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

非技术描述：热电器件具有直接将温度梯度转化为电能的能力，并将废热发电用于各种工业，汽车和空间应用。如果高性能的热电材料是由无毒和地球上丰富的元素制成的，热电技术可以在大多数应用中更有效。新兴的层状氧化钴材料由于其热稳定性、天然丰度、轻量化和无毒性而成为热电应用的有希望的候选者。单晶形式的钴酸钙表现出优异的能量转换效率，接近成熟的传统热电材料。然而，多晶陶瓷的性能仍然很低，只有单晶的30- 60%。该项目的目的是通过设计晶粒之间的界面来修饰多晶钴酸钙，旨在显著提高其热电性能，甚至超过单晶。除了对热电氧化物的发展产生直接影响外，本研究获得的对晶体界面工程的基本理解将有助于许多其他陶瓷系统的发展。该项目为许多学科的年轻科学家和研究生提供培训，特别是在西弗吉尼亚大学新推出的材料科学与工程项目。通过包括女性在内的代表性不足群体的本科生的参与，进一步加强了功能陶瓷和能量收集前沿研究的研究和教育活动的整合。技术细节：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。具有不相称性的多晶钴酸钙的低能量转换效率是由其低电导率和低塞贝克系数造成的。在本项目中，多晶钴酸钙的晶体结构和晶界密度都是由晶内掺杂控制的，特别是在晶界处适当的掺杂偏析或耗尽。掺杂剂在晶界处的偏析促进了晶体结构的形成，有利于大载流子迁移率和高导电性，而掺杂剂的偏析起到了载流子过滤器的作用，降低了载流子浓度，同时提高了塞贝克系数。此外，本项目旨在通过插入近似二次相，通过界面散射降低陶瓷的导热系数。晶界对材料力学性能的影响已经在许多材料中得到了广泛的研究。然而，对晶界对大多数热电材料的电和热输运性质的影响的理解目前非常有限。该项目的成功完成有望阐明潜在的原子结构起源和驱动掺杂偏析或耗尽晶体边界形成的热力学机制，了解沿设计边界/界面的载流子输运和散射，并最终利用这些知识来调整电陶瓷的物理性质。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。

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.