Thermal annealing effects on the energy storage properties of BST ceramics

Thermal annealing treatments with different atmospheres (air, oxygen, and reducing atmospheres, respectively) were employed for the conventionally sintered (Ba_0.4Sr_0.6)TiO₃ (BST) ceramics. The effect of thermal annealing on the energy storage properties of BST ceramics was investigated, where oxygen vacancies played an important role. The dielectric loss, bulk resistivity and dielectric breakdown strength (BDS) were found to be sensitive to the annealing process, leading to the different energy densities in the range of 0.30‐0.80 J/cm³. Temperature‐dependent dielectric measurement and thermally stimulated depolarization current analysis were conducted to understand the impacts of oxygen vacancies on the macroscopic properties, which were found to be closely associated with the annealing conditions.     

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.     

Colossal Permittivity in Microwave‐Sintered Barium Titanate and Effect of Annealing on Dielectric Properties

Colossal permittivity (ε′ = 301,484 at room temperature and 1 kHz) of barium titanate was induced in ceramics synthesized using the microwave sintering method. Three different sintering processes (conventional, spark plasma, and microwave) were performed to better understand colossal permittivity in sintered barium titanate. The dielectric permittivity measurements revealed that the appearance of colossal permittivity has strong dependence on the sintering temperature and atmosphere, and less on the grain size of the sintered ceramics. However, the as‐sintered barium titanate samples produced by microwave sintering show high dielectric loss (tanδ > 1) consistent with oxygen reduction during the microwave sintering process and consequent accumulation of oxygen vacancies and associated charge carriers at the grain boundary. Since the highly conductive state of as‐sintered ceramics precludes their use in dielectric applications, thermal annealing at different conditions was performed to recover insulating characteristics. Microwave‐sintered barium titanate with post annealing process (950°C for 12 h in air) showed low dielectric loss (tanδ = 0.045) at room temperature and 1 kHz, while still showing a much higher permittivity (ε′ = 36,055) than conventionally sintered barium titanate (ε′ = 3500).     

Effect of grain size on the energy storage properties of (Ba0.4Sr0.6)TiO3 paraelectric ceramics

(Ba_0.4Sr_0.6)TiO₃ (BST) ceramics with various grain sizes (0.5–5.6 μm) were prepared by conventional solid state reaction methods. The effect of grain size on the energy storage properties of BST ceramics (T_c ≈ −65 °C) was investigated. With decreasing grain sizes, a clear tendency toward the diffuse phase transition was observed and the dielectric nonlinearity was reduced gradually, which can be explained by the Devonshire's phenomenological theory (from the viewpoint of intrinsic polarization). Based on the multi-polarization mechanism model, the relationship between the polarization behavior of polar nano-regions (the extrinsic nonlinear polarization mechanisms) and grain size was studied. The variation of the grain boundary density was thought to play an important role on the improvement of dielectric breakdown strength, account for the enhanced energy density, which was confirmed by the complex impedance spectroscopy analysis based on a double-layered dielectric model.     

Effect of annealing atmosphere on leakage and dielectric characteristics of multiferroic gallium ferrite

Mutliferroic and magnetoelectric gallium ferrite (GaFeO₃) is plagued by substantial electrical leakage in polycrystalline form. Here, we report on understanding the conduction mechanism in gallium ferrite ceramic samples vis‐à‐vis processing conditions. The results show that oxygen annealed samples exhibit minimum electrical leakage as compared to air or nitrogen annealed samples suggesting the role of oxy3gen vacancies on electrical conduction. Detailed time and temperature‐dependent impedance spectroscopy analysis of the samples showed higher activation energy of conduction in oxygen annealed samples than in air or nitrogen annealed samples. The lower activation energies of 0.3‐0.4 eV in nitrogen/air annealed samples were attributed to higher oxygen vacancy concentration while oxygen annealed samples with low oxygen vacancy concentration exhibited higher activation energy of ~0.50 eV (high frequency, i.e., grain) and 0.98 eV (low frequency, i.e., grain boundary), latter due to superior level of oxygenation at the grain boundaries. Further, X‐ray photoelectron spectroscopy revealed that the oxygen vacancies are compensated by the valence fluctuation between Fe²⁺/Fe³⁺ ions whose extent is higher in air/nitrogen annealed samples than in oxygen annealed samples. The conduction mechanisms that could be active are most likely to be double ionization of oxygen vacancies and hopping from Fe²⁺ to Fe³⁺ states, latter especially in oxygen deficient samples.     

