Fine-grained BaTiO3 Ceramics Research Articles

Electrical Characteristics of Multilayered Ceramic Capacitors Depending on BaTiO3 Particle Size

Multilayer ceramic capacitors were prepared with BaTiO3-based ceramics of different grain sizes (150–500 nm), having appropriate dielectric properties and high-temperature stability. The grain size effect on the dielectric properties and insulation resistivity of fine-grained BaTiO3 ceramics at room temperature and high temperatures under electric fields were investigated. The reduction in particle size has a strong effect on DC-bias characteristics and withstanding voltage characteristics per unit thickness. BaTiO3 ceramic capacitor with smaller grains had higher reliability under the DC bias and higher withstanding voltage per unit thickness due to the shell with para-electricity. The highly accelerated lifetime test showed that the insulation resistance with smaller particle size exhibited one order higher as the voltage increases at higher temperatures and the samples with a size of 500 nm had a sharp drop in insulation resistance at 200 V. Small particles (150, 250 nm) have a smaller product of insulation resistance and capacity than larger particles (500 nm). But, the withstanding voltage per unit thickness of the small particles (150, 250 nm) was 82, 71 V/μm, respectively, which was greater than 58 V/μm of the larger particles (500 nm). This is thought to be an effect of suppressing the movement of defects in the smaller particles because small grains have many grain boundaries.

Large piezoelectricity and high transparency in fine-grained BaTiO3 ceramics

Grain size effect is considered as an important factor for the enhancement of dielectric and piezoelectric properties of the barium titanate (BT) ceramics. In this work, we prepared a dense fine-grained BT ceramic using the hot-press sintering method. This fine-grained BT not only exhibits superior electrical properties with a dielectric permittivity of 7200, piezoelectric coefficient of 420 pC/N, and field-induced electrostrain 0.2% at 30 kV/cm but also possesses high transmittance of 30%–60% over a wide range of wavelength from visible to middle infrared radiation spectrum. By a comparative study with the coarse-grained BT, the fine-grained BT is found to have a slight relaxation property due to its large volume fraction of grain boundaries. The coexistence of the fine domains in small grains and the relaxor regions of grain boundaries is responsible for the synchronous enhancement of electrical and optical properties of the fine-grained BT.

The effect of a barium titanate xerogel precursor on the grain size and densification of fine-grained BaTiO3 ceramics

In this study, we mixed a xerogel precursor of BaTiO3 (X-BTO) prepared by the sol-gel method into identical BaTiO3 powders obtained by co-precipitation during granulation to obtain fine-grained and dense BaTiO3 ceramics at a low sintering temperature (Ts). The effects of X-BTO, Ts, and dwelling time on the grain size and density of the BaTiO3 ceramics showed clear trends, and have predictable effects on the dielectric properties. With increasing X-BTO content, samples underwent a transition from the cubic phase to the tetragonal phase, the mean grain size increased, and the density and maximum dielectric constant first increased and then slightly decreased. Based on the requirements for dense and fine-grained ceramics with particle sizes comparable to powders, the BaTiO3 ceramic with 8 wt% X-BTO sintered at 940 °C exhibited improved permittivity (2445) and acceptable dielectric loss (0.019). This approach makes it possible to control the ceramic grain size by changing the powder's particle size, thereby creating fine-grained and dense BaTiO3 ceramics at a lower Ts.

Valence states and dielectric properties of fine-grained BaTiO3 ceramics co-doped with double valence-variable europium and chromium

(Ba1–xEux)(Ti1–xCrx)O3 (x = 0.01‒0.05) (BETC) and (Ba1–xEux)(Ti1.01–xCrx–0.01)O3 (x = 0.02‒0.05) (BETC1) ceramics with single-phase perovskite structures were prepared using a mixed-oxides method. The structure, microstructure, site occupations, valence states of Eu/Cr, and dielectric properties of these ceramics were investigated using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, fluorescence spectroscopy, emission spectroscopy, electron paramagnetic resonance, and dielectric measurements. The mixed valence states of Eu3+/Eu2+ and Cr3+/Cr5+ were confirmed. Co-doping with Eu and Cr greatly inhibited the grain growth of BaTiO3 ceramics, resulting in a fine-grained microstructure (0.7 µm). Two permittivity peaks of BETC with x ≥ 0.02 were characteristic of an inhomogeneous dopant concentration distribution forming a core–shell structure. For BETC1, the mixed valence states and site occupations of Eu/Cr were analyzed to elucidate an expansion in the unit-cell volume, very high Eu/Cr ratio (= 3.5) in coarser grains, regular diffuse phase transition behavior, rapid Tm-shifting rate of − 24 °C/at% (Eu/Cr), higher ε′RT (= 4550), and lower tan δ (= 0.0196) in BETC1, as well as the destruction of the core-shell structure in BETC.

