Importance of uniformity of grain size to reduce dc degradation and improve reliability of ultra-thin BaTiO3-based MLCCs

Excellent direct current (dc)-bias and reliability have become increasingly important for ultra-thin BaTiO₃-based multilayer ceramic capacitors (MLCCs). Herein, X5R-MLCCs with a thickness of ～1 μm are fabricated using BaTiO₃ with varied grain size. It is shown that the uniformity of the grain size plays an important impact on the direct current (dc)-bias and the reliability of ultra-thin MLCCs. Uniform grain size, which is indicative of good distribution of doping element contributes to improved temperature stability. By contrast, abnormal large grains induce reinforced space charge polarization. Chips with non-uniform grains exhibit dramatic dielectric constant change under dc-bias due to the high tetragonality and irreversible domain-wall motion. Weibull distribution and highly accelerated life test (HALT) reveal that non-uniform grains contain more oxygen vacancies supported by impedance spectra analysis at 300–450 °C. This study provides a feasible strategy to improve the dc-bias and the reliability of the ultra-thin MLCCs.     

Simultaneous enhancement of dielectric properties and temperature stability in BaTiO3-based ceramics enabling X8R multilayer ceramic capacitor

Modified BaTiO₃ ceramics that possess high dielectric permittivity and acceptable temperature stability have been widely utilized as multilayer ceramic capacitors (MLCCs) for high-frequency bypass and power filtering in automotive applications. However, since the increasing demand for high-capacity and small-size, high-permittivity materials that can serve as dielectric layers in MLCCs are urgently required. In this work, we design and fabricate a special BaTiO₃-0.03Mg-0.02Y-0.02CaZrO₃ ceramic with a high dielectric permittivity of 3000 and the dielectric variation below ±13% in the temperature range of -55–150°C, fulfilling the requirements of X8R capacitors. To achieve these results, we employed grain size engineering and cation doping, using BaTiO₃ precursors with a particle size of 240 nm to prepare the BaTiO₃-based ceramics with fine grains, while Mg and Y co-doping was used for improving the temperature stability due to dielectric dispersion. Utilizing these high-permittivity BaTiO₃-based materials, we fabricated MLCCs that satisfy the X8R criterion, possessing a high dielectric constant of 2950 and a high breakdown field (410 kV/cm).     

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.     

Defect control of Yb-doped dielectric ceramics on improving the reliability for MLCC application

A fine-grained Yb-doped (Ba_1-xCa_x)_mTiO₃ (BCT)-based ceramic, possessing high dielectric properties and outstanding reliability under a reducing atmosphere sintering, was synthesized by the chemical coating method. The mean grain size of the ceramics was 170 nm. A distinct core-shell structure of ceramics was observed by the TEM-EDS measurement. With increasing content of Yb, the curie temperature increased and the DC-bias capacitance change rate decreased under a DC field at 40 kV/cm. The optimal component showed a dielectric constant of 2200 and met the requirement of the X8R MLCCs application. Moreover, the outstanding highly accelerated life-time and higher grain boundary activation energy substantiated that Yb dopant prominently improved the reliability of dielectric materials of MLCCs. All these studies provide meaningful recommendations for the research on dielectric materials of high-performance, high-reliability base-metal MLCCs.     

Grain size effect on electrical and reliability characteristics of modified fine-grained BaTiO3 ceramics for MLCCs

Fine-grained BaTiO₃-based ceramics of different grain sizes (118–462 nm) with core–shell structures were prepared by a chemical coating method, having good dielectric properties and gentle temperature stability. The grain size effect on the dielectric properties and insulation resistivity of modified fine-grained BaTiO₃ ceramics under high temperatures and electric fields were investigated. The DC bias shows a strong effect on the dielectric properties with decreasing grain size. In the finest ceramics, the absolute value of the capacitance stability factor was the smallest, indicating that the modified-BaTiO₃ ceramic capacitor with smaller grains had higher reliability under the DC bias voltage. The highly accelerated lifetime test results showed that with decreasing the grain size, samples exhibited higher insulation resistance under elevated temperatures and high voltages. Impedance analysis proved that the finer-grained ceramic with core–shell structure had higher activation energy for both grain and grain boundary, whereas the proportion of ionic conductivity was lower.     

