主讲人：吕玮（美国佐治亚理⼯⼤学）

题目：Thermal Transport in Polymers and Amorphous Solids

时间：2015-12-25（星期五）14:30~16:30

地点：皇冠新体育app学术报告厅（新9教三楼）

举办单位：皇冠新体育app

摘要：

I) Understanding Phonon Transport in Amorphous Solids

The phonon gas model (PGM) originated from the behaviors observed and rationalized in homogenous crystalline solids and it has exhibited remarkable success in describing the behavior of a wide variety of solids, microstructures, nanostructures and molecules. Given its success, it has become the primary lens with which phonon transport is viewed. However for amorphous materials, or other structurally/compositionally disordered systems, due to the lack of periodicity, one cannot clearly define the group velocity. Since the PGM hinges on knowledge of the group velocities, application of the PGM to amorphous materials is highly questionable.

I developed a new method for direct calculation of the modal contributions to thermal conductivity, which is termed Green-Kubo modal analysis (GKMA). The GKMA method combines the lattice dynamics formalism with the Green-Kubo formula for thermal conductivity, such that the thermal conductivity becomes a direct summation of modal contributions, where one need not define the phonon velocity. The predicted temperature dependent thermal conductivity for amorphous silicon (a-Si)5 amorphous silica (a-SiO2), and amorphous carbon (a-C) shows the best agreement with experiments to date.

II) Understanding Divergent Thermal Conductivity in Single Polythiophene Chains

In 1955 Fermi, Pasta, and Ulam (FPU)made a“shocking little discovery”that even an anharmonic system can exhibit infinite thermal conductivity. The class of materials that most closely resemble the 1D chains that have been studied previouslyare polymers, specifically individual polymer molecules. Towards improving understanding of this phenomenon, I sought out to answer several key questions that lingered from previous analysis.

I used molecular dynamics simulations, the Green-Kubo Modal Analysis (GKMA) method as well as sonification to study the modal contributions to thermal conductivity in individual polythiophene chains. The simulations suggest that it is possible to achieve divergent/infinite thermal conductivity in individual polythiophene chains and the GKMA method allowed for exact pinpointing of the modes responsible for the anomalous behavior.

III) Design and Manufacture High Thermal Conductivity Polymer Composites

Recent experimental work has shown that mechanical stretching can increase polymer chain alignment in the stretching direction and the thermal conductivity can increase by more than two orders of magnitude. The studies where such high thermal conductivities were obtained, however, employed fabrication methods that are far from scalable. Part of my Ph.D. research is to develop of a scalable manufacturing process for machinable, high thermal conductivity, polymer composite bulk materials, which can be used as heat spreading chassis materials. I will show that we are able to reach more than 100 W/mK axial/in-plane thermal conductivity using a combination of mechanical stretched polymer fibers and high thermal conductivity graphite fillers.

简历：吕玮，1988年⽣，2010年毕业于华中科技⼤学建筑环境与设备⼯程系，获学⼠学位；2012年美国亚利桑那州立⼤学机械与航天航空⼯程系硕⼠研究⽣毕业，主攻纳米结构的光谱辐射特性及其在太阳能能量收集中的应用。研究⽣期间，在Patrick Phelan教授实验室发表论⽂两篇，并在美国霍尼韦尔公司作为系统⼯程师实习。现就读于美国佐治亚理⼯⼤学机械⼯程系，在Asegun Henry教授的分⼦模拟及能源实验室⼯作。预计2016年8月博⼠研究⽣毕业。主要从事凝聚态物理，⾼分⼦材料和计算物理的研究。运用多尺度计算物理和实验研究纳米材料的热学，光学和电学性质。目前主要研究项目是通过分⼦模拟，晶格动⼒学和第⼀性原理预测⾼分⼦材料和非晶固体的声⼦传热性质。对于⼯程⾼分⼦材料的设计和研发有着重要作用。未来可应用于电⼦封装，新型热交换器和航天航空材料。现于国际⼀流期刊（如Nature Nanotechnology, Physical Review Letters，Scientific Reports）、会议发表论⽂11篇（⼀作8篇），另有5篇论⽂筹备中。论⽂被SCI引用次数220次。博⼠就读期间，曾在美国希捷公司作为⾼级研发⼯程师实习，研究开发新型热辅助磁存储（HAMR）技术。