Numerical Analysis On The Mechanical Performance Of Carbon Nanotube Junction And Flexible Electronic Devices

Flexible electronic devices can undergo repeated tensile, compressive, folded and twisted deformations, and they are suitable to be applied to complex curvilinear surfaces. Therefore, compared with traditional electronic devices, flexible electronic devices have better application prospects in the fields of bio-medical systems, displaying systems, imaging systems, skin electronics and solar cell circuit board. There are basically two approaches to improve the flexibility of electronic devices:the material-induced flexibility and the structure-induced flexibility. The former is achieved by using new materials (such as carbon nanotube) with extraordinary mechanical and electrical properties instead of silicon, which has only1-2%failure strain. The latter is acquired by changing the structure of electronic device while still using silicon, which has the mature fabrication technology and low price.Although the researches on the flexible electronic devices have achieved great development, the mechanical analysis on the material-induced flexibility and the structure-induced flexibility is still at the exploratory stage. Based on the molecular dynamics (MD), the molecular structural mechanics (MSM), the analytical model and the finite element analysis (FEA) approaches, this dissertation studies the compressive buckling and the tensile failure behaviors of carbon nanotube (CNT) junctions in the filed of the material-induced flexibility, and the strain isolation effect, the velocity influence on the interfacial delamination due to the viscoelastic property of PDMS stamp and the strain distribution during the fabrication process of the flexible electronic devices. The accomplished studies are as follows:Based on the MD simulation, the compressive buckling behavior of CNT junctions has been studied under different strain rates. It is revealed that in the low strain rate range, the junction is prone to quasi-static buckling deformation and the influence of strain rate on the critical compressive strain is not evident; in the high strain rate range, the failure mode of junction is wave propagation. The MD simulation and the MSM approach reveal that the critical compressive strain appears to have an increasing-decreasing trend and the buckling mode transfers from the shell buckling to the column bucking with increase of the length. The increasing-decreasing trend suggests the optimal value of junction length, at which the junction can sustain the biggest load before the onset of buckling, namely the biggest buckling load. The MD simulations have been carried out to study the tensile failure behavior of the junction with different strain rates, temperatures and geometrical factors. It is found that the yield strain has a linear relationship with the temperature and the logarithmic strain rate, respectively. Meanwhile, based on the linear regression approach, the MTST model is capable of systematically predicting the yield strain at specified strain rate and temperature, and the predicted yield strain agrees well with that obtained by the MD simulation. The MD simulations reveal that with different temperatures and geometrical factors the junction has different failure modes:the brittle or ductile failure. This dissertation clarifies the conclusion that diameter determines the failure mode, and proposes the concept that the aspect ratio is the geometrical factor which determines the failure mode.This strain isolation effect invoked by the mesh structure, the structural substrate and the adhesive layer is studied. The FEA results show that the mesh structure (based on the bent bridge and the filamentary serpentine) and the structural substrate can isolate the strain from the substrate, lower the strain of the electronic component and improve the flexibility and stretchablity of the electronic devices. For example, when the flexible electronic devices based on the structural substrate undergo a124%deformation, the electronic component only has a0.3%compressive strain, which is much lower than its intrinsic limit strain. To minimize the stress of electronic component, the FEA analysis also suggests the optimal value of h/a ratio of the trench of the structural substrate. The analytical model based on the shear-lag assumption and the FEA results show that the strain of the electronic component is related to the length of the electronic component layer, the thickness and the Young’s modulus of the adhesive layer. It is also founded that a relatively thick, complicant adhesive layer is effective to reduce the strain of electronic component, and so is a relatively short electronic component layer.This dissertation proposed a simplifed analytical model (crack propagtion model) for the interfacial delamination during the transfer printing process. The Prony parameters are used to model the viscoelastic properties of PDMS stamp and the influence of the pick-up velocity on the interfacial delamination due to the viscoelastic properties of PDMS is explored. When the energy release rate of the crack tip exceeds the critical adhesive energy, the status of the crack will be transferred from close to open, resulting in an interfacial delamination. The analytical model and the FEA results reveal that the required pulling-off force increases with the increase of the velocity at a low pick-up velocity; when the velocity reaches50μm/s, the required pulling-off force will be saturated.This fabrication process of the flexible electronic devices with two-dimensional prestrain is investigated. A simplified analytical model is proposed to obtain the strain of elastic substrate, the profile and the max strain of the electronic component at flat, hemispherical and punched states. The analytical model and the FEA simulations results show that this analytical model can well predict the strain distribution of the elastic substrate and the electronic components.This dissertation further studies the material-induced flexibility and the structure-induced flexibility of the flexible electronic devices. This study is useful in guiding the design and application of the flexible electronic devices and promoting the development of the flexible electronic devices.