Gold is a classical metal with the Fermi level lying in the sp-band, while graphene is a zero-bandgap semiconductor with Dirac band structure. Thus, the photon emission from both gold and graphene can not be effectively achieved by electron–holes recombination as direct bandgap semiconductors. Alternatively, optical emission from hot carriers is possible in graphene and gold, but usually with very low efficiency. This is because the hot carrier relaxation time is much faster than the radiative lifetime in both graphene and gold. To solve this problem, here we show that in suspended graphene structures the photon emission intensity can be enhanced by increasing the hot carrier relaxation time, while in suspended gold/graphene hybrid structures, the photon emission intensity also increased with increasing holes sizes. Since nowadays most semiconductor light sources are based on bandgap design and also restricted by it, the study could help to develop materials with the emission wavelength not being limited by the bandgap.
Permeability of SiC preform is a key parameter during the pressure infiltration studies of molten metal (Copper and Aluminium alloy) into porous SiC preforms during preparation of SiC particle reinforced metal matrix composites (MMC) with high content of SiC reinforcement. Its characterization could be quite helpful to reveal the defect formation mechanism of the composite productions and further optimize the preparation processes of such composites. The objectives of this study were to assess the pore distribution and determine the permeability of porous SiC preforms. We compared the values of permeability obtained from experimental data, calculation by Carman-Kozeny equation and simulation of micro-models. The results of experiment illustrated that permeability were 0.64 × 10-12m2(ε=30%), 1.76 × 10-12m2(ε=37%) and 10.20 × 10-12m2(ε=44%) respectively, which are quite similar to the value both of calculation and computer simulation. Additionally, results indicate that the permeability substantially affected by porosity and both size and shape of filling particles into SiC preform.
To fabricate large-scale or unusually shaped composite structures, pieces of fabric plies can be spliced to match size and shape requirements, forming ply splice structures. The junction of different plies can be considered as a defect in the resulting composite material, affecting the overall mechanical properties. In this paper, unidirectional carbon fiber-reinforced plastic (CFRP) with ply splices was used as a research object to study these potential material defects. The effects of ply splices at different positions on the tensile properties of CFRP and the coupling between position of ply splicing were analyzed. Simultaneously, a finite element model was established to analyze the damage evolution, in which a continuous damage model and a cohesive zone model were used to describe the damage of the composite and interface layers, respectively. The model results were in good agreement with observed experimental results. Our results showed that there were three main factors for this failure mechanism: boundary effects, whether the ply splices were independent, or whether they were close to each other. In short, when two ply splices were located at the edge or independent of each other, the failure mode was first delamination and then fiber fracture, and the tensile strength was high. However, when the two ply splices were close to the edge or close to each other, the failure mode was first local fiber fracture and then delamination damage, and the resulting tensile strength was low. Finally, different reinforcement methods to improve the tensile properties of composites were adopted for the splicing layers at different positions through the analysis via model simulation. The two-side patch repair method was used to reinforce the ply splices on or near the edge. Additionally, increasing the toughness of the adhesive layer was used to reinforce the ply splices that were inside the material. These results showed that the tensile strength was enhanced by these two methods of reinforcement, and the initial damage load was especially increased.
To fabricate large-scale or unusually shaped composite structures, pieces of reinforcement plies can be spliced to match specific size and shape requirements, forming ply splice structures. The junction of different plies can be considered a defect in the final material, affecting the mechanical properties. In this paper, ply splice carbon fiber reinforced plastics were studied to analyze the fracture mechanism caused by the ply splice, including the effects of the junction geometry and the ply angle. Tensile tests were performed, assisted with digital image correlation for strain distribution analysis and acoustic emission for break mode analysis. The finite element method was also performed using ABAQUS software to study the fracture mechanism. In order to analyze the interlaminar fracture, the interface was simulated with cohesive elements. The results showed that, for a unidirectional carbon fiber reinforced plastics with ply splice, fracturing occurred first at the junction location and then at the interfaces between the splicing layers and the continuous layers. The final strength was determined by the number of continuous layers. The ply-angle had evident effects on the properties of carbon fiber reinforced plastics with ply splices. For a stacking sequence of [± 30°]5S, the effects of the ply splice on the strength and the fracture mode were similar to the effect with unidirectional plates. For a stacking sequence of [± 45°]5S, the ply splice had no effect on the fracture mode, but it did decrease the strength slightly. For a stacking sequence of [± 60°]5S, almost no effect of the ply splice structure could be observed. PACS (optional, as per journal): 75.40.-s; 71.20.LP
Punching shear failure of slab-column connections can cause the progressive collapse of a structure. In this study, a punching test database is first established. Then, based on the Levenberg–Marquardt (LM) algorithm and using the nonlinear function of the backpropagation neural network (BPNN), a prediction model of the punching capacity of slab-column connections without transverse reinforcement is established. Finally, the proposed model is compared with the formulas of the Chinese, American, and European standards using several methods. The statistical eigenvalue method shows that the BPNN model has the highest accuracy and the lowest dispersion. The defect point counting method shows that the BPNN model had the fewest total number of defects and was the safest and most economical. The influencing factor analysis suggests that factors in the BPNN model had the most reasonable influence on the punching bearing capacity of slab-column connections. Finally, the model is verified using a case study and the Matlab program. The results show that the average error of the formulas in the Chinese, American, and European standards are 21.08%, 30.21%, and 11.47%, respectively, higher than that of the BPNN model.
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