designed and developed carbon material molecular dynamics software (CMMD) based on object-oriented software engineering after a systematic analysis of existing molecular dynamic software. The main feature of CMMD is that it provides potential functions suitable for describing of the interaction between carbon atoms, thus it can simulate the formation and breaking of covalent bonds. We use model-view-controller (MVC) pattern in the system design. The software architecture includes three layers: the presentation layer, the control and transform layer and the model layer. The software was developed under RedHat Linux by using standard C++ language, GCC compiler, MPI, Qt and GLUT.Third, the physical mechanism of the interaction between laser and graphite is discussed. Graphite exhibits nonmetal feature during laser absorption process, but exhibits metal feature during laser energy transferring. The free-electron model is used to describe the energy coupling of laser and graphite lattice. At last, the heating and cooling of laser-induced graphite lattice were simulated by using CMMD. The results show that graphite lattice is heated to a high temperature (near 5100K) and it melts in a very short time. Subsequently, the liquid carbon cools down. The sp2, sp3 atoms co-exist during the cooling process, that is, graphite and diamond nucleus form and grow synchronously, but the sp2 atoms are the majority in the system. Both graphite and diamond structures are reserved because of the high cooling speed (1011K/s). The study implied that graphite transforms to diamond in the solid-liquid-solid pattern rather than the solid-solid pattern under laser irradiation.更多还原