Simulation study of phononic crystal structures

Abstract: Thermoelectric materials are important in today’s society with a variety of applications. The material properties that determinate the efficiency of a thermoelectric device are mostly constants. In order to develop more efficient thermoelectric devices new materials and solutions need to be made. Minimizing the thermal conductivity in thermoelectric materials is of great interest to make them more effective.   One effective way of lowering the thermal conductivity is to introduce holes in the lattice and create a phononic crystal. The main focus of this master thesis has been on how to place these holes and to optimize the geometry to minimize thermal conductivity. The method used in this report is based solely on simulations, which were done using Comsol Multiphysics 5.2a. The results are compared to already published results. Different geometries are tested to see how it affects the thermal conductivity and to optimize the phononic crystal. The Maxwell-Eucken model is also used to see how porosity affects the thermal conductivity and to analyze what the lowering in thermal conductivity depends on.   The result showed a substantial reduction in thermal conductivity when creating a phononic crystal compared to bulk silicon. The simulated reduction compared to bulk silicon is up to 98.5% when introducing holes in the lattice, while Maxwell-Eucken only predicted a 74% reduction due to porosity.   The conclusion is that by creating holes in a periodical pattern, phonon dispersion will occur, which lower the thermal conduction significantly.