Significantly Enhanced Dielectric Performances and High Thermal Conductivity in Poly(vinylidene fluoride)-Based Composites Enabled by SiC@SiO2 Core-Shell Whiskers Alignment

Introduction of Nanoscale Si3N4 to Improve the Dielectric Thermal Stability of a Si3N4/P(VDF-HFP) Composite Film

In order to improve the dielectric thermal stability of polyvinylidene fluoride (PVDF)-based film, nano silicon nitride (Si3N4) was introduced, and hence the energy storage performance was improved. The introduction of nano Si3N4 fillers will induce a phase transition of P(VDF-HFP) from polar β to nonpolar α, which leads to the improved energy storage property. As such, the discharging energy density of Si3N4/P(VDF-HFP) composite films increased with the amount of doped Si3N4. After incorporating 10wt% Si3N4 in Si3N4/P(VDF-HFP) films, the discharging density increased to 1.2 J/cm3 under a relatively low electric field of 100 MV/m. Compared with a pure P(VDF-HFP) film, both the discharging energy density and thermal dielectric relaxor temperature of Si3N4/P(VDF-HFP) increased. The working temperature increased from 80 °C to 120 °C, which is significant for ensuring its adaptability in high-temperature energy storage areas. Thus, this result indicates that Si3N4 is a key filler that can improve the thermal stability of PVDF-based energy storage polymer films and may provide a reference for high-temperature capacitor materials.

Core-Shell Engineering of Conductive Fillers toward Enhanced Dielectric Properties: A Universal Polarization Mechanism in Polymer Conductor Composites

Flexible dielectric and electronic materials with high dielectric constant (k) and low loss are constantly pursued. Encapsulation of conductive fillers with insulating shells represents a promising approach, and has attracted substantial research efforts. However, progress is greatly impeded due to the lack of a fundamental understanding of the polarization mechanism. In this work, a series of core-shell polymer composites is studied, and the correlation between macroscopic dielectric properties (across entire composites) and microscopic polarization (around single fillers) is investigated. It is revealed that the polarization in polymer conductor composites is determined by electron transport across multiple neighboring conductive fillers-a domain-type polarization. The formation of a core-shell filler structure affects the dielectric properties of tpolymer composites by essentially modifying the filler-cluster size. Based on this understanding, a novel percolative composite is prepared with higher-than-normal filler concentration and optimized shell's electrical resistivity. The developed composite shows both high-k due to enlarged cluster size and low loss due to restrained charge transport simultaneously, which cannot be achieved in traditional percolative composites or via simple core-shell filler design. The revealed polarization mechanism and the optimization strategy for core-shell fillers provide critical guidance and a new paradigm, for developing advanced polymer dielectrics with promising property sets.

Graphene Oxide/Polyvinyl Alcohol–Formaldehyde Composite Loaded by Pb Ions: Structure and Electrochemical Performance

An immobilization of graphene oxide (GO) into a matrix of polyvinyl formaldehyde (PVF) foam as an eco-friendly, low cost, superior, and easily recovered sorbent of Pb ions from an aqueous solution is described. The relationships between the structure and electrochemical properties of PVF/GO composite with implanted Pb ions are discussed for the first time. The number of alcohol groups decreased by 41% and 63% for PVF/GO and the PVF/GO/Pb composite, respectively, compared to pure PVF. This means that chemical bonds are formed between the Pb ions and the PVF/GO composite based on the OH groups. This bond formation causes an increase in the T_g values attributed to the formation of a strong surface complexation between adjacent layers of PVF/GO composite. The conductivity increases by about 2.8 orders of magnitude compared to the values of the PVF/GO/Pb composite compared to the PVF. This means the presence of Pb ions is the main factor for enhancing the conductivity where the conduction mechanism is changed from ionic for PVF to electronic conduction for PVF/GO and PVF/GO/Pb.

Wood-Derived Composites with High Performance for Thermal Management Applications

Fabricating advanced polymer composites with remarkable mechanical and thermal conductivity performances is desirable for developing advanced devices and equipment. In this study, a novel strategy to prepare anisotropic wood-based scaffolds with a naturally aligned microchannel structure from balsa wood is demonstrated. The wood microchannels were coated with polydopamine-surface-modified small graphene oxide (PGO) nanosheets via assembly. The highly aligned porous microstructures, with thin wood cell walls and large voids along the cellulose microchannels, allow polymers to enter, resulting in the fabrication of the wood-polymer nanocomposite. The tensile stiffness and strength of the resulting nanocomposite reach 8.10 GPa and 90.3 MPa with a toughness of 5.0 MJ m^-3. The thermal conductivity of the nanocomposite is improved significantly by coating a PGO layer onto the wood scaffolds. The nanocomposite exhibits not only ultrahigh thermal conductivity (in-plane about 5.5 W m^-1 K^-1 and through-plane about 2.1 W m^-1 K^-1) but also satisfactory electrical insulation (volume resistivity of about 10¹⁵ Ω·cm). Therefore, the results provide a strategy to fabricate thermal management materials with excellent mechanical properties.

