Flexible thermally conductive and electrically insulating silicone rubber composite films with BNNS@Al2O3 fillers

With the advent of 5G era, the electronic and electrical industries are in increasingly urgent demand for flexible materials with high thermal conductivity coefficients (λ). In this paper, alumina (Al2O3) was chemically grafted to the surface of boron nitride nanosheets (BNNS) to prepare hetero-structured thermally conductive fillers (BNNS@Al2O3), and two-component room temperature vulcanized silicone rubber (RTV-2SR) was used as the polymer matrix. Then BNNS@Al2O3/RTV-2SR flexible thermally conductive and electrically insulating composite films were prepared by shear-coating. Results of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscope (SEM) showed that the BNNS@Al2O3 hetero-structured thermally conductive fillers with “point-plane” structure were successfully prepared. The BNNS@Al2O3 hetero-structured thermally conductive fillers successfully achieved the synergy of Al2O3 and BNNS and had more significant improvement on λ than single Al2O3, single BNNS, and simply blending BNNS/Al2O3. When the amount of BNNS@Al2O3 was 30 wt% (BNNS/Al2O3, 1:1, wt:wt), the in-plane λ (λ∥) and through-plane λ (λ⊥) of the BNNS@Al2O3/RTV-2SR thermally conductive composite film reached 2.86 W/mK and 0.89 W/mK, 2.8 times λ∥ (1.01 W/mK) and 4.2 times λ⊥ (0.21 W/mK) pure RTV-2SR film, also higher than those of BNNS/RTV-2SR (with single BNNS, λ∥ = 2.46 W/mK, λ⊥ = 0.76 W/mK), Al2O3/RTV-2SR (with single Al2O3, λ∥ = 1.62 W/mK, λ⊥ = 0.49 W/mK), and (BNNS/Al2O3)/RTV-2SR (with simply blending BNNS/Al2O3, 1:1, wt:wt, λ∥ = 2.04 W/mK, λ⊥ = 0.62 W/mK) thermally conductive composite films with the same fillers amount. Meanwhile, the prepared BNNS@Al2O3/RTV-2SR thermally conductive composite film obtained excellent mechanical properties, electrical insulation properties, and thermal properties. Its tensile strength, elongation at break, hardness, dielectric constant (e), dielectric loss (tanδ), volume resistivity (ρv), glass transition temperature (Tg), and heat resistance index temperature (THRI), respectively, reached 0.84 MPa, 175%, 27 HA, 3.89, 0.0088, 5.76 × 1011 Ω·cm, − 40.1 °C and 185.3 °C. This work can provide a new idea for the preparation of hetero-structured thermally conductive fillers and flexible thermally conductive and electrically insulating composite films. Schematic diagrams for hetero-structured BNNS@Al2O3 fillers, BNNS@Al2O3/RTV-2SR thermally conductive composite films and corresponding thermal conduction mechanism.