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CERAMICS INTERNATIONAL Hydrothermal conditions for the formation of tetragonal BaTiO 3 particles from potassium titanate and barium salt

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Abstract

The formation of tetragonal BaTiO 3 from K 2 Ti 6 O 13 (K2T6) nanowires was examined under hydrothermal conditions at 100–240 1C. The morphology and the crystal structure of the BaTiO 3 particles that grew via a dissolution–precipitation mechanism were controlled by the temperature and the concentrations of the NaOH and the K2T6. The degree of tetragonality of the formed BaTiO 3 powders was quantitatively evaluated on the basis of a Rietveld crystal-structure refinement and compared with the results of Raman and DSC analyses. The powders consisted mainly of star-like BaTiO 3 particles that formed at 100 1C and contained cubic (22%) and tetragonal (78%) phases with a low c/a value of 1.0042. The BaTiO 3 particles changed their shape to dendritic and seaweed-like with an increase of temperature to 150 1C and 200–240 1C, respectively. According to the Rietveld analysis this increase of the temperature led to the formation of a tetragonal phase with a high c/a ratio (1.008–1.010), which dominated over the cubic and tetragonal phases with a low c/a (1.004–1.007). An increase in the K 2 Ti 6 O 13 and NaOH concentrations was found to decrease the tetragonality of the formed BaTiO 3 particles. The electron-diffraction patterns suggested a multidomain structure for the seaweed-like, mainly tetragonal, BaTiO 3 particles.

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... Along the same line, it can be noted from the literature that, for example, the synthesis step of "washing cycles" of the precipitate sample obtained by hydrothermal reaction has not been considered by many authors as an important issue when synthesizing BaTiO 3 powders. Generally speaking, the number of washing steps (W n ) remains unclear and an unspecified element in most experimental specifications where the washing approach used in most papers was not specified in detail in the majority of cases [1][2][3]9,10,12,13]. On one hand, many literary studies in this field tend to minimize some synthesis variable/final proprieties connectionin the sense that the experimental results are interpreted in an over-idealized mannerbecause maybe ignoring some details (i.e., washing cycles) of the synthesis history. ...
... It is also the object of the present work, therefore, studying the influenceif anyof washing cycles (W n o15) on the final BaTiO 3 material properties from a chemical and structural point of view. With this aim, the obtained BaTiO 3 cake is washed with distilled water at room temperature with different washing cycles from 0 to 14 times (W n o15) to remove impurities and the excess of Ba from the cake, as already reported in the literature [1][2][3]9,10,12,13]. The filtered solution was collected each time and analyzed by inductively coupled plasma spectroscopy (Perkin-Elmer Co., ICP-OES). ...
... In particular, this results is important because it also suggests that these experimental variables can control the Barium-Titanium stoichiometry of final sintered BaTiO 3 powder in the means of Ba under-stoichiometry (Ba 2 þ /Ti 4 þ o1 in the case of þ1% Ba mol%) and Ba over-stoichiometry (Ba 2 þ /Ti 4 þ 4 1 for þ2% Ba mol%). These experimental results may also shed new light on the effective influence of selected variables of the hydrothermal synthesis process and, in apparent contrast with previous studies [1][2][3]9,10,12,13], on the fact that the BaTiO 3 cake washing is a critical step in the hydrothermal synthesis of BaTiO 3 . ...
Article
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 (Wn<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.
... Some approaches such as sol-gel processing [26], molten salt reaction [27], chemical co-precipitation [28], solvothermal synthesis [29], solid-state calcination [30], and hydrothermal method [31], have been developed for preparing BaTiO 3 with different nanostructures (rod, wire, cube, tube, sphere, etc), which range in size from several nanometers to hundreds of nanometers. Among these approaches, hydrothermal treatment attracted much attention because of its low cost, environmental benignity, easy adjustment, and high product purity. ...
... The result indicated that the sphere-shape BaTiO 3 nano-particles with the average diameter of ∼112 nm, which were prepared under 1 M NaOH solution, exhibited relatively uniform size compared to those BaTiO 3 obtained without extra addition of alkali. This is possibly because of the re-dissolution of the BaTiO 3 from the surface of large particles and the re-precipitation of the dissolved TiO 3 2− onto small particles, as seen from equation (3) [31,33]. In detail, there are amounts of TiO 3 2− on the surface of the irregular BaTiO 3 particles which were initially formed under strong alkaline hydrothermal environment. ...
... As shown in the inset of figure 4(f), a splitting peak in BaTiO 3 -512 curve comes out at approximately 45.1°position. This further demonstrates the strengthening of the tetragonality of the BaTiO 3 -512[31]. ...
