Sol–gel synthesis and characterization of Ce doped-BaTiO3

Barium titanate (BaTiO₃) have been doped “in situ” with 5.5 mol% cerium by a sol–gel method using barium acetate, titanium (IV) isopropoxide, and cerium (III) acetylacetonate as starting materials. The dried gel showed a microstructure consisting of nano-sized grains (∼140 nm) with great tendency to agglomeration. Several thermal analysis techniques were used to study the decomposition process of the gel. The presence of hydroxyls up to 720 °C suggests a strong bonding TiOH that is responsible for the existence of aggregates even at high temperatures. The as-prepared gel powder was found to be amorphous, and then decomposes through oxides and barium carbonate around 500 °C and crystallizes on the perovskite structure of tetragonal BaTiO₃ at 1100 °C for 3h in air. A small influence of the frequency on the dielectric properties of the Ba_0.945Ce_0.055TiO₃ ceramics was observed in 100 Hz to 1 MHz domain. At the Curie temperature point (22 °C) the dielectric constant was 10130 at 100 Hz while the dielectric loss (tan δ) was 0.018.     

Dielectric properties of barium titanate–molybdenum composite

The barium titanate–molybdenum composites were prepared through solid state reaction method in argon atmosphere. The microstructure, resistivity, and dielectric properties of the composites were investigated. XRD results indicated that chemical reactions between barium titanate (BaTiO₃:BT) and molybdenum (Mo) have taken place during sintering, resulting in the formation of BaMoO₄ (BM) and BaTi₂O₅ (BT₂). The resistivity decreased with the increasing amount of Mo in the composites. The composites (when x = 5 and 20 wt.%) showed lower dielectric constant than pure BaTiO₃, especially, the dielectric constant (when x = 20 wt.%) reached a minimum value (<10⁴), while composites (when x = 10 and 15 wt.%) showed rather high dielectric constant at temperatures range from 25 °C to 160 °C. The dielectric constant of the composite gradually decreased with increase in frequency at the room temperature. The dielectric constant of composite (when x = 5 wt.%) comes up to 10⁴, and the T_c (Curie temperature) of the composite was relatively higher than that of BT (120 °C).     

Synthesis and characterization of high performance CaZrO3-doped X8R BaTiO3-based dielectric ceramics

A novel system of lead-free X8R BaTiO₃-based dielectric materials with high dielectric constants was prepared via conventional mixing method. The phase structure, dielectric and electrical properties and the “relaxorlike” characteristic of ceramic materials were systemically studied. The XRD results indicate that no secondary phase formed in the CaZrO₃-doped samples after sintering. SEM micrographs reveal that the grain size of CaZrO₃-doped BaTiO₃-based samples are not uniform, ranging from 0.2 to 0.9 µm. Bulk densities and dielectric properties were measured as a function of CaZrO₃ concentration. The Curie temperatures of all CaZrO₃ doped samples increase due to the existence of an internal stress between grain cores and shells caused by the diffusion of CaZrO₃. The oxygen vacancies have a profound effect on the dielectric loss and frequency characteristics. The 3.5 wt% CaZrO₃-doped sample sintered at 1250 °C showed the optimal dielectric performance (ε_r=4330, tan δ<1.5%, ΔC/C_20 °C≤±14%) that satisfied EIA-X8R specification.     

Dielectric,pyroelectric, and ferroelectric properties of gadolinium doped Sr0.53Ba0.47Nb2O6 ceramic

Strontium barium niobate doped with gadolinium, with the stoichiometric formula Gd_ySr_{(0.53–3y/2)}Ba_0.47Nb₂O₆ (GSBN) was synthesized using the solid-state reaction method, with varied mol% compositions of Gd (y=0.00, 0.01, 0.03, 0.05 and 0.07). Gadolinium was chosen as a dopant with the goal of enhancing the ferroelectric and pyroelectric properties of SBN. The X-ray diffraction spectra showed that all compositions exhibit a single-phase tetragonal tungsten bronze structure. The influence of Gd as dopant on the microstructure was examined by using field emission scanning electron microscopy. The dielectric characteristics of the samples showed diffuse phase transitions. The Curie temperature of the samples shifted to lower temperature with increasing Gd concentration. The relaxor characteristic of the GSBN (above and below the Curie temperature) was described using the Curie-Weiss Law, a Gaussian distribution, and a quadratic equation. SBN doped with 3 mol% of Gd exhibits the highest remnant polarization, P_r=8.8 μC/cm², while 1 mol% Gd-doped SBN shows the highest pyroelectric coefficient of 285 µC/m² K. These qualities can be useful in security, healthcare, pollution monitoring, fire sensing, and smart energy system applications.     

