High-temperature Capacitors Research Articles

Conduction mechanism of donor and acceptor doped sodium niobate-based ceramics

Recently, NaNbO3 (NN) has become a hot topic of current research due to its antiferroelectric energy storage properties, which demand that the ceramics withstand large applied electric fields. The breakdown strength is dependent on conduction properties, but there is limited research on the conduction mechanisms of NN. Here we report that A-site donor-doping of Bi3+ (BixNa1-3xNbO3) and B-site acceptor-doping of Mg2+ (NaNb1-2/5yMgyO3) in NN lead to dramatic changes in the magnitude of the bulk conductivity (σb) and the conduction mechanism of NN ceramics. Undoped NN exhibits mixed conduction behavior with an oxide ion transport number (tion) of ∼0.44, and σb of ∼10−6 S/cm at 600 °C. A low level of Bi3+ doping (x = 0.06) suppresses the NN mixed conduction mechanism to electron conduction (tion ∼ 0), and the bulk conductivity increases significantly, σb > 10−4 S/cm at 600 °C. On the other hand, Mg2+ doping (y = 0.06) samples mainly change the σb, with the mixed ion/hole conduction (tion ∼ 0.43) and σb > 10−4 S/cm at 700 °C. The results show that the conductivity of NN increases and exhibits different conduction mechanisms with the doping of Bi3+ and Mg2+. Aliovalent doping is not beneficial to improving the insulation properties. Thus, this work provides theoretical guidance for the study of energy storage characteristics and the suppression of leakage behavior of high-temperature dielectric capacitors.

Multi-layered boron nitride/polyimide high-temperature capacitor dielectric film

Polymer dielectric is the key material for film capacitor. With the development of advanced electronics and power systems, higher requirements are put forward for the temperature resistance of film capacitors (>150 °C). Unfortunately, common high-temperature polymers cannot be practically used due to their high conduction loss under high temperature and high electric fields. Herein, high-temperature dielectric polymer nanocomposites composed of polyimide (PI) and boron nitride nanosheets (BNNS) are presented. By designing an alternating multilayer structure of PI and BNNS, the conduction loss of nanocomposites is effectively reduced. At 150 °C, the discharge energy density of the nanocomposite containing four BNNS layers is 3.98 J/cm3, which is 530% higher than that of PI. Simulation and experimental results show that the alternating multilayered structure can reduce conduction loss and leakage current density more effectively comparing with conventional blended polymer-based composites with randomly distributed fillers. This work provides a feasible idea for the scalable design of high-temperature capacitor dielectrics.

Prediction of high-temperature polymer dielectrics using a Bayesian molecular design model

Machine learning has shown its great potential in the accelerated discovery of advanced materials in the field of computational molecular design. High-temperature polymer dielectrics are urgently required with the emerging applications of energy-storage dielectric film capacitors under high-temperature conditions. Here, we demonstrate the successful prediction of polymers with a high dielectric constant (ɛ) and high glass transition temperature (Tg) using a Bayesian molecular design model. The model is trained on a joint data set containing 382 computed ɛ values using density functional perturbation theory and experimentally measured Tg values of ∼7000 polymers to build relative quantitative structure–property relationships and identify the promising polymers with specific desired range of dielectric constant and glass transition temperature. From the hypothetical polymer candidates, ten promising polymers are proposed based on their predicted properties and synthetic accessibility score for high-temperature dielectric film capacitors’ application. Moreover, 250k novel polymer structures are generated with the model to support future polymer informatics research. This work contributes to the successful prediction of high-temperature polymer dielectrics using machine learning models.

