Dongrui Wang

Improved Dielectric Properties of Nanocomposites Based on Poly(vinylidene fluoride) and Poly(vinyl alcohol)-Functionalized Graphene

In this work, two series of nanocomposites of poly(vinylidene fluoride) (PVDF) incorporated with reduced graphene oxide (rGO) and poly(vinyl alcohol)-modified rGO (rGO-PVA) were fabricated using solution-cast method and their dielectric properties were carefully characterized. Infrared spectroscopy and atom force microscope analysis indicated that PVA chains were successfully grafted onto graphene through ester linkage. The PVA functionalization of graphene surface can not only prevent the agglomeration of original rGO but also enhance the interaction between PVDF and rGO-PVA. Strong hydrogen bonds and charge transfer effect between rGO-PVA and PVDF were determined by infrared and Raman spectroscopies. The dielectric properties of rGO-PVA/PVDF and rGO/PVDF nanocomposites were investigated in a frequency range from 10² Hz to 10⁷ Hz. Both composite systems exhibited an insulator-to-conductor percolating transition as the increase of the filler content. The percolation thresholds were estimated to be 2.24 vol % for rGO-PVA/PVDF composites and 0.61 vol % for rGO/PVDF composites, respectively. Near the percolation threshold, the dielectric permittivity of the nanocomposites was significantly promoted, which can be well explained by interfacial polarization effect and microcapacitor model. Compared to rGO/PVDF composites, higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz. This work provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectric materials.

Functionalized graphene–BaTiO3/ferroelectric polymer nanodielectric composites with high permittivity, low dielectric loss, and low percolation threshold

The fabrication and dielectric properties of a novel multi-component high-k composite system consisting of poly(vinylidene fluoride), surface-functionalized graphene nanosheets and BT nanoparticles (fRGO–BT/PVDF) were investigated. The fRGO nanosheets were prepared through the π–π stacking of polyaniline and GO following in situ hydrazine reduction. The fRGO–BT/PVDF nanocomposites were fabricated by a solution casting and hot-pressing approach. SEM results confirm that fRGO and BT are well dispersed within the PVDF matrix. The dielectric properties of the binary fRGO/PVDF nanocomposites exhibit a typical percolation transition with the percolation threshold of 1.49 vol%. This type of nanocomposite, co-filled with conductive graphene nanosheets and high-k ceramics, shows a high kr (65) and a relatively low dielectric loss (tan δ = 0.35) at a high frequency of 1 MHz. Meanwhile, the dielectric properties of the fRGO–BT/PVDF nanocomposites show temperature independent behavior over a wide temperature range. These flexible, high-k fRGO–BT/PVDF nanocomposites are potential flexible dielectric materials for use in high-frequency capacitors and electronic devices.

Surface-Functionalized MWNTs with Emeraldine Base: Preparation and Improving Dielectric Properties of Polymer Nanocomposites

A comparative study of the dielectric properties of poly(vinylidene fluoride) (PVDF) based nanocomposites with pristine multiwalled carbon nanotubes (MWNTs) and surface-modified MWNTs with core/shell structure (denoted as MEB) as fillers, was reported. Compared with MWNTs/PVDF composites, the MEB/PVDF composites exhibited lower loss tangent and higher dielectric permittivity. It is suggested that the conductive/nonconducting core/shell structure of the MEB filler is the main cause of the improved dielectric properties. Percolation based MWNTs networks is in charge of the improvement of dielectric permittivity, and the nonconducting emeraldine base layer of the MEB filler supports the low loss tangent and low conductivity in the MEB/PVDF composites.

Size-dependent low-frequency dielectric properties in the BaTiO3/poly(vinylidene fluoride) nanocomposite films

Effects of inorganic nanoparticles size and thermal agitation on dielectric properties of BaTiO3/poly(vinylidene fluoride) (BT/PVDF) nanocomposite films at low frequency were studied. The dielectric properties of the BT/PVDF nanocomposite films with three kinds of diameters of BT nanoparticles loading at 50 vol. % were studied in a wide frequency range from 10−2 Hz to 107 Hz by two measured processes. A significant low-frequency dielectric permittivity increase and the difference in dielectric properties between two measured processes were discussed. Interfacial polarization, crystal phase effect, and thermal agitation are considered to analyze the significant increase and difference in dielectric behaviors.

Positive piezoresistive behavior of electrically conductive alkyl-functionalized graphene/polydimethylsilicone nanocomposites

Piezoresistive nanocomposites using alkyl-functionalized graphene (G-ODA) as a conducting filler and polydimethylsilicone (PDMS) as the polymer matrix were prepared and their piezoresistivity behavior was investigated. One-pot synthesis of G-ODA from graphite oxide and octadecylamine improved its dispersion in nonpolar xylene and PDMS with low surface free energy. Results show that the graphene nanosheets were homogeneously dispersed in the PDMS matrix and an ultra-low percolation threshold (0.63 vol%) of the composites was obtained. The G-ODA/PDMS composites with 1.19 vol% content of G-ODA show a remarkable positive piezoresistivity of high sensitivity (R/R0 > 400 under the pressure of 1.2 MPa), excellent repeatability, small hysteresis, and long-term durability. Under uniaxial compression, the resistance of the composites exponentially increased with the pressure. The resistance–pressure curves remain nearly unchanged after 1000 loading–unloading cycles. The results suggest that the G-ODA/PDMS nanocomposites provide a new route toward fabrication of soft piezoresistive sensors with high performance.

