Hydrothermal Synthesis of Tetragonal Barium Titanate from Barium Chloride and Titanium Tetrachloride under Moderate Conditions

Tetragonal BaTiO₃powders were prepared hydrothermally at 240°C, in only 12 h, using BaCl₂·2H₂O and TiCl₄, which are rather easy to manipulate. Characterization via X-ray diffractometry, scanning electron microscopy, Brunauer–Emmett–Teller analysis, and differential scanning calorimetry confirmed that increasing the NaOH excess concentration (from 0.5 M to 2.0 M ) and decreasing the initial TiCl₄concentration (from 0.625 M to 0.15 M ) promotes the formation of tetragonal BaTiO₃powders. After reaction, the powders were proved to be phase-pure BaTiO₃, with no impurities, such as Cl⁻ and CO₃²⁻.     

醇对低温水热合成四方相钛酸钡的影响

以Ba(OH)2·8H2O和Ti(OC4H9)4为原料,氨水为矿化剂,以不同醇/水混合溶剂作为分散介质,在180℃下反应120 h,水热合成四方相钛酸钡粉体,研究不同醇对粉体的影响,并通过XRD、TEM、IR及激光纳米粒度分析对粉体进行了表征.结果表明以一元醇为溶剂有利于合成四方相钛酸钡,其中又以乙醇为最优,以乙醇/水为溶剂合成的四方相钛酸钡粉体的c/a最大可达1.0090,且粉体纯度高,粒径分布狭窄,晶粒形貌发育完整,呈现球形和四方形.     

Preparation of Tetragonal Barium Titanate Thin Film on Titanium Metal Substrate by Hydrothermal Method

Tetragonal BaTiO₃ thin films were prepared directly on Ti metal substrates in Ba(OH)₂ solutions by a hydrothermal method at temperatures 400° to 800°C for 5 to 240 min. The film thickness estimated from weight gain of Ti plate was in the range from 0.5 to 2.5 μm, and it increased with increasing treatment temperature, treatment time, and Ba(OH)₂ concentration. Rectangular crystals having {100} and {001} faces grew idiomorphically with approximate crystal size of 0.3 to 2.0 μm. The tetragonality of the BaTiO₃ films became apparent when the average crystal size exceeded about 1 μm. Lattice parameters of the films were a = 3.994 Å, c = 4.035 Å, and c/a = 1.010. The films formed above 600°C had preferred orientation showing stronger XRD peaks of h 00 and 00 l than the other peaks.     

Formation Mechanisms of Tetragonal Barium Titanate Nanoparticles in Alkoxide–Hydroxide Sol-Precipitation Synthesis

The early stage of barium titanate (BaTiO₃) nanoparticle formation is investigated by in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) using synchrotron radiation. BaTiO₃ nanoparticles are synthesized via dissolution of barium hydroxide octahydrate and hydrolysis of titanium (IV) isopropoxide in isopropanol. In the course of raising the temperature of the alkoxide–hydroxide mixture solution to 80°C, in situ synchrotron XRD reveals that BaTiO₃ nanocrystals smaller than 6 nm begin to nucleate at 50°C without intermediate TiO₂ anatase formation, and Ti K edge absorption spectra also confirm the formation of corner-sharing TiO₆ octahedra at 60°C. The average size of BaTiO₃ precipitates increases to about 7.5 nm at 80°C. The synthesized nanopowders show an anomalously high tetragonality according to the Rietveld refinement of synchrotron XRD data.     

Synthesis of Tetragonal Barium Titanate Nanoparticles Via Alkoxide–Hydroxide Sol-Precipitation: Effect of Water Addition

Barium titanate (BaTiO₃) nanoparticles are synthesized under N₂ atmosphere by the hydrolysis and condensation of barium hydroxide octahydrate and titanium (IV) isopropoxide with controlled water addition. With increasing the amount of water addition, the primary nanoparticles tend to grow from 7.5 to 68.5 nm in average diameter and to agglomerate to form spherical secondary particles of 0.5–3 μm in diameter. The influence of water content on the crystal size and structure was studied by X-ray diffraction via synchrotron radiation. As the amounts of added water increased, Rietveld analysis revealed that the lattice volume shrinks along the c -axis changing the crystal structure of BaTiO₃ nanoparticles gradually from tetragonal to cubic. FT-IR spectra also indicated that the concentration of hydroxyl ions in BaTiO₃ nanoparticles decreases with increasing water contents. It is thus concluded that adding sufficient amount of water is effective in reducing the concentration of protons in the oxygen sublattice during synthesis of BaTiO₃ nanoparticles via alkoxide–hydroxide sol-precipitation process.     

