Formation Mechanism Of Over Saturated Solid Solution A1(Ti) And Its Effect On The Reversible Hydrogen Absorption And Desorption Of NaAlH4

The main purpose of this paper is to improve the preparation method of NaAlH4 and to study its hydrogen storage properties.The dopant-NaAlH4 hydrogen storage materials were produced by mechanical milling and annealing process,and the effects of preparation technology and doping method on the hydrogen storage properties of doped-NaAlH4 were studied.Firstly,the supersaturated solid solution of Al(Ti)was produced by the addition of the process control agent in the ball milling process,and the solid solubility is about 2.2at%.During the milling process,the formation of Al(Ti)will undergo nucleation,growth and disappearance of the thin layer TiAl3 alloys.TiAl3 not only can promote the diffusion of Al,but also can improve the shedding rate of Ti particles increasing the contact surface with Al during the ball milling process,which plays a very important role in the formation of the supersaturated solid solution of Al(Ti).The first principle calculation method was used for Al(Ti)to investigate the reason of the lattice volume shrinkage,the low solid solubility limit,and the achieving a higher supersaturation.The formation process and mechanism of the supersaturated solid solution of Al(Ti)were studied by the theoretical and experimental methods.When annealed at 400 for 5h,the supersaturated solid solution of Al(Ti)will precipitate out the Al and small TiAl3 particles.Secondly,Ti-NaAlH4 was synthesized with the supersaturated solid solution of Al(Ti)and the commercial NaH by high-energy ball milling,and the effect of pre-milling time for the synthesis of Ti-NaAlH4 system was studied.The study found that with the increase of pre-milling time,the amount of NaAlH4 was increased.The only Na3AlH6 was synthesized for the no pre-milling samples and pre-milling 10h samples,the initial dehydriding capacity of which is only 1.33wt%and 1.62wt%respectively.The high-content NaAlH4 was synthesized for pre-milling 30h,50h and 70h samples,the initial dehydriding capacity of which is 3.48 wt%,4.91 wt%and 4.96 wt%respectively.With the increase of pre-milling time,two step dehydrogenation activation energy of NaAlH4 is more and more low,compared with the pre milling 30h samples,the first step and second step activation energy for the pre-milling 70h samples were decreased by 42kJ/moland 34 kJ/mol,respectively.With the increase of the pre-milling time,the hydrogen absorption kinetics of the samples was also increased,and the amount of hydrogen absorption for no pre-milling samples was about 1.51wt%during 3h,and for the pre-milling 70h samples was about 3.63 wt%.Finally,TiAl3-NaAlH4 was synthesized with the Al-TiAl3 precipitated out the supersaturated solid solution of Al(Ti)and the commercial NaH by high-energy ball milling,and the effects of different annealing conditions on the hydrogen storage properties of TiAl3-NaAlH4 system was studied.The study found that the only Na3AlH6 was synthesized with the pure commercial Al powder,TiAl3 powder and NaH powder via high-energy ball milling for 40h,the initial dehydrogenation of which is only 1.69wt%.The high-content NaAlH4 can be synthesized with the mixed powder annealed at 400 ? for 5h and the commercial NaH via high-energy vibrational ball milling for 40h,the initial dehydrogenation of which reached 4.50wt%.The dehydrogenation activation energy of the former sample was 173kJ/mol(only the second step),the first step and the second step dehydrogenation activation energy of the latter sample were 112kJ/mol and 74kJ/mol.The kinetics of hydrogen absorption after dehydrogenation of the samples with the annealed process has a great improvement.For example,the capacity of hydrogen absorption of the sample without the annealed process is only 1.40 wt%during the 3h,and the capacity of hydrogen absorption of the sample with the annealed process is 3.40wt%.