Nanocrystalline BaTiO3 Ceramics Research Articles

Single step densification of high permittivity BaTiO3 ceramics at 300 ºC

Dense nanocrystalline BaTiO3 ceramics are prepared in a single step by the Cold Sintering Process at 300 °C, under a uniaxial pressure of 520 MPa for 12 h using a molten hydroxide flux. Transmission electron microscopy reveals a dense microstructure with sharp grain boundaries. The average grain sizes are 75−150 nm depending on the flux amount. The dielectric permittivity is 700–1800 at room temperature at 106 Hz, with a dielectric loss, tan δ ∼ 0.04. The difference in permittivity and phase transition behavior are explained in terms of the intrinsic size effect of the BaTiO3. The nanocrystalline BaTiO3 ceramics still shows a macroscopic ferroelectric switching via a hysteresis loop. This work demonstrates that cold-sintering process could enable the densification of ferroelectric oxides in a single step. Futhermore, comparable dielectric properties to reported values for nanocrystalline grains are obtained, but at this time, with the lowest processing temperatures ever used.

Molten Salt Synthesis and Structural Characterization of BaTiO3 Nanocrystal Ceramics

A new synthesis route to obtain high-purity barium titanate powder, BaTiO3, using the molten salt method by reacting the raw materials (BaCO3 and TiO2) in an atmosphere of molten NaCl and KCl, has been developed. The synthesized BaTiO3 ceramic particles have been successfully carried out at the sintering temperature 950°C for 4 hours. The Rietveld refinement of the XRD diffraction patterns was employed to characterize the structural information of the nanocrystalline BaTiO3 ceramics. The lattice parameters (a=4.0043 Å, b=4.0308Å with space group P4mm) of tetragonal perovskite structure, as an indication of piezoelectric characteristics, have been successfully determined by the Rietveld refinement. While the crystallitte particle size and strains have been obtained for the values of 110.6 nm and 0.74 % respectively

High dielectric constant associated with the strain-induced phase transition of an ordered assembly of BaTiO3 nanocubes under three-dimensional clamping

Numerical calculations of the Gibbs function for an ordered assembly of BaTiO3 nanocubes (nanocrystals) under three-dimensional clamping have revealed that the phase transition from a tetragonal to cubic crystal structure can take place at room temperature at some misfit strain associated with a tilt angle of the attached nanocubes. The phase transition is second-order due to the three-dimensional clamping. Near and at the phase transition, the dielectric constant becomes extremely high owing to the second-order transition. This is considered to be the reason for the high dielectric constant of an assembly at room temperature, which has been experimentally observed. An ordered assembly of BaTiO3 nanocubes under rigid three-dimensional clamping is a completely different system from the normal nanocrystalline (polycrystalline) BaTiO3 ceramics under elastic clamping and from a BaTiO3 epitaxial thin film under two-dimensional clamping.

Defect Chemistry of Perovskite Titanates: From Bulk Materials to Nanostructured Materials

To describe the electrical conductivities of perovskite titanates (e.g. SrTiO3 and BaTiO3) and the relation to parameters like temperature, oxygen partial pressure or acceptor- or donor-type dopants, the defect chemistry has been extensively studied since the late 1960s. The studies finally lead to the formulation of the point defect chemistry on the basis of defect chemical reactions, exchange processes with the atmosphere and changes of the valence state regarding their individual thermal activation and thermo­dynamic equilibriums. Comparing with microstructured counterparts, nanostructured materials have significantly higher interface (surface and/or grain boundary) to bulk fraction, which leads to some unique effects in nanostructured perovskite titanates. One effect is that the space charge layer becomes increasingly important. Owing to the high surface-to-volume ratio, the surface space charge layer plays an important role in thin films. For example, the p-type conductivity of the epitaxial SrTiO3 thin films decreases, and the n-type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the space charge layer. Moreover, the defect thermodynamics at nanometer-scale is different. E.g. the oxidation enthalpy of nanocrystalline BaTiO3 ceramics is found to be about one-third of that of microcrystalline BaTiO3. As a result, the p-type electrical conductivity of nanocrystalline BaTiO3 is about 2 orders of magnitude higher than that of microcrystalline BaTiO3.

