THERMOMECHANICAL BEHAVIOR OF GAN NANOWIRES DURING TENSILE LOADING AND UNLOADING

Structural phase transformations from wurtzite to a graphite-like phase and from wurzite to a tetragonal phase in ZnO and GaN nanowires under tensile loading cause significant changes in thermal conductivity. Molecular dynamics simulations are carried out to investigate the effect of deformation and the phase transformations on the thermal conductivity of [0001]-oriented GaN nanowires. The calculation of conductivity at each state of the nanowires is based on the equilibrium Green-Kubo approach with quantum correction. Under tensile loading, initially wurtzite-structured wires transform into a tetragonal structure, causing the thermal conductivity to decrease as the strain increases. Unloading of the transformed wires is associated with a reverse transformation from the tetragonal structure to the wurtzite structure. Opposite to what is observed during the forward transformation during loading, the thermal conductivity increases as the strain decreases in the unloading process. However, during the reverse transformation, the nanowires consist of both tetragonal regions and wurtzite-structured regions. Such intermediate states are not observed during the loading process. Phonon scattering at the interfaces between the wurtzite and tetragonal regions during unloading causes the thermal conductivity of the nanowires to be lower compared with that during loading.