Significant improvement in electrical characteristics and energy storage performance of NBT-based ceramics

Dielectric capacitors with superior power density and ultra-fast charge/discharge capability have attracted growing attention in the pulse power field. In this work, rare earth oxide (Sm2O3) was introduced into (0.74Na0.5Bi0.5-0.26Sr)Ti0.9Zr0.1O3 matrix, to enhance the comprehensive energy storage performance. The structure, dielectric and ferroelectric characteristics of these materials were systematically studied, to assess the potential application in multilayer ceramic capacitors. All samples yielded a perovskite structure, as the addition of Sm2O3 results in a substantial increase in density, along with the enhancement of relaxor behavior and local nanodomains. With increasing Sm2O3 concentration, a distinctly slim ferroelectric hysteresis loop as well as frequency dispersive dielectric characteristics have been detected. The NBSZT-0.02Sm ceramic has realized an excellent energy storage density of 4.62 J/cm3, with a moderately good energy efficiency of 84.38% and favorable temperature stability over a broad temperature range of 102.7–397.1 °C. The activation energy (Ea) of the ceramics increased from 1.40 eV to 1.72 eV with Sm2O3 concentration increasing from x = 0 to x = 0.04, which inhibits the appearance of the local electrical branch and thus improves the breakdown strength of the NBSZT-xSm ceramics. This study provides a novel possibility for designing lead-free ceramic materials for high-temperature capacitors.

Enhancing high-temperature capacitor performance of polymer nanocomposites by adjusting the energy level structure in the micro-/meso-scopic interface region

The interface plays a major role in the conduction and breakdown behaviors of dielectric materials. Enhancing interface compatibility and Schottky barrier to reduce conduction loss and enhance breakdown strength of nanocomposites has been widely studied. Nevertheless, there are few reports on the effect of the energy level structure in filler/polymer and electrode/dielectric interface region on the breakdown strength and high-temperature energy storage performances. Herein, the polyimide (PI) films sandwiched by Al2O3 layers and filled with SiO2 shell-coated high-K BaTiO3 nanofibers were prepared. Our results reveal that the wide bandgap oxide layer can regulate the energy level structure of the interface region, introduce deep traps in the nanocomposites and increase the Schottky barrier at the electrode/dielectric interface to impede charge injection and transport. Moreover, the nanocomposites combine the advantages of anisotropic dielectric properties from the Al2O3 layer, SiO2 shell, and BaTiO3 core, enhancing dielectric constants of the nanocomposites. The optimal nanocomposites show greatly enhanced discharge energy density and breakdown strength at 150 °C, which are 370% and 38% higher than those of PI, respectively. This work provides more insight into the mechanism of electrical conduction and breakdown in polymer nanocomposites and offers an effective strategy for developing polymer nanocomposites with superior capacitive performance at elevated temperatures.

Polymer composites with high energy density and charge–discharge efficiency at high temperature using aluminum oxide particles

Polymer-based dielectric capacitors with excellent elevated-temperature capacitive performances are highly demanded in next-generation electronic power systems. Here, we design an effective and low-cost way to fabricate high-temperature polymer capacitors via adding Al2O3 nanoparticles (NPs) into polyetherimide (PEI) nanocomposites. Benefited from good thermal stability, high thermal conductivity of Al2O3 NPs and high glass transition temperature of PEI, the Joule heat caused by high temperature and large electric field, dissipate quickly. Therefore, the nanocomposites present low dielectric loss and superior high-temperature capacitive properties. Consequently, even at 150 °C, the Al2O3 NPs/PEI nanocomposite delivers still slim electric displacement-electric field (D-E) loops, small remnant electric displacement (Dr), and large maximum electric displacement (Dmax). Thus, it results in an ultra-high discharged energy density (Ue) of 6.68 J cm−3, which far exceeds the state-of-the-art polymer and polymer composites, and high efficiency (η) of 84.77% at 150 °C. This work addresses the paradox that large Ue and high η cannot be simultaneously achieved in high-temperature polymer composites.

