CAREER: Mean Free Path Spectroscopy - Experimental determination of the mean free path distribution in solids

1055317 dame固体材料的导热工程对于热管理和绝缘，发电和信息技术等许多应用都是必不可少的。许多现代系统，包括激光器、热电能量转换器和晶体管，都是基于包含比一微米小得多的结构的材料。与手册中报告的熟悉的体积值相比，这些结构可以导致导热系数的非常大的降低，有时会降低十倍或更多。为了预测和设计块状和微结构材料的导热性，必须了解能量载流子的平均自由程，即它们碰撞之间的平均距离。尽管各种热导率模型已经被广泛应用了半个多世纪，但当这些模型应用于微观结构时，它们往往表现出很大的分歧。模型分歧的核心是它们的平均自由程计算不同，但迄今为止还没有任何材料的平均自由程完整分布的直接实验测量报告。因此，本CAREER提案的主要目标是通过实验测量几种标准材料中平均自由程的完整分布。为了实现这一目标，本项目将采用两种互补的实验策略，旨在涵盖非常大的长度和时间尺度。第一种方法是系统地测量尺寸在50纳米到100微米之间的微结构的稳态传导。然后利用新的Fredholm积分方程将数据转换成平均自由程谱。第二种方法包括测量从0.1 ns到10微秒的超快热瞬变，并使用玻尔兹曼输运方程进行分析。这个CAREER提案的智力价值在于推进了对导热性的基本理解。通过实验量化实际平均自由路径分布，这些测量将有助于确定目前使用的标准模型中哪些实际上是正确的。该项目还将测试Y. K. Koh和D. Cahill最近提出的一个假设，该假设将他们观察到的频率相关的热导率与平均自由程分布联系起来。该项目还建议开发一种全电泵探测装置，它应该比重要的光泵探测实验技术更容易获得，成本更低。这个项目更广泛的影响包括工程相关性和教育/推广部分。所获得的基本知识将与广泛的材料系统相关，包括合金，晶体材料和非晶材料，以及包括激光，晶体管和热电能量转换在内的众多应用的热工。这些拓展活动围绕着在地区高中举办的讲习班展开，这些讲习班以热电能量转换为基础，并在人类尺度上可视化纳米世界。在每次工作坊结束时，工作坊的材料将被捐赠给教室供他们将来使用。工作坊的内容已经通过两次迭代得到加强，目标高中科学教师将被招募和支持一名教师，以优化工作坊内容并在网站上传播。

1055317DamesEngineering the conduction of heat in solid materials is essential for numerous applications ranging from thermal management and insulation, to power generation, to information technology. Many modern systems including lasers, thermoelectric energy converters, and transistors are based on materials containing structures much smaller than one micron. These structures can cause very large reductions in the thermal conductivity, sometimes by a factor of ten or more, as compared to the familiar bulk values reported in handbooks. To predict and engineer the thermal conductivity in both bulk and microstructured materials, it is essential to understand the mean free paths of the energy carriers, that is, the average distance between their collisions. Although various models of the thermal conductivity have been used widely for more than half a century, when these models are applied to microstructures they often exhibit large disagreements with each other. The heart of the model disagreements is their different mean free path calculations, yet direct experimental measurements of the full distribution of mean free paths have not been reported to date for any material. Therefore, the primary goal of this CAREER proposal is to experimentally measure the full distribution of mean free paths in several standard materials. To accomplish this objective, this project will pursue two complementary experimental strategies, designed to cover a very large range of length and time scales. The first approach involves systematic measurements of steady-state conduction in microstructures with sizes ranging from around 50 nm to around 100 microns. The data will then be transformed into mean free path spectra using a new Fredholm integral equation. The second approach involves measuring ultrafast thermal transients ranging from around 0.1 ns to around 10 microseconds, and analyzed using the Boltzmann transport equation. The intellectual merit of this CAREER proposal is in advancing the fundamental understanding of thermal conductivity. By experimentally quantifying the real mean free path distributions, these measurements will help identify which of the standard models in use today can actually be correct. This project will also test a postulate recently introduced by Y. K. Koh and D. Cahill to relate their observed frequency-dependent thermal conductivity to a mean free path distribution. The project also proposes to develop an all-electrical pump-probe apparatus that should be more accessible and less costly than the important optical pump probe experimental technique. The broader impacts of this project include both engineering relevance and an education/outreach component. The fundamental knowledge gained will be relevant for a wide range of materials systems including alloys, crystalline materials, and amorphous materials, and for thermal engineering of numerous applications including lasers, transistors, and thermoelectric energy conversion. The outreach activities are built around workshops at regional high schools based on thermoelectric energy conversion and visualizing the nanoworld at the human scale. At the end of each workshop the workshop materials will be donated to the classroom for their future use. The workshop content has already been strengthened by two iterations with the intended high school science teachers, and one teacher will be recruited and supported for a summer to optimize the workshop content and disseminate it on a website.