Modulating the cross-plane thermal conductivity of graphite by MnCl2 and FeCl3 co-intercalation

Understanding thermal transport in graphite intercalation compounds can facilitate the development of novel methods to design materials with tunable thermal conductivities. This study reports the effective modulation of the cross-plane lattice thermal conductivity of graphite by co-intercalation with MnCl2 and FeCl3. Scanning transmission electron microscopy and electron energy-loss spectroscopy indicated that the predominant phase in the system corresponded to stage-3 structural ordering with the preferential intercalation of MnCl2, despite the significantly higher amount of FeCl3 used for synthesis (300 times) compared with that of MnCl2, indicating an extremely high intercalation selectivity of MnCl2 in the adopted process-parameter window. Time-domain thermoreflectance measurements demonstrate that intercalation reduced the thermal conductivity of graphite by up to six times at 298 K and 10 times at lower temperatures. The significant tunability of thermal conductivity was obtained for a wide range of thicknesses, varying from 30 nm to 1.5 mu m. A semi-empirical Debye-Callaway model that considered the effect of intercalation in an interfacial-scattering fashion explains the temperature dependence of the thermal conductivity of graphite on intercalation. It revealed that intercalation suppressed the effective phonon-transport length of the system by two orders of magnitude. This study could guide future studies on the fabrication of novel materials with excellent cross-plane thermal-conductivity tunability through intercalation.