Dielectric characteristics of donor-acceptor modified BaTiO3 ceramics

Sm/Mn codoped BaTiO₃ ceramics were investigated for their microstructure and dielectric characteristics. The powders were prepared by the conventional solid state procedure. The concentration of Sm₂O₃ as a donor dopant has been kept from 0.1 up to 5.0 at%. The content of MnO₂ as acceptor was kept constant at 0.05 at% Mn in all samples. The specimens were sintered at 1290 °C, 1320 °C and 1350 °C in an air atmosphere for two hours.A mainly uniform and homogeneous microstructure with average grain size ranging from 0.3 µm to 2.0 µm was observed in low doped samples. In highly doped samples, apart from the fine grained matrix, the appearance of local area with secondary abnormal grains was observed.The dielectric properties were investigated as a function of frequency and temperature. The low doped samples exhibit the high value of dielectric permittivity at room temperature and the greatest change at the Curie temperature. The highest value of dielectric constant (ε_r=6800) was measured for 0.1Sm/BaTiO₃ samples sintered at 1350 °C. A nearly flat permittivity-temperature response and lower values of ε_r were obtained in specimens with 2.0 and 5.0 at% additive content. The dielectric constant increases with the increase of sintering temperature. The dissipation factor ranged from 0.01 to 0.22 and decreases with the increase of sintering temperature. The Curie constant (C), Curie-Weiss temperature (T₀) and critical exponent of nonlinearity (γ ) were calculated using a Curie-Weiss and modified Curie-Weiss law. The highest value of Curie constant (C=9.06·10⁵ K) was measured in 0.1 at% doped samples. The Curie constant decreased with increasing dopant content. The γ values, ranging from 1.001 to 1.58, point out the sharp phase transition in low doped samples, and the diffuse phase transition in heavily doped BaTiO₃ samples.     

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.     

Microstructural design of BaTiO3-based ceramics for temperature-stable multilayer ceramic capacitors

In order to satisfy EIA X8R specification, a new type of BaTiO₃-based ceramic with hierarchical structure in a formula scheme “a ferroelectric ABO₃ + another ferroelectric ABO₃”, was designed. There were (Ba, Bi)TiO₃ and Ba(Ti, Zr)O₃ phases with different Curie temperatures coexisting in the grains from inside to outside, prepared by wet chemical method under 100 °C. The hierarchical structure of the ceramic grains was proved by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The dielectric constant of (Ba, Bi)TiO₃–Ba(Ti, Zr)O₃ ceramic was ～6000, the ΔC/C_20 °C was ?12.0%, 14.1%, and ?8.3% at ?55 °C, 130 °C, and 160 °C, respectively, and the dielectric loss is less than 0.1, which is obviously superior to (Ba, Bi)TiO₃ and Ba(Ti, Zr)O₃. The results of this work showed that the formula scheme “a ferroelectric ABO₃ + another ferroelectric ABO₃” for solid solutions is a promising approach to prepare high performance temperature-stable capacitor materials.     

High-temperature dielectrics in CaZrO3-modified Bi1/2Na1/2TiO3-based lead-free ceramics

Materials in two composition regimes, Bi_1/2Na_1/2TiO₃–BaTiO₃–CaZrO₃ (BNT–BT–CZ) and Bi_1/2Na_1/2TiO₃–BaTiO₃–K_0.5Na_0.5NbO₃–CaZrO₃ (BNT–BT–KNN–CZ), were synthesized via the mixed oxide route and their structural, dielectric and electrical properties were investigated. CZ was identified to render the two local maxima in permittivity more diffused. This resulted in temperature-insensitive relative permittivity spectra with average values from ～470 up to ～2300 and operational windows of at least ～400 °C with less than 15% of variation in the temperature range from ?100 up to above 500 °C. Moreover, loss factors are below ～10% and RC constants range from ～0.03 s up to ～4 s at 300 °C. The materials of current investigation are highly attractive for developing capacitors of wide temperature usage.     

