An Investigation On The Hydrogen Storage Alloys Used For Metal Hydride Hydrogen Compression

In this thesis, based on a comprehensive review of the R&D on metal hydride hydrogen compressor and the hydrogen storage materials for hydrogen compressor, three typical families of hydrogen storage alloys, namely, AB5 type mischmetal-based alloys, AB₂ type Ti-based alloys and AB type TiFe-based alloys, were selected as the study object. By the mean of multicomponent alloying, superstoichiometry for A side element and partial substitution of B side element, the following series of alloys were designed and investigated , Mm_(1-xy)Ml_x(CaCu)_yNi_(5-x)M_x(M=Mn, Al), Ti_1+xCr_1.2Mn_0.8-yM_y(M= Fe, Ni, V-Fe, Cu, V) and Ti_1.1Fe+5wt.%M (M= Mg, Ca, Ml). And several alloys with good hydrogen storage properties were developed for metal hydride hydrogen compression.The XRD results show that Mm_(1-x-y)Ml_x(CaCu)_yNi_(5-x)M_x(M=Mn, Al) alloys possess the same crystal structure, CaCu₅-type hexagonal structure, and no second phases were detected. The ratio of Ce to La has great effect on the crystal structure parameters and on the hydrogen storage properties of the alloys. With increasing ratio of Ce to La, the ratio of structure parameter c/a increases linearly. Partial substitution of Mm with Ml, Ca and Cu can significantly improve the activation behavior and the hydriding/dehydriding pressure hysteresis of the alloys. The incubation periods for absorbing hydrogen were decreased and initial hydriding rates were increased. Partial substitution of A side element with Cu and B side with Al and Zr are effective for reducing the pressure hysteresis. The experimental results indicate that Mm_0.6Ml_0.1Ca_0.2Cu_0.1Ni_4.5Mn_0.4Al_0.1Zr_0.05 has good hydrogen storage properties and compression properties. The hydrogen storage capacities, room temperature hydriding plateau pressure and ΔH⁰ of dehydriding are 142 ml/g, 1.73MPa, 31.5 kJ/molH₂, respectively. With this alloy a single tube compressor was made and tested. The desorption pressure reach 18MPa when the desorption tempeture is about 100℃. And when the desorption temperature reaches 160℃, the desorption pressure reaches 40MPa.For Ti_1+xCr_1.2Mn_0.8-yM_y(M= Fe, Ni, V-Fe, Cu, V) series alloys , it was found that the main phase of all the tested alloys remains C14 type Laves phase. For Ti_1+xCr_2-yMn_y alloys, the superstoichiometry of A side Ti and partial substitution of Cr were concluded to explore the variationof hydrogen storage properties and compression characteristics. It is found that the activation characteristics and saturated absorption capacities of Ti_1+xCr_2-yMn_y alloys are greatly improved by increasing Ti. At the same time, the equilibrium pressure decreases and hysteresis factors are reduced. Partial substitution of Mn with Cr is effective for increasing of the equilibrium pressure plateau and reducing of the hysteresis. The test result show that among Ti_1+xCr_2-yMn_y alloys, Ti_1.1Cr_1.2Mn_0.8 possesses the best comprehensive properties, the hydrogen storage capacities of hydriding and dehydriding at 25℃ are 197 ml/g and 173 ml/g, respectively. And the temperature to desorb 40MPa hydrogen pressure is only 131℃. In order to get better hydrogen storage characteristics, the effects of substitution of Mn by Fe, Ni, V-Fe, Cu, V on the hydrogen storage properties are investigated. The hydrogen storage capacities evidently increase after alloying with Fe, Ni, V-Fe, Cu, V. TiCr_1.2Mn_0.5Fe_0.3 and Ti_1.1Cr_1.2Mn_0.5Cu_0.3 have good properties such as high hydrogen storage capacities (215ml/g and 219ml/g respectively) and high compression ratio. The desorption pressure can reach 31.1 MPa and 26.1 MPa at 100℃. And the temperature at which desorption pressure reaches 40MPa are 112℃ and 119℃, respectively.For AB type alloys, Ti_1.1Fe+5wt.%M (M= Mg, Ca, Ml) series alloys were designed and tested. The test results indicate that after addition of hydrogen asorbing element, such as Mg, Ca, and Ml, the main phase of Ti_1.1Fe+5wt.%M (M= Mg, Ca, Ml) remain TiFe phase. Ti_1.1Fe+5wt.%Ml and Ti_1.1Fe have single TiFe phase microstructure. However, for Ti_1.1Fe+5wt%Mg and Ti_1.1Fe+5wt.%Ca alloys, besides TiFe phase, a little of TiFe₂ phase is detected. The addition of Mg, Ca, Ml greatly improves the activation properties. But the equilibrium pressures decrease and the hysteresis is reduced. Ti_1.1Fe+5wt%Ml has the best hydrogen storage properties, which are necessary for hydrogen compression. The desorption pressure of Ti_1.1Fe+5wt%Ml reaches 9.10MPa at 100℃ and compression radio reaches 6.8. When the temperature is increased to 172℃, the obtained desorption pressure reaches 40MPa.