Finite size effects in submicron barium titanate particles

An experimental and theoretical analysis of finite size effects in submicron barium titanate particles is presented. The dielectric data show that the dielectric constant (ε = 734) of small particles (d = 20 nm) remains very similar to that of single crystals provided that the particles are grown from an amorphous phase and not mechanically crushed into powder. This shows that what has in the past been mistaken for true size effects (i.e., depolarization fields) is due to lattice strain. The X-ray data show that the Curie temperature (cubic-tetragonal transition temperature) decreases monotonically to zero degree Kelvin as particle size is reduced. However this is shown to be a simple function of lattice constant: The lattice constant expands as the particle diameter d contracts. This is similar to surface relaxation in thin films.     

Calculations of finite size effects in barium titanate

The polarisation and permittivity of barium titanate are calculated as a function of temperature from Landau-Ginzburg-Devonshire theory, taking into account effects due to the finite volume of coherently polarising regions. For regions of less than 10 nm in diameter, in which spatially uniform thermal fluctuations of the polarisation become significant, there are marked departures from the single crystal behaviour, particularly in the vicinity of the ferroelectric phase transitions. The behaviour is similar to aspects of that reported for fine-grained ceramics and thin films, suggesting that limited coherence of the magnitude of polarisation is responsible for a reduction in the permittivity and the appearance of diffuseness of the ferroelectric phase transitions.     

Effect of vibro-milling time on phase formation and particle size of barium titanate nanopowders

Barium titanate (BT) nanopowder was synthesized by a solid state reaction via a rapid vibro-milling technique. The effect of milling time on phase formation and particle size of BT powder was investigated. Powder samples were characterized using XRD (X-ray diffraction) and SEM techniques. It was found that the resulting BT powders have a range of particle size depending on milling times. Production of a single-phase BT nanopowder can be successfully achieved by employing a combination of 30 h milling time and calcination conditions of 1200 °C for 2 h.     

粉体的燃烧合成技术及其在钛酸钡粉体制备中的应用

综述了燃烧法合成陶瓷粉体的原理、方法及其在钛酸钡粉体制备中的应用 ,比较了各种燃烧法合成钛酸钡粉体工艺的优缺点 ,并指出目前经济、有效地制备钛酸钡粉体的方法是以有机燃料和金属硝酸盐为原料的低温燃烧合成法。     

钛酸钡纳米复合材料制备与性能研究

对钛酸钡颗粒进行羟基化和巯基官能化处理,将纳米银粒子成功接枝到钛酸钡颗粒表面制备了x Ag@BT颗粒(x为银质量分数,BT为钛酸钡),并用溶液共混法制备了Ag@BT/PVDF纳米复合材料(PVDF为聚偏氟乙烯)。结果表明,x Ag@BT颗粒为具有草莓结构的纳米颗粒,尺寸为2~16 nm的银粒子成功装饰在钛酸钡颗粒表面;纳米银粒子的引入可以降低复合材料在低频下的介电损耗和电导率,且当纳米银粒子质量分数为1.0%时复合材料的特征击穿强度达到最大值;与传统BT/PVDF纳米复合材料相比,1.0%Ag@BT/PVDF纳米复合材料具有更优良的电性能。     

Non-destructive three-dimensional Raman analysis of domain orientation in barium titanate single-crystal

Ferroelectric domain orientation in barium titanate (BaTiO₃; BT) single-crystal has been investigated with respect to its geometrical distribution in the solid angle by using polarised Raman spectroscopy. After retrieving the angular dependence of the intensity of selected Raman modes of BT and their defocusing properties at selected laser wavelength (in-depth probe response function), both in-plane and out-of-plane domain fractions could be visualised and mapped with microscopic resolution by Raman spectroscopic assessments. It is demonstrated that polarised Raman spectroscopy is a valuable and efficient tool for fully 3D, non-destructive assessments of domain orientation in ferroelectrics.     

In-situ observation of needle domain evolution in barium titanate single crystals

We report in-situ optical observations of the ferroelastic switching in a barium titanate single crystal under compressive stress. Optical micrographs were captured in two regions showing distinct arrangements of domains. Coarsely spaced needle domains in a matrix consisting of a differently oriented large single domain were found to retreat under the application of compressive stress of around 1–2 MPa. However, a comb of closely spaced needle domains was found to be more stable, retreating only slightly under a similar magnitude of applied load. The observations show that the pattern of needle domains influences the ferroelastic switching process and the observed coercive stress can depend strongly on domain arrangement.     