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Different sizes of barium titanate (BaTiO3) nano-particles were synthesized from precursor H2Ti3O7 nanotubes through facile hydrothermal synthesis by using EtOH/H2O as mixing solvent. Field-emission scanning electron microscopy (FESEM) was mainly used to investigate the effects of solution alkalinity, polarity, and hydrothermal temperature on the size and morphology evolution of BaTiO3 nanoparticles. The results indicate that the presence of strong alkalinity improved the size evenness of BaTiO3 nanoparticles possibly because of the re-dissolution and re-precipitation of TiO3²⁻ ions. BaTiO3 nanoparticles with the average diameters of approximately 21 and 53 nm were obtained in 9:1 and 1:1 EtOH/H2O (v/v) hydrothermal medium, respectively. Compared with that, the size of BaTiO3 particles prepared in pure distilled water reached up to ∼512 nm. As for the reason, the addition of EtOH could lower the polarity of hydrothermal medium and make the medium reach the supersaturation more easily, thus limiting the size growth of BaTiO3 particles. In addition, the presence of EtOH led to easier formation of BaTiO3 nanoparticles even at mild hydrothermal temperature, but the particle size was limited even though the temperature was much increased. This is possibly due to lowered interfacial activity in the presence of EtOH. By adjusting solution alkalinity, EtOH/H2O volume ratios, and hydrothermal temperature, BaTiO3 nanoparticles with the average sizes of approximately 21, 53, 104, 284, and 512 nm, were obtained, and more different and controlled nano-sizes can be expected by further hydrothermal adjustment. In the end, microwave absorption evaluation indicated that decreased size of the BaTiO3 particles enhanced the reflection loss. One possible reason is that the decreased nano-size led to the increased specific surface area of the BaTiO3 nanoparticles.
... Electrospinning and hydrothermal synthesis are two of the most commonly used methods to prepare BaTiO 3 nanowires [2,[19][20][21][22][23]. Nevertheless, the expensive raw materials/equipment, and low yield make the electrospinning not an ideal option for mass-manufacturing. ...
... Nevertheless, the expensive raw materials/equipment, and low yield make the electrospinning not an ideal option for mass-manufacturing. On the other hand, the hydrothermal approach, has been acknowledged as a simple, low cost and high-throughput production synthesis method, which is more suitable for industrial applications [19,20,24,25]. Previous research shows that the morphology of hydrothermal derived BaTiO 3 nanowires is highly dependent on the synthesis temperature, concentration of reactants, and the nature of the precursor materials [10,[26][27][28][29]. ...
Article
This work demonstrates a highly efficient method for large-scale synthesis of elongated BaTiO3 nanowires via a novel stirring hydrothermal process. Compared with conventional hydrothermal reaction, one-dimensional (1D) lead-free BaTiO3 nanowires can be realized at an even lower temperature with a shorter dwelling time. Results suggest that the mechanical force-driven stirring process improved the diffusion and surface reaction rate of BaTiO3 nanocrystal growth in the solution phase, apparently, leading to the growth of the BaTiO3 nanowires via an oriented attachment mechanism. Meanwhile, to the best of our knowledge, this method is also the first reported protocol which is capable to grow elongated BaTiO3 nanowires with length up to tens of micrometers. This research outcome provides a promising option for low cost, high-throughput fabrication of 1D nanostructure in respect of mass-manufacturing.
... Among other processing routes, hydrothermal synthesis provides the opportunity of direct synthesis of crystalline BaTiO 3 with cubic or tetragonal phases at significantly lower temperatures in strong alkaline solutions [18]. Kržmanc et al., and Zhang et al., demonstrated a two-step hydrothermal synthesis of BaTiO 3 fibers using potassium titanate (K 2 Ti 6 O 13 ) and sodium titanate (Na 2 Ti 3 O 7 ) fibers, respectively [19,20]. In these studies, layered titanates were prepared first and then they served as a template/Ti-source for the hydrothermal formation of BaTiO 3 in fiber morphology. ...
... The number of these examples can be increased based on the precursors and processing parameters. But, in general, the formation of BaTiO 3 under hydrothermal conditions is fundamentally governed by the size, shape, chemistry, and the amount of Tiprecursor [19]. Ti, TiO 2 powders, layered titanates, TiCl 4 , and Ti-containing solutions were the most employed precursors for hydrothermal synthesis of BaTiO 3 . ...
Article
Tetragonal BaTiO3 nanoparticles have been hydrothermally synthesized using the electrospun TiO2 nanofibers as the Ti-source. Microstructural and compositional analyses have been carried out using XRD, SEM, TEM, XPS, and Raman spectroscopy. Characterization studies confirmed that the BaTiO3 nanoparticles had a tetragonal structure with an average particle size of 100 nm. Photocatalytic activities of the nanoparticles were investigated via the decolorization of methylene blue dye aqueous solutions under visible light and UV-A irradiations. The degradation efficiency reached to 13 and 35% after exposing to light for 60 min with visible and UV-sources, respectively. Effect of sintering temperatures on the dielectric properties was also investigated. Among all the sintering temperatures employed, 1473 K was the optimum sintering temperature for these ceramics in terms of high relative density (95.3%), high dielectric constant (3162 at 1 MHz), and low loss tangent (15 × 10−3 at 1 MHz). Experimental results showed that electrospun TiO2 fibers can be used as a precursor template to produce nano-scale BaTiO3 particles which are suitable for various applications such as photocatalysis and capacitors.
