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

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(Elsevier Share Link: http://authors.elsevier.com/a/1SyqF~2-Enn2X) 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.
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... Based on these results, we examined the mechanism of BaTiO 3 formation during the chemical densification of bulk materials. Some studies have investigated the mechanism of the formation of perovskite oxides in liquid phase via hydrothermal synthesis and similar low-temperature process [28][29][30][31]. In particular, it has been investigated that the hydrothermal synthesis of BaTiO 3 by sealed heating above 100°C under high pressure caused the diffusion reactions proceed via the diffusion of Ba ion into titania particles with in-situ transformation to BaTiO 3 , and the dissolution-precipitation reactions involving the dissolution of titanium compounds in aqueous barium hydroxide solutions [28]. ...
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To achieve carbon neutrality by 2050, novel ceramic processes which can fabricate it at lower temperatures compared with conventional sintering process are required. Bulk barium titanate (BaTiO3) is directly synthesized from hydrous titania gel (TiO2·nH2O) pellets near room temperature via the acid–base chemical densification (ABCD) process, which affords perovskite oxides via neutralization reactions. TiO2·nH2O used as precursor material has amorphous structure and includes a trace amount of water in its structure. Scanning electron microscopy images show that the mechanism of BaTiO3 crystallization in TiO2·nH2O pellet is dependent on the reaction conditions in the barium hydroxide solution, including the heat-treating temperature and time. The bulk BaTiO3 are obtained from TiO2·nH2O pellets by treating below 100 °C for a long time more than 25 h; however, heat-treating at higher temperatures caused TiO2·nH2O pellets to disintegrate. It is consolidated that dominant reaction, whether the diffusion reaction or a dissolution–precipitation reaction, in this process depends on the reaction temperature and time. Then, at higher temperatures than 120 °C, a part of TiO2·nH2O pellets dissolve to barium hydroxide solution as a dissolution–precipitation reaction. In contrast, at below 120 °C, BaTiO3 bulk is obtained by diffusion reaction of Ba ion into TiO2·nH2O pellet. Graphical Abstract
... We assume that the presence of BaCO 3 is caused primarily by insufficient heating time. In this case, interaction with the environment during calcination leads to the formation of a small amount of barium carbonate in the form of an impurity, as evidenced by the studies presented in [64,65]. At the same time, it is worth considering the fact that there was no forced air pumping in the furnace. ...
... The secondary phase during the solidification process might impede grain growth [42]. Particles of calcium titanate could act as barriers during the solidification process, limiting the movement of grain borders and halting the coarsening or growth of the grains into bigger sizes [43]. In the microstructure, it is evident that second phases appear together with the grain boundary liquid phase and precipitate in BCT. ...
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Barium calcium titanate (BCT) ceramics with varying yttrium doping concentrations were fabricated using the solid-state compaction process to explore the attributes of dopants. (Ba0.75Ca0.25) TiO3 and (Ba0.75Ca0.25) (YyTi(1-y)) O3 where, y = 0.00, 0.10, 0.15, and 0.20 ceramics were synthesized by pressing isostatically in pellet press apparatus, then sintered at 1250 °C with consequent cooling in furnace ambient. The structural, morphological, and dielectric properties were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and impedance spectroscopy interpretations, respectively. The XRD analysis revealed that the cubic BCT lattice was transformed into a tetragonal structure with Yttrium doping. Scanning electron microscopy (SEM) disclosed that yttrium doping countered the liquid phase formation of BCT as well as influenced grain development and microstructure, leading to the formation of distinct grain boundaries and improved densification. The average grain size (18–29 nm) of the Y-BCT increases as the doping level rises. At 60 Hz, it was reported that the dielectric constant obtained a maximum value of 70000 with a resistivity of 5 × 10⁸ Ώ-cm for y = 0.15. The manifestation of the secondary phase confirmed from XRD, allocating an easy path for oxygen migration, might be responsible for the rise in oxygen vacancy, higher leakage current, and dielectric loss for y = 0.01. Co-doping of calcium and yttrium in BCT ceramics has modified the basic structure and ameliorated composites' structural stability and dielectric characteristics. The optimized sample, upon demonstrating outstanding efficiency, ought to be employed for specific uses such as energy storage devices and capacitors.
... The Raman spectra revealed a broad band at ~780 cm − 1 representing the amalgamation of La 3+ in the BTLa 10 Y 10 ceramics. This vibrational band may be related to the defects generated through charge recompense intended to entering La 3+ into the Ba 2+ site [22,23]. Moreover, R and T phases co-occurred for the BTLa 10 Fig. 2. FT-IR spectra of BTLa 10 and BTLa 10 Y 10 ferroelectric ceramics. ...
... The high concentration of surfactants in the microemulsion method restricts the development from the perspective of cost and environmental protection [10]. In contrast, the hydrothermal method makes it possible to prepare BT particles (BTPs) under milder conditions and is of particular interest due to its energy saving, facile operation, and eco-friendly feature [11]. Most importantly, a simple and eco-friendly synthesis strategy avoids the use of toxic surfactants and the introduction of surface impurities in particles [12]. ...
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