Engineering grain size and electrical properties of donor-doped barium titanate ceramics

A semiconducting lanthanum-doped barium titanate ceramic has been fabricated for battery safety applications by simple means from nanoparticles prepared at room temperature by kinetically controlled vapor diffusion catalysis. The material, characterized by electron microscopy, X-ray diffraction and electrical measurements, exhibits a difficult to achieve combination of submicron grain size (∼500 nm) and attractive electrical properties of room temperature resistivity below 100 Ω cm and a 12-fold increase in resistivity through the Curie temperature (positive thermal coefficient of resistivity, PTCR). Systematic investigation of sintering conditions revealed that a short period of heating at 1350 °C under air is necessary to suppress abnormal grain growth, while precise control of the cooling rate is needed to achieve the targeted electrical properties. Cooling must be sufficiently fast to avoid complete back-oxidation, yet slow enough to facilitate oxygen adsorption at the grain boundaries to produce the thin oxide layer apparently responsible for the observed PTCR.     

Effect of mechanical stress on phase stability and polarization states in ferroelectric barium titanate and lead titanate

The effect of normal and shear stress on phase transitions in BaTiO₃ and PbTiO₃ has been investigated using a modified Landau–Ginzburg–Devonshire phenomenological model based on assumption of constant stress boundary conditions. Stress–temperature phase diagrams have been developed, and the influence of stress on polarization stability has been analyzed. The results show monoclinic phases with various polarization states absent in stress-free BaTiO₃ may exist under uniaxial, biaxial, anisotropic three-dimensional, and shear stress conditions. For PbTiO₃, our calculations show that, under normal stress new phases cannot be generated and the only stable ferroelectric phase has tetragonal symmetry, but under shear stress orthorhombic, rhombohedral, and monoclinic phases can be stabilized.     

Dielectric and structural studies of ferroelectric phase evolution in dipole-pair substituted barium titanate ceramics

Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ ceramics with x equal to 0, 0.0025, 0.005, 0.01, 0.025, and 0.05 have been prepared by conventional solid-state reaction. Structural and dielectric characterization have been performed to investigate the effect of dipole-pair substitution concentration on the macroscopic dielectric properties. Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ evolves from a classic ferroelectric to a diffuse phase transition (DPT) as x increases. Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ for x ≥ 0.01 possesses diffuseness parameters comparable to Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) and recently reported (Ba_0.97Pr_0.03)(Ti_0.9425Ce_0.05)O₃ (BPTC), yet it lacks the frequency and temperature dependence of T_m necessary to be a strictly defined relaxor ferroelectric. Additionally, Ba{[Ga_0.05,Ta_0.05]Ti_0.9}O₃ possesses a relative permittivity, ɛ_r, of 700 ± 16% and dissipation factor less than 0.05 at 10 kHz within the temperature range [−75°C, 120°C]. In comparison to BaTiO₃, Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ possesses enhanced electrical resistivity at and above room temperature. In situ XRD, including Rietveld refinement, have been performed to determine the lattice parameter, coefficient of thermal expansion, and phase transition temperature (T_c) of each composition within the temperature range [RT, 1000°C], thus linking the dielectric properties with the material's structure. These studies have been corroborated by temperature-dependent Raman spectroscopy to compare the T_c determined by electrical and structural characterization. The properties of Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ are discussed in context with available models that describe donor and acceptor dopants spatially separated in the parent matrix, inter-relating lattice parameter, Curie temperature, and other material properties.     

Composition-driven broad phase boundary for optimizing properties and stability in lead-free barium titanate ceramics

A new piezoelectric system of (1−x−y)BaTiO₃-yCaZrO₃-xBaSnO₃ (BT-yCZ-xBS) was designed to achieve enhanced piezoelectric/strain properties and temperature stability. First, the relationships between composition, phase, and electrical properties are systematically investigated. The broad phase boundary with successive rhombohedral-orthorhombic (R-O) and orthorhombic-tetragonal (O-T) was obtained in 0.04 ≤ x ≤ 0.05 and 0.04 ≤ y ≤ 0.07 by tailoring the relationship of composition and phase structure, confirmed by X-ray diffraction, temperature-dependent dielectric constants, and Raman spectra. The optimized piezoelectric coefficient of d₃₃ = 560 pC/N, high strain of >0.20%, and large converse piezoelectric coefficient of d₃₃* = 1170 pm/V were realized. Second, the optimized piezoelectricity both demonstrate a stable performance with fluctuation <8% for d₃₃* and 20% for d₃₃ between 22 and 60°C since the broad phase boundary is exhibited in this temperature range. We believe that this work is a successful example to optimize piezoelectric properties and enhance the stability for piezoceramics.     

