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The effect of segregation behavior of non-metallic dopants (H/He/O) and metallic dopants (Be/Al/Mg/Y) on the performance of grain boundary (GB) in SiC has been systematically investigated by first-principles calculations. Firstly, the GB energy and excess volume of different GBs have been studied to evaluate the stability of GB and the capacity to accommodate dopant atoms. The solution energies of dopant atoms greatly reduce in the GB region compared with those in the bulk, which makes the dopant atoms inside the grain tend to segregate and aggregate near the GB. The driving force of GB on dopant segregation generally decreases with the increase of distance from GB plane, and the preferential site of dopant is closely correlated with the atomic size of dopant. In addition, H and Y atom possesses the lowest segregation energy at the interstitial and substitutional site near the GB, respectively. Next, the segregation of single dopant induced the changes in the strength and stability of GB have been explored. It is found that non-metallic dopants have the significant embrittlement effects on GB strength. However, the segregation of most metallic dopants could strengthen the GB and Mg atom has the most significant strengthening effect on the GB. The stability of GB can be greatly improved by segregation of Al and Y dopants. Besides, the aggregation of H atoms has the obvious embrittlement effect on the GB. Furthermore, the co-segregation behavior of different dopants has also been explored. Be and Mg dopants have the most significant inhibition effect on the segregation of detrimental impurities H/He/O due to the repulsive interaction between dopant atoms. The present results provide a new insight into the effect of dopant segregation on GB properties and are expected to be a useful guidance for screening the chemical composition and manipulating the performance of SiC-based ceramics. [ABSTRACT FROM AUTHOR]

The formation energies and formation volumes of the Ta, Al, and C vacancies in Ta2AlC have been calculated by using first-principles method. The results have shown that the vacancy formation is energetically most favorable for the C atom with formation energy of 1.72 eV. The formation energies of Ta and Al vacancies are 3.44 eV and 3.52 eV, respectively. The electronics properties show that the Ta vacancy has a clear effect on the conductivity of the Ta2AlC. This result indicates that Ta2AlC is a good candidate material for high-temperature and nuclear applications. [ABSTRACT FROM AUTHOR]

First-principles calculations have been conducted to gain insight into the vacancy trapping mechanism of H atoms produced by neutron induced transmutation in Be 12 Ti, which is a promising candidate for the plasma facing materials in fusion reactors. The microscopic mechanisms for H trapping by four kinds of vacancies, namely, Be1 , Be2 , Be3 , and Ti vacancy have been studied by calculating the solution energies and the trapping energies of hydrogen-vacancy complexes. We have included a zero-point energy correction that generally leads to a higher trapping energy, but the energy tendency remains unchanged. Vacancies serve as trapping centers to induce H atoms to segregate on their internal surface by providing an optimal H-embedding isosurface of charge density. The Ti vacancies exhibit a stronger H trapping ability than the Be vacancies. A Ti vacancy can capture up to ten H atoms, while the maximal number of H atoms that a Be vacancy can accommodate is four. But a Be1 vacancy can only capture two H atoms if the zero-point energy correction is included in the simulation. Among all H n V Be 1 (n = 1–5) complexes, the configuration of H 2 V Be 1 has the lowest H solution energy. The H 2 molecules are not formed in the most stable hydrogen-vacancy complexes explored. The present results provide a fundamental image of vacancy trapping mechanism for H atoms as well as H bubble formation in solid Be 12 Ti, which is extremely important for the reduction of radioactive tritium inventory within fusion reactor and appropriate handling of radioactive nuclear wastes during the decommissioning of the fusion blanket. [ABSTRACT FROM AUTHOR]

• The microstructures of helium bubbles in Be 12 Ti are studied detailedly. • The stabilities of helium bubbles are investigated by the calculations of solution energies. • The nucleation and growth mechanism of helium bubbles in Be 12 Ti are elaborated. First-principles calculations have been conducted to study the stability of He-vacancy clusters and the trapping behavior of vacancies for He atoms in Be 12 Ti. We considered the various metastable microstructures of four types of He-vacancy clusters, and the most stable configurations were determined by the calculations of solution energy. Some He-vacancy clusters in Be 12 Ti have specific geometrical shape, and grow along specific crystal plane. For example, three types of He 2 V clusters (two He atoms trapped by vacancy) form dumbbell-shaped structure, three He atoms of He 3 V clusters (three He atoms trapped by vacancy) are located in the specific crystal plane. The solution energies of four types of clusters increase with the increase of number of He atoms, and the increasing rate is basically identical. The solution energies of He n V B e 1 clusters are always the largest, and the solution energies of the He n V T i clusters are always the smallest, which mainly attribute to the larger vacancy volume and perfect spatial symmetry of Ti vacancies. The binding energies of a single He atom with He n V (n =0-3) clusters are all positive, and the segregation of He atoms on the inner surface of the vacancy is closer to the center of the vacancy, indicating that the He-vacancy has a stronger trapping ability for He atoms. The present research results provide a theoretical foundation of vacancy trapping mechanism for He atoms in Be 12 Ti, which is extremely important for the nucleation and growth mechanism of He bubbles in Be 12 Ti. [ABSTRACT FROM AUTHOR]