High-Energy Density Capacitors Utilizing 0.7 BaTiO3–0.3 BiScO3 Ceramics

A high, temperature-stable dielectric constant (∼1000 from 0° to 300°C) coupled with a high electrical resistivity (∼10¹²Ω·cm at 250°C) make 0.7 BaTiO₃–0.3 BiScO₃ ceramics an attractive candidate for high-energy density capacitors operating at elevated temperatures. Single dielectric layer capacitors were prepared to confirm the feasibility of BaTiO₃–BiScO₃ for this application. It was found that an energy density of about 6.1 J/cm³ at a field of 73 kV/mm could be achieved at room temperature, which is superior to typical commercial X7R capacitors. Moreover, the high-energy density values were retained to 300°C. This suggests that BaTiO₃–BiScO₃ ceramics have some advantages compared with conventional capacitor materials for high-temperature energy storage, and with further improvements in microstructure and composition, could provide realistic solutions for power electronic capacitors.     

High-Temperature Dielectrics in the BiScO3–BaTiO3–(K1/2Bi1/2)TiO3 Ternary System

Perovskite solid solution in the (1− x )[0.4BiScO₃–0.6BaTiO₃]+ x (K_1/2Bi_1/2)TiO₃ [BSBT–KBT x ] system was synthesized using conventional sintering and hot-isostatic pressing. Dielectric properties of BSBT ceramics with different dopant levels of KBT were characterized as a function of temperature and frequency for potential use of high-temperature capacitors. The BSBT ceramics with KBT exhibited high dielectric permittivities (ɛ_r) (>1700 at RT) and low dielectric loss over the temperature range from 100° to 300°C, with flat temperature coefficients of permittivity (TCɛs). In addition, BSBT ceramics with increasing KBT were observed to possess dielectric relaxation characteristics at temperatures (>RT) as observed in lead-based relaxors. Furthermore, high energy densities, being on the order of 4.0 J/cm³ at 220 kV/cm was observed for the BSBT–KBT20 ceramics from the electric-field polarization behavior.     

Dielectric Bodies in the Quaternary System BaTiO3–BaSnO3–SrSnO3–CaSnO3

Dense bodies were prepared of compositions in the quaternary system BaTiO₃–BaSnO₃–SrSnO₃–CaSnO₃ containing from 3 to 60 mole % stannate. The general effect of the stannate addition to barium titanate was to decrease the Curie temperature and broaden the peak. On a molar basis the three stannates were approximately equivalent in their effect on the dielectric properties of barium titanate, although the rate of shift of the Curie temperature was slightly greater when SrSnO₃ was used. Bodies containing calcium or strontium stannate had lower power factors than those containing barium stannate. Bodies compounded with calcium stannate matured most readily and at lower temperatures. Bodies having dielectric constants ( K ) of 2300 to 2800 at 1 kc. with low positive temperature coefficients up to about 55°C. were obtained with a 3 mole % addition of stannate. Bodies with minimum K 's of 3000 to 4000 at 1 kc. over the range 25° to 85°C. were obtained from BaTiO₃ with an addition of about 6 mole % BaSnO₃, SrSnO₃, or CaSnO₃. Bodies with negative temperature coefficients of K ranging from about 13,000 to about 1000 p.p.m. per °C. were obtained with stannate additions of from 10 to 60 mole %.     

Relaxor Behavior of the 0.9BaTiO3–0.1La(Mg1/2Ti1/2)O3 Solid Solution

Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO₃–0.1La(Mg_1/2Ti_1/2) O₃ (0.9BT–0.1LMT) has been studied in the temperature range of 12–550 K. In comparison with pure BT, in 0.9BT–0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12–150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole–Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La³⁺/Ba²⁺ and Mg²⁺/Ti⁴⁺ substitutions in the relaxor behavior formation is discussed.     

