SGER - Pulsed Electron-Beam Deposition of PbTe/CdTe Nanocomposites and Thermal Property Study

CBET-0829977Mu型纳米结构中的热输运呈现出表面和界面边界散射以及声子谱限制效应等新的令人兴奋的现象，这为设计制备具有理想性能的材料提供了新的自由度。研究表明，降低超晶格、纳米线和纳米复合材料的热导率可以显著提高热电优值，这可能对制冷和余热回收产生变革性的影响。目前，大多数用于热电转换的纳米结构材料要么是电子束外延生长的单晶超晶格薄膜，这是非常昂贵的；要么是单个纳米线，这给集成到功能复合材料带来了巨大的挑战。因此，在声子输运方面具有纳米限制效应的纳米结构材料备受关注。这一探索性研究项目为制备热电转换用纳米复合材料提供了一种新的途径。更具体地说，我们将使用脉冲电子束沉积(PED)这一新的、通用的、低成本的、用户友好的薄膜/纳米颗粒沉积技术来制备PbTe/CdTe纳米复合材料，并利用3？技术研究所制备材料的热输运。我们选择沉积PbTe/CdTe纳米复合材料，是因为块体PbTe在200℃到500℃的温度范围内表现出了最好的热电性能。此外，我们在实验室的PED系统中制备这些材料方面取得了初步进展。制备的PbTe/CdTe纳米结构薄膜将以较厚的PbTe层作为基质，以纳米级的CdTe层或纳米颗粒作为嵌入材料的纳米结构。我们预计，除了合金的散射外，基质材料还将有效地散射高能、短波长的声子，而纳米结构的CdTe将为长波长的声子提供额外的散射机制。因此，我们可以利用类似于单晶超晶格薄膜或量子复合材料的机制来有效地降低所制备材料的热导率。这项研究计划的最终目标是创造一种成本效益高的方法来大规模制备纳米结构高性能热电材料。提出的研究方案的智力价值在于利用新型PED系统制备了PbTe/CdTe纳米复合材料，并了解了所制备的纳米复合材料中的热输运。如果成功，这项研究将为低成本、大批量生产高性能热电材料铺平道路。此外，在这项研究中，我们将获得高能电子束与靶之间的相互作用以及这些相互作用将如何影响材料制备过程和所产生的材料的知识。就更广泛的影响而言，PED系统是一种相对较新的材料沉积技术，沉积过程还没有被很好地了解，因此所获得的关于沉积过程的知识将对使用PED技术的其他材料制造产生广泛的影响。SGER的这一支持将帮助HBCU下属的菲斯克大学建立一个长期的热科学研究和教育计划。这些活动将促进两所大学之间的跨学科互动，并作为加强代表不足的少数族裔学生教育的重要一步。

CBET-0829977MuThermal transport in nanostructures presents novel and exciting phenomena, such as surface and interfacial boundary scattering and phonon spectrum confinement effects, which offer new degrees of freedom to fabricate materials of desirable properties by design. It has been shown that reduced thermal conductivity in superlattices, nanowires, and nanocomposites can lead to a significant enhancement of thermoelectric figure of merit, which could produce transformative impacts to refrigeration and waste heat recovery. Currently, most nanostructured materials under investigation for thermoelectric energy conversion are either single crystalline superlattice thin films fabricated by e-beam epitaxial growth, which is very expensive, or individual nanowires, which pose great challenges in integration into functional composites. Therefore, nanostructured materials that can be fabricated cost-effectively, yet possess nanoconfinement effects on phonon transport are of great interest. This exploratory research project is a novel approach to fabricate nanostructured composite materials for thermoelectric energy conversion. More specifically, we will employ Pulsed Electron-beam Deposition (PED), a new, versatile, cost-effective and user-friendly thin film/nanoparticle deposition technique, to fabricate PbTe/CdTe nanocomposite materials and investigate the thermal transport through the fabricated material with the 3ù technique. We choose to deposit PbTe/CdTe nanocomposite materials because bulk PbTe has shown the best thermoelectric performance in the temperature range of 200 °C and 500 °C. In addition, we have made initial progress in fabricating these materials with the PED system in our lab. The fabricated PbTe/CdTe nanostructured thin films will have relatively thick PbTe layers serving as the matrix and nanometer scale CdTe layers or nanoparticles as the embedded nanostructure in the materials. We expect that in addition to the alloy scattering, the matrix materials will effectively scatter the high energy, short wavelength phonons, and the nanostructured CdTe will provide additional scattering mechanisms to the long wavelength phonons. Therefore, we can effectively reduce the thermal conductivity of the fabricated materials using similar mechanisms to those for single crystalline superlattice thin films or quantum composites. The ultimate goal of this research program is to create a cost-effective approach to fabricate nanostructured high performance thermoelectric materials at large scale. The intellectual merit of the proposed research program resides in the fabrication of the PbTe/CdTe nanocomposite materials with the novel PED system and the understanding of thermal transport through the fabricated nanocomposite materials. If successful, this research will pave the road to cost-effective, large-volume production of high-performance thermoelectric materials. In addition, in this research we will acquire the knowledge on the interactions between the energetic electron beam and the target and how these interactions will affect the material fabrication process and the resulted materials. With regard to broader impacts, the PED system is a relatively new materials deposition technique and the deposition process is not well understood yet, so the acquired knowledge on the deposition process will have extensive impacts on other material fabrication using the PED technique. This SGER support will help Fisk University, an HBCU Institution, to establish a long-lasting research and education program in thermal science. The activities will foster cross-disciplinary interactions between two universities and serve as a major step to enhance underrepresented minority student education.