Microscopic Mechanism of Cation Exchange Process

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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.***