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.     

Enhanced energy storage properties of Ba0.4Sr0.6TiO3 lead-free ceramics with Bi2O3-B2O3-SiO2 glass addition

In this study, we present an effective strategy to enhance the energy storage properties of Ba_0.4Sr_0.6TiO₃ (BST) lead-free ceramics by the addition of Bi₂O₃-B₂O₃-SiO₂ (BBS) glass, which were prepared by the conventional solid state sintering method. The phase structure, microstructure and energy storage properties were investigated in detail. It can be found that the Ba_0.4Sr_0.6TiO₃-x wt%(Bi₂O₃-B₂O₃-SiO₂) (BST- x wt%BBS, 0 ≤ x ≤ 12) ceramics possess large maximum polarization (P_max), low remanent polarization (P_r) and slim polarization electric field (P-E) hysteresis loops. The breakdown strength (BDS), recoverable energy storage density (W_rec) and energy storage efficiency (η) are enhanced obviously with the addition of BBS glass. The BST-9 wt%BBS ceramic is found to exhibit excellent energy storage properties with a W_rec of 1.98 J/cm³ and a η of 90.57% at 279 kV/cm. These results indicate that the BST-x wt%BBS ceramics might be good candidates for high energy storage applications.     

Improved energy storage performance of Bi(Mg0.5Ti0.5)O3 modified Ba0.55Sr0.45TiO3 lead-free ceramics for pulsed power capacitors

(1–x)Ba_0.55Sr_0.45TiO₃–xBi(Mg_0.5Ti_0.5)O₃ (x = 0, 0.08, 0.1, 0.12, 0.15, 0.2) ceramics were fabricated via a solid-state reaction route. The ultrahigh recoverable energy density (W_rec = 4.05 J cm^?3), efficiency (η = 78%), maximum polarization (P_max = 51.40 μC cm^?2), and high dielectric breakdown strength (BDS = 230 kV cm^?1) were achieved for the 0.9BST?0.1BMT ceramic. The fast discharge rate (t_0.9～0.14 μs), current density (C_D～637.02 A cm^?2), high power density (P_D～38.70 MW cm^?3), good temperature stability (20?180 °C), frequency stability (10?500 Hz), and fatigue endurance for cycling (10⁵) of 0.9BST?0.1BMT ceramic make it suitable for the development of energy-storage devices. The relaxor behavior with a high W_rec (3.06 J cm^?3) and η (93%) at BDS (220 kV cm^?1) was also achieved for the 0.8BST?0.2BMT ceramic. This study systematically investigates the correlation among the structural, dielectric, impedance, and energy storage properties of BMT-doped BST ceramics.     

Effect of grain size on the energy storage properties of (Ba0.4Sr0.6)TiO3 paraelectric ceramics

(Ba_0.4Sr_0.6)TiO₃ (BST) ceramics with various grain sizes (0.5–5.6 μm) were prepared by conventional solid state reaction methods. The effect of grain size on the energy storage properties of BST ceramics (T_c ≈ −65 °C) was investigated. With decreasing grain sizes, a clear tendency toward the diffuse phase transition was observed and the dielectric nonlinearity was reduced gradually, which can be explained by the Devonshire's phenomenological theory (from the viewpoint of intrinsic polarization). Based on the multi-polarization mechanism model, the relationship between the polarization behavior of polar nano-regions (the extrinsic nonlinear polarization mechanisms) and grain size was studied. The variation of the grain boundary density was thought to play an important role on the improvement of dielectric breakdown strength, account for the enhanced energy density, which was confirmed by the complex impedance spectroscopy analysis based on a double-layered dielectric model.     

Excellent energy storage performance of paraelectric Ba0.4Sr0.6TiO3 based ceramics through induction of polar nano-regions

Dielectric energy storage materials with congenitally high power densities and ultrafast discharge rates have been extensively studied for emergent applications. As a typical and traditional dielectric material, paraelectric Ba_0.4Sr_0.6TiO₃ (BST) ceramic exhibits a moderate dielectric constant (ε_r), low dielectric loss and slightly nonlinear P–E hysteresis. However, its energy storage density (W) is extremely low because of its low maximum polarisation (P_max) and weak breakdown strength (BDS). In this study, ferroelectric Na_0.5Bi_0.5TiO₃ (NBT) was introduced into paraelectric BST to enhance energy storage performance. The results show that the introduction of NBT induced polar nano-regions (PNRs) in the paraelectric matrix, resulting in a slim hysteresis loop with low remnant polarisation (P_r) and high P_max simultaneously. Furthermore, owing to a decrease in the oxygen vacancy concentration and an increase in the band gap energy, the BDS of the BST ceramic also significantly increased. As a consequence, a remarkable energy storage density (W_rec = 3.89 J/cm³) and a high energy storage efficiency (η = 83.8%) were realised in the 0.75Ba_0.4Sr_0.6TiO₃-0.25Bi_0.5Na_0.5TiO₃ (0.75BST–0.25NBT) ceramic under a practical electric field of 360 kV/cm. Moreover, the ceramic also exhibited an excellent current density (～1029.7 A/cm²) and ultrahigh power density (～128.7 MW/cm²). The attained energy storage performances indicate that the NBT-modified BST ceramics are promising materials for high energy storage capacitor applications field.     

Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics

SiO₂-added barium strontium titanate ceramics Ba_0.4Sr_0.6TiO₃-xwt%SiO₂ (x=0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO₂ additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO₂ additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO₂ additive led to improve the breakdown strength, further increase of SiO₂ deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86 J/cm³ and energy storage efficiency of 79% at 134 kV/cm.     

Impact of nanosilicates on poly(vinylidene fluoride) crystal polymorphism: Part 1. Melt-crystallization at high supercooling

Polymorphism of poly(vinylidene fluoride), PVDF, in the presence of Lucentite STN organically modified silicate (OMS) is investigated for PVDF nanocomposites melt-crystallized at high supercooling temperatures where neat PVDF crystallizes exclusively in the alpha crystalline phase. Nanocomposites were prepared from solution with 0-1.0 wt% OMS composition. Here we observed that clay addition promotes gamma phase formation in nanocomposites melt-crystallized at high supercooling (i.e., at low crystallization temperature), whereas previously we showed that even small amount of nanosilicates resulted in beta phase formation in cold-crystallized PVDF nanocomposites [1].Wide-angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) studies showed that α- and γ-phases co-existed in nanocomposites containing up to 0.1 wt% OMS, and the amount of α-crystals substantially diminished for higher OMS content. Formation of γ-crystal phase was confirmed with morphologic observation of spherulites of low-birefringence using polarizing optical and atomic force microscopies, and their crystalline structures were verified by FTIR and Raman microscopic spectroscopy. We also address in this work the ambiguities in assessing PVDF crystallographic phases, and correct the phase identification errors which have persisted up to this point in the literature based on melting point confusion. The crystal phase identification for PVDF nanocomposites is discussed and clarified, based on X-ray scattering, vibrational spectra, and thermal analysis. For reference, we provide a vibrational band list, indicating the close, or overlapping bands, of the three phases of PVDF: α, β and γ.     

High energy storage efficiency and fast discharge property of temperature stabilized Ba0.4Sr0.6TiO3–Bi(Mg0.5Ti0.5)O3 ceramics

(1?x)Ba_0.4Sr_0.6TiO₃-xBi(Mg_0.5Ti_0.5)O₃ ((1?x)BST-xBMT) relaxor ferroelectric ceramics were prepared by a conventional solid-state method. In this work, the microstructure, dielectric properties, and pulsed charge–discharge properties were investigated. The doping of BMT caused a decrease in the surface energy and grain boundary energy, and contributed to the formation of polar nanoregions (PNRs). The existence of PNRs was confirmed by piezoresponse force microscopy measurements (PFM) on the 0.8BST-0.2BMT ceramic. The PNRs responded quickly under an AC voltage, thus the pulsed charge–discharge time was short (<80 ns), aiming to realize time compression to improve the power density (P_D). The PNRs were not closely connected to each other and adverse to the formation of leakage current and pinning, thus inhibiting charge transfer at the grain boundaries and contributing to the high energy storage efficiency (η ～ 93%). In addition, the 0.8BST-0.2BMT ceramic also displayed excellent temperature stability. The capacitance-temperature dependence satisfied the requirement of X8R (?55–150 °C, ΔC/C_{25 °C} ≤ ±15%), and η had no obvious fluctuation in the temperature range from 25 °C to 150 °C. This study could provide a successful method to achieve a temperature stable and high η, and a fast charge–discharge process.     

