[en] The discharge characteristics of SF₆ dependent on gas pressure under extremely inhomogeneous electric fields are strongly nonlinear and must be carefully considered when optimizing gas-insulated equipment insulation design and defect detection. The main aim of this study was to determine the relationship between nonlinear breakdown characteristics and the pre-breakdown discharge mode transition. The breakdown model established by Niemeyer, as adapted under impulse voltage, was expanded. The images and the phase-resolved patterns of the pre-breakdown discharge as a function of gas pressure and applied voltage were analyzed to observe the relationship between them as per the spatial distribution of the spark paths. A universal model of the nonlinear breakdown phenomenon under steady-state voltage was established accordingly. Experimental results demonstrate the existence of a positive glow corona (PGC) discharge in the interval with the discontinuous breakdown voltage as well as significant interactions between the PGC discharge and leader discharge. The nonlinear characteristics can be attributed to the inception and quenching of the PGC discharge. The shielding effect induced by the PGC discharge is the neglected culprit of the strong nonlinear discharge characteristics under steady-state voltage. The nonmonotonic U–p curve in electronegative gases was also assessed in an effort to build a theoretical basis for the manufacture and condition monitoring of the gas-insulated equipment. (paper)

[en] Eutectic solidification has been investigated for decades; however, our understanding of the mechanism of eutectic nucleation remains very limited. In this work, we investigated the regular eutectic solidification process by an atomistic simulation method, and a new eutectic nucleation scenario is proposed. We found that, near the eutectic composition, the nucleation of a metastable triangle phase is more energy favorable for diffusion-controlled binary eutectic systems, and the solidification often passes through a transient state. With the proceeding of solidification, alternated A-rich and B-rich domains will be generated at the interface between the transient phase and liquid phase by pseudospinodal decomposition. When the compositions of alternated domains are close to the respective equilibrium values, eutectic phases will be formed inside the domains simultaneously, which further develops into coupling eutectic colonies. Furthermore, the construction of solute interdiffusion field and the formation of interface curvature are explained self-consistently with our eutectic nucleation mechanism.

[en] To understand and develop new nanostructure materials with specific mechanical properties, a good knowledge of the elastic strain response is mandatory. Here we investigate the linear elasticity response in the modified phase-field-crystal (MPFC) model. The results show that two different propagation modes control the elastic interaction length and time, which determine whether the density waves can propagate or not. By quantitatively calculating the strain field, we find that the strain distribution is indeed extremely uniform in case of elasticity. Further, we present a detailed theoretical analysis for the orientation dependence and temperature dependence of shear modulus. The simulation results show that the shear modulus reveals strong anisotropy and the one-mode analysis provides a good guideline for determining elastic shear constants until the system temperature falls below a certain value. (paper)

