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.     

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.     

Phase transformation and gas–solid reaction of Al2O3 during high-energy ball milling in N2 atmosphere

Al₂O₃ powders were milled in N₂ atmosphere using an attritor ball mill. The phase transformation and gas–solid reaction of Al₂O₃ during high-energy ball milling in N₂ atmosphere were investigated. It is found that the γ-Al₂O₃ transform to α-Al₂O₃ as milling time increases. Milled for 20 h in N₂ atmosphere, Al₂O₃ becomes partially amorphous and the gas–solid reaction between Al₂O₃ and N₂ takes place. A new phase with cubic aluminum nitride (AlN) structure forms during milling which is different from normal hexagonal AlN produced by carbon thermal reduction processing. As milling energy increases, the amount of this new phase with cubic AlN structure increases. The amount of this new phase is as high as 72% when Al₂O₃ is milled in N₂ atmosphere at 650 rpm for 40 h.     

Hydrothermal synthesized AgNbO3 powders: Leading to greatly improved electric breakdown strength in ceramics

Silver niobate AgNbO₃ ceramics have been regarded as a promising lead-free material for energy storage applications. In present work, pure and fine AgNbO₃ powders were successfully fabricated via the hydrothermal method using AgNO₃ as the raw material. It is found that the reaction products show strong dependence on the molar ratio of NH₄HF₂:AgNO₃:Nb₂O₅, pH value and reaction time. Pure AgNbO₃ powders were obtained when the process conditions are 3NH₄HF₂:2AgNO₃:xNb₂O₅ with x≤0.85 or yNH₄HF₂:2AgNO₃:1Nb₂O₅ with y = 4–5, pH = 5 and reaction time t ≥ 10 h. Benefitting from the hydrothermal synthesised AgNbO₃ powders, AgNbO₃ ceramics with fine-grain of ∼3.4 μm were obtained. The fine-grain leads to increased electric breakdown strength E_b up to 250 kV/cm, which is the highest among the pure AgNbO₃ ceramics to our best knowledge. The further enhancement in E_b and recoverable energy density could be highly anticipated if the antiferroelectric phase could be stabilized.     

B-site acceptor doped AgNbO3 lead-free antiferroelectric ceramics: The role of dopant on microstructure and breakdown strength

B-site aliovalent modification of AgNbO₃ with a nominal composition of Ag(Nb_1-xM_x)O_3-x/2 (x = 0.01, M = Ti, Zr and Hf) was prepared. The effects of dopants on microstructure, dielectric, ferroelectric and conduction properties were investigated. The results indicate that the introduction of acceptor dopant does not lead to grain coarsening. Zr⁴⁺ and Hf⁴⁺ doping are beneficial to stabilize the antiferroelectric phase of AgNbO₃. Among all the samples, Ti⁴⁺ doped AgNbO₃ has the minimum resistivity while Hf⁴⁺ doped AgNbO₃ has the maximum resistivity, therefore, Hf⁴⁺ doped AgNbO₃ has high BDS. The XPS results indicate that the conduction behaviour is associated with the concentration of oxygen vacancies. This work hints that acceptor dopant is also effective on the microstructure control and chemical modification of AgNbO₃-based ceramics.     

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.     

Structure and energy storage performance of Ba-modified AgNbO3 lead-free antiferroelectric ceramics

The AgNbO₃ antiferroelectric (AFE) ceramics have attracted increasing attention for their high energy storage performance and environmentally friendly characters. In this work, Ag_1–2xBa_xNbO₃ ceramics were successfully prepared by the conventional solid-state reaction method. The effect of Ba-modification on phase structure, microstructure, and electric properties was systematically investigated. The introduction of Ba²⁺ ion led to complex cell parameter evolution and significant refinement of grain size. Room temperature dielectric permittivity increased obviously from ~260 for the pure AgNbO₃ counterpart to ~350 for those after adding a small amount of Ba element. Slim P-E hysteresis loops with improved AFE phase were achieved after Ba modification, due to the decrease of tolerance factor. A high recoverable energy density up to 2.3?J/cm³ with energy efficiency of 46% can be obtained for the composition of Ag_0.96Ba_0.02NbO₃, in correlation with the enhanced AFE stability, reduced P_r, increased P_m and decreased ΔE. Moreover, the Ag_0.96Ba_0.02NbO₃ ceramics also exhibited excellent temperature stability in both energy density and efficiency with small variation of <?5% over 20–120?℃. The results suggest that the electric properties of AgNbO₃ system can be largely tuned after Ba modification, making it a promising candidate for energy storage applications.     

Fabrication of Transparent Pb(Mg1/3Nb2/3)O3–PbTiO3 Based Ceramics by Conventional Sintering

Transparent 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (PMN‐PT) based ceramics were prepared by a conventional solid‐state synthesis without using a hot‐press method. The ceramics became transparent when they were sintered in an O₂ atmosphere. The optical transmission increased with decreasing diameter of the calcined powder, which was controlled by the size of zirconia ball‐milling media. Substitution of 3 mol% La for Pb in PMN‐PT further increased the optical transmission to 68% at the wavelength of 2000 nm, which was comparable to that of hot‐pressed Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ based transparent ceramics.     

