Micromechanics of Ferroelectric Crystals
Professor George Weng received his B.S. from National Taiwan University in 1967 and Ph.D. from Yale University in 1974. He worked at Delft University of Technology, UCLA, and GM before joining Rutgers in 1977 as an Assistant Professor. He was promoted to an Associate Professor in 1980, Full Professor in 1984, and Distinguished Professor in 1992. His research interests are in theoretical and applied mechanics, with a strong link to materials science and applied physics. He has worked on the micromechanics of composites, phase transition in shape-memory alloys, coupled behavior in ferroelectric crystals, grain-size effects in nanocrystalline materials, and transport properties in CNT nanocomposites. Prof. Weng has been an Editor of Acta Mechanica since 1985, and was the Technical Editor of ASME Journal of Engineering Materials and Technology from 1992-1997. He also served as Chairman of ASME Materials Division during 1993-1994. He was elected Fellow of ASME in 1990 and Fellow of the American Academy of Mechanics in 1998. In 2013, Prof. Weng was awarded the William Prager Medal by the Society of Engineering Science, in recognition of his "outstanding research contributions in theoretical solid mechanics". A 3-day, 55-paper Prager Medal Symposium was held at the joint SES-ASME Summer Conference at Brown last year. A special issue of Acta Mechanica (Vol. 225, Nos. 4-5, 2014) was recently published to include many of the presented papers.
本讲教师：George J. Weng
时间： 8月6日上午9:00-10:00; 地点： 力学一楼二楼会议室 (239) Ferroelectric crystals possess several distinct characteristics that make it a unique class of multi-functional materials. In addition to its strong electromechanical coupling, there is the existence of polarization domains which can be reoriented under an external electric field and/or mechanical stress. Ferroelectric crystal also exhibit distinct crystal structures at different temperatures. Domain switch and phase transition are the two fundamental processes that control the ferroelectric characteristics. In this talk we will present a micromechanics approach that can have wide applicability for the study of domain switch and phase transition in bulk ferroelectrics. Our starting point is the Eshelby mechanics formulated in the electromechanical context, with the features of spontaneous strains and spontaneous polarizations. We first use it to derive the thermodynamics driving force for domain switch, and prove that, for a rank-1 and rank-2 laminated domain pattern, this driving force turns into the jump of Eshelby' s energy momentum tensor across the domain wall. We also use the derived theory to explain how a fixed axial compression can enhance the actuation strain of a  BaTiO3 crystal. We then extend the theory to phase transition, and use it to explain how the double and triple hysteresis loops develop during the cubic to tetragonal, and during the tetragonal to orthorhombic transition, respectively. We will also explain why the variation of dielectric constant near the transition temperature exhibits a mansion-like profile when it occurs between two ferroelectric states. 主办单位：材料力学行为与设计中科院重点实验室