The elastic properties and point defects of thorium monocarbide(ThC) have been studied by means of density functional theory based on the projector-augmented-wave method. The calculated electronic and elastic properties of ThC are in good agreement with experimental data and previous theoretical results. Five types of point defects have been considered in our study, including the vacancy defect, interstitial defect, antisite defect, schottky defect, and composition-conserving defect. Among these defects, the carbon vacancy defect has the lowest formation energy of 0.29 eV. The second most stable defect(0.49 eV) is one of composition-conserving defects in which one carbon is removed to another carbon site forming a C2 dimer. In addition, we also discuss several kinds of carbon interstitial defects, and predict that the carbon trimer configuration may be a transition state for a carbon dimer diffusion in Th C.
We investigated the effect of grain boundary structures on the trapping strength of He_N(N is the number of helium atoms) defects in the grain boundaries of nickel. The results suggest that the binding energy of an interstitial helium atom to the grain boundary plane is the strongest among all sites around the plane. The He_N defect is much more stable in nickel bulk than in the grain boundary plane. Besides, the binding energy of an interstitial helium atom to a vacancy is stronger than that to a grain boundary plane. The binding strength between the grain boundary and the He _N defect increases with the defect size. Moreover, the binding strength of the He_N defect to the Σ3(12)[110] grain boundary becomes much weaker than that to other grain boundaries as the defect size increases.
In this paper,we present a theoretical study on excitonic absorption spectra of one-dimensional semiconductor quantum wires.The carrier-carrier scattering is treated by the second Bom approximation in the Markovian limit.The absorption spectra of different carrier densities and temperatures are discussed.The excitonic absorption peak position and width show complicated dependence on carrier density and temperature,indicating the importance of carrier-carrier scattering.The behavior can be understood by the cooperative effects of exchange self-energy and Coulomb correlation due to carrier-carrier scattering.
We have investigated the expansion and bursting of a helium nano-bubble near the surface of a nickel matrix using a molecular dynamics simulation. The helium atoms erupt from the bubble in an instantaneous and volcano-like process,which leads to surface deformation consisting of cavity formation on the surface, along with modification and atomic rearrangement at the periphery of the cavity. During the kinetic releasing process, the channel may undergo the "open" and"close" states more than once due to the variation of the stress inside the nano-bubble. The ratio between the number of helium atoms and one of vacancies can directly reflect the releasing rate under different temperatures and crystallographic orientation conditions, respectively. Moreover, a special relationship between the stress and He-to-vacancy ratio is also determined. This model is tested to compare with the experimental result from Hastelloy N alloys implanted by helium ions and satisfactory agreement is obtained.
In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.