Effect of yttrium (Y) substitution on the structure and dielectric properties of BaTiO3

As the rate of application of multilayer ceramic capacitors (MLCCs) in small electronic devices increases, the use of the raw material barium titanate (BaTiO₃) with a small particle size and excellent dielectric properties becomes needed. Due to the size effect, small-sized BaTiO₃ generally has a cubic phase structure with a low dielectric constant, which limits its use in MLCCs. We report the preparation of small cubic phase Y-doped BaTiO₃ (BYT) nanoparticles by a hydrothermal method and the preparation of highly dielectric tetragonal phase BYT ceramics based on this method. XRD and Raman analysis showed that the BYT nanoparticles are in substable cubic phases. The particle size of the BYT nanoparticles, measured by TEM, XRD, and BET, was approximately 35 nm. The dielectric properties of the BYT ceramics were tested by an impedance analyzer, and the dielectric constant of the BYT ceramics was 7547 when the Y³⁺ doping amount was 0.5 mol%. In addition, the substitution mechanism of Y³⁺ doping in BaTiO₃ crystals was proposed from XPS and EPR analysis. The results demonstrate for the first time that the 50 nm cubic phase BaTiO₃ powder can meet the needs of next-generation high-capacity MLCCs. This work provides a reference for small cubic phase BaTiO₃ as a dielectric material for high-capacity MLCCs.     

Improved Energy Storage Properties of Fine‐Crystalline BaTiO3 Ceramics by Coating Powders with Al2O3 and SiO2

The multilayer structure of capacitor demands for fine grain size of dielectric ceramics in devices, because the thinner layer which needs ceramics with fine grain size is helpful in enlarging the capacitance. In this paper, the aqueous chemical coating method was utilized to modify the BaTiO₃ particles. The fine‐crystalline BaTiO₃ ceramics with an average grain size below 200 nm without abnormal grain growth by co‐coating Al₂O₃ and SiO₂ has been prepared. The phase composition, microstructures of coated particles and ceramics, and dielectric properties were investigated. For samples containing 3 wt% of Al₂O₃ and 1 wt% of SiO₂, the energy storage density is 0.725 J/cm³ and the efficiency of the ceramic samples can keep above 80%. The breakdown strength was improved to about 190 kV/cm.     

Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

Effects of grain size and temperature on the energy storage and dielectric tunability of non-reducible BaTiO3-based ceramics

BaTiO₃-based ceramics with various grain sizes (136–529 nm) are prepared through a chemical coating method followed by sintering in a reducing atmosphere. Effects of grain size and temperature on electric properties, energy-storage properties, and dielectric tunability are studied via Current-Field (J-E) curves, ferroelectric hysteresis loops, Capacitance-Voltage (C–V) curves and Thermally stimulated depolarization currents (TSDC). At all temperatures, fine-grain ceramics yield a lower energy density but a higher energy efficiency under the same electric field, owing to a lower ferroelectric contribution. Meanwhile, fine-grain ceramics exhibit a higher maximum energy density due to their higher breakdown strength. Fine-grain ceramics with the grain size of 136 nm have the maximum energy density of 0.41 J/cm³ under the breakdown strength of 75 kV/cm, the corresponding efficiency is 81%. C–V curves show that fine-grain ceramics have better bias-field stability. According to TSDC results, fine-grain ceramics exhibit fewer oxygen vacancies and a higher relaxation activation energy.     

Critical role of CuO doping on energy storage performance and electromechanical properties of Ba0.8Sr0.1Ca0.1Ti0.9Zr0.1O3 ceramics

CuO doped Ba_0.8Sr_0.1Ca_0.1Ti_0.95Zr_0.05O₃ (BSCTZ) ceramics were prepared by a modified mechano-chemical activation technique with the aim of improving energy storage properties for ceramic capacitor applications. CuO can effectively improve the microstructural characteristics along with a transformation of BSCTZ from classical ferroelectric to relaxor, which is the prime requirement for obtaining high discharge energy density and energy efficiency. The effect of CuO doping on the microstructural, ferroelectric, dielectric, and piezoelectric properties have been systematically studied. The study reveals that an appropriate amount of CuO doping can significantly enhance the morphological properties along with improvement in material density, which is very beneficial in a material for attaining improved energy storage performance. The BSCTZ sample with 3 mol% CuO doping has shown a highly dense microstructure, high saturation polarization (33.01 μC/cm²), low remnant polarization (6.74 μC/cm²), ultrahigh discharge energy density (1.81 J/cm³) and high energy efficiency (81.9%). The CuO doping in BSCTZ has also led to a slight improvement in breakdown strength and electromechanical properties compared to pure BSCTZ ceramics, which is mainly attributed to excellent density and optimum grain size of the material.     

