Surface modification of BaTiO3 inclusions in polydicyclopentadiene nanocomposites for energy storage

A new nanocomposite system displaying high breakdown strength, improved permittivity, low dielectric loss, and high thermal stability is presented. Free‐standing nanocomposite films were prepared via a solvent‐free in‐situ polymerization technique whereby 5 vol % BaTiO₃ (BT) nanocrystals with tailored surface chemistry were dispersed in dicyclopentadiene (DCPD) prior to initiation of ring opening metathesis polymerization by a second generation Grubbs catalyst. The relative permittivity was enhanced from 1.7 in the neat poly(DCPD) film to a maximum of 2.4 in the composite, while the dielectric loss tangent was minimized below 0.7%. Surface modification of the BT nanocrystals mitigated reduction in breakdown strength of the resulting nanocomposites such that only a 13% reduction in breakdown strength was observed relative to the neat polymer films. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40290.     

High energy density nanocomposites based on poly(vinylidene fluoride‐chlorotrifluoroethylene) and barium titanate

Three series of poly (vinylidene fluoride‐chlorotrifluoroethylene)/barium titanate (BT) nanocomposites with varied compositions were fabricated via solution cast process followed by thermally treated in different ways. Quenching the composite samples at lower temperature could effectively enhance their dielectric constant, breakdown strength as well as the energy density. The highest energy density (13.6 J/cm³) is observed in the sample quenched from 200°C to ?94°C with 5 vol% BT, which is much higher than nanocomposites reported in the current literature. The addition of ceramic particles leads to the improvement of dielectric permittivity and energy density measured under the same electric field. However, the dielectric breakdown strength and the energy density measured at breakdown strength of the resultant composites are reduced as a function of BT content. The fixed maximum electric displacement and reduction of saturation electric field suggest that the addition of ceramic particles with high dielectric constant may help increase the energy density of composites under low electric field but not for high electric field. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.     

A Ferroconcrete‐Like All‐Organic Nanocomposite Exhibiting Improved Mechanical Property,High Breakdown Strength,and High Energy Efficiency

Polymer‐based nanocomposite dielectrics with high energy storage capacity are crucial enablers for numerous applications in modern electronic and electrical industries. The energy density of parallel plate capacitors is determined by breakdown strength and dielectric permittivity of the inner dielectrics. Poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) (P(VDF‐TrFE‐CFE)), with the highest permittivity among all the dielectric polymers, is a promising candidate for high energy density capacitors. However, its relatively low breakdown strength and energy efficiency restrict the applications. In this work, a new method combining combinatorial‐electrospinning and hot‐pressing is proposed to fabricate P(VDF‐TrFE‐CFE)‐based all‐organic dielectrics with ferroconcrete‐like structure. In this structure, continuous fibers of polysulfone (PSF) with high Young's modulus act as tough scaffold to improve the mechanical properties of nanocomposites, and an over 750% enhancement of Young's modulus is obtained. The enhanced mechanical properties bring about significant improvement in Weibull breakdown strength to 485 MV m^?1, more than 50% higher than neat terpolymer. Furthermore, the suppressed leakage current and conduction loss, and hence the improved discharge energy efficiency under moderate electric field, are achieved due to the high insulation of PSF and its interfacial restriction on space charge mobility.     

Fabrication of highly refractive barium‐titanate‐incorporated polyimide nanocomposite films with high permittivity and thermal stability

Crystalline nanoparticles of barium titanate (BT) are incorporated into polyimide (PI) to fabricate highly refractive, anti‐UV‐degradable nanocomposite films with high permittivity and thermal stability. For homogeneous incorporation of BT nanoparticles into the PI matrix, the BT nanoparticles are surface modified by phthalimide with the aid of a silane coupling agent as a scaffold. The PI nanocomposites are prepared by in situ polymerization in which a diphthalic anhydride and a diamine are used to form the PI matrix in the presence of the surface‐modified nanoparticles. The refractive index of the transparent nanocomposite films reaches 1.85 at a nanoparticle content of 59 vol% with a high dielectric constant of ε = 37 and thermal stability up to 460 °C. Copyright © 2012 Society of Chemical Industry     

