Controlling Thermoelectric Properties of Complex Oxide Ceramics by Integrated Design of Grain Boundaries and Interfaces

非技术描述：热电装置具有直接将温度梯度转化为电能的能力，并从各种工业、汽车和空间应用的废热中发电。如果高性能热电材料是由无毒和地球上丰富的元素制成的，热电技术在大多数应用中可以更有效。新兴的层状钴氧化物材料由于其热稳定性、天然丰度、重量轻和无毒而成为热电应用的有希望的候选者。钴酸钙单晶的能量转换效率显示出接近成熟的传统热电材料的优异性能。然而，多晶陶瓷的性能仍然较低，仅为单晶的约30- 60%。本项目的目标是通过设计晶粒之间的界面对多晶钴酸钙进行改性，旨在显着提高其热电性能，甚至超过单晶。除了对热电氧化物的发展产生直接影响外，从这项研究中获得的晶体界面工程的基本理解将有助于许多其他陶瓷系统。该项目为许多学科的年轻科学家和研究生提供培训，特别是西弗吉尼亚大学新推出的材料科学与工程项目。通过女性等代表性不足的大学生的参与，进一步加强了功能陶瓷和能量收集领域的尖端研究的研究与教育活动的一体化。技术要求：高性能热电材料需要具有高导电性、高塞贝克系数和低导热性。具有无公度特性的多晶钴酸钙的低能量转换效率是由于其低电导率和低塞贝克系数造成的。在这个项目中，多晶钴酸钙晶体织构和晶界密度都由晶内掺杂，特别是适当的掺杂剂在晶界处的偏析或耗尽控制。虽然掺杂剂在晶界处的偏析促进晶体织构并促进大的载流子迁移率和高电导率，但掺杂剂偏析充当载流子过滤器以降低载流子浓度并同时增加塞贝克系数。此外，该项目旨在通过插入近似第二相的界面散射来降低陶瓷的热导率。在许多材料中，晶界对力学性能的影响已被广泛研究。然而，大多数热电材料的电和热输运性质的晶界的影响的理解是目前非常有限的。本项目的成功完成有望阐明潜在的原子结构起源和热力学机制，这些机制驱动了掺杂剂偏析或耗尽的晶界的形成，以了解载流子沿沿着设计的边界/界面的输运和散射，并最终利用这些知识来调整电瓷的物理性能。该奖项反映了NSF的法定使命，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，

NON-TECHNICAL DESCRIPTION: Thermoelectric devices possess the ability to directly transform temperature gradients into electrical power and generate electricity from waste heat for various industry, automobile and space applications. Thermoelectric technology could be more efficient in most applications if the high-performance thermoelectric materials were made of non-toxic and earth-abundant elements. The emerging layered cobalt oxide materials are promising candidates for thermoelectric applications due to their thermal stability, natural abundance, lightweight, and non-toxicity. The energy conversion efficiency, for calcium cobaltite, in single crystal form shows excellent performance that approaches the well-developed conventional thermoelectric materials. However, the performance of polycrystalline ceramics remains low and only ~30-60 % of that found for the single crystals. The objective of this project is to modify polycrystalline calcium cobaltite through designing the interfaces between crystalline grains, aiming to significantly improve their thermoelectric performance even over the single crystals. Besides the direct impact on the development of thermoelectric oxide, the essential understanding of crystal interface engineering gained from this research will be instrumental to many other ceramic systems. This project provides the training to young scientists and graduate students in many disciplines, especially the newly launched Materials Science and Engineering program at West Virginia University. Integration of research and education activities in cutting-edge research in functional ceramics and energy harvesting are further strengthened through the involvement of undergraduate students from underrepresented groups including women.TECHNICAL DETAILS: High-performance thermoelectric materials need to have high electrical conductivity, high Seebeck coefficient, and low thermal conductivity. The low energy conversion efficiency of polycrystalline calcium cobaltite with incommensurate character is caused by the low electrical conductivity and low Seebeck coefficient. In this project, polycrystalline calcium cobaltite crystal texture and grain boundary density are both controlled by intragranular doping and especially the appropriate dopant segregation or depletion at the grain boundaries. While dopants segregating at the grain boundaries promote crystal texture and facilitate large carrier mobility and high electrical conductivity, the dopants segregation acts as carrier filter to decrease the carrier concentration and simultaneously increase the Seebeck coefficient. Furthermore, this project aims to reduce the thermal conductivity of ceramics by interface scattering through the insertion of the approximate secondary phases. The effect of grain boundaries on the mechanical properties has been extensively investigated in many materials. However, the understanding of the impact of grain boundaries on both the electrical and thermal transport properties of most thermoelectric materials is currently very limited. Successful completion of this project is expected to elucidate the underlying atomic structure origin and thermodynamic mechanisms that drive the formation of the crystal boundaries with dopant segregation or depletion, to understand the carrier transport and scattering along the designed boundaries/interfaces, and to ultimately utilize such knowledge to tune the physical properties of electroceramics.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.