Cr Effect on Hydrogen Interactions with Intrinsic Point Defects and Hydrogen Diffusion in α-Al₂O₃ as Tritium Permeation Barriers

Abstract

Hydrogen transport in the outer Al2O3 scale is the rate-limiting step for H-permeation resistance of FeAl/Al2O3-type aluminide tritium permeation barriers (TPBs) in fusion reactors. With first-principle calculations, the effects of Cr, a main impurity element in the Al2O3 scale, on intrinsic point defect formation, hydrogen interactions with intrinsic point defects, and hydrogen diffusion in α-Al2O3 have been investigated under the working conditions of aluminide TPBs. It is found that Cr addition is favorable for the formation of VO, yet unfavorable for VAl formation in α-Al2O3. Compared with the α-Al2O3–H case, Cr is beneficial for the formation of H-related defects in α-Al2O3, whereas it is unfavorable for the Hi trapping ability of VAl and VO. HO– will dominate among Hi–, VAl3–, VO0, and [VAl3––H+]2–, and only one step of Hi reorientation will be involved for the Hi diffusion in Cr-doped α-Al2O3. Hi is the dominant diffusion species in both pure and Cr-doped α-Al2O3, whereas the activation energy of H diffusion in Cr-doped α-Al2O3 is sharply reduced, which is unfavorable for H-permeation resistance of aluminide TPBs. The Cr effect on hydrogen behaviors in α-Al2O3 can be attributed to the chemically unstable electron structure of Cr3+ and a relatively stronger bonding interaction between H and Cr than that between H and Al or O atoms.