Complex diffusion behavior of oxygen in nanocrystalline BaTiO3 ceramics

(18)O/(16)O exchange annealing and subsequent Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis is used to investigate oxygen transport in dense, nanocrystalline (average grain size d ≈ 300 nm) ceramics of nominally un-doped BaTiO3. Isotope penetration profiles are obtained as a function of temperature, 973 < T/K < 1173, at an oxygen activity aO2 = 0.20 and as a function of oxygen activity, 0.002 < aO2 < 0.20, at T = 1073 K. All isotope profiles show the same unusual shape: a flattened profile over the first ∼10(2) nm, followed by a short, conventional diffusion profile. We demonstrate that the entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation based on an increase in the local oxygen diffusion coefficient close to the surface. This position-dependent increase is attributed to additional oxygen vacancies that are generated by diffusion of chlorine impurities out of the ceramics. The presence of chlorine derives from the chemical route necessary to produce nanometric powders: it thus indicates a new manner in which nanocrystalline ceramics may differ from their microcrystalline counterparts.

Grain size effect on structures and dielectric constants for nanocrystalline BaTiO3 ceramic

Size effect on the crystalline phase contents has been investigated for BaTiO3 ceramics and varied from 100 to 8 nm by means of X-ray diffraction and Raman spectroscopy techniques. Mixed phases of tetragonal and orthorhombic at room temperature were corroborated by Raman spectroscopy. Rietveld refinement for X-ray diffraction data revealed that a locally orthorhombic structure preceded the globally tetragonal form with the grain decreasing, and a modified brick wall model was used to predict the dielectric constant between 100 and 8 nm, which demonstrated that grain boundary effects, coexisted structures took the dominant role in determining the ultrafine nanocrystalline BaTiO3 ceramics dielectric constant.

Effect of Grain Size on Phase Transitions and Dielectric Properties of Nano-Crystalline Barium Titanate Ceramics

Barium titanate (BaTiO3) ceramics with grain size varied from 1000 to 8 nm were prepared by two step sintering method (TSS) and spark plasma sintering (SPS), respectively. Mixture structures of BaTiO3 ceramics were proved by in-situ temperature high resolution x-ray diffraction. Multiple ferroelectric domains present in nano-crystalline BaTiO3 ceramics were observed by transmission electron microscope. The evolution of phase transitions supported the existence of intrinsic mechanism. Dielectric loss of fine grain size BaTiO3 was higher than coarse grain size during Curie phase transition due to diffuse phase transition and grain boundary effects.

The Changes of Hysteresis for Nanocrystalline BaTiO&lt;sub&gt;3&lt;/sub&gt; Ceramics at Different Impressing Rate of Voltage

For nanocrystalline BaTiO3 ceramics, the ferroelectric P-E hysteresis loop was measured with different impressing rate of field. The results are interesting for different grain size samples. Based on the combination of the theory of Avrami mode and the reorientation of polarization vector, the experimental results are analyzed. It is shown that the reorientation of polarization vector is the chief mechanism of hysteresis for large grain and the formation of new domain is important for samples of small grain size.

Grain-size effects on thermal properties of BaTiO3 ceramics

Dense nanocrystalline BaTiO3 ceramics are successfully prepared by the high pressure assisted sintering. Microstructures are observed by scanning electronic microscopes. The grain sizes are estimated to be about 30 and 150 nm. In comparison, BaTiO3 ceramics with the grain size of 600 nm and 1.5 μm are fabricated by conventional pressure-less sintering. The thermal properties of BaTiO3 ceramics with different grain sizes are investigated by differential scanning calorimetry and thermal expansion. The results suggest that the enthalpy values for the tetragonal-cubic transition decreased and the thermal expansion values increased with decreasing grain size. Furthermore, the Curie temperature shifts to lower temperature with decreasing grain size.

