Hydrogen Storage Properties and Catalytic Effect on Thermal Decomposition of Ammonium Perchlorate of Amorphous Mg-based Alloys
|School||Nanjing University of Technology and Engineering|
|Keywords||hydrogen storage alloys amorphous TiF3 hydrogen absorption and desorptionproperties catalysis ammonium perchlorate(AP)|
Compared to crystalline alloys for hydrogen storage, Mg-based amorphous alloys present great potential as hydrogen storage materials owing to high hydrogen storage capacity, good hydriding/dehydrogenation kinetics and relatively low dehydrogenation temperature. In this thesis, a review of researches of Mg-Ni based amorphous alloys for hydrogen storage was introduced. On this basis, Mg2Ni was selected for comprehensive study which was prepared as fully amorphous state by means of mechanical milling. The micro structure and hydrogenation effect of amorphous Mg-Ni alloy were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC), scanning electon microscope (SEM) and energy diffraction spectrum (EDS), as well as catalytic effect on or of amorphous Mg-Ni alloy. Hydriding/dehydrogenation kinetics and P-C-T curves were also tested for demonstrating hydrogen storage properties with or without TiF3. Thermogravimetry-mass spectrometry (TG-MS) coupling technique was suitable for preliminarily studying the mechanism of catalyzing of amorphous Mg-Ni alloy on thermal decomposition of AP. Conclusions are as following:(1) During mechanical milling period, not only the powder get smaller, but also surface area, amorphous character and crystallization temperature increase gradually by prolonging milling time. Mg-Ni alloy milled for80h is optimized and selected as subsequent study subject because of the formation of fully amorphous Mg2Ni phase, fine powder and high crystallization temperature. Hydriding has few impacts upon alloy’s microstructure with keeping amorphous which results in higher thermostability.(2) The hydrogen absorption capacity of a-Mg-Ni is about2.6wt.%at423K under the hydrogen pressure of3.0MPa. The absorption rate and hydriding kinetics are improved after doping TiF3. As the amount of TiF3increases from5mol.%to10mol.%, the hydrogen absorption capacity decreases from3.0wt.%to1.8wt.%. However, α-Mg-Ni still lacks of dehydrogenation capacity even though TiF3is mixed with a-Mg-Ni as a catalyst. For a-Mg-Ni+10mol.%TiF3, the higher the hydriding temperature is, the better the absorption rate and hydriding kinetics are improved. Meanwhile, it can be activated absolutely. The hydriding P-C-T curve at473K shows a wide plateau pressure at0.1MPa but the dehydrogenation P-C-T curve doesn’t.(3) The mechanism of thermal decomposition of pure AP is based on proton transfer. The thermal decomposition takes place in three stages:crystallographic transition, low-temperature decomposition (LTD) and high-temperature decomposition (HTD). Both a-Mg-Ni and a-Mg-Ni-H reduce HTD temperature, accelerate decomposition process and add decomposition heat. As the amount of addictive raises, HTD temperature gets lower and releases more heat. Differently, LTD temperature decreases after doping a-Mg-Ni.The more α-Mg-Ni is doped, the higher LTD temperature is, while a-Mg-Ni-H exhibits conversely. Catalytic mechanism can be described as following:At low temperature, crystallization of amorphous alloy is exothermal, which leads to AP’s temperature rising. Consequently, the LTD of AP is brought forward. At high temperature, Mg2Ni reacts with N of HNO and NH3to convert to complex compound. Then reduction in reaction’s activation energy occurs, which in turn lowers HTD temperature, accelerates the HTD and increases decomposition heat. Mg, Ni and H2resolved by a-Mg-Ni-H react with intermediate degradation products, which accelerates the decomposition process of AP. For a-Mg-Ni-H, dehydrogenation is exothermic which brings about AP’s temperature dropping and LTD temperature rising.