Synthesis of BaTiO<Subscript>3</Subscript> powder from barium titanyl oxalate (BTO) precursor employing microwave heating technique

Cubic barium titanate (BaTiO₃) powder was synthesized by heating barium titanyl oxalate hydrate, BaTiO(C₂O₄)₂·4H₂O (BTO) precursor in microwave heating system in air at 500°C. Heating BTO in micro-wave above 600°C yielded tetragonal form of BaTiO₃. Experiments repeated in silicon carbide furnace showed that BaTiO₃ was formed only above 700°C. The product obtained was cubic.     

PTCR ceramics produced from oxalate-derived barium titanate

BaTiO₃ powders with an average crystallite size from 68 to 2000 nm have been prepared by calcining barium titanyl oxalate at temperatures from 700 to 1150°C, and their morphology and recrystallization kinetics have been studied. The powders have been used to produce positive temperature coefficient of resistance (Ba,Ca,Pb)TiO₃ ceramics, and the microstructure and electrical properties of the ceramics have been investigated. The results indicate that improving the crystallinity of the barium titanate powder suppresses recrystallization in the ceramics, increases their resistivity, and has a significant effect on their resistance jump and electric strength. We have identified the optimal temperature range of barium titanyl oxalate calcination, which insures the highest electric strength of PTC thermistors with a resistance of 31 Ω. The corresponding crystallite size of the barium titanate powder is ?200 nm.     

Effect of the crystallinity of BaTiO<Subscript>3</Subscript> powders prepared using the merker method on the properties of PTCR ceramics

Barium titanate powders differing in particle size (110–740 nm) were prepared by calcining barium titanyl oxalate precipitated by the Merker method. The powders were sintered to produce PTCR ceramics with the composition 100(Ba_0.89Ca_0.08Pb_0.03)TiO₃ + ^0.8TiO₂ + ^0.7Y + ^0.1Mn + ^2.5SiO₂ and electrical properties of the ceramics were studied. The results demonstrate that improving the crystallinity of the barium titanate powder suppresses recrystallization of the ceramics and has a significant effect on their resistance ratio and electric strength. We found the optimal range of calcination temperatures (950–1000°C) for barium titanyl oxalate which ensures the highest electric strength of thermistors with a resistance of 31 Ω. The average crystallite size of the parent barium titanate powder is ∼250–320 nm.     

Synthesis and microstructure of SrTiO<Subscript>3</Subscript> and BaTiO<Subscript>3</Subscript> ceramics by a reaction-sintering process

Strontium titanate and barium titanate ceramics prepared by a reaction-sintering process were investigated. The mixture of raw materials of stoichiometric SrTiO₃ and BaTiO₃ was pressed and sintered into ceramics without any calcination stage involved. A density 4.99 g/cm³ (97.5% of the theoretic value) was found in SrTiO₃ after 6 h sintering at 1,370 °C. Grains less than 1.5 μm were formed at 1,300–1,330 °C and became 2.2–3.3 μm at 1,350–1,370 °C SrTiO₃. A density 5.89 g/cm³ (97.9% of the theoretic value) was found in BaTiO₃ after 6 h sintering at 1,400 °C. Merged grains were observed in BaTiO₃ and were less than 10 μm after sintered at 1,400 °C.     

Solid-state synthesis of nano-sized BaTiO<Subscript>3</Subscript> powder with high tetragonality

A solid-state reaction method was used to synthesize nano-sized, Ca-doped BaTiO₃ powder with high tetragonality (=c/a) in order to increase the volumetric efficiency of multilayer ceramic capacitors (MLCCs). The reaction temperatures for three different starting material combinations were examined by thermogravimetric/differential thermal analysis (TG/DTA). Nano-sized starting materials and the mechanochemical activation of the needle-shaped BaCO₃ via high-energy milling were effective in decreasing the reaction temperature. In addition, the results showed that the tetragonality of fine Ca-doped BaTiO₃ could be enhanced by 2-step heat treatment, consisting of holding at 800 °C for 1 h followed by consecutive heating to the target temperature, without any significant grain growth than that of the conventional 1-step calcination. The synthesized particles heat-treated at 950 and 1,000 °C by 2-step heat treatment were confirmed by characterization to have an average size of 128 and 212 nm, and a tetragonality of 1.0097 and 1.0105, respectively, which are higher tetragonality values than those previously reported for similar sized particles.     

