Effect Of Pr₃Al₁₁ On Crystallite Growth Behavior Of Magnesium Based Hydrogen Storage Materials

Based on the review of the development and research status of Mg based hydrogen storage materials,the nanostructured Mg-based composite were prepared by mechanical ball milling,and the effect of Pr₃Al₁₁nanoparticles on crystallite growth behavior of nano Mg-based hydrogen storage materials was systematically studied by means of X-ray diffraction（XRD）,transmission electron microscope（TEM）,Rietveld full-spectrum fitting and gaseous P-C-T hydrogen storage test.Relevant research contents and results are as follows:（1）In order to understand the effect of Pr₃Al₁₁nanoparticles on isothermal crystallite growth kinetics of nanocrystalline Mg,the crystallite growth characteristics in a Mg-Pr₃Al₁₁composite and a pure Mg sample were comparatively investigated in this work.The crystallite growth exponents of nanocrystalline Mg in the Mg-Pr₃Al₁₁composite and pure Mg were determined as n=5 and n=4,respectively.Meanwhile,the activation energy for crystallite growth in Mg-Pr₃Al₁₁composite was calculated to be 118.8 k J/mol,which is higher than 97.1 k J/mol in pure nanocrystalline Mg.Further studies reveal that the rise of crystallite growth exponent and increase of activation energy in the Mg-Pr₃Al₁₁composite are primarily attributed to the pinning effect of Pr₃Al₁₁nanoparticles at crystallite boundaries of Mg.（2）Similarly,to clarify the growth characteristics of MgH₂,the growth characteristics of MgH₂nanocrystallites in pure MgH₂and MgH₂-10 wt%Pr₃Al₁₁samples were comparatively investigated in the present work.The growth exponents of MgH₂nanocrystallites in pure MgH₂and MgH₂-10 wt%Pr₃Al₁₁samples were evaluated to be n=5 and n=6,respectively.Their activation energies for crystallite growth were also determined to be 109.2 k J/mol and 144.2 k J/mol,respectively.The increase of crystallite growth exponent and rise of activation energy for crystallite growth in MgH₂-10 wt%Pr₃Al₁₁composite are ascribed to the inhibition role of nano-sized Pr₃Al₁₁phase in MgH₂crystallite growth.（3）Mg crystallites grow up very fast during hydrogen desorption,leading to the degradation of hydrogen storage properties.Therefore the growth behavior and mechanism of Mg crystallites during hydrogen desorption of nanocrystalline MgH₂were investigated in this work.It was found that the transformation from MgH₂to Mg occurred by the surface-controlled‘nucleation and growth’mechanism.After the instantaneous nucleation of Mg at free surfaces of MgH₂particles,Mg crystallites grew through three stages,namely one-dimensional,then two-dimensional and finally one-dimensional growths.In the second stage,Mg crystallites grew quickly as compared with other stages.Meanwhile,after complete hydrogen desorption,the average Mg crystallite size in MgH₂-10 wt%Pr₃Al₁₁sample was smaller than that in pure MgH₂sample due to the presence of Pr₃Al₁₁.