Dense BaTiO3 Ceramics Research Articles

A new strategy to achieve the controllable preparation of nanoceramics with BaTiO3@resin core–shell nanoparticles

In this paper, phenolic resin (PR) is coated on the monodispersed BaTiO3 nanopowders (C-BTO) prepared by the co-precipitation method via a core–shell manner. This PR can optimize nanopowders’ dispersibility in the granulation, and act as a grain growth inhibitor during sintering. So BaTiO3 nanoceramics with dense and size-controlled morphology were achieved at a low sintering temperature. The microstructural and dielectric studies show that the grain size and dielectric properties initially increase and then decrease as the amount of PR increases. Moreover, the activation of PR is optimal at 20 wt%, and obtained ceramics have a high density and uniform grain size, with a relatively high dielectric constant (ɛmax) of 3545 and low dielectric loss (tanδ) of 0.011. Besides, by tailoring the particle size of C-BTO@PR powders, dense BaTiO3 ceramics with mean size of 135–308 nm can be obtained with sizes similar to initial particle. The results demonstrate that PR coated BaTiO3 nanopowders in core–shell structure is an effective strategy to prepare BaTiO3 nanoceramics controllably.

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.

Unfolding grain size effects in barium titanate ferroelectric ceramics.

Grain size effects on the physical properties of polycrystalline ferroelectrics have been extensively studied for decades; however there are still major controversies regarding the dependence of the piezoelectric and ferroelectric properties on the grain size. Dense BaTiO3 ceramics with different grain sizes were fabricated by either conventional sintering or spark plasma sintering using micro- and nano-sized powders. The results show that the grain size effect on the dielectric permittivity is nearly independent of the sintering method and starting powder used. A peak in the permittivity is observed in all the ceramics with a grain size near 1 μm and can be attributed to a maximum domain wall density and mobility. The piezoelectric coefficient d33 and remnant polarization Pr show diverse grain size effects depending on the particle size of the starting powder and sintering temperature. This suggests that besides domain wall density, other factors such as back fields and point defects, which influence the domain wall mobility, could be responsible for the different grain size dependence observed in the dielectric and piezoelectric/ferroelectric properties. In cases where point defects are not the dominant contributor, the piezoelectric constant d33 and the remnant polarization Pr increase with increasing grain size.

Impedance and modulus spectroscopy characterization of lead free barium titanate ferroelectric ceramics

In this work, dense BaTiO3 ceramics were successfully prepared by the solid state reaction process. X-ray diffraction technique showed single-phase polycrystalline sample with perovskite structure. Dielectric behavior and the impedance relaxation were investigated in a wide range of temperature (room temperature (RT) – 40°C) and frequency (1kHz–1MHz). A broad dielectric constant peak was observed over a wide temperature range around the phase transition temperature. The complex impedance plot exhibited two impedance semicircles identified over the frequency range of 1kHz–1MHz, which is explained by the grain and grain boundary effects. The presence of non-Debye type of relaxation has been confirmed by the complex impedance analysis. The centers of the impedance semicircles lie below the real axis, which indicates that the impedance response is a Cole–Cole type relaxation.

Influence of nanogold additives on phase formation, microstructure and dielectric properties of perovskite BaTiO3 ceramics

The formation of perovskite phase, microstructure and dielectric properties of nanogold-modified barium titanate (BaTiO3) ceramics was examined as a function of gold nanoparticle contents by employing a combination of X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, Archimedes principle and dielectric measurement techniques. These ceramics were fabricated from a simple mixed-oxide method. The amount of gold nanoparticles was found to be one of the key factors controlling densification, grain growth and dielectric response in BaTiO3 ceramics. It was found that under suitable amount of nanogold addition (4 mol%), highly dense perovskite BaTiO3 ceramics with homogeneous microstructures of refined grains (~0.5–3.1 μm) and excellence dielectric properties can be produced.

Phase evolutions and electric properties of BaTiO3 ceramics by a low-temperature sintering process

BaTiO3 (BT) nanoparticles were synthesized by a modified polymeric precursor method in a weak acid solution. The synthesized process of BT precursor with increasing calcination temperature was investigated through thermal analysis (DTA/TG), X-ray diffraction, transmission electron microscope and Fourier-transform infrared spectroscopy. Good dispersive and homogeneous cubic BT nanoparticles were calcined at 800 °C, whereas dense BT ceramics were sintered at ~1,160 °C. The present results showed that the dielectric, piezoelectric and ferroelectric properties of BT ceramics were dependent on the ceramics densification and crystallographic structure. The excellent electric properties (P r = 10.5 μC/cm2, d 33 = 217 pC/N, k p = 0.32 %) were found at a sintering temperature of 1,160 °C, which was due to the coexistence of tetragonal and orthorhombic phase. The depressed electric properties at higher sintering temperature were associated to oxygen vacancies and impurity phases. In addition, phase evolutions of BT nanoparticles and ceramics were all stated in detail.

