DYNAMICS OF BORON NITRIDE NANOTUBE BASED NANORESONATORS

Abstract

Boron nitride nanotubes were hypothetically predicted in the year 1994 by Rubio et al. and Blasé et al. The same were experimentally realised by Chopra and co-workers in 1995 and laid foundation for the scientific community to conduct further research. As boron nitride nanotube possesses many remarkable properties, research in this field advanced at a breath taking pace, ever since with many unexpected discoveries. The real progress in nanotechnology in the last two decades, has been due to a series of advances in a variety of complementary areas, such as: the discoveries of atomically precise materials like nanotubes associated with development of manipulation techniques to image and manipulate atomic and molecular configurations in real materials. Simultaneously, the conceptualization and demonstration of individual electronic and logic devices with materials at an atomic or molecular level and the advances in computational nanotechnology, i.e., modelling and simulation based on actual physics and chemistry of possible nanomaterials, devices and applications, also complemented the progress in above field. It turns out that at nanoscale, devices and system sizes are sufficiently small, so that, it is possible to describe their behaviour in a more accurate manner. The modelling and simulation techniques have also become predictive in nature and many novel concepts and designs were initially proposed followed by their realization or verification through experiments. Further, the advances in nanotechnology encouraged many researchers to believe that the incorporation of BNNTs in a matrix makes them the ultimate reinforcing material. The primary objective of the present work is to study the dynamic behaviour of boron nitride nanotubes with focus on their mass sensing ability. The computational methods employed in this work include: continuum mechanics, finite element method and hybrid modelling approach. The analytical formulation is based on energy approach and 4th order Runga-Kutta method and Galerkin approach have been employed for obtaining solution of the equations. Continuum mechanics approach is employed to study the effect of mass variations on the single walled and multi walled boron nitride nanotube as well as nanocomposites reinforced with SW-BNNT. The effects of different boundary conditions, viz., cantilever and bridged, on the computed frequencies are investigated for possible applications in vibrating mass sensors or nanoresonators of the femtogram scale mass. Euler-Bernoulli’s principle is utilized for modelling of a straight and wavy BNNT based mass sensor. The excitations are due to the iv changes in mass as well as parametric variations. The results obtained are validated with previously conducted experimental studies. The analytical formulation takes into account the cubic nonlinearity induced in the system due to stretching of the mid-plane for doubly clamped boundary condition and quadratic nonlinearity due to the curvature (waviness) of the nanotube. It is observed that the mass sensitivity of SW-BNNT reaches up to 10-26 kg. The effect of variation in length on frequency is found to be more significant as compared to that of diameter of nanotube, thereby suggesting the suitability of smaller BNNTs in mass sensing applications due to enhanced sensitiveness. The excitations induced in the BNNT system due to presence of defects are also analysed. A hybrid modelling approach is used for development of 3D atomistic FE model consisting of a beam element that represents a BN bond and the point masses at the ends correspond to the atomic mass of boron and nitrogen. The effect of various atomic vacancy defects and their positions is analysed. It is observed that the effect of atomic vacancy is maximum when it is nearer to the fixed end. Further, movement of the vacancy towards the free end results in negative frequency shift. This indicates that the exciting frequency of defective nanotube is larger than that of pristine one. A finite element based continuum mechanics approach is employed for modelling of waviness and pinholes in BNNT. The vibration responses of wavy doubly clamped BNNT for varying waviness factor and length are analysed using techniques such as time series, phase space, Poincaré maps and fast Fourier transforms. Regions of periodic, sub-harmonic, quasiperiodic and chaotic response for wavy SW-BNNT based mass sensing system are observed. A comprehensive numerical study is conducted to investigate the elastic behaviour of SW-BNNT based nano-composite under the effect of various types of loads, using RVE approach. The dynamic behaviour of the same is also investigated for exploring its suitability as a nanoresonator for sensing mass of the order of femtogram (fg) level. Feasibility of SW-BNNT, MW-BNNT and BNNT reinforced composites is explored in view of mass sensing and for bio-medical applications. It is found that SW-BNNT based nanoresonator system can be utilized for monitoring of acetone level (a biomarker for diabetes) present in exhaled human breath in a non-invasive way. The shift in the resonant frequency due to the change in the attached virus/bacterium on MW-BNNT is analysed for identifying its suitability as a bio-nano-mass sensor. The bio compatibility of BNNT is explored by analysing BNNT reinforced composite as a stent material for prolonged use in human heart arteries. v Thus, the present work has been able to establish a basis for development of real time sensors to diagnose diabetes in a non-invasive manner using SW-BNNT; to detect presence of various microbes/viruses having a mass of zeptogram (zg) level using MW-BNNT and to detect masses of fg level using BNNT reinforced composite based nanoresonators working under extreme environment.