Lead-free Ag1−3xLaxNbO3 antiferroelectric ceramics with high-energy storage density and efficiency

Environmentally benign lead-free bulk ceramics with high recoverable energy density (W_rec) are very attractive in advanced pulsed power capacitors. In this work, composition engineering was adopted by La³⁺ modification to improve the energy storage performance of Ag_1−3xLa_xNbO₃ ceramics. It was found that the antiferroelectric (AFE) phase was stabilized after La³⁺ substitution, as a result of the reduced tolerance factor t. Significant improvement of W_rec and energy storage efficiency (η) were achieved with value of 3.12 J/cm³ and 0.63 for x = 0.02 at an electric field of 230 kV/cm, more than 1.5 times the value of pure AgNbO₃ (W_rec = 1.9 J/cm³, η = 0.40). The excellent energy storage properties are resulted from the increased antiferroelectric-ferroelectric phase transition electric field (E_F), ferroelectric-antiferroelectric phase transition electric field (E_A), and breakdown electric field (E_b). The enhanced E_b was ascribed to the decreased grain size and increased electrical resistivity upon La³⁺ modification. The feature makes Ag_1−3xLa_xNbO₃ a potential candidate for energy storage applications.     

Grain size tailoring and enhanced energy storage properties of two-step sintered Nd3+-doped AgNbO3

AgNbO₃ as a lead-free antiferroelectric material, has received widespread attention in recent years due to its promising application in the aspects of energy storage devices. However, the high remnant polarization and low breakdown strength limits its energy storage properties. In this work, Nd³⁺-doped AgNbO₃ (Ag_1−3xNd_xNbO₃, x=0−0.015) ceramics were prepared and a two-step sintering method was employed. The introduction of Nd³⁺ leads to the enhanced stability of the antiferroelectric phase, refined grain size and increased resistivity. Furthermore, by adjusting the pre-heating temperature in the two-step sintering, the homogeneity of microstructure is improved and the resistance of pre-heated samples increases by one order of magnitude compared with normally sintered samples, leading to the enhanced breakdown strength. Ag_0.97Nd_0.01NbO₃ pre-heated at 1100 °C for 2 h exhibits promising energy storage properties, with a recoverable energy storage density of 3.2 J/cm³ and energy efficiency of 52 % under an applied electric field of 210 kV/cm.     

Enhanced the energy storage performance in AgNbO3‑based antiferroelectric ceramics via manipulation of oxygen vacancy

The applications of silver niobate (AgNbO₃)-based antiferroelectric (AFE) ceramics for potential energy storage are limited by the introduction of oxygen vacancies (OVs). The inevitable OVs narrow the band gap and promote grain growth, resulting in poor breakdown strength and low recoverable energy density (W_rec). Here, we report a significant energy density performance of (Ag_1–2xSr_x)(Nb_0.78Ta_0.22)O₃ AFE ceramics designed by restraining OVs. Electron paramagnetic resonance (EPR) and UVvis absorption spectra experiments demonstrate that the OV content gradually decreases and the band gap increases with increasing Sr content. Donor doping of Sr leads to the generation of silver ion vacancies, thus, the OV concentration decreases to maintain the electrical neutrality of the system. As a result, a high W_rec of ∼5.6 J/cm³ together with an energy efficiency of 70.1% at 300 kV/cm is achieved in the (Ag_0.92Sr_0.04)(Nb_0.78Ta_0.22)O₃ ceramic. This work offers a novel strategy for improving the energy storage properties of AgNbO₃-based AFE ceramics.     

Antiferroelectric‐ferroelectric phase transition in lead‐free AgNbO3 ceramics for energy storage applications

The high‐energy storage density reported in lead‐free AgNbO₃ ceramics makes it a fascinating material for energy storage applications. The phase transition process of AgNbO₃ ceramics plays an important role in its properties and dominates the temperature and electric field dependent behavior. In this work, the phase transition behavior of AgNbO₃ ceramics was investigated by polarization hysteresis and dielectric tunability measurements. It is revealed that the ferrielectric (FIE) phase at room temperature possesses both ferroelectric (FE)‐like and antiferroelectric (AFE)‐like dielectric responses prior to the critical AFE‐FE transition point. A recoverable energy storage density of 2 J/cm³ was achieved at 150 kV/cm due to the AFE‐FE transition. Based on a modified Laudau phenomenological theory, the stabilities among the AFE, FE and FIE phases are discussed, laying a foundation for further optimization of the dielectric properties of AgNbO₃.     

