Thermal conductivity of skutterudite CoSb3 from first principles: Substitution and nanoengineering effects

CoSb₃-based skutterudites are promising intermediate-temperature thermoelectric materials and fundamental understanding of the thermal transport in CoSb₃ is crucial for further improving its performance. We herein calculate the lattice thermal conductivity κ_L of CoSb₃ with first-principles methods and conduct a comprehensive analysis on phonon mode contribution, relaxation time and mean free path (MFP) distributions. The contribution of optical phonons is found to be significant (28% at 300 K) and important optical modes usually involve two or more pnicogen atoms moving synchronously. The MFP (~135 nm at 300 K) corresponding to 50% κ_L accumulation in CoSb₃ is much larger than that predicted from the kinetic theory (~4 nm), providing an opportunity to reduce κ_L by nanoengineering. The effects of elemental substitution and nanoengineering on κ_L are therefore investigated. A 10% substitution of Sb by As results in 57% reduction of κ_L while the in-plane (cross-plane) κ_L of a 50-nm CoSb₃ thin film is only 56% (33%) of the bulk κ_L at 300 K. The impurity scattering and boundary scattering mainly suppress phonons in different frequency regimes. By combining these two effects, κ_L can be reduced by more than 70% at 300 K, potentially leading to much improved ZT near room temperature.