Research On Hydrogen Storage Characteristics And Phase Composition Of TiCr Based Alloys

To develop the alloys for high efficiency hydrogen storage device, TiCr based hydrogen storage alloys have been studied systematically in this work. X-ray diffraction, pressure composition temperature (PCT) tests, metallography, X-ray photoeletron spectroscopy (XPS), thermogravimetry-differential scanning calorimetry (TG-DSC) and mass spectrometry (MS) have been employed. Hydrogen storage performance and its dependence on composition, microstructure. thermodynamics and dynamics of the alloys from TiCr binary alloy to alloys containing five elements were investigated elaborately. Some experimental rules between hydrogen absorption-desorption characteristics and alloy composition have been acquired. Moreover, several alloys with superior performance have been developed, which show a high prospect for practical application.TiCr₁.8 showed the best hydrogen absorption-desorption performance among all the TiCr binary alloys. So it was selected as the fundamental composition in this research.Hydrogen storage performance of ternary TiCr₁.8-xM_x alloys (M=V, Mo and Mn) was first examined. Results showed that their phase composition transited from Laves phase to BCC phase with increasing M (M=V or Mo) content. However, the alloy with the substitution of Mn for Cr maintained Laves phase. Crystal cell parameters were enlarged with the increasing M content in TiCr₁.8-xM_x alloys. The maximum hydrogen storage capacity and the content of M for the individual alloy followed the Gauss equation. Hysteresis between hydrogen absorption and desorption enhanced upon the substitution of M for Cr. TG-DSC tests of TiCr₁.8-xV_x alloys indicated that hydrogen desorption were impeded with increasing V content accompanied with the increase of activation energy for dehydrogenation. MS tests revealed that hydrogen released is not only either in atomic and molecular state or in tri-atom state.The Laves phase of TiCr₁.8 alloy gradually transformed into BCC phase in TiCr₁.8 x(VFe)_x with the substitution of more VFe alloy for Cr. The maximum and reversible hydrogen storage capacity alloy with VFe content of TiCr₁.8-x(VFe)_x alloy followed Gauss equation. The increased VFe content inhibited hydrogen desorption from the quaternary alloys. All TiCr₂x(VFe)_x alloys had BCC structure in spite of their stoichiometry. The crystal parameters decreased with increasing X. At the same time, the activation performance of the alloys first was improved and than deteriorated. The phasecompositions of initially activated alloys were dependent on the operating temperature. Among all the quaternary alloys under study TiCri 2(VFe)06 had the highest reversible hydrogen storage capacity and was the most suitable one for practical application. As MS tests showed, the desorbed hydrogen was mostly in atomic state at lower temperature in contrast to mostly in molecular state at higher temperature. FCC structure of the fully hydrogenated alloy gradually transformed into BCC structure accompanied with hydrogen desorption. The phase constituent of the quaternary TiCri 8-2xMnxVx alloy from single Laves phase gradually became the coexistence of Laves phase and BCC phase with increasing X, which resulted in the increase of maximum hydrogen storage capacity. The hysteresis between hydrogen absorption and desorption was also enhanced simultaneously. The hydrogen storage performances of five element alloys (TiCrVFeM, M=Ni, Mo, Mn and Zr) were also investigated. The increasing Ni addition resulted in the shrinkage of crystal cell, decrease of hydrogen storage capacity and deterioration of thermodynamics of hydrogen absorption of the alloys. The increasing substitution of Zr for Ti increased the crystal parameters and decreased the hydrogen storage capacity. The introduction of Mo had similar effect on the characteristics of hydrogen storage alloys. The Mo addition resulted in the enhancement of hysteresis between hydrogen absorption and desorption. Both the as-cast and the melt-spinning TiCrVFeMn alloy had BCC and Laves phases compositions before hydrogenation. In contrast, the hydrogenated melt-spinning alloy had a duplex phase composition of BCC phase and TiH phase, while there was simply Laves phase in the hydrogenated as-cast alloy. The melt-spinning alloy had higher hydrogen storage capacity and higher hysteresis between hydrogen absorption and desorption than the as-cast alloy.