Excellent energy storage performance in La and Ta co-doped AgNbO3 antiferroelectric ceramics

Lead-free dielectric materials with high breakdown electric field strength and energy density are required for pulsed power devices with high level of integration. This work describes: (Ag_0.94La_0.02)(Nb_1-xTa_x)O₃ lead-free antiferroelectric ceramics, which were prepared by rolling process. Following composition engineering, an outstanding energy density of 6.9 J cm^-3 at electric field of 490 kV cm^-1 was achieved, coupled with remarkable frequency stability (<1% over 1-100 Hz under E = 420 kV cm^-1) for (Ag_0.94La_0.02)(Nb_0.80Ta_0.20)O₃ ceramics. Moreover, it also shows excellent charge-discharge properties (discharge density = 1429 A cm^-2, power density = 144 MW cm^-3). The addition of La³⁺ and Ta⁵⁺ induced a disordered local structure, which gradually decreased the phase transition temperature of M₂-M₃ to room temperature, reflecting the enhanced antiferroelectricity. All advantageous properties observed for the La and Ta co-doped AgNbO₃ ceramics highlight their significant potential for energy storage applications.     

Effect of compositional variations on phase transition and electric field-induced strain of (Pb,Ba) (Nb,Zr, Sn,Ti)O3 ceramics

(Pb_0.97Ba_0.02)Nb_0.02(Zr_0.55Sn_0.45?xTi_x)_0.98O₃ (PBNZST, 0.03≤x≤0.06) ceramics were prepared by conventional solid state synthesis and their crystal structure, ferroelectric, dielectric, and electric field-induced strain properties were systemically investigated. A transformation from antiferroelectric (AFE) phase to ferroelectric (FE) phase was observed at 0.05<x<0.06. Besides, with the increase of Ti content, the electric field-induced strain decreased, due to the larger strain of AFE ceramics compared to FE ceramics. Further, when the measuring frequency decreased, the strain improved, because the electric field at low frequency allows a more efficient switching of domains, resulting in larger strain. The maximum strain of 0.55% was obtained in (Pb_0.97Ba_0.02)Nb_0.02(Zr_0.55Sn_0.45?xTi_x)_0.98O₃ antiferroelectric ceramics with x=0.03 at 2 Hz.     

The effect of A-site nonstoichiometry on the microstructure,electric properties,and phase stability of NaNbO3 polycrystalline ceramics

The impact of A-site nonstoichiometry on the microstructure, electric properties, and phase stability of sodium niobate ceramics (Na_1+xNbO₃, x = ?2 to 1 mol %) was investigated. All the components maintained an orthorhombic antiferroelectric (AFE) structure. The grain size increased from 3.9 to 14.3 μm with the variation in x from ?2 to 1. The AFE–FE phase transition electric field dramatically increased from 100 kV cm^?1 at x = 0 to 170 kV cm^?1 at x = ?2, confirming the enlarged energy barrier between AFE Pbma and FE Pmc2₁ phase under external field in A-site deficient components. This is attributed to the lattice compressive stress generated by introducing proper A-site vacancies. Combined results of transmission electron microscopy and Raman spectroscopy indicated that the AFE distortion of Pbma phase was significantly enhanced in A-site deficient components, which jointly contributed to the stability of AFE phase in A-site deficient NaNbO₃ material.     

Li+ and Sm3+ co-doped AgNbO3-based antiferroelectric ceramics for high-power energy storage

Ag_1-x-3yLi_xSm_yNbO₃ (x≤0.05, y≤0.05) (ALSN) antiferroelectric ceramics were successfully prepared via conventional solid-state reaction and sinter routes in oxygen atmosphere for improving the energy storage characteristic of pure AgNbO₃. The results indicate that all of the studied compositions display a pure orthorhombic antiferroelectric (AFE) perovskite structure, while their key parameters of electric-field-induced antiferroelectric-ferroelectric transition can be affected by Li⁺ or/and Sm³⁺ doping contents. The Sm³⁺ doping can enhance the stability of antiferroelectric state, giving rise to higher antiferroelectric-ferroelectric transition electric-field (E_F and E_B), while Li⁺ doping can reduce E_F and E_B for Sm³⁺ doped AgNbO₃ with low Sm³⁺ content (y≤0.03). When co-doping the same amounts of Li⁺ and Sm³⁺ at x=y≤0.03, both E_F and E_B almost remain unchanged. At x=y=0.05, the diffuse phase transition (DPT) behavior of antiferroelectric-paraelectric (AFE-PE) phase transition occurred, resulting in a “slim-like” double-polarization hysteresis with significantly enhanced E_F. Due to these features, both W_rec and η are improved compared with pure AgNbO₃. The W_rec and η with composition at x=y=0.05 is 2.33 J/cm³ and 58% under applying electric field of 240 kV/cm, respectively. The results suggest that building DPT behavior of AFE-PE phase transition could be an alternative strategy to improve the energy storage characteristic of AgNbO₃.     