Strong piezoelectricity exhibited by large-grained BaTiO3 ceramics

A good understanding of the strong piezoelectric mechanism of BaTiO3 ceramics is very important from both scientific and technological viewpoints. This Letter reports an unusual piezoelectric phenomenon observed in a group of large-grained dense BaTiO3 ceramics with average grain sizes ranging from 5.1 to 16.3 μm, which are prepared with a hydrothermally synthesized BaTiO3 fine powder by hot-press sintering. Among these BaTiO3 ceramics, the one possessing an average grain size of 5.6 μm exhibits a particularly prominent piezoelectricity with piezoelectric constants d33 = 485 pC/N and d33* = 725 pm/V at room temperature and a peak d33 value of 574 pC/N around 8.5 °C. This ceramic is further investigated from the aspects of domain patterns and polarization versus electric field (P-E) loops and compared with a conventionally sintered BaTiO3 ceramic prepared using a solid-state reaction method. The investigation shows that it has the domain patterns of simple parallel stripes inside the grains and the square-like P-E loops and differs significantly from the conventionally sintered ceramic, and indicates that the observed strong piezoelectricity is strongly related to the large remanent polarization.

Insulator-to-semiconductor transition of nanocrystalline BaTiO3 at temperatures ≤200 °C.

As a classic dielectric material, BaTiO3 is one of the most important materials used in electronic applications. In this work, highly dense BaTiO3 ceramics with an average grain size of 35 nm were prepared, and dielectric and electrical properties were investigated. Microcrystalline BaTiO3 is an insulator at low temperatures; however, nanocrystalline BaTiO3 shows considerable semiconductivity with an activation energy of only 0.27 eV at temperatures ≤200 °C. At room temperature, the conductivity of nanocrystalline BaTiO3 is about fourteen orders of magnitude higher than that of the microcrystalline counterpart. Only by decreasing the grain size, one can transform BaTiO3 from an insulator to a semiconductor.

Preparation of Barium Titanate Porous Ceramics and their Piezoelectric Power Generation Property

Barium titanate (BaTiO3) porous ceramics were prepared by conventional sintering method, and their dielectric and piezoelectric properties were measured using 31-resonators. With decreasing sintering temperature, dielectric constant showed a maximum of 5500 at 1300 °C, while piezoelectric constant and elastic compliance increased. These resonators were developed to unimorph-type vibrators and their instantaneous electric powers were measured. As the results, the maximum electric power of 129 μW was measured for the BaTiO3porous ceramics sintered at 1200 °C, and this value was 20 times greater than that for dense BaTiO3ceramics.

Peculiar size effect in nanocrystalline BaTiO3

Dense BaTiO3 ceramics with grain sizes of 35nm to 5.6μm were prepared, and the electrical properties investigated in the temperature range 500–700°C by means of impedance spectroscopy. Charge carriers (oxygen vacancies and holes) are depleted in the space charge regions at the BaTiO3 grain boundaries. When the grain size is ⩾250nm, the width of the space charge region was determined to be ∼40nm. Therefore, the depletion regions were expected to overlap when the grain size decreases to 35nm; in such a situation, charge carriers would be depleted over the entire grain, resulting in depressed conductivity. However, the conductivity of the 35nm grain size sample was measured to be one to two orders of magnitude higher than those of the microcrystalline samples, and the activation energy markedly lower. Moreover, we determined a width of ∼7nm for the space charge regions in the 35nm grain size sample; therefore, the space charge regions do not overlap. The enhanced conductivity is ascribed to a reduced oxidation enthalpy in nanocrystalline BaTiO3, and the distorted grain boundaries in nanocrystalline BaTiO3 are believed to be the atomic level origin of the reduced oxidation enthalpy.

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.

Grain size effect on the nonlinear dielectric properties of barium titanate ceramics

The nonlinear dielectric properties of dense BaTiO3 ceramics with grain size of 1 μm–90 nm were investigated. In the finest ceramics, the permittivity reduces below 1000 and a remarkable nonhysteretic linear dc-tunability [ε(E)] is obtained at high field, above 40 kV/cm. The observed behavior was explained by considering the nanostructured ceramic as a composite formed by ferroelectric grains, whose nonlinearity is reducing, and by low-permittivity nonferroelectric grain boundaries, whose volume fraction increases when decreasing the grain size. Reducing the grain size in ferroelectric dense materials is an alternative route to accomplish the application requirements: nonhysteretic tunability and permittivity below 1000.

Ferroelectric and Optic Properties of the Translucent BaTiO3 Ceramics Derived from Nanocrystalline Monolith

ABSTRACTPolycrystalline, translucent barium titanate (BaTiO3) ceramics were successfully fabricated from partially crystallized, transparent monolithic xerogels produced by a high concentration sol-gel method using metal alkoxides. The xerogels yielded dense and translucent BaTiO3 ceramics after heat treatment at 1100°C for 1 h in oxygen flow, with an average grain size of about 0.5 μ m and a relative sintered density of 97%. The transmittance of a ceramic (0.2 mm thick) was observed to increase almost linearly with wavelength of light, showing a transmittance of 32% at 800 nm. The dielectric constant of the ceramics at room temperature was found to be 1500, and the remanent polarization and coercive field were 2 μ C/cm2 and 5 kV/cm, respectively. The variation of transmittance for a ceramic as a function of applied electric field to examine the material's electro-optic property was also measured.

