Department of Materials Science and Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
Department of Materials, University of Oxford , Oxford OX1 3PH, United Kingdom.
ACS Nano. 2018 Feb 27;12(2):1359-1372. doi: 10.1021/acsnano.7b07732. Epub 2018 Jan 25.
Thin film nonstoichiometric oxides enable many high-temperature applications including solid oxide fuel cells, actuators, and catalysis. Large concentrations of point defects (particularly, oxygen vacancies) enable fast ionic conductivity or gas exchange kinetics in these materials but also manifest as coupling between lattice volume and chemical composition. This chemical expansion may be either detrimental or useful, especially in thin film devices that may exhibit enhanced performance through strain engineering or decreased operating temperatures. However, thin film nonstoichiometric oxides can differ from bulk counterparts in terms of operando defect concentrations, transport properties, and mechanical properties. Here, we present an in situ investigation of atomic-scale chemical expansion in PrCeO (PCO), a mixed ionic-electronic conducting oxide relevant to electrochemical energy conversion and high-temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with in situ heating, we characterized chemical strains and changes in oxidation state in cross sections of PCO films grown on yttria-stabilized zirconia (YSZ) at temperatures reaching 650 °C. We quantified, both statically and dynamically, the nanoscale chemical expansion induced by changes in PCO redox state as a function of position and direction relative to the film-substrate interface. Additionally, we observed dislocations at the film-substrate interface, as well as reduced cation localization to threading defects within PCO films. These results illustrate several key aspects of atomic-scale structure and mechanical deformation in nonstoichiometric oxide films that clarify distinctions between films and bulk counterparts and that hold several implications for operando chemical expansion or "breathing" of such oxide films.
薄膜非化学计量氧化物使许多高温应用成为可能，包括固体氧化物燃料电池、致动器和催化。大量的点缺陷（特别是氧空位）使这些材料具有快速离子导电性或气体交换动力学，但也表现为晶格体积和化学成分之间的耦合。这种化学膨胀可能是有害的，也可能是有益的，特别是在薄膜器件中，通过应变工程或降低工作温度可以提高性能。然而，薄膜非化学计量氧化物在操作缺陷浓度、输运性质和力学性质方面可能与体相材料不同。在这里，我们通过电子能量损失谱和透射电子显微镜与原位加热相结合，对电化学能量转换和高温致动相关的混合离子-电子导电氧化物 PrCeO（PCO）的原子尺度化学膨胀进行了原位研究。我们在温度达到 650°C 的情况下，对生长在氧化钇稳定氧化锆（YSZ）上的 PCO 薄膜的横截面进行了化学应变和氧化态变化的表征。我们静态和动态地量化了 PCO 氧化还原态变化引起的纳米尺度化学膨胀，以及相对于薄膜-基底界面的位置和方向。此外，我们还观察到了薄膜-基底界面处的位错，以及 PCO 薄膜内穿位错的阳离子定位减少。这些结果说明了非化学计量氧化物薄膜中原子尺度结构和力学变形的几个关键方面，澄清了薄膜与体相材料的区别，并对这类氧化物薄膜的操作化学膨胀或“呼吸”具有重要意义。
