Hydrogen interactions with intrinsic point defects in hydrogen permeation barrier of α-Al₂O₃: a first-principles study

Abstract

It is crucial to understand hydrogen interactions with intrinsic point defects in the hydrogen permeation barrier (HPB) of α-Al2O3, finding underlying reasons for the not-so-low H-permeability of the barrier, and thereby produce samples with tailored defects for optimal performance. Using density functional theory (DFT), the formation energies of intrinsic point defects in an α-Al2O3 lattice, including extrinsic H-related defects (H(i), V(Al)-H complex, HO(i) and H(O)), in all possible charged states, are first calculated under HPB working conditions, to determine the dominant basic defect species for hydrogen. We find that the stable forms of H-related defects in α-Al2O3 are charged H interstitials (H(i)(q), where q is the charge state of the defect) and hydrogenation of the bulk V(Al)(3-) ([V(Al)(3-)-H(+)](q)), under hydrogen-rich conditions. As the system reaches equilibrium, H in α-Al2O3 is mainly present in the H(i)(+) state, and preferentially exists in the form of [V(Al)(3-)-H(+)] and H(O)(+). Migration processes of the dominant defects are further investigated, predicting that H(i)(+) is the predominant diffusion species in α-Al2O3. [V(Al)(3-)-H(+)](2-) and H(O)(+) can release trapped hydrogen during high temperature annealing, contributing to the H-transport in α-Al2O3. The formation energy is much higher than the migration energy for H(i)(+), suggesting that the migration of H(i)(+) is the bottleneck for creating low enough H-permeation in α-Al2O3, and corresponding strategies for optimum H-suppressing performance for an α-Al2O3 HPB are proposed.

Identification of hydrogen defects inα−Al2O3by first-principles local vibration mode calculations

“…We found that H i is an amphoteric defect with the defect transition level at ε(+1/−1) = 4.19 eV, as shown in Fig. 1, in agreement with the previous work [17]. Furthermore, we investigated the other feasible forms of H defect and found that H could substitute for the O site forming H O defect with two defect transition levels at ε(+1/0) = 5.67 eV and ε(0/−1) = 6.46 eV.…”

“…We found that H i could hop from one site to another site in a crystal with the migration barrier of ∼0.84 eV, which can be converted to the temperature of ∼400 K [51]. This agrees well with a previous theoretical work [17]. For V Al defect, we found that the migration barrier is much higher at ∼1.7 eV, meaning that V Al starts to be mobile at a temperature of ∼800 K [51].…”

“…α-Al 2 O 3 is an important material for a variety of applications. For example, it can be used as a substrate for epitaxial growth [15], a surface passivation layer of Si solar cells [16], and a hydrogen permeation barrier [17,18]. In addition, α-Al 2 O 3 can be used in optical devices when it is doped with Cr or Ti [19,20].…”

“…In addition, by using the polarized IR absorption measurements, all vibrational modes associated with the O-H bonds are suggested to lie very close to the basal plane [5,11]. Recently, hydrogen in α-Al 2 O 3 has been theoretically studied by first-principles calculations [17]. Zhang et al suggested that hydrogen in α-Al 2 O 3 is mainly presented in the form of hydrogen interstitial (H i + ) defect and some H could exist in the form of substitution for oxygen site (H O + ) as well as a complex defect between hydrogen and aluminum vacancy (H-V Al ) [17].…”

“…Recently, hydrogen in α-Al 2 O 3 has been theoretically studied by first-principles calculations [17]. Zhang et al suggested that hydrogen in α-Al 2 O 3 is mainly presented in the form of hydrogen interstitial (H i + ) defect and some H could exist in the form of substitution for oxygen site (H O + ) as well as a complex defect between hydrogen and aluminum vacancy (H-V Al ) [17]. Moreover, they reported that H i + was suggested to be the predominant diffusion species in α-Al 2 O 3 resulting in the low efficiency for protection against H permeation through α-Al 2 O 3 .…”