Free volume dependence of dielectric behaviour in sandwich-structured high dielectric performances of poly(vinylidene fluoride) composite films

High output flexible polyvinylidene fluoride based piezoelectric device incorporating cellulose nanofibers/BaTiO3@TiO2 piezoelectric core-shell structure

Flexible composite film has gained increasing attention in the fields of wearable devices and portable electronic products. In this work, a novel core-shell structure of cellulose nanofibers/BaTiO3@TiO2 (CNF/BTO@TiO2) was synthesized with the assistant of the biological macromolecule material of cellulose nanofiber (CNF), in which the CNF can improve the stability and dispersibility of BaTiO3 (BTO) in the aqueous phase and elevate the integrity of the core-shell structure. The core-shell structure can reduce the agglomeration of fillers in polyvinylidene fluoride (PVDF) and improve the structural defects of the composite film. Meanwhile, the core-shell structure can promote the polarization of the electric dipole and the formation of β phase in PVDF due to the generated interface spatial polarization between the shell of TiO2 and the core of BTO. When the content of the core-shell structure was 5 wt%, the β phase content reaches 61.89 %, and the piezoelectric coefficient of composite film reaches 84.29 pm/V. Thus the maximum output open-circuit voltage (VOC) and short-circuit current (ISC) of the piezoelectric composite film is as high as 13.10 V and 464.3 nA. In addition, its excellent pressure sensing capability allows for its application in various flexible electronic devices.

Achieve High Dielectric and Energy-Storage Density Properties by Employing Cyanoethyl Cellulose as Fillers in PVDF-Based Polymer Composites

Fluoropolymer/inorganic nanofiller composites are considered to be ideal polymer dielectrics for energy storage applications because of their high dielectric constant and high breakdown strength. However, these advantages are a trade-off with the unavoidable aggregation of the inorganic nanofillers, which result in a reduced discharge of the energy storage density. To address this problem, we developed polyvinylidene fluoride (PVDF) graft copolymer/cellulose-derivative composites to achieve high-dielectric and energy-storage density properties. An enhanced dielectric constant and improved energy density were achieved with this structure. The optimal composites exhibited a high discharge energy density of 8.40 J/cm³ at 300 MV/m. This work provides new insight into the development of all-organic composites with bio-based nanofillers.

Structure and Dielectric Properties of Poly(vinylidenefluoride-co-trifluoroethylene) Copolymer Thin Films Using Atmospheric Pressure Plasma Deposition for Piezoelectric Nanogenerator

This study investigates the structural phase and dielectric properties of poly(vinylidenefluoride-co-trifluoroethylene) (P[VDF-TrFE]) thin films grown via atmospheric pressure (AP) plasma deposition using a mixed polymer solution comprising P[VDF-TrFE] polymer nano powder and dimethylformamide (DMF) liquid solvent. The length of the glass guide tube of the AP plasma deposition system is an important parameter in producing intense cloud-like plasma from the vaporization of DMF liquid solvent containing polymer nano powder. This intense cloud-like plasma for polymer deposition is observed in a glass guide tube of length 80 mm greater than the conventional case, thus uniformly depositing the P[VDF-TrFE] thin film with a thickness of 3 μm. The P[VDF-TrFE] thin films with excellent β-phase structural properties were coated under the optimum conditions at room temperature for 1 h. However, the P[VDF-TrFE] thin film had a very high DMF solvent component. The post-heating treatment was then performed on a hotplate in air for 3 h at post-heating temperatures of 140 °C, 160 °C, and 180 °C to remove DMF solvent and obtain pure piezoelectric P[VDF-TrFE] thin films. The optimal conditions for removing the DMF solvent while maintaining the β phases were also examined. The post-heated P[VDF-TrFE] thin films at 160 °C had a smooth surface with nanoparticles and crystalline peaks of β phases, as confirmed by the Fourier transform infrared spectroscopy and XRD analysis. The dielectric constant of the post-heated P[VDF-TrFE] thin film was measured to be 30 using an impedance analyzer at 10 kHz and is expected to be applied to electronic devices such as low-frequency piezoelectric nanogenerators.

Novel Functionalized Boron Nitride Nanosheets Achieved by Radiation-Induced Oxygen Radicals and Their Enhancement for Polymer Nanocomposites

Boron nitride nanosheets (BNNSs) exfoliated from hexagonal boron nitride (h-BN) show great potential in polymer-based composites due to their excellent mechanical properties, highly thermal conductivity, and insulation properties. Moreover, the structural optimization, especially the surface hydroxylation, of BNNSs is of importance to promote their reinforcements and optimize the compatibility of its polymer matrix. In this work, BNNSs were successfully attracted by oxygen radicals decomposed from di-tert-butylperoxide (TBP) induced by electron beam irradiation and then treated with piranha solution. The structural changes of BNNSs in the modification process were deeply studied, and the results demonstrate that the as-prepared covalently functionalized BNNSs possess abundant surface hydroxyl groups as well as reliable structural integrity. Of particular importance is that the yield rate of the hydroxyl groups is impressive, whereas the usage of organic peroxide and reaction time is greatly reduced due to the positive effect of the electron beam irradiation. The comparisons of PVA/BNNSs nanocomposites further indicate that the hydroxyl-functionalized BNNSs effectively promote mechanical properties and breakdown strength due to the enhanced compatibility and strong two-phase interactions between nanofillers and the polymer matrix, which further verify the application prospects of the novel route proposed in this work.