Influences of dielectric and conductive fillers on dielectric and mechanical properties of solid silicone rubber composites

Dielectric elastomers are materials being used for electromechanical transduction applications. Their electromechanical response depends on permittivity, Young’s modulus and electric breakdown strength. A factor that limits its application is high operating voltages that can be reduced through improvement in permittivity. One of the methods is by incorporating high permittivity fillers into polymer matrix to obtain dielectric–dielectric composites (DDC).These composites show high permittivity at the cost of reduced flexibility. An alternative solution is development of composites by incorporating organic or inorganic conductive fillers into polymer matrix. These composites show high permittivity with high dielectric loss and low breakdown strength. To overcome both the above limitations both dielectric and conductive fillers are incorporated into dielectric polymer matrix to obtain conductor–dielectric composites (CDC). In this study, high temperature vulcanized solid silicone rubber as matrix has been used to prepare DDC composites with barium titanate (BT) filler and CDC composites with both BT as dielectric and ketjenblack as conductive fillers, using Taguchi design of experiments. The effect of factors such as amount of fillers and curing agent, mixing time in roll mill and curing temperature on the dielectric and mechanical properties are reported. Lichtenecker model predicts the permittivity of the DDC composite more accurately. For the CDC composites permittivity increased by 390%, effective resistivity decreased by 80%, Young’s modulus increased by 368% and Shore A hardness increased by 90% as compared to those of reference matrix. Important interaction effects are observed among both the fillers that are uniformly dispersed without any aggregation.

     

Fast discharge and high energy density of nanocomposite capacitors using Ba0.6Sr0.4TiO3 nanofibers

Nanocomposites combining high breakdown strength (BDS) polymer and high dielectric permittivity ceramic fillers have shown great potential for pulsed power application. Here a new composite material based on surface-functionalized Ba_0.6Sr_0.4TiO₃ nanofibers/poly(vinylidene fluoride) (BST NF/PVDF) has been prepared by solution casting. The nanocomposites containing 2.5 vol% isopropyl dioleic(dioctylphosphate) titanate (NDZ 101)-functionalized BST NF (N-h-BST NF) have large energy density of 6.95 J cm⁻³ at 380 MV m⁻¹, which is 1.85 times larger than that of the pure PVDF at the same electric field. Also, the discharge speed of the nanocomposites containing 7.5 vol% N-h-BST NF is approximately 0.11 μs. The good properties, together with the large energy density and fast discharge speed, make this material a promising candidate for pulsed power capacitor.     

Gradient dielectric constant sandwich-structured BaTiO3/PMMA nanocomposites with strengthened energy density and ultralow-energy loss

High energy storage density with low-energy loss polymer films are essential for high-performance electric devices. To avoid the high-energy loss of utilizing nonlinear polymer materials, a sandwich nanostructure comprising a linear polymer poly(methyl methacrylate) (PMMA) matrix embedded with a high dielectric constant BaTiO₃ (BT) interlayer and poly(vinylidene fluoride) (PVDF) binder was constructed using a solution casting strategy. This structural design takes advantage of each component in the composite. The good dispersion of BT particles in the binder, which was incorporated between PMMA, enabled a high dielectric constant and fewer defects. Additionally, the excellent film formation ability of the PVDF binder guarantees the uniform thickness and stable structure of the BT mid-layer, and good miscibility between PVDF and PMMA enhanced the interaction between each layer. Interestingly, since the dielectric constant of PVDF was between BT fillers and PMMA, a dielectric gradient distribution mitigated the local electric field concentration, as proven by the simulation results. Consequently, a low-loss linear PMMA composite film exhibited satisfying breakdown strength and excellent discharged energy density, which were 25% and 460% higher than those of pristine PMMA, respectively.     

