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

One-step formation of three-dimensional interconnected T-shaped microstructures inside composites by orthogonal bidirectional self-assembly method

The fillers inside a polymer matrix should typically be self-assembled in both the horizontal and vertical directions to obtain 3-dimentional (3D) percolation pathways, whereby the fields of application can be expanded and the properties of organic-inorganic composite films improved. Conventional dielectrophoresis techniques can typically only drive fillers to self-assemble in only one direction. We have devised a one-step dielectrophoresis-driven approach that effectively induces fillers self-assembly along two orthogonal axes, which results in the formation of 3D interconnected T-shaped iron microstructures (3D-T CIP) inside a polymer matrix. This approach to carbonyl iron powder (CIP) embedded in a polymer matrix results in a linear structure along the thickness direction and a network structure on the top surface of the film. The fillers in the polymer were controlled to achieve orthogonal bidirectional self-assembly using an external alternating current (AC) electric field and a non-contact technique that did not lead to electrical breakdown. The process of 3D-T CIP formation was observed in real time using in situ observation methods with optical microscopy, and the quantity and quality of self-assembly were characterized using statistical and fractal analysis. The process of fillers self-assembly along the direction perpendicular to the electric field was explained by finite element analogue simulations, and the results indicated that the insulating polyethylene terephthalate (PET) film between the electrode and the CIP/prepolymer suspension was the key to the formation of the 3D-T CIP. In contrast to the traditional two-step method of fabricating sandwich-structured film, the fabricated 3D-T CIP film with 3D electrically conductive pathways can be applied as magnetic field sensor.</p

Enhanced capacitive pressure sensing performance by charge generation from filler movement in thin and flexible PVDF-GNP composite films

This study introduces an approach to overcome the limitations of conventional pressure sensors by developing a thin and lightweight composite film specifically tailored for flexible capacitive pressure sensors, with a particular emphasis on the medium and high-pressure range. To accomplish this, we have engineered a composite film by combining polyvinylidene fluoride (PVDF) and graphite nanoplatelets (GNP) derived from expanded graphite (Ex-G). The uniform sized GNP with average lateral size of 2.55av and the average thickness of 33.74 av with narrow size distribution was obtained a gas induced expansion of expandable graphite (EXP-G) combined with tip sonication in solvent. By this precisely controlled GNP within the composite film, a remarkable improvement in sensor sensitivity has been achieved, surpassing 4.18 MPa−1 within the pressure range of 0.1 to 1.6 MPa. This enhancement can be attributed to the generation of electric charge from the movement of GNP in polymer matrix. Additionally, stability testing has demonstrated the reliable operation of the composite film over 1000 cycles. Notably, the composite film exhibits exceptional continuous pressure sensing capabilities with a rapid response time of approximately 100 milliseconds. Experimental validation using a 3 × 3sensor array has confirmed the accurate detection of specific contact points, thus highlighting the potential of the composite film for selective pressure sensing. These findings signify an advancement in the field of flexible capacitive pressure sensors that offer enhanced sensitivity, consistent operation, rapid response time, and the unique ability to selectively sense pressure. Our study provide the effect of GNP in polymer composite system with a facile and easy process, to enhance the capacitive pressure sensing properties, particularly at high pressure range applications.</p