Atomistic modelling of electromechanical response of pristine and defective boron nitride nanotubes

Abstract

Recent breakthroughs in the synthesis of boron nitride nanotubes (BNNTs) attracted researchers’ attention again for developing their nanocomposites. This is due to the fact that BNNT possesses a wide band gap (~5.5 eV, independent of geometry), strong hardness, chemically and thermally stable, and excellent piezoelectric properties than its carbon-based counterpart. Furthermore, BNNTs have comparable mechanical and thermal properties compared to carbon nanotubes. In this thesis, molecular dynamics (MD) simulations were carried out with a three-body Tersoff potential force field to predict the electromechanical response of BNNTs. The investigation of transversely isotropic elastic and electronic properties of pristine and defected single-walled BNNTs (SWBNNTs) is accomplished by imposing uniaxial tension, twisting moment, in-plane shear and in-plane biaxial tension to the BNNTs. Two types of defects, namely, vacancy and Stone-Wales (SW) defects were considered. In addition, effects of various factors such as chirality and diameter of BNNTs, vacancy concentration, distribution of vacancy pores along the length and circumference of tubes, and SW defect density were critically examined. Our study reveals that the elastic coefficients of BNNTs decrease as their diameter increase, except axial Young’s modulus. Young’s modulus of SWBNNT increases with the diameter and reaches its maximum value when the tube diameter is ~14 Å, and then it starts decreasing. We also found that the axial Young’s modulus of a BNNT increases as its aspect ratio increases and stabilizes at a particular value of aspect ratio (L/D~15). The vacancies and SW defect greatly affect the elastic properties of SWBNNTs. The failure mechanism of SWBNNT under each loading condition was discussed in detail. Our study reveals that the elastic moduli of zigzag SWBNNTs are higher than the armchair tubes and decrease as the diameter of a tube increases. The effect of SW defects is found to be higher on the elastic properties of smaller diameter SWBNNTs than the larger diameter tubes regardless of chirality. The temperature-dependent transversely isotropic elastic properties of multi-walled BNNT (MWBNNTs) were also determined. The effect of chirality, number of layers and aspect ratio (AR) were taken into consideration. The results reveal that the elastic constants of MWBNNTs decrease as their number of layers increase. The elastic moduli of MWBNNTs do not depend on the AR but are the function of chirality. Furthermore, the effect of temperature on the transversely isotropic elastic constants of MWBNNTs was studied. The higher temperature considerably affects the mechanical properties of MWBNNTs. The results reveal that the mechanical properties and failure behavior of MWBNNTs significantly depend on the number of layers, chirality and temperature. The piezoelectric coefficients were determined by applying the electric field in the axial direction of BNNTs. The effect of diameter and different types of atom vacancies and their positions were taken into consideration. Our results reveal that the vacancy defects significantly influence the piezoelectric coefficients of BNNTs, in some cases, increase the electromechanical response of defective tubes over pristine ones. The results showed that the piezoelectric coefficients of BNNTs strongly depend on the position of vacancies and the maximum enhancement in their values was observed as 17% and 13% for B mono vacancy and di vacancy of B–N bond II, respectively, over that of pristine BNNT. The increase in the B mono-vacancies and removal of B–N bonds II improve the piezoelectric coefficients of BNNTs. Moreover, the enhancement in the piezoelectric coefficients of a BNNT is found to be significant when the vacancy defect breaks its symmetry. The current results are explained using the axial viral stresses generated in the BNNTs subjected to the electric field. The critical buckling load and strain of SWBNNTs containing various vacancy and SW defects with their piezoelectric response also investigated. Our result reveals that the critical buckling load and strain of vacancy and SW defected zigzag SWBNNTs are lower than the pristine one. The critical buckling load and strain decrease with increase in vacancy concentration and the reverse is true in case of SW defect density. The electric field has a strong influence on the buckling behavior of zigzag SWBNNTs compare to armchair ones. The piezoelectric coefficient of zigzag SWBNNTs decreases with increase in vacancy concentration while SW defect does not affect the piezoelectric coefficient of zigzag SWBNNTs. The present work shows that we can enhance/alter the electromechanical properties of BNNTs via a novel pathways of defect engineering by introducing different types of defects and changing their positions to suit a particular NEMS applications. Keywords: Atomistic modeling; Boron nitride nanotube; Buckling behavior; Elastic properties; Electric field; Molecular dynamics simulation; Piezoelectric coefficient; SW defects; Transversely isotropic elastic properties; Vacancy defects; fracture behavior.