Grain size and grain boundary-related effects on the properties of nanocrystalline barium titanate ceramics

Dense nanocrystalline BaTiO₃ ceramics with grain size (GS) down to 50 nm were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), impedance spectroscopy and Raman spectroscopy. A continuous reduction of the tetragonal distortion towards the pseudo-cubic state was obtained when the GS was reduced. Therefore, even the finest structure (ceramic with average GS of 50 nm) is still non-centrosymmetric. The dielectric constant (K) shows relative thermal stability in a large range of temperatures and is strongly depressed in the nanocrystalline ceramics, in comparison with the micrometric ones (K being below 1000 for the ceramic with 50 nm GS). The losses are smaller than 5% in the frequency range of 10²–10⁶ Hz and temperatures below 200 °C. As the GS decreases, the structural phase transitions assume a more diffuse character. A decrease of the Curie temperature with reducing the GS was confirmed by X-ray, calorimetric and permittivity data. The Raman spectra collected for the range 80–800 K provided evidence for the presence of all the crystalline phases of BaTiO₃, as in single-crystal and micrometric ceramics; a few differences can be attributed to GS effects and to the high density of the non-ferroelectric grain boundaries. Evidence for the different phase transitions were provided by the disappearance of some bands and by anomalies in positions and intensities of selected Raman modes. The overall properties of the nanocrystalline BaTiO₃ ceramics can be explained as a combination of intrinsic effects, associated with the decrease of tetragonality and heat of transition with reducing GS, and extrinsic contributions due to the non-ferroelectric grain boundaries causing a “dilution” of the ferroelectric properties.     

Inversion domain network stabilization and spinel phase suppression in ZnO

The development of inversion domain networks consisting of basal‐plane and pyramidal‐plane inversion domain boundary (b‐IDB and p‐IDB) interfaces within grains in Sn‐Al dual‐doped ZnO (Zn_0.98Sn_0.01Al_0.01O) polycrystalline ceramics has been confirmed using transmission electron microscopy. The atomic structure of the b‐IDB and p‐IDB interfaces has been analyzed using atomic‐resolution scanning transmission electron microscopy. The localization of Sn and Al at the respective sites of the b‐IDBs and p‐IDBs was confirmed by energy‐dispersive X‐ray spectroscopy. In contrast to Sn or Al single‐dopant addition to ZnO, which results in the formation of spinel phase precipitates without the development of inversion domain networks, Sn‐Al dual‐doping caused the suppression of spinel phase formation and the formation of monophasic inversion domain networks composed of RMO₃(ZnO)_n homologous phase compound members, where R and M represent dopants substituting at the b‐IDB and p‐IDB sites, with a general formula of SnAlO₃(ZnO)_n. The results of this study demonstrate that the formation of inversion domain networks in ZnO‐based ceramics can be stabilized via multiple‐dopant addition. This finding has potential implications for the modification of the bulk or nanoscale properties based on the choice of the specific dopants, R and M, the control of the ratio R:M and the value of n in the RMO₃(ZnO)_n homologous phase compound members constituting the inversion domain networks.     

Doping influence on structural ferroelectric phase transitions and electrical features of barium calcium titanate

We comprehensively report Sr, Cr, Mn, and Mg doping ions' influence on structural and dielectric properties of barium calcium titanate (BCT) ceramics over a wide temperature and frequency range. X-ray diffraction and electrical studies showed the phase transition (PT) from paraelectric (PE) to ferroelectric (FE) phase. The linear and nonlinear susceptibility measurement identified the discontinuous PT of BCT and Sr-doped BCT. Moreover, this measurement also distinguished diffused PE-FE PT features induced in BCT by the Cr and Mn ions. Exceptionally, the Mg ion doping caused the relaxor-like features in BCT. Distribution of relaxation times indicated a mixed order-disorder and displacive character of PT. The doping of Cr, Mn, and Mg ions shortened the relaxation times related to the B-sublattice ions jumping. We also confirmed low-temperature FE-FE PT via linear and nonlinear dielectric susceptibility and pyroelectric measurements. We showed that non-zero net polarization could be induced by an applied electric field.     