Stress-induced structural changes in electrospun polyvinylidene difluoride nanofibers collected using a modified rotating disk

By attaching separate, parallel electrodes onto a rotating disk collector, well aligned electrospun polyvinylidene difluoride (PVDF), PVDF/carbon nanotube nanocomposite and vinylidene fluoride-trifluoroethylene copolymer nanofibers are directly deposited onto flat substrates forming relatively large, uniform and compact fibrous thin films. The attachments alter the electric-field distribution on the rotating disk, which fosters the fanning of the nanofibers, while the electric field between the separate electrodes and the mechanical force exerted by the rotational disk facilitate the alignment. X-ray diffraction and infrared spectroscopic studies show that the specific environment and force fields created on the modified rotating disk cause the electrospun fibers being effectively stretched to form highly oriented β-form crystallites with slightly reduced inter-chain distance. They also lead to slight increases in crystallinity and crystal size. A mechanism is proposed to account for the structural alteration induced by the modified rotating disk collector. Ferroelectricity of the aligned electrospun PVDF fibrous thin films is also demonstrated.     

High energy-storage performance of lead-free Ba0.4Sr0.6TiO3–Sr0.7Bi0.2TiO3 relaxor-ferroelectric ceramics with ultrafine grain size

Relaxor-ferroelectric (RFE) ceramics possess slender ferroelectric hysteresis loop and low remnant polarization (P_r). They have great potential to provide excellent energy-storage performance as dielectric energy-storage materials. Herein, a lead-free 0.8Ba_0.4Sr_0.6TiO₃–0.2Sr_0.7Bi_0.2TiO₃ (0.8BST–0.2SBT) RFE ceramic with high energy-storage performance has been realized successfully. The addition of Bi³⁺ and increase in Sr²⁺content at the A site of the BST can effectively inhibit the growth of grains for high breakdown strength (E_b). As a result, an ultrafine average grain size of 0.7 μm was obtained in 0.8BST–0.2SBT RFE ceramic, affording a high E_b of 300 kV/cm. Further investigation revealed that the mutual conversion of short-range polar nanoregions and long-range-ordered ferroelectric domains upon application and withdrawal of a 300 kV/cm applied electric field resulted in a high maximum polarization (P_max) of 31 μC/cm² and a low P_r of 2.5 μC/cm². Hence, the 0.8BST–0.2SBT RFE ceramic simultaneously exhibited a high recoverable energy-storage density of 3.3 J/cm³ and a high energy-storage efficiency of 85% at 300 kV/cm. Additionally, a good energy-storage performance was reported over a temperature range of 50°C-120 °C and frequency from 10 to 1000 Hz, making the 0.8BST-0.2SBT RFE ceramic a potential lead-free dielectric energy-storage material.     

Remarkably enhanced energy storage properties of lead-free Ba0.53Sr0.47TiO3 thin films capacitors by optimizing bottom electrode thickness

The lead-free Ba_0.53Sr_0.47TiO₃ (BST) thin films buffered with La_0.67Sr_0.33MnO₃ (LSMO) bottom electrode of different thicknesses were fabricated by pulsed laser deposition method on a (001) SrTiO₃ substrate. It was found that the roughness of electrode decreases and substrate stress relaxes gradually with the increase of LSMO thickness, which is beneficial for weakening local high electric field and achieving higher E_b. Therefore, the recoverable energy density (W_rec) of BST films can be greatly improved up to 67.3 %, that is, from 30.6 J/cm³ for the LSMO thickness of 30 nm up to 51.2 J/cm³ for the LSMO thickness of 140 nm after optimizing the LSMO thickness. Furthermore, the thin film capacitor with a 140 nm LSMO bottom electrode shows an outstanding thermal stability from 20 °C to 160 °C and superior fatigue resistance after 10⁸ electrical cycles with only a slightly decrease of W_rec below 1.6 % and 3.7 %, respectively. Our work demonstrates that optimizing bottom electrodes thickness is a promising way for enhancing energy storage properties of thin-film capacitors.     

Relaxor behavior of BaTiO3-BiYO3 perovskite materials for high energy density capacitors

Dielectric materials with high dielectric constant and breakdown voltage are very promising for pulsed energy storage applications. In this paper, (1-x) BaTiO₃-xBiYO₃ (x=0–0.5) ceramics were synthesized using conventional solid-state reaction method. The ceramic structure transformed from ferroelectric tetragonal phases (x≤0.5) to pseudo-cubic phases (x≥0.1). When x=0.2, beyond the solid solubility limit of BaTiO₃-BiYO₃, the second phase and glassy phases were formed, accompanying lattice parameter excursion. It revealed a gradual change from classic ferroelectric behavior in pure BaTiO₃ to highly diffusive and dispersive relaxor-like characteristics with BiYO₃ content increasing. It exhibited high polarization maximum and low remnant polarization, which was favorable for energy storage in (1-x)BaTiO₃-xBiYO₃ ceramics, due to the disrupted long polarization, the created weak coupling and the formed second phase. Furthermore, the nonlinearity of the (1-x)BaTiO₃-xBiYO₃ ceramics were weakened obviously. A maximum energy storage density of 0.316 J/cm³ at 66 kV/cm with relative high energy efficiency of 82.7% was achieved in 0.8BaTiO₃-0.2BiYO₃ ceramic, which indicated that (1-x)BaTiO₃-xBiYO₃ ceramics were promising lead-free relaxor materials for energy storage applications.     