Enhanced grain size effect on electrical characteristics of fine-grained BaTiO3 ceramics

Fine-grained BaTiO3 ceramics with different grain sizes (96–194 nm) were prepared by microwave sintering, and the grain size (GS) effect of fine-grained BaTiO3 ceramics on the electrical properties were investigated. A change from the tetragonal to pseudocubic structure gradually takes place with the decrease of GS. As the GS increased from 96 to 194 nm, the dielectric constant increased from 1300 to 1560. The temperature capacitance coefficients satisfied the EIA X7R specifications for all the samples. Impedance analysis proved that the fine-grained ceramics have higher activation energy for both grain and grain boundary. The remnant polarization (P r ) values decrease with the decreasing of the GS, which is ascribed that the increasing of the grain boundary and internal stress would weaken the polarization of the direction of electric field. The grain boundary and defects have the pinning effects on the domain, leading the increase in E c .

Structural and dielectric properties, electron paramagnetic resonance, and defect chemistry of Pr-doped BaTiO3 ceramics

Fine-grained Pr-doped BaTiO3 ceramics (BPT) were prepared according to the nominal formula (Ba1−xPrx)Ti1−x/4O3 (0.02⩽x⩽0.05) using a mixed oxide method. Structure, microstructure, dielectric properties, and defect chemistry were investigated by X-ray diffraction (XRD), Raman spectroscopy, electron paramagnetic resonance (EPR), scanning electron microscopy (SEM), and dielectric measurements. Crystal structure changed from tetragonal (0.02⩽x⩽0.04) to cubic (x=0.05), accompanied by a linear decline of the c/a axial ratio with x. BPT exhibited a slight diffuse phase transition behavior and its dielectric-peak temperature (Tm) decreased rapidly at a rate of −22°C/at.% Pr. Pr ions were substituted predominantly for Ba sites as Pr3+ and induced Ti-vacancy defects, which are evidenced by a change in unit cell volume, a rapid Tm – shifting rate, a new Raman band at ∼837cm−1, and an EPR signal at g=2.002. A very small number of Pr ions inevitably entered Ti sites as Pr4+, which is evidenced by a thermally accessible signal at g=3.77 related to the preservation history of the x=0.05 sample. The effect of electron free radicals on EPR signal is discussed. Another thermally accessible EPR signal associated with a rhombic g-tensor of the stronger g=2.002 signal was observed. This signal is also related to the preservation history of the sample.

Grain size effect on electrical and reliability characteristics of modified fine-grained BaTiO3 ceramics for MLCCs

Fine-grained BaTiO 3 -based ceramics of different grain sizes (118–462 nm) with core–shell structures were prepared by a chemical coating method, having good dielectric properties and gentle temperature stability. The grain size effect on the dielectric properties and insulation resistivity of modified fine-grained BaTiO 3 ceramics under high temperatures and electric fields were investigated. The DC bias shows a strong effect on the dielectric properties with decreasing grain size. In the finest ceramics, the absolute value of the capacitance stability factor was the smallest, indicating that the modified-BaTiO 3 ceramic capacitor with smaller grains had higher reliability under the DC bias voltage. The highly accelerated lifetime test results showed that with decreasing the grain size, samples exhibited higher insulation resistance under elevated temperatures and high voltages. Impedance analysis proved that the finer-grained ceramic with core–shell structure had higher activation energy for both grain and grain boundary, whereas the proportion of ionic conductivity was lower.

Effect of Hydrothermal Treatment on the Piezoelectric Response of Oriented Barium Titanate Ceramics

The piezoelectric response of [11-oriented, fine-grained BaTiO3 ceramics was investigated. It was found that an exposure of the BaTiO3 compacts to high-pressure, high-temperature steam in an autoclave promoted the orientation of the sintered ceramics. As temperature in the autoclave increased, the orientation increased. A large small-field piezoelectric constant of d33 = 517 pC/N was observed for the oriented BaTiO3 ceramics with the grain size of 1.76 μm and the Lotgering factor of 85.3 %.

Polarization Mechanism of Fine-Grained BaTiO<sub>3</sub> Ceramics at High Temperature Region

Pure Barium titanate (BaTiO3) ceramics with grain sizes of 0.6, 1.5, and 18 μm on average were prepared by using a two-step sintering method. The wideband dielectric spectra in the range from kHz to THz were measured at 25 and 150 °C to understand the polarization mechanism of fine-grained BaTiO3ceramics. It was revealed that the dipole and ionic polarizations decreased with decreasing grain size at 25 and 150 °C when the grain size was smaller than 1 μm. These mechanisms were interpreted as a grain boundary effect.