In situ TEM Investigations of the High-Temperature Relaxor Ferroelectric BiScO3–Pb(Mg1/3Nb2/3)O3–PbTiO3 Ternary Solid Solution

Compositions near the morphotropic phase boundary (MPB) of the BiScO₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ternary system showed high-temperature relaxor properties (1 kHz) of T _max∼250°C and a permittivity maximum of ∼16 000. Transmission electron microscopy (TEM) was used to investigate the nature of the microstructure under ambient conditions and with in situ heating studies for samples with different composition and electric field-temperature histories. A mottled domain structure was observed with diffraction contrast TEM techniques and associated with frozen in polar micro-domains. These observations are consistent with the low field permittivity temperature measurements made under various frequencies (100 Hz–100 kHz) with a freezing temperature, T _f∼160°C. Field-cooled (FC) samples exhibited a macro-domain structure similar to normal ferroelectric behavior. On heating the FC samples to approximately the T _max and above, the domain contrast was no longer apparent. When subsequently cooled to room temperature conditions, a micro-domain structure was observed, similar to the zero FC samples. The results are discussed with respect to permittivity measurements and phenomenological mechanisms contributing to the dispersion in the permittivity below the Curie maximum.     

Phase Equilibria in the System BaTiO3–SrTiO3

Phase equilibria in the binary system BaTiO₃–SrTiO₃ were studied above 1200°C. This system is characterized by a complete series of solid solutions with a minimum at 2.5 mole % SrTiO₃ and 1585°C. The hexagonal BaTiO₃ phase is suppressed to a region extending no farther than 0.5% SrTiO₃ at 1600°C. No immiscibility gap was found.     

Orientation Control in Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films were fabricated via a sol–gel method on Pt(111)/Ti/SiO₂/Si(111) substrates. The effects of different factors on the orientation of the sol–gel-derived BSPT thin films were investigated. The results showed that a higher lead excess concentration, longer drying time, higher pyrolysis temperature, longer pyrolysis time, higher crystallization temperature, and longer crystallization time could enhance the (100) orientation of the BSPT thin films. Based on the experimental results, a mechanism for the orientation evolution in the sol–gel-derived BSPT thin films was proposed. The production of the (100) orientation was attributed to the (100)-oriented PbO nanocrystals forming during the pyrolysis process due to the lattice match.     

Fabrication and Properties of Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films near the morphotropic phase boundary were successfully fabricated on Pt(111)/Ti/SiO₂/Si substrates via an aqueous sol–gel method. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 50 nm. The dielectric, ferroelectric, and piezoelectric properties of the sol–gel-derived BSPT thin films were investigated. A remanent polarization of 74 μC/cm² and a coercive field of 177 kV/cm were obtained. The local effective piezoelectric coefficient d ^*₃₃ was 23 pC/N at 2 V, measured by a scanning probe microscopy system. The dielectric peak appeared at 435°C, which was 80°C higher than that of Pb(Ti, Zr)O₃ thin films.     

Effects of Niobium Doping on the Microstructure and Electrical Properties of 0.36BiScO3–0.64PbTiO3 Ceramics

Recently, a new family of piezoelectric perovskite materials based on the solid solution (1− x )BiScO₃– x PbTiO₃ was developed. This system was found to have a Curie temperature higher than 450°C and excellent piezoelectric properties near the MPB composition. Niobium, as a donor dopant in the piezoelectric system Pb(Zr,Ti)O₃ and other lead - based perovskite materials, has commonly been used to increase the electrical resistivity, dielectric, and piezoelectric properties. In the current work, the effect of niobium substitution in the BS–PT system has been reported. The results of niobium additions in the BS–PT system showed no large enhancement of the piezoelectric properties. Niobium doping also led to lower Curie temperatures and higher dielectric loss. Further grain size effects in niobium - doped BS–PT compositions provided experimental evidence of significant extrinsic contributions to the piezoelectric properties in this system.     

