Microstructure,Hydrogen Absorption/Desorption Behavior and Structural Stability of Mg-based Alloys

Author:Jie Li Shuai

Supervisor:li jin shan


Degree Year:2018





Mg with high theoretical hydrogen capacity,environmental friendliness and low cost is generally regarded as one of the most promising candidates for hydrogen storage applications.But high dehydrogenation temperature,slow absorption/desorption kinetics,poor cycling stability and sensitiveness to oxygen limit the large-scale application of Mg.The hydrogen storage properties of Mg have been improved by alloying,catalysis and crystalline refinement.However,a single method can not make the de-/hydrogenation performance of Mg meet the requirements of on-board hydrogen storage system.Researchers still hold conflicting opinions on limiting steps and determining factors of MgH2 dehydrogenation.The lack of effective measures to improve the oxidation resistance of Mg severely impedes the long-life application of Mg based alloys.Considering phase composition optimization and microstructure refinement,Mg-riched Mg-Ni-Ce alloys with different Ni and Ce contents have been prepared under the protection of covering regent in the resistance furnace.The as-cast alloys are high-energy ball milled for 2 h to achieve microstructure refinement.After activation,nanocrystallin Mg-based composites with uniformly distributed Mg2Ni and CeH2.73 are obtained.The effects of alloying and high energy ball milling on the phase composition,microstructure,activation performance,hydrogen absorption/desorption kinetics and cycle stability of Mg based alloys have been investigated in this work.Experimentally systematical comparisons are carried out to clarify the dehydrogenation steps of Mg-based hydrogen storage alloys during the overall desorption process.This work also attempts to explore the ameliorate oxidation resistance through microstructural evolutions during air contact and absorption/desorption kinetics change of Mg-based hydrogen storage alloys with different Ce contents.The main research contents and the innovation results are as follows:XRD,SEM,TEM and EPMA methods are employed to characterize the microstructure and phase composition of as-cast and ball milled Mg-Ni-Ce alloys.It is found that,for Mg-20Ni and Mg-20Ce alloys,the eutectic Mg/Mg2Ni and Mg/Mg12Ce mixtures distribute as a network and both alloys preserve lamellar microstructure,which result from the eutectic reaction between Mg and Ni/Ce.A new 18R-type long-period stacking ordered phase(LPSO)was formed coherently with Mg,Ni-substituted Mg12Ce and Mg2Ni in Mg-riched Mg-Ni-Ce ternary alloys.The 18R-type LPSO structure is a variant of Mg12Ce rather than Mg and the orientation relationship between the Mg12Ce matrix and the 18R phase is(002)Mg12Ce//(0018)18R.HEBM produces some Mg,Mg2Ni and Mg12Ce nanocrystals with grain size less than 8 nm,which probably decompose from the LPSO phase during HEBM process.After 2 h high energy ball milling,the mean particle size of each sample is30μm and nanocrystallin Mg-based composites are obtained.Three cycles of hydrogen absorption and desorption at 350℃are enough to fully activate ball milled samples.In the first hydrogen absorption,Mg12Ce reacts with hydrogen to form MgH2 and CeH2.73.CeH2.73 remains stable during subsequent hydrogen absorption and desorption.Mg2Ni absorbs hydrogen to form Mg2NiH0.3 and Mg2NiH4,with garin size10 nm.A large number of nanocrystals of Mg,Mg2Ni and CeH2.73 with grain size less than 10 nm are observed in as-activated Mg-Ni-Ce ternary alloys.It is found that CeH2.73 facilitates hydrogenation of Mg better than Mg2Ni.Conversely,Mg2Ni is more conducive to desorption.A synergistic catalytic effect between Mg2Ni and CeH2.73 is observed.It is the differences of composition and microstructure evolution that dictate the discrepancies of cycling behavior among Mg-20Ce,Mg-20Ni and Mg-5Ni15Ce alloys.The average particle size of each experimental alloy decreases gradually during 100 absorption/desorption cycles at320℃.Correspondingly,more newly exposed secondary phases uniformly distribute on the particle surface.The grain size of Mg increases significantly for binary alloys,while introducing Mg2Ni and CeH2.73 simultaneously can effectively inhibit the growth of Mg grains.Both the absorption kinetics and absorption capacity improve gradually for Mg-5Ni15Ce alloy,while the desorption kinetics remain stable.But the absorption/desorption kinetics and absorption capacity deteriorate in varying degrees for Mg-20Ni and Mg-20Ce binary alloys.The effects of Mg nucleation,hydrogen diffusion and hydrogen recombination on the overall dehydrogenation process of MgH2-CeH2.73 composite are investigated by control experiments.The MgH2-CeH2.73 particles with different grain/particle sizes preserve extremely close desorption activation energies and kinetics,indicating that the desorption kinetics does not seem to be associated with hydrogen diffusion in Mg.For partially dehydrogenated samples,the desorption temperature and desorption activation energy decrease significantly considering the fact that primary-precipitated metal Mg phase on the surface of MgH2 can act as nucleate precursors.As minor Ni is distributed on the surface,both onset and peak temperatures in thermal desorption decrease for MgH2-CeH2.73 composite.The reduced activation energy by Ni addition is comparable to the value caused by partial dehydrogenation.Recombination of hydrogen atoms and Mg nucleation play important roles during dehydrogenation of Mg based alloys.CeH2.73 spontaneously transforms into CeO2 during air exposure,preventing forming a continuous and dense magnesium oxide layer on the particle surface.The formed CeH2.73/CeO2 particles act as catalysts accelerating the hydrogen dissociation and nucleation sites for the MgH2 formation during hydrogenation.The absorption/desorption kinetics and capacity of air-exposed samples improve with the increase of Ce content.For Mg-20Ce alloy,the desorption activation energy and absorption capacity of air-exposed sample is extremely close to those of the as-activated one,suggesting superior oxidation resistance.But the hydrogen storage properties of Mg-20Ni alloy deteriorate significantly.The effects of CeO2 on the oxidation resistance of MgH2 are investigated by high energy ball milling MgH2 with or without CeO2(20 wt%)in Ar and air atmosphere,respectively.Minor CeO2 reacts with MgH2 to form CeH2.73 and MgO during ball milling in Ar.Both MgH2+CeO2 milled in Ar and air can rapidly absorb and desorb hydrogen.MgH2+CeO2 milled in air can absorb5 wt%hydrogen within 5 min at300℃.However,only 1wt.%hydrogen can be released within 20 min for pure MgH2milled in air.It is believed that the uniformly distributed cerium oxides,which serve as catalysts promoting recombination of hydrogen and nucleation of new phase,can effectively prevent forming a dense and continuous MgO layer on the MgH2 surface during air exposure due to the unique instincts of CeO2.