MSE Seminar Series: Alexander Roytburd
Friday, November 16, 2012
1:00 p.m.-2:00 p.m.
Room 2108 Chemical and Nuclear Engineering Bldg.
301 405 5240
Engineering Self Assembled Nanostructures by Constrained Phase Transformation
Department of Materials Science and Engineering
University of Maryland College Park
Phase transformations in solids are always accompanied by some spontaneous strain. Therefore, imposing external constraint makes it possible to obtain self assembled polydomain nanostructure consisting of single phase or heterophase domains. With decreasing size of the system down to the nanoscale, this mechanism of elastic relaxation becomes more preferable than plastic deformation, particularly in thin epitaxial films. The formation and evolution of single phase (twin) polydomain structures in epitaxial films are well studied experimentally and theoretically while there are only few experimental observations of heterophase polydomain structures. Therefore, the recent discovery of rhombohedral-tetragonal polydomain structures with large extrinsic electromechanical response in epitaxial ferroelectric BiFeO3 films is especially important for development of theory and expanding it for other materials. Theoretical analysis of this structure shows that the stability of two-phase equilibrium is determined by competition of phase-substrate elastic interaction which initiates mixture formation and phase-phase elastic interaction which opposes phase mixture. If phase-substrate interaction is dominant, polydomain heterophase structure is stable. If phase-phase interaction dominates, the two-phase state is unstable and direct and reversible transformation proceeds with hysteresis. For stable two phase state, changing temperature or external electrical or mechanical fields leads to domain evolution which results in enhanced extrinsic properties, including large electric susceptibility, piezo effect, or elastic compliance. As an expansion of the theory, we consider metal-hydride transformation in epitaxial film. It is predicted that formation of equilibrium metal-hydride polydomain nanostructures leads to reversible metal-hydride transformation and consequently sustainable hydrogenation-dehydrogenation.
The general principle of formation of heterophase polydomain nanostructures can be applied to engineering of self assembled nanocomposites, including multiferroics, through epitaxial growth from liquid or vapor phase condensation. The possibility of engineering polydomain nanostructures in constrained crystals of different shape is mentioned. In conclusion, I would like to make a connection between the subject discussed in this talk and the recently awarded Kyoto Prize for outstanding achievements in materials science.