Nanosized Boron Nitride/Alumina Hybrid Filler-Incorporated PVA Composites as Thermal Interface Materials Enabling Tunable Isotropic/Anisotropic Thermal Conductivity

<b>(Thermal Management Challenges in Electronics)</b><br/>Effective thermal management in miniaturized, densely packed electronics necessitates the development of thermal interface materials (TIMs) possessing a multifunctionality in limited form factors. Hexagonal boron nitride (h-BN) has attracted a lot of attention as functional fillers for next-generation TIMs owing to its exceptional thermal conductivity (185–300 W/mK), excellent electrical insulation (wide band gap of 5.5 eV), and low dielectric constant. However, incorporating high-aspect-ratio, platelet-like h-BN in polymeric backbones significantly increases viscosity, limiting usable amounts and hindering achievable thermal conductivity. Additionally, the intrinsic layered structure of BN causes it to align horizontally under film processing (e.g., shear forces during mixing or casting), leading to a significant reduction in through-plane thermal conductivity.<br/><br/><b>(A Simpler Approach to Isotropic Thermal Conductivity)</b><br/>A common approach to improve the isotropy of BN involves creating spherical shapes derived from the platelet morphology of hexagonal boron nitride (h-BN). This is typically achieved through a complex process involving high-temperature heat treatment followed by grinding/sieving or spray-drying granulation and then vacuum sintering. However, these methods often involve high temperatures, multiple processing steps, and increased costs, limiting their practicality.<br/><br/>In this study, we present novel thermal interface materials (TIMs) utilizing a polyvinyl alcohol (PVA) matrix filled with nanosized boron nitride/alumina hybrid filler to address aforementioned challenges.<br/><br/>We employed a hydroxide-assisted ball milling process to synthesize nano-sized BN with an average lateral size of less than 100 nm, a thickness of 3-4 nm, and an aspect ratio of around 25. We subsequently achieved selective deposition of the nanomaterial onto the surface of micrometer-sized Al2O3 particles through controlled electrostatic interactions in solution. This approach yielded core-shell structures with a uniform, spherical morphology.<br/><br/><b>(Hybrid Fillers and Enhanced Isotropy)</b><br/>Then, we investigated the impact of BN-coated Al2O3 particles on the isotropy of thermal conductivity in TIM films. Hybrid particles with weight ratios of BN to Al2O3 of 1:4 and 4:1 were used as fillers. The particles were mixed with PVA (1:1 weight ratio) and solvent-cast into films.<br/><br/>Compared to films made solely of bare BN (fabricated under the same conditions), the hybrid particle-based samples exhibited a remarkable 180% enhancement in the isotropic ratio (through-plane vs. in-plane thermal conductivity).<br/><br/><b>(Understanding the Mechanism)</b><br/>To elucidate the underlying mechanism, the intensity ratio of the (002) and (100) miller indices of BN in the X-ray diffraction (XRD) profiles of the films was analyzed.<br/><br/>The analysis revealed that the BN particles on the Al2O3 surface were arranged more randomly compared to the bare BN films. This random arrangement allows for better heat flow in all directions, leading to preserved thermal conductivity through the thickness of the film (through-plane direction). This implies that optimizing hybridized fillers consisting of spherical Al2O3 particles with highly conductive BN particles can achieve a directional control of thermal conductivity, even though using a low-cost yet highly feasible process.<br/><br/><b>(Beyond Thermal Performance: Promising Applications)</b><br/>Beyond their impressive thermal conductivity, these films exhibit remarkable bending durability, flexibility, and efficient heat dissipation. This unique combination of properties suggests broad applicability in various fields requiring both in-plane and through-plane thermal conduction, making them highly promising candidates for use as heat spreaders and heat sinks.