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

1055317 DamesEngineering固体材料中的热传导对于从热管理和绝缘到发电到信息技术的众多应用至关重要。 许多现代系统，包括激光器、热电能量转换器和晶体管，都是基于包含远小于1微米的结构的材料。 与手册中报道的熟悉的体积值相比，这些结构可以导致热导率非常大的降低，有时降低十倍或更多。 为了预测和设计块体和微结构材料的热导率，必须了解能量载体的平均自由程，即它们碰撞之间的平均距离。 尽管半个多世纪来各种导热系数模型得到了广泛的应用，但当这些模型应用于微结构时，它们往往表现出很大的差异。 模型分歧的核心是它们不同的平均自由程计算，但迄今为止还没有任何材料的平均自由程的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，本项目将采取两种互补的实验策略，旨在涵盖非常广泛的长度和时间尺度。 第一种方法涉及尺寸范围从约50 nm到约100微米的微结构中的稳态传导的系统测量。 然后将数据转化为平均自由程光谱使用一个新的Fredholm积分方程。 第二种方法涉及测量范围从约0.1 ns到约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建议的智力价值在于推进对热导率的基本理解。 通过实验量化的真实的平均自由程分布，这些测量将有助于确定哪些标准模型在今天使用的实际上可以是正确的。 这个项目也将测试最近由Y. K. Koh和D. Cahill将他们观察到的频率依赖性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浦探测装置，该装置应比重要的光泵浦探测实验技术更容易获得且成本更低。 该项目的更广泛影响包括工程相关性和教育/外联部分。 所获得的基础知识将与广泛的材料系统（包括合金，晶体材料和非晶材料）以及众多应用（包括激光器，晶体管和热电能量转换）的热工程相关。 外联活动围绕在区域高中举办的讲习班展开，这些讲习班以热电能转换为基础，并以人类尺度展示世界。 在每个研讨会结束时，研讨会的材料将捐赠给教室，供未来使用。 讲习班的内容已经通过两次迭代得到了加强，培训对象是中学科学教师，将招聘一名教师，并为他们提供一个夏天的支持，以优化讲习班的内容，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.