Doping behaviors of NiO and Nb2O5 in BaTiO3 and dielectric properties of BaTiO3-based X7R ceramics

Doping behaviors of NiO and Nb₂O₅ in BaTiO₃ in two doping ways and dielectric properties of BaTiO₃-based X7R ceramics were investigated. When doped in composite form, the additions rendered higher solubility than that doped separately due to the identical valence between the complex (Ni_1/3²⁺Nb_2/3⁵⁺)⁴⁺ and Ti⁴⁺. NiO–Nb₂O₅ composite oxide was more effective in broadening dielectric constant peaks which was responsible for the temperature-stability of BaTiO₃ ceramics. A reduction in grain size was observed in the specimens with 0.5–0.8 mol% NiO–Nb₂O₅ composite oxide, whereas the abnormal growth of individual grains took place in the 1.0 mol% NiO–Nb₂O₅ composite oxide-doped specimen. When the specimen of BaTiO₃ doped with 0.8 mol% NiO–Nb₂O₅ composite oxide was sintered at 1300 °C for 1.5 h in air, good dielectric properties were obtained and the requirement of (EIA) X7R specification with a dielectric constant of 4706 and dielectric loss lower than 1.5% were satisfied.     

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).     

Duplex structure in K0.5Na0.5NbO3-SrZrO3 ceramics with temperature-stable dielectric properties

Temperature-stable dielectric properties have been developed in the 0.86 K_0.5Na_0.5NbO₃-0.14SrZrO₃ solid solution system. High dielectric permittivity (ε′ = 2310) with low loss sustained in a broad temperature range (−55–201 °C), which was close to that of the commercial BaTiO₃-based high-temperature capacitors. Transmission electron microscopy with energy dispersive X-ray analysis directly revealed that submicron grains exhibited duplex core-shell structure. The outer shell region was similar to the target composition, whilst a slightly poor content of Sr and Zr presented in the core region. Based on Lichtenecker’s effective dielectric function analysis along with Lorentz fit of the temperature dependence of dielectric permittivity, a plausible mechanism explaining the temperature-stable dielectric response in present work was suggested. These results offer an opportunity to achieve the X8 R specification high-temperature capacitors in K_0.5Na_0.5NbO₃ based materials.     

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.     

High temperature properties of multiferroic BiFeO3–DyFeO3–BaTiO3 solid solutions

Dielectric and magnetic properties of the xBiFeO₃–yDyFeO₃–zBaTiO₃ solid solution ceramics at high temperature range of RT ∼600 °C have been characterized. For the more detailed understandings of the multiferroic property, the relation between the crystal structure transition, magnetic transition, dielectric transition with increasing temperature have been analyzed. Residual magnetization M_r under the low and high applied magnetic fields (H = 20 Oe, 8 kOe) and the dielectric properties, ɛ_r and tan δ, with varying measuring frequency and temperature have been characterized using the vibrating sample magnetometer and LCR meter, respectively. The neutron diffraction data has been collected at the temperature range of RT ∼800 °C. The low DyFeO₃ concentration samples (y = 0, 0.025) show the magnetic transitions at temperature range of 410–430 °C, while the high DyFeO₃ samples (y ≥ 0.05) show the additional transition at 250–290 °C. The magnetic transition at 410–430 °C corresponds to the crystal structural transition to the tetragonal P4mm from the rhombohedral R-3c, at which the BiFeO₃ and the DyFeO₃ samples lose their antiferromagnetic ordering.     

Chemical corrosion of basic refractories by cement kiln materials

The study of basic refractories corrosion by cement kiln materials was carried out with powder tests and coating tests at the typical temperature range of the sintered material in the transition zone and the sintering zone (1200–1450 °C). The tested basic refractories were magnesia-spinel (MSp) and magnesia–zirconia (MZ) refractory bricks. The industry cement kiln materials were rich of sulphur and chlorine. The microstructures of the as-delivered and tested samples were researched by XRD and SEM–EDS techniques. The new phases detected in the samples with MSp (with and without ZrO₂) bricks after tests at the temperature of 1200 °C were binary (C₁₂A₇ t_mp ? 1392 °C, CaZrO₃: t_mp ? 2345 °C)¹: and ternary (C₂AS: t_mp ? 1593 °C or C₃MS₂: t_dp ? 1573 °C) phases. After tests at the temperature of 1300 °C and higher, ternary phases of C₇A₃Z (t_mp ? 1550 °C) and CaZrO₃ and quaternary phases Q-C₂₀A₁₃M₃S₃ or C₆A₄(M,f)S (t_dp ? 1380 °C) were detected. The C₃A₃·CaSO₄ phase was formed in the samples after the corrosion tests performed up to the temperature of 1300 °C. The new phases formed in the sample with the MZ bricks were clinker phases (β-C₂S, C₃A and C₂(A,F)/C₄AF) and the C₇A₃Z phase.     