Microstructure and microwave-frequency electromagnetic properties of Ni0.4Zn0.6Fe2O4/Ba0.6Sr0.4TiO3 composites

(x)Ni_0.4Zn_0.6Fe₂O₄+(1−x)Ba_0.6Sr_0.4TiO₃ composite ceramics with x=0.6, 0.7, 0.8, 0.9 and 1 were synthesized by solid state reaction method. The high dense composites have only two phases, i.e., Ni_0.4Zn_0.6Fe₂O₄ and Ba_0.6Sr_0.4TiO₃. The permittivity ε′ of the composites decreases slightly with the frequency increasing from 3 MHz to 1 GHz. The permittivity ε′′ of the composites also shows a little increase with frequency in the 3 MHz–1 GHz range. The permeability displays a relaxation resonance within the 3 MHz–1 GHz frequency range. The permeability μ′ increases while the cut-off frequency decreases with the Ni_0.4Zn_0.6Fe₂O₄ concentration, obeying the Snoek's law μ_if_r=constant. The permittivity ε′ of the composites decreases with Ni_0.4Zn_0.6Fe₂O₄ concentration. The composites have a relatively higher ε′ than the pure Ni_0.4Zn_0.6Fe₂O₄ at 1–10 GHz. In the frequency range of 1–10 GHz, the magnetic permeability μ′ reaches its maximum and μ′′ shows a minimum for the composite with x=0.6 in all ceramics. The permeability μ′ of the composites decreases with dc magnetic field at 1–10 GHz. The permeability shows a domain wall resonance, and the resonance frequency shifts to high frequency with the dc magnetic field. The permittivity was also influenced by the dc magnetic field due to a magnetodielectric effect.     

Grain size-driven effect on the functional properties in Ba0.6Sr0.4TiO3 ceramics consolidated by spark plasma sintering

Dense fine-grained Ba_0.6Sr_0.4TiO₃ ceramics with submicronic grains sizes (GS) have been prepared using nanopowders synthesized via sol-gel route and consolidated by Spark Plasma Sintering (SPS). By changing SPS parameters, the GS was reduced from 214 nm to 74 nm. Diffuse ferroelectric-paraelectric phase transitions and low values of dielectric permittivity (<1000) at the Curie temperature (T_C ∼280 K) were revealed by Impedance Spectroscopy in all sintered ceramics. The GS reduction from submicron to nanoscale range reflects in a gradually diminishment of dielectric constant, tunability, polarisation and storage energy properties. Raman spectroscopy investigations pointed out the presence of polar nanoclusters above the T_C. The short-range polar order is affected by the GS decrease, but becomes more thermally stable. The observed properties of Ba_0.6Sr_0.4TiO₃ nanostructured ceramics are interpreted by considering the interplay between the GS reduction, the role of low-permittivity grain boundaries and the diffuse character of the ferroelectric-to-paraelectric transformation.     

Effects of raw materials on nonhydrolytic sol-gel synthesis of Ba0.6Sr0.4TiO3

The effects of raw materials on the nonhydrolytic sol-gel synthesis of Ba_0.6Sr_0.4TiO₃ were systematically studied in this work. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM). Ba(CH₃COO)₂, Sr(CH₃COO)₂, BaCO₃ and SrCO₃ can undergo deesterification substitution reaction with Ti(OC₄H₉)₄ to synthesize Ba_0.6Sr_0.4TiO₃ phase. Sample taking Ba(CH₃COO)₂ and Sr(CH₃COO)₂ as barium and strontium source has better Ba_0.6Sr_0.4TiO₃ synthesis results than that of samples using BaCO₃ or SrCO₃. Ba_0.6Sr_0.4TiO₃ phase can not be synthesized at low temperature taking BaCl₂, SrCl₂ and TiOSO₄ as barium, strontium or titanium source due to their strong ionic bonding. TiCl₄ undergoes an alcoholysis reaction with (oxygen donor) alcohol to form a multimolecular association structure and solvated complex chlorotitanium alkoxide, which reduces the possibility of a deesterification polycondensation reaction. The Ba_0.6Sr_0.4TiO₃ phase cannot be synthesized at low temperatures when CH₃CH₂OH or C₃H₇OH is used as the solvent. The synthesis of Ba_0.6Sr_0.4TiO₃ is insufficient because of the coordination ability and solvation effect when CH₃COOH is used as the solvent. The synthesis of Ba_0.6Sr_0.4TiO₃ is the best when Ba(CH₃COO)₂, Sr(CH₃COO)₂, Ti(OC₄H₉)₄ and glycerol (C₃H₈O₃) are used as the barium source, strontium source, titanium source and solvent, respectively. Pure Ba_0.6Sr_0.4TiO₃ phase with a particle size of 17–43 nm and high dispersion is synthesized when the molar ratio of barium-strontium-titanium is 0.6:0.4:1.2.     