Preparation of Fine Tetragonal Barium Titanate Powder by a Microwave–Hydrothermal Process

A microwave–hydrothermal (MH) process was performed at 240°C to prepare tetragonal BaTiO₃ from TiCl₄ and Ba(OH)₂. No alkali hydroxide was used to avoid contaminations. MH BaTiO₃ powder with a c / a ratio of 1.010 and a mean size of 180 nm was synthesized within only 9 h. The MH BaTiO₃ contains a very low concentration of lattice hydroxyl group, associated with a very small lattice strain. The measured density of the MH BaTiO₃ is favorably consistent with the theoretical value, and the Ba/Ti stoichiometry determined is 0.996. The formation of a tetragonal structure in BaTiO₃ and the particle growth were strongly promoted by the MH process. The effects of lattice defects on the stoichiometry and the determination of transition enthalpy were discussed.     

Kinetics and Mechanisms of Hydrothermal Synthesis of Barium Titanate

Reaction mechanisms for the hydrothermal synthesis of barium titanate are evaluated. Feedstocks of barium hydroxide octahydrate and anatase titania are reacted for varying durations (1–72 h) to provide intermediate-stage samples for characterization by transmission electron microscopy/ energy-dispersive spectroscopy (TEM/EDS), X-ray diffractometry (XRD), and inductively coupled plasma spectroscopy (ICP). Quantitative evaluation of the extent of reaction by ICP and XRD methods permits the analysis of data with the Johnson-Mehl-Avrami equation. This analysis reveals two reaction-rate regimes. Kinetic analysis, based on reaction progress, yields insight into the first reaction-rate regime but is inconclusive in the analysis of the second reaction-rate regime. In the first regime, at the early stage of barium titanate formation, a dissolution-precipitation mechanism dominates. In contrast, in the second regime, at longer reaction times, an in-situ transformation mechanism is probably dominant. However, multiple reaction mechanisms (e.g., in-situ transformation and dissolution-precipitation) may be competing for rate control. Alternatively, dissolution-precipitation may be the dominant mechanism throughout the barium titanate synthesis, with nucleation and growth controlling the first regime and dissolution rate controlling the second regime.     

Synthesis of Single-Crystal Barium Titanium Isopropoxide Complex to Form Barium Titanate

Single-crystal barium titanate double alkoxide complex with a composition presumed to be BaTi(OCH(CH₃)₂)₆·xC₆H₆ was successfully synthesized in this work. The crystal was converted to BaTiO₃ at 100°C. The preparation of the Oxides via single crystals of metal alkoxides has some advantages over the well-known sol-gel method in that it results in oxides with well-defined and homogeneous compositions at the atomic level and in fine particle sizes, smaller than 50 nm.     

Hydrothermal Synthesis of Nanometer-Sized Barium Titanate Powders: Control of Barium/Titanium Ratio, Sintering, and Dielectric Properties

Nanometer-sized BaTiO₃ powders have been synthesized hydrothermally from Ba(OH)₂ and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH)₂ prevents full incorporation of barium, and barium-deficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. K _s(BaTiO₃,-25°C) = 7 × 10^-8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO₃ are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Room-temperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995-1.000.     

水热合成法制备钛酸钡纳米粉体的研究进展

钛酸钡纳米粉体是非常重要的工业基础材料之一,关于钛酸钡纳米粉体的制备及形貌的控制一直是研究的热点,特别是水热合成方法,因其独特的优势越来越引起人们的关注.就水热法合成法制备钛酸钡纳米粉体的反应机理、影响因素以及存在的问题进行了系统综述,并对其发展前景进行了展望.     

水热反应条件对合成BaTiO3纳米粉体影响的研究

通过正交实验法,对不同钡钛比、反应温度、保温时间进行了水热合成钛酸钡纳米粉体的实验研究,对所得粉体进行了DTA,XRD,TEM等分析表征,分析研究了不同水热反应条件对合成钛酸钡纳米粉体的影响。     

Crystallographic Orientation Relationships Between Hexagonal and Tetragonal Barium Titanate

The high-temperature hexagonal (h) phase has been found to retain metastability at room temperature and co-exist with the room-temperature tetragonal (t) phase in hot-pressed BaTiO₃ ceramics. The crystallographic orientation relationships between two polytypes have been analyzed by TEM and selected-area diffraction patterns (SADPs). The relationships are illustrated using transformation matrices for both the forward h→t phase transformation ( M _f) and the backward t→h phase transformation ( M _b) based on two independent microstructures containing h-particle in t-matrix and t-particle in h-matrix, respectively. Self-consistent transformation matrices are derived from the coincident reflections in corresponding SADPs. The forward and backward matrices are inverse matrix to each other.     

水热法制备钕掺杂BaTiO3粉体及其介电性能研究

以BaCl2·2H2O、TiCl4和NdCl3为原料,采用水热法制备了掺钕的BaTiO3纳米粉体,并经高温烧结后得到瓷体。利用DTA、XRD、SEM等测试手段,分析了钕掺杂对钛酸钡粉体及其陶瓷电性能的影响。研究表明,掺杂后,钕固溶到了钛酸钡的晶格中并取代钡位或钛位。钕的掺杂有助于获得细晶高致密的陶瓷,当W(NdCl3)%为0.6时,致密度最高,常温下相对介电常数高达5650,击穿场强达到3.5kV/mm。     

Dielectric Aging in Tetragonal Solid Solutions of Calcium Titanate in Barium Titanate

The dielectric aging phenomenon was observed in tetragonal polycrystalline solid solutions of calcium titanate in barium titanate between 30° and 90°C, and the 90° ferroelectric domain microstructures were analyzed using replica electron microscopy and statistical procedures. The results show a correlation between aging rate and 90° domains that may be satisfactorily interpreted in terms of the internal residual stress relaxation theory of aging.     

