(571d) Molecular Dynamics Simulation Study of Transition Metal Nanowires under Uniaxial Tension: Temperature and Strain Rate Effects

Nanomaterial sensing layers are employed for chemical and biological species detection in surface acoustic wave (SAW) sensors¹. The propagation of SAW results in these nanowires being subjected to continuous stresses. Knowledge of the mechanical properties of these nanowires is important to establish the stability and robustness of nanomaterial based SAW sensors.

Molecular dynamics (MD) simulation of infinitely long, cylindrical bimetallic Pd-Pt nanowires, with an approximate diameter of 2.3 nm and varying compositions (25 and 50% Pt) are used to investigate its mechanical properties. The nanowires are subjected to uniaxial tensile strain along the [001] axis. The empirical quantum Sutton-Chen potential function is used to describe the inter-atomic potential between the various transition metal atoms. A loose-coupling thermostat (Berendson) is selected for finite-temperature control of the simulated system, with a time constant of 25% of the total relaxation time during each strain increment. Our previous study indicates that the low temperature stable phases of 25 and 50% Pt nanowires exhibit hcp and fcc crystal type, respectively². In the present simulation, these nanowires are subjected to varying strain rates of 0.05%, 0.5%, and 5.0% ps^-1, at simulation temperatures of 50 and 300 K, in order to study the effects of different strain rates and thermal conditions on the deformation characteristics and mechanical properties of the nanowire. Analyses of the changes in crystal structure associated with the wire deformation have been carried out and are used to deduce its mechanical properties. Comparisons to the behavior exhibited by pure Pd and Pt nanowires of similar diameter are also carried out in the present study.

At low temperature and strain rate, where crystal order and stability are highly preserved, the calculated stress-strain response of pure Pt³ and Pd nanowires showed clear periodic, stepwise dislocation-relaxation recrystallization behavior. At low strain rates (0.5% ps^-1), the onset of amorphous crystal deformation occurs⁴, and fully amorphous deformation takes place at high strain rates (5.0% ps^-1), with amorphous melting detected at 300 K. Due to higher entropy of the nanowire at higher temperature and strain rate, periodic stress-strain behavior becomes less clearly defined, and superplasticity behavior is observed. Deformation of nanowire at low strain rates results in the development of a single-walled helical substructure at 300 K and lower strain rates, when the superplasticity characteristic is significantly enhanced. We find that the Young's modulus is about 50%-75% of the bulk Pd and Pt value, while the Poisson ratio is not significantly changed at the nanoscale. The regimes over which similar crystal structure transformation take place in bimetallic nanowires are identified and compared to those seen in pure Pd and Pt nanowires. The results are expected to throw light on the higher mechanical stability exhibited by Pd-Pt nanowire alloys of certain composition range over pure Pd and Pt nanowires.

Reference:

1. Srinivasan, K; Cular, S; Bhethanabotla, V. R.; Lee, S-Y; Harris, M. T. and Culver, J. N. Proceedings of IEEE Ultrasonics symposium (2005)

2. Sankaranarayanan. S.K.R.S, Bhethanabotla, V. R. and Joseph, B. Accepted in Phys. Rev. B (2006)

3. S. J. A. Koh, H. P. Lee, C. Lu, and Q. H. Cheng, Phys. Rev. B 72, 085414 (2005)

4. Hideyuki, I., Y. Qi, T. Cagin, K. Samwer, W. L. Johnson and W. A. Goddard III. Phy. Rev. Letts. 82 (14), 1999.