Carbon nanotube covalent bonding mediates extraordinary electron and phonon transports in soft epoxy matrix interface materials

Soft thermal interface materials (TIMs) are essential components for the thermal management of electrical and mechanical devices. However, the low thermal conductivity (κ) still limits efficient heat dissipation. Here we achieve a record high κ (101.7 W m⁻¹ K⁻¹) among soft isotropic TIMs, mediated by the covalent bonding between hydroxyl groups of multiwalled carbon nanotubes (MWNTs) and ester groups of soft epoxy (SE) matrix. It is important to maintain a high relative humidity (60%) during synthesis since the binding reaction is initiated by capturing a proton from environmental moisture. The covalent bonding uniformly distributes MWNTs (0.5 vol%) which construct percolation networks with additional Ag flake fillers (33.5 vol%). The solid electron and phonon contact at the interface is achieved by the coalescence between Ag flakes and Ag nanoparticles pre-functionalized on MWNTs (nAgMWNTs). This dramatically increases electron (11.9 nm) and phonon (105.7 nm) mean free paths and κ. The dispersion of nAgMWNTs is poor without the hydroxyl-ester binding, and the identical filler combination provides very low κ (9.7 and 0.9 W m⁻¹ K⁻¹) in polyurethane and silicone rubber matrices. The κ in SE matrix is 11,300% greater than that in silicone rubber matrix. The soft isotropic nAgMWNT‒Ag flake‒SE TIM shows excellent heat dissipation performance of a cellular phone application processor.