Bragg coherent diffractive imaging of defect dynamics in individual nanoparticles and thin film grains

Elucidating the effects of defects like grain boundaries and dislocations on the properties of polycrystalline materials is a topic of both scientific and technological importance, relevant to designing strain-tolerant materials, controlling ion intercalation, and designing new catalysts. However, complete understanding at the atomic level of how these defects influence material properties requires the ability to image them during dynamic processes. Bragg coherent diffractive imaging (BCDI) is promising in this regard as it uses x-rays to nondestructively obtain three-dimensional images of the strain and defect network within individual nanocrystals in-situ. The first chapter of this dissertation gives an overview of the principles behind how the BCDI technique works and the process by which we collect and analyze our data. The second chapter discusses the extension of the BCDI technique for use in tracking the defect dynamics of grains in polycrystalline thin films. BCDI has previously only been used to track the defect dynamics of nanoparticle systems. The study discussed is the first example of the technique being used to track the defect dynamics of an individual grain within the constraints of surrounding grains. The third and fourth chapters discuss using the BCDI technique to study the palladium hydride phase transformation, which involves the intercalation of hydrogen atoms into the palladium lattice. Phase transitions involving solute intercalation are essential to a wide range of applications such as hydrogen storage, hydrogen sensing, battery charging, catalysis, etc. Chapter 3 specifically talks about the utilization of the newly extended BCDI technique to study the hydrogen absorption dynamics of individual palladium grains in polycrystalline thin films. We investigate in detail the role of dislocations and grain boundaries on three-dimensional strain and displacement field dynamics during the hydriding phase transformation. We observe very different behavior for thin film palladium grains compared to previous reports on palladium nanoparticles, including a lack of a hydrogen-rich surface layer and an increase in grain boundary mobility. This hydrogen-enhanced plasticity provides a plausible explanation for the switch in the size-dependent behavior of single crystal nanoparticles to the lower transformation pressures of bulk polycrystalline materials and has interesting implications for the mechanism of hydrogen embrittlement. The study in chapter 4 involves a more traditional use of the BCDI technique to study the hydrogen absorption reaction in palladium nanoparticles. We observe that dislocations nucleated deep in a palladium nanoparticle during the forward hydriding phase transformation heal during the reverse transformation, demonstrating that the traditional view of irreversible defect formation in the absence of heating is incorrect. Finally, we show that defective palladium nanoparticles exhibit sloped isotherms, indicating that defects act as barriers to the phase transformation.

Keywords: Materials Science and Engineering.
Advisor: Chueh William
Kanan Matthew William
Cui Yi