Improved Energy Storage Properties Accompanied by Enhanced Interface Polarization in Annealed Microwave‐Sintered BST

Microwave‐sintering (MWS) technique was employed to fabricate dense (Ba_0.4Sr_0.6)TiO₃ (BST) ceramics. With respect to the high dielectric loss at room temperature, induced by the formation of oxygen defects during the MWS under vacuum atmosphere (?60 kPa), the as‐sintered samples were thermally annealed in air to reduce tanδ and recover the insulating performance. Accompanied by the decreased tanδ, the energy storage properties for annealed MWS BST were optimized, with increasing energy density (γ) from 0.77 to 1.15 J/cm³ and energy efficiency (η) from 60% to 82%. The lower oxygen vacancy concentrations were believed to account for the enhanced insulating characteristics of grain boundaries and contribute to the improved properties after annealing. Electrical characterization of grain and grain boundary by impedance spectroscopy demonstrated that the annealing preferentially modified the grain boundary. In addition, resistances extracted from the high temperature impedance analysis were found to be inadequate for evaluating the electrical characteristics of materials affected by extrinsic mechanisms, such as the interfacial polarization. For comparison, annealing effect on energy storage properties were also discussed for conventionally sintered BST.     

Annealing effect on relaxor behaviours of spark plasma sintered Pb(Sc1/2Nb1/2)O3 superfine ceramics

Abstract

Abstract

Relaxor ferroelectric Pb(Sc_1/2Nb_1/2)O₃ (PSN) superfine ceramics were fabricated by spark plasma sintering process and subjected to an annealing in oxygen ambience at a low temperature of 500°C. The microstructure of the PSN ceramics was examined by X‐ray diffraction and scanning and transmission electron microscopy, while the dielectric properties were measured in the range of 25–200°C. The dielectric measurements revealed a reverse change in the frequency dispersion ΔT and diffusive factor λ as annealing extended. Transmission electron microscopy observation and conductivity measurement revealed that the uncommon performance in relaxor properties could be attributed to the complex effects of oxygen vacancies and domain structures induced by the annealing.     

Effect of layered structure on dielectric properties and energy storage density in xBa0.7Sr0.3TiO3-SrTiO3 multilayer ceramics

xBa_0.7Sr_0.3TiO₃-SrTiO₃ (BST-ST) multilayer ceramics with different BST layers (x=1, 3, 5) were designed and fabricated by lamination of Ba_0.7Sr_0.3TiO₃ (BST) and SrTiO₃ (ST) tapes. Dielectric and energy storage properties of the multilayer ceramics were investigated. BST-ST multilayer ceramics exhibited enhanced temperature- and frequency-stability of dielectric properties, accompanying high permittivity (~2000) and low dielectric loss (<0.005) at room temperature. P-E loops revealed that BST-ST multilayer ceramics displayed low remnant polarization and favorable maximum polarization. The optimal energy storage performance was obtained in the composition of x=5 with dielectric breakdown strength of 220 kV/cm and energy storage density of 2.3 J/cm³. These results indicate that BST-ST multilayer ceramics can be a favorable candidate for dielectric capacitor applications.     

Oxygen‐Vacancy‐Related High Temperature Dielectric Relaxation in (Pb1−xBax)ZrO3 Ceramics

(Pb_1?xBa_x)ZrO₃ (x = 0, 0.025, 0.05, 0.075, 0.1) ceramics were synthesized by a traditional solid‐state reaction method, and the pure phase was obtained of all sintered samples. For all compositions, substitution of Pb²⁺ by Ba²⁺ reduced the phase transition temperature of antiferroelectric to ferroelectric and Curie temperature. Polarization–electric field hysteresis loops were conducted and typical ferroelectric hysteresis loops were observed in higher temperature range. Impedance and dielectric measurements were studied on the high temperature relaxation. Relaxation behavior could be suppressed after annealing treatment in oxygen atmosphere. Value of activation energy calculated from impedance was lower than that calculated from conduction measurements. It was concluded that short‐range hopping of oxygen vacancy contributes to the dielectric relaxation and long‐distance movement of doubly ionized oxygen vacancies contributes to the conduction.     

