▎ 摘　　要

Due to the ultra-high thermal conductivity (k) of graphene, graphene-based materials are expected to be good thermal conductors. Here, however, we uncovered extremely low k of ultralight graphene aerogels (GAs). Although our GA (similar to 4 mg cm(-3)) is about two times heavier than air (similar to 1.2 mg cm(-3)), the k (4.7 x 10(-3)-5.9 x 10(-3) W m(-1) K-1) at room temperature (RT) is about 80% lower than that of air (0.0257 W m(-1) K-1 at 20 degrees C). At low temperatures, the GA's k reaches a lower level of 2 x 10(-4)-4 x 10(-4) W m(-1) K-1. This is the lowest k ever measured to our best knowledge. The mechanism of this extremely low k is explored by studying the temperature variation of GA's k, thermal diffusivity (alpha) and specific heat (c(p)) from RT to as low as 10.4 K. The uncovered small, yet positive partial derivative alpha/partial derivative T reveals the dominant interface thermal contact resistance in thermal transport. For normal materials with thermal transport sustained by phonon-phonon scattering, partial derivative alpha/partial derivative T always remains negative. The study of c(p) suggests highly disordered and amorphous structure of GAs, which also contributes to the ultralow k. This makes the GA a very promising thermal insulation material, especially under vacuum conditions (e.g. astronautics areas). (C) 2015 Elsevier Ltd. All rights reserved.