Ambarish J. Kulkarni

Size-dependent thermal conductivity of zinc oxide nanobelts

The thermal conductivity of [011¯0]-oriented ZnO nanobelts 19–41A in size is characterized over the temperature range of 500–1500K using the Green-Kubo approach. Values obtained are one order of magnitude lower than that for bulk ZnO single crystal. Surface scattering of phonons and the high surface-to-volume ratios of the nanobelts are primarily responsible for the significantly lower values and the size dependence observed. The conductivity is also found to decrease with temperature and this decrease is attributed to thermal softening of the material, three- and four-phonon processes, and optical phonon interactions.

A semi-analytical method for quantifying the size-dependent elasticity of nanostructures

In this paper, a semi-analytical method is developed to compute the elastic stiffness of nanostructures such as nanowires, nanotubes and nanofilms. Compared with existing methods for such computations, this new method is more accurate and significantly reduces the computational time. It is based on the Taylor series expansion of an interatomic potential about the relaxed state of a nanostructure and implicitly accounts for the effects of shape, size and surface of the nanostructures. To analyze the applicability and accuracy of this method, as a case study, calculations are carried out to quantify the size dependence of the elastic moduli of nanofilms and nanowires with [0 0 1], [1 1 0] and [1 1 1] crystallographic growth orientations for groups 10 and 11 transition metals (Cu, Ni, Pd and Ag). The results are in excellent agreement with data in the literature and reveal consistent trends among the materials analyzed.

Tunable thermal response of ZnO nanowires

ZnO nanowires with the growth orientation undergo a reversible phase transformation from wurtzite (WZ) to a graphitic phase (HX) under tensile loading, leading to a pseudoelastic behavior with recoverable strains up to 16%. Here, we report that this phase transform causes a novel transition in thermal response. Molecular dynamics simulations with the Green?Kubo approach are carried out to determine the thermal responses of wires in the 18.95?40.81???size range. Results obtained show that the thermal conductivity in the unstressed WZ state is 8.3?8.6?W?m?1?K?1 for the sizes considered and is an order of magnitude lower than the corresponding bulk value. Under loading, elastic stretching and the formation of interfaces cause the thermal conductivity to first decrease significantly. As the transformation progresses, the conductivity increases rapidly to 10.1?11.1?W?m?1?K?1, or 20.5?28.5% higher than that of the initial WZ-structured wires. The enhancement is primarily due to an increase in atomic packing density, lower anharmonic coupling of phonons and higher surface specularities of the HX-structured wires. This phenomenon offers a means for tuning the thermal behavior of ZnO nanowires.

Characterization of novel pseudoelastic behaviour of zinc oxide nanowires

We recently reported the discovery of a novel pseudoelastic behaviour resulting from a reversible phase transformation from wurtzite (P63 mc) to a novel graphite-like hexagonal (P63/mmc) structure in -oriented ZnO nanowires under uniaxial loading [Phys. Rev. Lett. 97 105502 (2006)]. This previously unknown phenomenon is observed in nanowires and has not been reported for bulk ZnO. In this paper, molecular dynamics simulations are carried out to characterize the tensile behaviour dominated by this transformation of nanowires with lateral dimensions of 18–41 Å over the temperature range of 100–700 K. Significant size and temperature effects on the behaviour are observed. Specifically, the critical stress for the initiation of the phase transformation, the recoverable strains associated with the pseudoelasticity and the hysteretic energy dissipation are found to be both size and temperature dependent and can vary by as much as 59%, 32% and 57%, respectively. The large recoverable strains of 10–16% are unusual for the normally rather brittle ZnO ceramic and are due to both elastic stretch and the phase transformation in the slender one-dimensional nanowires. The hysteretic energy dissipation is in the range 0.05–0.14 GJ m−3 per cycle and such low levels are attributed to the relatively low energy barrier for the transformation. Unlike the pseudoelasticity in fcc metal nanowires of Cu, Ni and Au, which leads to a novel shape memory effect, the pseudoelasticity quantified here does not result in a shape memory of ZnO nanowires. The primary reason is the absence of an energy barrier for the phase transformation at zero stress.

Thermomechanical Behavior of Zinc Oxide Nanobelts

Belt-like ZnO nanostructures with rectangular cross-sections have recently been synthesized through vapor deposition (Pan et al.) [1]. These nanobelts can serve as building blocks for functional nano-systems. The integration of these nanostructures in systems requires understanding of their inherent properties, functionalities and behavior. An atomistic framework is developed to evaluate the thermomechanical behavior of these nanobelts. Molecular dynamics simulations are performed to characterize the response to uniaxial tensile loading. The ultimate tensile strength, strain at failure and Young’s modulus are obtained as functions of temperature, size and growth orientation. The results are compared with the behavior of bulk ZnO.