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

(1-x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃-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.     

High energy-storage performance of lead-free AgNbO3 antiferroelectric ceramics fabricated via a facile approach

AgNbO₃ lead free AFE ceramics are considered as one of the promising alternatives to energy storage applications. In the majority of studies concerning the preparation of AgNbO₃ AFE ceramics, an oxygen atmosphere is required to achieve high performance, increasing the complexity of the fabrication process. Herein, a facile approach to preparing AgNbO₃ ceramics in the ambient air was reported, in which the AgNbO₃ ultrafine powder with stable perovskite structure was synthesized by hydrothermal method instead of the conventional ball milling process, leading to a lower temperature of phase formation and thus smaller grain size. The resulting ceramics sintered at 940 °C displayed high breakdown strength (216 kV/cm) and a recoverable energy density of 3.26 J/cm³ with efficiency of 53.5 %. Also, the high thermal stability of recoverable energy density (with minimal variation of ≤20 %) and efficiency (≤ 10 %) over 30–150℃, enables AgNbO₃ ceramics achieved to be a promising candidate for energy storage applications.     

Significantly enhanced energy storage performance in Sm-doped 0.88NaNbO3-0.12Sr0·7Bi0·2TiO3 lead-free ceramics

Lead-free antiferroelectric (AFE) ceramic materials have attached increasing attention in application of high-power capacitors for the past few years, due to their high energy storage density and environmental protection. However, the related applications are seriously restricted because of the limited number of environment friendly AFE candidate materials, high cost and low energy storage efficiency. In this work, the A-site ion Sm³⁺ doped 0.88NaNbO₃-0.12Sr_0·7Bi_0·2TiO₃ lead-free AFE P phase ceramics (0.88Na_1-3xSm_xNbO₃-0.12Sr_0·7Bi_0·2TiO₃, abbreviated as NN-SBT-100xSm) were prepared and characterized. With the increase of Sm doping amount, a relaxor-like behavior was found in the dielectric-temperature curves of NN-SBT-100xSm, indicating the AFE orthorhombic P phase is gradually replaced by an AFE orthorhombic R phase. As a result, double-like and slim P-E curve with near-zero residual polarization and suppressed hysteresis loss was obtained at x > 0.01. More encouragingly, a good discharge energy storage density (W_rec = 3.58 J/cm³) and a high efficiency (η = 82%) at a low electric field (E = 200 kV/cm) has been recorded simultaneously for NN-SBT-2Sm relaxor AFE ceramic, which are better than the other lead-free energy storage ceramics under the same E. In addition, the energy storage properties of NN-SBT-2Sm ceramics exhibit outstanding temperature and frequency stability. These results indicate that NN-SBT-2Sm relaxor AFE ceramic has a great practical value in pulse power capacitors.     

High energy storage density and stable fatigue resistance of Na0.46Bi0.46Ba0.05La0.02Zr0.03Ti0.97-xSnxO3 ceramics

Energy density and fatigue resistance properties were investigated for lead-free Na_0.46Bi_0.46Ba_0.05La_0.02Zr_0.03Ti_0.97-xSn_xO₃ (for 0 ≤ x ≤ 0.15) ceramics, synthesized via solid-state reaction technique. Perovskite pseudo-cubic crystal structure was revealed for all ceramics using X-ray diffraction. Polarization and current density versus electric field were perceived and suggested the relaxor behavior with increasing composition. A high storage energy density ~1.58 J/cm³, and conversion efficiency ~71.7% at ~110 kV/cm applied field was obtained for x = 0.03 composition at room temperature. Energy storage density was revealed ~1.53 J/cm³, and efficiency ~88.6% at 110 °C with a 100 kV/cm applied field. In addition, ceramic x = 0.03 was fatigue-free from 1 to 10⁵ cycles. Hence, the composition x = 0.03 might be applicable for high energy storage devices.     

