Nd-doped barium titanate ceramics with various Ti/Ba ratios prepared by sol-gel method

The Nd-doped BaTiO3 nanocrystalline powders and ceramics with different Ti/Ba ratios were prepared by sol-gel method. Phases and microstructures of the Nd-doped BaTiO3 based powders and ceramics were characterized by XRD, SEM and TEM methods. The results revealed that the powders synthesized by sol-gel method were nanometer scale (30-60 nm) and were mainly composed of cubic BaTiO3 with a small amount of BaCO3. After sintering at high temperature, both cubic BaTiO3 and BaCO3 were transformed into tetrahedron BaTiO3 phase. The dielectric properties of the ceramics were also determined and the influence of Ti/Ba ratio on the dielectric properties was discussed. The Tc did not change with the variation of Ti/Ba ratio, while theεmax increased firstly and then decreased. The excess TiO2 is benefit for the modification of ceramics' microstructure and dielectric properties.     

Barium titanate via thermal decomposition of Ba,Ti-precursor complexes: The nature of the intermediate phases

The thermal decomposition of Ba,Ti-precursor complexes, containing organic ligands and suitable for the single-source preparation of nanocrystalline BaTiO₃, leads firstly to the segregation of specific Ba-rich and Ti-rich phases. Quantitative electron energy loss spectroscopy and powder X-ray diffraction data indicated that the (i) Ba-rich phase is a BaO-stabilised variant of the calcite-type high-temperature modification of BaCO₃ and (ii) Ti-rich phases are represented by low crystalline barium titanates with the general Ba:Ti ratio close to 1:4. The subsequent solid state reaction between these phases results then in the formation of BaTiO₃.     

Sol-Gel Synthesis of Nano-Scaled BaTiO3, BaZrO3 and BaTi0.5Zr0.5O3 Oxides via Single-Source Alkoxide Precursors and Semi-Alkoxide Routes

Sol-gel synthesis of nano-sized BaTiO₃, BaZrO₃ and BaTi_0.5Zr_0.5O₃ ceramics using alkoxide and semi-alkoxide routes has been investigated and the pervoskites obtained have been compared with respect to crystallisation temperature, crystallite size and compositional purity. Heterometal alkoxides containing two (for BaTiO₃ and BaZrO₃) and three (for BaTi_0.5Zr_0.5O₃) different metals were used as single-source precursors in the alkoxide route while semi-alkoxide synthesis was performed by reacting barium hydroxide or acetate with Ti and/or Zr alkoxides. Semi-alkoxide synthesis also produces stoichiometric and phase-pure oxides, however, at temperatures higher than 1000°C. At temperatures below 1000°C, BaCO₃ and small amounts of other undesired phases (e.g., BaTi₂O₄) were present in the oxides derived from semi-alkoxide synthesis. Thermal behaviour, studied by TGA/DTA measurements, shows that thermal decomposition occurs in three major steps and depends on the educt composition and the synthesis route. Among alkoxide derived powders, crystalline BaTi_0.5Zr_0.5O₃ phase is formed at 400°C while complete crystallisation of BaMO₃ ceramics occurs around 600°C. The cubic to tetragonal phase transition for BaTiO₃ is clearly observed at relatively low-temperature of 800°C. The stoichiometry and phase homogeneity of the obtained powders were demonstrated by energy dispersive X-ray analysis and powder diffractometry. The averaged crystallite size of the obtained nano-ceramics was evaluated using the FormFit programme. SEM and TEM observations revealed a high microstructural uniformity.     

Detailed FT-IR spectroscopy characterization and thermal analysis of synthesis of barium titanate nanoscale particles through a newly developed process

BaTiO₃ nanopowders were prepared at lower temperature and in shorter time span through a newly developed process. To reach an in-detail understanding of mechanism of the process, thermal analysis were performed using DTA/TGA and FT-IR techniques aided by in-depth discussions. The results indicated that thermal evolution of sol–gel-powder transformation consists of following steps: the vaporization of volatile organic solvents, the deformation of gel structure and pyrolysis of the Ba–Ti organic precursors, the decomposition of BaCO₃ into BaO and CO₂, reaction of resulted BaO with TiO₂ to form BaTiO₃. The results also elucidated the reaction pattern and formation mechanism of BaTiO₃ nanopowders from gel-derived precursor.     

