Structure-Property Phase Diagram of BaZrxTi1−xO3 System

In the course of searching environmental friendly lead-free relaxor ferroelectrics a complete phase diagram of barium zirconate titanate, Ba(Zr _x Ti_{1− x} )O₃ system with compositions 0.00≤ x ≤1.00 has been developed based on their dielectric behavior. It has been shown that BaZr _x Ti_{1− x} O₃ system depending on the composition, successively depicts the properties extending from simple dielectric (pure BaZrO₃) to polar cluster dielectric, relaxor ferroelectric, second order like diffuse phase transition, ferroelectric with pinched phase transitions and then to a proper ferroelectric (pure BaTiO₃). A comprehensive structure–property correlation of BaZr _x Ti_{1− x} O₃ ceramics has been studied to understand the various ferroelectric phenomena in the whole phase diagram.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

Phase Relations in (Pb,La)Zr0.65Ti0.35O3

Low-temperature phase relations in (Pb,La)Zr_0.65Ti_0.35O₃ ceramics were determined as a function of La content and temperature. Dielectric and piezoelectric measurements and X-ray data were used to locate and identify phase transitions. The FE₁-FE₂ transition between rhombohedral ferroelectric phases occurs at higher temperatures as the La eontent is increased. For x >4, where x is the atom % La substituted for Pb, a phase region exists between the paraelectric and ferroelectric states. Dielectric data suggest that this region is antiferroelectric. For 6< x <9 a field-induced phase transition accompanies poling. Poled material with x ∼8 exhibits unusual mechanical properties.     

Dielectric and Ferroelectric Properties of Nanocrystalline Bi2Ti2O7 Prepared by a Metallorganic Decomposition Method

Nanopowders of Bi₂Ti₂O₇ were synthesized by a metallorganic decomposition (MOD) technique. Pure Bi₂Ti₂O₇ nanocrystals formed after annealing at 550°C for 5 min. X-ray patterns show that Bi₂₀TiO₃₂ is a metastable phase during Bi₂Ti₂O₇ formation. It was found that there were two peaks in the curves of the dielectric response as a function of temperature for pressed nanocrystalline Bi₂Ti₂O₇ samples. The Curie temperature decreases with decrease of grain size whereas the ferroelectric-ferroelectric phase transition temperature increases. The hysteresis loops observed also suggest that Bi₂Ti₂O₇ might belong to a ferroelectric material.     

Relaxor Behavior and Dielectric Properties of MgTiO3-Doped BaZr0.35Ti0.65O3 Composite Ceramics for Tunable Applications

MgTiO₃-doped BaZr_0.35Ti_0.65O₃ (BZT) composite ceramics have been prepared by the conventional solid-state route. The dielectric nonlinear characteristics and relaxor behavior of these composite ceramics have been investigated. The secondary-phase BaMg₆Ti₆O₁₉ is formed among BZT composite ceramics with the increase of MgTiO₃. BZT composite ceramics show typical diffuse phase transition characteristic and ferroelectric relaxor behavior. The dielectric constant of BZT composite ceramics can be tailored from thousands to hundreds by manipulating the addition of MgTiO₃. The dielectric loss still keeps around 0.001 and the tunability is above 20% at a dc-applied electric field of 25 kV/cm. Suitable dielectric constant, low dielectric loss, and high tunability of this kind of composite ceramics can be useful for potential microwave tunable applications.     

Mechanism of Texture Development in Bi0.5(Na,K)0.5TiO3 prepared by the Templated Grain Growth Process

The mechanisms of texture evolution in Bi_0.5(Na,K)_0.5TiO₃ (BNKT) prepared by the templated grain growth method are examined using platelike SrBi₄Ti₄O₁₅ and Al₂O₃ powders and SrTiO₃ and Al₂O₃ single crystals as templates. These templates give rise to a 〈100〉 texture in the BNKT matrix. The mechanism of texture evolution is dependent on the template species. When SrBi₄Ti₄O₁₅ and SrTiO₃ are used, a new grain (terrace) forms between the matrix and the template grains. The terrace has the same crystallographic orientation as the template. The terrace grows at the expense of the matrix grains, resulting in texture evolution. For the Al₂O₃ template, no terrace forms between the matrix and the template grains. Instead, the matrix grains directly attach to the template surface. The formation of a phase boundary with a specific orientation gives rise to texture evolution for this template.     