Giant permittivity up to 100 MHz in La and Nb co-doped rutile TiO2 ceramics

Dielectric spectroscopy was carried out for reduced and stoichiometric La_0.0025Nb_0.0025Ti_0.995O₂ ceramics synthesized by sintering in different atmospheres. A giant permittivity (~1 × 10⁴) was obtained at a frequency of 100 MHz and temperature range from 170 to 350 K. Three dielectric relaxation mechanisms were observed within the temperature range of 10-300 K via dielectric spectroscopy. A low temperature dipole relaxation peak (in the temperature range of 10-30 K) in the spectra was identified to be associated with the giant permittivity specifically measured at 100 MHz. The origin of such giant permittivity was attributed to dipole orientation polarization. Hopping polaron and interfacial effect contributed to giant permittivity. After annealing treatment, all the relaxation contributions were weakened. Low dielectric loss was attributed to high resistance of grain and grain boundaries. Annealing in ambient conditions led to decreased relaxation times which gives the signature of decreased concentration of oxygen vacancies and Ti³⁺. Dipoles which were related to oxygen vacancies and Ti³⁺, resulted in giant permittivity up to 100 MHz.     

Effect of High‐Pressure Oxygen Annealing on Electrical and Magnetoelectric Properties of BaSrCo2Fe11AlO22 Ceramics

The effect of high‐pressure oxygen (HPO) annealing on the electrical, magnetic, and magnetoelectric (ME) properties of room‐temperature multiferroic BaSrCo₂Fe₁₁AlO₂₂ ceramics was investigated. The electrical resistivity of the ceramics was found to strongly depend on the partial oxygen pressure during annealing at 1040°C. Samples annealed under ~10 atm of oxygen exhibited a resistivity of up to 1.6 × 10⁹ Ω·cm at room temperature, more than two orders of magnitude higher than that of samples without oxygen annealing. Thermally stimulated current and complex impedance measurements suggested that the enhancement of the resistivity by the HPO annealing originated from a decrease in the amount of defects related to oxygen vacancies and an increase in the resistance of grains and interfaces. HPO annealing also affects the magnetic‐field response of spiral magnetic ordering, which is ascribed to the ME properties. Furthermore, samples subjected to HPO annealing exhibited a lower contribution of the space charges trapped at the grain boundaries and/or defects to the magnitude of the measured magnetoelectrically induced electric polarization P. The present results indicate that HPO annealing is an effective method to evaluate authentic ME effects in multiferroic BaSrCo₂Fe₁₁AlO₂₂ ceramics.     

Dielectric relaxation and electrical conduction in (BixNa1−x)0.94Ba0.06TiO3 ceramics

The dielectric relaxation and electrical conduction were investigated in (Bi_xNa_1?x)_0.94Ba_0.06TiO₃ (Abb. xBNBT6, x = 0.5, 0.495, 0.485, and 0.475) ceramics prepared by solid state reaction. With a decrease in x, the dielectric properties of the ceramics decreased, whereas the electrical conduction increased, resulting in a transition from insulator to oxide‐ions conductor. When x = 0.475, the ceramics exhibited large conductivity (~10^?3 S cm^?1 at 575°C) and low activation energy (~0.45 eV), indicating their potential application in solid oxide fuel cells. A mixed conduction mechanism with oxide‐ions, electrons, and holes was proposed. With a decrease in x from 0.495 to 0.475, it was found that the p‐type conduction was switched to n‐type conduction. The dielectric relaxation of the x = 0.495 sample was associated with short‐range hopping of oxygen vacancies. However, the dielectric properties of the x = 0.485 and 0.475 samples can be explained by Maxwell‐Wagner interface relaxation.     

