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The increasing demand for high energy storage devices calls for concurrently enhanced dielectric constants and reduced dielectric losses of polymer dielectrics. In this work, we rationally design dielectric composites comprising aligned 2D nanofillers of reduced graphene oxide (rGO) and boron nitride nanosheets (BNNS) in a polyvinylidene fluoride (PVDF) matrix through a novel press-and-fold technique. Both nanofillers play different yet complementary roles: while rGO is designed to enhance the dielectric constant through charge accumulation at the interfaces with polymer, BNNS suppress the dielectric loss by preventing the mobility of free electrons. The microlaminate containing eight layers each of rGO/PVDF and BNNS/PVDF films exhibits remarkable dielectric performance with a dielectric constant of 147 and an ultralow dielectric loss of 0.075, due to the synergistic effect arising from the alternatingly electrically conductive and insulating films. Consequently, a maximum energy density of 3.5 J/cm3—about 18 times the bilayer composite counterpart—is realized. The high thermal conductivities of both nanofillers and their alignment endow the microlaminate with an excellent in-plane thermal conductivity of 6.53 Wm−1K−1, potentially useful for multifunctional applications. This work offers a simple but effective approach to fabricating a composite for high dielectric energy storage using two different 2D nanofillers.

HighlightsHighly porous aerogel with longitudinally aligned channels and whisker-like ligaments is constructed by solvent-assisted unidirectional freezing.The thermal insulation and solar reflection capabilities of the composite aerogel reach a state-of-the-art level.The composite aerogel capable of infrared stealth and temperature preservation presents great potential for application in energy-saving buildings.With the mandate of worldwide carbon neutralization, pursuing comfortable living environment while consuming less energy is an enticing and unavoidable choice. Novel composite aerogels with super thermal insulation and high sunlight reflection are developed for energy-efficient buildings. A solvent-assisted freeze-casting strategy is used to produce boron nitride nanosheet/polyvinyl alcohol (BNNS/PVA) composite aerogels with a tailored alignment channel structure. The effects of acetone and BNNS fillers on microstructures and multifunctional properties of aerogels are investigated. The acetone in the PVA suspension enlarges the cell walls to suppress the shrinkage, giving rise to a lower density and a higher porosity, accompanied with much diminished heat conduction throughout the whole product. The addition of BNNS fillers creates whiskers in place of disconnected transverse ligaments between adjacent cell walls, further ameliorating the thermal insulation transverse to the cell wall direction. The resultant BNNS/PVA aerogel delivers an ultralow thermal conductivity of 23.5 mW m−1 K−1 in the transverse direction. The superinsulating aerogel presents both an infrared stealthy capability and a high solar reflectance of 93.8% over the whole sunlight wavelength, far outperforming commercial expanded polystyrene foams with reflective coatings. The anisotropic BNNS/PVA composite aerogel presents great potential for application in energy-saving buildings.

Thermal management can be broadly divided into two, based on the thermal conductivity (TC) requirement. The first involves thermal management requiring very low TC which is expected to be lower than the TC of air (24 mW/mK) and they are often referred to as thermally insulating materials. Thermally insulating materials are commonly used to reduce energy consumption in buildings. Most commercially available thermally insulating products possess only low thermal conductivities but poor insulating capabilities in the daytime with little sunlight reflectance and thermal emittance. The second involves thermal management requiring high TC and they are often used as thermal interface materials to extract heat from electronic components and transfer to heat sinks. Achieving high TC involves tailoring the structure to minimize the interfacial thermal resistance (ITR) and creating a conduction pathway to enhance phonon transport. This thesis proposes different approaches to tailor the structures of composite to achieve desired thermal and optical properties, as summarized below.To develop thermally insulating materials with excellent optical properties, anisotropic boron nitride nanosheet (BNNS)/polyvinyl alcohol (PVA) composite aerogels are developed using the unidirectional freeze-casting technique. Benefitting from the aligned porous structure, the composite aerogel with an optimal BNNS content exhibits a combination of an ultralow TC of 20.3 mW/mK in the through-thickness direction, a high solar-weighted reflectance of 95.0 % over the whole sunlight wavelength and a high emittance of above 93% within the atmospheric transparency window. These exceptional thermo-optical properties enable the composite aerogel to maintain the interior temperature much cooler than commercially available foams, making them promising candidates as super-insulating envelopes for energy saving in buildings towards carbon neutrality.To minimize the ITR in nanocomposite and to develop a thermally conductive yet electrically insulating material, BNNS nanocomposite films were fabricated by infiltrating functionalized BNNS into the pores and microchannels of PVA aerogel. The composite was subsequently hot pressed to compact the available pore channels for better contact between the segregated cell walls. The developed segregated BNNS/PVA (SBP) nanocomposite film with 40 wt% of BNNS exhibits high TC of 5.2 W/mK, which is about 1400% improvement on the neat PVA film. The high thermal conductivity can be attributed to interconnected BNNS within the pore channel and surface of the film, which creates a phonon pathway within and outside the composite film. The BNNS also endows the composite film with very low dielectric loss of about 10-2 without significantly deteriorating the dielectric constant of the composite film. Thus, the low dielectric loss composite film can ensure reliability of electronics during service. The efficient infiltration of BNNS into the PVA microchannel enhances the overall TC, thus providing a framework for developing and scaling highly thermal conductive nanocomposite films. The high TC, low dielectric constant and very low dielectric loss position of the nanocomposite film serve as potential materials for thermal management in microelectronics and integrated circuits.The findings of this study highlight the excellent contribution of BNNS design structures towards thermal management for applications with extreme TC requirements and it provides insight into the development of insulating aerogels and thermally conductive films.