Varistor and Magnetic Properties of Nickel Copper Zinc Niobium Ferrite Doped with Bi2O3

Bi₂O₃ was added into nickel copper zinc niobium ferrite and treated with different thermal processes to change the grain‐boundary chemical composition. The relationship between the grain‐boundary composition and varistor properties were investigated using scanning electron microscopy, transmission electron microscopy, energy dispersion spectroscopy, and X‐ray photoelectric spectroscopy. The experimental results show that Bi₂O₃ reacts and diffuses into the spinel ferrite grain, forming bismuth iron compounds, causing the spinel ferrite chemical composition near grain boundary becomes iron deficient. The Fe deficiency spinel ferrite near the grain boundary then changes into p‐type conduction. The annealing process after sintering improves the bismuth oxide diffusion and chemical reaction near the grain boundary, which can increase the grain‐boundary resistivity. The n‐type semiconductive grain interior and p‐type spinel ferrite near the grain‐boundary combination can form a double Schottky barrier, leading the specimen to exhibit varistor properties. A multifunctional varistor‐magnetic material with a nonlinear coefficient of 10 and initial permeability of about 225 at 10 MHz can be successfully fabricated by sinteringNi_0.2881Cu_0.1825Zn_0.4802Nb_0.0096Fe_2.0168O₄ ferrites added with 5 mol% Bi₂O₃ sintered at 950°C, then annealed at 650°C for 1 h.     

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.     

Effects of MnO2 concentration on dielectric properties of barium strontium titanate glass ceramics

The phase development, microstructural evolution and dielectric properties of manganese-doped barium strontium titanate glass ceramics have been studied. The specimens with (Ba,Sr)TiO₃ (BST) as the major crystalline phase were prepared by bulk crystallization process. The results show that the dielectric constant and the dielectric loss measured at room temperature pass through a maximum with increasing MnO₂ concentration. This MnO₂ concentration dependence of dielectric properties was also investigated by impedance analyses. The evidence of impedance spectroscopy indicates that the activation energy values of grain and grain boundary coincide with the change in dielectric properties.     

Effect of Sn4+–Isovalent doping concentration on giant dielectric properties of SnxTa0.025Ti0.975-xO2 ceramics

The Sn_xTa_0.025Ti_0.975-xO₂ (x%Sn(TTO)) ceramics with x = 2.5–10% were prepared using a standard mixed-oxide method and sintered at 1450 °C for 3 h to achieve a dense microstructure. The effects of the isovalent–Sn⁴⁺ doping concentration on the crystal structure, microstructure, giant dielectric behavior, and electrical properties were systematically investigated. Continuously enlarged lattice parameters and bond lengths with a single rutile–TiO₂ phase were observed as x% increased. The mean grain size was slightly reduced (～17.3–14.6 μm) due to an increased oxygen vacancy and the solute drag effect. The dielectric permittivity (ε′) decreased with increasing x%, whereas the loss tangent (tanδ) was remarkably reduced. The semiconducting grain resistance of the x%Sn(TTO) ceramics remained unchanged owing to the same Ta⁵⁺ donor concentration. The insulating grain boundary (GB) resistance was extremely increased by more than two orders of magnitude when x% increased from 2.5 to 5.0%, leading to the significantly improved giant dielectric properties. The optimized low tanδ～0.02 and high ε′～10⁴ with temperature coefficient less than ±15% in the range of -60–210 °C were reasonably described by the internal barrier layer capacitor model. Improved dielectric properties can be obtained by engineering GB by varying the Sn⁴⁺–isovalent doping concentration. This study provides an important approach for improving the dielectric properties of co–doped TiO₂ without the creation of complex defect clusters inside the grains.     

PTCR behaviour of highly donor doped BaTiO3

The electrical properties of donor doped BaTiO₃ samples with a donor concentration >0·3 mol% were studied. Samples were sintered at a low partial pressure of oxygen in order to facilitate the anomalous grain growth and donor incorporation. In order to optimise the PTCR anomaly the samples were annealed in an oxidising atmosphere. The samples were characterised using impedance spectroscopy and SEM. Results show that by the use of a specific sintering profile PTCR ceramics containing a higher amount of donor dopant can be prepared.     

Densification and properties of oxygen sintered CuO-doped PIN-PMN-PT ceramics

Relationships between sintering temperature and annealing atmosphere on microstructure and dielectric, ferroelectric, and piezoelectric properties of reactively sintered CuO-doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) ceramics were investigated. Uniform 2−3 μm grain size, dense CuO-doped PIN-PMN-PT ceramics are obtained when oxygen sintered versus a bimodal grain size microstructure when sintered in air. Oxygen sintered ceramics have excellent properties including d₃₃ = 300–315 pC/N, E_C = 7.7–8 kV/cm, and tan δ < 1.5%. The MPB region was mapped for ternary compositions doped with 0.5 mol% CuO and sintered in O₂. MPB 25PIN-40PMN-35PT demonstrated the maximum piezoelectric properties with d₃₃ = 565 +/− 23 pC/N and k_p = 0.64 +/− 0.01. Sintering from 1050 °C to 1200 °C increased the coercive field from 8.5 to 11.5 kV/cm and reduced dielectric losses from tan δ = 1.8% to 0.8% by facilitating diffusion of CuO into the lattice and creating domain wall pinning defect dipoles as evidenced by an increase in the internal field bias of P-E loops.     

Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics

SrSn_xTi_(1−x)O₃ (x=0, 0.01, 0.03, 0.05, 0.07) dielectric ceramics were fabricated by the solid state reaction method. Significant refinement of grain size and improved resistivity were observed with the addition of Sn, accounting for effectively enhanced dielectric breakdown strength, beneficial for the energy storage applications. Impedance analysis was employed to calculate the conductivities of grain and grain boundary and resistance ratios (R_gb/R_g) of grain boundary to grain. The grain boundary effect was believed to dominate the modified macroscopic performance, which was confirmed by the complex impedance analysis. The optimal properties were achieved for samples with x=0.05, exhibiting a charge energy density of 1.1 J/cm³ and an energy efficiency of 87%.     

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 PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Effects of annealing temperature on structure and electrical properties of sol–gel derived 0.65PMN-0.35PT thin film

0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃ (0.65PMN-0.35PT) thin films were deposited on Pt/Ti/SiO₂/Si substrates annealed from 550 to 700 °C using sol-gel process. The effects of annealing temperature on microstructure, insulating, ferroelectric and dielectric properties were characterized. The result reveals that 0.65PMN-0.35PT thin films possess a polycrystalline structure, matching well with the perovskite phase despite the existence of a slight pyrochlore phase. The film samples annealed at all temperatures exhibit relatively dense surfaces without any large voids and the grain size increases generally with the increase of the annealing temperature. Meanwhile, pyrochlore phase is considerably generated because of the deformation of perovskite phase caused by volatilization of Pb at an excessive high-temperature. The film annealed at 650 °C exhibits superior ferroelectricity with a remanent polarization (P_r) value of 13.31 μC/cm², dielectric constant (ε_r) of 1692 and relatively low dielectric loss (tanδ) of 0.122 at 10⁴ Hz due to the relatively homogeneous large grain size of 130 nm and low leakage current of approximately 10^-6 A/cm².     

Defect redistribution along grain boundaries in SrTiO3 by externally applied electric fields

During thermal annealing at 1425 °C nominal electric field strengths of 50 V/mm and 150 V/mm were applied along the grain boundary planes of a near 45° (100) twist grain boundary in SrTiO₃. Electron microscopy characterization revealed interface expansions near the positive electrode around 0.8 nm for either field strength. While the interface width decreased to roughly 0.4 nm after annealing at 50 V/mm, the higher field strength caused decomposition of the boundary structure close to the negative electrode. Electron energy-loss and X-ray photoelectron spectroscopies demonstrated an increased degree of oxygen sublattice distortion at the negative electrode side, and enhanced concentrations of Ti³⁺ and Ti²⁺ compared to bulk for both single crystals and bicrystals annealed with an external electric field, respectively. Oxygen migration due to the applied electric field causes the observed alteration of grain boundary structures. At sufficiently high field strength the agglomeration of anion vacancies may lead to the decomposition of the grain boundary.     

Effect of annealing environment on dielectric properties of erbium oxide

Erbium oxide due to its large polarizability and wide bandgap possesses superior dielectric properties and low leakage current. To study the effect of annealing environments on dielectric properties of erbium oxide, it was annealed under argon and air environments. Complex impedance analysis was performed to investigate dielectric properties, relaxation and conduction processes of the annealed samples. The decrease in grain size and increase in surface area, due to annealing in argon environment, resulted in over 2-fold increase in dielectric constant as compared to conventionally used SiO₂ (Effendy et al., 2017) [1]. The annealing in reducing environment also happened to widen the bandgap energy of erbium oxide. Annealing of erbium oxide in inert environment was proved to be better for the enhancement of dielectric properties, owing to creation of smaller grains with larger grain boundaries.     

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.     

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.     

Microwave Sintering of Ti3Si(Al)C2 Ceramic

In this work, microwave sintering (MWS) method was successfully applied for fabrication of dense layered ternary Ti₃Si(Al)C₂ ceramic. Compared to conventional pressureless approaches, MWS could significantly decrease preparation temperature from 1600°C to 1400°C. The activation energy of the MWS process was estimated as 233 ± 18 kJ/mol, which was much lower than those in previous sintering techniques. The low sintering temperature likely originates from the low activation energy during MWS process. Such low temperature do not only make the as‐received Ti₃Si(Al)C₂ ceramic much smaller grain size and better mechanical properties, but also indicate higher energy converting efficiency during the sintering processes. Wide application of MWS techniques in MAX phases is expected to promote the practical applications of these materials and contribute to the energy saving during sintering process.