Grain-size effects on dielectric and piezoelectric properties of poled BaTiO3 ceramics

Barium titanate (BaTiO3) is known historically as the first ceramic piezoelectric material and shows a high possibility of being revitalized as a popular lead-free piezoelectric material. However, few systematic studies of the grain-size effects on the dielectric and piezoelectric properties of poled BaTiO3 ceramics have been carried out to date. In this work, a series of dense BaTiO3 ceramics with uniform grain-size distribution were successfully prepared by the conventional solid-state reaction, and the grain-size effects on the dielectric and piezoelectric properties were explored in their poled states. An interesting physical phenomenon has been found: dielectric permittivity ε′ and the piezoelectric constant d33 increase significantly at room temperature with the reduction of the average grain size g and reach the maxima at g=0.94μm. It was revealed that domain structure and relative density play substantial roles in determining the ε′ and d33 behaviors. The average 90° domain width decreases monotonically with g. Both the 90° domain wall density and the area dimension of area dimension domain wall are considered as important factors that greatly influence the d33 value. Extrinsic contribution comprises the major part of the observed d33 values in the fine-grained BaTiO3 ceramics.

The effect of Ta2O5- and ZnO-doping on the Curie temperature of BaTiO3

The phase, microstructure and Curie temperature of Ba1−2xTa2xTi1−xZnxO3 (0.005 ≤ x ≤ 0.1) have been investigated in an attempt to widen its temperature stable region. The compositions formed dense, single-phase, fine-grained BaTiO3 ceramics at x≤0.01 and multiphase ceramics at x ≥ 0.015. The dielectric constant decreased with an increase in the dopant concentration due to second phase formation. The Curie temperature increased from ∼123°C at x = 0.005 to ∼148°C at x = 0.1 and the optimum sintering temperature decreased from ∼1430°C at x = 0.005 to 1276°C at x = 0.1.

Development of Dielectric Ceramics for Nickel Electrode Multilayer Capacitors

Calcium-doped BaTiO3-based dielectrics have a high resistance to reduction. This can be explained by taking the following three factors into account: 1) decrease in conduction electron density caused by acceptor Ca ions at the Ti site, 2) low freezing temperature of the equilibrium reaction by Ca ions at the Ti site, and 3) large enthalpy change in the reduction reaction by Ca ions at the Ba site. Fine-grained BaTiO3 ceramics can be prepared using the tetragonal BaTiO3 powder using ultra-fine starting BaTiO3 powder synthesized by hydrolysis method. The non-uniform distortion is caused in the grains during sintering, which is attributed to the surface tension and clamping stress from the thin grain boundary layer. This suppresses the tetragonality symmetry of the grains and provides the BaTiO3 ceramics with a high dielectric constant, low loss, and flat capacitance temperature characteristics. The Ca-doped ultra-thin dielectric layer compatible with nickel electrode can realize the nickel electrode multilayer capacitors of large capacitance and large volumetric efficiency, which is competitive with the Ta and Al-electrolytic capacitors and as reliable as conventional precious metal electrode multilayer capacitors.

Dielectric Properties of Fine-Grained BaTiO3 Ceramics Doped with CaO

Fine-grained dielectric ceramics, i.e., of 0.2 µm grain diameter, were obtained using Ca-doped BaTiO3 powders consisting of particles 0.2 µm in diameter. The (Ba1-xCax)mTiO3 ceramics (x: 0–0.10, m: 1.003–1.009) showed a high dielectric constant and stable temperature dependence, which were provided by interfacial stress due to the grain boundary, even though the ceramics has no core-shell structure. The failure time under an accelerated life condition was remarkably improved with Ca doping, which may be caused by the decrease in BaTiO3 unit volume with Ca2+ at the Ba2+ lattice site, and Ca2+ acceptor ions at the Ti4+ site.

Sintering and dielectric properties of fine-grained BaTiO3 ceramics

A new sintering method, spark plasma sintering (SPS), is described for the sintering of fine-grained BaTiO3 ceramics. Dense ceramics with fine grain size of near 170 run are obtained using SPS at low temperature of 900°C without holding time. The dielectric measurements of various grain size specimens show that the dielectric constants decrease with the reduce of grain size, and diffuse phase transition showed in the dielectric-temperature spectra. The variation in dielectric properties with grain size is explained.

Effects of Porosity on Dielectric Properties of BaTiO3 Ceramics

We investigate the porosity dependence of the ferroelectric domain density and the internal stress of fine-grained (∼1 µ m) BaTiO3 ceramics using transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The ferroelectric domain density and the internal stress exerted by the neighboring grains of the fine-grained BaTiO3 were examined to elucidate the effect of porosity on the dielectric properties of BaTiO3 ceramics.