Controlling and Understanding Thermal Energy Exchange at Single Domains of Functional Materials

非技术描述：在比电更长的历史上，热对人类来说是熟悉和重要的，但人们控制和理解热的能力远远落后于电。关键在于材料科学和工程：如何设计、合成和开发支持、引导和测量热流的材料和工具(就像半导体利用电流工作一样)。这个项目试图使用一类特定的材料来解决这个根本问题：纳米级功能材料。由于热是由电子和原子振动(所谓的“声子”)携带的，要实现这一目标，需要对电子和声子的行为及其相互作用进行精细的控制。纳米级和高质量的功能材料允许这种控制，因为它们可以用来调节热流的方向和大小(以及从电流和电场的转换/转换为电流和磁场)。通过吴教授所在的大学和当地中学的合作，这个项目所获得的知识被用来教育代表人数不足的学生。技术细节：功能材料支持特殊的热能交换过程，如热电、热整流和电热效应。这些材料中普遍存在各种结构域(包括功能结构域和晶界)和结构域壁，它们对热能交换过程有很强的影响，有时甚至起主导作用。由于在纳米尺度上测量热流的挑战，以前对这些效应的研究大多局限于具有大量无序磁区的材料。结果，内在效应被域上的系综平均所隐藏，留下了许多关键问题悬而未决。这个项目超越了总体平均，使用吴教授实验室最近开发的微加工工具和纳米材料加工技术，测量、理解、控制和优化功能材料单域或单域壁上的热能交换。该项目通过电子的工程密度和迁移率以及晶格的结晶度和界面，实现了对功能材料热流的精细控制；确定了界面热整流、复合热电材料和电热冷却等热能交换过程中的基本限制；并为各种热设备的新颖设计和性能改进打开了巨大的机遇。与研究活动相结合，吴教授还在运营一个教育项目，旨在刺激和准备大学预科学生在与热应用有关的材料科学和工程领域的职业生涯。

NON-TECHNICAL DESCRIPTION: Heat has been familiar and important to humankind for a much longer history than electricity, yet people's ability of controlling and understanding heat lags much behind that of electricity. The key lies in materials science and engineering: how to design, synthesize and develop materials and tools that support, direct and gauge heat flow (in the same manner in which semiconductors work with electric current). This project seeks to tackle this fundamental problem using a specific class of materials: nanoscale functional materials. As heat is carried by both electrons and atomic vibration (the so-called "phonons"), achieving this goal requires exquisite control of behavior of both electrons and phonons, as well as their interactions. Functional materials made in the nanoscale and with high quality allow such control because they can be used to regulate the direction and magnitude of heat flow (as well as its conversion from/to electric current and fields). Knowledge gained in this project is used to educate underrepresented students through a partnership between Prof. Wu's university and local middle-high schools. TECHNICAL DETAILS: Functional materials support extraordinary thermal energy exchange processes such as thermoelectrics, thermal rectification, and electrocaloric effect. Domains (including functional domains and crystal grains) and domain walls of various types ubiquitously exist in these materials and strongly influence, sometimes dominate, the thermal energy exchange processes. Hampered by challenges in measuring heat flow at the nanoscale, previous studies of these effects are mostly limited to materials with a large number of disordered domains. As a result, intrinsic effects are hidden by ensemble averaging over the domains, leaving many key questions unsettled. This project goes beyond this ensemble averaging, to gauge, understand, control and optimize the thermal energy exchange at the level of single domain or single domain wall of functional materials, using microfabricated tools and nanomaterials processing techniques recently developed in Prof. Wu's lab. The project enables exquisite control of heat flow with functional materials by engineering density and mobility of electrons as well as crystallinity and interface of the lattice; identifies fundamental limits in such thermal energy exchange processes as interfacial thermal rectification, composite thermoelectrics, and electrocaloric cooling; and opens tremendous opportunities for novel design and improved performance of various thermal devices. Integrated with the research activities, Prof. Wu is also running an education project that stimulates and prepares pre-college students for careers in materials science and engineering pertaining to thermal applications.