Study of the structure,electrical properties,and energy storage performance of ZnO-modified Ba0.65Sr0.245Bi0.07TiO3 Pb-free ceramics

Barium titanate ceramic is frequently used as a ferroelectric material and can be applied in the pulse power field in energy storage devices. Its properties, including dielectric, ferroelectric, and energy storage properties, can be significantly improved through doping. In this work, we prepared a series of (1-x)Ba_0.65Sr_0.245Bi_0.07TiO₃-xZnO (x = 0.005, 0.01, 0.02, 0.03) lead-free bulk relaxor ferroelectric ceramics by a traditional die-pressing processing route. The uniformity of the grain sizes for these ceramics was improved, and the grains were refined when a certain amount of ZnO was introduced into BaTiO₃-based ceramics. In addition, the breakdown strength was improved in the case where the relaxor behavior was not significantly improved. It should be noted that the sample doped with 0.02 mol Zn showed the maximum room-temperature storage density (1.51 J/cm³) at the largest electric field strength (210 kV/cm). At the same time, this ceramic exhibited good stability to temperature (60–150 °C) and frequency (10–100 Hz) variations, as well as fantastic fatigue resistance (10,000 charge-discharge cycles). This paper presents in-depth studies of the structure, morphology, electrical properties, and energy storage performance of ZnO-modified BaTiO₃-based ceramics.     

Glass additive in barium titanate ceramics and its influence on electrical breakdown strength in relation with energy storage properties

Glass additive was employed to improve the microstructures and energy storage properties of barium titanate ceramics using liquid phase sintering technology. Microstructural observation indicated that the average grain size reduced obviously with increasing glass concentration. Also, the dielectric constant decreased and the dielectric breakdown strength increased as glass concentration increased. The increase in the breakdown strength with decreasing grain size was consistent with the well-known relationship for the mechanical failure. The activation energies of bulk grain and grain boundary as well as their differences were calculated using measured impedance values. Good inverse dependence of the dielectric breakdown strength on the difference between activation energies of bulk grain and grain boundary was obtained for the glass-added BaTiO₃ ceramics. It was also found that the energy storage density of the ceramics increased gradually with increasing glass concentration. Possible effect of the interfacial polarization in degrading the energy storage property was discussed.     

Microstructure,dielectric, piezoelectric,and ferroelectric properties of fine-grained 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

For preparing fine-grained 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free ferroelectric ceramics, the precursor powders were synthesized via sol-gel method and calcined at various temperatures. The precursor powders calcined at 520 °C, 550 °C, and 600 °C exhibit mean grain sizes of 30 ± 4 nm, 54 ± 3 nm, and 78 ± 5 nm, respectively. By optimizing the synthesis parameters, the fine-grained ceramics with high relative densities (>97%) and mean grain size around 100 nm were prepared. The ferroelectric, dielectric, and piezoelectric behavior were investigated. The ceramics prepared using the different precursor powders show different piezoelectric, ferroelectric, and dielectric behavior. The ceramic calcined at 550 °C and sintered at 900 °C exhibits the breakdown strength higher than 85 kV/cm, which exhibits the maximum polarization of 38.4 ± 0.3 μC/cm², remanent polarization of 20.6 ± 0.4 μC/cm².     

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.     

Amelioration on energy storage performance of KNN-based transparent ceramics by optimizing the polarization and breakdown strength

Transparent ceramic capacitors have broad application prospects in electronic devices due to their excellent optical transparency and energy storage properties. However, the low polarizability and high remnant polarization of the existing transparent dielectric ceramics limit the promotion of energy storage performance. Here, Bi(Li_0.5Nb_0.5)O₃ (BLN) was chosen to modify the (K_0.5Na_0.5)NbO₃ (KNN)-based ceramics to optimize the optical transmittance and energy storage characteristics simultaneously. On the one hand, the grain growth is inhibited, contributing to the improved breakdown strength and transmittance. On the other hand, the doping BLN could reduce the polar nanoregions size, which makes them respond more quickly to the external electric field and, thus, improves the energy storage efficiency. As a consequence, 0.95KNN–0.05BLN ceramic possesses the excellent W_rec of 4.39 J/cm³, η of 81.4%, and transparency of 77.9% with an average grain size of ∼109 nm. This work opens up a paradigm to develop a transparent pulse capacitor.     