Tuning of high-temperature dielectric properties in the system (Bi0.5Na0.5)TiO3–BaTiO3–CaZrO3

Solid solutions of the (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xCaZrO3 system are regarded as promising dielectrics for high-temperature capacitors as they exhibit a remarkable flat trend of the permittivity over a large temperature range coupled with comparable low dielectric losses. In this work, the composition 0.8(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.2CaZrO3 was chosen in an attempt to optimize especially the high-temperature dielectric properties above 200 °C. In particular, the influence of excess bismuth to account for element losses caused by evaporation, and the effect of manganese as acceptor dopant are reported. Conventional solid-state reaction route was used to synthesize selected compositions. X-ray diffraction was used to confirm a pseudo-cubic perovskite main phase in all examined compositions, although small traces of a zirconia secondary phase were also detected. All samples exhibit an expected flat trend of the relative permittivity with a maximum deviation of the permittivity lower than 15% between −80 °C and 300 °C. The unmodified base composition shows small dielectric loss (<2%) between −55 °C and 265 °C. By using small quantities of manganese doping, the small-loss temperature range was extended (−70 °C and 300 °C). Excess bismuth also affects the temperature-dependent dielectric losses, resulting in a narrowed temperature range, eventually limiting the application possibilities.

NaNbO3 modified BiScO3-BaTiO3 dielectrics for high-temperature energy storage applications

Among the lead-free compositions identified as potential capacitor materials, BiScO3-BaTiO3 (BS-BT) relaxor dielectrics exhibit good energy storage performance. In this research, 0.4BS-0.6BT is considered as the parent composition, with NaNbO3 (NN) addition intended to substitute the A and B site cations. The NN modified BS-BT ceramics exhibit excellent temperature stability in terms of their dielectric properties, with the room-temperature dielectric constant on the order of 500–1 000 and variation less than 10% up to 400 °C. In addition, NN has a high band-gap energy leading to increased breakdown strength and energy storage properties in modified compositions. The highest breakdown strength was achieved for 0.4BS-0.55BT-0.05NN, being on the order of 430 kV/cm, and a high energy density of 4.6 J/cm3 with high energy efficiency of 90% was simultaneously achieved. Of particular importance is that the variation of the energy density was below 5% due to the temperature-insensitive dielectric constant, while ∼90% energy efficiency was retained over the temperature range of 25–160 °C. The improved temperature stability with NN addition makes this composition promising for high temperature capacitor and dielectric energy storage applications.

Structural and electrical characteristics of manganese modified Bi0.5K0.5TiO3 ceramic

The Rietveld analysis of X-ray diffraction spectrum confirms the tetragonal (P4bm) symmetry of the manganese doped Bi0.5K0.5TiO3 (BKT) compound synthesized by the solid-state reaction route. The dielectric analysis of doping of 5% manganese (Mn) ions at the Ti-sites in the host BKT is analyzed on the basis of the Maxwell–Wagner mechanism in the material. The P-E loop indicates the ferroelectric nature of the compound. Correlated barrier hopping is found to be the dominant conduction mechanism in the compound. The leakage current confirms the negative temperature coefficient of resistance nature of the compound and the Ohmic electrode contact. The substitutions of smaller Mn ion with multiple valencies at the Ti site enhances the dielectric constant substantially, decreases the loss tangent, and lower the leakage current. These features are applicable for energy storage devices, memory devices, high-temperature sensors, and high-temperature capacitors.

Construction of lead-free dielectrics for high temperature multilayer ceramic capacitors and its inner electrode matching characteristics

High-temperature multilayer ceramic capacitors (HT-MLCCs) are indispensable key basic components in high-temperature electronic equipment. Obtaining practical HT-MLCCs requires not only a high-temperature dielectric with excellent temperature stability, but also a good match between the inner electrode and the dielectric. In this work, a quaternary lead-free system (1-x) (0.64Na0.5Bi0.5TiO3-0.16 K0.5Bi0.5TiO3-0.20Bi(Mg2/3Nb1/3)O3)-xCaZrO3 (NBT-KBT-BMN-xCZ) was investigated as candidates for high-temperature dielectric. The introduction of CZ can adjust the composition of different types of polar nanoregions (PNRs) and weaken the ferroelectric coupling behavior, thereby achieving the flattening of the dielectric temperature curve. Among all the components, NBT-KBT-BMN-0.06CZ (0.06CZ) has the most excellent high-temperature dielectric properties, and is further used to match the two inner electrodes of Pt and Ag/Pd to make HT-MLCCs. The results show that for Pt-MLCC, co-firing causes a second phase MgTi2O5 to appear at the heterogeneous interface, resulting in incomplete inner electrodes, lower dielectric permittivity and increased loss in the high-temperature region. In contrast, for Ag/Pd-MLCC, the heterogeneous interface is well bonded, the inner electrode is continuous and complete and there is no second phase, which makes the capacitor show excellent dielectric performance (εr=1393±15%, tanδ≤0.025) over a wide range of 129–404 °C. This work provides an important solution for the development of a new generation of HT-MLCCs.