Effect of oxygen treatment on structure and electrical properties of Mn-doped Ca0.6Sr0.4TiO3 ceramics

Different oxygen treatment methods, including O₂ and N₂ annealing, were conducted on Ca_0.6Sr_0.4TiO₃ (CST) ceramics with varying Mn content (0?mol%, 0.5?mol% and 2.0?mol%). Structure characterization, including XRD and SEM, indicated the minimal effect of annealing on the microstructure. Grain boundaries were found to be sensitive to oxygen treatments, and annealing in O₂ resulted in increased grain boundary resistance, while in N₂ led to the opposite result. The insulating properties of bulk ceramics were found to be dominated by grain boundaries. Both the concentration and mobility of oxygen vacancies were confirmed to affect the energy storage properties to some extent in this work.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Microstructure and Electrical Properties of Nonstoichiometric 0.94(Na0.5Bi0.5+x)TiO3–0.06BaTiO3 Lead‐Free Ceramics

0.94(Na_0.5Bi_0.5+x)TiO₃–0.06BaTiO₃ (x = ?0.04, 0, 0.02; named NB_0.46T‐6BT, NB_0.50T‐6BT, NB_0.52T‐6BT, respectively) lead‐free piezoelectric ceramics were prepared via the solid‐state reaction method. Effects of Bi³⁺ nonstoichiometry on microstructure, dielectric, ferroelectric, and piezoelectric properties were studied. All ceramics show typical X‐ray diffraction peaks of ABO₃ perovskite structure. The lattice parameters increase with the increase in the Bi³⁺ content. The electron probe microanalysis demonstrates that the excess Bi₂O₃ in the starting composition can compensate the Bi₂O₃ loss induced during sample processing. The size and shape of grains are closely related to the Bi³⁺ content. For the unpoled NB_0.50T‐6BT and NB_0.52T‐6BT, there are two dielectric anomalies in the dielectric constant–temperature curves. The unpoled NB_0.46T‐6BT shows one dielectric anomaly accompanied by high dielectric constant and dielectric loss at low frequencies. After poling, a new dielectric anomaly appears around depolarization temperature (T_d) for all ceramics and the T_d values increase with the Bi³⁺ amount decreasing from excess to deficiency. The diffuse phase transition character was studied via the Curie–Weiss law and modified Curie–Weiss law. The activation energy values obtained via the impedance analysis are 0.69, 1.05, and 1.16 eV for NB_0.46T‐6BT, NB_0.50T‐6BT and NB_0.52T‐6BT, respectively, implying the change in oxygen vacancy concentration in the ceramics. The piezoelectric constant, polarization, and coercive field of the ceramics change with the variation in the Bi³⁺ content. The Rayleigh analysis suggests that the change in electrical properties of the ceramics with the variation in the Bi³⁺ amount is related to the effect of oxygen vacancies.     

Highly enhanced dielectric loss of MgO-doped Ba0.67Sr0.33TiO3 ceramics prepared by a non-contamination DCC-HVCI method

MgO-doped Ba_0.67Sr_0.33TiO₃ (BST) ceramics with uniform microstructure and enhanced dielectric loss were prepared by direct coagulation casting via high valence counter ions (DCC-HVCI) method. MgO was utilized as acceptor dopant as well as coagulating agent to release Mg²⁺ solidifying ceramic suspensions. Effect of sintering temperature and MgO doping content on microstructure and dielectric properties of BST ceramics was investigated. It was found that the loss tangent value (tan δ) decreased remarkably from 0.025 to 0.004 with increasing MgO concentration from 0.5 to 1.5 mol%. The 1.5 mol% MgO-doped BST ceramics processed high density of 99.5% and tan δ of 0.004 which is improved by 150% compared with that of dry-pressed samples. This improvement can be attributed to the ubiquitous distribution of MgO in BST ceramic matrix leading to the enhanced inhibitive effect. This paper provides a novel facial route to prepare high-performance functional ceramics with high reliability and low cost.     

Ink-jet printed BNT thin films with improved ferroelectric properties via annealing in wet air

In this work, an ink-jet printing process based on the sol-gel route was applied to prepare lead-free ferroelectric Na_0.5Bi_0.5TiO₃(BNT) thin films for the first time. Dense and crack-free films with perovskite structure were obtained from a modified precursor solution through multiple printing and pyrolysis processes. The ferroelectric, dielectric and electrical properties were significantly affected by the annealing temperature and atmosphere. The film annealed at 670?°C in wet air showed a high remnant polarization of 24.7?μC/cm² with a low coercive field of 263?kV/cm, the dielectric constant and loss were 185 and 0.1 at 10?kHz, respectively. It was found that wet air was an alternative to reduce oxygen vacancies and enhance properties of ferroelectric films, which can be explained by the defect chemical reaction between water and oxygen vacancies. X-ray photoelectron spectroscopy(XPS) confirmed the decrease of oxygen vacancies after annealing with water presence, with a formation of Ohmic conduction mechanism dominated by charged hydroxyl groups.     

