The Morphology Control of BaTiO3 Particles Synthesized in Water and a Water/Ethanol Solvent

The experimental conditions for the growth of shape‐controlled BaTiO₃ particles in NaOH and Ba(NO₃)₂ aqueous and water/ethanol solutions using various TiO₂‐containing precursors were studied at 80°C–100°C. The different chemistries and physical characteristics of the precursors resulted in different BaTiO₃ formation rates and morphologies_. Nanocrystalline anatase led to irregularly shaped BaTiO₃ particles, whereas star‐like, single‐crystalline BaTiO₃ particles grew from aerogel TiO₂ and sodium titanate (NT) belts in alkaline aqueous solutions. With the addition of ethanol, the star‐like BaTiO₃ particles changed to square‐like, the size of which decreased with an increase in the ethanol content. The electron microscopy observations supported a dissolution–precipitation mechanism as the primary reaction mechanism for the formation of BaTiO₃ nanocrystals, which further aggregated into single‐crystalline star‐ or square‐like particles by oriented attachment. The modification in the water solution with ethanol is believed to influence both the nucleation and aggregation process and consequently influence the particle shape and size.     

Reactive sintering of phosphorous coated BaTiO3

Doping BaTiO₃ with P⁵⁺ has been reported to decrease the sintering temperature of the ceramic, allowing homogeneous fine grained microstructures to be obtained. According to the present paper, the formation of a phosphorous-BaO rich phase covering the BaTiO₃ particles seems to be the origin of the improved porosity coalescence and removal observed at the earlier sintering stage of these materials. The formation and development of phosphorous-barium rich phases have been followed by means of DRX, and high temperature NMR. Phosphorous cations incorporated from phosphate ester form a surface layer covering the BaTiO₃ particles which reacts during heating to form Ba₂TiP₂O₉ and/or Ba₃(PO₄)₂. Formation of these phases seem to occur by solid-solid reaction involving phosphorous diffusion through the BaTiO₃ particles.     

Short-range dissolution–precipitation crystallization of hydrothermal barium titanate

A modified autoclave was used to investigate the crystallization mechanism of BaTiO₃ during the hydrothermal reaction of Ba(OH)₂ and TiO₂ anatase. An uneven distribution of the crystallized BaTiO₃ particles was observed: more than 99 wt.% of total BaTiO₃ particles remained where the precursor TiO₂ was put; less than 1 wt.% was collected from the areas away from the TiO₂. According to the experimental observations in this work and proofs reported in the literature, we propose that the crystallization mechanism is dissolution–precipitation in nature, but the soluble Ti⁴⁺ species can only redisperse in a short distance away from TiO₂ particles before precipitation. In other words, the nucleation of hydrothermal BaTiO₃ starts at a low concentration of Ti⁴⁺. The mechanism of the Ba(OH)₂–Ti(OH)₄ reaction is a fast dehydration process.     

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.     

Formation of an amorphous product phase during the solid state reaction between a vitreous SiO2 thin film and a (001) BaTiO3 substrate

The solid state chemical reaction of a thin SiO₂ film with a single-crystal (001) BaTiO₃ substrate has been investigated as a model system for processes occurring during the sintering of BaTiO₃ ceramics with the sintering additive SiO₂. Thin amorphous SiO₂ films on (001) BaTiO₃ substrates react with the substrate during annealing at 1000°C for 5 to 15 min. Transmission electron microscopy (TEM) has revealed the formation of a glassy reaction product after 5 min of heating. The composition of this glass has been analyzed by energy dispersive X-ray spectrometry (EDX), showing mainly SiO₂ and about 16 at.% of BaTiO₃. The Ba:Ti ratio of the glass is 1:1. During prolonged heating, or if the initial BaTiO₃ surface is decorated with Pt markers, the reaction product crystallizes, with the fresnoite Ba₂TiSi₂O₈ forming as the dominant crystalline phase.     

Formation mechanism of barium titanate by thermal decomposition of barium titanyl oxalate

This study examined the formation mechanism of BaTiO₃ from the thermal decomposition of barium titanyl oxalate. A significant amount of O₂ evolution near 357 and 720 °C was observed by gas chromatography/mass spectroscopy, except for the previously known H₂O, CO₂, CO evolution. The metastable Ba₂Ti₂O₅CO₃(CO₂) intermediate phase seemed to be transformed mainly to Ba₂Ti₂O₅CO₃, while a certain amount of crystalline BaCO₃ and amorphous Ti-rich phase were formed simultaneously at 450-600 °C in air. A modification of the decomposition mechanism reported by Gopalakrishnamurthy et al. was proposed based on the experimental findings.     

