Direct molecular dynamics simulation of piezoelectric and piezothermal couplings in crystals

The thesis is focused on investigating the effect of strain on the thermal conductivity of piezoelectric materials. Piezoelectric materials are crystals which display a mechanical deformation upon application of an electric field. Examples of such material are ZnO, AlN, and SiO2. Using Molecular Dynamics simulations, we calculate the thermal conductivity of unstrained and strained ZnO and AlN crystals. We also calculate the thermal resistance of SiO/graphene interfaces under strain.We calculate the piezoelectric and elastic properties of ZnO. These will serve as confirmation of the correctness of the inter-atomic potential used, and will serve to show the magnitude of strain that is possible to apply. Using non-equilibrium molecular dynamics, we determine the elastic coefficient of ZnO c33, and we see that it agrees with experimental values. We also determine that the elastic limit of a perfect ZnO crystal is 6 GPa which corresponds to a 6% strain. We also determine the piezoelectric coefficient of ZnO using NEMD, and we find that the piezoelectric coefficient d33 also agrees with literature values.Second, we look at the effect of strain on the intrinsic thermal conductivity of ZnO and AlN. We use reverse non-equilibrium molecular dynamics to calculate the conductivity because the computational costs are significantly lower than those for the equilibrium method; especially for ZnO whose inter-atomic potential contains Coulomb interaction. We also study the size-effect on the thermal conductivity of ZnO and AlN. We show that the Schelling formula can indeed be implemented to both crystals for different values of strain. The infinite length thermal conductivity for ZnO is extracted from the formula, and it is found to be 410 W/mK. We then calculate the thermal conductivity of strained ZnO crystals. We show that after correcting for the size effect the thermal conductivity follows power-law dependence to uniaxial strain. Also, we demonstrate that the thermal conductivity of ZnO can be affected by a static external field due to the induced strain. The infinite length thermal conductivity of AlN is found to be 3000 W/mK. We show that for the case of AlN the effect of strain does not affect the thermal conductivity due to the different inter-atomic bonding. Hence, AlN might not be a useful material for piezothermal application.Third, we explore the effect of piezoelectric strain on the thermal conductance of SiO2/graphene and ZnO/graphene superlattices. Using EMD we calculate the thermal conductivity of a superlattice composed of silica and graphene monolayers. The thermal conductance of the superlattice was evaluated under different values of external electric field. We find that applying a positive electric field parallel to the Z-direction leads to reduction of the thermal conductance by a factor of 2 for an electric field of 20 MV/m. On the other hand, no change in the thermal conductance is noted for ZnO/graphene superlattice. The effect is due to the non-uniform strain induced at the superlattice junctions. The effect is recreated in Si/Ge superlattice by mechanically applying a non-uniform strain at the interface. This approach might be responsible for the scattering of phonons.