The main focus of this thesis is the characterization of defects and microstructure in high-energy ball milled magnesium hydride powder and magnesium-based multilayered composites. Enhancement in kinetics of hydrogen cycling in magnesium can be achieved by applying severe plastic deformation. A literature survey reveals that, due to extreme instability of -MgH2 in transmission electron microscope (TEM), the physical parameters that researchers have studied are limited to particle size and grain size. By utilizing a cryogenic TEM sample holder, we extended the stability time of the hydride phase during TEM characterization. Milling for only 30 minutes resulted in a significant enhancement in desorption kinetics. A subsequent annealing cycle under pressurized hydrogen reverted the kinetics to its initial sluggish state. Cryo-TEM analysis of the milled hydride revealed that mechanical milling induces deformation twinning in the hydride microstructure. Milling did not alter the thermodynamics of desorption. Twins can enhance the kinetics by acting as preferential locations for the heterogeneous nucleation of metallic magnesium. We also looked at the phase transformation characteristics of desorption in MgH2. By using energy-filtered TEM, we investigated the morphology of the phases in a partially desorbed state. Our observations prove that desorption phase transformation in MgH2 is of nucleation and growth type, with a substantial energy barrier for nucleation. This is contrary to the generally assumed core-shell structure in most of the simulation models for this system. We also tested the hydrogen storage cycling behavior of bulk centimeter-scale Mg-Ti and Mg-SS multilayer composites synthesized by accumulative roll-bonding. Addition of either phase (Ti or SS) allows the reversible hydrogen sorption at 350C, whereas identically roll-bonded pure magnesium cannot be absorbed. In the composites the first cycle of absorption (also called activation) kinetics improve with increased number of fold and roll (FR) operations. With increasing FR operations the distribution of the Ti phase is progressively refined, and the shape of the absorption curve no longer remains sigmoidal. Up to a point, increasing the loading amount of the second phase also accelerates the kinetics. Microscopy analysis performed on 1-2 wt.% hydrogen absorbed composites demonstrates that MgH2 formed exclusively on various heterogeneous nucleation sites. During activation, MgH2 nucleation occurred at the Mg-hard phase interfaces. On the subsequent absorption cycles, heterogeneous nucleation primarily occurred in the vicinity of internal free surfaces such as cracks. / Materials Engineering
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:AEU.10048/1638 |
| Date | 06 1900 |
| Creators | Danaie, Mohsen |
| Contributors | Mitlin, David (Chemical and Materials Engineering), Elias, Anastasia (Chemical and Materials Engineering)- Chair, Zhang, Hao (Chemical and Materials Engineering), Cadien, Ken (Chemical and Materials Engineering), Nychka, John (Chemical and Materials Engineering), Brett, Michael (Electrical and Computer Engineering), Botton, Gianluigi (Materials Science and Engineering- McMaster University) |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Format | 8642006 bytes, application/pdf |
| Relation | http://dx.doi.org/10.1016/j.jallcom.2008.09.078, http://dx.doi.org/10.1016/j.actamat.2010.01.055, http://dx.doi.org/10.1017/S1431927608088429 |
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