| With the further investigation of hydrogen storage materials, there are more kinds of media having been developed to serve our lives. As one of the solid substrates for hydrogen storage, Ca(BH4)2 is considered as excellent candidate because of its high hydrogen storage mass percentage up to 11.6wt%, good reversibility after modification and the abundance of its contained atoms calcium (Ca) and boron atoms (B) in nature reserves.In chapter 3, based on density functional theory, the crystal structure, electronic structure and elastic properties of Ca(BH4)2 have been calculated for the Ca(BH4)2 (low temperature: a-phase, high temperature:β-phase). According to the calculations of elctronic structure of the two phases, we found that:a-Ca (BH4)2 and β-Ca (BH4)2 are wide band gap insulators and their band gaps are very similar, about 4.9 eV. Ca+2 and [BH4]- are combined by ionic bonding. In the [BH4]", B and H atoms are combined by the covalent bond. In addition, combining the theory of elasticity and density functional theory, we calculated and compared the elastic anisotropic properties and the Debye temperatures, which are consistent with the experimental results. It is proved that average atomic bond of β-Ca(BH4)2 is stronger than that of the a-Ca(BH4)2. The mechanical stability criterions are met, which means the two phases are mechanical stable. Meanwhile, both of α-Ca(BH4)2 and β-Ca(BH4)2 exhibit the characteristics of a brittle material and the volume change of a-Ca(BH4)2 is much more obvious than that of β-Ca(BH4)2 under a uniaxial deformation.For practical application and wide scale industrial use of Ca(BH4)2 as hydrogen media, effective measures must be taken to make it with considerable stability and the ability to better absorption and desorption of hydrogen. So we will demonstrate how to make the Ca(BH4)2 system have a good dynamic performance and thermodynamic properties in the following sections.In chapter 4, we choose the low temperature phase a-Ca (BH4) 2 as the doping system, we select three elements in the fifth subgroup from the top to down{ V (vanadium), Nb (Niobium), Ta (tantalum)} replace Ca (calcium), B (boron) atoms and occupy interstitial sites in α-Ca(BH4)2 System. First principle calculations are performed on the doped new systems to study their electronic structure and other physical characteristics (energy, charge, etc.). Basedon their electronic structure, we theoretically analyze the effects of the three elements on the hydrogen desorption mechanism of the catalytic a-Ca(BH4)2 system. Substantially, the catalytic mechanism of the fifth subgroup elements on doped a-Ca(BH4)2 can be understood as:the newly generated chemical bonds between the atoms of the system restructure, where between the doping element and the B atom from adjacent BH4 has a strong interaction. It turns that the distance between the doping element and B atom shorten obviously, which will benefit the formation of compound A-B between A (A=V, Nb, Ta) and B. Thus, the interaction between B and H in the corresponding BH4 among weaken and H slowly departure from the shackles of B to be in a near-free state, thus optimizing the desorption hydrogen capacity of doped system, improving its thermodynamic properties. Our results are in good accordance with the results of other research groups and some experimental results of the calculation are given. There are some minor differences in value, the reason is that the calculation software and calculation methods (pseudopotential we used to select specific precision selection, etc.) are different. Overall, our calculations are accurate and reliable, and can provide guidance for the subsequent part of the experiment to explore. |
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