Enhanced Piezoelectric Properties in Mn-Doped 0.98K0.5Na0.5NbO3–0.02BiScO3 Lead-Free Ceramics

Mn-doped 0.98K_0.5Na_0.5NbO₃–0.02BiScO₃ (0.98KNN–0.02BS) lead-free piezoelectric ceramics have been prepared by a conventional sintering technique and the effects of Mn doping on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of Mn can improve the densification of the ceramics effectively. Because of the high densification, fine grain, and Mn doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. Very good piezoelectric and dielectric properties of d ₃₃=288 pC/N, k _p=0.46, ɛ_r=1591, and T _C=328°C were obtained for the 0.98KNN–0.02BS ceramics doped with 0.8 mol% Mn. Therefore, the 0.98KNN–0.02BS ceramics containing a small amount of Mn are a good candidate material for lead-free piezoelectric ceramics.     

Transmission Electron Microscopy Microstructure of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

Microstructural characterizations using transmission electron microscopy on 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ ceramics sintered at 1030°–1150°C for 2 h were carried out. The liquid phase was found at the triple junction of the grains in all specimens and abnormal grain growth occurred in the presence of the liquid phase. Abnormally grown grains whose shapes were cuboidal were well developed. Anisotropically faceted amorphous liquid phase pockets were observed inside the grain in a specimen sintered at 1060°C for 2 h. The interface between the grain and the liquid matrix was flat and some were identified to be {100} planes of the grains. A certain amount of liquid at the sintering temperature of 1060°C enhanced the abnormal grain growth and contributed to the improvement of the piezoelectric properties.     

Dielectric Properties and Microstructure of Nb-Co Codoped BaTiO3–(Bi0.5Na0.5)TiO3 Ceramics

X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and an impedance analyzer were used to examine the Nb–Co codoping effects on the densification, crystalline phase, microstructure development, and dielectric–temperature characteristics of BaTiO₃–(Bi_0.5Na_0.5)TiO₃ ceramics. The results indicate that the Curie temperature shifted to a higher temperature (above 140°C) by adding BNT. The dielectric constant–temperature (ɛ– T ) curve broadened at the Curie temperature due to the small grain size (0.3–0.4 μm). A core-shell structure was developed, which is helpful to flatten the ɛ– T curve of BaTiO₃ ceramics at high temperatures.     

Impedance Spectroscopy of Undoped BaTiO3 Ceramics

The electrical properties of ceramic BaTiO₃ were investigated by ac impedance spectroscopy over the ranges 25°-330°C and 0.03 Hz-1 MHz. Results are compared with those obtained from fixed-frequency measurements, at 1 kHz and 100 kHz. Fixed-frequency Curie-Weiss plots show deviations from linearity at temperatures well above t _c. The ac measurements show that grain boundary impedances influence Curie-Weiss plots in two ways: at high temperatures, they increasingly dominate the fixed-frequency permittivities; at lower temperatures, closer to T ^c, the high-frequency permittivity contains a contribution from grain boundary effects. Methods for extraction of bulk and grain boundary capacitances from permittivity and electric modulus complex plane plots are discussed. The importance of selecting the appropriate equivalent circuit to model the impedance response is stressed. A constriction impedance model for the grain boundary in BaTiO₃ ceramics is proposed: the grain boundary capacitance is neither temperature-independent, nor shows Curie-Weiss behavior. The grain boundary is ferroelectric, similar to the grains, but its impedance is modified by either air gaps or high-impedance electrical inhomogeneity in the region of the necks between grains; the activation energy of the constriction grain boundary impedance differs from that of the bulk, suggesting differences in defect states or impurity levels.     

Sintering Behavior of Cadmium-Doped Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 Ceramics

Substituting Cd²⁺ ions into the A and B sites in a Pb(Ni_1/3Nb_2/3)O₃PbZrO₃PbTiO₃ (PNNPZPT) ternary perovskite material made it possible to determine the effects of the Cd²⁺ ion substitution site on sintering behavior. The substitution site of the Cd²⁺ ion was identified by using X-ray photoelectron spectroscopy spectra. Although Cd²⁺ ions were substituted into the A and B sites in PNNPZPT, the Cd²⁺ ions preferred the A site over the B site. When Cd²⁺ ions replaced Pb²⁺ ions, a weight gain was observed during sintering. On the other hand, replacing Ni2 ions with Cd²⁺ ions promoted weight loss. Those weight changes indicated that Cd²⁺ ions change the bonding strength between the B-site cations and the oxygen of the octahedron in a perovskite structure. Density was influenced by the Cd²⁺ ion substitution site, and the A-site-doped compositions had higher densities than the B-site-doped compositions.     