Constructing Sandwich-Structured Poly(vinyl alcohol) Composite Films with Thermal Conductive and Electrical Performance

With the miniaturization and high integration development in microelectronic devices, the problem of heat dissipation has attracted widespread attention. Highly thermal conductive and electrical insulation polymer composites show great advantages to solve the problems of heat dissipation. Nevertheless, the fabrication of polymer composites with both excellent thermal conductivity and electrical performance is still a great challenge. Herein, to coordinate the thermal and electrical properties of the composite film, the sandwich-structured poly(vinyl alcohol) (PVA)/boron phosphide (BP)-boron nitride nanosheet (BNNS) composite films were prepared, with the PVA/BP composite film as the top and bottom layers and the BNNS layer as the middle layer. When the filler loading was 31.92 wt %, the sandwich-structured composite films showed excellent in-plane thermal conductivity (9.45 W·m^-1·K^-1), low dielectric constant (1.25 at 10² Hz), and excellent breakdown strength. In the composite film, the interconnected BP particles and BNNS layer formed several heat dissipation pathways to increase the thermal conductivity, while the insulated BNNS layer hampered the electron transformation to enhance the electrical resistivity of films. Therefore, the PVA/BP-BNNS composite films showed a potential application in heat dissipation of high power electronic devices.

Application Progress of PALS in the Correlation of Structure and Properties for Graphene/Polymer Nanocomposites

Giving a deep insight into the microstructure, and realizing the correlation between microstructure and properties is very important to the precise construction of high-performance graphene/polymer nanocomposites (GPN). For the promising application in microstructure characterization, much attention has been focused on the effective technique of positron annihilation lifetime spectroscopy (PALS). Based on the introduction of the basic principle, this review summarized the application progress of PALS in the correlation of microstructure and properties for GPN, especially for the characterization of free volume and interfacial interaction, and the correlation of these microstructures and properties.

Optimizing nanostructures to achieve enhanced breakdown strength and improved energy storage performances in dipolar polymers

Polymer dielectrics have attracted ever-increasing attention for electrical energy storage applications in recent years. Typically, polymer-based nanocomposite films are adopted to obtain polymer dielectrics with high energy density but sometime suffer from the inhomogeneous distribution of fillers. In this work, enhanced breakdown strength, suppressed dielectric loss and improved energy storage performances of PVDF film are concurrently achieved via the regulation of the sub-nano free volume, without introduction of any kinds of fillers and also minimizing the uneven distribution of the local electric field. The most improved breakdown strength is up to 488 MV m^-1, with an enhancement of 60% compared with that of pristine PVDF, which enables the irradiated PVDF film to exhibit an improved polarization strength and charged energy density, giving rise to a maximum polarization strength of 3.55 μC cm^-2 and a charged energy density of 9.75 J cm^-3. More importantly, the irradiated PVDF film exhibits a superior discharged energy density of 7.91 J cm^-3 which is 261% that of the pristine PVDF film, while maintaining the charge-discharge efficiency above 80%. In addition, the alteration of experimental breakdown strength with the increase of irradiation dose is found to be inversely correlated with the size variation of free volume holes, and the theoretical simulation of local electric field distribution further proves that the breakdown strength enhancement originates from the size shrinkage of free volume holes. The adjustment of free volume provides a potentially effective way to regulate the dielectric properties and energy storage performances of dipolar polymers.

Two-Dimensional Fillers Induced Superior Electrostatic Energy Storage Performance in Trilayered Architecture Nanocomposites

Dielectric capacitors with ultrahigh power densities and fast charging/discharging rates are of vital relevance in advanced electronic markets. Nevertheless, a tradeoff always exists between breakdown strength and polarization, which are two essential elements determining the energy storage density. Herein, a novel trilayered architecture composite film, which combines outer layers of two-dimensional (2D) BNNS/poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) with high breakdown strength and an intermediate layer made of blended 2D MoS₂ nanosheets/P(VDF-HFP) with large polarization, is fabricated using the layer-by-layer casting method. The insulating BNNS with a wide band gap is able to largely alleviate the distortion of the local electric field, thereby suppressing the leakage current and effectively reducing the conductivity loss, while the 2D MoS₂ nanosheets act as microcapacitors in the polymer composites, thus significantly increasing the permittivity. A finite element simulation is carried out to further analyze the evolution process of electrical treeing in the experimental breakdown of the polymer nanocomposites. Consequently, the nanocomposites possess an excellent discharged energy density of 25.03 J/cm³ accompanied with a high charging/discharging efficiency of 77.4% at 650 MV/m, which greatly exceeds those of most conventional single-layer films. In addition, the corresponding composites exhibit an outstanding reliability of energy storage performance under continuous cycling. The excellent performances of these polymer-based nanocomposite films could pave a way for widespread applications in advanced capacitors.