Microwave synthesis of nano-sized barium titanate

Nano-sized barium titanate powders have been synthesized by microwave processing at 2.45 GHz. Using barium titanyl oxalate (BTO) as a precursor, microwave processing was carried out by heating the precursor to a temperature between 600 °C and 750 °C with different heating rates from 10 °C/min to 20 °C/min without isothermal holding. X-ray diffraction analysis indicates that the decomposed product at 680 °C was pure cubic BaTiO₃. The BET specific surface area of barium titanate powder, after microwave heating to 680 °C, was 14.2 ± 0.5 m²/g, corresponding to an average particle size of 70 nm. This particle size was confirmed by the scanning electron microscopy (SEM). Parallel study shows that the conventional heating in a regular resistance furnace using a similar heating schedule did not result in complete conversion of BTO. This study shows that the microwave processing significantly accelerated the decomposition of barium titanyl oxalate and reduced the temperature of barium titanate nano-powder formation, resulting in nano-sized pure cubic barium titanate powder.     

Facile synthesis and electrochemical properties of layered Li[Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>]O<Subscript>2</Subscript> as cathode materials for lithium-ion batteries

A layered oxide Li[Ni_1/3Mn_1/3Co_1/3]O₂ was synthesized by an oxalate co-precipitation method. The morphology, structural and composition of the as-papered samples synthesized at different calcination temperatures were investigated. The results indicate that calcination temperature of the sample at 850°C can improve the integrity of structural significantly. The effect of calcination temperature varying from 750°C to 950°C on the electrochemical performance of Li[Ni_1/3Mn_1/3Co_1/3]O₂, cathode material of lithiumion batteries, has been investigated. The results show that Li[Ni_1/3Mn_1/3Co_1/3]O₂ calcined at 850°C possesses a higher capacity retention and better rate capability than other samples. The reversible capacity is up to 178.6 mA?h?g^-1, and the discharge capacity still remains 176.3 mA?h?g^-1 after 30 cycles. Moreover, our strategy provides a simple and highly versatile route in fabricating cathode materials for lithium-ion batteries.     

Effects of doping with La and Mn on the crystallite growth and phase transition of BaTiO3 powders

The effects of La and Mn dopants on the crystallite growth and the phase transformation of BaTiO₃ powders were studied. The barium titanate powders were obtained by calcining barium titanyl oxalate tetrahydrate in the temperature range 800 to 1200 °C. Crystallite growth of BaTiO₃ powders was promoted by the use of Mn dopant due to the increase of oxygen vacancies. The dissolution of La dopant into BaTiO₃ structure may decrease the oxygen vacancies so that the growth of BaTiO₃ crystallites is inhibited at high temperature ( 900 °C). When the crystallite size is small, the barium titanate can exist as a cubic phase due to the manifestation of the surface energy. Undoped cubic BaTiO₃ powders can be stable at a size < 30 nm. Doping with La and Mn would bring the crystallite size for the cubic-to-tetragonal phase transformation to 100 nm, resulting from the presence of cation or oxygen vacancies.     

Oxalate-precursor processing for high quality BaZrO<Subscript>3</Subscript>

BaZrO₃ is by far the most inert crucible material that has been used for melt processing of high quality single crystal YBCO superconductors. To overcome the processing difficulties of existing solid-state methods, solution processing methods are increasingly important in powder synthesis. This study investigates several methods of producing oxalate precursors for subsequent thermal decomposition to BaZrO₃ with a view to producing high quality BaZrO₃ ceramics. The most favourable system used barium acetate, ammonium oxalate and zirconium oxychloride, which unlike other previously reported oxalate processes allowed near stoichiometric precipitation without requiring a large excess of Ba reagents, elevated precipitation temperatures or slow addition of reagents. Precise control over precipitate stoichiometry was achieved by variation of the solution Ba:[Zr+Hf] mole ratio without requiring accurate control over oxalate addition. XRF, XRD, N₂ BET adsorption, DTA/TGA and TEM analysis showed this process to be capable of producing BaZrO₃ powders suitable for ceramics applications. The phase purity, particle size and surface areas of BaZrO₃ powders produced by calcination of these precursors can be adjusted by variation of stoichiometry and calcination temperature. Crucibles formed from oxalate precursors have been able to contain Y₂O₃-BaCuO₂-CuO melts for up to seven days.     

