Mechanical Property and Hydrogen Sorption in Mg Based Nanolayers

Hydrogen storage technology is vital for the application of hydrogen as an alternative fuel for sustainable energy related applications. Mg is a promising light-weight material that has superior hydrogen storage capacity (7.6 wt. % of hydrogen) and low cost for large scale applications. However, the stability of Mg hydride is a key challenge for on-board applications of Mg. Bulk Mg hydride has a tetragonal crystal structure (referred to as T-MgH_(2)), and desorbs H at ~ 573K. In contrast the application of H for automobile fuel cells requires a H_(2) desorption temperature at ~ 350K. In spite of active studies in the past decade, such a goal has not been achieved for practical application of T-MgH_(2).

This thesis focuses on tackling this challenge and consists of several major components.

First we have demonstrated that stress-induced orthorhombic Mg hydride (O-MgH_(2)) is thermodynamically destabilized at ~ 373K or lower. This destabilization arises from a large tensile stress in single layer O-MgH_(2) bonded to a rigid substrate, or a compressive stress due to the large volume change incompatibility in Mg/Nb multilayers. H desorption occurred at room temperature in O-MgH_(2) 10 nm / O-NbH 10 nm multilayers. These studies provide key insight into the mechanisms that can significantly destabilize Mg hydride and other type of metal hydrides.

Second, we have shown that the morphology of DC magnetron sputtered Mg thin films on rigid SiO_(2) (substrate) varied from a continuous dense morphology to a porous columnar structure when the films grew thicker. Thermal desorption spectroscopy studies show that thinner dense MgH_(2) films desorb H_(2) at a lower temperature than thicker porous MgH_(2) films. The influence of stress on the formation of the metastable MgH_(2) phase and consequent reduction of H sorption temperature are discussed.