Chemical Expansion of Mixed Conducting Ceramics

0074539高温应用中使用的添加剂陶瓷必须在热膨胀系数(CTE)上紧密匹配，以避免在温度变化较大时与应变差相关的过度应力。电化学陶瓷(用作离子传输膜或固体氧化物燃料电池)是一个额外的挑战。这些材料不仅随着温度的升高而膨胀，而且还随着化学氧化状态的变化而膨胀。这些材料在高温下的大部分膨胀可能是化学效应而不是热效应造成的。然而，今天人们对化学膨胀、其后果和技术应用知之甚少。这个项目将扩大我们对陶瓷膨胀的理解，将这些化学效应包括在内。提出了一种用温度和氧含量从热力学上定义膨胀的新的连续介质理论，并对几种混合导电氧化物的膨胀作为阳离子化学计量比、温度和氧分压的函数进行了实验研究。通过在理论框架内考察这些数据，将对陶瓷中的膨胀以及膨胀与电子结构和缺陷结构的关系有一个更完整和更一般的理解。作为第二个目标，将开发新的实验技术，包括非平衡膨胀，用于直接测量氧表面交换动力学和化学扩散。所研究的材料体系包括镧-锶-钴-氧化铁(LSCF)和锶-铁-钴-氧化物(SFC)。LSCF与固体氧化物燃料电池和电驱动空气分离直接相关，而SFC对开发甲烷部分氧化制合成气的致密膜的工人特别感兴趣。LSCF也是一个很好的模型材料系统，因为它的缺陷热力学被很好地理解，而且它的阳离子组成覆盖了大量的缺陷和电子结构(包括离子和电子结构)。电化学陶瓷在包括燃料加工、电力生产和空气分离在内的广泛应用中具有重要意义。对膨胀的充分理解是促进电化学陶瓷知识和应用的关键。该项目还将提供一个框架，以接近所有固体中的化学膨胀，从而进一步扩大其影响。

0074539AdlerCeramics used in high-temperature applications must match closely in coefficient of thermal expansion (CTE) to avoid excessive stress associated with strain differences over large changes in temperature. Electrochemical ceramics (used as ion-transport membranes or in solid oxide fuel cells) present yet an additional challenge. These materials not only expand with temperature, but also with changes in chemical oxidation state. The majority of expansion at high temperatures in these materials may result from chemical rather than thermal effects. Yet today there is little knowledge or understanding of chemical expansion, or its consequences and technological applications. This project will enlarge our understanding of expansion in ceramics to include these chemical effects. A new continuum theory that defines expansion thermodynamically in terms of temperature and oxygen content will be presented and then an experimental study of expansion in several mixed conducting oxides as a function of cation stoichiometry, temperature, and oxygen partial pressure will be performed. By examining this data in the framework of the theory, a more complete and general understanding of expansion in ceramics will be gained, as well as into how expansion relates to electronic and defect structure. As a secondary goal, new experimental techniques will be developed involving non-equilibrium expansion for directly measuring oxygen surface exchange kinetics and chemical diffusion. The material systems under study include lanthanum-strontium-cobalt-iron-oxide (LSCF) and strontium-iron-cobalt-oxide (SFC). LSCF is of direct relevance to solid-oxide fuel cells and electrically driven air separation, while SFC is of particular interest to workers developing dense membranes for partial oxidation of methane to syngas. LSCF is also a good model material system because its defect thermodynamics are well understood, and its cation composition covers an enormous range of defect and electronic structure (both ionic and electronic). Electrochemical ceramics are of importance in a broad spectrum of applications, including fuels processing, electricity production, and air separation. A full understanding of expansion in is critical to advancing knowledge and application of electrochemical ceramics. This project will also provide a framework for approaching chemical expansion in all solids, thus extending the impact of it much further.