Study of flexible nanodielectric materials (FNDMs) with high permittivity is one of the most active academic research areas in advanced functional materials. FNDMs with excellent dielectric properties are demonstrated to show great promise as energy-storage dielectric layers in high-performance capacitors. These materials, in common, consist of nanoscale particles dispersed into a flexible polymer matrix so that both the physical/chemical characteristics of the nanoparticles and the interaction between the nanoparticles and the polymers have crucial effects on the microstructures and final properties. This review first outlines the crucial issues in the nanodielectric field and then focuses on recent remarkable research developments in the fabrication of FNDMs with special constitutents, molecular structures, and microstructures. Possible reasons for several persistent issues are analyzed and the general strategies to realize FNDMs with excellent integral properties are summarized. The review further highlights some exciting examples of these FNDMs for power-energy-storage applications.

Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage

Core-shell structured BaTiO3/poly(methyl methacrylate) (PMMA) nanocomposites were successfully prepared by in situ atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) from the surface of BaTiO3 nanoparticles. A broadband dielectric spectrometer was used to investigate the temperature dependence of the dielectric properties of the nanocomposites in a frequency range from 0.1 Hz to 1 MHz. It was found that the nanocomposites not only showed a significantly increased dielectric constant when compared with pure PMMA, but also showed the inherent low loss of the base polymer in a wide range of frequencies. Only in the very low frequency/high temperature range, can a higher dielectric loss can be observed in the nanocomposites. It was also found that the effective dielectric constant of the core-shell structured hybrid nanoparticles can be tailored by varying the polymer shell thickness. The dielectric response of beta relaxation of PMMA was also studied and the results showed that the nanoparticles had no influence upon the relaxation activation energy. Fourier-transform infrared spectroscopy (FTIR) and 1H NMR spectra confirmed the chemical structure of the PMMA shell on the surface of the BaTiO3 nanoparticles. Transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) results revealed that the PMMA shell thickness could be well controlled by tuning the feed ratio of MMA to BaTiO3.

Core-shell structured poly(methyl methacrylate)/BaTiO3 nanocomposites prepared by in situ atom transfer radical polymerization: a route to high dielectric constant materials with the inherent low loss of the base polymer

Calcium barium zirconate titanate (Ba0.95Ca0.05Zr0.15Ti0.85O3, BCZT) ceramic particles were prepared by a conventional solid-state method. BCZT powders were modified by dopamine through a chemical coating method. The composite flexible films based on dopamine@BCZT and polyvinylidene fluoride were fabricated via a solution casting method. The microstructure and morphology were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and field emission scanning electron microscopy. A precision impedance analyzer and a dielectric withstand voltage test were used to test the dielectric constant, loss tangent, and breakdown strength. TEM results showed that dopamine was uniformly coated on the surface of BCZT particles with an average thickness of 20 nm. SEM results showed that the ceramic particles were dispersed homogeneously in the matrix. The dielectric constant increased with the increase of BCZT contents, while the loss tangent remained constant in the frequency range of 103 to 105 Hz. Different theoretical models were employed to predict the effective dielectric constants of the composite films, and the estimated results were compared with the experimental data. Weibull distribution was used to analyze the dielectric breakdown strength, and the results showed that the breakdown strength decreased then stayed over 60 kV mm−1.

Fabrication, characterization, properties and theoretical analysis of ceramic/PVDF composite flexible films with high dielectric constant and low dielectric loss

Flexible polymer composite films are prepared by a solution cast method with polar polyvinylidene fluoride (PVDF) or non-polar epoxy as the polymer matrix. BaTiO3 nanoparticles and BaTiO3 nanofibers with large aspect ratio are used as dielectric fillers after surface modification by polydopamine. The effects of filler shape, surface modification and polarity of polymer matrix on the microstructure, dielectric constants and breakdown strength of polymer composites are investigated in detail. Surface modification by polydopamine improves the compatibility between BaTiO3 and polymer as well as passivating the surfaces of BaTiO3. At the same volume fraction, composites filled with BaTiO3 nanofibers exhibit greater dielectric constants than the composites filled with BaTiO3 nanoparticles. The polydopamine layers on BaTiO3 nanofibers give rise to stronger interfaces between the fillers and polymer matrices. Improved breakdown strengths are achieved in both composites. This work may provide a general strategy for flexible polymer nanocomposites with greatly enhanced dielectric constants and breakdown strength.

Improving the dielectric constants and breakdown strength of polymer composites: effects of the shape of the BaTiO3 nanoinclusions, surface modification and polymer matrix

BaTiO3 nanofibers with a large aspect ratio prepared via electrospinning and modified by dopamine were used as dielectric fillers in poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)-based nanocomposites. Highly flexible polymer nanocomposite films were fabricated via a simple solution-cast method. Enhanced dielectric permittivities were obtained at a low volume fraction of BaTiO3 nanofibers. The breakdown strength of the polymer nanocomposites was also improved, which is favorable for enhanced ferroelectric properties in the nanocomposites. Pr ∼9.1 μC cm−2 was achieved in the nanocomposites with 10.8 vol% BaTiO3 nanofibers. The improved breakdown strength and enhanced ferroelectric properties are attributed to the combined effect of the surface modification by dopamine, the large aspect ratio of the BaTiO3 nanofibers and the improved crystallinity of the polymer nanocomposites induced by the BaTiO3 nanofibers.

Enhanced dielectric and ferroelectric properties induced by dopamine-modified BaTiO3 nanofibers in flexible poly(vinylidene fluoride-trifluoroethylene) nanocomposites

Ferroelectric ceramics (e.g., lead zirconate titanate, PZT) and polymers exhibit extraordinary dielectric and piezoelectric properties and processability, respectively. It is, however, difficult to retain the ideal dielectric constant (er) and piezoelectric coefficient (d33) in their composites. Here we show that an interfacial adhesion mechanism (afforded by the chelation technique) in PZT-polymer composites can lead to a dielectric constant that is, remarkably, seven times greater than what is usually found. At frequencies below 40 Hz, the dielectric constant of the composite is higher than in PZT alone (i.e., er > 1300), and this is the first achievement of the ceramic–polymer 0–3 composites as reported. Additionally, a super-high piezoelectric coefficient (d33 > 170) is also obtained owing to the interfacial mechanism. Our finding can lead to a novel way for preparing ceramic–polymer composites with an ultrahigh dielectric constant and ultrahigh piezoelectric coefficient, which are required in many modern electric systems and energy converters.

Cheng Chen

Papers

Enhancement of dielectric constant and piezoelectric coefficient of ceramic-polymer composites by interface chelation