High-temperature dielectric properties of Bi0.5Na0.5TiO3–NaNbO3–Sr0.8Na0.4Nb2O6 ceramics

Dielectric materials 0.99((1-x)Bi0.5Na0.5TiO3-xNaNbO3)-0.01Sr0.8Na0.4Nb2O6 (BNT-xNN-SNN) with x = 0.15, 0.20, 0.25 and 0.30 have been developed for high-temperature capacitors. Temperature-dependent dielectric permittivity ε′ and loss tanδ, DC resistivity and ferroelectric properties are reported. The ε′ and tanδ of the BNT-0.25NN-SNN ceramic are very stable over a wide temperature range (<-55 °C ∼ >350 °C), in which the variable rates of ε′ (∣ε′- ε′25 °C∣/ε′25 °C), and tanδ are no more than 15% and 0.02, respectively. In addition, ε′ at a reference temperature 25 °C is 1250. The resistivity is also high on the order of magnitude of 1011 Ω m at 100 °C and 108 Ω m at 300 °C. The RC constant is approximately 2.8 s at 300 °C. The results show that the BNT-0.25NN-SNN ceramic is a good candidate material for high-temperature capacitors.

Effects of Nd3+ Donor Doping on Dielectric, Electrical Conductivity and Ferroelectric Properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 Lead-Free Ceramics

In this study, lead-free (Bi0.465Na0.465Ba0.07)1− x Nd x TiO3 (x = 0.00, 0.01, 0.02 and 0.03) piezoceramics were prepared via conventional solid state mixed-oxide route. The role of A-site Nd3+ donor doping was investigated through systematic studies of crystal structure, morphology, dielectric, electrical conductivity and ferroelectric properties. The X-ray diffraction data revealed single-phase perovskite structure with pseudocubic symmetry for all samples. The morphology exhibited dense microstructure with the average grain size that tended to decrease with increasing x. Compared to undoped BNBT (x = 0), the dielectric loss (tanδ) at high temperature (> 350 °C) significantly decreased with increasing Nd3+ doping level. With increasing x, the impedance spectroscopy data revealed more electrically homogeneous microstructure with a significant increase in resistivity (ρ) from Ω.m (BNBT) to Ω.m at x = 0.02. In addition, Nd3+ doping can induce a transition from a square-like to a slant P-E hysteresis loop with a significant drop in coercive field (E c) (from 25.81 for BNBT to 8.48 kV/cm for x = 0.03) and remanent polarization (P r) (from 27.93 to 3.67 µC/cm2), corresponding to the ergodic relaxor. These results suggest A-site Nd3+ doped 0.93BNT-0.07 ceramic as a promising candidate for high temperature capacitors and actuator materials.

Scalable Polyimide-Poly(Amic Acid) Copolymer Based Nanocomposites for High-Temperature Capacitive Energy Storage.

The developments of next-generation electric power systems and electronics demand for high temperature (≈150°C), high energy density, high efficiency, scalable, and low-cost polymer-based dielectric capacitors are still scarce. Here, the nanocomposites based on polyimide-poly(amic acid) copolymers with a very low amount of boron nitride nanosheets are designed and synthesized. Under the actual working condition in hybrid electric vehicles of 200 MV m-1 and 150°C, a high energy density of 1.38 J cm-3 with an efficiency higher than 96% is achieved. This is about 2.5 times higher than the room temperature energy density (≈0.39 J cm-3 under 200 MV m-1 ) of the commercially used biaxially oriented polypropylene, the benchmark of dielectric polymer. Especially, the energy density and efficiency at 150°C show no sign of degradation after 20000 cycles of charge-discharge test and 35 days' high-temperature endurance test. This research provides an effective and low-cost strategy to develop high-temperature polymer-based capacitors.