[en] Objective: To explore the value of chest CT features and clinical indexes in the differential diagnosis between suspected COVID-19 with two or more negative nucleic acid tests and confirmed COVID-19. Methods: The clinical data and chest CT images of 105 cases with confirmed COVID-19 (55 males and 50 females, aged from 2 month to 88 years) and 97 cases with suspected COVID-19 (59 males and 38 females, aged from 1 month to 93 years) were analyzed retrospectively in Shiyan Taihe Hospital from January 21 to February 10, 2020. X² test and two independent sample t test were used to analyze the clinical data and CT signs of the two group cases, with P < 0.05 for statistically significant difference. Results: Compared with the suspected patients, the average age of confirmed ones was higher (t = 2.460, P = 0.01). The main pathological changes were pure ground glass (68 cases) and mixed ground glass density (53 cases) (X² = 50.016, P < 0.01). Interstitial thickening (83 cases) (X² = 55.395, P < 0.01), vascular widening (73 cases) (X² = 57.527, P < 0.01), air bronchogram sign or bronchiectasis (67 cases) (X² = 17.899, P < 0.01), fibrous streak shadow (54 cases) (X² = 5.500, P = 0.02), commonly distributed under the pleura and the long axis of the lesion was parallel to the pleura (89 cases) (X² = 23.597, P < 0.01), most of them had no pleural effusion (X² = 7.017, P < 0.01); both lesions were mainly distributed in patches (89 confirmed cases, 87 suspected cases) (X² = 19.573, P < 0.01). In addition, the lesions of patients with confirmed COVID-19 showed progress in short term (72/87, 82.76%), and those with suspected COVID-19 showed remission in short term (63/89, 70.78%). The difference was statistically significant (X² = 51.114, P < 0.01). There was no significant difference in gender and distribution of pulmonary lobes (X² = 1.462, P = 0.23; X² = 7.381, P = 0.19). The number of white blood cells (X² = 17.891, P < 0.01) and the percentage of lymphocytes (X² = 11.151, P < 0.01) of COVID-19 were mostly normal or decreased, creatine kinase (X² = 9.589, P < 0.01) was mostly normal, and erythrocyte sedimentation rate was mostly normal or increased (X² = 4.240, P = 0.04). Conclusions: The imaging features and biochemical indexes of diagnosed COVID-19 are different from those of suspected ones. The comparative analysis of imaging features, clinical indexes and follow up examination are helpful for the differential diagnosis of COVID-19. (authors)

[en] The Kármán vortex shedding is totally suppressed in flows past a wavy square-section cylinder at a Reynolds number of 100 and the wave steepness of 0.025. Such a phenomenon is illuminated by the numerical simulations. In the present study, the mechanism responsible for it is mainly attributed to the vertical vorticity. The geometric disturbance on the rear surface leads to the appearance of spanwise flow near the base. The specific vertical vorticity is generated on the rear surface and convecting into the near wake. The wake flow is recirculated with the appearance of the pair of recirculating cells. The interaction between the upper and lower shear layers is weakened by such cells, so that the vortex rolls could not be formed and the near wake flow becomes stable. (fundamental areas of phenomenology (including applications))

[en] Grain boundary directed spinodal decomposition has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The morphological selection mechanism of spinodal decomposition at grain boundaries is a major challenge to reveal, and remains elusive so far. In this work, the effect of grain boundaries on spinodal decomposition is investigated by using the phase-field model. The simulation results indicate that the spinodal morphology at the grain boundary is anisotropic bicontinuous microstructures different from the isotropic continuous microstructures of spinodal decomposition in the bulk phase. Moreover, at grain boundaries with higher energy, the decomposed phases are alternating α/β layers that are parallel to the grain boundary. On the contrary, alternating α/β layers are perpendicular to the grain boundary. (paper)

[en] Reverse austenite transformation (RAT) is critical for designing advanced high-strength steels (AHSS), which, however, has not been sufficiently studied in nanostructured (NS) steels or Fe alloys, and hence not fully understood yet. Herein, the RAT (e.g. ferrite to austenite) kinetics in the NS Fe alloy upon continuous heating was experimentally and theoretically investigated, where, the ultrafine austenite characterized by a sluggish growth velocity and a high thermal stability, and additionally, an appreciable solute partitioning detected using atom probe microscopy, indicate the diffusion-controlled mechanism of RAT. The double-edged role of grain boundaries (GBs) in the NS alloy is elucidated, i.e. enhancing the diffusivity due to the type-A kinetics, and simultaneously, facilitating the formation of constrained diffusion field mainly due to the segmented effect of GB nucleation. On this basis, a modified diffusion model incorporating the effect of GBs is implemented to understand the GB-constrained austenite growth and the associated partitioning behavior, and further complemented with Cahn model, an austenite growth model is applied to predict the overall kinetics of RAT in the NS Fe alloy. It then follows that a strategy by combination of diffusion-controlled growth model and microstructure model could serve as a framework to predict the kinetics of RAT in the NS alloys. Regarding the RAT kinetics in the NS alloys, the present work uncovers the ‘GB-constrained’ mechanism, which is expected to offer the potential application for nanostructure manipulation in the development of AHSS.