Collaborative Research: Net-Shape and Scalable Additive Manufacturing for Thermoelectric Waste Heat Recovery Materials and Devices using Selective Laser Melting

在美国，超过55%的能源消耗是作为废热释放的。仅就制造业而言，每年未回收的废热总量估计为2500万亿BTU。美国汽车产生的废热相当于每年损失500多亿美元。在各种余热回收技术中，固态热电发电机（teg）是一种很有前途的策略，可以提高能源效率，减轻空气污染，减少碳排放。传统的TEG制造包括材料合成、模块组装和器件集成，生产率低，成本高。只有解决TEG制造中的以下关键挑战，才能实现TEG在现有能源系统中的广泛部署：大量、低成本、可靠和高ZT（性能值）热电材料的经济高效合成；TEG器件的规模化制造；在温度梯度环境下实现功能梯度。该项目采用基于增材制造（AM）的净形纳米制造工艺，利用材料科学、传热和制造领域的最新进展来应对这些挑战。为了实现这一雄心勃勃的目标，弗吉尼亚理工大学、卡内基梅隆大学和华盛顿大学与增材制造行业的领导者合作，组建了一个跨学科的能量收集、材料科学家、传热和制造团队。来自不同背景的学生将接受21世纪劳动力的培训。此外，还将加大对K12学生的外展力度。该项目的目标是利用基于选择性激光熔化（SLM）的增材制造方法，为高性能热电材料和功能器件开发一种新的集成纳米制造工艺。此外，将建立激光加工变量与热电材料特性之间的相关性，以提供对激光-材料相互作用的基本理解，从而实现热电器件的净形状和可扩展的AM方法。具体而言，本文将验证以下假设：(1)激光增材制造过程中产生的非平衡条件会引入大量的纳米缺陷、纳米级颗粒和丰富的多尺度晶界，从而通过声子散射显著降低热导率。(2)在纳米制造过程中使用掺杂Si或其他纳米颗粒作为添加剂材料，以改善机械性能，增强电导率，增加塞贝克系数。(3)基于激光的AM可以很容易地实现热电元件沿长度的梯度掺杂和具有温度变化的可变截面面积，从而充分利用温度相关的材料特性来实现高性能热电器件。(4)利用激光增材制造技术，直接制造热电材料、保温层、电导体层和热交换器作为一个功能完整的能量收集系统，与传统制造方法相比，可以获得更高的机械稳定性和热可靠性。这项技术的特点是低成本、高效率和工业可扩展的清洁能源系统纳米制造，如果成功测试和验证，将对许多与能源和制造系统相关的行业，如汽车、发电站、钢铁厂等，具有极大的吸引力。了解热电材料的电子和声子输运的基本原理，开发下一代制造工具，设计新的传热系统，将提高能源系统的效率，减少美国对外国能源的依赖。产业伙伴关系加速了基础科学研究与产业实践的融合。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Over 55 percent of the energy consumed in the US is released as waste heat. For the manufacturing sector alone, the total unrecovered waste heat is estimated to be 2,500 trillion BTU per year. The waste heat from American automobiles is equivalent to losing over $50 billion each year. Among various waste heat recovery technologies, solid-state thermoelectric generators (TEGs) are a promising strategy to increase energy efficiency, alleviate air pollution, and reduce carbon emissions. Traditional TEG manufacturing includes material synthesis, module assembly, and device integration, which has low productivity and high cost. A widespread deployment of TEGs in existing energy systems can be achieved only by resolving following key challenges in TEG manufacturing: cost-effective synthesis of abundant, low cost, reliable, and high ZT (figure of merit) thermoelectric materials; scalable manufacturing of TEG devices; function graded realization in the temperature gradient environment. This project has an additive manufacturing (AM) based net-shape nanomanufacturing process, that takes the advantages of the latest advances in materials science, heat transfer, and manufacturing, to tackle these challenges. To accomplish this ambitious goal, an interdisciplinary team of energy harvesting, material scientist, heat transfer and manufacturing is assembled at Virginia Tech, Carnegie Mellon, and UW, in collaboration with an industry leader of AM. Students from diverse background will be trained for the twenty-first century workforce. Great efforts will also be made for outreaches to K12 students. The objective of this project is to develop a novel integrated nanomanufacturing process for high-performance thermoelectric materials and functional devices using the selective laser melting (SLM) based AM method. Furthermore, a correlation between the laser processing variables and thermoelectric material characteristics will be established to provide fundamental understanding of laser-material interactions to achieve a net-shape and scalable AM method for thermoelectric devices. Specifically, the following hypotheses will be tested: (1) The non-equilibrium conditions produced during the laser-based AM process can introduce numerous nano-defects, nanoscale particles, and abundant multi-scale grain boundaries, which can reduce the thermal conductivity dramatically by phonon scatterings. (2) doped Si or other nano-particles will be used as additive materials in the nanomanufacturing process to improve the mechanical properties, enhance the electrical conductivity, and increase the Seebeck coefficient. (3) The laser-based AM can readily realize the graded doping and variable cross-section areas along the length of the thermoelectric elements with temperate variance to make the best use of the temperature-dependent material properties for achieving high performance thermoelectric devices. (4) Using the laser-based AM, the direct manufacturing of thermoelectric materials, thermal insulation layers, electrical conductor layers, and heat exchangers as a functional and integrated energy harvesting system, can result in higher mechanical stability and thermal reliability as compared to the traditional manufacturing approaches. Characterized as low-cost, high-efficiency, and industry-scalable nanomanufacturing of clean energy systems, this technology, if successfully tested and validated, will become extremely attractive for many industries associated with energy and manufacturing systems, such as automobiles, power stations, steel plants and many more. Understanding the fundamentals of electron and phonon transport for thermoelectric materials, developing next generation manufacturing tools, and designing novel heat transfer systems will result in increased efficiency of the energy system and reduced of US dependency on foreign energy sources. The industrial partnership accelerates the assimilation of basic science research into industrial practice.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.