Enhancing the thermal conductivity and dielectric properties of polymer composite film through segregated boron nitride nanosheets

Boron nitride nanosheets (BNNS)/polyvinyl (PVA) nanocomposite films with anisotropic thermal conductivities are fabricated by sequential unidirectional freeze casting (UFC). High in-plane TC is made possible by the interconnection and alignment of thermally conductive BNNS through segregating BNNS in between PVA matrix. The nanocomposite films also possess very low dielectric loss arising from the inherent electrically insulating characteristics of BNNS. The unique combination of properties make them a potential candidate for thermal management in integrated circuits and microelectronics. • Segregated and aligned BNNS in PVA matrix are achieved using sequential freeze-casting. • The sequential freeze-casting involves infiltrating BNNS into the microchannels of PVA aerogel followed by hot pressing. • The unique structure yields 267% higher thermal conductivity than conventional composites with dispersed BNNS. • The nanocomposite films possess high in-plane thermal conductivity, good electrical resistivity, and low dielectric loss. • These unique properties make the composite film an ideal candidate for heat dissipation in microelectronics. High interfacial thermal resistance (ITR) between thermally conductive nanofillers and polymer matrix, and lack of good orientation of nanofillers are primary limiting factors in harnessing their inherent thermal conductivity in polymer nanocomposites. Thus, exploiting ultrahigh thermal conductivities of nanofillers involves developing methods or mechanisms that can minimize the ITR. In this work, boron nitride nanosheets (BNNS)/polyvinyl alcohol (PVA) nanocomposite films with segregation-induced interconnection among BNNS are fabricated by a sequential unidirection freeze-casting (UFC) technique. A PVA aerogel is first made by UFC followed by infiltrating functionalized BNNS into its pores and microchannels which is subjected to a second UFC process. The composite aerogel is subsequently hot pressed to compact the available pore channels for reduced ITR arising from better contact between the segregated BNNS cell walls. The resulting segregated BNNS/PVA (SBP) nanocomposite film with 40 wt% BNNS exhibits high thermal conductivity of 5.2 W/mK, which is about 267 % higher than the nanocomposite film containing dispersed BNNS made by conventional UFC. The SBP film also possessed high electrical insulation characteristics and a very low dielectric loss of 10 - 2 at a frequency of 1 kHz, properties arising directly from the segregated BNNS. The sequential UFC provides an effective method to incorporate aligned and interconnected BNNS through segregation for enhanced thermal conductivity and electrical resistivity for thermal management in microelectronics and integrated circuits.