Giant permittivity and low dielectric loss of SrTiO3 ceramics sintered in nitrogen atmosphere

The dielectric properties of SrTiO₃ ceramics sintered in nitrogen (N₂) exhibit a weak temperature- and frequency-dependent giant permittivity (>10⁴) as well as a very low dielectric loss (mostly < 0.02) over a broad temperature range from −100 to 200 °C. Based on the results of ac conductivity and structural analysis, the giant permittivity and low dielectric loss were due to the fully ionized oxygen vacancies and giant defect-dipoles. When further sintering these ceramics in air, the materials exhibit a large temperature- and frequency-dependent high dielectric loss, which were due to the ionization and motion of oxygen vacancies.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Enhanced magnetic and dielectric properties of Ti‐doped YFeO3 ceramics

Polycrystalline YFeO₃ (YFO) and YFe_1?(4/3)xTi_xO₃(YFTO) ceramics were prepared using the powder synthesized from the sol‐gel route. X‐ray diffraction analyses of the polycrystalline ceramics revealed the crystallization of the phase in orthorhombic crystal structure associated with the space group Pnma. The magnetization versus magnetic field hysteresis loops were obtained at room temperature for YFO and YFTO ceramics. The magnetic property changes from weak ferromagnetic in YFO to ferromagnetic in YFTO ceramics. The dielectric constant recorded at room temperature for YFTO ceramics was six times higher than that of YFO, whereas the dielectric loss gets reduced to 0.06 from 0.3 for YFO at 1 kHz. Impedance spectroscopy study carried out on YFO and YFTO ceramics confirmed the existence of non‐Debye‐type relaxation. Observed single semicircle in Z′ vs ?Z′′ plot established the incidence of intrinsic (bulk) effect and ruled out any grain boundary or electrode effects. The mechanism for the dielectric relaxation and electrical conduction process observed in YFO and YFTO ceramics was discussed by invoking electric modulus formalisms. Activation energy obtained by ac conductivity study suggested that the conduction process in YFO was linked up with the existence of the polaron and oxygen vacancies, whereas only oxygen vacancies contribute to the conduction process in YFTO ceramics.     

Energy storage and discharge performance for (1−x)BaSrTiO3−xBi(Zn1/2Ti1/2)O3 ceramics

Although lead-free dielectric ceramics have been widely studied to obtain excellent dielectric properties and good energy storage properties, the primary challenge of low energy storage density has not yet been resolved. Here, we introduce the concept of crossover relaxor ferroelectrics, which represent a state intermediate between normal ferroelectrics and relaxor ferroelectrics, as a solution to address the issue of low energy density. The (1−x)BaSrTiO₃−xBi(Zn_1/2Ti_1/2)O₃ (x = 0,.05, .1, .15, .2) ceramics were prepared by a solid-state method. Remarkably, 0.85BST–0.15BZT ceramics achieved a high recoverable energy density (W_rec) of 2.18 J/cm³ under an electric field of 240 kV/cm. BST–BZT materials exhibit substantial recoverable energy density, high breakdown strength, and superior energy efficiency, positioning them as a promising alternative to meet the diverse demands of high-power applications.     

Influence of solid loading on densification behavior,micromorphology and dielectric properties of Ba0.67Sr0.33TiO3 ceramics fabricated by a novel DCC-HVCI method

Ba_0.67Sr_0.33TiO₃ (BST) ceramics with highly improved dielectric performance were fabricated by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method. The influence of solid loading on densification behavior, micromorphology, and dielectric performance of the samples was investigated. With the increase of solid loading from 40 to 50 vol%, the maximum densification rate of BST ceramics increased from 0.090 to 0.122 s⁻¹, and the densification temperature decreased from 1424 to 1343 °C, which indicated that high solid loading could promote the densification behavior of samples during sintering. BST ceramics fabricated by the DCC-HVCI method showed uniform grain size and microstructure, which was beneficial for the dielectric properties of BST ceramics. Samples obtained from 45 vol% suspensions possessed the lowest dielectric permittivity (ε_r ≈ 2801), and the dielectric loss (tanδ≈0.0262) was about 1/10 of that of dry-pressed samples (tanδ≈0.301), which could be attributed to the composition homogenization.     