Synthesis and piezoelectric properties of Li-doped BaTiO3 by a solvothermal approach

Li-doped BaTiO₃ particles with the Li⁺ mole fraction, x, of 0–0.06 were synthesized by a solvothermal approach at 200 °C. The products consisted of nanoparticles of 50–100 nm in diameter. The sinterability and piezoelectric property of Li-doped BaTiO₃ were improved by doping with Li ion, i.e., the Li-doped BaTiO₃ samples could be sintered to almost full theoretical density (>95%) at a low temperature such as 1100 °C, and the highest piezoelectric constant, d₃₃ (260 pC/N) and electromechanical coupling factor, k_p (43.7%) could be realized at x value of 0.03. The Curie temperatures of all samples were around 130 °C, and did not change very much depending on the amount of Li-doping.     

Microwave sintered sol–gel derived BaTiO3 and Ba0.95La0.05TiO3 ceramic samples for capacitor applications

Sol–gel derived BaTiO₃ and Ba_0.95La_0.05TiO₃ powders were calcined at 700 °C for 40 min and sintered at 1100 °C for 1 h in a microwave furnace to obtain single-phase perovskite ceramic samples. About 98% of the theoretical density was obtained in the sintered BT ceramic samples. Room temperature (RT) dielectric constant (ε_r) and dielectric loss (tan δ) at 1 kHz frequency of the BLT ceramic samples were found to be ~2220 and 0.005, respectively. High value of ε_r, low value of tan δ and negligible temperature coefficient of capacitance from RT to 60 °C suggested the suitability of BLT ceramic samples for multi-layer capacitor applications.     

Microstructure and dielectric properties of Dy/Mn doped BaTiO3 ceramics

Dy/Mn doped BaTiO₃ with different Dy₂O₃ contents, ranging from 0.1 to 5.0 at% Dy, were investigated regarding their microstructural and dielectric characteristics. The content of 0.05 at% Mn was constant in all the investigated samples. The samples were prepared by the conventional solid state reaction and sintered at 1290°, and 1350 °C in air atmosphere for 2 h. The low doped samples (0.1 and 0.5 at% Dy) exhibit mainly fairly uniform and homogeneous microstructure with average grain sizes ranged from 0.3 μm to 3.0 μm. At 1350 °C, the appearance of secondary, abnormal, grains in the fine grain matrix and core–shell structure were observed in highly doped Dy/BaTiO₃. Dielectric measurements were carried out as a function of temperature up to 180 °C. The low doped samples sintered at 1350 °C, display the high value of dielectric permittivity at room temperature, 5600 for 0.1Dy/BaTiO₃. A nearly flat permittivity–temperature response was obtained in specimens with 2.0 and 5.0 at% additive content. Using a Curie–Weiss and modified Curie–Weiss low, the Curie constant (C), Curie like constant (C′), Curie temperature (T_C) and a critical exponent (γ) were calculated. The obtained values of γ pointed out the diffuse phase transformation in highly doped BaTiO₃ samples.     

Low temperature synthesis of tetragonal BaTiO3 by a novel composite-hydroxide-mediated approach and its dielectric properties

High purity tetragonal BaTiO₃ powders were synthesized by a composite-hydroxide-mediated approach at low temperature using a novel hydrothermal reaction apparatus with a rolling system. The optimum synthesis conditions were explored, and the obtained samples were characterized by their XRD, TEM, TG-DTA and SEM. The powders with an average size of 150 nm in diameter were sintered to almost full theoretical density (ca. 99%) at 1200 °C for 5 h and the obtained ceramics presented a high dielectric constant (9500 at the Curie temperature).     