Preparation of Hierarchically Porous Nanocrystalline CaTiO3, SrTiO3 and BaTiO3 Perovskite Monoliths

Hierarchically porous nanocrystalline CaTiO3, SrTiO3, and BaTiO3 ceramics have been produced by impregnating corresponding alkaline‐earth metal ions into preformed macroporous TiO2 monoliths in a solution containing urea, followed by calcination. The macroporous TiO2 had been obtained via the sol–gel process accompanied by phase separation utilizing a chelating agent, ethyl acetylacetonate (EtAcAc), together with mineral salt and ammonium chloride, to decrease the reactivity of titanium alkoxide. Formations of CaCO3, SrCO3, and BaCO3 on the surface of TiO2 monoliths are promoted by CO2 generated by the concurrent two processes during impregnation; hydrolysis, and decarbonation of the chelating agent EtAcAc, and hydrolysis of urea at 60°C. The latter also raises pH of the impregnating solution which further promotes the mineralization of the carbonate salts. Calcination of the resultant monolithic composite of metal carbonate/TiO2 allows the crystallization of metal titanate. The addition of urea to the impregnating solution is found to be an effective strategy for the formation of perovskite monoliths by the impregnation process. This study provides a versatile approach to the preparation of hierarchically porous titania‐based perovskites.

Simultaneous Observation of Nanocrystalline BaTiO&lt;sub&gt;3&lt;/sub&gt; Ceramics Surface Morphology and Ferroelectricity Using Scanning Nonlinear Dielectric Microscopy

A scanning nonlinear dielectric microscopy (SNDM) with an additional electrically conducting atomic force microscope cantilever as a probe needle is adopted to simultaneously observe the surface topographic and domain images of nanocrystalline BaTiO3 ceramics with super-high resolution. The sample exhibits a uniform grain size distribution and the average grain sizes are calculated to be about 30 nm. Some regions are brighter than the others in SNDM image, indicating the existence of ferroelectric domains structure. In addition, P-E hysteresis and piezoresponse loops are found in nanocrystalline BaTiO3 ceramics. The experimental results demonstrate that the SNDM with the function of atomic force microscopy is very useful for understanding domain structures of nanocrystalline ferroelectric materials.

Ferroelectricity of nanocrystalline BaTiO3 ceramics by first principle calculation

The vibrational modes of BaTiO3 in different crystalline phases have been investigated by means of density functional theory pseudo potential method. The evidence of tetragonal to orthorhombic and orthorhombic to rhombohedral phase transitions present for 8 nm BaTiO3 ceramics further demonstrates that the similar successive transitions in 8 nm BaTiO3 ceramics from rhombohedral to orthorhombic, tetragonal and cubic transitions are as those in coarse BaTiO3 ceramics.

Preparation of nanocrystalline BaTiO3 ceramics

The high-dense nanocrystalline BaTiO3 (BT) ceramics with grain size smaller than 100 nm have been successfully prepared by the two step sintering and the spark plasma sintering (SPS) process. The successive transitions in nanograin BT ceramics from rhombohedral to orthorhombic, tetragonal and cubic transitions, similar to those in coarse BT ceramics, were revealed by in-situ temperature dependent Raman spectrum. The multiphase coexistence and the diffused phase transition character were demonstrated in the 8 nm nanocrystalline BT ceramics.

Processing, Microstructure and Properties of Nanograin Barium Titanate Ceramics by Spark Plasma Sintering

Bulk dense nanocrystalline BaTiO3 ceramics ranging from 15 nm to 100 nm have been successfully prepared by the spark plasma sintering method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometre scale for BaTiO3 ceramics. Scanning nonlinear dielectric microscopy measurements revealed temperature-dependent variations in contrast, which were attributed to domain rearrangements in BaTiO3 ceramics below 100 nm. Furthermore the piezoresponse hysteresis loops showed that the nanocrystalline BaTiO3 ceramics were switchable and ferroelectricity was retained at the high temperature of 290 oC, demonstrating the existence of nano ferroelectric domains and the ferroelectric phase still retained above Curie temperature, which confirmed the diffused phase transition character in nanograin BaTiO3 ceramics. The dielectric data also show a ferroelectric to paraelectric phase transition in nanograin BaTiO3 ceramics.