Spray Pyrolysis of Fe3O4–BaTiO3 Composite Particles

Fe₃O₄–BaTiO₃ composite particles were successfully prepared by ultrasonic spray pyrolysis. A mixture of iron(III) nitrate, barium acetate and titanium tetrachloride aqueous solution were atomized into the mist, and the mist was dried and pyrolyzed in N₂ (90%) and H₂ (10%) atmosphere. Fe₃O₄–BaTiO₃ composite particle was obtained between 900° and 950°C while the coexistence of FeO was detected at 1000°C. Transmission electron microscope observation revealed that the composite particle is consisted of nanocrystalline having primary particle size of 35 nm. Lattice parameter of the Fe₃O₄–BaTiO₃ nanocomposite particle was 0.8404 nm that is larger than that of pure Fe₃O₄. Coercivity of the nanocomposite particle (390 Oe) was much larger than that of pure Fe₃O₄ (140 Oe). These results suggest that slight diffusion of Ba into Fe₃O₄ occurred.     

Sintering Behavior and Surface Microstructure of PbO-Rich PbNi1/3Nb2/3O3–PbTiO3–PbZrO3 Ceramics

The sintering behavior and surface microstructure of PbNi_1/3Nb_2/3O₃–PbTiO₃–PbZrO₃ (PNiNb-PT-PZ) ceramics were investigated. The PNiNb-PT-PZ ceramics with the stoichiometric composition and the addition of excess lead oxide (PbO-rich ceramics) were sintered by liquid-phase sintering in accordance with the solution-reprecipitation mechanism at temperatures below the melting point of PbO. The temperature at which the liquid phase forms fell to near the eutectic point of the PbO–Nb₂O₅ and the PbO–TiO₂ system (868°C) with the addition of 5 mol% PbO. As the calcination temperature influenced the sinterability of the stoichiometric PNiNb-PT-PZ ceramic, unreacted PbO was considered to be the source of the liquid phase in the sintering of the stoichiometric powder. The secondary phase was observed at the surface of PbO-rich ceramics and was suggested to be a liquid phase expelled from inside the ceramic. A sintering scheme of PNiNb-PT-PZ ceramics was proposed, and the high sinterability of PNiNb-PT-PZ ceramics was attributed to the low formation temperature of the liquid phase.     

Low-Temperature Sintering and Piezoelectric Properties of CuO-Added 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

The sintering temperature of 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ (NKN–BT) ceramics needs to be decreased below 1000°C to prevent Na₂O evaporation, which can cause difficulties in poling and may eventually degrade their piezoelectric properties. NKN–BT ceramics containing CuO were well sintered at 950°C with grain growth. Poling was easy for all specimens. Densification and grain growth were explained by the formation of a liquid phase. The addition of CuO improved the piezoelectric properties by increasing the grain size and density. High piezoelectric properties of d ₃₃=230 pC/N, k _p=37%, and ɛ₃^T/ɛ₀=1150 were obtained from the specimen containing 1.0 mol% of CuO synthesized by the conventional solid-state method.     

Investigation of Twin Lamellae in BaTiO3 Ceramics

The microstructure of many electroceramic components based on barium titanate is dominated by anomalous grain growth. At firing temperatures below the eutectic temperature nearly all large crystallites were found to contain lamellae. The crys-tallographic orientation of these lamellae was studied by TEM. By combining image and diffraction information it could be directly shown that the lamellae lie on (111) planes and represent twins, which are ∼0.5 μm thick. The relevance of twin formation for the mechanism of anomalous grain growth is briefly discussed.