Significantly enhanced phonon mean free path and thermal conductivity by percolation of silver nanoflowers

Soft thermal interface materials (TIMs) composed of thermally conductive fillers and polymer matrixes have been widely employed for thermal management in electronic and energy devices. However, the thermal conductivity (κ) of TIMs is significantly smaller than the intrinsic κ of fillers due to the large interfacial thermal contact resistance between fillers. Here we achieve a very efficient thermal percolation network of flower-shaped silver nanoparticles (silver nanoflowers, Ag NFs) in soft polyurethane (PU) matrix TIMs. A record high κ (42.4 W m−1 K−1) is achieved compared with soft isotropic TIMs in the literature. Ag nanoflake–PU and Ag nanosphere–PU TIMs provide significantly smaller κ (7.9 and 15.0 W m−1 K−1) at an identical filler concentration (38 vol%). Surprisingly, the phonon transport of the Ag NF–PU TIM dramatically increases (κlat = 22.2 W m−1 K−1) compared with Ag nanoflake–PU and Ag nanosphere–PU (κlat = 0.2 and 1.2 W m−1 K−1) TIMs. Kinetic theory reveals that the phonon mean free path (39.6 nm) is significantly increased for the Ag NF–PU TIM by the active coalescence of metallic Ag NFs. The hierarchically structured Ag NFs construct an excellent thermal percolation network in soft isotropic TIMs.