An investigation of cathode erosion has been conducted for conditions similar to those encountered in a plasma cutting process. A hafnium insert in a water-cooled copper sleeve serves as the cathode. Modifications to the torch allowed the observation of the cathode surface during operation and measurement of material loss from the cathode during different phases of an operating cycle. Erosion has been found to be predominantly due to ejection of molten material droplets. Most ejection events are associated with changes in the conditions of the plasma, e.g. during start-up, change of gas flow and shutdown. The ejections can be explained by imbalances of the forces acting on the molten surface, those associated with the arc current, those due to surface tension, and those associated with the fluid dynamics within the torch.
Here, we report a type of aluminum-based condensate microdrop self-propelling (CMDSP) functional films based on the controllable fabrication of anodic alumina rod-capped nanopores, which can be realized by a three-step method based on the skillful combinations of well-established hard anodization, mild anodization and chemical etching techniques. Such a surface nanoengineering strategy is verified to be feasible via our exemplified experiments and scanning electronic microscopy characterizations. After fluorosilane modification, the surface nanostructure can induce the efficient self-jumping of small-scale condensate microdrops, especially below 50 μm. This work offers an avenue for developing CMDSP aluminum surfaces with self-cleaning, antifrosting, and antidewing functions.
Seven composite models of meta-aramid fibers with different moisture contents were studied using molecular dynamics simulation. The effects of moisture on the thermal stability and mechanical properties of the fibers and their mechanisms were analyzed, considering characteristics such as hydrogen bonding, free volume, mean square displacement, and mechanical parameters. The simulation results showed that the large number of hydrogen bonds between water molecules and meta-aramid fibers destroyed the original hydrogen-bond network. Hydrogen bonds between the molecular chains of meta-aramid fibers were first destroyed, and their number decreased with increasing moisture content. The free volume of the fibers thereby increased, the interactions between fiber chains weakened with increasing moisture content, and the fiber chain movement intensified accordingly. The ratio of diffusion coefficients of the water molecules to moisture contents of the composite models increased linearly, and the water molecule diffusion increased, which accelerated the rate of damage to the original hydrogen-bond network of the meta-aramid fibers and further reduced their thermal stability. In general, the mechanical properties of the composites were negatively related to their moisture content.
Reducing the dielectric constant and loss of cellulose insulation can make the electric field distribution uniform in oil-paper insulation systems and decrease the heat generation in dielectric materials, thus ensuring a reliable transformer operation. To guide the experimental design from the molecular level, the fluctuation method was introduced into the molecular dynamics simulation to evaluate the static permittivity of cellulose insulation, ε s. The correlation between the dynamics parameter mean square displacement (MSD) and dielectric loss induced by orientational polarization, was investigated. The simulation results and experimental values of five types of cellulose insulation were compared to verify the rationality of the proposed method. The results indicated that the simulation values of ε s for five models were agreed well with the experimental values in terms of both the magnitude and the variation trend. The simulation time and permittivity of the surrounding medium ε RF are two key parameters, which determine the accuracy of the simulation results of ε s. Considering the convergence, 15 ns was chosen as the lower limit of simulation time. The reaction field approximation was adopted to calculate the dipole-dipole interaction instead of true interaction, that is, ε RF →∞. The MSD results reproduced the experimental trend in dielectric loss, indicating that the method can qualitatively predict the dielectric loss of cellulose insulation. Hence, these methods are sufficient to guide design experiments and provide a route to understand the mechanism of the change in dielectric properties at the molecular level. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
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