Evidence of a dissolution–precipitation mechanism in hydrothermal synthesis of barium titanate powders

In our study of the system barium hydroxide-titanium tetraisopropoxide–water–isopropanol between 85 and 150 °C, we have observed the following evidence of a mechanism by dissolution-precipitation: (1) when varying the water/isopropanol ratio in synthesis at 150 °C, the grain size of barium titanate decreases when the amount of alcohol increases, i.e. when the solubility of the precursors decreases; (2) TEM observations of incompletely reacted powders showed that the grains are either amorphous or entirely crystalline BaTiO₃, which means that homogeneous nucleation and growth is occurring instead of heterogeneous nucleation; (3) high resolution TEM observations of fully reacted powders revealed the presence of necks between particles. These three experimental observations in the same reaction system provide strong evidence of dissolution-precipitation as the primary reaction mechanism.     

The effect of the hydrothermal synthesis variables on barium titanate powders

In this work, the influence of (a) Ba excess in the starting hydrothermal mixture with TiO₂, (b) hydrothermal reaction temperature, and (c) washing cycles on the hydrothermal synthesis of barium titanate (BaTiO₃) were investigated to assess their relative contributions to the final characteristics of the sintered oxide. BaTiO₃ cake was prepared by hydrothermal synthesis at 150 °C and 180 °C using BaOH₂·8H₂O and TiO₂·xH₂O as starting hydrothermal mixture with an excess of Barium (+1 Ba mol% and +2 Ba mol%). The obtained BaTiO₃ cake was washed several times from 0 to 14 (W_n<15) using simple de-ionized water and then sintered at 1120 °C for 3 h. All considered hydrothermal syntheses variables strongly contribute to the final characteristics of the sintered BaTiO₃ powders in terms of Ba²⁺/Ti⁴⁺ molar ratio, crystalline structure and mean particle size. In particular, it is clear from these experiments that the removal of the unfavorable barium salts from BaTiO₃ cake by long washing cycles before final calcination is a critical step in the hydrothermal synthesis of BaTiO₃.     

A novel route to produce BaTiO3 glass-ceramics with nanosized cubic BaTiO3 phase precipitating for high energy-storage applications

To meet requirements of miniaturization devices in high pulsed power technology, super dielectric energy storage performance, such as high dielectric breakdown strength (DBS), large energy storage density with high power density, is extremely important in dielectric materials. However, for BaTiO₃ based ceramics and glass ceramics, there is still a critical challenge to achieve high DBS and large energy storage density. Herein, a novel route was proposed to precipitate nanocrystals with cubic BaTiO₃ phase from glass matrix, which can elevate dielectric constant and meanwhile maintain high DBS compared to parent glass. A high recoverable energy storage density of ∼ 3.66 J cm⁻³ at 1000 kV cm⁻¹ and high discharge energy density of ∼3.57 J cm⁻³ with good thermal stability and ultra-high peak power density of ∼ 910 MW cm⁻³ can be achieved in BaTiO₃ glass ceramic, which implies this type of glass ceramics is suitable for high pulsed power technology application.     

The isoelectric point of BaTiO3

The value of the isoelectric point (IEP) of BaTiO₃ is highly significant for the development of a fully rational basis for the successful processing of BaTiO₃ powder by aqueous routes. We report here experimental evidence that the true IEP value for BaTiO₃ is in the neutral pH range (6–7). However, Ba²⁺ ions are also selectively adsorbed at BaTiO₃ particle surfaces, raising the ζ-potential; IEP values greater than pH 7 can occur when the concentration of Ba²⁺ in the aqueous phase is high. Studies of the effects of Ba²⁺ and CO₃²⁻ additions on the ζ-potential of BaTiO₃ and on the rheology of aqueous suspensions, indicate that the reason for the wide range of ζ-potential and IEP values found for BaTiO₃ is in large measure the presence of impurity BaCO₃ in the BaTiO₃ powders used. Fully dehydrated BaTiO₃ powder has a low IEP value (∼4), which slowly rises on ageing in water. The IEP appears not to be influenced by Nb₂O₅ doping at the levels normally used.     

