On the role of amorphous intergranular and interfacial layers in the thermal conductivity of a multi-walled carbon nanotube–copper matrix composite

The interface in multi-walled carbon nanotube (MWCNT)–metal matrix composites significantly effects the interfacial thermal diffusion characteristics vary due to the presence of an interphase. The thermal conductivities of MWCNT–copper (Cu) composites with and without amorphous impurity layers at the MWCNT–Cu interface as well as the Cu grain boundary have been studied. The measured thermal conductivities of composites containing chemically treated MWCNT with no amorphous impurity layers were much better than those containing pristine MWCNT. Interfacial and intergranular matrix amorphous layers, thought to originate from Cu oxide in Cu powders and amorphous carbon on the surface of the pristine MWCNT, were observed in the pristine MWCNT–Cu composite matrix. Interfacial amorphous layers a few nanometers thick generated at pristine MWCNT–Cu interfaces act as a thermal barrier and due to the relatively large thermal resistance of these layers the MWCNT are expected to act like elongated pores. Intergranular amorphous layers at the Cu grain boundaries also show thermal resistance, resulting in a decrease in the effective thermal conductivity of the Cu matrix. It is expected that amorphous carbon is partially removed by extended chemical treatment with sulfuric acid (H2SO4) and nitric acid (HNO3), inhibiting amorphous layer formation and resulting in an increase in thermal conductivity of the composites. Our results suggest that extended chemical treatment with H2SO4 and HNO3 is necessary not only for uniform dispersion of MWCNT in the ethanol solution, but also for preparation of clean surfaces free of amorphous carbon in the initial MWCNT for effective MWCNT–Cu composite fabrication.