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 k(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 (similar to 135 nm at 300 K) corresponding to 50% k(L) accumulation in CoSb3 is much larger than that predicted from the kinetic theory (similar to 4 nm), providing an opportunity to reduce k(L) by nanoengineering. The effects of elemental substitution and nanoengineering on k(L) are therefore investigated. A 10% substitution of Sb by As results in 57% reduction of k(L) while the in-plane (cross-plane) k(L) of a 50-nm CoSb3 thin film is only 56% (33%) of the bulk k(L) at 300 K. The impurity scattering and boundary scattering mainly suppress phonons in different frequency regimes. By combining these two effects, k(L) can be reduced by more than 70% at 300 K, potentially leading to much improved ZT near room temperature.