The role of diffusion behavior on the formation and evolution of the core-shell structure in BaTiO3-based ceramics

In this work, the influence of starting particle size and sintering conditions on the microstructures and dielectric properties of BaTiO₃-based ceramics coated with 0.3Bi(Zn_1/2Ti_1/2)O₃-0.7BaTiO₃ were investigated to reveal the core-shell structure by using high resolution transmission electron microscopy technique coupled with energy-dispersive spectrometer analysis. The ion-diffusion behavior plays a critical role in the formation and evolution of the core-shell structure and, therefore, significantly influences the dielectric properties. When using starting powders containing BaTiO₃ particles larger than 100 nm in size and sintering for shorter dwelling times (0.5-2.0 hours), a core-shell structure could be formed and retained owing to the limited diffusion behavior, enabling BaTiO₃-based ceramics to meet the X8R specification for multilayer ceramic capacitors applications at high temperatures. However, when using 80 nm BaTiO₃ nanopowders and further extending the dwelling time to 6.0 hours, more driving energy was provided to prompt ion diffusion, which led to the compositional inhomogeneity becoming homogenized.     

Effect of Sr(Zn1/3Nb2/3)O3 modification on the energy storage performance of BaTiO3 ceramics

Lead-free (1-x)BaTiO₃-xSr(Zn_1/3Nb_2/3)O₃ (abbreviated as BT-xSZN, x = 0–0.08) relaxor ferroelectric ceramics were prepared using the traditional solid phase technology, and the effects of SZN modification on their phase structures, microstructures, dielectric performance, ferroelectricity and energy storage performance were studied in detail. A pure perovskite phase was observed in the BT-xSZN ceramics. The BT-based ceramics modified by SZN exhibited refined grain size. As the SZN content was increased, the breakdown strength initially increased and then decreased, and the ferroelectric loops gradually became ‘slim’. The BT-xSZN (x = 0.07) ceramics demonstrated a favourable energy storage performance with high recoverable energy density (W_rec = ~1.45 J/cm³) and energy storage efficiency (η = ~83.12%) at 260 kV/cm. Results indicate that the energy storage performance of BaTiO₃ ceramics modified by SZN can be remarkably improved, widening their applications in energy storage at low temperatures.     

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.     

Grain size effect on electrical properties of Mn-modified 0.67BiFeO3–0.33BaTiO3 lead-free piezoelectric ceramics

To investigate how grain size affects the dielectric, ferroelectric, and piezoelectric properties of Mn-modified 0.67BiFeO₃–0.33BaTiO₃ ceramics, we prepared samples with a wide variety of grain sizes from 4.1 μm to 0.59 μm via a conventional solid-state process that use the normal and the two-step sintering methods. Small-signal dielectric measurements show that all the samples exhibit a relaxor-like behavior and that grain size has little influence on the room-temperature dielectric permittivity. For grain sizes below 2 μm, the remanent polarization P_r and piezoelectric coefficient d₃₃ decrease with the grain size, whereas they remain almost constant near P_r = 27 μC/cm² and d₃₃ = 70 pC/N in samples with grain sizes exceeding 2 μm. The mechanism underlying the observed grain size effect is discussed in terms of the electric-field-induced formation of macroscopic ferroelectric domains.     

High recoverable energy storage density and large energy efficiency simultaneously achieved in BaTiO3–Bi(Zn1/2Zr1/2)O3 relaxor ferroelectrics

Relaxor ferroelectrics have attracted much attention as electric energy storage materials for intermittent energy storage because of their high saturated polarization, near-zero remnant polarizations, and considerable dielectric breakdown strength (BDS). Despite the numerous efforts, the dielectric energy storage performance of relaxor ferroelectric ceramics is incomplete or unsatisfactory. The enhancement of recoverable energy storage density W_rec usually accompanies with the sacrifice of discharge-to-charge energy efficiency η; therefore, it is an important issue to achieve high recoverable W_rec and large efficiency η simultaneously. In this work, the (1-x)BaTiO₃-xBi(Zn_1/2Zr_1/2)O₃ (abbreviated as BT-100xBZZ, 0 ≤ x ≤ 0.20) ferroelectric ceramics were prepared using the conventional solid-state reaction method. The phase structure, microstructural morphology, dielectric and ferroelectric properties, relaxation behaviors, and energy storage properties of BT-BZZ ceramics were investigated in detail. X-ray powder diffraction, dielectric spectra, and ferroelectric properties confirm the transformation of tetragonal phase for normal ferroelectrics (BT) to pseudo-cubic phase for relaxor ferroelectrics (BT-8BZZ). A high recoverable energy storage density W_rec of 2.47 J/cm³ and a large energy efficiency η of 94.4% are simultaneously achieved in the composition of BT-12BZZ, which presents typical weakly coupled relaxor ferroelectric characteristics, with an activation energy E_a of 0.21 eV and a freezing temperature T_f of 139.7 K. Such excellent energy storage performance suggests that relaxor ferroelectric BT-12BZZ ceramics are promising dielectric energy storage materials for high-power pulsed capacitors.