Research On The Inlfuence Of Soft-chemistry Technology On Microstructure Of The Nano Precursors Of La-Mg Hydrogen Storage Alloy

Sweeping through the world in the1970s oil crisis prompted countries beganto seek new and sufficient energy. Hydrogen as a rich source of high energydensity,clean energy,would have to cause the attention. Phillips Laboratory of theNetherlands found that the LaNi5materials with reversible characteristics ofabsorption and release hydrogen in1970, and have high safety performance. Theyalso noted that the hydrogen storage materials have broad prospects fordevelopment,which cause lots of people to research on them. Soft chemicalsynthesis method has achieved great develop in prepareding hydrogen storagematerials.Soft chemical routes have been reported to prepare alloys with goodhomogeneity, high purity and sub-micron size particles, which are importantfactors for high hydrogen storgen capacityIn this article, soft chemical is adopted to synthesis La-Mg hydrogen storagealloys precursor and its structures were characterized.Nano precursor powders wereprepared by the sol-gel method using lanthanum nitrate （La（NO₃）3·6H₂O） analyticalgrade) and magnesium nitrate （Mg（NO₃）₂·6H₂O analytical grade） as startingmaterials, distilled water as solvent and citric acid （C6H8O7·H₂O） as chelatingagent.The effect of amount of citric acid and calcination temperature onmicrostructure of the nano precursors of La-Mg hydrogen storage alloy had beenstudied by Infrared Radiation （IR）,thermogravimetric and differential thermalanalysis （TG/DTA）, X-ray diffraction analyzer （XRD） and Transmission electronmicroscopy （TEM）;The results shows the molar ratio of citric acid to metal cationsincreases from1to1.5, the amount of citrate increases correspondingly, when themolar ration increases to2.0, the amount of citrate goes up to maximum. Hydrogenof hydroxyl in carboxyl is easy to run away, the COO-can form stable chelates（-O-M-）with metal cations. The chelates can easily form citrate which is of threedimensional skeleton structure.With the increase the quantity of carboxyl groups,much more carboxyl groups are used to complex metal cations and then form citrate,so the quality of citrate is increase.TG/DTA results show that with increase ofcalcination temperature the amount of impurity phase, La（OH）₃, decreasesmarkedly. There is the presence of impurity phase, La（OH）3, calcinated at750℃and850℃. When calcinate at950℃, impurity phase disappear and only La2O3andMgO two-phase mixture present and the nano powders with good crystallization.The effect of pH value on microstructure of the nano precursors of La-Mghydrogen storage alloy had also been studied. At the pH value of0.5and1.48, themulti-stage ionization of citric acid are restrained, which result that carboxyl groups complex metal cations insufficiently. In basic solution, the ionization ofcitric acid will be accelerated and the ability of citric acid complexing metalcations will be improved. At the pH value of0.5and1.48,the gel is preparedwithout the addition of ammonia or NH₄NO₃as fuel for self propagating, whichresult in local burning and longer combustion, the combustion takes place with lowrate of the flame propagation, which caused the longer combustion time and thegrain initial synthesized have long time to grow up then the non-uniform particlesize are formed. At the pH value of8.0and9.0,the grain synthesised instantly andhad no time to grow up because of the gel combustion rapidly. The average particlesize of powders is about30nm at the pH of8.0.