Synthesis of CaTiO<Subscript>3</Subscript> from calcium titanyl oxalate hexahydrate (CTO) as precursor employing microwave heating technique

Calcium titanate, CaTiO₃, an important microwave dielectric material and one of major phases in synroc (synthetic rock), a titanate ceramic with potential application for fixation of high level nuclear waste was synthesized from calcium titanyl oxalate [CaTiO (C₂O₄)₂·6H₂O] (CTO) by employing microwave heating technique. CTO heated in microwave heating system in air at 500°C for 1 h gave a perovskite, CaTiO₃. The product obtained by heating of CTO in the same system at 700°C for the same duration was however, much more crystalline. CaTiO₃ obtained by the present method was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and BET surface area measurement.     

Preparation and characterization of Mn-doped BaTiO<Subscript>3</Subscript> thin films by magnetron sputtering

Barium titanate (BaTiO₃) thin films doped with Mn (0.1–1.0 at%) were prepared by r.f. magnetron sputtering technique. Oxygen/argon (O₂/Ar) gas ratio is found to influence the sputtering rate of the films. The effects of Mn doping on the structural, microstructural and electrical properties of BaTiO₃ thin films are studied. Mn-doped thin films annealed at high temperatures (700 °C) exhibited cubic perovskite structure. Mn doping is found to reduce the crystallization temperature and inhibit the grain growth in barium titanate thin films. The dielectric constant increases with Mn content and the dielectric loss (tan δ) reveals a minimum value of 0.0054 for 0.5% Mn-doped BaTiO₃ films measured at 1 MHz. The leakage current density decreases with Mn doping and is 10⁻¹¹ A/cm⁻² at 6 kV/cm for 1% Mn-doped thin films.     

Processing of chemical solution-deposited BaTiO<Subscript>3</Subscript>-based thin films on Ni foils

BaTiO₃ films on base metal foils are of interest for capacitor applications, but the processing requires reducing atmospheres that influence the film defect chemistry and density. In this study, powders dried from barium titanate solutions and barium titanate films were studied by X-ray diffraction, differential scanning calorimetry, thermal gravimetric analysis, infrared spectroscopy, and spectroscopic ellipsometry at various points in the processing. It was found that atmospheres designed to minimize Ni oxidation delay decomposition of organics, leading to retained carbonate phases. Thus, crystallization of the barium titanate occurs via decomposition of a barium carbonate phase. Retained organics that are present during high temperature processing can cause porosity in the films. On annealing at 1000 °C, there is slightly increase in the refractive index of the film due to further crystallization and densification. The final refractive index is comparable to that of 95% dense barium titanate ceramics. Re-oxidation did not change the refractive index of the film over the wavelength range from 350 to 650 nm.     

Microwave synthesis and sintering characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

CaCu₃Ti₄O₁₂ (CCTO) was synthesized and sintered by microwave processing at 2·45 GHz, 1·1 kW. The optimum calcination temperature using microwave heating was determined to be 950°C for 20 min to obtain cubic CCTO powders. The microwave processed powders were sintered to 94% density at 1000°C/60 min. The microstructural studies carried out on these ceramics revealed the grain size to be in the range 1–7 μm. The dielectric constants for the microwave sintered (1000°C/60 min) ceramics were found to vary from 11000–7700 in the 100 Hz–00 kHz frequency range. Interestingly the dielectric loss had lower values than those sintered by conventional sintering routes and decreases with increase in frequency.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Characterization of dielectric barium titanate powders prepared by homogeneous precipitation chemical reaction for embedded capacitor applications

