A Comparative Study on Hydrogen Diffusion in Amorphous and Crystalline Metals Using a Molecular Dynamics Simulation
- PDF / 1,017,997 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 47 Downloads / 243 Views
CTION
HYDROGEN diffusion in amorphous metals has attracted a steady interest during the last two or three decades, since it presents different behaviors from that in crystalline metals with periodic atomic arrangements.[1–10] An amorphous structure involves various energy states of interstitial sites and saddle points due to its disorder,[1–4,9,11] which causes two distinct features of interstitial hydrogen diffusion. The first is that the hydrogen diffusivity rises with increasing hydrogen content. As hydrogen concentration goes up, the number of mobile hydrogen atoms at high energy interstitial sites increases, and consequently, the average diffusion rate of hydrogen becomes faster. This phenomenon is accepted as an established fact supported by theoretical calculations[2,3,9,10] and experimental results.[12–17] The second is that the temperature dependence of hydrogen diffusivity deviates from linear Arrhenius plot,[5,9,18–24] appearing in a curvature form. This indicates that the principal diffusion mechanism of hydrogen changes along with temperature. However, the curvature form is not unique and varies depending on experiments and theoretical modeling, sometimes downward,[5,20–24] or sometimes upward.[9,18–20] This is thought to originate from the variances of distributions in site and saddle point energies according to the systems constituting amorphous alloys.[25,26] On the other hand, as shown in recent experimental results,[27–30] hydrogen permeability in amorphous alloys may be comparable to that in Pd, which has BYEONG-MOON LEE, Ph.D. Student, and BYEONG-JOO LEE, Professor, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted July 31, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
triggered research activities[10,31–33] to use amorphous alloys as hydrogen purification membranes. The permeability is determined by the product of hydrogen solubility and diffusivity.[34] Hydrogen solubility in amorphous alloys has been reported to be higher than that in crystalline structures,[9,12,16] which has been explained by the low packing density of amorphous structures (the packing efficiency of theoretical random packed model is 0.64,[35] smaller than 0.68 of a bcc (body-centered cubic crystal). Based on this high solubility, many attempts have been made to improve the cost-effectiveness of hydrogen membrane materials, by replacing expensive Pd alloys currently employed in industry for hydrogen purification.[34] However, the permeability is affected not only by the solubility but also by the diffusivity. Therefore, for more effective material screening toward competitive prices, it is necessary to figure out quantitatively the hydrogen diffusivity in amorphous alloys with respect to crystalline structures. The difference in hydrogen diffusivity between amorphous and crystalline metals can only be evaluated correctly when the diffusivity is compared at the same composition. Furthe
Data Loading...
