Continuum characterization of novel pseudoelasticity of ZnO nanowires

A novel pseudoelastic behavior was recently discovered in [0 1 1¯ 0]-oriented ZnO nanowires under uniaxial tensile loading and unloading. This behavior results from a reversible transformation from the parent wurtzite (WZ) structure to a previously unknown graphitic structure (HX) and is associated with recoverable strains up to 16%. In this paper, a micromechanical continuum model is developed to characterize this behavior. Using the first law of thermodynamics, the model decomposes the transformation into an elastic process of structural transitions between WZ and HX through a sequence of thermodynamically reversible phase equilibrium states and a thermodynamically irreversible process of interface propagation. The elastic equilibrium transition process is modeled with strain energy functions of the two constituent phases which are obtained from independent molecular dynamics calculations. The dissipative interface propagation process is modeled phenomenologically with a function which relates dissipation to the interfacial area between the two phases. The model captures major characteristics of the behavior of wires with lateral dimensions between 20 and 40 Å over the temperature range of 100–500 K.