Evolution of the electrical contact of dynamic pantograph–catenary system

A good contact between the pantograph and catenary is critically important for the working reliability of electric trains, while the basic understanding on the electrical contact evolution during the pantograph--catenary system working is still ambiguous so far. In this paper, the evolution of electric contact was studied in respects of the contact resistance, temperature rise, and microstructure variation, based on a home-made pantograph-catenary simulation system. Pure carbon strips and copper alloy contact wires were used, and the experimental electrical current, sliding speed, and normal force were set as 80 A, 30 km/h, and 80 N, respectively. The contact resistance presented a fluctuation without obvious regularity, concentrating in the region of 25 and 50 mf~. Temperature rise of the contact point experienced a fast increase at the first several minutes and finally reached a steady state. The surface damage of carbon trips in microstructure analysis revealed a complicated interaction of the sliding friction, joule heating, and arc erosion.

Influence of Surface Condition on Expulsion in Spot Welding AZ31B Magnesium Alloy

Experiments were carried out to study the influence of surface condition on expulsion during the spot welding of AZ31B Mg alloy. A general electrical contact resistance theory for conductive rough surfaces and the relation between maximum temperature Tm in the contact and voltage-drop V across interface of two surfaces were employed to understand the reason of expulsion in Mg alloy spot welding. The main reason of expulsion is that the high electrical contact resistance induced by large roughness of the surface and oxide film covered on the surface leads to local melting of metal in the interface of two surfaces, and liquid metal of the local area ejected from the specimen under electrode force forms expulsion.

A novel contact engineering method for transistors based on two-dimensional materials

Contact engineering is of critical importance for two-dimensional(2D)transition metal dichalcogenide(TMD)-based devices.However,there are only a few solutions to overcome this obstacle because of the complexity of the TMD-contact interface.In this work,we propose a novel method using a soft plasma treatment followed by the seamless deposition of a metal electrode to reduce the contact resistance of MoS_(2)field effect transistors(FETs).The treated FETs exhibit three times higher mobility than the control FETs without plasma treatment.The soft plasma treatment can remove the facial sulfur atoms and expose the middle Mo atoms so that they come into direct contact with the metal electrode,thus greatly improving the contact behavior.First-principles calculation is also performed to support the experimental results.Our potentially scalable strategy can be extended to the whole family of TMD based FETs to provide a possible route of device processsing technology for 2D device application.

Effect of assembly pressure on the performance of proton exchange membrane fuel cell

The assembly force is a crucial factor in the process of proton exchange membrane fuel cell(PEMFC)stacking,and has significant effects on the fluid flow,mass transfer,and water and thermal management,which affect the fuel cell performance.In this study,from the most deformable component,the gas diffusion layer(GDL),combining with a finite-element analysis,and computational fluid dynamic method,the impact of the assembly force on a single-channel PEMFC is analyzed.A nonlinear stress-strain curve obtained from a microanalysis is creatively introduced into the two-dimensional compression model.The gas diffusion coefficient in the three-dimensional model is also obtained from the microscopic simulation.The simulated effective oxygen diffusion coefficient of the compressed GDL is approximately 0.86 times the Bruggemann estimated value.When the contact resistance is ignored,the output voltage at 2.5 MPa is decreased by approximately 15.4%at 1.7 A·cm^(−2)compared with that at 0.5 MPa.After the contact resistance is considered,the effects of the assembly pressure on the cell performance(V-I curve)are qualitatively different.The pressure drop of the 2.5 MPa case is 20%higher than that of the 1.4 MPa case at 1.7 A cm^(−2).O_(2)is hard to flow into the region under the rib where the porosity and permeability are lower.The results indicate that both liquid water and membrane water contents increase when the assembly force increases.The effect of the assembly force on the temperature is also analyzed.

Spatial evolution of friction of a textured wafer surface

Mechanical failure of integrated circuits and micro-electro-mechanical systems(MEMS)demands new understanding of friction in small devices.In present research,we demonstrated an in situ approach to measure sliding friction of a patterned surface composing multi-materials and structures.The effects of materials and surface morphology on friction and electrical contact resistance were investigated.The material transfer at the interface of dissimilar materials was found to play dominating roles in friction.The current work provides important insights from the fundamentals of friction that benefit the design of new micro-devices.