arXiv subjects

B. C. Pan

Publications and source records attributed to B. C. Pan.

7 recordsLinked to original sources

arXiv2017

Site preference and diffusion behaviors of H influenced by the implanted-He in 3C-\b{eta} SiC

SiC materials are potential plasma facing materials in fusion reactors. In this study, site preference and diffusion behaviors of H in pure 3C-\b{eta} SiC and in He-implanted 3C-\b{eta} SiC are investigated, on the basis of the first-principles calculations. We find that the most stable sites for H in pure 3C-\b{eta}SiC is the anti-bond site of C (ABc) in Si-C, while it becomes the bond-center (BC) site of Si-C bonds in the He-implanted 3C-\b{eta} SiC. Analysis on the electronic structures reveals that such change is attributed to the reduction of hybridization of C-Si bonds induced by He. Moreover, the presence of He strongly affect the vibrational features in the high frequency region, causing a blue shift of 25 cm-1 for C-H stretch mode with H at ABc site and a red shift of 165cm-1 for that at BC site, with respect to that in the pure system. In pure 3C-\b{eta} SiC, H is diffusive with an energy cost of about 0.5 eV, preferring to rotate around the C atom in a Si-C tetrahedron with an energy barrier of just about 0.10 eV. In contrast, in He-implanted 3C-\b{eta} SiC, the energy barriers for H migration goes up to be about 0.95 eV, indicating the implanted-He blocks the diffusive H to some extent. Our calculations also show that the influence of He on H diffusion is effective in a short range, just covering the nearest neighbor.

cond-mat.mtrl-sci↗

arXiv2015

Self-blocking of interstitial clusters near metallic grain boundaries

Nano-crystallize materials have been known for decades to potentially owe the novel self-healing ability for radiation damage, which has been demonstrated to be especially linked to preferential occupation of interstitials at grain boundary (GB) and promoted vacancy-interstitial annihilation. A major obstacle to better understanding the healing property is the lack of an atomistic picture of the interstitial states near GBs, due to severely separation of the timescale of interstitial segregation from other events and abundance of interstitials at the GB. Here, we report a generic "self-blocking" effect of the interstitial cluster (SIAn) near the metallic GB in W, Mo and Fe. Upon creating a SIAn near the GB, it is immediately trapped by the GB during the GB structural relaxation and blocks there, impeding GB's further spontaneous trapping of the SIAn in the vicinity and making these SIAns stuck nearby the GB. The SIAn in the stuck state surprisingly owes an exceptionally larger number of annihilation sites with vacancies near the GB than the SIAn trapped at the GB due to maintaining its bulk configuration basically. Besides, it also has an unexpectedly long-ranged repelling interaction with the SIA in the bulk region, which may further affect the GB's trap of the SIAn. The self-blocking effect might shed light on more critical and extended role of the GB in healing radiation-damage in NCs than previously recognized the GB's limited role based on GB's trap for the SIA and resulted vacancy-SIA recombination.

cond-mat.mtrl-sci↗

arXiv2013

Atomistic Mechanism from Vacancy Trapped H/He Atoms to Initiation of Bubble in W under Low Energy Ions Irradiation

With the first principles calculations of H and He induced energetics change we demonstrate that in W the accumulation of H (up to 9) and He (up to 4) in a single vacancy (V) surprisingly reduce the formation energy of first and second nearest vacancy (as low as 0 eV), which gives the direct evidence of V-H (He) complex mutation mechanism from V-Hn (Hen) to V2-Hn (Hen) and with the potential to lead to the growth of H (He)-vacancy complexes: an initial step to H and He bubble. This finding well explains the long-standing problem of why H and He bubbles being produced on W surface exposed to low-energy (far lower than displacement threshold energy) D or He ions irradiation. The further identified repulsive (attractive) interaction between V-H12 (V-He14) and additional H (He) illustrates the experimentally observed big difference of deposition depth of H (micron) and He (100 angstrom) bubbles in W even the migration rate of He is far larger than that of H.