Dielectric Behavior and Second Phases in X7R-Formulated BaTiO3 Sintered in Low-Oxygen Partial Pressures

Multi-layer ceramic capacitor chips prepared from an X7R-formulated BaTiO₃ powder and nickel base-metal electrodes were sintered at 1200°C and re-oxidized at 1000°C in low-oxygen partial pressures (pO₂). While chips A and B, sintered in pO₂≈10⁻⁹ and 10⁻¹¹ atm, respectively, exhibited a typical temperature coefficient of capacitance resembling the X7R characteristics, normal dielectric behavior was retained in chip C sintered at the lowest pO₂ of ∼10⁻¹³ atm with the Curie point resurged at ∼120°C. The chips were analyzed using X-ray diffractometry, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The dielectric layer contains a siliceous residual glassy second phase in grain boundaries, triple-grain junctions, and quadruple-grain corners and crystalline second phases in locations scattered inhomogeneously. A crystalline second phase, common to all chips, was determined to hexagonal silicate oxyapatite Ca₂Y₈Si₆O₂₆. Tetragonal Ba₂TiSi₂O₈ was another crystalline second phase specific to chip C. Eutectic liquids have also formed principally among BaO, SiO₂, and solid-state additives of CaO and Y₂O₃ below or at 1200°C to aid the densification of BaTiO₃ dielectrics. They were solidified upon cooling to a residual glassy second phase in the ceramics. Sintered BaTiO₃ grains of 250–400 nm in both chips A and B contained the characteristic X7R core–shell structure. Those in chip C have grew significantly to 5–8 μm but lost the core–shell completely. With almost all additives in chip C reacting to form second phases, the microstructure is represented by the {111} single and double twins resembling that of undoped BaTiO₃ ceramics sintered at low temperatures.     

Phase evolution and <110>-orientation mechanism in RTGG-processed BaTiO3 ceramics with electrical properties

The <110>-oriented BaTiO₃ ceramics were fabricated using BaCO₃ matrix and H_1.08Ti_1.73O₄.nH₂O (HTO) template particles, and the mechanism of BaTiO₃ phase formation was investigated. The dielectric, ferroelectric, and piezoelectric properties were also investigated. The transformation of the HTO phase into the TiO₂ bronze or TiO₂ (B) phase was observed at 600°C, where the BaTiO₃ nucleation was accompanied by the formation of a Ba₂TiO₄ phase. The TiO₂ phase reacted with the Ba₂TiO₄ phase at 800°C to give a BaTiO₃ phase, whereas its reaction with the BaTiO₃ resulted in the formation of BaTi₂O₅ phase that got decomposed into BaTiO₃ and Ba₆Ti₁₇O₄₀ phase at sintering temperature ≥1300°C. Sintering with samples’ embedding in BaTiO₃ powders prevented the formation of the Ba₆Ti₁₇O₄₀ secondary phase. The crystallographic orientation along the <110> direction (F₁₁₀) was developed by the epitaxial grain growth mechanism. In addition to the contribution of the grain-size increment for enhancing the F₁₁₀, the preservation of the platelike structure was also found to have a significant impact. The ceramics prepared by the embedded sintering (grain size ≈12.4 µm and F₁₁₀ = 83%) exhibited the room-temperature dielectric constant of 1708 and piezoelectric strain constant of 445 pm/V, which are higher than those of the BaTiO₃ ceramics with randomly oriented grains.     

Rapid synthesis of submicron crystalline barium zirconate BaZrO3 by precipitation in aqueous basic solution below 100 °C

Pure crystalline BaZrO₃ powders can be produced by precipitation in highly basic aqueous solution. The influence of several synthesis parameters is studied. At high OH^? concentration ([NaOH] = 20 mol/l), it is possible to obtain the well-crystallized stoichiometric perovskite phase at relatively low temperature (～80 °C), after a short reaction time (15 min) and without requiring any precaution to avoid the presence of CO₂. This synthesis method yields spherical particles, whose size can be controlled by changing the concentration of the Ba + Zr solution. No calcination treatment is necessary since the precipitate is crystalline. Suitable choice of the synthesis parameters ([NaOH] = 20 mol/l, [Ba + Zr] = 1 mol/l, reaction time = 15 min) yields a sub-micron precipitate with excellent densification behaviour. Corrosion tests in BaO–CuO melt show that ～98% dense BaZrO₃ obtained by sintering at 1650 °C for 13 h could be used for crucibles in the synthesis of YBa₂Cu₃O₇ superconducting single crystals.     

