Phonon scattering at the interfaces of epitaxially grown Fe2VAl/W and Fe2VAl/Mo superlattices

Abstract

We investigated the cross-plane thermal conductivity and boundary thermal resistance of epitaxially grown superlattice samples. These included iron vanadium aluminum alloy (Fe2VAl) and tungsten (W) or molybdenum (Mo) prepared by radio frequency magnetron sputtering. We used out-of-plane X-ray diffraction measurements to confirm the epitaxial growth of the superlattices. The superlattice with the shortest period of 1.9 nm showed satellite reflections indicating a highly ordered structure. Misfit dislocations in the 2-nm-thick Mo layer were also identified by high-resolution transmission electron microscopy. We attribute these features to the large lattice mismatch between the Fe2VAl and Mo lattices. The cross-plane thermal conductivity of the superlattice decreased as the number of interfaces increased because of thermal resistance at interlayer boundaries. A simulation of the thermal conductivity by the constant boundary resistance model reproduced the experimental data but with some deviations for samples with a smaller period (less than 20 nm). This deviation from the simulation results indicates that the phonon modes contributing to the thermal transport changed and/or the phonon scattering probability at the interfaces decreased. The superlattice of Fe2VAl/W with a period thickness greater than 20 nm had a boundary thermal resistance approximately twice as large as that of the Fe2VAl/Mo superlattice. This result provides clear evidence for the effects of the mass difference of layers on phonon scattering at an interface.