Core-shell structured BaTiO3@SiO2@PDA for high dielectric property nanocomposites with ultrahigh energy density

Flexible nanocomposite dielectrics with high dielectric constant and discharge energy density have broad application prospects in the field of energy storage. However, dielectrics with high dielectric constant tend to have a high dielectric loss. Herein, we prepared a dielectric composite material with ultra-high discharge energy density by modifying the interface between nanoparticles and poly(vinylidene fluoride-co-hexafluoropropylene) (P[VDF-HFP]). After coating a shell of insulating amorphous SiO₂ (~7 nm) outside the barium titanate (BT), the electric field concentration and current density inside BT particles can be significantly reduced. In addition, coating the SiO₂ shell with a polydopamine (PDA) shell (~7 nm) not only enhances the interface interaction between the nanoparticles and the polymer matrix, but also can form lots of microcapacitors in the composite. As a result, an ultra-high discharge energy density of 13.78 J/cm³ at the expense of relatively inconspicuous efficiency (~59.8%) in the BT@SiO₂@PDA/P (VDF-HFP) with 2.5 wt% loading has been achieved under 460 kV/mm. This is mainly attributed to the increases of dielectric constant from 12.1 to 14.2 and the relatively low dielectric loss (0.086) at 100 Hz. Moreover, compared with the pure P (VDF-HFP) (400 kV/mm), the breakdown voltage of the composite with 2.5 wt% loading is surged to 460 kV/mm, which benefited from the hindrance of nanoparticles on carrier migration at low content. This work has realized a thin-film dielectric with ultra-high discharge energy density through a novel design of the nanoparticle structure, providing a theoretical direction for the development of polymer dielectric capacitors.     

Fabrication of highly refractive,transparent BaTiO3/poly(methyl methacrylate) composite films with high permittivities

Incorporation of crystalline barium titanate (BT) nanoparticles into poly(methyl methacrylate) (PMMA) was carried out to prepare highly refractive polymer nanocomposite films that have transparency and high permittivities. The BT nanoparticles were prepared by hydrolysis of a barium/titanium complex alkoxide in 2‐methoxyethanol, then surface‐modified with a silane coupling agent (3‐methacryloxypropyltrimethoxysilane) to improve their affinity for PMMA. The incorporation of the surface‐modified nanoparticles into PMMA was performed up to a nanoparticle content almost equivalent to particle close‐packing state. The refractive index of the composite films increased with nanoparticle incorporation, keeping the relative transmittance normalized with PMMA film above 90%. A high refractive index of 1.82 was reached at a nanoparticle content of 53 vol% with a dielectric constant as high as 36 and a dissipation factor as low as 0.05. The results demonstrate that the crystalline BT nanoparticles are useful fillers for effectively increasing both refractive index and dielectric constant of polymer nanocomposites. Copyright © 2011 Society of Chemical Industry     

Grain‐size–dependent dielectric properties in nanograin ferroelectrics

A series of ferroelectric ceramic models with grain and grain‐boundary structures of different sizes are established via Voronoi tessellations. A phase‐field model is introduced to study the dielectric breakdown strength of these ferroelectric ceramics. Afterward, the relation between the electric displacement and electric field and the hysteresis loop are calculated using a finite element method based on a classical and modified hyperbolic tangent model. The results indicate that as the grain size decreases, the dielectric strength is enhanced, but the dielectric permittivity is reduced. The discharge energy density and energy storage efficiency of these ferroelectric ceramics extracted from the as‐calculated hysteresis both increase along with a decrease in their grain size at their breakdown points. However, under the same applied electric field, the ferroelectric ceramic with a smaller grain size possesses a lower discharge energy density but a higher energy storage efficiency. The results suggest that ferroelectric ceramics with smaller grain sizes possess advantages for applications in energy storage devices.     