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.     

Effect of niobium valence on the mechanochemical activation of niobium oxides–hematite magnetic ceramic nanoparticles

Three types of nanostructured systems: xNbO·(1?x)α-Fe₂O₃, xNbO₂·(1?x)α-Fe₂O₃, and xNb₂O₅·(1?x)α-Fe₂O₃ were synthesized by ball milling at different molar concentrations (x=0.1, 0.3, 0.5, and 0.7). The effect of Nb valence and milling time on mechanochemical activation of these systems were studied by X-ray diffraction and the Mössbauer spectroscopy measurements. In general, Nb-substituted hematite was obtained at lower molar concentrations for all Nb oxides. For the NbO–Fe₂O₃ system the favorable substitution of Fe²⁺ for Nb²⁺ in the octahedral sites in the NbO lattice was observed after 12 h milling for x=0.7. In the case of the NbO₂–Fe₂O₃ and Nb₂O₅–Fe₂O₃ systems a formation of orthorhombic FeNbO₄ compound was observed, in which Fe³⁺ cations were detected. For the highest concentration of NbO₂ (x=0.7) iron was completely incorporated into the FeNbO₄ phase after 12 h milling. The molar concentrations of x=0.3 and 0.5 were the most favorable for the formation of ternary FeNbO₄ compound in the Nb₂O₅–Fe₂O₃ system. Influence of ball milling on thermal behavior of the powders was investigated by simultaneous DSC–TG measurements up to 800 °C.     

Anisotropic grain growth of mullite in high-energy ball milled powders doped with transition metal oxides

Dense mullite ceramics with anisotropic grains were derived from the high-energy ball milled mixtures of Al₂O₃ and amorphous silica with the presence of transition metal oxides (FeO_1.5, CoO and NiO). The mullitization and grain growth behavior of the unmilled mixture without the addition of the transition metal oxides and the undoped system of Al₂O₃ and amorphous silica with and without milling were also investigated and compared. The mullitization temperature was about 1200 °C in the milled systems, 100 °C lower than that required by the conventional solid-state reaction process. The lowered mullitization temperature, as well as the anisotropic grain growth, was attributed to the refined structure of the oxide powders, as a result of the high-energy ball milling. The experimental results have been explained by a dissolution-precipitation mechanism.     

Effect of Ag+ doping and Ag addition on the thermoelectric properties of KSr2Nb5O15

The electron and phonon thermal transport behavior of Ag ⁺ doped KSr₂Nb₅O₁₅ were discussed by using the first-principles calculations. The band gap was reduced after Ag⁺ doping, and the electrons near the Fermi level had stronger transition capability, which effectively increased the carrier concentration and electrical conductivity and reduced the thermal conductivity, thereby improving the ZT of the doped KSr₂Nb₅O₁₅ from 0.6298 to 0.7214 (1200 K) under ideal conditions. In addition, the solid-state reaction method was used to prepare Ag nanoparticle added KSr₂Nb₅O₁₅ samples, and their thermoelectric performance was tested. The experimental results and the calculated results showed a good consistent trend in which Ag improved the thermoelectric properties of KSr₂Nb₅O₁₅. When the amount of addition of nanosized Ag was 20 wt%, the power factor and ZT of the material were the highest at 1073 K, which were 0.228 mW/(K²·m) and 0.1090, respectively. This research shows how to improve the thermoelectric performance of KSr₂Nb₅O₁₅ ceramics and broaden their temperature range for application.     

The growth of LiNbO3 thin film by LPMOCVD using β-diketonate complexes

We studied the material characteristics of polycrystalline films of lithiumniobate (LiNbO₃) and its components (Li₂O and Nb₂O₅) prepared by Low Pressure Metal Organic Chemical Vapor Deposition (LPMOCVD). Precursors are Li(DPM) and Nb(DPM)₂Cl₃, and the carrier gas is nitrogen or argon with 50 % of oxygen at 5 Torr. We proposed a quantitative model for Nb₂O₅ film growth. Li₂O film grows on alumina substrate under argon+oxygen atmosphere, but Li₂CO₃ grows under nitrogen+oxygen atmosphere. On silicon or silica substrate, both react to form lithium silicates. By feeding both precursors, we found the optimum condition for preparing LiNbO₃ film from a film composition map as a function of the reaction temperature vs Li mol % in feed gas.     