Dielectric behaviors of Nb2O5–Co2O3 doped BaTiO3–Bi(Mg1/2Ti1/2)O3 ceramics

Nb₂O₅ and Nb–Co doped 0.85BaTiO₃–0.15Bi(Mg_1/2Ti_1/2)O₃ (0.85BT–0.15BMT) ceramics were investigated. From XRD patterns, undesired phase was observed when the (Nb₂O₅/Nb-Co) doping levels exceed 3 wt.%/2 wt.%, giving rise to the deteriorate dielectric constant. The 0.85BT–0.15BMT ceramics doped with 2 wt.%Nb₂O₅ was found to possess a moderate dielectric constant (? ～ 1000) and low dielectric loss (tan δ = 0.9%) at room temperature and 1 kHz, showing flat dielectric behavior over the temperature range from ?55 to 155 °C. It was found that the formation of core–shell structure in the BT based ceramics is controlled by the doping sequence of Nb- and Bi-oxides.     

Structural,dielectric, and piezoelectric properties of fine-grained NBT–BT0.11 ceramic derived from gel precursor

(Na_1/2Bi_1/2)TiO₃ doped in situ with 11 mol% BaTiO₃ (NBT–BT_0.11) powders were synthesized by a sol–gel method, and the electrical properties of the resulting ceramics were investigated. The powders consisting of uniform and fine preliminary particles of about 50 nm were prepared by calcining the gel precursor at 700 °C. (Na_1/2Bi_1/2)_0.89Ba_0.11TiO₃ ceramics, sintered at temperatures up to 1150 °C have a rhombohedral symmetry, while the ceramic sintered at 1200 °C exhibits a tetragonal crystalline structure. The ceramics show high dielectric constant (?_r ～ 5456), dielectric loss of 0.02, depolarization temperature T_d ～ 110 °C and temperature corresponding to the maximum value of dielectric constant T_m ～ 262 °C. The dielectric constant (?₃₃) and the piezoelectric constant (d₃₃) attain the maximum values of 924 and 13 pC/N, respectively, while the electromechanical coupling factor (k_p) value is 0.035. The NBT–BT_0.11 ceramics derived from sol–gel present high mechanical quality factor (Q_m ～ 860). The dielectric and piezoelectric properties values of NBT–BT_0.11 ceramics derived from sol–gel are smaller than those of samples produced by the conventional solid state reaction method, due to the grains size and oxygen vacancies that generate dipolar defects.     

Microstructure effect on dielectric Properties of MgO-doped BaTiO3–BiYO3 ceramics

MgO-doped 0.97BaTiO₃–0.03BiYO₃ (0.97BT–0.03BY) polycrystalline ceramics were prepared by the solid-state sintering method. Then the structural, dielectric and resistant properties were investigated as functions of MgO addition. Microstructure was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). The results show that Bi³⁺, Y³⁺ and Mg²⁺ ions exhibit nonuniform distribution behavior in BT–BY ceramics, demonstrating the existence of a “core–shell” structure, which plays important roles in the capacitance-temperature characteristics, where 0.97BT–0.03BY with the addition 2.2–2.8 at% MgO meets the Electronic Industries Association (EIA) X8R (−55 to 150 °C, ΔC/C_25 °C=±15% or less) specification. Moreover, the fine-grained samples with core–shell structure show much higher bulk resistance than the coarse-grained samples over the studied temperature range, which is attributed to the higher proportion of grain boundaries and the lower concentration of the effective acceptor.     

B-site donor and acceptor doped Aurivillius phase Bi3NbTiO9 ceramics

The electrical properties of B-site donor and acceptor doped Aurivillius phase Bi₃NbTiO₉-based ceramics have been investigated. The effect of donor and acceptor doping on the dielectric constant, coercive field, dc conductivity and piezoelectric constant are presented. The band gap of Bi₃NbTiO₉ (BNTO) is about 3.4 ± 0.2 eV, determined from high-temperature dc conductivity measurements. All of the ceramics are ferroelectrics with high Curie points (∼900 °C). In acceptor doped ceramics, a low-temperature peak in the dielectric loss tangent is explained in terms of a Debye-type relaxation that results from an oxygen ion-jump mechanism. The activation energy for the relaxation is calculated as 0.93 ± 0.05 eV. The reduction of the piezoelectric constant below 500 °C is produced by depolarization, which is produced by the switching of thermally unstable non-180° domain walls.     