... Two major processing techniques are commonly used to fabricate BT-based materials: conventional solid-state reaction process [7,9,13,[16][17][18] and wet chemical processing [19][20][21][22] techniques such as sol-gel, hydrothermal and co-precipitation. The solid state reaction process is one of the most practical and efficient approaches of BTbased materials fabrication for numerous advantages like lower cost, higher productivity and simplicity of preparation. ...
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This paper investigates the non-isothermal kinetic analysis of Ca2+ incorporation into BaTiO3 in Ba0.85Ca0.15TiO3 (BCT15) lead-free ferroelectric material, prepared via the solid-state reaction method, using differential scanning calorimetry (DSC) technique at multiple heating rates. The X-ray diffraction technique (XRD) has demonstrated the formation of pure cubic BaTiO3 phase when the BCT15 precursors were calcined at 1000 °C. Incorporation kinetics was analyzed by Freidman, Ozawa-Flynn-Wall (OFW) and Kissinger-AkahiraSunose (KAS) isoconversional methods. The results indicate that the incorporation process was carried out through a single-step. The kinetic parameters were determined through the combined kinetic analysis method. It was established that the incorporation mechanism follow perfectly the Avrami-Erofeev (A3) kinetic model. The values of activation energy (Ea) and preexponential factor (A) were found to be 388 kJ/mol and 4.1 × 1017 min−1, respectively. The obtained kinetic parameters are critical to better understand and optimize the incorporation process for BCT15 preparation with suitable and promising properties.
... Precipitation from the solution of the crystalline material occurs at temperatures between the boiling point and the critical point of water. Various and more complex morphologies including randomly-shaped [137][138][139], spheres [140][141][142][143][144], cubes [141,[145][146][147]163], nanowires [67], nanotubes [148], as well as more complex dendrite [149] or star-like [150] structures have been synthesized by this method compared to the simple structures synthesized by the sol-gel method. The combination of hydrothermal with sol-gel method has been used for the synthesis of rounded/randomly shaped [36,[155][156][157][158][159], hollow [160], or elongated [161] nanocrystals. ...
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The high demand for energy consumption in everyday life, and fears of climate change are driving the scientific community to explore prospective materials for efficient energy conversion and storage. Perovskites, a prominent category of materials, including metal halides and perovskite oxides have a significant role as energy materials, and can effectively replace conventional materials. The simultaneous need for new energy materials together with the increased interest for making new devices, and exploring new physics, thrust the research to control the structuring of the perovskite materials at the nanoscale. Nanostructuring of the perovskites offers unique features such as a large surface area, extensive porous structures, controlled transport and charge-carrier mobility, strong absorption and photoluminescence, and confinement effects. These features together with the unique tunability in their composition, shape, and functionalities make perovskite nanocrystals efficient for energy-related applications such as photovoltaics, catalysts, thermoelectrics, batteries, supercapacitor and hydrogen storage systems. The synthesis procedures of perovskite nanostructures in different morphologies is summarized and the energy-related properties and applications are extensively discussed in this paper.
... The degradation plots revealed also that impurities such as H 2 O and other oxides remaining from the preparation were present in an amount of 21, 11, 9, and 2% for 0, 1, 2, and 3 Ni-KBTO samples, respectively. The results are in agreement with that detected by Maček Kržmanc et al [26]. ...
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Nano-structure ferromagnetic-ferroelectric composite materials produced by the sol gel method have great attention in different applications due to their desired properties and applications. Potassium Barium titanate doped with different concentrations of Ni2+ nanoparticles (K0.1Ba0.9Ti(1-x)NixO3; x = 0.0-0.3) were prepared by sol-gel process. The final nanoparticles were obtained through drying and calcination at 200 C and 600 C, respectively. The crystalline, nanoparticles' nature and the optical characteristics of the Ni-KBTO nanoparticles are investigated using XRD, SEM, TEM, and optical properties. The XRD and TEM results demonstrate that the Ni2+ NPs were superlatively incorporated within the semi-crystalline KBTO structure. TGA thermogram of all samples shows two weight loss events. The first event is assigned for impurities and/or adsorbed water releasing. The second event was at a higher temperature and assigned as degradation of the nanoparticles. The thermal stability was enhanced by Ni2+ incorporation. The KBTO nanoparticles exhibit enhancement optical bandgap, which decreases from 3 eV to 2.3 eV with increasing Ni2+ in KBTO NPs. This can be ascribed to the change in nanoparticles arrangement and resembles band structures between Ti4+ and Ni2+, resulting in the efficient transfer of charge carriers. The dielectric constant was in the range 106 to 107 at low frequency and about 101 to 102 at high frequency. The Curie temperature of pure Potassium barium titanate is 373 K and 328 K for Potassium barium titanate doped with 20% of Ni2+.
... [17][18][19][20] BT was synthesized by the hydrothermal method, which allows for synthesis in various particle forms, for example, spherical shape, coral-like shape and rod-like shape. [21][22][23] In particular, spherical BT have been used in many studies for composites with high dielectric constants, and in the case of rod-like shape BT, many studies have investigated the aspect ratio which affects the dielectric constant. 24,25 On the other hand, few studies have investigated composites of BT in the coral-like form, 26 and the dielectric properties of BT dispersed in PDMS elastomer have rarely been reported. ...
