High Quasi-Isotropic Thermal Conductivity of Polydimethylsiloxane Nanocomposites with Hexagonal-Boron Nitride Microspheres and Flakes

Boron nitride has been widely utilized as a filler to increase the thermal conductivity of polymers due to its unique features, such as electrical insulation, high thermal conductivity, high breakdown strength, and physical and chemical stability. However, the two-dimensional hexagonal-boron nitride (h-BN) has an anisotropic structure, showing a tendency to align horizontally under external pressure, resulting in a limited increase of through-plane thermal conductivity for polymer composites. To address this, we conducted a study where we synthesized h-BN microspheres (sph-nano-BN) using a spray-drying technique from nanosized h-BN (nano-BN). These microspheres, along with a combination of microsized h-BN flakes, are incorporated into polydimethylsiloxane (PDMS) to improve the quasi-isotropic thermal conductivity of the composites. The resulting composites, which contained both large lateral h-BN flakes and sph-nano-BN, exhibited the highest through-plane thermal conductivity of over 6 W/mK, which increased by 3000% compared to that of pure PDMS, thanks to the prevention of the in-plane arrangement of h-BN flakes by sph-nano-BN particles. Additionally, the composites showed a high thermal isotropy (through-plane thermal conductivity/in-plane thermal conductivity: λ ⊥ /λ // ) of 0.75, representing an increase of over 1.5 times compared to that of previous literature. This study presents a simple yet straightforward approach to enhance quasi-isotropic thermal conductivity in polymer composites, making it immediately applicable to the future design of composites for heat removal in versatile electronics.