Ultrahigh Temperature Lead-Free Film Capacitors via Strain and Dielectric Constant Double Gradient Design.

With the development of miniaturization, lightweight and integration of electronic devices, the demand for high-temperature dielectric capacitors is becoming urgent. Nevertheless, the breakdown strength and polarization are deteriorated at high temperatures due to the thermal energy assisting the electron transport and impeding the dipole alignment. Here, a structure of capacitor with double gradients of dielectric constant gradient and strain gradient is designed to achieve high breakdown strength, high working temperature, and high energy storage density simultaneously. It is found that the designed structure of BaHf0.17 Ti0.83 O3 /1mol% SiO2 doped BaZr0.35 Ti0.65 O3 /0.85BaTiO3 -0.15Bi(Mg0.5 Zr0.5 )O3 exhibits excellent energy storage performance. The energy storage density of 127.3 J cm-3 with an energy storage efficiency of 79.6% is realized in the up-sequence multilayer with period N= 2 at room temperature. Moreover, when the working temperature varies from -100 to 200 °C, the energy storage density of the N= 4 capacitor keeps stably at 84.62 J cm-3 with an energy storage efficiency 78.42% at 6.86 MV cm-1 . All these properties promise great potential applications of the designed multilayer capacitors with the double gradients in harsh environments, and the design principle can be applicable to other systems to boost working temperature.

Improving the Energy Density and Efficiency of the Linear Polymer PMMA with a Double-Bond Fluoropolymer at Elevated Temperatures.

A variety of applications can be found for high-temperature film capacitors, including energy storage components and pulsed power sources. In this work, in order to increase the energy density (Ue), poly(vinylidene fluoride-chlorotrifluoroethylene-double bond) (P-DB) is introduced into poly(methyl methacrylate) (PMMA) to manufacture composite films by a solution casting process. In the case of the pure PMMA film, there is significant improvement in the polarization (Pmax) and breakdown field (Eb) of the composite film. These improvements can effectively increase the Ue of the composite film at room temperature and the elevated temperature. The results show that at an elevated temperature of 90 °C and at 350 MV/m, the Ue of 40 vol % P-DB reaches 8.7 J/cm3, and the efficiency (η) of 77% is also considerable. Compared with biaxially oriented polypropylene (2.0 J/cm3), the proposed film exhibits 4 times enhancement in the energy storage density, meaning that it can be an energy storage capacitor with huge potential at high temperatures.

Synthesis and properties of lead-free BNT-BT-xCZ ceramics as high-temperature dielectrics

Bulk ceramic samples of the perovskite series (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xCaZrO3 with 0 ≤ x ≤ 0.2 were prepared using the conventional solid-state route. It is shown that careful characterization of Bi2O3 raw materials is essential to avoid deviations from the nominal composition. X-ray diffraction analysis reveals formation of perovskite main phase, and a relaxor-like response of the relative permittivity over temperature was observed. Flattening of the permittivity-temperature curves is observed with increasing CaZrO3 fraction, while maintaining low dielectric losses over a wide temperature range. Samples with x = 0.2 exhibit a temperature-stable permittivity of εr ≈ 600 (Δε’r/ε‘r,mid ≈ 10 % between -80°C and 300°C) along with small dielectric losses (tan δ < 0.02 between -55°C and 265°C) at 1 kHz. These attributes qualify these compositions as suitable high temperature capacitor dielectric materials.

2D MoS2 Nanosheet‐Based Polyimide Nanocomposite with High Energy Density for High Temperature Capacitor Applications

Front Cover: In article number 2100079 by Kailiang Ren and co-workers, a MoS2-g-PMMA/PI (MPP, PI is polyimide) nanocomposite consisting of a thin layer of poly(methyl methacrylate) coated 2D MoS2 nanosheets and PI is reported. The energy density of MPP-3% nanocomposite (3% MoS2) reaches 8.6 J cm−3. Furthermore, the highest energy density of MoS2-g-PMMA/PI reaches 3.92 J cm−3 at 150 °C, which is 47.9% higher than that of the PI film.