Silver-Nanoparticle-Embedded Hybrid Nanopaper with Significant Thermal Conductivity Enhancement

Nanopapers derived from nanofibrillated cellulose (NFC) are urgently required as attractive substrates for thermal management applications of electronic devices because of their lightweight, easy cutting, cost efficiency, and sustainability. In this paper, we provided a facile fabrication strategy to construct hybrid nanopapers composed of dialdehyde nanofibrillated cellulose (DANFC) and silver nanoparticles (AgNPs), which exhibited a favorable thermal conductivity property. AgNPs were in situ proceeded on the surface of DANFC by the silver mirror reaction inspired by the aldehyde groups. Owing to the intermolecular hydrogen bonds inside the hybrid nanopapers, the DANFC enables the uniform dispersion of AgNPs as well as promotes the formation of the hierarchical structure. It was found that the AgNPs-coated DANFC (DANFC/Ag) hybrid nanopapers could easily form an effective thermally conductive pathway for phonon transfer. As a result, the thermal conductivity (TC) of the obtained DANFC/Ag hybrid nanopapers containing only 1.9 vol % of Ag was 5.35 times higher than that of the pure NFC nanopapers along with a significantly TC enhancement per vol % Ag of 230.0%, which was supposed to benefit from the continuous heat transfer pathway constructed by the connection of AgNPs decorated on the cellulose nanofibers. The DANFC/Ag hybrid nanopapers possess potential applications as thermal management materials in the next-generation portable electronic devices.

Highly Thermally Conductive and Superior Electrical Insulation Polymer Composites via In Situ Thermal Expansion of Expanded Graphite and In Situ Oxidation of Aluminum Nanoflakes

Polymer composites with highly thermally conductive and electrical insulation are urgently demanded for thermal management in modern electrical and energy applications. However, the incorporation of metal fillers in traditional polymeric composites usually fails to meet the requirements for simultaneously high thermal conductivity and high electrical insulation. Here, we successfully fabricated composites with high thermal conductivity and high electrical insulation by in situ thermal expansion of expandable graphite (EG) and in situ oxidation of aluminum (Al) nanoflakes in aluminum-plastic package waste (APPW). Due to the synergistic effect of the hybrid filler framework, the maximum thermal conductivity reached as high as 8.7 W m^-1 K^-1 for APPW/EG₁₀/Al₆₀-F composites. In addition, the formation of the nano Al₂O₃ layer around the Al filler surface brings extremely low electrical conductivity (<10^-14 S cm^-1) and low dielectric loss (<0.06). Based on the results of finite element simulation, the heat flowed mainly along the effective filler framework and the high thermal conductivity is attributed to the interconnection of the high aspect ratio filler. Furthermore, the strong thermal management capability of the prepared composites was demonstrated in the heat dissipation experiment. The present work suggests that surface-oxidized Al nanoflakes demonstrate fascinating performance and show promising application as thermal management materials in emerging electrical systems and electronic devices.

Enhanced interfacial polarization in poly(vinylidene fluoride-chlorotrifluoroethylene) nanocomposite with parallel boron nitride nanosheets

The miniaturization of electronics provides an opportunity for the polymer film capacitor due to its lightweight and flexibility. In order to improve energy density and charge-discharge efficiency of the film capacitor, the development of a polymer nanocomposite is one of the effective strategies, in which the distribution of the fillers plays a key role in the enhancement of the electrical energy capability. In this work, the few-layer boron nitride nanosheets (BNNSs) was exfoliated with assistance of the fluoro hyperbranched polyethylene-graft-poly(trifluoroethyl methacrylate) (HBPE-g-PTFEMA) copolymer as stabilizer, which was adsorbed on the surface of the nanosheets via a CH-π non-covalent interaction. The morphological results confirm the lateral size of ∼0.4 μm for resultant nanosheets with the intact crystal structure. The loading of 0.5 vol% BNNSs was embedded into poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE)) matrix by solution casting method, and then the nanocomposite film was uniaxial stretched to achieve the orientation of nanosheets in polymer host. The dielectric constant of stretching nanocomposite with ratio of 4 at 50 mm min^-1 reaches 51.1 at 100 Hz with low loss as 0.016, while the energy density of 7.0 J cm^-3 at 250 MV m^-1 with charge-discharge efficiency of 56% is obtained in current nanocomposite film, which is attributed to the interfacial polarization as well as parallel nanosheets blocking the growth of electrical treeing branches. This strategy of the aligned nanosheets/polymer nanocomposite establishes a simple route to construct heterogeneity in polymer films with enhanced electrical energy capability for flexible capacitors.

