Nanometric Effects at Ultra-Small Crystallite Size: Investigation of Low-Temperature Protonic Conductivity in Dense Functional Oxide Ceramics

这项研究项目是加州大学戴维斯分校化学工程和材料科学系(Z.A.Munir教授和S.Kim教授)和德国RWTH亚琛大学物理化学研究所(M.Martin教授)共同努力的成果。这项研究的目的是提供对致密的纳米晶氧化物中迄今未观察到的质子和氧气传输过程的理解，作为功能氧化物中纳米尺度效应的一个例子。研究人员能够制备具有超小颗粒尺寸(接近10纳米)的氧化物，这为研究这一点和相关的纳米效应打开了一扇窗。其目的是制备和研究致密、块状、颗粒尺寸更小(10 Nm)的此类材料。观察到未水合氧化钇稳定的氧化锆和掺杂氧化铈中出现的低温质子导电性，为纳米结构电陶瓷的独特行为这一基本问题打开了一扇新的大门。到目前为止还没有观察到这样的行为，因为先前以散装形式制备这种材料的尝试没有成功。研究人员的成功归功于他们独特的能力(通过一种新的压力辅助场激活烧结法)，制备了颗粒尺寸为20纳米的高密度(98%)块状纳米氧化物。他们观察到的基本问题包括：这种质子传导的性质是什么？与质子迁移率相关的机制是什么？由于这种现象只在具有超小颗粒尺寸的材料中观察到，晶界如何在质量和电荷输运中发挥作用？从薄膜的观察来看，当颗粒很小时，晶界的作用和性质是否不同？如果有的话，掺杂剂产生的点缺陷对工艺有什么贡献？这些和相关基本问题的答案应该会为我们理解这些功能氧化物中的纳米尺度效应提供重要的智力贡献，并鼓励在这一重要领域进行新的研究。在低温下(甚至在室温下的水中)使用具有机械完整性的稳定氧化物作为质子导体对质子导体的应用考虑产生了巨大的影响。质子导体有广泛的应用领域，包括用作氢分离器(用作混合导体时)或发电(用于燃料电池)。它们也可用于电解制氢，以及用于有机化合物的加氢和脱氢反应。目前的固体氧化物燃料电池需要高温(800-1000摄氏度)，这一条件会带来材料降解问题，以及其他技术复杂和经济障碍。单是经济上的考虑，就让广泛的商业化变得望而却步。降低成本的一个有效途径是降低操作温度，同时又不损伤电解液的快速电极动力学和高离子电导率，我们的结果证明了其可行性。需要强调的是，观察到的低温质子电导率是在室温下发生的，不需要使用催化剂。结果表明，通过优化，在室温下利用水浓缩池进行可行发电是一个可能的目标。研究的一个重要方面是研究生、本科生和博士后研究员的参与和交流。除了教员互访外，还计划为学生和博士后研究员开展交流项目。每个研究生(来自德国和美国)将经历从合成和加固、结构和电学表征(在加州大学戴维斯分校)到SIMS测定(在RWTH亚琛大学)的整个过程。

This research project is a collaborative effort between the Department of Chemical Engineering and Materials Science (Professors Z. A. Munir and S. Kim) at the University of California, Davis and the Institute of Physical Chemistry (Professor M. Martin) at RWTH Aachen University, Germany. The goal of this research is to provide an understanding of the heretofore-unobserved proton and oxygen transport processes in dense, nanocrystalline oxides, as an example of the nanoscale effect in functional oxides. The investigators' ability to prepare oxides with ultra-small grain size (approaching 10 nm) has opened a window of investigation on this and on related nanometric effects. The aim is to prepare and investigate such materials in dense, bulk form with even smaller grain size ( 10 nm). Observation of the occurrence of low-temperature protonic conductivity in unhydrated yttria-stabilized zirconia and doped ceria, typical predominant oxygen ion conductors, opens a new door on the fundamental issue of a unique behavior of nanostructured electroceramics. Heretofore such a behavior has not been observed since prior attempts to prepare such materials in bulk form had not been successful. The researchers' success was made possible by their unique ability (through a novel pressure assisted field activated sintering method) to prepare highly dense ( 98%), bulk, nanometric oxides with a grain size of 20 nm. The fundamental questions that arise from their observations include: what is the nature of this protonic conduction? What is the mechanism associated with protonic mobility? Since this phenomenon is only observed in materials with ultra-small grain size, how do grain boundaries play a role in mass and charge transport? In view of observation on thin films, are the role and nature of grain boundaries different when the grains are very small? What, if any, do dopant-generated point defects contribute to the process? The answers to these and related fundamental questions should provide a significant intellectual contribution to our understanding of the nanoscale effect in these functional oxides and stimulate new research in this important area. The use of stable oxides with mechanical integrity as protonic conductors at low temperatures (even in water at room temperature) has an immense impact on application considerations for protonic conductors. Protonic conductors have an extensive field of application, including their use as hydrogen separators (when used as mixed conductors), or to produce power (when used in fuel cells). They can also be used in electrolysis for hydrogen production, and for reactions to hydrogenate and dehydrogenate organic compounds. Current solid oxide fuel cells require high temperatures (800 - 1000C), a condition that presents material degradation problems, as well as other technological complications and economic obstacles. The economic considerations alone make broad commercialization prohibitive. An effective way to reduce the cost is to reduce the operating temperature without scarifying fast electrode kinetics and high ionic conductivity of the electrolyte, which our results has demonstrated its feasibility. It is to be emphasized that the observed low-temperature protonic conductivity occurs at room temperature without the need to apply a catalyst. The results show that with optimization, viable power generation using water concentration cells at room temperature is a possible goal. An important aspect of the research is the participation and exchanges of graduate and undergraduate students and postdoctoral fellows. In addition to faculty exchange visits, an exchange program for students and postdoctoral fellows is planned. Each graduate student (both from Germany and the US) will go through the entire process from synthesis and consolidation and structural and electrical characterization (at UC Davis) to SIMS determinations (at RWTH Aachen University).