2D MoS2 Nanosheet‐Based Polyimide Nanocomposite with High Energy Density for High Temperature Capacitor Applications

AbstractWith the development of electrical vehicles and power electronics, the demand for high‐temperature energy storage capacitors with high energy density has grown rapidly. In this investigation, 2D MoS2 nanosheets are coated with a thin layer of poly(methyl methacrylate) and then mixed with polyimide (PI) solution to fabricate nanocomposites. The dielectric constant of MoS2‐g‐PMMA/PI (MPP‐3%) reaches 4.2, which is 20% higher than that of a pristine PI film. The energy density of the MPP‐3% nanocomposite reaches 8.6 J cm−3, which is 40% higher than the highest energy density of a pristine PI film. In addition, the electric breakdown field of the MPP‐3% composite is 50% higher than that of the MoS2/PI nanocomposite without a surface coating. Furthermore, it is found that the highest energy density of MoS2‐g‐PMMA/PI reaches 3.92 J cm−3, which is 47.9% higher than that of the PI film at 150 °C. The charge–discharge efficiency of the nanocomposite film reaches 61.7% at 150 °C. This result is much higher than the previously reported research results for high‐temperature capacitor applications. The MoS2‐g‐PMMA/PI‐based nanocomposite shows great promise for use in high‐temperature capacitor applications.

Superior energy storage properties in (1−x)(0.65Bi0.5Na0.5TiO3-0.35Bi0.2Sr0.7TiO3)-xCaZrO3 ceramics with excellent temperature stability

Novel (1−x)(0.65Bi0.5Na0.5TiO3-0.35Bi0.2Sr0.7TiO3)-xCaZrO3 (BNT-BST-xCZ, x = 0–0.15) ceramics were manufactured through a solid phase sintering process and their structural and electrical performance have been systematically investigated. By doping CaZrO3, a higher recoverable energy storage density of 2.9 J/cm3 and energy storage efficiency of 80% at 330 kV/cm can be simultaneously obtained in BNT-BST-0.1CZ ceramic. Further analysis with the aid of finite element simulation indicates that the improvement of energy storage performance lies in the refined grain size and reduced defects, which are both conductive to improve the dielectric breakdown strength. More importantly, incorporating CaZrO3 gives slimmer hysteresis loops as well as excellent temperature (20–150 °C) and frequency (5–200 Hz) stability of energy storage properties. Moreover, a stable dielectric permittivity of in an ultra-wide range from 44 °C to 483 °C was achieved in BNT-BST-0.10CZ ceramic. Our findings suggest that the BNT-BST-0.1CZ ceramic has great potential applications in pulsed power systems and high-temperature capacitors.

Large field-induced strain with enhanced temperature-stable dielectric properties of AgNbO3-modified (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(1-x)(Bi0.5Na0.5)0.94Ba0.06TiO3-xAgNbO3 lead-free piezoelectric ceramics (abbreviated as BNBT-100xAN) were prepared using the conventional solid-state sintering method. The effects of the introduction of AgNbO3 (AN) dopants for the dielectric and piezoelectric performances of BNBT-100xAN ceramics were systematically studied. The XRD patterns and Raman spectra demonstrated that AN as a modifier was successfully diffused into the BNBT-100xAN lattice and revealed a pseudo-cubic symmetry structure. All samples exhibited a dense surface morphology accompanied by the uniform distribution of elements. A large bipolar strain of ~0.501% and unipolar strain of ~0.481% corresponding to the normalized strain d33* of ~740 p.m./V were achieved for BNBT-1AN ceramic at 65 kV/cm field. The BNBT-4AN ceramic exhibited an excellent temperature-stable permittivity with the range from 59 to 380 °C and its dielectric loss was less than 0.02 between 97 °C and 329 °C. These results revealed that BNBT-100xAN ceramics were more hopeful candidates for actuators, strain sensors, and high-temperature capacitors.