Ferroelectric behaviour of 30 nm BaTiO 3 ceramics prepared by high pressure assisted sintering

Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 °C, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the 'dilution' effect of grain boundaries.

Investigation of ferroelectric properties on nanocrystalline barium titanate ceramics

Dense nanocrystalline BaTiO3 (BT) ceramics with 50 nm average grain size obtained by spark plasma sintering were investigated. The dielectric data showed a ferropara phase transition with a maximum permittivity of 1,602 at 120°C and at 1 kHz. The polarization reversal characteristics were measured, and the typical piezoelectric hysteresis loop was recorded. The asymmetry of permanent polarization and coercive field in polarization-electric field curves were obtained and attributed to the existence of an internal electric field. The present results provide experimental evidence, indicating that if a critical grain size exists for ferroelectricity, then it is less than 50 nm for polycrystalline BT ceramics.

Grain-size effects on the hardness of nanograin BaTiO3 ceramics

Dense nanocrystalline BaTiO3 (BT) ceramics with grain size (GS) 50 nm were successfully prepared by spark plasma sintering (SPS) method. The nanoindentation experiment was used to test the hardness of different GS BT ceramics. It was found that the hardness of 50 nm nanograin BT ceramics increased 82% than that of 1.2 μm BT ceramics fabricated by conventional sintering (CS) process. Dislocation pinning resulting from the increase of grain boundary by ultrafine GS are believed to be the dominant factor in raising strength. Restriction on dislocation generation and mobility due to the domain wall motion and sliding imposed by ultrafine GS are expected to be exceptionally important. At the same time, the large stress in nanograin BT ceramics may also play a role in producing high strength. These differences of the hardness behavior between BT ceramics are attributed to the variation of the resultant microstructure, especially the GS of the ceramics.

Bulk dense nanocrystalline BaTiO3 ceramics prepared by novel pressureless two-step sintering method

In this paper, we investigate preparation of bulk dense nanocrystalline BaTiO3 ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 73 to above 99.6%. Isothermal and constant grain size sintering have been carried out to high density in pure BaTiO3 system. The kinetics of this method is also discussed, which exploits the different kinetics between densification diffusion and grain boundary mobility. Using this method, bulk dense ceramics with a grain size of 8–10 nm was obtained successfully at a very low sintering temperature.

First Principle Study on Electronic Structure of Nanocrystalline BaTiO&lt;sub&gt;3&lt;/sub&gt; Ceramics

The full potential linearized augmented plane wave method within the generalized gradient approximation was used to calculate electronic structure of nanocrystalline BaTiO3 ceramics. We calculated the total and partial density of states of 50 nm BaTiO3 ceramics. The results show that the atoms distribution of nanograin BaTiO3 ceramics is different from those of coarse BaTiO3 ceramics. It is also revealed that the hybridization between Ti 3d and O 2p is very strong, which is very important to the ferroelectric stability of nanocrystalline BaTiO3 ceramics.

Effect of Annealing on Ferroelectric Properties of NanometreBaTiO3 Ceramics Prepared by High Pressure Sintering Method

Dense nanocrystalline BaTiO3 ceramics with a grain size of 50 nmare prepared under 6 GPa at 1273 K using a high pressure sinteringmethod. The sintered bulk is uniform and the relative density is above97%. We anneal the ceramic samples in oxygen with various temperaturesand for different times without apparent grain growth. After theannealing, several broadened peaks can be observed at about 378 K bydielectric measurements. However, these peaks are very different fromthose of coarser-grained ceramics. It is indicated that both theelimination of oxygen vacancies and the release of residual stressescaused by high pressure greatly improve the overall ferroelectricproperties of BaTiO3 ceramics. The observation of nearly linearpolarization hysteresis loop after anneal provides the solid evidenceof ferroelectricity in these nano-sized BaTiO3 ceramics. It isbelieved that the absence of 90° domains and the existence ofpoor-permittivity nonferroelectric grain boundaries contribute to theslim loop.