Subsolidus Phase Equilibria in the System Na2O-Bi2O3-TiO2 at 1000°C

Subsolidus phase relations in the system Na₂O-Bi₂O₃-TiO₂ at 1000°C were investigated by solid-state reaction techniques and X-ray diffraction methods. Five ternary compounds were observed in the system: Na_0.5Bi_4.5Ti₄O₁₅; Na_0.5Bi_0.5TiO₃; a cubic pyrochlore solid solution composed of xNa₂O.25Bi₂O₃.(75−;x) TiO₂ where x is 2.5 to 3.75; a new compound Na_0.5Bi_8.5Ti₇O₂₇ indexed with the orthorhombic cell of a = 5.45, b = 5.42, and c = 36.8 Å; and an unidentified phase with the probable composition NaBiTi₆O₁₄.     

Lead-Free SrBi4Ti4O15 and Bi4Ti3O12 Material Fabrication Using the Microwave-Assisted Molten Salt Synthesis Method

In this paper, the microwave-assisted molten salt method (MAMSS) and molten salt method (MSS) were used to synthesize SrBi₄Ti₄O₁₅ (SBT). The phase constitution was determined by powder X-ray diffraction and the microstructure of powder was examined by scanning electron microscopy. In contrast to the conventional MSS method, MAMSS produces more distinct plate-like grains and synthesizes both SBT and Bi₄Ti₃O₁₂ (BTO) at 600°C with a 30-min soaking time. The increase of temperature and soaking time can make the plate-like grains of BTO more distinct.     

Electrical Properties of Superlattice-Structured Bi4Ti3O12–PbBi4Ti4O15 Single Crystals

Single crystals of superlattice-structured ferroelectrics composed of Bi₄Ti₃O₁₂ and PbBi₄Ti₄O₁₅ were grown and the properties of polarization hysteresis and leakage current along the a -axis were investigated. Oxidation treatment led to a marked increase in leakage current at room temperature, showing that electron hole acts as a detrimental carrier for electrical conduction. A well-developed polarization hysteresis with a remanent polarization of 41 μC/cm² was observed, which is suggested to originate from the peculiar ferroelectric displacement of Bi in the Bi₂O₂ layers.     

Piezoelectricity and Phase Transitions of the Mixed-Layer Bismuth Titanate Niobate Bi7Ti4NbO21

The piezoelectiric properties of the bismuth titanate niobate Bi₇Ti₄NbO₂₁ have been studied. The two phase transitions at 677° and 856°C are anlayzed based on the measured piezoelectric properties. It is shown that the Bi₇Ti₇NbO₂₁ compound is a ferroelectric until 856°C. Bi₃TiNbO₉ clusters are shown to exist within Bi₇Ti₇NbO₂₁ granis. These clusters may be responsible for the small additional piezoelectric activity of the studied material at temperatures higher than 856°C.     

Subsolidus Phase Equilibria in the System Bi2O3-TiO2-Nb2O5

The subsolidus phase equilibria in the system Bi₂O₃-TiO₂-Nb₂O₅ at 1100°C were determined by solid-state reaction techniques and X-ray powder diffraction methods. The system was found to contain 4 ternary compounds, i.e. Bi₃TiNbO₉, Bi₇Ti₄NbO₂₁, a cubic pyrochlore solid solution having a compositional range of 3Bi₂O₃· x TiO₂ (7– x )Nb₂O₅ where x ranges from 2.3 to 6.75, and an unidentified phase, 4Bi₂O₃·11TiO₂·5Nb₂O₅.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Ferroelectric and Piezoelectric Properties of Na1−xBaxNb1−xTixO3 Ceramics

Piezoelectric ceramics Na_{1− x} Ba _x Nb_{1− x} Ti _x O₃ with low BaTiO₃ concentrations x have been prepared by the solid-state reaction method, and their ferroelectric and piezoelectric properties have been studied. The ceramics are classic ferroelectrics when x ≤0.10, and the ferroelectric–paraelectric phase transition becomes diffusive when x ≥0.15. A low doping level of BaTiO₃ changes the NaNbO₃ ceramics from antiferroelectric to ferroelectric. With the increase in BaTiO₃ doping level, the Curie temperature of ceramics decreases linearly and the remnant polarization and coercive field also decrease, while their dielectric constant increases. Na_0.9Ba_0.1Nb_0.9Ti_0.1O₃ ceramics show the largest piezoelectric constant d ₃₃ (147 pC/N) and good sinterability, suggesting that it is a good candidate for lead-free piezoelectric ceramics.     