Role of A-site (Sr), B-site (Y), and A,B sites (Sr,Y) substitution in lead-free BaTiO3 ceramic compounds: Structural,optical, microstructure,mechanical, and thermal conductivity properties

Strontium and Yttrium-doped and co-doped BaTiO₃ (BT) ceramics with the stoichiometric formulas BaTiO₃, B_1-xSr_xTiO₃, Ba_1-xY_xTiO₃, BaTi_1-xY_xO₃, Ba_1-xY_xTi_1-xY_xO₃, and Ba_1-xSr_xTi_1-xY_xO₃ (x = 0.075) noted as BT, BSrT, BYT, BTY, BYTY, and BSrTY have been synthesized through sol-gel method. X-ray diffraction (XRD) patterns of the prepared ceramics, calcined at a slightly low temperature (950 °C/3h), displayed that BT, BSrT, and BYT ceramics possess tetragonal structures and BTY, BYTY, and BSrTY have a cubic structure. The incorporation of the Ba and/or Ti sites by Sr²⁺ and Y³⁺ ions in the lattice of BaTiO₃ ceramic and the behaviors of the crystalline characteristics in terms of the Y and Sr dopant were described in detail. The scanning electron microscopy (SEM) images demonstrated that the densification and grain size were strongly related to Sr and Y elements. UV–visible spectroscopy was used to study the optical behavior of the as-prepared ceramic samples and revealed that Sr and Y dopants reduce the optical band gap energy to 2.74 eV for the BSrTY compound. The outcomes also demonstrated that the levels of Urbach energy are indicative of the created disorder following the inclusion of Yttrium. The measurements of the thermal conductivity indicated the influence of the doping mechanism on the thermal conductivity results of the synthesized samples. Indeed, the thermal conductivity of BaTiO₃ is decreased with Sr and Y dopants and found to be in the range of 085–2.23 W.m^-1. K^?1 at room temperature and decreases slightly with increasing temperature from 2.02 to 0.73-W.m^-1. K^?1. Moreover, the microstructure and grains distribution of the BT, BSrT, BYT, BTY, BYTY, and BSrTY samples impacted the compressive strength, hence; the compressive strength was minimized as the grain size decreased.     

Difference in the Nature of Eu3+ Environment in Eu3+‐Doped BaTiO3 and BaSnO3

BaTiO₃ and BaSnO₃ samples doped with Eu³⁺ ions were prepared using glycine‐nitrate gel combustion method. Relative intensities and line shapes of magnetic dipole allowed ⁵D₀ → ⁷F₁ and electric dipole allowed ⁵D₀ → ⁷F₂ transitions of Eu³⁺ from the hosts, BaTiO₃ and BaSnO₃, are significantly different. Based on detailed structural investigations, it is confirmed that synthesizedBaTiO₃ sample is tetragonal with no center of symmetry around Ba²⁺ ions. Unlike this BaSnO₃ is cubic with centrosymmetric Ba²⁺ site. From X‐ray diffraction and experimentally obtained Judd–Ofelt parameters (Ω₂ and Ω₄ values), it is confirmed that in BaTiO₃ there is a decrease in the average Ba–O and Ba–Ba distances compared with that in BaSnO₃. This leads to higher Eu–O bond polarizability and adds to the distortion in its environment around Eu³⁺ in BaTiO₃:Eu compared with BaSnO₃:Eu. This is responsible for the observed difference in the luminescence properties.     

