Vacancy cluster in graphite: Migration energy and aggregation mechanism
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Vacancy clusters in graphite have been investigated using Density Functional T heory (DFT) within B3LYP exchange-functional. The smallest size of vacancy clusters (V4) has been chosen to study the migration energy and aggregation mechanism. Two main types of V4 vacancy clusters have been modeled, the disc (V4d) and the line vacancy clusters; including boat vacancy (V4b) and zig-zag vacancy (V4z). The results show that the presence of unst able V3 vacancy may induce the mono-vacancy to migrate with low energy and vanish through forming stable V4 vacancy cluster. Also, the calculated energy barriers required to form the boat vacancy cluster (V4b), the zig-zag vacancy cluster (V4z) and disk vacancy cluster (V4d) support that the disc and the boat vacancy clusters co-exist. However the zig-zag type might only exist by knocking-out mechanism for highly irradiated graphite.Crystal (programming language)
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The Molecular Dynamics Method with the Finnis-Sinclair many-body potential is used to calculate the formation energies and combination energies of some small vacancy clusters with different configurations in TiAl. The most stable configuration of a vacancy cluster and its effect on vacancy migration nearby the cluster are simulated and discussed. The results show that, in the most stable configuration, each vacancy makes its best endeavor to form nearest neighbor relationship with the other vacancies, and there are anti-site defect nearby the cluster. The atoms nearby the vacancy cluster would move towards the cluster center for bigger cluster, and it seems that there exist interstitials in the vacancy cluster. The existing vacancy cluster as the aggregation center of vacancy has the ability of trapping or absorbing the near vacancy.
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Vacancy clusters in graphite have been investigated using Density Functional T heory (DFT) within B3LYP exchange-functional. The smallest size of vacancy clusters (V4) has been chosen to study the migration energy and aggregation mechanism. Two main types of V4 vacancy clusters have been modeled, the disc (V4d) and the line vacancy clusters; including boat vacancy (V4b) and zig-zag vacancy (V4z). The results show that the presence of unst able V3 vacancy may induce the mono-vacancy to migrate with low energy and vanish through forming stable V4 vacancy cluster. Also, the calculated energy barriers required to form the boat vacancy cluster (V4b), the zig-zag vacancy cluster (V4z) and disk vacancy cluster (V4d) support that the disc and the boat vacancy clusters co-exist. However the zig-zag type might only exist by knocking-out mechanism for highly irradiated graphite.
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Abstract The formation energy of the mono-vacancy and both the formation energy and binding energy of the di-and tri-vacancy in BCC alkali metals and transition metals have been calculated by using the modified analytical embedded-atom method (MAEAM). The formation energy of each type of configuration of the vacancies in the alkali metals is much lower than that in the transition metals. From minimum of the formation energy or maximum of the binding energy, the favorable configuration of the di-vacancy and tri-vacancy respectively is the first-nearest-neighbor (FN) or second-nearest-neighbor (SN) di-vacancy and the [112] tri-vacancy constructed by two first-and one second-nearest-neighbor vacancies. It is indicated that there is a concentration tendency for vacancies in BCC metals.
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To modify the shape difference between the spherical vacancy and the realistic vacancy, a shape factor is introduced into Brooks' vacancy model. It is shown that the modified model is more efficient in predicting the vacancy volume of metals than the original one, and the calculated vacancy volumes of 17 metals by the modified model agree with the values reported in literatures. Furthermore, the present results show that the vacancy shape may contribute 2%-9% to the whole vacancy volume.
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Molecular dynamics simulation was performed to obtain the formation energy of vacancy clusters in silicon crystal and learn relevant factors,in which the influence laws and their mechanisms of model system size,configuration of vacancy cluster and vacancy number on the formation energy of vacancy cluster were revealed.The results show that the size of model system and configuration of vacancy cluster have little effect on the formation energy of vacancy cluster in general;the size of 3×3×3 model system is more suitable for the calculation and analysis of the formation energy;the minimum formation energy increases linearly as vacancy number increasing is less than 6;the formation energy of vacancy cluster in essence depends on the number of broken Si-Si bonds as well and energy.
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