| Hydrogen atoms mainly occupy the interstitial site among metal atoms in metal hydrides. In comparison with FCC and HCP structures, BCC structures have more interstitial site to hold much more hydrogen. Mg and Ti are more popular metallic elements which form stable hydrides because of their higher hydrogen capacities (theoretically up to7.6wt%) and lower costs (they form MgH2and TiH2hydrides including7.6mass%and4.0mass%hydrogen, respectively), so that MgH2and TiH2hydrides are excellent materials for vehicle fuel and nickel-metal hydride batteries. In the present work, we have explored the preparation method to synthesize Mg-Ti BCC alloys and tried to improve the hydrogen storage performance of these alloys by adding different kinds of additives.Mg76Ti12Ni12-xCrx (x=0,3,6,9) alloys were prepared by mechanical alloying. For Mg76Ti12Ni12alloy, the binary alloy phase of Mg2Ni and Ti2Ni phase formed after milled30h. Besides Ti2Ni phase and Mg2Ni phase, Cr1.75Nio.25Ti phase also formed in the Mg76Tii2Ni12-xCrx (x=3,6,9) alloys. For Mg76Ti12Ni12-xCrx (x=0,3,6,9) alloys, the amorphous degree increased with the increase of Cr content. The hysteresis coefficient of hydrogen absorptiondes-orption decreased, as well as the hydrogen absorption rate could be improved by adding proper Cr content.The Ti and Mg content were changed based on Mg76Ti,2Ni9Cr3alloy. Mg76-xTi12+xNi9Cr3(x=4,8,12,16) alloys were prepared by mechanical alloying. For Mg76-xTi12+xNi9Cr3(x=4,8,12,16) alloys, the main binary alloy phase was Mg2Ni and Ti2Ni after milled. And the hydrogen storage capacity was3.93wt.%,3.82wt.%,3.64wt.%and2.81wt.%after milled20h. However, it decreased to2.36wt.%,2.16wt.%,1.81wt.%,2.0wt.%when the milling time increased to80h. It was found that the hydrogen storage capacity decreased with the increase of the milling time and the ratio of Ti/Mg. It was also found from DTA results that the hydrogen desorption temperature of the Mg76-xTi12+xNi9Cr3(x=4,8,12,16) hydride were550K,528K,518K and506K, respectively. The hydrogen desorption temperature decreased with increasing the rate of Ti/Mg. The decomposition enthalpies of the Mg76-xTi12+xNi9Cr3(x=4,8,12,16) hydride were-80.2kJ/mol,-78.5kJ/mol,-73.5kJ/mol and-80.0kJ/mol, respectively. The decomposition enthalpy decreased with the increase of Ti content firstly, then increased with increasing Ti content. The stability of the hydride can be reduced with proper Ti content.Mg76-xTi12Ni12Mnx (x=2,4,6,8) alloys were prepared by mechanical alloying and the effects of Mn content on the hydrogen storage properties were investig-ated systematically. For Mg76-xTi12Ni12Mnx (x=2,4,6,8) alloys, the main binary alloy phase consist of Mg2Ni and Ti2Ni too. It was found that TiMn2phase appeare when x=6and x=8. The hydrides decomposition enthalpies of Mg76-xTi12Ni12Mnx (x=2,4,6,8) alloys were-79.2kJ/mol,-78.0kJ/mol,-73.7kJ/mol and-73.6kJ/mol. The hydrogen storage capacity increased, as well as the stability of the hydride reduced with the addition of the Mn element. The thermodynamics performance was also improved with addition of the element Mn.According to the good hydrogen storage properties of Mg70Ti12Ni12Mn6alloy mentioned above, Mg70-xTi12+xNi12Mn6(x=8,16,24,32) alloys were prepared by mechanical alloying. The alloys were mainly composed of HCP and FCC structures after ball-milling for50h. When the milling time increased to100h, it was shown that the diffraction peak of Mn element disappeared, as well as the diffraction peak of Mg and Ni weakened gradually. However, the peak intensities of Ti increase significantly. It was suggested that Mg, Ni and Mn dissolve into Ti lattice to form solid solution. When the milling time increased to200h, X-ray diffraction (XRD) testing showed that the diffraction peak of Mg2Ni occurs, and the diffraction peak of Mg and Ni decreased while the intensity of X-rays diffraction pattern of Ti was improved at2θ=35.3°,38.4°,40.2°nd43.1°. These results demonstrated that Mg and Ni further dissolve into Ti lattice. For Mg46Ti36Ni12Mn6and Mg38Ti44Ni12Mn6alloys, the peaks with the BCC structure appeared at20=43.1°,63.1°and70.7°. After ball-milling for400h, the diffraction peak of Mg disappeared, and part of the diffraction peak of Ni also disappeared, only FCC and BCC structure exist. These results were also due to that Mg and Ni further dissolved into Ti lattice. And, the FCC structure which corresponded to Ni was due to Ni element not yet diffusing into Ti lattice. The hydrogen storage capacity of Mg46Ti36Ni12Mn6alloy with BCC structure was1.36wt.