Effects of Bi4Ti3O12 addition on the microstructure and dielectric properties of modified BaTiO3 under a reducing atmosphere

The effects of Bi₄Ti₃O₁₂ addition on the microstructure and dielectric properties of Mn-modified BaTiO₃ were investigated to develop low temperature fired BaTiO₃-based ceramics with stable temperature characteristics. The sintering temperature of Mn-doped BaTiO₃ could be reduced to 1200 °C by adding more than 1 mol% Bi₄Ti₃O₁₂. TEM results show an apparent core–shell structure with 2 mol% Bi₄Ti₃O₁₂ addition. However, it was destroyed when the Bi₄Ti₃O₁₂ content increased from 2 to 4 mol%. The permittivity decreased and the Curie temperature shifted to higher temperature when the Bi₄Ti₃O₁₂ content increased from 0 to 3 mol%. The temperature characteristic of capacitance was very close to the EIA X8R specification when 2 mol% Bi₄Ti₃O₁₂ was added due to the presence of the core–shell grain structure and raised Curie temperature. With adequate Bi₄Ti₃O₁₂ addition, the BaTiO₃-based system shows great potential for applications in EIA X8R-type multilayer ceramic capacitors.     

Ionic conductivity and mechanical properties of Y2O3-doped CeO2 ceramics synthesis by microwave-induced combustion

We developed a new method, i.e. microwave-induced combustion synthesis to produce highly sinterable Y₂O₃-doped CeO₂ nanopowders. The process took only 15 min to yield Y₂O₃-doped CeO₂ powders. We also investigated the conductivity of Y₂O₃-doped CeO₂ ceramics. It was found Y₂O₃ concentration to have a large effect on the morphology, activation energy, ionic conductivity, and mechanical properties of Y₂O₃-doped CeO₂ ceramics. The results revealed that the bulk densities of Y₂O₃-doped CeO₂ ceramics sintered at 1420 °C for 5 h were all higher than 92% of the theoretical densities, and the maximum ionic conductivity, σ_800 °C = 0.023 S/cm at 800 °C, the minimum activation energy, E_a = 0.954 eV determined in the temperature of 300–800 °C and the maximum fracture toughness, K_IC = 1.825 ± 0.188 MPa m^1/2 were found for 9 mol.% Y₂O₃-doped CeO₂ specimen. The grain size of CeO₂ decreases with increasing Y₂O₃ concentration. The fracture toughness was found to increase at increased Y₂O₃ concentration, because of the decrease of CeO₂ grain size.     

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.     

Hydrothermally synthesized BaTiO3 textured in a strong magnetic field

Hydrothermally synthesized BaTiO₃ was colloidally consolidated by slip casting and by electrophoretic deposition in a high strength magnetic field of 17.4 T. Textured BaTiO₃ was successfully prepared by both processing techniques, with the {001} plane aligning normal to the direction of the magnetic field. The initial alignment in the green powder compact was preserved and enhanced after sintering from 1400 °C to 1500 °C. The sintering temperature hardly influenced the degree of texture in slip cast samples but did influence the grain size of all BaTiO₃ ceramics and the degree of texture in the electrophoretically deposited samples. The BaTiO₃ ceramics processed in the strong magnetic field showed crystallographic texture but not morphological, so the microstructure did not show any anisotropy. The highest Lotgering factor (0.85) was measured for the ceramics made by electrophoretic deposition and sintered at 1500 °C.     

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.     

Synthesis and dielectric anomalies of CdCu3Ti4O12 ceramics

CdCu₃Ti₄O₁₂ ceramics were successfully synthetized by the conventional solid-state reaction method. The influences of sintering parameters on phase structure, microstructure and dielectric properties were investigated systematically. CdCu₃Ti₄O₁₂ ceramics sintered at 1020 °C for 15 h exhibited high temperature stability and outstanding dielectric properties, evidenced by the △C_T/C_25 °C ranges from −14.8% to 12.1% measured from −55 to 125 °C at 1 kHz, and the giant dielectric constant ε′=2.4×10⁴ as well as dielectric loss tanδ=0.072. Four dielectric anomalies were evidenced in dielectric temperature spectra and the related physical mechanisms were discussed in detail. The oxygen vacancies play an important role in dielectric anomalies in the high temperature range.     