Tuned Local Surface Potential of Epoxy Resin Composites by Inorganic Core-Shell Microspheres: Key Roles of the Interface

Designing and controlling the interface interaction between polymer and filler is a challenge for nanocomposite insulation materials with the enhanced insulating and thermal conductive properties simultaneously. Meanwhile, the roles of the interface in the charge distribution of the composite on the macroscale are well studied. However, the effects of the interface on the nanoscale are not clear. In this work, first, we have demonstrated a method to modify the dielectric constant of composites by introduced air into the core-shell-structured M-SiO₂@Al₂O₃ particles. To clarify the electric interfacial region, we use Kelvin probe force microscopy (KPFM) to image with high spatial resolution of the surface charge distribution around an individual M-SiO₂@Al₂O₃ particle embedded in the epoxy matrix. We find that the KPFM results of the distinct electric interfacial region are consistent with the finite element simulation. Moreover, the charge accumulation is much easier in the presence of the M-SiO₂@Al₂O₃ particles because of the increasing concentration of traps. This work provides significant insight into understanding the intrinsic interfacial behavior in insulating polymeric composites.

Interface design for high energy density polymer nanocomposites

This review provides a detailed overview on the latest developments in the design and control of the interface in polymer based composite dielectrics for energy storage applications. The methods employed for interface design in composite systems are described for a variety of filler types and morphologies, along with novel approaches employed to build hierarchical interfaces for multi-scale control of properties. Efforts to achieve a close control of interfacial properties and geometry are then described, which includes the creation of either flexible or rigid polymer interfaces, the use of liquid crystals and developing ceramic and carbon-based interfaces with tailored electrical properties. The impact of the variety of interface structures on composite polarization and energy storage capability are described, along with an overview of existing models to understand the polarization mechanisms and quantitatively assess the potential benefits of different structures for energy storage. The applications and properties of such interface-controlled materials are then explored, along with an overview of existing challenges and practical limitations. Finally, a summary and future perspectives are provided to highlight future directions of research in this growing and important area.

Inorganic-Organic Hybrid Janus Fillers for Improving the Thermal Conductivity of Polymer Composites

Janus fillers represent a combination of inorganic thermally conductive silver nanoparticles and organic polystyrene brushes on one entity but different sides. They are of practical importance for polymer composites with high thermal conductivity because of the improved dispersion and reduced interfacial heat resistance. Moreover, benefiting from the sheetlike structure and single-side deposition of inorganic particles, Janus fillers tend to align such that the heat pathway is constructed in the composite films, when fabricated by layer-by-layer doctor blading. As a result, the in-plane thermal conductivity of the polymer composite is as high as 4.57 W m^-1 K^-1, with only 10 vol % Janus filler loading.

Fabrication of Morphologically Controlled Composites with High Thermal Conductivity and Dielectric Performance from Aluminum Nanoflake and Recycled Plastic Package

Polymer composites with high thermal conductivity are highly desirable for modern electronic and electrical industry because of their wide range of applications. However, conventional polymer composites with high thermal conductivity usually suffer from the deterioration of electrical insulation and high dielectric loss, whereas polymer composite materials with excellent electrical insulation and dielectric properties usually possess low thermal conductivity. In this study, combining surface-oxidized aluminum (Al) nanoflake and multilayer plastic package waste (MPW) by powder mixing technique, we report a novel strategy for polymer composites with high thermal conduction, high electrical insulation, and low dielectric loss. The resultant MPW/Al, MPW/Al₄₀₀, and MPW/Al₅₀₀ composites exhibited the maximum thermal conductivity of 4.8, 3.5, and 1.4 W/mK, respectively, which exceeds those of most of the corresponding composites reported previously. In addition, all the composites still have high insulation (<10^-13 S/cm) and maintain dielectric loss at a relatively low level (<0.025). Such a result is ascribed to the formation of an insulating Al₂O₃ shell and the continuous three-dimensional filler network, which is revealed by Agari model fitting coefficient. The model of effective medium theory qualitatively demonstrates that the lower interfacial thermal resistances of the MPW/Al composite can also benefit the high thermal conduction. This interfacial engineering strategy provides an effectively method for the fabrication of polymer materials with high-performance thermal management.