Thermal and mechanical response of [0001]-oriented GaN nanowires during tensile loading and unloading

Abstract

Molecular dynamics simulations are carried out to investigate the thermal and mechanical responses of GaN nanowires with the [0001] orientation and hexagonal cross sections to tensile loading and unloading. The thermal conductivity of the nanowires at each deformed state is calculated using the Green-Kubo approach with quantum correction. The thermal conductivity is found to be dependent on the strain induced by tensile loading and unloading. Phase transformations are observed in both the loading and unloading processes. Specifically, the initially wurtzite-structured (WZ) nanowires transform into a tetragonal structure (TS) under tensile loading and revert to the WZ structure in the unloading process. In this reverse transformation from TS to WZ, transitional states are observed. In the intermediate states, the nanowires consist of both TS regions and WZ regions. For particular sizes, the nanowires are divided into two WZ domains by an inversion domain boundary (IDB). The thermal conductivity in the intermediate states is approximately 30% lower than those in the WZ structure because of the lower phonon group velocity in the intermediate states. Significant effects of size and crystal structure on mechanical and thermal behaviors are also observed. Specifically, as the diameter increases from 2.26 to 4.85 nm, the thermal conductivity increases by 30%, 10%, and 50%, respectively, for the WZ, WZ-TS, and WZ-IDB structured wires. However, change in conductivity is negligible for TS-structured wires as the diameter changes. The different trends in thermal conductivity appear to result from changes in the group velocity which is related to the stiffness of the wires and surface scattering of phonons.

Surface-induced structural transformation in nanowires

“…The aforementioned surface-induced structural transformation can be extended to metal oxide [96,100] and nitride [101] nanowires. This broadens the scope because of the extreme sensitivity of electronic and optical properties to the structures in metal oxides and nitrides.…”

“…Hence, surface effects are dominant factors affecting the structure of nanowires and can even induce thorough structural transition. On the heels of these discoveries, other interesting materials properties such as surfaceinduced lattice reorientation, martensitic phase transformation (MT), pseudo-elastic behavior, and shape memory effects (SMEs) have been studied and observed from fcc [14,[19][20][21][22][23]28,[63][64][65][66][67][68][69][70][71][72][73][74][75], bcc [76][77][78][79][80][81][82], and hcp [83,84] single-element, layered composite [85][86][87][88][89], intermetallic alloy [90][91][92][93][94][95], and even metal oxide [96][97][98][99][100] or nitride [101] compound nanowires. The reversible strain can be as high as 40-70% and is much lar...…”

Thermal conductivity prediction for GaN nanowires from atomistic potential

“…This model fails to explain the thermomechanical behaviors of [0001]-oriented GaN nanowires in Ref. 23 because thermal conductivities are different at the same strain in the loading and unloading processes due to the difference in crystal structures. This model is valid only for materials with constant structures.…”

“…This trend is similar to the result for GaN nanowires reported by us previously. 23 However, the dependence of thermal conductivity on the crystalline structures cannot be described properly. This model fails to explain the thermomechanical behaviors of [0001]-oriented GaN nanowires in Ref.…”

Influence of Prestress Fields on the Phonon Thermal Conductivity of GaN Nanostructures

“…The Callaway's relaxation-time theory was also utilized to simu late the thermal conductivity of bulk and films of GaN through taking into account different phonon scattering mechanisms [18]. Besides of the continuum models, the atomic methods are also performed to calculate the thermal conductivity of GaN bulks as well as GaN nanostructures [18][19][20]. However, it is needed to point out that these experimental and theoretical studies have not accounted for the influence of the stress fields on the thermal con ductivity of GaN nanostructures.…”