Research On The Mechanism Of Nanodiamond Synthesis By Laser Irradiation On Carbon Material

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.