Effect of annealing atmosphere on the structure and dielectric properties of unfilled tungsten bronze ceramics Ba4PrFe0.5Nb9.5O30

Unfilled tungsten bronze ceramics with the nominal formula Ba₄PrFe_0.5Nb_9.5O₃₀ were synthesized via the standard solid-state sintering route, and the effects of oxygen vacancies on the dielectric and electrical properties were investigated in addition to the structure. Room-temperature X-ray diffraction showed that the N₂-annealed sample had the largest cell volume. Low-temperature spectrum showed that N₂ annealing rendered the dielectric constant and dielectric loss more frequency dispersive, whereas O₂ annealing inhibited the frequency dispersion. The dc conductivity of all the samples originated from the electrons produced in the second ionization of oxygen vacancies and was most likely controlled by a mixed conduction mechanism of the electron and oxygen-vacancy ions. The N₂-annealed sample has the highest dc conductivity owing to its high concentration of oxygen vacancies. The broadening of the Raman lines and the decrease of Raman intensity for the N₂-annealed sample originated from a significant structural disorder. X-ray photoelectron spectra demonstrated that the increased oxygen vacancies caused by the change of valences of Fe and Pr ions contributed to the structural disorder.     

Colossal permittivity in BaTiO3-0.5wt%Na0.5Ba0.5TiO3 ceramics with high insulation resistivity induced by reducing atmosphere

The donor-acceptor co-doping grain-fine BaTiO₃-0.5wt%Na_0.5Ba_0.5TiO₃ (BT-0.5wt%NBT) ceramics are obtained by conventional solid-state reaction method and sintered in different atmospheres. The dielectric properties are sensitively influenced by the sintering atmosphere, which the colossal permittivity can be activated and increased by enhanced atmospheric reducibility with the increase of H₂ content. However, the excessive H₂ content can make a significant deterioration in dielectric loss and insulation resistivity. The impedance spectra, XPS and EPR measurements indicate that sintering atmosphere can effectively regulate the concentration and distribution of charge carriers (delocalized electrons, oxygen vacancies and defects), which induce the interfacial polarization and hopping polarization. When sintered in 0.1% H₂/N₂, the sample possesses a relatively good comprehensive performance with colossal permittivity (ε_γ>4×10⁴), ultra-low dielectric loss (tanδ=0.0218) and high insulation resistivity (ρ_v>10¹¹Ω·cm), which related to the moderate values of resistance and conduction activation energy for the grain boundaries.     

Thermally-stimulated defect relaxations and microwave/terahertz dielectric response of La,Al co-doped (Ba,Sr)La4Ti4O15 ceramics

The thermally-stimulated relaxation of defects in La,Al co-substituted (Ba,Sr)La₄Ti₄O₁₅ dielectric ceramics with hexagonal perovskite structure were investigated systematically with the thermally stimulated depolarization current (TSDC) measurements. The ion substitution decreased the lattice parameters and the bond lengths of the cations and O²⁻. TSDC spectra indicated the main extrinsic defects in the ceramics were oxygen vacancies and the concentration of oxygen vacancies decreased with the increasing ion substitutions, which was derived from the decrease in the lattice parameters. The terahertz time-domain spectra indicated the damping factor of lattice vibration was reduced with the decline of the concentration of oxygen vacancies, which decreased the microwave/terahertz dielectric loss. High-performance (Ba_0.2Sr_0.8)_0.75La_4.25Ti_3.75Al_0.25O₁₅ (ε_r = 43.9, Q × f = 84300 GHz, τ_f = -18.0 ppm/°C) ceramics were obtained. The correlations between crystal structures, defect behaviors and dielectric properties discussed in this work could provide guidance for the modification of microwave dielectric ceramics.