BaTi1−x SnxO3 Solid Solutions: Solid-Phase and Sol-Gel Syntheses and Characterization

Ba(Ti_1?x Sn_x)O₃ solid solutions were synthesized by the sol-gel process and solid-phase reactions, and their electrophysical properties studied. SnCl₄ · 5H₂O, TiCl₄, and BaCO₃ were precursors in the sol-gel process. IR spectroscopy, X-ray powder diffraction, and differential thermal analysis were used to study the formation conditions for BaTiO₃, BaSnO₃, and BaTi_0.85Sn_0.15O₃.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

溶胶-凝胶法制备掺锰钛酸钡纳米粉体及其陶瓷

The Mn-doped barium titanate nanosized powders and ceramics were prepared via the sol-gel process. The powders and ceramics were characterized by XRD， SEM and TEM. The dielectric properties of the ceramics were also measured. The influences of calcination temperature and Mn concentration on the microstructure， dielectric properties and phase composition of BaTiO₃ nano-powders and ceramics were discussed. The results indicated that the BaTiO₃-based powders doped with 1.0mol% Mn were mainly in cubic BaTiO₃ phase， but the tetragonal phase became more evident when the calcination temperature increased. After sintering， Mn-doped ceramics were mainly composed of cubic BaTiO₃. Specially， a new phase of hexagonal crystal BaTiO₃ and BaMnO₃ existed in the ceramics doped with 5.0mol% Mn and the ceramic grains were in ‘clintheriform’. The structure of ‘clintheriform’ led to the poor densification of ceramics， reducing the dielectric constant obviously. The dielectric constants of BaTiO₃ ceramics first increased and then decreased as the Mn concentration increased. The room temperature dielectric constant was 2 290 and the lowest dielectric loss was 0.004 when the Mn concentration was 0.5mol%.     

High-throughput first-principle calculations of the structural,mechanical, and electronic properties of cubic XTiO3 (X = Ca,Sr, Ba,Pb) ceramics under high pressure

High-throughput first-principle calculations are implemented to study the structural, mechanical, and electronic properties of cubic XTiO₃ (X = Ca, Sr, Ba, Pb) ceramics under high pressure. The effects of applied pressure on physical parameters, such as elastic constants, bulk modulus, Young's modulus, shear modulus, ductile-brittle transition, elastic anisotropy, Poisson's ratio, and band gap, are investigated. Results indicate that high pressure improves the resistance to bulk, elastic, and shear deformation for XTiO₃ ceramics. Pugh's ratios B/G reveal that CaTiO₃ and PbTiO₃ ceramics are ductile, but SrTiO₃ and BaTiO₃ ceramics are brittle under the ground state. The brittle-to-ductile transition pressures are 24.26 GPa for SrTiO₃ and 43.23 GPa for BaTiO₃. Under high pressure, the strong anisotropy promotes the cross-slip process of screw dislocations, and then enhances the plasticity of XTiO₃ ceramics. Meanwhile, XTiO₃ (X = Ca, Sr, Ba) is intrinsically an indirect-gap ceramic, but PbTiO₃ is a direct-gap ceramic. High pressure increases the band gap of XTiO₃ (X = Ca, Sr, Ba) ceramic, but decreases that of PbTiO₃ ceramic. This work is helpful for designing and applying XTiO₃ ceramics under high pressure.     

Preparation and electrorheological characteristic of Y-doped BaTiO3 suspension under dc electric field

The electrorheological (ER) effects of BaTiO₃ or other perovskite materials with high dielectric constant are presumed to be large. However, their weak ER activity is very puzzling. In this study, we choose cubic BaTiO₃ and first achieve its ER enhancement under dc electric field by modifying its intrinsic structure with doping rare earth Y ions, which are synthesized by means of sol-gel technique. DSC-TG, FT-IR, XRD, ICP and XPS techniques are used to characterize thermal, structure and component change of materials. It is demonstrated that Y³⁺ substitutes for Ba²⁺, which causes lattice-distorting defects. Rheological experiments show that Y-doped BaTiO₃ suspension has notable ER effect and clear fibrillation structure under dc electric field, while the pure cubic BaTiO₃ suspension suffers from electrophoretic effects and its ER effect is very weak. The ER effect of typical Y-doped BaTiO₃ ER suspension is ten times that of pure BaTiO₃ ER suspension. Based on the electrical measurements, the enhancement of ER activity of BaTiO₃ may be attributed to the increase of conductivity due to Y-doping. The enhancement in ER activity of cubic BaTiO₃ under dc electric field by doping rare earth Y ions is helpful to further understand the perovskite-based ER materials with high dielectric constant but low ER activity.     

Thermal Properties of Mechanochemically Pretreated Precursors of BaTiO3 Synthesis

The changes of physico-chemical properties of mechanochemically pretreated (BaCO₃ +TiO₂ +PbO) powders were investigated. The values of apparent activation energy of BaTiO₃ formation calculated by the Freeman and Carroll method decrease with milling time. The changes of precursors density may be interpreted as a consequence of mechanochemical reactions during milling. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterization of core-shell structured TiO2–BaCO3 particles

Preparation of core-shell structured TiO₂–BaCO₃ particles as precursor of BaTiO₃ genesis, proceeds using a two step procedure, by first coating the TiO₂ core by Ba(OH)₂ shell followed by conversion of the shell region with CO₂ gas by the formation of BaCO₃. Straightforward experimental results reveal environmental scanning electron microscopy (ESEM) and scanning transmission electron microscopy (STEM) as suitable methods for analytical characterization of the core and shell regions from individual TiO₂–BaCO₃ grains. Evidence of coating the whole ensemble of TiO₂ particles is possible using Photo Electro Motive Force (Photo EMF, PEMF) measurements. This method is able to indicate very sensitively changes of surface properties of TiO₂ after coating with Ba(OH)₂ and BaCO₃, respectively. PEMF measurements were used for the first time with concern to this topic.     