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Coral-like structured barium titanate (BaTiO3) nanoparticles were synthesized as filler for a high dielectric elastomer. The nanoparticle size, and shape, and the reactivity of the synthesis were modified according to temperature, time, pH, and precursor materials. Dielectric properties of poly(dimethylsiloxane) (PDMS) composites were estimated by volume fractions of BaTiO3 of 5, 10, and 15 vol% for both sphere and coral-like shapes. As a result, coral-like BaTiO3–PDMS composites had the highest dielectric constant of 10.97, which was 64% higher than the spherical BaTiO3–PDMS composites for the 15 vol% fraction. Furthermore, the phase transition process and surface modification were applied to increase the dielectric properties through calcination and improved particle dispersion in the elastomer using polyvinylpyrrolidone (PVP). The dispersion of the PVP coated BaTiO3–PDMS composite was improved compared to pristine BaTiO3 as shown by SEM imaging. The coral-like BaTiO3 embedded composite could be used for electronic devices such as piezoelectric devices or electro-adhesive grippers, which require flexible and high dielectric materials.
... In general, the process uses titanium alkoxide and Ba (or Sr) salts, with and without occurrence of organic species. The reaction modifies and stabilize cations agents preventing the accumulation of the particles [35,36]. The main advantages of this method are: low working temperatures, large control of the particle size (from 10 to 100 nm) and the possibility of synthesize mixed oxide networks. ...
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Barium titanate (BaTiO3) powders of varying particle sizes were grown under hydrothermal conditions at a temperature of 240 °C. This was accomplished by varying the hydroxide concentration of the reaction medium as well as the time required for synthesis. The spectroscopic, thermal, and microscopic characteristics of three BaTiO3 powders with average particle sizes 0.09, 0.3, and 0.5 μm were examined. Transmission electron microscopy indicated that all the particles were of single domain. The powder with smallest particle sizes (0.09 μm) contained unreacted titanium dioxide, whereas the larger particles (0.3, 0.5 μm) were pure barium titanate. The Raman spectra showed that all these crystals were tetragonally distorted. Infrared spectra showed primarily Frohlich modes, whose intensities also decreased with particle size. Analysis of the broadening of the powder diffraction patterns suggested that the BaTiO3 powders exhibited small strains. Differential scanning calorimetry showed a decrease in enthalpy of transition (ΔH) values with particle size. The particle size dependence on ΔH is attributed to the transition from a polar to a nonpolar state which occurs due to the bulk dipoles disordering due to interaction with the surface dipoles.
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Hydrothermal synthesis of barium titanate (BaTiOâ) at 240°C using different barium salts as starting reactants is examined. It is found that halide salts (I⁻, Br⁻, Cl⁻) result in the highest tetragonal content, followed by acetate, nitrate, and hydroxide salts. The tetragonal content was quantified from differential scanning calorimetry by following the endothermic Curie transition around 130°C and reached as high as 37-39% for the materials made with the barium halides. The particle sizes ranged from 0.15 to 0.37 μm, as estimated by scanning electron microscopy. Sintering and dielectric properties of the barium titanates synthesized from different barium sources were examined. BaTiOâ made from the chloride salt sintered with high densities at 1250°C with grain sizes of 1-5 μm and a room-temperature dielectric constant (1 kHz) of 2960. 35 refs., 10 figs., 1 tab.
Article
The formation of nano-sized barium titanate powders by hydrothermal reaction between barium hydroxide octahydrate and hydrous titania was studied at various temperatures, times and reactant concentrations without mineralizer. FTIR, TGA, SEM, XRF, BET and XRD were used to investigate the effects of temperature, time and concentration (in reactivity, particle size, morphology and secondary phase of BaTiO(3) powder prepared hydrothermally. ne main impurities were BaCO(3), TiO(2), surface H(2)O and lattice OH(-) radical at low conversion while secondary phase such as the lattice hydroxyl radical and trace BaCO(3) remained at high conversion. As the temperature and time increased in low concentration reaction below 0.5 M, (he maximum conversion could not exceed 98% but the conversion increased with increasing concentration, reaching over 99% above 1.5 M. In addition, concentration of more than 1.5 M was favorable to smaller particle size, better sphericity and Ba/Ti ratio (sic) 1. Kinetic analysis by the Johnson-Mehl-Avrami plot shows hydrothermal reaction progresses through solidification path. Initially, dissolution-precipitation mechanism takes place to form the nuclei of BaTiO(3) through fast dissolution of TiO(2)center dot xH(2)O and reaction with Ba(2) ions and then the BaTiO(3) crystal begins to grow.
