Strongly Anisotropic Thermal Conductivity of Free‐Standing Reduced Graphene Oxide Films Annealed at High Temperature

Thermal conductivity of free‐standing reduced graphene oxide films subjected to a high‐temperature treatment of up to 1000 °C is investigated. It is found that the high‐temperature annealing dramatically increases the in‐plane thermal conductivity, K , of the films from ≈3 to ≈61 W m −1 K −1 at room temperature. The cross‐plane thermal conductivity, K ⊥ , reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m −1 K −1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal conductivity, K/K ⊥ ≈ 675, which is substantially larger even than in the high‐quality graphite. The electrical resistivity of the annealed films reduces to 1–19 Ω □ −1 . The observed modifications of the in‐plane and cross‐plane thermal conductivity components resulting in an unusual K/K ⊥ anisotropy are explained theoretically. The theoretical analysis suggests that K can reach as high as ≈500 W m −1 K −1 with the increase in the sp 2 domain size and further reduction of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.