Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Poh, Chung-Kiak, Metallic nanostructures, ultrathin films and optical technologies for hydrogen storage and switchable mirrors, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3291
The ability of some hydrides to reversibly absorb hydrogen under the right conditions makes them potential candidates for hydrogen storage, while the change in electrical or optical properties during the metal-insulator transition can be used to realize devices of technological interests such as two-dimensional hydrogen diffusion indicators and smart windows.
This thesis focuses primarily on the investigation of metal hydrides based on magnesium and palladium, in the form of both nanopowders and thin films. In general, samples prepared in the form of nanopowders were intended for hydrogen storage applications, while samples prepared in the form of thin film were intended for switchable mirror applications. Nanopowder samples were synthesized by ball milling, while the thin films were prepared by physical vapor deposition techniques such as pulsed laser deposition and thermal evaporation.
The desorption capacity, thermodynamics, and kinetics of Ti- and Ni/Ti-catalyzed Mg hydrides were investigated using Sieverts-type apparatus and differential scanning calorimetry. Based on analysis of the van’t Hoff equation and the Kissinger equation, the addition of Ti and Ni as catalysts has been found to play a key role in improving the thermodynamic and kinetic properties of magnesium hydride by decreasing the desorption temperature and the activation energy. A combination of Ti and Ni is a more effective catalyst than either Ti or Ni alone, suggesting the existence of a synergetic effect.
We propose and demonstrate a simple but effective real-time optical method to determine the Tdes and kinetics of freshly fabricated magnesium nanowires inside a transparent quartz tube, while it is still under the protective gas environment. The proposed characterization technique based on optical reflection requires only milligrams of sample and helps to eliminate the common problem of oxidation associated with removal and transport of the freshly fabricated nanostructures into an inert protective environment. This optical technique could be applied to any hydrogen storage material in powder form which shows a significant difference in its optical absorption between the hydride and the non-hydride phase. A three-color, 8-bit, 240 × 320 pixel imager was used to acquire the optical signals, and the image processing was peformed in MATLAB®.
Magnesium films of various thicknesses were fabricated by pulsed laser deposition and capped with a palladium protective layer. The change in the kinetics as a function of film thickness was measured. Raman spectroscopy on the 11 nm magnesium hydride film reveals a small but detectable peak arising from the Eg phonon mode. The Raman frequencies of bulk magnesium hydride predicted by CASTEP calculations are included for comparison.
We propose and demonstrate a multi-stacked structure intended for low-nanometer ultrathin films of various metal hydrides, which would enable them to simultaneously achieve an enhanced optical contrast equivalent to that of thicker films, yet be able to retain much of the rapid switching kinetics characteristic of low-nanometer ultrathin films. Such improvement in the performance will help to further extend the scope of applications, particularly in the area of dynamic switchable mirrors, where switching speed and high switching contrast are crucial.
Future work will also be based on resonant photodesorption of hydrogen, as well as surface plasmon nanophotonics, both of which are aimed at improving the efficiency of hydrogen desorption of promising hydrides such as MgH2 or LiBH4.