Significant enhancement in breakdown strength and energy density of the BaTiO3/BaTiO3@SiO2 layered ceramics with strong interface blocking effect

The BaTiO₃/BaTiO₃@SiO₂ (BT/BTS) ceramics with layered structure, where grain size was about 1–2 μm in the BT layer while it was about 300–400 nm in the BTS layer, were fabricated by the tape-casting and lamination method. With the increasing of SiO₂ content in the BTS layer, the dielectric constant decreased gradually, and the breakdown strength was remarkably improved. Compared to the SiO₂-added BaTiO₃ bulk ceramics, the layered ceramics displayed significant enhancements in dielectric properties, breakdown properties and energy storage properties. The enhancement in dielectric properties was mostly attributed to the diluting effects created by this structure to SiO₂. Based on the finite element analysis with the dielectric breakdown mode, it was regarded that the electric field redistribution and the interface blocking effect led to the enhancement of breakdown strength. Finally, the maximum energy density of 1.8 J/cm³ was obtained at a breakdown strength of 301.4 kV/cm for the BT/BTS3 ceramic.     

Relaxor Ferroelectric BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for Energy Storage Application

Perovskite solid solution ceramics of (1 ? x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (BT–BMN) (x = 0.05–0.2) were synthesized by solid‐state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT‐based ceramics. X‐ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor‐like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature‐stable permittivity plateau (from ?50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 10¹³ Ω cm at x = 0.15, and an energy density of about 1.13 J/cm³ is achieved in the sample of x = 0.1.     

Eco-friendly poly(vinyl alcohol)/delaminated V2C MXene high-k nanocomposites with low dielectric loss enabled by moderate polarization and charge density at the interface

High-dielectric-constant (high-k) polymer/conductor composites with low dielectric loss are desirable for energy storage. However, high leakage currents from interfacial regions with high charge density are difficult to handle. In this work, high permittivity and low dielectric loss were achieved in poly(vinyl alcohol) (PVA)/V₂C MXene nanocomposite films fabricated by solution casting by taking advantage of the interfacial compatibility and moderate interfacial charge density of the nanocomposites. Water-soluble PVA was utilized as the polymer matrix. Delaminated V₂C MXene nanosheets with appropriate conductivity were prepared and used as the filler. The mild interface polarization of the nanocomposites was responsible for achieving favourable permittivity values. The small gap between the work functions of PVA and V₂C contributed to moderate interfacial charge density values and thus low dielectric loss values. A proportional correlation between the interfacial charge density and the conductivity of composites was also verified. The depth of charge injection from the MXene to PVA was found to be half of the interlamellar spacing of the delaminated MXene. The dependence of the electrical properties of the nanocomposites on the frequency and MXene content was also studied. The composite with 4 wt% MXene exhibited a permittivity of ~24 (16 times that of PVA) and a dielectric loss of ~0.14 (1.5 times that of PVA) at 1 kHz, as well as breakdown strength of ~31 MV m⁻¹ (63% of PVA). This work might enable environmentally friendly fabrication of promising composite dielectrics.     

Dielectric properties and energy-storage performance of two-dimensional molybdenum disulfide nanosheets/polyimide composite films

Flexible dielectric materials with high electric energy density and high-temperature resistant characteristic are of great importance for modern electronics and electrical systems. Herein, two-dimensional molybdenum disulfide (MoS₂) nanosheets were efficiently produced via liquid-phase exfoliation and then incorporated into polyimide (PI) to prepare MoS₂/PI dielectric nanocomposites. Compared to the pristine PI, MoS₂/PI nanocomposite films exhibited much larger dielectric permittivity while their dielectric losses still maintained relatively low levels. On the other hand, the Weibull breakdown strength of these nanocomposite films initially increased and then decreased with the increase in the MoS₂ content and gave rise to a maximum value of 395 MV m⁻¹ at 1 vol % loading. Combination of the improved dielectric permittivity and breakdown strength makes the MoS₂/PI nanocomposite film with 1 vol % MoS₂ possess an elevated energy density of about 3.35 J cm⁻³. Moreover, good tensile and thermal properties of the nanocomposite films hold great promise for their applications in high-temperature and harsh conditions. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47991.     

