ABSTRACT
We have investigated the nano-indentation responses of graphene/aluminum systems via computational nano-indentation processes by using molecular dynamics simulations. The effects of system temperature, grain-orientation and bilayer graphene are also investigated. We demonstrate that, the graphene coating enlarges the load-carrying area by about 5.36 times and changes the deformation behaviors of aluminum substrate during nano-indentation processes. The load bearing capacity of graphene/Al system is significantly improved by about 4.7 times compared with that of bare Al system. It is revealed that higher system temperature weakens the ultimate indentation depth and corresponding load. The grain orientation of aluminum substrate hardly affect the indentation mechanical properties of graphene/Al system. The strengthening effect of bilayer graphene is about 1.5 times that of monolayer graphene.
ABSTRACT
We investigate chirality and grain boundary (GB) effects on indentation mechanical properties of graphene coated on nickel foil using molecular dynamics simulations. The models of graphene with different chirality angles, different numbers of layers and tilt GBs were established. It was found that the chirality angle of few-layer graphene had a significant effect on the load bearing capacity of graphene/nickel systems, and this turns out to be more significant when the number of layers is greater than one. The enhancement to the contact stiffness, elastic capacity and the load bearing capacity of graphene with tilt GBs was lower than that of pristine graphene.
ABSTRACT
Distributed Bragg reflectors (DBR) with metal film on the bottom have been demonstrated to further improve the light output power of GaN-based light-emitting diodes (LEDs). Periods of TiO2/SiO2 stacks, thickness of metal film, and material of metallic reflector were designed and optimized in simulation software. The maximal bandwidth of double-DBR stacks have reached up to 272 nm, which was 102 nm higher than a single-DBR stack. The average reflectance of LEDs with wavelength from 380 nm to 780 nm in double-DBR stacks is 95.09% at normal incident, which was much higher than that of a single-DBR stack whose average reflectance was 91.38%. Meanwhile, maximal average reflectance of LEDs for double-DBR stacks with an incident angle from 0 to 90° was 97.41%, which was 3.2% higher than that of a single-DBR stack with maximal average reflectance of 94.21%. The light output power of an LED with double-DBR stacks is 3% higher than that of an LED with a single-DBR stack, which was attributed to high reflectance of double-DBR stacks.