Ferroelectric properties of Li‐doped BaTiO3 ceramics

We prepared 3 kinds of Li⁺‐doped BaTiO₃ ceramics by the solid‐state reaction method: (i) (Ba_1?xLi_x)TiO_3?x/2 having A‐site Li⁺, (ii) Ba(Ti_1?xLi_x)O_3?3x/2 having B‐site Li⁺, and (iii) x/2 Li₂CO₃+BaTiO₃ mixed one, for which we investigated the stable site of Li. The density of all prepared ceramics is above 95%. The results show that the lattice structure, the grain size, and the electric properties of Li⁺‐doped BaTiO₃ ceramics are dependent on Li⁺ site. According to the increase in Li content, the cell volume of Ba_1?xLi_xTiO_3?x/2 decreases, but that of BaTi_1?xLi_xO_3?3x/2 increases. That of x/2Li₂CO₃+BaTiO₃ decreases by the small addition of Li, but increases by the large addition of Li. All Li⁺‐doped ceramics show antiferroelectric‐like double hysteresis loops. The shape of loops and the dielectric properties are also dependent on the Li site. We suggest that the role of oxygen vacancy accompanied by the Li‐doping is important. By comparison with the results of 3 type ceramics, it is concluded that at x/2Li₂CO₃+BaTiO₃ ceramics, the Li⁺ prefers to favorably substitute Ba²⁺ at A site for the low concentration of Li but its location was changed to Ti⁴⁺ site for the high concentration of Li.     

Kinetic studies of the chemical polymerization of substituted aniline in aqueous solutions and characterization of the polymer obtained Part 1. 3‐Chloroaniline

Aqueous polymerization of 3‐chloroaniline (mCA) was studied using sodium dichromate as oxidant in the presence of hydrochloric acid. The effect of hydrochloric acid, sodium dichromate and monomer concentration on the polymerization rate, specific viscosity of the obtained polymer and ac conductivity was investigated. The initial and overall reaction rates increase with increasing hydrochloric acid concentration or sodium dichromate concentration, but decrease with increasing monomer concentration. The specific viscosity values (η_sp) increase with increasing hydrochloric acid concentration or monomer concentration, which means that the molecular weight of the polymer samples increases accordingly. On the contrary, the molecular weight decreases with increasing sodium dichromate concentration. The highest ac conductivity value of the obtained polymer was found for 0.0255 mol l⁻¹ of Na₂Cr₂O₇, 0.8 mol l⁻¹ HCl and 0.0956 mol l⁻¹ monomer concentration in the reaction medium. The order of the polymerization reaction with respect to hydrochloric acid, Na₂Cr₂O₇ and monomer concentration was found to be 1.0, 0.9 and 0.75, respectively. The apparent activation energy (E_a) for this polymerization system was found to be 13.674 × 10⁴ mol⁻¹. The obtained poly(3‐chloroaniline) was characterized by UV–visible, IR and ¹H NMR spectroscopy. X‐ray diffraction analysis and electron microscopy studies were carried out. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results were used to confirm the structure. © 2001 Society of Chemical Industry     

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.     

Hydrothermal preparation of BaTiO3 thin films

In preparing BaTiO₃ thin films under hydrothermal conditions, the effects of concentrations of nutrient and mineralizer, and reaction time on crystallinity, grain size, surface roughness, and film thickness were investigated. Experiments were performed in the ranges of 0.1-1.5M BaCl₂ · 2H₂O or Ba(OH)₂ · 8H₂O and 0-1.5 M KOH with varying reaction time from 0.16 to 8 hours at 140 °C. Bimodal dispersion of crystalline grains on the surface of BaTiO₃ thin films was predicted through nucleation and crystal growth reaction. As the concentrations of nutrient and/or mineralizer increased, grain size of the thin film became smaller, but more uniform and compact. When 0.4 M Ba(OH)₂ · 8H₂O was used with 1.0 M KOH, a reaction time longer than 4 hours was required in order to fabricate BaTiO₃ thin films.     

Temperature dependence of electrical resistivity and dielectric characteristics of poled and BaZrO3 modified BaTiO3 ceramics

Samples of pure BaTiO₃, pure BaZrO₃ and intermediates containing from 5 up to 30% BaZrO₃ were carefully prepared by the usual ceramic procedure followed by X-ray diffraction analysis to ensure the complete reaction. Well sintered and translucent ceramic bodies were obtained. Measurements of dielectric constant (?), dielectric loss (tan δ) and a.c. resistivity (?) were undertaken as a function of temperature up to 250°C and at various frequencies of 10–50 Kc/s before and after polarization (800 V). The effect of poling field is discussed on the basis of the presence of spontaneous polarization in BaTiO₃ lattice. Finally, it was established that the introduction of Zr⁴⁺ ions in BaTiO₃ lattice occurs in three steps: firstly, filling of Ba²⁺ vacancies, secondly solid solution formation from BaZrO₃ in BaTiO₃ lattice and thirdly, occupying interstitial sites in BaTiO₃ lattice.