Manufacture and measurement of combinatorial libraries of dielectric ceramics: Part I: Physical characterisation of Ba1−xSrxTiO3 libraries

The application of combinatorial methods to materials science offers the opportunity to accelerate the discovery of more efficient dielectric ceramics. High throughput methods have the potential to investigate the effects of a wide range of dopants on the dielectric properties, and to optimise existing systems, encouraging the short innovation cycles that the communications technology industry requires. The London University Search Instrument (LUSI) is a fully automated, high-throughput combinatorial robot that has the potential capability to produce 1000's of sintered bulk ceramic samples with varying composition in 1 day, as combinatorial libraries on alumna substrates. The LUSI robot was demonstrated to be able to automatically print and sinter libraries of bulk ceramic Ba_1−xSr_xTiO₃ (BST), for x = 0–1 in steps of 0.1. The samples were prepared from inks consisting of suspensions of BaTiO₃ and SrTiO₃ nanopowders, stabilised with 1.2 wt% dispersant. The samples were printed as arrays of 22 2mm diameter dots, two of each composition, on alumina substrates. These were then sintered at 1350 and 1400 °C/1 h by LUSI. EDXA and XRD confirmed the compositional gradient throughout the libraries, and SEM showed the samples to be well sintered, with a large 20 μm grain size for pure BaTiO₃ decreasing rapidly for increasing x values.     

Effects of Sn on the microstructure and dielectric properties in BaTiO3-based ceramics

Ba_0.985Bi_0.01TiO₃–BaTi_1?xSn_xO₃ powders were synthesized by a two-step soft chemical method. Ceramics with core–shell structure could be easily obtained by using these uniformly distributed powders. The ceramics not only satisfied the requirement of EIA-X8R specification, but also were near to that of EIA-X9R specification. Microstructural evaluation conducted by X-ray diffraction and scanning electron microscopy confirmed the hierarchical structure of the ceramic grains. The shape of the ε–T curves near the dielectric peak became broad when x increased from 0.001 to 0.02. The permittivity of Ba_0.985Bi_0.01TiO₃–BaTi_0.98Sn_0.02O₃ ceramic was ～23,000, ΔC/C_20 °C was ?15%, 14.4% and ?15% at ?55 °C, 120 °C and 170 °C, respectively, and the dielectric loss was 0.5. The results showed that the content of Sn had a strong impact on the diffusion and the dielectric properties of the ceramics.     

Simultaneous enhancement of dielectric properties and temperature stability in BaTiO3-based ceramics enabling X8R multilayer ceramic capacitor

Modified BaTiO₃ ceramics that possess high dielectric permittivity and acceptable temperature stability have been widely utilized as multilayer ceramic capacitors (MLCCs) for high-frequency bypass and power filtering in automotive applications. However, since the increasing demand for high-capacity and small-size, high-permittivity materials that can serve as dielectric layers in MLCCs are urgently required. In this work, we design and fabricate a special BaTiO₃-0.03Mg-0.02Y-0.02CaZrO₃ ceramic with a high dielectric permittivity of 3000 and the dielectric variation below ±13% in the temperature range of -55–150°C, fulfilling the requirements of X8R capacitors. To achieve these results, we employed grain size engineering and cation doping, using BaTiO₃ precursors with a particle size of 240 nm to prepare the BaTiO₃-based ceramics with fine grains, while Mg and Y co-doping was used for improving the temperature stability due to dielectric dispersion. Utilizing these high-permittivity BaTiO₃-based materials, we fabricated MLCCs that satisfy the X8R criterion, possessing a high dielectric constant of 2950 and a high breakdown field (410 kV/cm).     

The effects of raw materials particle size and salt type on formation of nano-CaZrO3 from molten salts

Nano-CaZrO₃ was successfully synthesized at 800 °C using the molten-salt method, and the effects of salt type and raw materials particle size on the formation of CaZrO₃ were investigated. Na₂CO₃, CaCl₂, nano-ZrO₂ and micro-ZrO₂ were used as starting materials. On heating, Na₂CO₃ reacted with CaCl₂ to form NaCl and in situ CaCO₃. Na₂CO₃–NaCl molten eutectic salt provided a liquid medium for reaction of CaCO₃ and ZrO₂ to form CaZrO₃. The results demonstrated that in both nano- and micro-ZrO₂ inclusive samples, CaZrO₃ started to form at about 700 °C and that, after the temperature was increased to 1000 °C, the amounts of CaZrO₃ in the resultant powders increased with a concomitant decrease in CaCO₃ and ZrO₂ contents. After washing with hot-distilled water, the samples containing nano- and micro-ZrO₂ heated for 3 h at 800 °C and 1000 °C, were single-phase CaZrO₃ with 70–90 nm and 400–450 nm particle size, respectively. Also, the synthesis process was completed in lower temperatures using eutectic salts. Furthermore, the synthesized CaZrO₃ particles retained the size and morphology of the ZrO₂ powders, which indicated that a template formation mechanism dominated the formation of CaZrO₃ by molten-salt synthesis.