Article
The hydrothermal synthesis of barium titanate particles by reaction of titanate layered nanotubes and barium hydroxide in aqueous solution at 90, 110, and 200 °C from 1 to 24 h has been studied. Anisotropic growth of barium titanate particles was observed when the synthesis temperature was 90 °C resulting in the formation of pseudo-cubic BaTiO3 single-crystal particles with a “wild”-type dendritic shape. Synthesis at 110 °C led to the formation of round-shaped, pseudo-cubic, and tetragonal BaTiO3 particles. At 200 °C, single-crystal barium titanate “seaweed”-type dendritic particles with predominantly tetragonal structure were obtained. This study demonstrates that the anisotropic growth of barium titanate crystals can be controlled by manipulating the temperature and time of reaction and highlights the influence of the synthesis parameters on the anisotropic growth of BaTiO3 crystals under hydrothermal conditions when using layered titanates with nanotube morphology as Ti precursor.
Article
Effects of the chloride ions on the synthesis of tetragonal BaTiO3 in the microwave-hydrothermal (MH) and conventional hydrothermal (CH) processes were investigated. Such effects were found to be dependent of the synthesis method and temperature. In the MH process performed at 240 °C, tetragonal BaTiO3 powders of c/a ratios larger than 1.009 were readily prepared whether the chloride ions were present or not. At the lower temperature of 220 °C, however, the c/a ratio in the MH BaTiO3 was only 1.0061 in the absence of the chloride ions but increased to 1.0084 aided by the chloride ions. In the CH process, the chloride ions were observed to play an invariably important role in the preparation of tetragonal BaTiO3. In contrast, the chloride ions appeared to have less influence on the particle size of the hydrothermal BaTiO3 in a relatively short period of time.
Article
Tetragonal barium titanate with c/a ratio of 1.0093 and average particle size of 240 nm was synthesized by the microwave-hydrothermal (MH) method at 240 °C in only 12 h. Temperatures above 200 °C were first introduced to the MH process in this study, since the temperature has a critical effect on the formation of tetragonal BaTiO3, shown by the experiments here. Hydrous titanium oxide and Ba(OH)2 were used as precursors, without halide anions and alkali-metal cations to avoid contamination. The kinetics of tetragonal phase formation in BaTiO3 was considerably promoted in the MH processing, in comparison with the conventional hydrothermal (CH) route.
Article
A systematic study of the particle size dependence of the ferroelectric domain, internal stress and dielectric constant (K) of BaTiO3 powders was performed using scanning electron microscopy, X-ray diffractometer and differential scanning calorimetry. The dielectric constant of BaTiO3 powder was lower than that of the BaTiO3 sintered body. Low values of the dielectric constant were observed for BaTiO3 powder, which was ascribed to low domain density and constrained forces existing in the particles. As the particles became smaller due to the decrease in the constrained force powders of lower domain density are formed.
Article
The effect of poly(acrylic acid) (PAA), poly(vinylpyrrolidone) (PVP), sodium dodecylsulfate (SDS), hydroxypropylmethylcellulose (HPMC), and D-fructose additives on the growth of BaTiO3 particles by aqueous synthesis is studied. Through different mechanisms, all the tested additives influenced the growth of BaTiO3. For high concentrations, PAA adsorbed on specific crystallographic faces changing the growth kinetics and inducing the oriented attachment of the particles acting as a crystal growth modifier. PVP, SDS, andHPMC behave as growth inhibitors rather than crystal habit modifiers, and barium titanate crystals as small as 26 nm were obtained. D-Fructose appeared to increase the activation energy for barium titanate nucleation when the additive concentration increases. The present study gives new insights into how additives control barium titanate particle growth in aqueous media.
Article
The formation of fine BaTiO3 particles by reaction between liquid TiCl4 and Ba(OH)2 in aqueous solution at 85 °C and pH⩾13 has been studied for 0.062⩽[Ba2+]⩽0.51 mol l−1. The concentration of Ba2+ ions has a strong influence on reaction kinetics, particle size and crystallite size. When [Ba2+]>≈0.12 mol l−1, the precipitate consists of nanosized (≈30 nm) to submicron (100–300 nm) particles of crystalline BaTiO3. At lower concentrations, the final product is a mixture of crystalline BaTiO3 and a Ti-rich amorphous phase even for very long reaction times. A two-steps precipitation mechanism is proposed. Initially, a Ti-rich amorphous precipitate is rapidly produced. Reaction between the amorphous phase and the Ba2+ ions left in solution then leads to crystallisation of BaTiO3. In addition to nucleation and growth of nanocrystals, the final size and morphology of BaTiO3 particles obtained at low concentration can be determined by aggregation of nanocrystals and heterogeneous nucleation on existing crystal surfaces.
Article
The mechanisms driving the mutual crystallographic alignment of nanocrystals in mesocrystals are various and not yet fully understood. As discussed in this paper, formation of mesocrystals can result from a topochemical reaction between single crystal particles or templates suspended in a liquid phase and ionic/molecular species in solution. In such a case, the initial particle morphology is retained in the final mesocrystal. If the transformation is not topotactic, the final product will maintain no memory of the precursor morphology. The magnitude of the lattice mismatch as well as the defective state of the precursor surface probably determine the degree of mutual crystallographic alignment of the nanocrystals which nucleate and grow on the substrate. Although identified by studying the hydrothermal crystallization of SrTiO3 from different single crystal titania precursors, this mechanism is very general and applicable to a variety of compounds and experimental situations, including solvothermal and molten salt syntheses.
