Strain effects on the thermal conductivity of nanostructures

Applying stress/strain on a material provides a mechanism to tune the thermal conductivity of materials dynamically or on demand. Experimental and simulation results have shown that thermal conductivity of bulk materials can change significantly under external pressure (compressive stress). However, stress/strain effects on the thermal conductivity of nanostructures have not been systematically studied. In this paper, equilibrium molecular-dynamics (EMD) simulation is performed to systematically study the strain effects on the lattice thermal conductivity of low-dimensional silicon and carbon materials: silicon nanowires (one dimensional) and thin-films (two dimensional), single-walled carbon nanotube (SWCNT, one dimensional) and single-layer graphene sheet (two dimensional). Spectral analysis of EMD is further developed and then applied to avoid the numerical artifacts such as the neglect of long-wavelength phonons that are often encountered when using EMD with periodic boundary conditions. Intrinsic thermal conductivity of the simulated bulk and nanostructures can be obtained using spectral analysis of EMD. The thermal conductivity of the strained silicon and diamond nanowires and thin films is shown to decrease continuously when the strain changes from compressive to tensile. However, for SWCNT and single-layer graphene, the thermal conductivity has a peak value, and the corresponding applied strain is at $\ensuremath{-}0.06$ or $\ensuremath{-}0.03$ for SWCNTs depending on the chirality and at zero for graphene, respectively. The following two reasons could explain well the effects of strains on the thermal conductivity of the nanowires and thin films that decreases continuously from compressive strain to tensile strain: (1) mode-specific group velocities of phonons decrease continuously from compressive strain to tensile strain and (2) the specific heat of each propagating phonon modes decrease continuously from compressive strain to tensile strain. However, for SWCNT and single-layer graphene, the mechanical instability induces buckling phenomenon when they are under compressive strains. The phonon-phonon scattering rate increases significantly when the structure buckles. This results in the decreasing behavior of thermal conductivity of SWCNT and graphene under compressive stress and explains the peak thermal conductivity value observed in SWCNT and single-layer graphene when they are under strain. The results obtained in this paper has important implications of challenging thermal management of electronics using advanced materials such as carbon nanotubes and graphene. It also points to a potentially new direction of dynamic thermal management.