Raman microscopic investigations of BaTiO<Subscript>3</Subscript> precursors with core–shell structure

Due to their outstanding dielectric and ferroelectric properties, barium titanate (BaTiO₃)-based ceramics have found many applications in electronic devices. To optimise the final quality of such ceramics, a detailed knowledge of the complex processes involved in the formation of BaTiO₃ is required. The phase formation process in ordered structures of the BaCO₃/TiO₂ system was analysed by X-ray diffraction and by Raman spectral imaging (RSI) as a function of the annealing temperature. RSI was used for the first time as a locally resolving method for phase analysis, and proved to be a useful tool in examining the formation process of BaTiO₃ starting from spherical, core–shell structured precursors of the type TiO₂ core/BaCO₃ shell. The Raman spectra of different BaO–TiO₂ phases appearing as intermediate phases during the formation of BaTiO₃ were recorded for separately-prepared pure substances. Using these spectra as fingerprints, and choosing phase filters by setting wave number windows, phase landscape pictures of the samples at different temperatures during the genesis of BaTiO₃ could be created with a lateral resolution of up to 200 nm. These pictures confirm shell-like formation of the different barium titanate phases according to the diffusion of barium and oxygen ions from the Ba-rich shell into the TiO₂ core. At an intermediate state of the phase formation process, the phase sequence Ba₂TiO₄, BaTiO₃, BaTi₂O₅, BaTi₄O₉ and BaTi₅O₁₁ to TiO₂ was detected from the outer to the inner parts of the core–shell structures.     

EPMA and Microstructural Characterization of Yttrium Doped BaTiO3 Ceramics

 Yttrium-doped BaTiO₃ ceramics have been studied as a potential material for positive temperature coefficient resistors (PTCR). The mechanism of Y incorporation into BaTiO₃ plays an important role for displaying good electrical properties. Determination of the amount of yttrium in the BaTiO₃ as well as microstructure characterization of the samples were performed using SEM, EDS and WDS analysis. An optimized trace element WDS quantitative analysis was applied to determine elemental concentrations for Ba, Ti and Y in the samples as accurately as possible. BaTiO₃ and Y₂O₃ were used as standards. Analysis was undertaken using a JEOL JXA 840A electron probe microanalyzer. WDS X-ray intensity measurements were performed under 20 kV, 50 nA beam current and 0.2% preset standard counting deviation (σ_c) using a PET crystal. Measured k-ratios were quantified by ZAF matrix correction. Average results of WDS quantitative analysis showed 20.17 ± 0.08 at % Ti, 19.95 ± 0.09 at % Ba, 0.22 ± 0.03 at % Y, and 59.66 at % O. The results suggest incorporation of yttrium in the BaTiO₃ preferentially at the Ba-sites, however partial incorporation of Y at Ti-sites could not be excluded.     

A water-soluble precursor of BaTiO3 and transparent BaTiO3 thin-films prepared from the solution by a water-based dip-coating technique

A new water-soluble precursor of BaTiO₃ was prepared from citratoperoxotitanate and barium citrate as the Ti and Ba sources, respectively. The water-soluble precursor was easily solved in water to form a stable solution, which produced BaTiO₃ by heat-treatment at 500 °C and above. A water-based dip-coating technique demonstrated a potential application as the coating solution of BaTiO₃. Transparent BaTiO₃ films were formed on the quartz-glass substrates with an increment of typically 9 nm per coating with 0.05 mol dm^?3 solution. The transmittance of the 180 nm-thick film attained almost 90 % at the maximum and the overall transmittance was above 60 % over the visible region. The polycrystalline film was composed of BaTiO₃ grains smaller than 200 nm. Although the film was an insulator, it was not suited for the dielectric application because of the structural problems due to the relatively low density and the thinness of the BaTiO₃ layer. The BaTiO₃ pellet obtained from the water soluble precursor by condensation, pyrolysis and sintering showed the good dielectric properties with ε_r = 3,500 and tan δ = 0.027 with a sintering temperature of 1,375 °C.     

