Microscopic Mechanism of Cation Exchange Process

该项目研究了用于合成高挥发性化合物的阳离子交换过程的微观机制，例如汞基高T_c超导体。这些材料具有超导器件技术所需要的特性：TC~130K，在77K以上的温度下载流能力大，但它们的高度挥发性限制了传统的热反应过程，因为合成高纯度样品所需的相平衡的精细控制很难实现。该方法在非常规生长机制中采用了阳离子交换过程。选择具有与所需挥发性化合物相关的晶体结构和化学成分的较不挥发的前体基质。然后，通过扰动前体基质上的一个或多个阳离子并用挥发性阳离子取代它们，可以形成挥发性化合物。虽然阳离子交换过程已经成功地应用于高质量的Hg-HTS薄膜和块材的合成，但在微观水平上对阳离子交换的机理还知之甚少。了解这一机制是这项研究的中心目标，因为它不仅对开发汞高温超导电子/电气设备的能力至关重要，而且对合成其他重要技术材料的过程的推广也至关重要，这些材料可能在传统工艺中无法实现。我们将在微观水平上研究生长缺陷、晶界和化学掺杂对阳离子交换行为的影响，以便更深入地了解作为工艺参数函数的阳离子交换的宏观行为。还将对阳离子交换过程进行理论模拟，以更深入地了解所涉及的基本物理。%该项目解决与技术相关的材料科学领域的基础合成和加工研究问题。该项目与堪萨斯大学、威斯康星大学、斯坦福大学、芝加哥大学伊利诺伊分校、几个政府实验室(AFRL、ORNL、SANDIA、LANL和ANL)以及工业组织超级管道技术公司的研究人员合作。研究生和本科生在该项目中发挥主要作用，并接触到几个学科的研究机会，可以接触到广泛的研究设备和设施。*

This project addresses the microscopic mechanism of a cation-exchange process for synthesis of highly volatile compounds, such as Hg-based high-Tc superconductors. These materials have desirable features for superconducting device technologies: Tc~130K and large current-carrying capability at temperatures above 77 K. Their highly volatile nature, however, limits the conven-tional thermal-reaction process since a delicate control of phase equilibrium required for synthesis of high-purity samples is difficult to achieve. The approach employs a cation-exchange proc-ess in an unconventional growth mechanism. A less-volatile precursor matrix is selected with a related crystalline structure and chemical composition to a desired volatile compound. Volatile compounds can then be formed by perturbing one or more cations on the precursor matrix and replacing them with volatile cations. Although the cation exchange process has been applied suc-cessfully to synthesis of high- quality Hg-HTS films and bulks, the mechanism of cation exchange is barely understood at the microscopic level. Understanding this mechanism is a central goal of this research as it is crucial not only to the ability to develop Hg-HTS electronic/electrical devices, but also to generalization of the process for synthesizing other technological important materials that may not be achievable in conventional processes. The effect of growth defects, grain boundaries, and chemical doping on the cation exchange behavior at the microscopic level will be studied so as to achieve a thorough understanding of the macroscopic behavior of the ca-tion exchange as a function of processing parameters. Theoretical modeling of the cation ex-change process will also be carried out to gain a more thorough understanding of the fundamental physics involved. %%% The project addresses fundamental synthesis and processing research issues in a topical area of materials science having technological relevance. The project is collaborative with interactions among researchers at U. Kansas, U. WI, Stanford U., U. IL at Chicago, several government labo-ratories (AFRL, ORNL, Sandia, LANL, and ANL), and an industrial organization, Supercon-ducting Technologies, Inc. Graduate and undergraduate students play a primary role in the project and are exposed to research opportunities across several disciplines with access to a broad range of research equipment and facilities.***