Crystallization behavior,ultrahigh power density and high actual discharge energy density of lead-free borate glass-ceramics containing TiO2

This work presents SrO-BaO-Nb₂O₅-B₂O₃-P₂O₅-K₂O-TiO₂ glass-ceramics prepared by controlled crystallization method. The uniformly dense microstructure with fine grains can be achieved by introducing TiO₂. The structural changes were confirmed by the results of SEM and TEM. The 0.5 mol% TiO₂ added glass heated at 750 °C for 2 h demonstrates excellent comprehensive properties of ε_r= 110, BDS = 1408 kV/cm, high energy storage efficiency (η) of 92% and energy storage density of 9.65 J/cm³. The as-prepared glass-ceramic exhibits ultrahigh power density (86.21 MW/cm³), actual discharge energy density (1.00 J/cm³) and excellent temperature stability. These findings qualify this environment-friendly glass-ceramics as one potential candidate for energy storage applications, especially in high power and pulsed power system field.     

Novel polymer composites of RuO2@nBaTiO3/PVDF with a high dielectric constant

The dielectric properties of a novel polymer dielectric material were investigated. The conductive phase of RuO₂ was synthesized for deposition on the surface of a nanosized BaTiO₃ (nBT). The RuO₂@nBT hybrid particles were incorporated into a poly (vinylidene fluoride) (PVDF) as a three-phase composite (RuO₂@nBT/PVDF). The obtained dielectric constant (ε′) was significantly high (3837.16) for the composite with a volume fraction of $f_{Ru{O}_2@nBT}$ = 0.50. The large interfacial polarization between the RuO₂?nBT and RuO₂?PVDF interfaces considerably increased the value of ε′. Therefore, interfacial polarization is a critical factor in improving the dielectric properties. The dielectric behavior of the RuO₂@nBT/PVDF composites can be described using the effective medium percolation theory model, which indicates the significant contributions of the conductive RuO₂ phase and high-permittivity nBT phase.     

Enhanced energy storage performance in Sn doped Sr0.6(Na0.5Bi0.5)0.4TiO3 lead-free relaxor ferroelectric ceramics

SnO₂ doped Sr_0.6(Na_0.5Bi_0.5)_0.4TiO₃ (NBT-ST) ceramics were prepared by a conventional solid-state reaction method. Their phase structures, microstructures and electrical properties were characterized in details. It is found that SnO₂ doping could increase the lattice parameters, density and average grain size. A suitable amount of SnO₂ can improve dielectric properties, and affect the relaxor behavior of the NBT-ST matrix, thereby it can effectively reduce the energy loss and optimize the energy storage performance. Furthermore, the energy storage properties are improved with SnO₂ doping. Especially, the 1 at. % SnO₂ doped NBT-ST achieves a high recoverable energy density of 2.35 J/cm³, which is mainly attributed to large maximum polarization of 43.2 μC/cm², small remnant polarization of 5.83 μC/cm² and high breakdown strength of 180 kV/cm. Also, relatively good temperature stability for dielectric performance and excellent fatigue resistance are observed in this composition. These properties are attractive for lead-free energy storage applications.     

Impact of microwave sintering on dielectric properties of screen printed Ba0.6Sr0.4TiO3 thick films

The influence of 30 GHz microwave sintering compared to conventional sintering has been investigated on polycrystalline Ba_0.6Sr_0.4TiO₃ (BST60) thick films with respect to an application as tunable dielectrics. The BST thick films were prepared as metal–insulator–metal (MIM) capacitors on alumina substrates. The average grain size (440 nm) and the porosity (approx. 30%) of the sintered films are only little affected by the sintering method. However, permittivity, dielectric loss and tunability have been influenced substantially. The dielectric improvement by microwave sintering is interpreted in terms of an increased crystal quality (ξ_S) and/or a decrease of defect concentrations. It is assumed that microwave sintering preferably heats up parts of the film where an increased defect density exists and therefore causes a selective heating process. This may heal up charged defects, inhomogeneities, and structural defects.