Ferroelectric properties of dense nanocrystallineBaTiO3 ceramics

Dense BaTiO3 ceramics with 50 nm average grain size obtained by spark plasma sintering wereinvestigated. The dielectric data show a broad ferro–para phase transition with a maximumpermittivity of at 390 K and 1 kHz. The local ferroelectric switching behaviour was investigated bypiezoresponse force microscopy. Typical piezoelectric hysteresis loops were recordedat different positions of the sample. The present results provide experimentalevidence for polarization switching at the local scale, indicating that the criticalgrain size for the disappearance of ferroelectric behaviour in dense, bulkBaTiO3 nanocrystalline ceramics is below 50 nm.

Local switching properties of dense nanocrystalline BaTiO3 ceramics

The switching properties of dense BaTiO3 ceramics with 50 nm average grain size were investigated at local scale by piezoresponse force microscopy. Large areas with low piezoelectrical activity beside islands with strong piezoresponse were found. The application of electric fields induces stable domain structures and changes in the polarization state far away from the probing area, probably via trans-granular dipole interactions. Piezoelectric hysteresis loops were recorded on various positions, even in regions with initial zero piezoresponse, which possibly showed a superparaelectric behavior. The results are incontestable proof that 50 nm BaTiO3 ceramics retain ferroelectricity at a local scale.

Preparation of Fine-grained BaTiO3Ceramics by Spark Plasma Sintering

Dense BaTiO3ceramics consisting of fine grains were prepared using fine powder (average grain size of 0.06 μm; BT006) as a starting material and the spark plasma sintering (SPS) method. The powder was densified to >95% of theoretical x-ray density by the SPS process, and the average grain size of the resulting ceramics was <0.5 μm; the particle size of the initial powder significantly affects the grain size of the resulting SPS pellets. Fixed-frequency (100 kHz), room-temperature permittivity measurements of the BT006-SPS ceramics showed relatively low values (3000–3500) compared with those (typically 5000) for SPS ceramics consisting of larger grains (approximately 1 μm). Lower permittivity was attributed to poor development of ferroelectric domains in the ceramics, which originated from incomplete development of the tetragonal structure as well as the presence of a local orthorhombic structure.

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense BaTiO3 ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO3 (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X‐ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000‐1200°C, because of the short sintering period (5 min). Cubic‐phase BaTiO3 coexisted with tetragonal BaTiO3 at room temperature in the SPS pellets, even when well‐defined tetragonal‐phase BaTiO3 powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent100‐106 Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.

ChemInform Abstract: Preparation of Dense BaTiO3 Ceramics from Sol‐Gel Derived Monolithic Gels.

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Preparation of Dense BaTiO3 Ceramics from Sol‐Gel‐Derived Monolithic Gels

Dense, small‐grained BaTiO3 ceramics, with a grain size around 1 μm and a relative sintered density >98%, were obtained at 1100°C from sol‐gel‐derived gel monoliths without using any sintering additives. The monolithic gels asprepared had a relative density of about 50% and consisted of ultrafine pseudo‐cubic BaTiO3 particles (<50 nm). These gels, with a significantly high density compared with that of previous ones (∼30%), have been synthesized at room temperature from a sol solution with a concentration of equimolar mixture of titanium isopropoxide and barium ethoxide (0.8 mol/L), using the methanol/2‐methoxyethanol mixed‐solvent system. Microstructural development of the gel monoliths with increasing sintering temperature and the dielectric properties of the obtained dense BaTiO3 ceramic have been investigated.

Microstructure and Dielectric Properties of BaTiO<sub>3</sub> Prepared by Colloidal Processing

An ultrafine BaTiO3 powder of 80 nm diameter was consolidated to powder compacts with 58% TD and uniform structures by filtration of 2-methoxyethanol, 2-ethoxyethanol or methanol suspension and subsequent isostatic pressing under a pressure of 294MPa. These compacts were sintered into dense BaTiO3 ceramics with average grain sizes of 1-2μm (>95% TD) within 2h at 1200-1250°C. Increasing the heating rate to the sintering temperatures of 1000-1300°C was effective in increasing the density. Grain growth from 0.5 to 2μm caused significant increase of the c-parameter of BaTiO3, resulting in the increase of tetragonality (c/a ratio). The phase transition from tetragonal to cubic form at room temperature was estimated to occur below 0.4μm average grain size. The relative dielectric constant (e′) at room temperature was highest (≈6000 at 1kHz) in the high-density BaTiO3 with an average grain size of 1.4μm, sintered at 1200°C. Increase of the grain size to 2.3μm in dense BaTiO3 caused the decrease of the e′ value. The low-density and small-grained (0.5μm) BaTiO3 also showed a low e′ value (1400 at 1kHz). The e′ values of all samples were almost independent of frequency in the range of 100Hz to 500kHz. The dielectric loss was strongly affected by the porosity and decreased at higher density. The decrease of grain size reduced the e′ values below and above the Curie temperature, leading to the broadening of e′ in a wide temperature range (18-220°C).