A TEM and HREM study of particle formation during barium titanate synthesis in aqueous solution

The formation mechanisms of barium titanate particles from an amorphous TiO₂ gel during synthesis in aqueous solution at temperatures between 20 and 80°C have been investigated. It was found that barium ions diffuse into the gel almost immediately, with nanocrystalline BaTiO₃ particles being formed after heating to just 40°C. These particles grew to dimensions of about 100 nm as the temperature was increased to 80°C, consuming the remaining TiO₂ gel. Some remnants of gel were found on particle surfaces in a sample taken at this temperature but after holding the sol at 80°C for 2 or 4 h, the particle surfaces became “cleaner” and more rounded. It is proposed on the basis of these observations that the BaTiO₃ particles were formed by an in-situ transformation of the amorphous TiO₂ gel. The mechanism by which (i) the particles were then rounded off and (ii) the final gel fragments were incorporated into the main BaTiO₃ particles was, however, less clear.     

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO₃) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5 µm range were formed at temperatures above 80 °C. Also, the cube-shaped morphology of the BaTiO₃ product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO₃ proceeded alongside secondary BaTi₂O₅ and Ba₂TiO₄ phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO₃ as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO₃ crystallization by the Johnson-Mehl-Avrami method showed that BaTiO₃ crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.     

Influence of feedstock concentration on tetragonality and particle size of hydrothermally synthesized barium titanate powders

Barium titanate (BaTiO₃) powders were synthesized through hydrothermal process with Ba(OH)₂·8H₂O and TiO₂. By increasing the feedstock concentration (FC) from 0.25 to 1.50 M, BaTiO₃ powders maintain a stable average particle size (~180 nm and ~6.4441 m²/g) with an increasing tetragonality (c/a: 1.0065–1.0075). Johnson-Mehl-Avrami and standard reaction rate equations were adopted to analyze the kinetic process of BaTiO₃ formation. The reaction is governed by first-order and phase-boundary-controlled mechanism for 0.25 M and 1.50 M, respectively. Lower extent of reaction is believed to lead to the better tetragonality for BaTiO₃ powders fabricated with higher FC. On the other hand, the relative stable particle size is correlated with the unvaried nucleation frequency and grain growth rate with various FC. This work can provide a guideline to manipulate the properties of BaTiO₃ powders used in electronic industry.     

Effects of phase constitution and microstructure on energy storage properties of barium strontium titanate ceramics

Barium strontium titanate (Ba_0.3Sr_0.7TiO₃, BST) ceramics have been prepared by conventional sintering (CS) and spark plasma sintering (SPS). The effects of phase constitution and microstructure on dielectric properties, electrical breakdown process and energy storage properties of the BST ceramics were investigated. The X-ray diffraction analysis and dielectric properties measurements showed that the cubic and tetragonal phase coexisted in the SPS sample while the CS sample contained only tetragonal phase. Much smaller grain size, lower porosity, fewer defects and dislocation were observed in SPS samples, which greatly improved the electrical breakdown strength of the Ba_0.3Sr_0.7TiO₃ ceramics. The enhanced breakdown strength of the SPS samples resulted in an improved maximum electrical energy storage density of 1.13 J/cm³ which was twice as large as that of the CS sample (0.57 J/cm³). Meanwhile, the energy storage efficiency was improved from 69.3% to 86.8% by using spark plasma sintering.     

Combined shock-wave synthesis of noble spinel and laves cubic phase

The paper described the shock-wave synthesis of high-pressure high-temperature mineral MgAl₂O₄ with the parameter a = 8.085(3) Å and the Laves phase MgCu₂ with a = 7.076(2) Å from a mixture of MgO and aluminum powder placed into a copper insert inside a steel conservation ampoule. Spinel MgAl₂O₄ with a smaller lattice parameter a = 8.0798 Å has been formed in an aluminum insert cup. __________ Translated from Steklo i Keramika, No. 6, pp. 21–22, June, 2006.