| Tritium permeation barrier(TPB)technology is one of the key scientific technologies to realize tritium self-sufficiency in fusion reactors,which is one of the top priorities of the Chinese Fusion Engineering Test Reactor(CFETR).Alumina(Al2O3)has received significant attentions as typical candidate TPB materials,due to its excellent comprehensive properties.It has many different phases such as ?,?,?,?,?,? and ?.Among them,?-Al2O3 is considered to be the most promising candidate for TPB with high thermodynamic stability and high permeation reduction factor(PRF).However,an elevated temperature higher than 1000? is usually required to obtain ?-Al2O3 films by the normal thermal oxidation approaches,which can reduce the mechanical properties of the substrate materials like the reduced activation ferritic/martensitic steels(RAFMs).Hence,the Al2O3 film prepared by low temperature thermal oxidation approaches is mainly ? alumina.Besides,the TPBs would be exposed to high-energy,high-flux neutrons(14MeV)in future fusion reactors that many defects,such as vacancies,interstitial atoms,dislocations,cavities,etc.,can be produced through the collision cascade.The irradiation-induced defects could sum up on fabrication-induced defects so to reduce drastically the barrier performance.However,the underlying mechanism is still not settled.In this paper,a series of systematic first-principles calculations were performed to investigate the structure,clustering and diffusion behavior of H in irradiated and unirradiated Al2O3.The interactions between H atoms and irradiation-induced defects has been studied to explore the configurations of H.The underlying mechanism of irradiation defects on the permeation in Al2O3 has also been analyzed,which will provide theoretical assistance for hydrogen isotope transport analysis and relevant experimental research.Firstly,we have studied the structure,energetic and diffusion behavior of H in bulk?-Al2O3.In ?-Al2O3,the binding energy of two H atoms located in first nearest neighbor octahedral interstitial sites(OISs)is 2.41 eV,while the value is 2.15 eV for two H atoms located in second nearest neighbor OISs,which indicates that a strong attractive interaction occurs even if the distance between the two H atoms is 3.912 A.Therefore,when multiple H atoms are dissolved in ?-Al2O3,they can diffuse toward their adjacent H atoms and form clusters that prevent further diffusion of H.Every OIS can hold at most 3 H atoms.However,the most stable existence configuration of H in ?-Al2O3 is 2Hi due to its smallest formation energy and largest average binding energy.The distance and binding energy of 2Hi are 0.72 A and 4.72 eV,respectively,which are similar to those of H2 in air.The migration barrier of a H pair jumping from an OIS to a nearest OIS is 2.27 eV,which is much closer to the experimental values for single crystal ?-Al2O3.Thus,the most likely existence configurations of H in bulk ?-Al2O3 without defects is a H pair due to the smallest formation energy and the largest average binding energy.Both the dissolution energy and migration barrier of the H pair are so high that its dissolution and diffusion in ?-Al2O3 are very difficult,resulting in a high theoretical PRF.Secondly,we have systematically studied the influence of irradiation-induced point defects on the dissolution and diffusion properties of H in ?-Al2O3.The formation energy of H-defect complexes is much lower than that of H in perfect ?-Al2O3 that H can easily dissolve into ?-Al2O3 TPB under irradiation environment.Among these H-defect complexes,[H+-VAl3-]and[H+-VO0]are the dominant H-defect complexes with much lower formation energies at the middle of bandgap in both Al-rich and O-rich growth environments.Thus,H is easily captured by vacancy-type irradiation-induced point defects.As a result,higher H retention can be expected,which leads to enormous H waste.This is consistent with the previous experimental results in the literature.Moreover,we also calculated several different diffusion pathways of H-defect complexes in ?-Al2O3 and predicted its corresponding diffusion coefficient.The migration barrier of vacancy-type point defect and H diffusion as a bound entity is so high that it can hardly diffuse in such form.Thus,isolated vacancy-type irradiation-induced point defects can trap multiple H atoms to form H-defect complexes and impede H diffusion,thus decreasing its diffusivity.This could enhance the efficiency in preventing H permeation through ?-AlO3 TPB.However,when a large amount of Al vacancies gather together to form clusters or cavities,H atoms can migrate quickly in the shared part of Al vacancies.The effective distance of H diffusing through ?-Al2O3 TPB samples can be reduced,thus decreasing the efficiency in preventing H permeation through ?-Al2O3.In addition,the migration barrier of OHi migration to the INN O interstitial site is 0.44 eV,which is very low,leading to a higher diffusivity.This is considered to be a possible underlying reason for the low permeation efficiency of ?-Al2O3 TPBs in irradiation environments.We also suggested that the irradiated ?-Al2O3 TPB will have a more effective efficiency in preventing H permeation through ?-Al2O3 in H2O environments than in H2 environments.Thirdly,we have identified the ?-Al2O3 structure by calculating the vibration frequencies of H-related defects to compare with the experimental results in the literatures.We find that the most stable charge states for respective defect species at EG/2 are VAl-OIS-3,VAl-TIS-3,Oi-2,and Ali+3 in these two ?-Al2O3 structures,which is similar to that in ?-Al2O3.Under O-rich environments,the formation energies of VAAl-OIS-OIS-3 is a little lower than that of VAl-TIS-3 and far lower than 0 eV that they are easy formed by exposing to H2O vapor.Thus,the stability of VAl-OIS-3 is higher than that of VAl-TIS-3,which is in well agreement with the precious researches.When H atoms are introduced to ?-Al2O3 the stability of[Val-TIS-H]-2 is higher than that of[VAl-OIS-H]-2 in defective spinel ?-Al2O3 structure,while it is opposite in the non-spinel ?-Al2O3 structure.The calculated vibrational frequencies of OH-in[VAl-TIS-H]-2 and[VAl-OIS-H]-2 for defective ?-Al2O3 are 3608 cm-1 and 3374 cm-1,which is in excellent agreement with observed IR absorption peaks at?3500 and?3300 cm-1.However,the vibrational frequencies of OH-in[VAl-TIS-H]-2 and[VAl-OIS-H]-2 of non-spinel ?-Al2O3 structure is far away of the experimental results.From this point,the defective spinel model are more adequate to describe ?-Al2O3 structure,which is in consistent with the literatures.Lastly,we have systematically explored the permeation behavior of H in irradiated and unirradiated ?-Al2O3 and compared it with ?-Al2O3 to find the reason for the higher permeation efficiency of ?-Al2O3 than ?-Al2O3.We found that there are many native Al vacancies in ?-Al2O3 structure,which is arranged in a straight line along the X-axis.These Al vacancies can provide a rapid diffusion channel for H molecues.Moever,the formation energy of H pairs in ?-Al2O3 much lower than that in ?-Al2O3 resulting in a higher H retention.Thus,the permeation efficiency of H in ?-Al2O3 is much higher than that in ?-Al2O3 leading to a lower PRF.Besides,similar to that in ?-Al2O3,isolated vacancy-type irradiation-induced point defects in ?-Al2O3 can trap multiple H atoms to form H-defect complexes and impede H diffusion.thus enhancing the efficiency in preventing H permeation through ?-Al2O3 TPB.In addition,the migration barrier of OHi migration to the 1NN O interstitial site is 0.085 eV,which is very low,leading to a higher diffusivity.Our results can help researchers to gain an in-depth understanding of the transport mechanism for H in irradiated Al2O3 and provide a sound theoretical explanation for recent experimental results of H permeation in Al2O3 under irradiation environments. |
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