Multi-scale Simulation On Tensile Mechanical Properties Of Single-walled Branched Carbon Nanotubes

Branched carbon nanotubes(BCNTs) can realize even more complex function in nanometer devices because of their multilevel structure. Before playing important potential applications, a comprehensive research of branched carbon nanotubes for their mechanical properties should be done to provide theoretical and experimental foundation for future applications. According to this, the atomic-scale finite element method(AFEM) and the molecular dynamics method(MD) are adopted to simulate the mechanical properties of the single-walled straight carbon nanotubes(CNTs) and Y shaped branched carbon nanotubes(Y-BCNTs) under axial tension. The effects of the diameter and the chirality of carbon nanotubes on the tensile mechanical behavior are discussed. The following conclusions can be drawn from the present study:(1) First, atom models of branched carbon nanotube and straight carbon nanotube are built, and then by combining the atomic coordinates both programs of node transformation and element topology for AFEM simulations are developed to construct AFEM models of Y-BCNTs and CNTs.(2) We investigate the tensile strengths of CNTs under axial tensile. Both AFEM and MD numerical results show that the tensile strengths of armchair and chiral CNTs are greater than that of the zigzag CNTs when their diameters are similar. The results also indicate that the tensile strengths of armchair and zigzag CNTs increase gradually with increasing the diameter while that of chiral CNTs fluctuates due to the influence of the diameter and chiral angle.(3) Based on AFEM and MD method, the mechanical behavior of Y-BCNTs under axial tensile is investigated as a function of diameter and chirality. The results show that the tensile strengths of zigzag Y-BCNTs are greater than those of armchair and chiral Y-BCNTs, which is different from the CNTs. We find that the structure of the Y-BCNTs is not perfect. The C bonds at the junction of armchair and chiral Y-BCNTs are subjected to greater load due to some C-C bonds having various angles along the loading direction, thus accelerating the fracture of Y-BCNTs. Both of armchair and chiral Y-BCNTs fracture due to the stress concentration at the junctions,while the zigzag Y-BCNTs fracture near the boundary because the C-C bonds are more stable at the junction. At the same time, the change trend of the tensile strengthsof Y-BCNTs with respect to the diameter is different from CNTs. The tensile strength first decreases to reach the minimum at the diameter about 1.36 nm and then increases with increasing the diameter. The study find that the defects at the junction of the armchair and chiral Y-BCNTs with a diameter of 1.36 nm result in the fracture of C-C bonds at the junction during the initial state.(4) Comparing the numerical results of Y-BCNTs and CNTs, it can be found that the tensile strength of the Y-BCNTs is dependent on the defects at the junction, which is less than the corresponding CNTs. The numerical results also indicate that the chiral angle still has slight influence on the strength of chiral Y-BCNTs. Our study shows that the AFEM and MD methods agree well with each other in predicting the tensile mechanical properties of the Y-BCNTs and CNTs.