cond-mat.mtrl-sci↗

arXiv2013

An operational window for radiation-resistant materials based on sequentially healing grain interiors and boundaries

Design of nuclear materials with high radiation-tolerance has great significance1, especially for the next generation of nuclear energy systems2,3. Response of nano- and poly-crystals to irradiation depends on the radiation temperature, dose-rate and grain size4-13. However the dependencies had been studied and interpreted individually, and thus severely lacking is the ability to predict radiation performance of materials in extreme environments. Here we propose an operational window for radiation-resistant materials, which is based on a perspective of interactions among irradiation-induced interstitials, vacancies, and grain boundaries. Using atomic simulations, we find that healing grain boundaries needs much longer time than healing grain interiors. Not been noticed before, this finding suggests priority should be thereafter given to recovery of the grain boundary itself. This large disparity in healing time is reflected in the spectra of defects-recombination energy barriers by the presence of one high-barrier peak in addition to the peak of low barriers. The insight gained from the study instigates new avenues for examining the role of grain boundaries in healing the material. In particular, we sketch out the radiation-endurance window in the parameter space of temperature, dose-rate and grain size. The window helps evaluate material performance and develop resistant materials against radiation damage.

cond-mat.mtrl-sci↗

arXiv2010

Tuning electronic structure of graphene via tailoring structure: theoretical study

Electronic structures of graphene sheet with different defective patterns are investigated, based on the first principles calculations. We find that defective patterns can tune the electronic structures of the graphene significantly. Triangle patterns give rise to strongly localized states near the Fermi level, and hexagonal patterns open up band gaps in the systems. In addition, rectangular patterns, which feature networks of graphene nanoribbons with either zigzag or armchair edges, exhibit semiconducting behaviors, where the band gap has an evident dependence on the width of the nanoribbons. For the networks of the graphene nanoribbons, some special channels for electronic transport are predicted.

cond-mat.mes-hall↗

arXiv2009

Electronic structures and Raman features of a carbon nanobud

By employing the first-principles calculations, we investigate electronic properties of a novel carbon nanostructure called a carbon nanobud, in which a $C_{60}$ molecule covalently attaches or embeds in an armchair carbon nanotube. We find that the carbon nanobud exhibits either semiconducting or metallic behavior, depending on the size of the nanotube, as well as the combination mode. Moreover, with respect to the case of the corresponding pristine nanotubes, some new electronic states appear at 0.3-0.8 eV above the Fermi level for the carbon nanobuds with the attaching mode, which agrees well with the experimental reports. In addition, the vibrational properties of the carbon nanobuds are explored. The characteristic Raman active modes for both $C_{60}$ and the corresponding pristine nanotube present in Raman spectra of the carbon nanobuds with attaching modes, consistent with the observations of a recent experiment. In contrast, such situation does not appear for the case of the carbon nanobud with the embedding mode. This indicates that the synthesized carbon nanobuds are probably of the attaching configuration rather than the embedding configuration.

cond-mat.mes-hall↗

arXiv2009

Hydrogen in Ag-doped ZnO: theoretical calculations

Based on density functional theory calculations, we systematically investigate the behaviors of a H atom in Ag-doped ZnO, involving the preference sites, diffusion behaviors, the electronic structures and vibrational properties. We find that a H atom can migrate to the doped Ag to form a Ag-H complex by overcoming energy barriers of 0.3 - 1.0 eV. The lowest-energy site for H location is the bond center of a Ag-O in the basal plane. Moreover, H can migrate between this site and its equivalent sites with energy cost of less than 0.5 eV. In contrast, dissociation of such a Ag-H complex needs energy of about 1.1 - 1.3 eV. This implies that the Ag-H complexes can commonly exist in the Ag-doped ZnO, which have a negative effect on the desirable p-type carrier concentrations of Ag-doped ZnO. In addition, based on the frozen phonon calculation, the vibrational properties of ZnO with a Ag-H complex are predicted. Some new vibrational modes associated with the Ag-H complex present in the vibrational spectrum of the system.

cond-mat.mtrl-sci↗