High performance Aurivillius-type bismuth titanate niobate (Bi3TiNbO9) piezoelectric ceramics for high temperature applications

This paper reports the significant improved piezoelectric properties of high temperature bismuth titanate niobate (Bi₃TiNbO₉, BTN) polycrystalline ceramics. The piezoelectric performance of BTN ceramics is significantly enhanced by cerium modifications. The dielectric measurements indicate that the Curie temperature T_c gradually decreases over the temperature range of 907–889 °C with cerium contents increasing up to 0.7 wt%. The BTN-5Ce (BTN+0.5 wt% CeO₂) exhibits optimized piezoelectric properties with a piezoelectric constant d₃₃ of 16 pC/N, which is five times the value of unmodified BTN (d₃₃~3 pC/N), while BTN-5Ce maintains a high Curie temperature T_c of 894 °C. The temperature-dependent electrical impedance and electromechanical coupling factors (k_p, and k_t) reveal that the BTN-5Ce exhibits thermally stable electromechanical coupling characteristics up to 500 °C but significantly deteriorates at 600 °C due to high conductivity at a higher temperature. The thermally stable electromechanical properties in combination with the ceramics׳ high electrical resistivity (10⁶ Ω cm at 500 °C) and high Curie temperature (~900 °C) demonstrate that cerium-modified BTN ceramics are good materials for high temperature sensing applications.     

Microstructure and dielectric characteristics of Nb2O5 doped BaTiO3-Bi(Znl/2Til/2)O3 ceramics for capacitor applications

The effects of Nb₂O₅ addition on the dielectric properties and phase formation of 0.8BaTiO₃-0.2Bi(Zn_l/2Ti_l/2)O₃ (0.8BT-0.2BZT) ceramics were investigated. The desired perovskite phase was achieved with Nb₂O₅ doping levels being in the range of 0.5 wt.%–3.0 wt.%. The 0.8BT-0.2BZT ceramics doped with 1.5 wt.% Nb₂O₅ was found to possess a moderate dielectric constant (ε = 1170) and low dielectric loss (tanδ = 1%) at room temperature and 1 kHz frequency, showing a flat dielectric behavior over the temperature range of −55 °C–200 °C. Based on this composition, the X9R-MLCC (multilayer ceramic capacitor) with Ag0.7-Pd0.3 electrode was sintered at 1060 °C. The optimized capacitance of the MLCC is 26.5 nF, with dielectric loss tanδ of 0.9% and electrical resistance of 4.50 × 10¹¹ Ω at room temperature, leading to a high time constant of 11,900 s, decreasing to 175 s at 200 °C, being one order higher than those of commercial X7R MLCC. In addition, the equivalent series resistance (ESR) was found to be on the order of 0.2 mΩ at 2 MHz, much lower than that of the DC Bus Capacitor Bank for the automotive inverters (where the desired characteristic is <3 mΩ). All these characteristics of the newly developed MLCC will benefit the high temperature and high power capacitor applications.     

Effect of Na0.5Bi0.5TiO3 on dielectric properties of BaTiO3 based ceramics

In this work, Na_0.5Bi_0.5TiO₃ (NBT) was used to improve the high temperature dielectric properties of Nb, Co-doped BaTiO₃ (BT). Different x was selected (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.3, 0.4) to optimize the ratio of BT to NBT in (1 ? x) BT–xNBT solid solution. The dielectric constant of the original X7R material is about 4900 at room temperature, decreasing to 2500 with NBT addition (x = 0.2). Of important is that the temperature stability was improved with dielectric constant variation being less than ±15% up to 160 °C.     

Studies of dielectric properties of rare earth (Dy,Tb, Eu) doped barium titanate sintered in pure nitrogen

This paper investigated dielectric properties of rare earth (Dy, Tb, Eu)-doped barium titanate sintered in pure nitrogen. The substituting concentration of rare earth (Dy, Tb, Eu) was 2.0 mol%. The doping behaviors of intermediate rare-earth ions (Dy, Tb, Eu) and their effects on the dielectric property of barium titanate were investigated. Eu³⁺ ion was substituted in the A-site of the perovskite lattice. Dy³⁺ and Tb³⁺ ions substituted partially for Ti⁴⁺ site and partially for Ba²⁺ site. The different rare earth element had a crucial effect on dielectric properties of rare-earth-doped BaTiO₃. Among these doped samples, Tb-doped BaTiO₃ had the largest dielectric constant (70,000–80,000); the smallest dielectric loss (less than 4%), and good capacitance-temperature coefficient, which satisfies the X7R specification of the Electronic Industries Association Standards (TCC within ±15% from ?55 °C to 125 °C).     

