|Ph.D Student||Nahor Hadar|
|Subject||Structure and Thermodynamics of Solid-Solid Ni-YSZ|
|Department||Department of Nanoscience and Nanotechnology||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
Atomistic structural characterization of interfaces mostly focuses on systems having low mismatch and a low index orientation relationship between the phases, where semi-coherent interfaces are formed and can be described as regions of coherency separated by misfit dislocations. However, many systems form incoherent interfaces due to significant difference in lattice parameters and/or lattice symmetry. Mechanisms for strain energy reduction and for mismatch accommodation at incoherent interfaces have been usually ignored.
In this work, solid-state dewetting of continuous Ni films deposited on the (111) and (001) surfaces of yttrium stabilized zirconia (YSZ) was used to produce equilibrated Ni particles on the substrates. Orientation relationships were determined using selected area diffraction (SAD) patterns and transmission electron microscopy (TEM), and the solid-solid interfacial energy was measured for each of the low-index orientations using Winterbottom analysis. The orientation distribution was determined using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD), and the solid-solid interface structure of the Ni(111)-YSZ(111) interface was determined using aberration corrected TEM and scanning transmission electron microscopy (STEM).
The results reveal that despite the 31% lattice mismatch between Ni and YSZ, the nominally incoherent interface reconstructs to form a 2-D array of high density, semi-periodic misfit dislocations. In addition, Ni particles were doped with Cr, or with Fe and Cr. Chemical analysis of the interfaces was conducted using energy dispersive spectroscopy (EDS) in STEM, and the equilibrium crystal shapes (ECS) were simulated using "Wulffmaker" software. Cr was found to adsorb to the Ni-YSZ interface, and in parallel to create a Cr2O3 phase which partially wets the interface. The presence of Fe in solution in the Ni was found to reduce the activity of Cr, thus reducing the amount of adsorption to the interface, and preventing formation of Cr2O3. Furthermore, in both doped systems, the same orientation relationship as in the un-doped system was found, regardless of the significant change in the equilibrium crystal shape.
These results strengthened the conclusion that the concept of coherent-incoherent interfaces as a means for describing interface atomistic structure is over simplified, where the concept of interfacial reconstruction is more complete, and serves to connect the atomistic structure to a thermodynamic description of the equilibrium interface state.