Advanced TEM investigation of the elementary plasticity mechanisms in palladium thin films at the nano scale

Date: 22 April 2015

Venue: UAntwerp, Campus Middelheim, Room G0.10 - Middelheimlaan 1 - 2020 Antwerpen

Time: 4:00 PM

Organization / co-organization: Department of Physics

PhD candidate: Behnam Aminahmadi

Principal investigator: Dominique Schryvers & Hosni Idrissi

Short description: PhD defence of Behnam Aminahmadi - Faculty of Science, Department of Physics



Abstract

Pd has been known for some time to be an enabling material for future hydrogen technologies. For example, thin membranes of Pd can play a role in hydrogen purification and sensing applications. The thesis first focuses on the processing of Pd films. The influence of the deposition rate on the formation of growth twins in nanocrystalline Pd films deposited by electron beam evaporation is investigated using transmission electron microscopy. Based on the results, an optimal deposition rate was achieved. Furthermore, nanocrystalline systems show moderate to high strain rate sensitivity at room temperature which might help restoring the ductility but can have disadvantage of creep/relaxation effects in applications. Therefore, in the present project a novel technique for stress and strain evolution measurement called "on-chip testing" which also allows in-situ high resolution transmission electron microscopy observations has been used for the first time to perform creep/relaxation experiments of nanocrystalline Pd free standing beams. Large creep rates are unexpectedly observed at room temperature and which are mainly controlled by dislocation mediated processes.

The mechanical stability and response to hydrogen pressure of Pd thin films, often having a nanoscale interior structure, is still insufficiently understood. In the present work, we have also performed advanced transmission electron microscopy characterizations on nanocrystalline Pd thin films prepared using sputter deposition and hydrided at low (for α-phase transformation) and high pressures (for β-phase transformation) to unravel nano-plasticity mechanisms. Statistical analyses of the grain size/morphology as well as the crystallographic texture have been performed using automated crystal orientation mapping in transmission electron microscopy while aberration corrected high resolution transmission electron microscopy has been used to unravel the nature and the near-core properties of the defects generated during hydriding/dehydriding cycles. The results revealed strong interaction of hydrogen with extended defects as well as a clear effect of hydrogen on both the stable and unstable planar fault energies of Pd. These results predict an important contribution of stacking faults and deformation twins on the mechanical behavior of nanocrystalline Pd films hydrided to β-phase when subjected to an external applied stress.