报告题目：Multi-Phase Field Models of Nanoscale Synthesis and 
 Deformation
报 告 人：Prof. 蔡巍 (Stanford University, USA) 
报告时间：6 月 30 号（星期四） 下午 15 点 40 分钟
报告地点：南一楼东边三层力学系会议室
邀 请 人：李振环  黄敏生  教授 
   The phase field model was originally developed to simulate the morphology of crystals as they solidify from melt.  Here we describe extensions of the phase field method to new applications where vapor, liquid and solid phases coexist.  The competition between interface energies (and in some cases elastic energies) gives rise to abnormal behaviors observed during synthesis and deformation of nanostructures. The first application is the catalyzed growth of semiconductor nanowires (NWs) by the vapor-liquid-solid (VLS) process.  A 3D phase field model for the VLS growth process is developed, which captures the realistic NW morphology, and provides an explanation of the growth kinking behavior observed in experiments. The second application is the rupture of single crystal silver NWs under constant load slightly below the yield point.  We hypothesize that the observed rupture of the NWs is caused by instability of the surface due to diffusion.  The phase field model, when extended to include elastic energy, provides support to this hypothesis. The third application is the collapse of slender, vertical solid (silicon) structures (patterns) when the liquid between them evaporates (as part of a cleaning process). To model this “ pattern collapse” process, the phase field model now needs to include not only the elastic energy, but also the elastic deformation of the solids.  The resulting phase field model predicts the stability range of the solid structure that is consistent with existing experimental observations. 
   Bio: Wei Cai received his B.S. degree in optoelectronic engineering from Huazhong University of Science and Technology, P. R. China in 1995, and his PhD degree in nuclear engineering from Massachusetts Institute of Technology in 2001.  He was a Lawrence Postdoctoral Fellow at the Lawrence Livermore National Laboratory from 2001 to 2004.  He is currently an Associate Professor in the Department of Mechanical Engineering at Stanford University.  He received the Presidential Early Career Award for Scientists and Engineers in 2004, and the American Society of Mechanical Engineers Hughes Young Investigator Award in 2013. His research interests include dislocation dynamics and metal plasticity, atomistic simulations of deformation, synthesis and transport mechanisms at the nanoscale.  He is co-author of 90 publications in these and related fields, and a co-author of a book “ Computer Simulations of Dislocations”.  He also co-authored a senior-undergraduate/junior-graduate textbook, “ Imperfections in Crystalline Solids”, to be published in Aug 2016.