Article
Fine particles of barium titanate single crystal with an average particle size of about 66 nm were prepared by a hydrothermal method. In the as‐prepared sample, there was a large amount of the hydroxyl group and barium vacancy, and its crystal structure was assigned to cubic with an expanded lattice using a Rietveld method. The hydroxyl group desorbed with increasing calcination temperature, especially desorbing remarkably in the range from 200 to 300 °C. Above 700 °C, there was no hydroxyl group in the particle, and the crystal structure of the sample treated at 800 °C was assigned to tetragonal with tetragonality of 1.001 using a Rietveld method. The state of the lattice vibration was measured by an infrared reflection method, and analyzed using a nonlinear least‐squares method with a four‐parameter semiquantum model. As the result, the crystal structure of the as‐prepared sample estimated using Fourier transform infrared and Raman was assigned to tetragonal. On the O6 octahedra deformation mode, the resonance frequency was independent of the concentration of the lattice hydroxyl group while the damping factor decreased with decreasing concentration of the lattice hydroxyl group. The role of the lattice hydroxyl group on the O6 octahedra deformation mode is discussed. © 1996 American Institute of Physics.
Article
Hydrothermal microwave method was used as a new route to synthesize pure BaTiO3 (BT) nanoparticles at 140°C for 10 min under rapid reacting with stoichiometric Ba/Ti ratio. The crystalline products were characterized by X-ray powder diffraction (XRD) and the structure was refined by the Rietveld method from the tetragonal structure, which was supported by the Ti K-edge X-ray absorption near-edge structure (XANES). The pre-edge of Ti in the XANES spectra indicated that titanium ions are localized in a nonregular octahedron. Typical FT-Raman spectra for tetragonal BaTiO3 nanoparticles presented well-defined peaks, indicating a substantial short-range order in the system. However, a scattering peak at 810 cm−1 was attributed to the presence of lattice OH− groups, commonly found in materials obtained by hydrothermal process. Besides, the peak at 716 cm−1 can be related to eventual Ba2+ defects in the BaTiO3 lattice. BaTiO3 (BT) nanoparticles presented spherical morphology with a non-uniform distribution of particle sizes. An intense and broad photoluminescence band was observed around the green color emission at room temperature. By means of an excitation energy of 2.54 eV (488 nm), it was noted that the maximum profile emission (2.2 eV) is smaller than the forbidden band gap energy of BaTiO3, indicating that certain localized levels within the band gap must exist.
Article
A thermodynamic model for hydrothermal synthesis of alkaline-earth titanates has been utilized to predict the optimum conditions for the synthesis of phase-pure CaTiO3. The predictions have been experimentally validated using Ca(NO3)(2) or Ca(OH)(2) as sources of calcium and crystalline or hydrous TiO2 as a source of titanium at moderate temperatures (433-473 K). Practical experimental techniques have been developed to avoid the contamination of the calcium titanate with undesirable solid phases (e.g., calcium carbonate or hydroxide). These conditions were compared with those previously determined for the Ba-Ti and Sr-Ti hydrothermal system.
Article
The bulk and surface structure of two BaTiO3 powders has been investigated by FT-IR and FT-Raman spectroscopies. A high-area sample (39 m2/g), produced by a hydrothermal method, cubic to XRD, appears to have lower symmetry to Raman spectroscopy, with unit-cell distortions similar to those of tetragonal ferroelectric BaTiO3. It contains BaCO3 impurities, leading to a bulk Ba/Ti ratio lower than 1, as also evidenced by Raman spectra. Bulk hydroxy groups are evident, possibly needed to balance such a charge defect. Calcination at 1223 K causes the disappearance of these impurities and converts this material in the usual tetragonal phase. The surface properties of both powders, investigated through water and pyridine adsorption, are similar and appear to be dominated by Ba2+ ions, so leading to a solid characterized by extremely weak Lewis acidity. An interpretation for the solid-state behavior of the starting powder is proposed.
Article
A thermodynamic method is proposed for analyzing the hydrothermal synthesis of ceramic materials. The method utilizes standard-state thermodynamic data for solid and aqueous species and a comprehensive activity coefficients model to represent solution nonideality. The method is used to generate phase stability diagrams for the species that predominate in the system. The stability diagrams can be used to predict the optimum suspension synthesis conditions (i.e., feedstock composition, pH and temperature) for hydrothermal synthesis of ceramic materials. The synthesis of barium titanate (BaTiO3(s)) and lead titanate (PbTiO3(s)) are discussed as examples. In the case of the synthesis of BaTiO3(s), which can be obtained at temperatures as low as 363 K, it is important to use solutions of appropriate pH. Practical techniques have been suggested to maintain the required pH by using a correct molar ratio of feedstocks, such as barium hydroxide (Ba(OH)2(s)) to titanium dioxide (TiO2(s)) or use of a mineralizer such as sodium hydroxide (NaOH(s)). It has been shown that contact with atmospheric carbon dioxide (CO2(g)) will always lead to the contamination of the product with barium carbonate (BaCO3(s)). Also, a low-temperature synthesis of PbTiO3(s) has been proposed.
