The Effects Of Structural Defects And Grafts On Elastic Properties Of Carbon Nanotubes

Based on lots of theory calculations and experimental results, some excellent mechanics properties of carbon nanotubes are shown. Then it will be applied widely in the future. But the researches on carbon nanotubes are only in pure ideal structure, and are far away from engineering practice. The mechanism that carbon nanotubes have good mechanics properties is not very clear at present. There are a few researches on mechanics properties of carbon nanotubes affected by the producing defect and impurity, how to obtain carbon nanotubes composite material with good mechanics properties, and other problems in engineering practice.Using the molecular dynamics method, in this paper we have calculated the basic elastic parameters of the armchair and zigzag single-walled carbon nanotubes, and investigated the effects of different vacancy defects, the impurities, and grafts on the elastic moduli. We have given a detail analysis on the numerical results based on the electron cloud coupling laws between two atoms which is determined by the local density approximation (LDA) based on the density functional theory (DFT), the structure laws of their deformation electron density, and the chemical bond theories. The researches in this paper are as follows.The Young's moduli, the balance base vectors, the Poisson's ratios and the energy of single-walled carbon nanotubes are calculated by the first principles. The relationships between basic elastic parameters, structure stability of the carbon nanotubes and their model and diameter are discussed. The results show that the Young's moduli of armchair carbon nanotubes decreases as the structural parameters n of the nanotubes increases; the Young's moduli of zigzag carbon nanotubes increases fluctuately as the chiral vector increases, and the Young's moduli of the metal type tubes are smaller. The Poisson's ratio of the armchair and zigzag single-walled carbon nanotubes almost remain constant and independent of the chiral vector. The diameter of stability carbon nanotubes should have the lower limit value.The effects of the form, ratio, distance and distribution of the vacancy defect in the carbon nanotubes on the Young's moduli of the single-walled carbon nanotubes are studied. The results are analyzed by considering the features of the bondσand bond n between molecules and the theory of the short-range coupling of electron clouds between two defects as well as the theory of 5-1DB defect formations in vacancy defects. It is found that, as the vacancy defect ratio in the carbon nanotubes increases, the Young's moduli decrease. The Young's moduli of the carbon nanotubes display a platform type decrement with increasing of the vacancy defect ratio; the effects of bi-vacancy defects in the carbon nanotubes on the Young's moduli depend on their orientations. As the number of atoms between the two single-vacancy defects increases, and the defects are located in the direction of the axes, the Young's modulus decreases to certain value and fluctuates around it; when the defects are located in the direction of the circumference, the Young's modulus decreases first, then oscillates up and finally becomes a constant, and the Young's moduli will decrease a little with increasing of the distance between the two single-vacancy defects.The effect of doping simplex impurities elements B, N, O, Si, P, S and molecule BN on the Young's moduli of the armchair (5, 5) and zigzag (9, 0) single-walled carbon nanotubes are studied. The results are analyzed by the laws of the potential energy varieties of carbon nanotubes with impure and the electron cloud's coupling between two atoms from LDA based on DFT. The results indicate that the Young's moduli of the carbon nanotubes approximately are a linear decreasing function with increasing of the impure concentration. The decreasing ratio depends strongly on the kinds and doping style of impurity element. When the atomic radius difference between the impurity and carbon element is small, the effects of the impurities on the Young's moduli of carbon nanotubes are relatively small. When impurities molecule BN doped, the Young's modulus effect of the high impurities concentration on the zigzag single-walled carbon nanotubes are little.Grafted by different numbers of carboxylic or hydroxyl functional groups on its ports, the structure of armchair (5, 5) and zigzag (9, 0) single-walled carbon nanotubes is optimized and calculated, the effects of grafts on the Young's moduli of the carbon nanotubes are investigated, and the reasons are analyzed in terms of the isoline structure of deformation electron density, the bond-length and the system potential energy varieties of the grafted carbon nanotubes. The results show that the effects of the grafted carboxyl on the Young's moduli of the zigzag carbon nanotubes are great, the effects of the grafted carboxyl on the Young's moduli of the armchair carbon nanotubes are relatively small. While the effects of the grafted hydroxyl on the Young's moduli of the armchair carbon nanotubes are great, the effects of the grafted hydroxyl on the zigzag carbon nanotubes have few varieties. The conclusion is put forward both the bond cr and bond n have an influence on the Young's moduli of carbon nanotubes, but the effects of the bondσis primary.The researches in this paper can provide not only new conclusions for the mechanics properties of carbon nanotubes, but also important data references for the applied research of carbon nanotubes.