%at573K, and the dehydriding temperature was lower than that of Mg62Ti2oNi12Mn6, Mg54Ti28Ni12Mn6and Mg38Ti44Ni12Mn6hydrides.To improve the hydrogen storage performance of BCC solid solution Mg46Ti36Ni12Mn6alloy, we added xwt.%TiF3(x=2,5,8,11) to Mg46Ti36Ni12Mn6alloy. The hydrogen storage capacity of Mg46Ti36Ni12Mn6+xwt.%TiF3(x=2,5,8,11) was1.65wt.%,2.33wt.%,1.95wt.%and1.87wt.%, respectively. Among these alloys, the Mg46Ti36Ni12Mn6+5wt.%TiF3alloy presented the best hydrogen storage performance.To study the effects of different kinds of catalyst on the hydrogen storage performance of the Mg46Ti36Ni12Mn6alloy with BCC structure,5wt.%M (M=LiH,TiF3,Nb205and C) was added. For Mg46Ti36Ni,2Mn6+5wt.%M (M=LiH,TiF3,Nb205and C) alloys, X-ray diffraction studies indicated that the full width at half maximum (FWHM) of main diffraction peaks corresponding to Mg46Ti36Ni12Mn6+5wt.%C was the smallest. The results revealed that the crystal lattice distortion of Mg46Ti36Ni12Mn6alloy was the least when adding C, the next was TiF3, LiH and Nb2O5. After ball-milling for200h, the hydrogen storage capacity of Mg46Ti36Ni12Mn6+5wt.%M(M=TiF3,C,Nb205,LiH) alloys were2.33wt.%,0.68wt.%,2.36wt.%and1.49wt.%respectively. Among these alloys, the particle size of Mg46Ti36Ni12Mn6+5wt.%M(M=C,LiH) was smaller. The result implied that C and LiH could raise mechanical milling efficiency but did not improve the hydrogen storage performance. However, the hydrogen storage performance can be improved by adding TiF3and Nb2O5. It could be seen from the DTA results that the initial exothermic temperature for Mg46Ti36Ni12Mn6+5wt.%M(M=TiF3,C,Nb205,LiH) hydride were568K,608K,535K and627K, respectively.The effect of metal hydride on hydrogen storage properties of Mg46Ti36Ni12-Mn6alloy were investigated By adding5wt.%M (M=MgH2, CaH2, LiAlH4, NaH). It could be found that the BCC structure came into being after ball-milling for200h. SEM results showed that the particle size was smaller, the solubility was better and the efficiency of amorphousness was higher when NaH and CaH2other than MgH2and LiAlH4were used as additives. Furthermore, other elements were easier to dissolve into the Mg-Ti matrix and to form stable BCC structure when NaH and CaH2were used as additives. At303K, the hydrogen storage capacity of Mg46Ti36Ni12Mn6+5wt.%M(M=MgH2, CaH2, LiAlH4, NaH) alloys were2.78wt.%,2.16wt.%,0.71wt.%and0.85wt.%, respectively, and were3.1wt.%,2.76wt.%,3.2wt.%and1.95wt.%as the temperature increased to573K. That was to say, the hydrogen storage capacity increased with increasing the temperature. The differential thermal analysis (DTA) results showed that the first endothermic peaks of Mg46Ti36Ni]2Mn6+5wt.%M(M=MgH2, CaH2, LiAlH4, NaH)hydride were534K,517K,521K and553K, respectively. The hydrogen desorption temperature were deduced by adding MgH2, CaH2, LiAlH4and NaH additives.The effect of substituting elements M (M=Co, Cr, Fe, V) for element Mn on hydrogen storage performance of Mg46Ti36Ni12M6alloy has been studied. For Mg46Ti36Ni12Cr6, Mg46Ti36Ni12Fe6and Mg46Ti36Ni12V6alloys, BCC structure, FCC structure as well as diamond cubic structure were observed, and the BCC structure appeared at29=40.4°,58.6°nd73.9°. For Mg46Ti36Ni12Co6alloy, they were only BCC structure and Primitive Cubic structure, not FCC structure and diamond Cubic structure, and the BCC structure appeared at2θ=42.9°and62.2°. The crystal lattice parameter of the BCC solid solution of Mg46Ti36Ni12M6(M=Co,Cr,Fe,V) alloys were2.9828,3.1475,3.1403and3.3764nm, respectively. The crystal lattice parameter increased with the increase of atomic radius of substitute elements.It was found that synthesized Mg46Ti36Ni12M6(M=Co, Cr, Fe, V) alloys were able to absorb hydrogen up to1.14wt.%,1.59wt.%,1.55wt.%and2.38wt.%at303K, respectively. And it increased to1.95wt.%,2.25wt.%,2.01wt.%and2.95wt.%when the temperature rised to573K. The hydrogen storage capacities of Mg46Ti36Ni12M6(M=Co, Cr, Fe, V) increased with the increasing of the substitution elements atomic radius. The initial hydrogen desorption temperature of Mg46Ti36Ni12M6(M=Co,Cr,Fe,V) hydride was460K,441K,441K and453K, and, the first endothermic peak appeared at624K,530K,514K and569K, respectively. |
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