Dielectric Breakdown of Polycrystalline Alumina: A Weakest‐Link Failure Analysis

The effects of varying electrode geometry (ball and ring) and size (radius), dielectric liquid (castor oil and Diala^® oil), and specimen thickness on the dielectric breakdown of a commercial‐grade alumina were investigated. The breakdown strength was expressed in terms of the maximum electric field in the ceramic calculated by finite element analysis (FEA) at the breakdown voltage. The breakdown strength decreased systematically with increasing electrode radius and specimen thickness, and the strength was higher in the Diala^® oil (dielectric constant, ε_r = 2.3 ± 0.06) as compared to the castor oil (ε_r = 4.6 ± 0.13). These effects of the electrode geometry, specimen thickness, and of the dielectric liquid on the breakdown strength of the alumina were analyzed with a weakest‐link failure model employing Laplace and Weibull distributions for a population of defects in the material. The measured size or scaling effects of the electrodes, specimen thickness, and of the dielectric liquid on breakdown strength were in better agreement with the Laplace distribution for a population of surface defects. The dielectric breakdown is likely initiated at surface pits produced by grain pullout. The measured area concentration of surface pits agreed with the defect density analyzed from the weakest‐link failure theory. FEA of specimens containing surface and subsurface cavities revealed that electric field concentrations were always greater for surface pits as compared to subsurface cavities. There is, in fact, no electric field concentration at a subsurface cavity located more than about 100–800 μm below the surface depending on the top electrode size.     

Enhancement of dielectric properties and energy storage performance in 3Y-TZP ceramics with BaTiO3 additives

With the fast charge-discharge speed and ultrahigh power density, electrostatic energy storage materials offer great potential in the applications for pulsed power systems. As a very important member of structural ceramics, 3Y-TZP (3 mol% Y₂O₃ doped tetragonal zirconia polycrystals) has shown extraordinary mechanical properties. However, the research on their energy storage performance is still lacking. Herein, a ferroelectric phase, BaTiO₃ (BT), was introduced and demonstrated to improve the dielectric properties and energy storage performance of 3Y-TZP ceramic matrix via the conventional solid-state reaction method. With increasing the BT content from 0 to 15 mol%, the permittivity of the composite ceramics could be effectively increased from 40.2 to 64.1 measured at 1 kHz. Simultaneously, the dielectric loss could be effectively decreased by depressing the response of charged defects, which was further interpreted by the thermally stimulated depolarization current technique. Meanwhile, the breakdown strength showed a typical increase-then-decrease trend with increasing BT content, and reached their maximum values when doped with 7 mol% BT. Together with the enhancement of dielectric properties, the 7 mol% BT-doped 3Y-TZP ceramics exhibited a maximum energy storage density of 0.42 J/cm³, which was approximately 150% larger than that of the pure 3Y-TZP ceramics.     

Enhancing the dielectric and energy storage properties of lead-free Na0.5Bi0.5TiO3–BaTiO3 ceramics by adding K0.5Na0.5NbO3 ferroelectric

A novel strategy of enhancing the dielectric and energy storage properties of Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBT–BT) ceramics by introducing a K_0.5Na_0.5NbO₃ (KNN) ferroelectric phase is proposed herein, and its underlying mechanism is elucidated. The lead-free KNN ceramic decreases the residual polarisation and increases the electric breakdown strength of the NBT–BT matrix through the simultaneous modification of its A-sites and B-sites. The obtained NBT?BT?x?KNN ceramics have a perovskite structure with unifying grains. A bulk 0.9NBT–BT–0.1KNN ceramic sample with a thickness of 0.2 mm possesses a high energy storage density of 2.81 J/cm³ at an applied electric field of 180 kV/cm. Moreover, it exhibits good insulation properties and undergoes rapid charge and discharge processes. Therefore, the obtained 0.9NBT–BT–0.1KNN ceramic can be potentially used in high-power applications because of its high energy density, good insulation properties, and large discharge rate.     