High energy storage density and efficiency in AgNbO3 based relaxor antiferroelectrics with reduced silver content

Silver niobate based lead-free antiferroelectric ceramics have demonstrated great advantages, but the high consumption of noble metal silver may restrict their commercial application. In this work, Na⁺ and Ta⁵⁺ co-modified (Ag_1-xNa_x)(Nb_1-yTa_y)O₃ (100xNa-100yTa) ceramics were investigated, aiming to reduce the silver consumption and achieve good energy storage properties. The Na⁺ tended to increase M2-M3 phase transition temperature (T_M2-M3), while Ta⁵⁺ was more likely to reduce T_M2-M3. A new current peak (E_R) was observed for the first time in all current-electric field curves. As expected, a room temperature M2-M3 phase boundary with relaxor AFE property was realized in 40Na-65Ta with obviously reduced silver content, in which high recoverable energy storage density (W_rec) of 6.5 J/cm³ and good energy storage efficiency (η) of 78% were achieved. This work demonstrates a strategy to realize relaxor antiferroelectrics in AgNbO₃ based ceramics for energy storage performance, and promotes the commercialization potential.     

Silver deficiency and excess effects on quality,dielectric properties and phase transitions of AgNbO3 ceramics

Silver niobate AgNbO₃ is a promising basic compound for high permittivity microwave dielectrics and lead-free piezoelectrics. Stoichiometric and nonstoichiometric Ag_xNbO_2.5+x/2 ceramics were obtained for x equal to 0.95, 0.98, 1.00, 1.02, and 1.05. Scanning electron microscopy and X-ray diffraction studies showed that the homogeneity of the perovskite phase and thus the ceramics quality improved with the increase of x. Dielectric and thermal measurements (DSC) were carried out in wide temperature range. They revealed that the phase transitions become less diffused and shift towards higher temperatures with the increase of silver concentration. These temperature shifts are more significant for the low temperature phase transitions, which are connected with appearance of the polar ferroelectric and antiferroelectric states, than for high temperature transitions related to oxygen octahedra tilts. Influence of silver concentration on interaction between electric dipoles and on stability of polar phases is discussed. A few percent silver excess improve significantly ceramic quality.     

Fabrication,structural and dielectric property of lead-free perovskite silver niobate ceramics

Dielectric capacitors with high recoverable energy density are in high demand for their application in electrical and electronic systems. Among lead-free dielectric materials, silver niobate (AgNbO₃) has attracted growing interest due to its superior energy storage density at room temperature. The field-induced phase transition from antiferroelectric (AFE) phase to ferroelectric (FE) phase contributes to its large energy density. In this work, pure perovskite silver niobate ceramics were fabricated in an oxygen atmosphere by the solid-state reaction technique. The Pbcm orthorhombic phase of AgNbO₃ was closely observed using the Rietveld refinement method to provide explanation for the origin of high spontaneous polarization within a unit cell. Local structural analysis via piezoelectric force microscopy revealed the existence of ferroelectric nano domains, which may contribute to the high energy storage efficiency (η = 99.9926%) in AgNbO₃ at low electric fields. The phase transitions of AgNbO₃ were also investigated via the dependence of the dielectric permittivity (ε′ and ε″) and loss angle tangent (tanδ) on temperatures, providing insights into the further modification of AgNbO₃.     

Achieving high energy storage performance of Pb(Lu1/2Nb1/2)O3 antiferroelectric ceramics via equivalent A-site engineering

Manipulating the critical switching field between antiferroelectric (AFE) state and ferroelectric (FE) is an important concept for tuning the energy storage performance of AFEs. As one of the lead-based AFE systems, Pb(Lu_1/2Nb_1/2)O₃ promises high potential in the miniaturization of pulsed power capacitors, but the extremely high critical switching field and low induced saturated polarization demonstrate severe drawbacks with respect to temperature stability and flexibility. Here, A-site Ba²⁺ doping engineering is used to effectively reduce the critical switching field and improve the saturated polarization in Ba_xPb_1-x(Lu_1/2Nb_1/2)O₃ (0.01 ≤ x ≤ 0.08, abbreviated as xBa-PLN) ceramics. We found the AFE-FE phase transition can be occurred at 80ºC with a high energy storage density of 4.03 J/cm³ for Ba_0.06Pb_0.94(Lu_1/2Nb_1/2)O₃ ceramic. Our results show that Ba²⁺ additions destroy the antiparallel structure of AFE phase, and finally reduce the critical switching field, demonstrating a potential alternative to modulate the energy storage performance of AFEs.     

Large energy storage density in BiFeO3-BaTiO3-AgNbO3 lead-free relaxor ceramics

Lead-free (0.70-x)BiFeO₃-0.30BaTiO₃-xAgNbO₃+5‰mol CuO (abbreviated as BF-BT-xAN) ceramics were fabricated using a modified thermal quenching technique. BF-BT-xAN ceramics are of a perovskite structure with morphotropic phase boundary (MPB) and show strong relaxor properties. Remarkably, the high recoverable energy storage density of 2.11 J/cm³ is obtained for BF-BT-xAN with x = 0.14. For the x = 0.14 ceramics, its energy storage efficiency is as high as 84 % at relative low field of 195 kV/cm, together with an outstanding thermal stability in a broad temperature range from 25 °C to 150 °C. In addition, this ceramic maintains superior energy storage performance even after 8 × 10⁴ electrical cycles due to its high densification after doping Ag₂O and Nb₂O₅. The result suggests that lead-free BF-BT-xAN ceramics may be promising candidate for dielectric energy storage application.     

