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Hierarchy of short-circuit diffusion paths on the nanoscale revisited
Jun. 04, 2014, 16:30 - 18:00
Institute of
Talk given by Prof. Eugen Rabkin We studied the solid state dewetting of thin Au, Ni and Fe films, and Au-Fe bilayers deposited on sapphire substrates. Both Au and Fe films were nanocrystalline and exhibited strong fiber <111> texture, while the Ni and Au-Fe films exhibited a strong heteroepitaxy to the substrate. The single crystalline particles formed at the late stages of dewetting exhibited stable faceted shapes that were far from those predicted by equilibrium considerations. We discussed high stability of faceted particles in terms of very slow mass transport along the singular atomically flat facets. In the case of heteroepitaxial Ni films and Au-Fe bilayers, the formation of highly faceted pinholes to the substrate is accompanied by the nucleation and growth of twin boundaries parallel to the substrate. These boundaries allow nucleating of defects at the side facets of the pinhole, thus enabling its growth. We argue that the slow pace of diffusion on atomically flat facets of nanoparticles and textured thin films brings to the forefront other types of short-circuit diffusion, such as grain boundary diffusion and metal self-diffusion along the interphase boundaries (i.e. along the film-substrate interface). We propose a general framework for treating the size and shape evolution in nanocrystalline and nano-particulate solids controlled by diffusion along the interphase boundaries between two immiscible, non-reactive phases. Like in the case of surface diffusion, the atomic diffusion flux along the interphase boundary is driven by the gradient of chemical potential of the atoms at the interface. Yet while in the case of surface diffusion this chemical potential is a local function of surface curvature and surface energy, in the case of interphase boundary the chemical potentials of the components depend on the overall geometry of the system and on boundary conditions at the triple lines. We suggest a variational method of calculating the chemical potentials at the interphase boundary. This method relies on calculating the variations of the total energy of the system caused by infinitesimal relative translations and rotations of the two solid phases abutting the interface. Our theoretical models are illustrated by the experimental data on sintering of model Cu-W agglomerates, on phase transformations in the nanoparticles of Au-Fe alloys, and on thermal grooving in thin Ni films with “mazed bicrystal” microstructure.
This event is part of the eventgroup INT Talks
Prof. Eugen Rabkin

Technion - Israel Institute of Technology
Materials Engineering
Prof. Dr.-Ing. Horst Hahn
Institute of Nanotechnology (INT)
Karlsruhe Institute of Technology (KIT)
Mail:horst hahnKem9∂kit edu
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