CaTiO3 linear dielectric ceramics with greatly enhanced dielectric strength and energy storage density

CaTiO₃ is a typical linear dielectric material with high dielectric constant, low dielectric loss, and high resistivity, which is expected as a promising candidate for the high energy storage density applications. In the previous work, an energy density of 1.5 J/cm³ was obtained in CaTiO₃ ceramics, where the dielectric strength was only 435 kV/cm. In fact, the intrinsic dielectric strength of CaTiO₃ is predicted as high as 4.2 MV/cm. Therefore, it should be a challenge issue to enhance the dielectric strength and energy storage density of CaTiO₃ ceramics by optimizing the microstructures. In the present work, dense CaTiO₃ ceramics with fine and uniform microstructures are prepared by spark plasma sintering, and the greatly enhanced dielectric strength (910 kV/cm) and energy storage density (6.9 J/cm³) are obtained. This can be ascribed to the improved resistivity and thermal conductivity, associated with the fine and uniform microstructures. The different post‐breakdown features of CaTiO₃ ceramics prepared by different process well interpret why the enhanced dielectric strength is achieved in the SPS sample. The energy storage density can be further improved to 11.8 J/cm³ by introducing the amorphous alumina thin films as the charge blocking layer, where the dielectric strength is 1188 kV/cm.     

Ultrathin ceramic nanowires for high interface interaction and energy density in PVDF nanocomposites

Dielectric nanocomposites with ceramic fillers have a crucial role in energy storage applications. Therefore, poly(vinylidene fluoride) (PVDF) based nanocomposites filled with 20 nm diameter, surface hydroxylated BaTiO₃ nanowires (BTnws) were produced by solution-casting method in this work, in which BTnws were synthesized via solvothermal method. The dielectric constant of BTnws/PVDF nanocomposites was 24 when the content of fillers was 10vol% at 100 Hz and the breakdown strength could increase up to 417 kV/mm before decreasing. The nanocomposites showed enhanced energy density performance and the maximum energy density could reach to 8.1J/cm³ at 320 kV/mm with 10vol% BTnws, nearly tripled that of pure PVDF at 300 kV/mm. Finite element and molecular dynamic simulation results revealed that thin BTnws could create dielectric homogeneity in the nanocomposites and have strong interface interaction with PVDF molecular. The ultrathin BTnws provided the possibility that single PVDF molecular could wrap on its surface, but this molecular wrapping pattern would not occur when the diameter of BTnws was large. Besides, the wrapping pattern could be reinforced by interactions between surface hydroxyl groups of BTnws and F atoms of PVDF molecular. Such contributions could induce good interface compatibility and lead to the improvement of energy density.     

Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles

Ceramics-polymer nanocomposites consisting of core-shell structured BaTiO₃@Al₂O₃ (BT@Al₂O₃) nanoparticles as the filler and poly(vinylidene fluoride) (PVDF) as the polymer matrix were fabricated by solution casting. At the same volume fraction, the BT@Al₂O₃/PVDF nanocomposites, with larger dielectric constant and higher energy density, outperformed the BT/PVDF nanocomposites. The 2.5 vol% BT@Al₂O₃/PVDF nanocomposites at 360 MV/m had a double more energy density than pure PVDF at 400 MV/m (6.19 vs. 2.30 J/cm³), and a remarkably 42% lower remnant polarization than the 2.5 vol% BT/PVDF nanocomposites (0.99 vs. 1.69 μC/cm² at 300 MV/m). Such significant enhancement was closely related to the surface modification by Al₂O₃, which improved the insulation of BT nanoparticles and reduced the contrast of dielectric constant between the filler and the PVDF matrix.