Increased electroaction through a molecular flexibility tuning process in TiO2–polydimethylsilicone nanocomposites

Flexible polymer materials with obvious electrostriction characteristics display a significant potential for application as novel potential actuators in the future. We report advanced TiO2–polydimethylsilicone (TiO2–PDMS) nanocomposites with electroaction that is effectively increased through a molecular flexibility tuning process. The increase in the electromechanical sensitivity (by 550%) and actuation strain (by 230%) under a low electric field in low elastic modulus TiO2–PDMS composites originates from the flexibility tuning process by the introduction of dimethylsilicone oil (DMSO). The DMSO is miscible with PDMS resulting in a uniform composition at the molecular level, which can significantly decrease the elastic modulus of the dielectric elastomer composites from 820 kPa to 95 kPa. The experimental results are interpreted using the swelling elastomers theory. It suggests that reducing the elastic modulus could be a good strategy to improve the actuation performance with a low electric field.

Experimental study and theoretical prediction of dielectric permittivity in BaTiO3/polyimide nanocomposite films

Theoretical models were used to predict dielectric permittivities of the thermosetting polyimide (PI) matrix nanocomposite films loading with BaTiO3 (BT) nanoparticles prepared by the alkoxide route. The observed dielectric permittivities are in good agreement with calculated values using Jayasundere equation and effective medium theory when the interactions of nanoparticle-nanoparticle and nanoparticle-polymer are considered. Additionally, temperature dependence of dielectric permittivity of the BT/PI nanocomposite films at 103 Hz was also studied for both heating from −50 to 150 °C and cooling from 150 to −50 °C. The transformation in crystal phase of BT and changes of free volume in PI were considered to be the main factors influencing the dielectric permittivities of the BT/PI nanocomposite films.

Dielectric properties of poly(vinylidene fluoride) nanocomposites filled with surface coated BaTiO3 by SnO2 nanodots

SnO2 nanoparticles with an average diameter of about 4 nm were coated on the surface of BaTiO3 (BT) (∼100 nm) by chemical treatment. With the introduction of BT@SnO2, the dielectric permittivity of poly(vinylidene fluoride) (PVDF) composite was significantly increased to 90 at 103 Hz, which is ∼40% higher than that of the BT/PVDF composites. It was attributed to the enhanced interfacial polarization in the interlayers between BT and PVDF due to the addition of SnO2 nanodots. The distance of SnO2 nanodots on the adjacent BT surfaces is close enough for the electron transport in the matrix by tunneling effect. Besides, the semiconductive SnO2 leads to the weak insulating-conducting transition close to the percolation threshold.

Highly improved electro-actuation of dielectric elastomers by molecular grafting of azobenzenes to silicon rubber

Herein we report a novel and efficient approach to fabricate dielectric elastomers with enhanced dielectric constant and high dielectric strength. Azobenzenes with strong permanent dipole moments were synthesized to co-crosslink with hydroxyl-terminated polydimethylsiloxane through a simple one-step process, which realized a type of robust, molecularly homogenous silicone rubber (SR). The chemical structure, dielectric and mechanical properties of the resultant azo-g-PDMS elastomers with azobenzne contents ranging from 0 to 13.2 wt% were carefully characterized. The dielectric constant of azo-g-PDMS films at 1 kHz increased from 2.72 to 4.88 with the increase of azobenzene contents. By grafting with 4.0 wt% of azobenzene, the breakdown strength of azo-g-PDMS reached 89.4 V μm−1, which is 36% higher than that of pristine SR. The electric field induced deformation of silicone rubber could be enhanced by grafting with azobenzenes. The azo-g-PDMS film with 7.1 wt% of azobenzenes displayed a maximum area strain of 17%. Meanwhile, the azo-g-PDMS films exhibited a short response time (about 0.5 s) to the change in the electric field. Some prototype electromechanical actuators based on this type of azo-g-PDMS films were fabricated, demonstrating that the azo-g-PDMS dielectric elastomer is a very promising candidate for artificial muscle applications.

High-permittivity polymer nanocomposites: influence of interface on dielectric properties

Flexible dielectric composites with high permittivity have been extensively studied due to their potential applications in highdensity energy capacitors. In this review, effects of interface characteristics on the dielectric properties in the polymer-based nanocomposites with high permittivity are analyzed. The polymer-based dielectric composites are classified into two types: dielectric–dielectric (DD, ceramic particle-polymer) composites and conductor–dielectric (CD, conductive particle-polymer) composites. It is highly desirable for the dielectric–dielectric composites to exhibit high permittivity at low content of ceramic particles, which requires a remarkable interface interaction existing in the composite. For conductor–dielectric composites, a high permittivity can be achieved in composite with a small amount of conductor particle, but associated with a high loss. In this case, the interface between conductor and polymer with a good insulating characteristic is very important. Different methods can be used to modify the surface of ceramic/conductor particles before these particles are dispersed into polymers. The experimental results are summarized on how to design and make the desirable interface, and recent achievements in the development of these nanocomposites are presented. The challenges facing the fundamental understanding on the role of interface in high-permittivity polymer nanocomposites should be paid a more attention.