Enhanced kinetics of hydride-metal phase transition in magnesium by vacancy clustering

R. CHECCHETTO; N. BAZZANELLA; A. KALE; A. MIOTELLO; S. MARIAZZI; R. S. BRUSA; P. MENGUCCI; C. MACCHI; A. SOMOZA; W. EGGER; L. RAVELLI

PHYSICAL REVIEW B

AMER PHYSICAL SOC

Lugar: New York; Año: 2011 vol. 84 p. 1 - 7

1098-0121

The relation between vacancies and vacancy clusters evolution and the H2 desorption kinetics was studied in nanocrystalline Mg samples submitted to successive H2 sorption cycles. Vacancy defects were detected by positron annihilation lifetime spectroscopy while the desorption process was monitored measuring the H2 desorption flux. During H2 sorption cycles, vacancies disappear, the number of vacancy clusters increases, and the crystalline quality of the Mg grains increases. The disappearance of intragranular vacancies is followed by an acceleration of the H2 desorption process. This is attributed to the increase of vacancy clusters at grain boundaries which assist the Mg nucleation in the hydride to metal phase transition. For H2 sorption cycles, the values of vacancy and vacancy cluster concentrations were obtained into the frame of the positron diffusion trapping model and the size of the involved vacancy clusters was evaluated by ab initio calculations of positron annihilation rates in Mg.