Article
Morphology-controlled barium titanate (BaTiO3) nanostructures, from single-crystal nanorods to nanoparticles, were synthesized by tuning the reaction time of a soft chemical process. Based on the morphology change of the nanostructures, the growth mechanism of the shape-controlled BaTiO3 nanostructures by the soft chemical reaction method was suggested. With the reaction time of 42 h, single-crystal BaTiO3 nanorods with flat surface of facets and defined crystal axis were obtained. As the reaction time was extended to 50 h, a certain volume of nanorods converted to nanoparticles generated by a redissolution−growth reaction. When the reaction time was further elongated to 72 h, the volume of nanoparticles in the product notably increased with single grains of particles that have diameters up to 200 nm. It was determined that an ion exchange and the structural transformation from the layer structure of the precursor into the perovskite structure prevailed at an early stage of the soft chemical reaction, and then the nanorods changed to nanoparticles due to the redissolution−growth reaction for reaction times over 42 h.
Article
A kinetic study of the growth of BaTiO3 under hydrothermal conditions using layered titanate nanotubes as precursors has been performed at 110 and 200 °C. At the early stages of crystallization, irrespective of temperature, pseudocubic BaTiO3 nucleates on the surfaces of the nanotubes. Growth subsequently proceeds by a phase-boundary mechanism with exponent m ≈ 1. During this first regime, “wild type” dendritic particles form with a rough surface and a microstructure which contains planar defects. As the reaction proceeds beyond 70% completion, the nanotubes fully dissolve, and nucleation and growth of tetragonal rather than pseudocubic BaTiO3 take place. During this second regime, the morphology of the BaTiO3 particles remains predominantly dendritic with “seaweed” morphology and a smooth surface, but smaller spherical precipitates and particles with a small number of dendritic arms are also observed. For this stage in the hydrothermal process, the exponent, m ≈ 0.2, does not correspond to any known kinetic mechanisms.
Article
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. Ks(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.
Article
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 0.5M to 2.0M) and decreasing the initial TiCl4concentration (from 0.625M to 0.15M) 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−.
Article
Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO3 from particulate titania in Ba(OH)2 solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)2] > ca. 0.1M the rate-controlling step is the Ba reaction with TiO2 at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)2 concentration, indicating that the TiO2 interface is saturated. At [Ba(OH)2] < ca. 0.1M the rate-determining step shifts to diffusion through the product BaTiO3 layer, the rates are concentration dependent, and the BaTiO3 particle sizes are inversely proportional to the Ba(OH)2 concentrations used.
Article
Tetragonal BaTiO3 powders were prepared hydrothermally, using Ba(OH)2·8H2O and TiO2 (anatase), in the absence of anions such as chloride ions, at a temperature of 220°C for several days. Characterization via X-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry confirmed that increasing the Ba:Ti molar ratios (from 1:1 to 4:1) and alkaline concentrations (from 1.0M to 3.0M) promotes the formation of tetragonal BaTiO3.
Article
BaTiO3 powders with various crystallite sizes, which were prepared through microemulsion-mediated synthesis, were thoroughly studied by Raman spectroscopy. Clear evidence for the presence of the tetragonal phase was found for ultrafine powders with an average crystallite size above 30 nm. The lifetime of phonons that are specifically assigned to the tetragonal phase decreased with decreasing crystallite size below a critical size of 100 nm. In particles as fine as 100 nm, the short mean free path of phonons, mainly due to internal pressure, causes decoupling of the coupled A1 (TO) phonons and a diffuse phase transition behaviour (TC = 115 °C). Coupled A1(TO) phonons, which give a spectral dip at around 180 cm−1 and a lesser extent of diffuseness, were revealed for powders consisting of particles as large as 0.17 µm (TC = 123 °C). Further coarsening upon annealing induced the formation of aggregates, resulting in the shift of phase transition points to higher temperatures for the rhombohedral to orthorhombic and the orthorhombic to tetragonal transitions and to lower temperatures for the tetragonal to cubic transition, respectively. Phase stability in powders is discussed by considering possible factors such as internal pressure in isolated particles and internal stress in aggregates. Copyright © 2007 John Wiley & Sons, Ltd.
Article
Clustering has an important effect on the tetragonal-cubic transformation of barium titanate (BaTiO3) particles. Small particles that would be cubic if they were by themselves can be tetragonal if they are in a cluster. The effects of clustering are shown in the behavior of the c/a ratio of the particles and the enthalpy change, ΔH, of transition as a function of particle size. The c/a ratio and the value of ΔH both decrease at a smaller particle size than those which are observed in samples where clustering is minimal. Our results are consistent with the observation that very small grains in polycrystalline samples can remain tetragonal even though the grain size is so small that it would be cubic if it were an individual particle. The transition temperature, TC, on the other hand, is insensitive to the particle size, which is similar to the observation in polycrystalline BaTiO3 that TC is insensitive to the grain size. The observed clustering effect is suggested to result from the reduction of depolarization energy of particles in clusters.
