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 (P6(3)mc) to a novel graphite-like hexagonal (P6(3)/mmc) structure in [01 (1) over bar0]-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 angstrom 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 GJm(-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.