Effect Of Mg Content On Absorption-desorption Characteristics Of Sm-Mg-Ni Compounds

Based on the research progress and existing problems of hydrogen storage compounds,this thesis systematically investigated the effect of Mg content on the structural and absorption-desorption characteristics of AB_y?y=2,3,3.5?type Sm_2-x Mg_x Ni₄,Sm_3-x Mg_x Ni₉,Sm_4-x Mg_x Ni₁₄?x=0,0.25,0.5,0.75?compounds by the combination of XRD?X-ray diffraction analysis?,the Retiveld refinement program,the pressure-composition-temperature?P-C-T?testing apparatus and transmission electron microscopy?TEM?,providing a clear interpretation of internal relations between the structure features and the absorption/desorption characteristics of these compounds.Furthermore,the main research conclusions are given as follows:?1?Employing intermetallic SmNi_x?SmNi,SmNi₂,SmNi₅?powders,Mg powders and Ni powders as starting materials,Sm_2-x Mg_x Ni₄,Sm_3-x Mg_x Ni₉ and Sm_4-x Mg_x Ni₁₄?x=0,0.25,0.5,0.75?compounds were successfully synthesized by a process of ball-milling and subsequent sintering.The abundance of stoichiometric phases in the sintered products were high enough to satisfy our research.The results of structure characterization depicted that Mg atoms preferred to occupy the A site in the[A₂B₄]subunit while not in the[AB₅]subunit.Moreover,for AB_y phases that had the same structure type and various y values,lattice parameters and subunit sizes would decrease with Mg concentration.Moreover,this behavior would be more obvious in the case of a smaller y value.?2?The P-C-T measurements were carried out for virgin samples under a hydrogen pressure of 6 MPa at 298 K,corresponding curves indicated that the hydrogen absorption plateaus would be elevated and the hydrogen absorption/desorption reversibility would also be improved as increasing the Mg concentration in the compounds.When the x value was constant,a larger y value was associated with a better hydrogen absorption/desorption reversibility.According to the P-C-T curves,it is unusual that two hydrogen absorption/desorption plateaus were observed for Sm_4-x Mg_x Ni₁₄?x=0.25,0.5,0.75?compounds.However,only one plateau was observed in Sm_2-x Mg_x Ni₄,Sm_3-x Mg_x Ni₉(AB₃ phases)and Sm₂Ni₇ compounds.?3?One P-C-T hydrogen absorption/desorption cycle was performed for all the compounds concerned in this study.A P-C-T cycle lead to absolute amorphization in the SmNi₂ and Sm_1.75Mg_0.25Ni₄ compounds and amorphous hydride were formed consequently.Otherwise,partial amorphization and hydrogenation induced micro-phase separation?HIMPS?occured in the Sm_1.5Mg_0.5Ni₄ compounds,with amorphous hydride,Sm_1.5-x Mg_0.5+x Ni₄H_y phase and Ni-rich phase formed after one cycle.Structure of SmNi₃ compounds was partially transformed into SmNi₃H_x phase which has a different space group after one cycle.While for other compounds that have different compositions,original structures were retained after one P-C-T hydrogen absorption/desorption test.?4?Hydrogen absorption/desorption cycling measurements and corresponding structure characterization and calculation were performed for those compounds where amorphization,HIMPS and structure transformation didn't occur after a P-C-T hydrogen absorption/desorption test.After 50 hydrogen absorption/desorption cycles,the capacity retention rate of Sm_3.25Mg_0.75Ni₁₄,Sm_3.5Mg_0.5Ni₁₄,Sm_3.75Mg_0.25Ni₁₄,Sm_2.25Mg_0.75Ni₉,Sm_2.5Mg_0.5Ni₉,Sm_2.75Mg_0.25Ni₉,Sm₂Ni₇ and Sm_1.25Mg_0.75Ni₄ were 98%,97%,72%,87%,71%,66%,44%and 44%,respectively.This trend was mostly ascribed to the preferred occupation of Mg atoms and a higher fraction of[AB₅]subunits,both of which could restrict the lattice strain upon cycles,thus leading to a better cycle stability.