Excellent energy-storage properties of NaNbO3-based lead-free antiferroelectric orthorhombic P-phase (Pbma) ceramics with repeatable double polarization-field loops

The energy-storage performance of stable NaNbO₃-based antiferroelectric (AFE) ceramics was for the first time reported in (0.94-x)NaNbO₃-0.06BaZrO₃-xCaZrO₃ lead-free ceramics. A gradual evolution from an instable AFE phase (x≤0.01) to an orthorhombic AFE P phase (Pbma) (0.01<x≤0.05) was found to accompany the appearance of repeatable double-like polarization versus electric field loops although poled samples (x<0.01) own an AFE monoclinic phase (P2₁). Interestingly, compared with x≤0.01 samples with instable antiferroelectricity, a relatively high recoverable energy storage density W_rec ? 1.59 J/cm³ (@ 0.1 Hz) and a storage efficiency η of ?30% were achieved in the x = 0.04 ceramic. Moreover, a high W_rec of > 1.16 J/cm³ and an outstanding charge-discharge performance with fast discharge rate (t_0.9 < 100 ns) were generated in the temperature range from room temperature to 180 °C in the x = 0.04 ceramic. These results suggest that NaNbO₃-based AFE P-phase ceramics could be new potential dielectric materials for high-energy storage capacitors.     

Pulse energy-storage performance and temperature stability of Bi2O3-added BaTiO3 based ceramics

The temperature stability and temperature stability range of barium titanate-based pulse energy-storage ceramics were modified by Bi₂O₃ tailoring in (Ba_0.98-xLi_0.02Bi_x) (Mg_0·04Ti_0.96)O₃ (x = 0, 0.025, 0.05, 0.075, 0.1) and (Ba_1.03-1.5xLi_0.02Bi_x) (Mg_0·04Ti_0.96)O₃ (x = 0.125, 0.15, 0.2, 0.25) ceramics. Excellent pulse energy-storage performances of ceramic films are achieved via the new dual priority strategy of establishing cationic vacancies and forming a liquid phase. The dielectric constant plateau appears due to the cubic phase and space charges. Outstanding temperature stability, frequency stability and antifatigue performance are obtained in the ceramics, and their variations are all less than 15%. The comprehensive energy-storage properties with dual priority parameters of energy-storage density and efficiency of 3.13 J/cm³ and 91.71%, accompanied by an excellent pulse discharge energy density of 2.48 J/cm³, current density of 1313.23 A/cm² and power density of 195.26 MW/cm³ are gained at x = 0.1. The perfect pulse energy-storage performances as well as ultrahigh stability are correlated with synergistic effects of multiphase coexistence, cubic phase, liquid-phase sintering, grain size, ceramic resistance, space charges and polar nanoregions. The comprehensive parameters indicate that the (Ba_0·88Li_0·02Bi_0.1) (Mg_0·04Ti_0.96)O₃ ceramics have potential application in high precision fields.     

Simultaneously optimizing both energy storage density and efficiency in a novel lead-free relaxor antiferroelectrics

Dielectric capacitors with both high energy density and efficiency are highly demanded in pulsed power systems. Relaxor antiferroelectrics have attracted much attention due to their unique advantages in optimizing both properties of a dielectric capacitor. In this work, a novel relaxor antiferroelectric ceramic with composition of (1-x)Bi_0.5Na_0.5TiO₃-xAg_0.91Sm_0.03NbO₃ was developed, where the antiferroelectricity was stabilized with the increase of Ag_0.91Sm_0.03NbO₃ counterpart. Moreover, the relaxor feature was also obviously improved, as a result of chemical and structural disorder introduced by hetero cations with different radii and valence states. As expected, a high recoverable energy density of 2.1 J/cm³ accompanied with efficiency of 83 % was simultaneously achieved at x = 0.15. The as-prepared ceramics also exhibited good thermal stability in energy storage performance with small variations (energy storage density <10 % and efficiency <5 %) over 30−130 °C. All these merits demonstrate that the 0.85Bi_0.5Na_0.5TiO₃-0.15Ag_0.91Sm_0.03NbO₃ ceramic has great potential for high power energy storage applications.