| 其他摘要 | As a secondary energy source, hydrogen is a promising clean fuel in the 21st century. Therefore, it is globally recognized that hydrogen is an ideal candidate to replace the limited primary fossil fuel. The development and employment of hydrogen related new energy technologies are treated as the important programs in many countries. For industries where high pressure and high purity hydrogen are needed, the employment of metal hydrogen storage materials which have functions of both purifying and compressing will endow the metal hydride hydrogen compressors (MHHC) with incomparable merits over traditional ones.
In order to develop the hydrogen storage materials used for hydrogen compressing, and optimize the operating conditions as well as the integrated hydrogen storage properties, based on the domestic and overseas research development on metal hydrides, vanadium (V) and its solid solution alloys were investigated in this dissertation. By means of X-Ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive spectrum (EDS), X-Ray diffraction lattice strain analysis and hydrogen absorbing/desorbing tests, the thermodynamic and kinetic characteristics, absorption/desorption cycling properties, surface morphologies, crystal structure changes after absorption/desorption cycles and lattice strain, etc., were systematically studied and analyzed. Main conclusions in this dissertation are summarized as follows:
1．By comparing two kinds of activation procedures, it was found that low temperature activation was effective for the V1-xCrx system while high temperature activation for the (V0.9Ti0.1)1-xFex system. XRD confirmed that both alloy systems belong to the single bcc solid solution structure. The thermodynamic parameters of the V1-xCrx and the (V0.9Ti0.1)1-xFex are as follows:
The thermodynamic parameters (△H0, △G0 and lnPd) and lattice parameters of both alloy systems changed linearly with the solute concentrations. This linear relationship showed that the lattice parameters decreased with Cr or Fe content, leading to the reduction of the accommodation interstitial sizes for hydrogen atoms and consequently the increase of dissociation plateau pressures, which was responsible for instability of the hydrides. In addition, the desorption entropy varied slightly with Cr or Fe additive, while the desorption enthalpy and Gibbs free energy reduced linearly with the solute contents which facilitated the desorption reactions. In V-based solid solutions, some energy states which could accommodate the hydrogen electrons were likely to be filled with alloying elements, resulting in a slight diminish of the maximum absorbing hydrogen content.
2．For the V1-xCrx alloys, the absorption plateau pressure became higher with Cr content. At the same temperature and with the same initial hydrogen pressure, the driving force of hydrogen absorbing ΔP(=P0－Pe) was weakened with Cr content, as a result the kinetics of hydrogen absorption was slowed. For the (V0.9Ti0.1)1-xFex alloys, with appropriate Fe content, the absorption rate was improved and the activation was accelerated. However, excessive Fe content produced an inverse influence.
3．For more than 100 absorption/desorption cycles, the thermodynamic and kinetic properties of V0.9Cr0.1 alloy were changed in the same trend as those of V. Firstly, both absorption and desorption plateau pressures enhanced with cycling. Secondly, the hydrogen absorption capabilities under the same pressures decreased, i.e. the hydrogen absorption became difficult as the cycles proceeded. The changes of cyclic behavior of both V and V0.9Cr0.1 were attributed to the relaxation of the internal strain introduced by ball milling and the contraction of the cell volumes. The two aspects made the hydrogen atom accommodation interstices sizes decrease and the hydrides become more unstable. Besides, X-Ray diffraction lattice strain analysis indicated a decrease of lattice strain in cycled V0.9Cr0. 1Hx which was correlated with the increase in absorption pressure during cycling. Thirdly, the variations of hysteresis of both materials were not noticeable with cycling.
The absorption kinetics of V and V0.9Cr0.1 declined with the absorption/desorption cycles, which was linked to the increase of absorption plateau pressures. SEM observation revealed that during the absorption/desorption cycles, numerous cracks and fissures formed both on the surface and inside of the samples, which could shorten the hydrogen atoms diffusion distance and provide more fresh surfaces for hydrogen atoms association, enhancing the hydrogen desorption plateau pressure and improving the desorption kinetics.
Cr played an important role in improving the hysteresis loss of V after activation. It also destabilized the γ-phase, promoting the effective hydrogen desorption capacity. Besides, Cr was beneficial to cyclic stability of V according to the changes of absorption kinetics of both V and V0.9Cr0.1 alloy during cycling.
4．V and V0.9Cr0.1 alloy still retained bcc structures after more than 100 absorption/desorption cycles and evacuation for 60min to a vacuum of 2×10-4Pa at 873K. The β-phase hydrides were so stable that they kept bct structure under the low vacuum of 0.1Pa at room temperature. V、V-Cr and V-Ti-Fe alloys had the same structure transformation under the above treatment.
5．It was found that the dissociation pressures for (V0.9Ti0.1)0.94Fe0.06 alloy and V were very closed at the same temperature, and both the enthalpy and entropy of (V0.9Ti0.1)0.94Fe0.06 alloy were slightly larger than those of V. However, the effective hydrogen desorption capacity of (V0.9Ti0.1)0.94Fe0.06 alloy was noticeably smaller than that of V. |
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