Ultralow Thermal Conductivity in Nanoporous Crystalline Fe<sub>3</sub>O<sub>4</sub>
Jin Gu Kang, Hyejin Jang, Jun Ma, Qun Yang, Khalid Hattar, Zhu Diao, Renliang Yuan, Jian‐Min Zuo, Sanjiv Sinha, David G. Cahill, Paul V. Braun
Abstract
While there is no known fundamental lower limit to the thermal conductivity of a material, the lowest thermal conductivities are typically found in amorphous and strongly disordered materials, not highly crystalline materials. Here, we demonstrate a surprising nanostructuring route to ultralow thermal conductivity in a large-unit-cell oxide crystal (Fe3O4) containing close-packed nanoscale pores. The electrical conductivity of this material reduces by a factor of 5 relative to dense Fe3O4, independent of pore size. In contrast, thermal conductivity has a strong dependence on pore size with a factor of 40 of suppression relative to dense Fe3O4 for 40 nm pores vs a factor of 5 for 500 nm pores. The matrix thermal conductivity of Fe3O4 containing 40 nm pores falls below the predicted minimum thermal conductivity by a factor of 3. We attribute this to strong acoustic phonon scattering and intrinsically limited contributions to thermal conductivity from optical phonons with small dispersion.