Effects of K0.5Bi0.5TiO3 addition on dielectric properties of BaTiO3 ceramics

(1?x)BaTiO₃–xK_0.5Bi_0.5TiO₃ (abbreviated as BT–KBT, 0.10≦x≦0.15) dielectric ceramics were prepared by a conventional oxide mixing method. The effects of KBT content on the densification, microstructure and dielectric properties of BT ceramics were investigated. The density characterization results show that the addition of KBT significantly lowered the sintering temperature of BT ceramics to about 1280 °C. The XRD results showed that the phase compositions of all samples were pure tetragonal phases. The dielectric constant and dielectric loss firstly increased and then decreased with the increase of KBT. In addition, dielectric constant and dielectric loss versus frequency were characterized in the frequency range from 100 Hz to 2 MHz. It is found that the dielectric constant and the dielectric loss changed with the increase of KBT contents regularly.     

Characterization of 0.93Pb(Zn1/3Nb2/3)O3–0.07BaTiO3 ceramics derived from a novel Zn3Nb2O8 B-site precursor

In this work, perovskite relaxor ferroelectric lead zinc niobate–barium titanate (0.93PZN–0.07BT) ceramics were fabricated by using a combination of Zn₃Nb₂O₈ B-site precursor and reactive sintering process. The effects of sintering condition on phase formation, densification, microstructure and dielectric properties of the final products have been investigated using a combination of X-ray diffraction, Archimedes density measurement, scanning electron microscopy and dielectric measurement techniques. It is seen that pure perovskite phase of PZN-BT solid solutions can be achieved in all samples. Density and average grain size values of sintered samples increased with sintering temperatures and dwell time. With appropriate sintering at 1150 °C for 5 h, 0.93PZN–0.07BT ceramics exhibited a peak dielectric constant of 11,497 and dielectric loss of 0.05 at the Curie temperature of 99 °C measured at 1 kHz.     

Preparation and microwave dielectric properties of low-loss MgZrNb2O8 ceramics

A novel low-loss microwave dielectric material MgZrNb₂O₈ was reported for the first time. Single-phase MgZrNb₂O₈ was prepared by a conventional mixed-oxide route and sintered in the temperature range of 1280–1360 °C. The microstructure and microwave dielectric properties were investigated systematically. The X-ray diffraction results showed that all samples exhibit a single wolframite structure. When the sintering temperature was lower than 1340 °C, the Q×f value mainly depended on the relative density. However, when the sintering temperature was above 1340 °C, the Q×f value mainly relied on the grain morphology in addition to the density. The MgZrNb₂O₈ ceramic sintered at 1340 °C for 4 h exhibited excellent microwave dielectric of ε_r=26, Q×f=120,816 GHz (where f=6.85 GHz), and τ_f=?50.2 ppm/°C. These results demonstrate that MgZrNb₂O₈ could be a promising candidate material for the application of highly selective microwave ceramic resonators and filters.     

Nano-sized Ag–BaTiO3 composite powders with various amount of Ag prepared by spray pyrolysis

The preparation of precursors of BaTiO₃ nanopowders with various amounts of Ag by spray pyrolysis is reported. The precursor powders obtained with hollow and thin-wall particles are composed of uniformly dispersed Ba, Ti, and Ag components. After post-treatment and a simple milling process, the precursor powders, irrespective of the amount of Ag, are transformed into Ag–BaTiO₃ composite nanoparticles. The mean particle size of the Ag (10 mol%)–BaTiO₃ powders is 142 nm. BaTiO₃ pellets containing Ag exhibit dense structures even at a low sintering temperature of 1000 °C. BaTiO₃ pellets with 10 mol% Ag show the highest dielectric constant of 2950, as opposed to the pure BaTiO₃ pellets (without Ag), whose dielectric constant is 1827.