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

职业：平均自由程光谱 - 固体中平均自由程分布的实验测定

• 批准号: 1358370
• 负责人: Chris Dames
• 金额: $ 28.72万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-01 至 2016-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1358370&HistoricalAwards=false
• 关键词: CAREER; Mean; Free; Path; Spectroscopy

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.

从热管理和绝缘到发电，再到信息技术，固体材料中的热传导工程对于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，都是基于含有远小于一微米的结构的材料。与手册中报告的常见体积值相比，这些结构会导致导热系数大大降低，有时会降低十倍或更多。为了预测和设计块体和微结构材料的导热系数，必须了解载流子的平均自由程，即它们之间的平均碰撞距离。尽管各种导热系数模型已经广泛应用了半个多世纪，但当这些模型应用于微结构时，它们往往表现出很大的不一致。模型分歧的核心是他们不同的平均自由程计算，然而到目前为止，还没有关于任何材料的关于平均自由程完全分布的直接实验测量的报道。因此，这项职业计划的主要目标是通过实验测量平均自由程在几种标准材料中的完全分布。为了实现这一目标，该项目将采取两种相辅相成的实验战略，旨在涵盖非常大的长度和时间范围。第一种方法包括系统地测量尺寸从大约50纳米到大约100微米的微结构中的稳态导电性。然后使用新的Fredholm型积分方程式将数据转换成平均自由程光谱。第二种方法包括测量从大约0.1 ns到大约10微秒的超快热瞬变，并使用玻尔兹曼输运方程进行分析。这项职业计划的学术价值在于促进了对导热系数的基本理解。通过实验量化真实的平均自由程分布，这些测量将有助于识别当今使用的哪些标准模型实际上是正确的。这个项目还将测试Y.K.Koh和D.CaHill最近提出的一个假设，该假设将他们观测到的与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动泵浦探测装置，该装置应该比重要的光泵浦探测实验技术更容易获得，成本更低。该项目的更广泛影响包括工程相关性和教育/宣传部分。所获得的基础知识将与广泛的材料体系相关，包括合金、晶体材料和非晶态材料，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程。外展活动围绕区域高中的讲习班展开，以热电能量转换为基础，并在人的尺度上可视化纳米世界。在每个工作坊结束时，工作坊的材料将捐赠给教室，供将来使用。讲习班的内容已经通过与目标高中科学教师的两次迭代而得到加强，将招聘一名教师并为其提供一个暑期支助，以优化讲习班的内容并在网站上传播。