纳米结构中的声子弹道扩散导热研究

The key problems in terms of high density heat dissipation in micro/nano-electro-mechanical systems and thermophysical property tuning in micro/nanoenergy devices desire in-depth understanding about thermal transport in dielectric nanostructures, which is a frontier and hot topic in micro/nanoscale heat transfer. At nanoscale where the mean free path of phonons is comparable to the characteristic length of systems, heat transports in the ballistic-diffusive way dominated by the ballistic transport, and the classical Fourier’s heat conduction law breaks down. The present project proposes to study the phonon ballistic-diffusive transport in typical nanostructures, such as nanofilms, nanowires, nanoporous media of Si/Ge/SiC. The schemes combining phonon Boltzmann transport equation, phonon Monte Carlo, molecular dynamics simulations and experimental measurements will be applied. The proposers will be focused on the following three key scientific points: the physical mechanisms of the invalidation of the Fourier’s heat conduction law, the size effects of the effective thermal conductivity of typical nanostructures, the temperature jump and heat flux profile near boundaries of nanostructures. The analytical models of the boundary temperature jump, heat flux profile and the effective thermal conductivity will be derived based on the phonon Boltzmann transport equation, and are validated via numerical simulations and experimental data. The studies try to present theoretical guidance for the thermal management in micro/nanoscale devices and for tuning the thermophysical properties of nanomaterials.

微纳电子机械系统的高热流密度散热和微纳能源系统的热物性调控等高新技术领域的问题都迫切需要研究半导体纳米结构中的热量输运规律，这也是目前微纳尺度传热领域的国际研究前沿和热点。在纳米尺度下，特征尺寸与声子平均自由程相当，热量以弹道输运主导的弹道扩散的方式传递，经典的傅里叶导热定律不再适用。本项目计划以硅/锗/碳化硅等典型半导体纳米结构（纳米薄膜、纳米线、纳米多孔材料等）为研究对象，采用声子玻尔兹曼方程理论分析、声子蒙特卡罗和分子动力学模拟、热导率测试实验相结合的方法，系统深入地研究纳米结构导热中傅里叶导热定律失效的物理机理、典型纳米结构等效热导率的尺寸效应及纳米结构的界面温度跳跃及热流密度分布规律三个关键科学问题，基于声子玻尔兹曼输运方程建立相应的边界温度跳跃、热流密度分布及等效热导率的模型，通过数值模拟和实验进行验证，为微纳电子器件热管理以及纳米材料热物性调控提供指导。

本项目围绕典型半导体纳米结构中的声子弹道扩散导热机理展开研究，主要取得了以下研究进展：（1）发展了基于声子追踪的声子蒙特卡洛模拟方法，实现了数值求解声子玻尔兹曼输运方程，用于模拟研究稳态/瞬态、U过程/N过程主导、纳米到微米尺度的声子热输运过程；（2）基于声子玻尔兹曼输运理论系统研究了纳米结构导热中傅里叶导热定律失效的物理机理，揭示了纳米结构声子弹道输运、边界散射以及边界局域模式的耦合机理；（3）揭示出纳米结构中弹道扩散导热对热输运的影响体现为温度跳跃和热流密度减小的边界条件，基于此建立了典型纳米结构在不同加热条件下的统一等效热导率模型；（4）通过数值、理论和实验研究获得了典型半导体纳米结构的等效热导率数据资料，用于实际电子器件近节点热管理和热功能纳米材料的热分析和热优化设计。本项目执行期间发表SCI论文47篇（其中国际期刊46篇），包括Nanoscale、Applied Physics Letters、International Journal of Heat and Mass Transfer、International Journal of Thermal Sciences、Nanoscale and Microscale Thermophysical Engineering、Physical Review E、Journal of Applied Physics、The Journal of Chemical Physics等权威期刊，发表EI检索的论文4篇，受邀在国际会议上作主旨报告9次，申报国家发明专利5项。项目负责人曹炳阳获2018年国家自然科学杰出青年基金资助（No. 51825601），2020年入选国际先进材料学会Fellow。