Thermal Conductivity Characteristics of Three-Layer Superlattices

For thermoelectric applications, the thermal conductivity of the material needs to be reduced as low as possible in order to achieve higher thermoelectric efficiency of devices, as the device efficiency depends on the thermoelectric figure of merit ($Z*T)$. Both theoretical and experimental data show that the cross-plane thermal conductivity of superlattices is much lower than that of the bulk materials The cross-plane thermal conductivity of three-layer superlattices is calculated by a numerical method, which is developed from the lattice dynamical theory. The phonon mean free path is included into the calculation, thus the minimum thermal conductivity occurs at the crossover of the particle-like model and wave-like model of the phonons. The studies focus on the effect of mass ratio, layer thickness, and mean free paths on the minimum thermal conductivity of the three-layer superlattice design. The minimum thermal conductivity of the three-layer superlattice structure is approximately half of that of the conventional two-layer superlattice structure. This finding indicates that the thermoelectric figure of merit for superlattice structure can further be enhanced by 2 times from the use of the three-layer superlattice design.