LEC Preparation Method Of La-Mg-Ni Based Hydrogen Storage Alloys And The Electrochemical Properties Of The La_0.75Mg_0.25Ni_3.5-xM_x (M=Si, Zn) Hydrogen Storage Alloys

In this thesis, previous research works on the La-Mg-Ni based hydrogenstorage alloys have been extensively reviewed . On this basis, the La-Mg-Nibased hydrogen storage alloys are researched as followed:At first, the effect of replacing part of Ni by Si and Zn in La_0.75Mg_0.25Ni_3.5 on the structure and electrochemical properties of thus formedLa_0.75Mg_0.25Ni_3.5-xM_x (M=Si,Zn,x=0.05～0.5) quaternary alloys wasinvesgated. XRD analyses shows that La_0.75Mg_0.25Ni_3.5-xSi_x consists ofCaCu₅ phase, Ce₂Ni₇ phase and LaMgNi₄ phase, La_0.75Mg_0.25Ni_3.5-xZn_xconsists of CaCu₅ phase, Ce₂Ni₇ phase and LaNi₃ phase. The increase of Sicontent leads to an increase of content of LaNi₅ phase. The increase of Zncontent leads to an decrease of content of Ce₂Ni₇ phase, but the LaNi₃phase and the CaCu₅ phase increase at same time. The electrochemical studyindicate that the high rate dischargeability (HRD) of the alloy electrodesdecreases when ether the two element was added. The maximum dischargecapacity and the exchange current density(I₀) of the alloy electrodesdecreases ,but the activation properties increases and the cyclic stability ofthe alloys has been improved with the increase of Si content. The maximumdischarge capacity of the alloy electrodes little decreased when the Zn wasadded.Secondly, the LEC preparation technics of the La-Mg-Ni based hydrogenstorage alloys is researched. It is feasible that the La-Mg-Ni based hydrogenstorage alloys is prepared by the LEC method, when we strict control thesmelting current, the smelting time and the pressure of the gas. At sametime, we have known the perfect technics. The content of Mg averagelylosses 1.08%, it is better than the old method. So, the content could becontrolled in a exact range, the average error is smaller than 0.13 centigrade.The boil and the surface tension of liquid is the great factor effecting thelosing of the content. The alloys is protected when the LEC method is usedin the anneal. The loss of the Mg is higher with the longer time and highertemperature. The LEC method can clearly reduce the loss of Mg than the oldmethod. The loss of LEC method cut down 62% at same condition.Lastly, we try to find out the shortage of the LEC method by researchingthe structure and electrochemical properties of the alloys. Then, we can improve the technics. The XRD result showed that the alloys prepared byLEC method were composed of Gd₂Co₇-type, Ce₂Ni₇-type, LaNi₅-type andLaNi₃-type phases. The anneal process has 3 major transformation: thetransformation from LaNi₃-type and LaNi₅-type to Ce₂Ni₇-type; thetransformation from Ce₂Ni₇-type to LaNi₅-type because of the Mg content loss; thetransformation from Ce₂Ni₇-type to Gd₂Co₇-type. The A₂B₇-type of the alloysmostly is Gd₂Co₇-type, if the alloys is cooled in the air. Annealing treatmentcould obviously improve overall electrochemical properties of alloyelectrodes. The reasons are that the anneal treatment can reduce theLaNi₃-type, and homogenize the alloys. But it is limited that theelectrochemical properties of alloy electrodes is more improved, becausethe alloys is not cooled in the water cooled pattern and the Mg content loss.