Various articles have reported that a highly pure and uniform form of barium titanate can be prepared by homogeneous precipitation. However, most of these works emphasize the mechanism of thermal decomposition of barium titanyl oxalate tetrahydrate, and only a few have discussed morphology or particle size. The morphology and particles size of barium titanyl oxalate tetrahydrate are governed by reaction temperature, pH value and solvent ratio; the barium titanate structure can be obtained by calcinating barium titanyl oxalate tetrahydrate above 600 °C or hydrothermally in a basic solution at 200 °C. The final morphology of barium titanate in this investigation was similar to that of barium titanyl oxalate tetrahydrate and the particle size of barium titanate increased with the calcination temperature. Using this barium titanate in a polymer/ceramic composite provided better dielectric characteristics than commercial ceramic powders use in embedded capacitor applications.     

Preparation of barium titanate by homogeneous precipitation

The barium titanyl oxalate, which was the precursor of barium titanate, was prepared by homogeneous precipitation using the diethyl oxalate as a precipitating agent. The oxalate dried at 80 °C was the crystalline phase, which was converted to amorphous-like phase after drying at 120 °C. The weight loss of oxalate was 47.3 wt%. The chemical formula of the oxalate could correspond to BaTiO(C₂O₄)₂ · 4H₂O. The barium titanate obtained from the calcination of oxalate at 850 °C was a tetragonal phase with a particle size of 0.2 m.     

Synthesis and X-ray diffraction study of radium plumbate

Previously unknown mixed oxide RaPbO₃ was prepared by thermal decomposition of a mixture of radium and lead carbonates. An X-ray diffraction study showed that radium plumbate crystallizes in the cubic perovskite structural type. The unit cell parameter (a = 4.303 Å) and crystallographic density (ρ = 10.42 g cm^?3) were determined. Formation of a new compound, RaPtO_3?x , upon sintering of mixtures of radium and lead carbonates at 800°C in a platinum crucible was detected. A similar barium compound can be formed upon calcination of barium compounds in platinum vessels in air.     

Effect of BaTiO<Subscript>3</Subscript> heat treatment on the microstructure and dielectric properties of BaTiO<Subscript>3</Subscript>-based ceramics

We have studied the effect of heat treatment of the starting BaTiO₃ powder on the dielectric properties and microstructure of X7R-type BaTiO₃-based ceramics. The results demonstrate that annealing of BaTiO₃ stabilizes the degree of tetragonality in the crystal lattice of the ceramics. Microstructural analysis shows that the annealing temperature has no effect on the average grain size of the ceramics. Increasing the BaTiO₃ annealing temperature increases the dielectric permittivity of the core phase and reduces the temperature coefficient of capacitance (TCC). We obtained an X7R-type BaTiO₃-based ceramic material (BaTiO₃ annealing temperature, 1150°C; firing temperature, 1160°C) with the following properties: ɛ_25°C = 2230, TCC = ±12% (−55 to 125°C), and tanδ_25°C = 0.013.     

Effect of Li and Bi co-substituted on structural and physical properties of BaTiO<Subscript>3</Subscript> ceramics

Polycrystalline samples of Li and Bi co-substituted BaTiO₃ were synthesized using microwave assisted heating of the starting materials. This synthesis process extraordinarily reduced the processing time to 40 min, which includes heating and the dwell durations. The room temperature powder X-ray diffraction patterns reveals that the obtained compounds were of pure BaTiO₃ phase (BTO). The structural, morphological and dielectric behaviour of these compositions were studied. Improved dielectric properties have been observed with the substitute of Bi and Li. It is interesting to note that the loss tangent of the co-substituted compositions are lower than that of the parent composition and it decreases approximately with increasing extent of co-substitution. This property is quite useful to develop this material further for capacitor applications. The transition temperature has shifted from 120 °C of pure BaTiO₃ towards higher temperatures to 150, 160 and 175 °C with (Bi, Li)_x where x = (0.02, 0.04 and 0.08), respectively of co-substitution BaTiO₃. The change is linear with the degree of co-substitution.