EAGER: Modeling and Characterization of Mesoscale Nondiffusive Heat Transfer

#1637370 Ma，延保随着微/纳/光电子器件和结构尺寸的不断减小，热传输的操纵和控制正成为许多纳米技术发展的瓶颈。宏观热传导中的导热系数是一种材料性质，与样品大小和加热方式无关，而微/纳米系统中测量的导热系数可能取决于样品大小和非稳态加热的频率。尺寸相关或频率相关的导热系数表明，描述微/纳米系统中非扩散热传递的傅立叶定律失效。本项目将开发的统一的非扩散-扩散模型将为微/纳米系统的热管理提供强大的理论和数值设计工具，这对打破纳米技术的发展瓶颈至关重要。综合研究和教育计划将鼓励更多来自传统代表性不足社区的妇女和学生进入STEM职业生涯，并参与加州大学默塞德分校拟议的研究活动。声子是绝缘体和半导体中的主要热载体。傅立叶-S定律的崩溃是由于当特征长度尺度变得与声子的平均自由程相当或甚至远小于声子的平均自由程时，存在显著的弹道热传导。目前，在理解非扩散换热方面存在两大技术障碍：(A)缺乏可用于实验数据分析的实用中尺度非扩散换热模型；(B)缺乏专门的参数来表征非扩散换热的独特特征。因此，目前对非扩散换热的研究大多集中在傅立叶定律框架内对有效导热系数(ETC)的预测和测量，而对非扩散换热的独特特性缺乏深入的认识。虽然分子尺度模型可以描述几个纳米尺度的热传导，傅里叶定律描述了几十微米或更大的宏观尺度的热传递，但分子尺度和宏观尺度的热传导模型之间存在差距。声子玻尔兹曼输运方程(BTE)本应填补这一空白，但由于有如此多的未知参数，求解成本高得令人望而却步。因此，为了弥补这一差距，从几十纳米到几十微米的长度尺度上描述声子的热传递，实用且计算成本低廉的中尺度非扩散热传输模型是必不可少的。该项目的研究目标是开发和验证一个中尺度非扩散换热(包括弹道和弹道扩散)模型，以阐明不能用导热系数来表征的独特特征。该项目的成功不仅将通过建立高保真的多尺度热传递的非扩散-扩散统一模型来弥合宏观尺度和分子尺度模型之间的热传导理论差距，而且还将为在傅立叶定律框架外研究非扩散热传递的独特特征开辟新的场所。

#1637370Ma, YanbaoWith continuous decrease in the size of micro-/nano-/optoelectronic devices and structures, the manipulation and control of heat transport is becoming a bottleneck for the development of many nanotechnologies. While thermal conductivity in macroscale heat transfer is a material property and independent of the sample size and heating method, the measured thermal conductivity in micro-/nanosystem may depend on the sample size and the frequency in unsteady heating. The size-dependent or frequency-dependent thermal conductivity indicates the breakdown of Fourier's law to describe nondiffusive heat transfer in micro-/nanosystems. The unified nondiffusive-diffusive model to be developed in this project will provide powerful theoretical and numerical design tools for thermal management in micro/nanosystems that is crucial for breaking the developmental bottleneck of nanotechnologies. The integrated research and education plan will encourage more women and students from traditionally underrepresented communities to enter STEM careers and participate in the proposed research activities at UC Merced. Phonons are the dominant heat carriers in insulators and semiconductors. The breakdown of Fourier?s law is due to the fact that there is significant ballistic heat conduction when the characteristic length scale becomes comparable to or even much smaller than the mean-free-path (MFP) of phonons. Currently, there are two major technical barriers in understanding nondiffusive heat transfer: (a) a lack of practical mesoscale nondiffusive heat transfer models that can be applied to experimental data analysis; and (b) a lack of ad hoc parameters to characterize the unique features of nondiffusive heat transfer. Consequently, most current studies of nondiffusive heat transfer focus on the predictions and measurements of effective thermal conductivity (ETC) within the framework of Fourier's law, but these provide little insight on the unique features of nondiffusive heat transfer. Although molecular scale models can describe heat conduction over a few nanometers, and Fourier's law delineates macroscale heat transfer over tens of microns or larger, a gap exists in heat conduction models between the molecular scale and the macroscale. The phonon Boltzmann transport equation (BTE) was supposed to fill this gap, but it is prohibitively expensive to solve with so many unknown parameters. Therefore, practical and computationally inexpensive mesoscale nondiffusive heat transfer models are indispensable to bridge this gap to describe heat transfer by phonons over length scales ranging from tens of nanometers to tens of microns. The research objective for this project is to develop and validate a mesoscale nondifussive heat transfer (including ballistic and ballistic-diffusive) model to elucidate unique features that cannot be characterized by thermal conductivity. The success of this project will not only bridge the gap in heat conduction theories between macroscale and molecular scale models by developing high-fidelity unified nondiffusive-diffusive models for multiscale heat transfer, but also open up new venues to study unique features of nondiffusive heat transfer outside the framework of Fourier's law.