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Partial discharge at the oil-paper interface is one of the main reasons for the failure of oil-paper insulation in oil-immersed transformers. In this article, based on the numerical model of multi-branching streamer discharge, a multi-branching streamer discharge model in oil-paper insulation system is constructed by further considering the charge transport characteristics of oil-paper interface. The surface discharge characteristics under the influence of the dielectric constant ratio between natural ester insulating oil and insulating paper, and the spacing between needle electrode and paperboard are studied by finite element method, respectively. The results show that the distance between the needle electrode and the paperboard, and the dielectric constant ratio of insulating oil and insulating paper both significantly affect the streamer branching. The smaller the clearance between the needle electrode and paper-board is, the more serious the inhibition of streamer branches at the oil-paper interface is, which resulting in the streamers near the oil-paper interface are more significant effect on the propagation of the z-axis streamer. When the dielectric constant of the insulating oil is larger than that of the paperboard, the development and branching of the streamer in the oil will be more significant. On the contrary, the streamer is more likely to develop along the surface of the paperboard and accumulate charge at the oil-paper interface.

Carbon doped into Bi2Se3 as substitution for Se was reported inducing simultaneous magnetic and hole doping. In this work, based on theoretical and experimental approaches, we find that carbon doped Bi2Se3 is indeed spin polarized, while the magnetic moments are small and hole doping is little. Most carbon atoms energetically favor to be dimer- and trimer-substitutions for Se which induce neither magnetism nor charge doping. A few isolated carbon atoms doped at interstitial sites in the vdW gap or at Se vacancies result in spontaneous spin polarization and charge doping. The diffusion of a single carbon dopant into Bi2Se3 is difficult, while if dopants are close, carbon clusters can easily form, which means that the isolated, pairwise, and trimeric carbon dopants should coexist. These studies suggest that to obtain strong ferromagnetic C-Bi2Se3, experiments need careful design to dope single carbon atoms into bulk dispersedly. [ABSTRACT FROM AUTHOR]

Carbon doped into Bi2Se3 as substitution for Se was reported inducing simultaneous magnetic and hole doping. In this work, based on theoretical and experimental approaches, we find that carbon doped Bi2Se3 is indeed spin polarized, while the magnetic moments are small and hole doping is little. Most carbon atoms energetically favor to be dimer- and trimer-substitutions for Se which induce neither magnetism nor charge doping. A few isolated carbon atoms doped at interstitial sites in the vdW gap or at Se vacancies result in spontaneous spin polarization and charge doping. The diffusion of a single carbon dopant into Bi2Se3 is difficult, while if dopants are close, carbon clusters can easily form, which means that the isolated, pairwise, and trimeric carbon dopants should coexist. These studies suggest that to obtain strong ferromagnetic C-Bi2Se3, experiments need careful design to dope single carbon atoms into bulk dispersedly. [ABSTRACT FROM AUTHOR]

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al 2 O 3 (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H 2 molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al 2 O 3 (0001) surface. These findings suggest that W surface doping is beneficial to α-Al 2 O 3 as tritium permeation barrier. • Tungsten doping promotes spontaneous dissociation of H 2 on α-Al 2 O 3 surface. • H atoms prefer to be adsorbed at W site and hardly to diffuse to surface O or to Al sites. • H migration to subsurface should occur around W. • Energy barrier of H invasion on W doped surface is much higher than the undoped α-Al 2 O 3 (0001) surface. [ABSTRACT FROM AUTHOR]