Thermal Expansion and High-Temperature Phase Transition of Ba6−3xLn8+2xTi18O54 (Ln=La, Nd, and Sm) Ceramics

Thermal expansion behaviors of Ba_{6−3 x} Ln_{8+2 x} Ti₁₈O₅₄ (Ln=La, Nd, and Sm, x =0.5, 0.67, and 0.75) ceramics were determined by dilatometric measurement. The samples of all investigated compositions expanded nearly linearly with increasing temperature in the range of 20°–1200°C. Their thermal expansion coefficients were determined to be 10.7–11.4 ppm/°C. A discontinuous change in sample size was observed at about 1350°C for each composition, indicating the existence of a phase transition. As determined by the high-resolution X-ray diffraction analysis, the phase constitution and the lattice parameters of Ba_{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄ ceramics were maintained in the quenched samples. The origin of the phase transition was discussed thoroughly. Phase equilibrium in the BaTiO₃–Ln_2/3TiO₃ system was reviewed by considering the phase transition.     

Synthesis and Structural, Ferroelectric, and Piezoelectric Properties of SrBi4Ti4O15 Ceramics

SrBi₄Ti₄O₁₅ ceramics were prepared via ordinary firing (OF) and hot forging (HF). Characterization using scanning electron microscopy and X-ray diffractometry shows platelike grains that were highly oriented ( F = 0.95) after hot forging. Ferroelectric, dielectric, and piezoelectric characterizations revealed that polarization reorientation was restricted to the ab plane of the orthorhombic structure, parallel to the (Bi₂O₂)²⁺ layers. The thickness coupling factor for OF samples was only half that for HF samples oriented parallel to the HF direction (in the ab-plane), as a consequence of poling restrictions in randomly oriented grains.     

New Yellow Ceramic Pigment Based on Codoping Pyrochlore-type Y2Ti2O7 with V5+ and Ca2+

A new yellow pigment with the pyrochlore structure Ca_xY_{2− x} V _x Ti_{2− x} O₇ was prepared as a substitute for the decreasing variety of available yellow ceramic pigments due to the severe regulation of toxic lead and cadmium. The solubility limit of vanadium in this pigment was found to be 1.5 wt% as V₂O₅ or 0.13 as x in the above formula expression. Characterization of vanadium in the vanadium pyrochlore yellow pigment by electron spectroscopy for chemical analysis and electron spin resonance showed that the oxidation state of vanadium was V⁵⁺ and its yellow color mostly originated from V⁵⁺ substituted for Ti⁴⁺. Comparison of color characteristics of Ca_xY_{2− x} V _x Ti_{2− x} O₇ with those of commercial V–SnO₂ and V–ZrO₂ revealed that Ca _x Y_{2− x} V _x Ti₂₋O₇ had better color strength and brightness than the commercial pigments.     

Factors Affecting the Formation of Ba2Ti9O20

The phase development sequence based on a composition equivalent to Ba₂Ti₉O₂₀ during heating is found to be in the following order: BaTi₅O₁₁ > BaTi₄O₉ > Ba₂Ti₉O₂₀. The lowest rate of formation of Ba₂Ti₉O₂₀ is caused by its high surface energy and interface energy, which result in a low nucleation rate. The existence of BaTi₅O₁₁ in calcined powder helps to form Ba₂Ti₉O₂₀ in sintered compacts. The effect of BaTi₅O₁₁ on Ba₂Ti₉O₂₀ formation can be explained by their similar oxygen packing and by reduced volume change during transformation. The amount of BaTi₅O₁₁ formed during heating depends greatly on the compositional homogeneity of powders. The addition of SnO₂ aids the formation of Ba₂Ti₉O₂₀ by reduced strain energy at transformation and reduced surface energy.     

Processing and Characterization of BaTi4O9

BaTi₄O₉ powder prepared by calcining BaCO₃ and TiO₂ powders was sintered to over 97% of theoretical density. Less than 5% Ba₂Ti₉O₂₀ occurred as a second phase in "pure" BaTi₄O₉, and Al₂O₃ impurities from processing formed isolated hollandite (∼BaAl₂Ti₆O₁₆) grains, which were identified by fringes in bright-field TEM images. For pure BaTi₄O₉ at 1 MHz, a dielectric loss (tan δ) of 5 × 10⁻⁴ and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.