Article
BaTiO3 dense ceramics with different grain sizes from 5.6 µm down to 35 nm were thoroughly studied by Raman spectroscopy. The temperature characteristics of optical phonons were compared with those obtained for powders. The micrograined ceramic revealed the well-known spectrum profiles and transitions, typical for bulk BaTiO3. On the other hand, the Raman spectra obtained for a nanograined ceramic with an average grain size of 35 nm revealed a tetragonally distorted pure BaTiO3 phase showing a diffused phase transition behaviour with respect to temperature. Abnormality of phonon damping characteristics for the nanograined ceramic was demonstrated through comparison with powders with various crystallite sizes and the micrograined ceramic. The Curie temperature of the nanograined ceramic was estimated to be 105 °C from the temperature characteristic of a sharp peak at 307 cm−1, which is one of the most specific tetragonal features for bulk BaTiO3. In the present study, local stabilization of the tetragonal phase in ultra-fine grains was experimentally demonstrated from comparison between the Raman spectroscopic results for powders and ceramics prepared through microemulsion-mediated synthesis. Rather long phonon mean free paths can exist even in such ultra-fine grains, but the phonon characteristics originating from various grains are diffused mainly because of the effect of internal stress. Copyright © 2007 John Wiley & Sons, Ltd.
Article
Single-crystal barium titanate (BaTiO3) nanorods were synthesized by a hydrothermal reaction using precursors of potassium titanate (K2O·nTiO2, n = 4 or 6) nanostructures. The precursors of potassium tetratitanate (K2Ti4O9) and potassium hexatitanate (K2Ti6O13) nanostructures were prepared by a sol–gel method in which a growth of K2O·nTiO2 (n = 4 or 6) nanorods was induced by a role of pre-crystallized K2O phase at defined heating temperatures. The specific morphologies of BaTiO3 nanorods featured with flat or stepped surfaces and rectangular or polygonal cross-section, were obtainable by selecting the structure of precursors.
Article
Tetragonal BaTiO3 powders were synthesized by microwave-hydrothermal (MH) method. The effects of reaction time, reaction temperature, and reactant composition on the MH synthesis were investigated. Typical experiments were performed at 240 °C, a temperature above 200 °C in MH processing was not reported by previous workers, and used hydrous titanium oxide and Ba(OH)2 as precursors, in the absence of chloride ions and alkali metal ions to avoid contaminations. In MH process, the formation of tetragonal BaTiO3 was strongly enhanced. Tetragonal BaTiO3 formed in only 3 h and approached full tetragonality (c/a = 1.010) when aged for 20 h. At lower temperature of 220 °C, the rate of the formation of tetragonal BaTiO3 and the growth of particles slowed down, substantially. It was found that the excess Ba2+ in hydrothermal solution played an important role on promoting the formation of tetragonal BaTiO3. A few conventional hydrothermal (CH) experiments were carried out for comparison with the MH experiments.
Article
We report for the first time on the controlled hydrothermal synthesis of barium titanate nanostructures using Na 2 Ti 3 O 7 nanotubes and nanowires as synthetic precursors. A variety of nanostructured BaTiO 3 have been prepared, exhibiting either simple shapes of nanowires, nanosheets, nanocubes, and hexagonal nanoparticles or ordered architectures of coral-like nanostructures of assembled nanorods, starfish-like nanostructures, and sword-like nanostructures. The shapes of the various BaTiO 3 products are found to be dependent on the concentration of Ba(OH) 2 , the temperature, and the nature of the precursors. The synthesis route exploits the differences in the hydrothermal stability of the Na 2 Ti 3 O 7 nanotubes and nanowires and the temperature-dependent crystal structure of barium titanate. Various nanoblocks, including nanosheets and nanorods formed from the Na 2 Ti 3 O 7 nanotubes and nanowires, respectively, grow and assemble to form the ordered BaTiO 3 nanostructures. This represents a new approach that is capable of assembling ordered perovskite nanostructures using relatively large nanoblocks formed from layered alkali-metal titanates. The process offers more flexibility than those using inorganic titanium salts or organometallic titanium compounds, which commonly leads to the formation of only BaTiO 3 nanoparticles.
Article
We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) by using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes by using both Rietveld refinement and pair distribution function (PDF) analysis. We observe the ferroelectric tetragonal phase even for the smallest particles at 26 nm. By using temperature-dependent Raman spectroscopy and XRD, we find that the phase transition is diffuse in temperature for the smaller particles, in contrast to the sharp transition that is found for the bulk sample. However, the actual transition temperature is almost unchanged. Rietveld and PDF analyses suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that although structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles.
Inorganic crystal structure database version 1.2.1., Faschinformations zentrum
  • Fiz Nist
FIZ/NIST Inorganic crystal structure database version 1.2.1., Faschinformations zentrum. National Institute of Standards and Technology, Karlsruhe, Germany.
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  • W Zhu
P.K. Dutta, R. Asiaie, S.A. Akbar, W. Zhu, Hydrothermal synthesis and dielectric properties of tetragonal BaTiO 3, Chem. Mater. 6 (1994) 1542-1548.