TECHNICAL: This project addresses structure-property relationships in polycrystalline thin films, and will seek to exploit the intrinsic structural anisotropy that exists when the films have columnar structures. In these films, grains, grain boundaries, phase boundaries (if they exist), and triple junctions all can be made to extend from the bottom of the film to the top, and therefore provide continuous paths across the film. Some of these paths, however, can be disconnected in the plane of the film: isolated grains of one phase and/or interfacial triple-junctions do not form a continuous path in the plane of the film. When these structural components can be engineered to be the active paths for electron transport, for example, the film will be conductive through its thickness, but not in its plane. A number of other potential applications of isolated columnar structural elements are conceivable, including the low-cost manufacture of large areas covered with field-emitter tips, for use in display systems. The project seeks, first, to develop suitable processing methods to create columnar microstructures with application potential, using two different strategies. The first strategy is the creation of two-phase films, using a binary eutectic alloy system. The goal here is to create a system in which one of the phases forms isolated columns that can be used for a variety of purposes, depending upon the properties that can be associated with the phase in the columns. The second strategy is the creation of columnar active regions by doping the regions defined by triple junctions, in an originally single-phase film. These two strategies are likely to provide differing opportunities for microstructural control, and will also be varyingly applicable in different materials systems. Our exemplar for the first strategy will be the Cu-Ag eutectic system, and for the second strategy we will base our development on films of LiF. Significant new findings are expected relative to the processing and microstructural control of both of these thin film systems. Controllable films with distinct columnar phases or compositional regions (which we identify generically as .components.) will enable the exploration of a number of attractively simple structure-property relationship hypotheses, and the testing of these hypotheses will be an important component of this research.
The project will train two PhD students in advanced processing-structure-property relationships, providing a very sound training for all of materials science, addressing both modeling and experimentation. In addition, as many as twelve undergraduate students may be involved in the research project through summer research programs, or special topics research courses as part of their undergraduate programs. The selection of students for these opportunities will be based upon existing programs at Purdue University, which have significant outreach to minorities and women. The demonstration of fundamentally simple ideas for the creation of special properties via microstructural control is likely to provide a number of opportunities for informal education and outreach, that will be fully exploited in the recruitment of students into the Materials Engineering program at Purdue, and also in presentations to the general public. Strong relationships with the microelectronics industry will ensure that novel properties such as anisotropic electrical conductivity, optoelectronic properties, and others, will be exploited efficiently.