Bi1.5ZnNb1.5O7 cubic pyrochlore ceramics prepared by aqueous solution–gel method

Bi_1.5ZnNb_1.5O₇ cubic pyrochlore ceramic was successfully prepared by the aqueous solution method. The preparation, microstructure development and dielectric properties of ceramics were investigated. Homogeneous precalcined ceramics powders have a cubic pyrochlore phase after thermal treatment at the temperature as low as 450 °C. The aqueous solution–gel method, which Bi, Zn and Nb ions are chelated to form metal complexes, leading to the formation of cubic pyrochlore phase at low firing temperatures. No detectable intermediary phase such as BiNbO₄ or pseudo-orthorhombic pyrochlore is observed in the XRD patterns of ceramics at the sintering temperature range from 850 to 1,000 °C. The dielectric properties study revealed that the ceramics sintered at 900 °C show excellent performance with dielectric constant of 111 and dielectric loss of 2.3871 × 10⁻⁴ under 1 MHz at room temperature.     

Synthesis and Microwave Absorption Properties of BaTiO3-polypyrrole Composite

BaTiO₃ powders are prepared by sol-gel method by cotton template. Polypyrrole is pre-pared by chemical oxidation route in the emulsion polymerization system. Then BaTiO₃-polypyrrole composites with different mixture ratios are prepared by as-prepared material.The structure, morphology, and properties of the composites are characterized with Infrared spectrum, X-ray diffraction, scanning electron microscope, and net-wok analyzer. The com-plex permittivity and reflection loss of the composites are measured at different microwave frequencies in S-band and C-band (0.03—6 GHz) employing vector network analyzer model PNA 3629D vector. The effect of the mass ratio of BaTiO₃ to polypyrrole on the microwave loss properties of the composites is investigated. A possible microwave absorbing mechanism of BaTiO₃-polypyrrole composite is proposed. The BaTiO₃-polypyrrole composite can find applications in suppression of electromagnetic interference and reduction of radar signature.     

Formation of metastable calcite-type barium carbonate during low-temperature decomposition of (Ba,Ti)-precursor complexes

(Ba,Ti)-precursor complexes, important for the production of advanced BaTiO₃ perovskite-type materials, undergo structural transformations and complex reactions during their thermal decomposition. Based on XRD phase analysis, combined with Rietveld refinement of crystal structure data, and on IR analysis, the intermediate formation of calcite-type BaCO₃ is evidenced, which can be explained by the stabilization of this metastable modification in the form of an oxycarbonate phase down to room temperature. Two possible processes, leading to such an oxycarbonate, are discussed: (i) partial substitution of CO₃²⁻ by O²⁻ in the anionic sublattice, and (ii) topotaxial formation of calcite-type structural domains of BaCO₃ by templating with oxygen-deficient titanates, resulting in the oxide–carbonate intergrowth structures.     

BaPbO3的生成及EXAFS研究

采用低Pb/Ba摩尔比及低的反应温度，以BaCO₃和PbO为原料，固相反应合成了BaPbO₃无机微粉。通过XRD分析，确定了不同Pb、Ba比例下生成的Ba-Pb-O化合物均为立方钙钛矿结构。首次采用EXAFS对BaPbO₃微粉中Pb原子的近邻结构进行了分析，分析了在立方钙钛矿BaPbO₃结构中，氧空位的存在是导致BaPbO₃具有类似金属导电性的主要原因。     

Influence of barium source on the characteristics of sol-precipitated BaTiO<Subscript>3</Subscript> powders and related ceramics

In order to obtain pure and fine BaTiO₃ powders with controlled morphology, sol-precipitation methods involving the use of titanium iso-propoxide and of two different barium sources, i.e. barium nitrate and barium acetate, were proposed in this work. The thermal behaviour of the synthesized gels and the X-ray diffraction data obtained for the oxide powders pointed out that, by using Ba(NO₃)₂ as barium source, the decomposition process was completed at lower temperature (750°C) and was accompanied by a more pronounced tendency to obtain a single phase BaTiO₃ composition, by comparison with the synthesis where barium acetate was used as raw material (1100°C). Scanning electron microscopy investigations emphasized the effect of the nature of barium source and synthesis conditions on the morphology of the oxide powders, as well as on the microstructure of the related ceramics.     

AgNbO<Subscript>3</Subscript> ceramics synthesized by aqueous solution-gel method

AgNbO₃ powders and ceramics were prepared by aqueous solution-gel method. The phase evolution of the powders was investigated by TG/DSC and XRD. The results showed that the pure AgNbO₃ phase was obtained at 600 °C without special treatment. The sintering behavior and dielectric properties of the AgNbO₃ ceramics were also investigated. It showed the dense ceramics were obtained as lower as 925 °C, which had the excellent dielectric properties with the permittivity of 291 and dielectric loss of about 1.7% at 1 MHz. The coarse grains were observed for the sample sintered over 975 °C, and then they decreased with the sintering temperature further increasing to 1,050 °C.