The Hydrogen Storage Alloy Positron Annihilation And Perturbed Angular Correlation Study

Positron annihilation and perturbed angular correlation are two nuclear analytical techniques based on the mass-energy transformation of positron-electron annihilation and the hyperfine interaction of nuclei with the electric field produced by the neighboring electrons and nuclei, respectively. These techniques are capable of investigating microstructure of materials on an atomic scale and become indispensable methods in condensed matter physics and materials science.Hydrogen storage alloys are important materials for energy, weapon and engineering. The investigation of microstructure and hydrogen behavior in hydrogen storage alloys is a currently interesting topic in materials science.In the present work two hydrogen storage alloys Pd_0.75Ag_0.25 å'Œ LaNi_4.25Al_0.75 are studied by the positron annihilation and perturbed angular correlation techniques.The Pd_0.75Ag_0.25H_x alloys with a hydrogen concentration ranging from 0 to 0.35 are investigated by the positron annihilation lifetime measurement method in the temperature region between 77-295 K. The measured lifetime spectra are well characterized by two positron annihilation lifetimes. The short lifetime Ï„₁ is ascribed to the free positron annihilation lifetime and is independent of temperature and hydrogen concentration. This indicates that no phase transition occurs. The long lifetime Ï„₂ is attributed to the one of annihilation of positrons trapped by the defects of hydrogen bubbles. Ï„₂ and its intensity I₂ do not vary with the temperature, while Ï„₂ increases and I₂ decreases with the increase of hydrogen concentration. The obtained results reveal the growth of the hydrogen bubbles in size and the reduction of bubble concentration with the increase of the hydrogen concentration.The Pd_0.75Ag_0.25H_xalloys are also studied by the perturbed angular correlation method at room temperature. The quadrupole interaction frequency is determined to be zero for thePd_0.75Ag_0.25 alloy with no hydrogen charging, showing that the 111Ag/111Cd probe nuclei are located in the substitutional site in the Pd_0.75Ag_0.25 alloy with a fcc structure, and do notexperience any electric field gradient interaction. After hydrogen charging a part of the probe nuclei feel perturbation from hydrogen bubble defects, and the quadrupole interactions are observed at the hydrogen concentrations of x =0.1, 0.2 and 0.35. The experimental results illustrate that the quadrupole interaction frequency increases and its intensity decreases with the increase of hydrogen concentration, demonstrating the growth of the hydrogen bubble size and the reduction of bubble concentration with the hydrogen concentration increase. The width of the quadrupole interaction frequency distribution is measured as well, which shows the hydrogen diffusion effect to some extent. The measured widths tell us that diffusion effect is less with the hydrogen concentration increase.The experimental results obtained by the positron annihilation and perturbed angular correlation are in good agreement. In addition the hydrogen diffusion is detected by the perturbed angular correlation.The perturbed angular correlation is used to investigate at room temperature the LaNi4.25Alo.75Hx alloys with the concentrations of x = 0 and 3. Only one quadrupole interaction frequency of "i= 14.39 Mrad/s (V1^ 10.1 X 1017V/cm2) is detected for the LaNi4.25Alo.75 alloy without hydrogen charging. An additional quadrupole interaction frequency of "2= 20.63 Mrad/s (V2ZZ= 14.48X 1017V/cm2) appears in LaNi4.25Alo.75Hx with the hydrogen concentration of x = 3. The fraction of w 1 is fi = 25.1%, and f*2= 74.9% for d>2. The measured U| is ascribed to the probe nuclei whose neighboring interstitial site are filled with no hydrogen, and w 2 is interpreted as related to the probe nuclei in the basal plane next to hydrogen-filled octahedral interstitial sites in the same plane. The experimental results show that hydrogen fills preferably the octahedral interstitials, and the partial substitution of Al for Ni would cause the lattice expansion and deformation, which favors the filling of hydrogen into octahedral interstitials.The Pdo.75Ago.25Hx alloys are investigated by both the positron annihilation and perturbed angular correlation for the first time and the experimental results obtained by them are well consistent. The present work first investigates the LaNi4.25Alo.75Hx alloyswith 14OLa-14OCe probe nuclei directly produced in the alloys by nuclear reaction, and observe the preferable filling of hydrogen into octahedral interstitials.