Relaxor behavior and photocatalytic properties of BaBi2Nb2O9

Lead-free Aurivillius phase BaBi₂Nb₂O₉ powders were prepared by solid-state reaction. Ferroelectric measurements on BaBi₂Nb₂O₉ (BBNO) ceramics at room temperature provided supporting evidence for the existence of polar nanoregions (PNRs) and their reversible response to an external electric field, indicating relaxor behavior. The photocatalytic degradation of Rhodamine B reached 12% after 3 hours irradiation of BBNO powders under simulated solar light. Silver (Ag) nanoparticles were photochemically deposited onto the surface of the BBNO powders and found to act as electron traps, facilitating the separation of photoexcited charge carriers; thus, the photocatalytic performance was significantly improved. The present study is the first examination of the photochemical reactivity of a relaxor ferroelectric within the Aurivillius family with PNRs.     

Effect of milling on the anisotropic grain growth in a sintered Ba5Nb4O15 specimen and its effect on microwave dielectric properties

In this study, relationships between the processing, microstructure, and properties of barium niobate (Ba₅Nb₄O₁₅) are investigated. The milling of a Ba₅Nb₄O₁₅ powder in water is effective with respect to size reduction. However, after milling in water, BaCO₃ is formed within the slurry. With the increase in the amount of BaCO₃ formed, the aspect ratio of the elongated Ba₅Nb₄O₁₅ grains increases. The formation of the elongated Ba₅Nb₄O₁₅ grains prohibits the densification. Hence, the microwave dielectric properties, including permittivity and quality factor, are poor because of the low density. In addition, milling in ethanol is carried out for comparison: A lower amount of BaCO₃ is formed after milling in ethanol; the extent of anisotropic grain growth is thus reduced.     

Assessment of redox behavior of nickel ferrite as oxygen carriers for chemical looping process

This study investigated the suitability of using nickel ferrite (NiFe₂O₄) oxygen carriers for a chemical looping process. NiFe₂O₄ powder was prepared by ball milling equimolar NiO and Fe₂O₃ in a high temperature solid-state reaction. Material characteristics of NiFe₂O₄ samples were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area measurements, and scanning electron microscopy (SEM). Redox cycling of NiFe₂O₄ oxygen carriers was performed by thermogravimetric (TGA) measurement under pure CH₄ gas and O₂/Air atmospheres, respectively. After five successive cycles, NiFe₂O₄ powder with a single phase of spinel structure demonstrated higher redox cycling behavior and better stability than standard NiO and Fe₂O₃. We also addressed the mechanism underlying the redox cycling by NiFe₂O₄ spinel powder. Our results demonstrate the feasibility of using the proposed preparation of NiFe₂O₄ as an oxygen carrier in a reversible chemical looping process (CLP).     

Mechanochemical reaction in the K2CO3–Nb2O5 system

We studied the mechanochemical synthesis of KNbO₃, starting from a powder mixture of K₂CO₃ and Nb₂O₅. The milling experiments were designed with different ball-impact energies in order to investigate the mechanochemical reactions. X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, Raman spectroscopy and transmission electron microscopy were used to characterize the samples. Based on the results, we propose a mechanism for the mechanochemical reaction between K₂CO₃ and Nb₂O₅. The first stage of the reaction is characterized by the formation of an amorphous carbonato complex, which decomposes after prolonged milling at higher ball-impact energy giving rise to the crystallization of KNbO₃ and other niobate phases with a molar ratio K/Nb < 1. The reaction course is discussed and compared with the Na₂CO₃–Nb₂O₅ system.     

Influence of ball milling contamination on properties of sintered AlN substrates

The AlN substrate was fabricated by the tape casting process, and its thermal conductivity and electrical conductivity were investigated for various ball milling times and types of milling media. The oxygen content was measured after ball milling, de-binding process, and sintering. The oxygen content after the de-binding process was 1.2–2.3 wt%, similar to that after milling. After the de-binding process, the specimens were sintered at 1850 ℃ for 3 h in nitrogen atmosphere. The thermal conductivity of the sintered specimens decreased from 158 W m⁻¹ K⁻¹ to 100 W m⁻¹K⁻¹ with increasing milling time. Simultaneously, the electrical conductivity decreased from approximately 10⁻⁷ Ω⁻¹ cm⁻¹ to 10⁻⁹ Ω⁻¹ cm⁻¹ at 500 °C when Al₂O₃ or ZrO₂ balls were used, whereas the electrical conductivity did not decrease when Si₃N₄ balls were used.     

In situ synthesis of NbB2–Al2O3 nanocomposite powder by mechanochemical processing

This research is based on the production of NbB₂–Al₂O₃ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of niobium, aluminium and boron oxide powders was subjected to high-energy ball milling. The structural evaluation of powder particles after different milling times was conducted by the X-ray diffractometry (XRD), scanning electron microscopy, and transmission electron microscopy. The results showed that during ball milling the Nb/Al/B₂O₃ reacted with a combustion mode producing NbB₂–Al₂O₃ nanocomposite. The XRD analyses exhibited that the NbB₂–Al₂O₃ nanocomposite was formed after 10?h milling time and increasing milling time up to 30?h had no significant effect other than refining the crystallite size. In the final stage of milling, the crystallite sizes of NbB₂ and Al₂O₃ were estimated to be less than 50?nm.