Modified Phase Diagram for the Barium Oxide–Titanium Dioxide System for the Ferroelectric Barium Titanate

The ferroelectric phase transition behavior in BaTiO₃ was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X-ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO₂) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO₂ partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba_1.054Ti_0.946O_2.946→Ba₂TiO₄+BaTiO₃ and second 1250°C for BaTi₂O₅→Ba₆Ti₁₇O₄₀+BaTiO₃, respectively. All other regimes of the BaO–TiO₂ were found to be consistent with the reported diagrams in the literature.     

Crystallographic Properties of Hydrothermal Barium Titanate Crystallites

The crystallographic properties and defects of hydrothermally grown barium titanate crystallites are reported. When a precursor with a Ba:Ti molar ratio of 1.0 was used, cubic barium titanate crystallites were formed. However, when the precursor Ba:Ti molar ratio was 3.0, the product was tetragonal. The defect concentration (including OH defects and barium vacancies) was higher in cubic crystallites than in tetragonal crystallites. Because of the defects in the crystallites, the displacement of Ba²⁺ cations and the deformation of the Ti-O₆ octahedrons was suppressed; thus, no phase transition from cubic to tetragonal occurred at the Curie point. When the defects were eliminated by thermal treatment, the normal crystallographic properties were obtained. A high reaction temperature and a high Ba:Ti molar ratio in the precursor were beneficial to limiting defect dormation in the crystallites; thus, tetragonal crystallites were obtained.     

Defect Chemistry and Microstructure of Hydrothermal Barium Titanate

Hydrothermal powders of BaTiO₃ and (Ba,Ca)(Ti,Zr)O₃ contain large amounts of protons in the oxygen sublattice. The proton defects are compensated by vacancies on metal sites. When the powder is annealed, water is released and the point defects disappear in the temperature range of 100°–600°C. Metal and oxygen vacancies combine to small nanometer-sized intragranular pores. At temperatures of >800°C, the intragranular pores migrate to the grain boundaries and disappear. In multilayer ceramic capacitors that have been prepared from hydrothermal powders, the intragranular pores are preferentially collected at the inner electrodes, which results in "bloating," cracks, and delamination.     

Exaggerated Growth of Hexagonal Barium Titanate under Reducing Sintering Conditions

The influence of reducing sintering conditions on anisotropic grain growth in BaTiO₃ was studied above the BaTiO₃-Ba₆Ti₁₇O₄₀ eutectic temperature. The morphology and structure of exaggeratedly grown grains was examined by X-ray powder diffractometry, scanning electron micros-copy, and high-resolution transmission electron microscopy. The results show that all anomalously grown anisotropic grains were hexagonal BaTiO₃ in the form of platelike crystals with a/c ratios up to 10. The direction of preferential growth of hexagonal grains is crystallographically analogous with that of parallel (111) twins in a cubic phase. Ti³⁺ ions, induced by reducing atmosphere, play an important role in the formation of hexagonal stacking.     

Hydrothermal Synthesis and Photocatalytic Activity of Whisker-Like Rutile-Type Titanium Dioxide

Nanowhiskers of rutile-type titanium dioxide (TiO₂) were synthesized by a simple hydrothermal treatment of a water-soluble titanium–glycolate complex at 473 K. The aspect ratio of the obtained rutile particles could be controlled by changing the concentration of the glycolic acid additive in the starting aqueous solution of the titanium complex. According to high-resolution transmission electron microscopy, whisker-like nanoparticles grow along the c -axis of rutile, the side facets of which are (110), (−110), (1–10), and (−1–10). Glycolic acid controls promotion of the rutile particles in the specified direction due to its preferred adsorption on the titanium-rich facets. Evaluation of the photocatalytic activity of the synthesized TiO₂ was performed in terms of decomposition of oxalic acid in an aqueous system under ultraviolet irradiation. It turned out that this particular whisker-like shape is responsible for the tremendous enhancement of rutile photocatalytic activity, and even rutile samples with low surface area demonstrated photocatalytic performance comparable with one of the best nanosized anatase photocatalytic materials. Therefore, it can be concluded that the (110) rutile facet plays an important role in the photocatalytic oxidation reaction, and that it exhibits higher photocatalytic activity than the (001) facet in the decomposition of oxalic acid.     

Zn掺杂BaTiO3微粉的微波水热合成

以钛酸丁酯、硝酸钡和醋酸锌为原料,采用微波水热法制备了掺锌的BaTiO3微粉。利用XRD和SEM对产物进行了表征。研究表明,掺杂后锌固溶到了钛酸钡的晶格中并取代钡位或钛位。微波水热合成锌掺杂钛酸钡微粉温度更低,同时反应速率明显提高。