PTCR barium titanate ceramics obtained from oxalate-derived powders with varying crystallinity

Barium titanate powders with average crystallite sizes of 68–2000 nm have been prepared by the calcination of barium titanyl oxalate (BTO) at temperatures of 700–1150 °C. The morphology and recrystallization kinetics of the powders have been studied using the SEM and X-ray methods. Samples of PTCR (BaCaPb)TiO₃ ceramics have been made from these powders and their microstructure and electrical properties have been investigated. It has been found that the increase of the crystallinity of the starting powders suppresses recrystallization of the ceramics, leading to growth in resistivity and significantly influencing on the resistance jump and breakdown strength of the ceramics. An optimal temperature range for the calcination of BTO has been found to ensure maximum breakdown strength of the PTC thermistors with the resistance of 31 Ω. At this temperature range the barium titanate powders had crystallite sizes of ~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.     

Preparation and properties of barium titanate glass-ceramics sintered from sol–gel-derived powders

Homogeneous Ba–Ti–B–Si, Ba–Ti–Al–Si and Ba–Ti–B gels have been successfully prepared by the sol–gel process. A novel method is presented for fabricating barium titanate glass-ceramics by sintering the gel powders with small barium titanate crystallites. The structural development, grain size, crystallization process and dielectric properties were systematically studied by differential thermal analysis, thermogravimetric analysis, X-ray diffraction techniques, scanning electron microscopy and dielectric measurements. The glass-ceramic samples were sintered at lower temperatures compared to the barium titanate ceramic sintering, and showed improved dielectric properties. It was found that the small size effect of the barium titanate grains on the dielectric constant in the glass-ceramics was quite evident. Ferroelectric hysteresis loop analyses were also performed to manifest the ferroelectric nature of the barium titanate grains in situ grown from the gels. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of Ultrasonic Processing on the Synthesis of Barium Titanyl Oxalate and the Characteristics of the BaTiO<Subscript>3</Subscript> Powder Prepared from It

Barium titanyl oxalate (BTO) with small deviations from stoichiometry has been synthesized by a chemical and a sonochemical method (under ultrasonication). Ultrasonic processing has been shown to reduce the particle size of the resultant BTO powder by about ten times and ensure a nearly spherical shape of the particles. The morphology of barium titanate powders prepared by decomposing the BTO at a temperature of 900°C is similar to that of the parent BTO and independent of stoichiometry. The powders have a barium to titanium ratio Ba/Ti = 1.002 and 0.987. The barium titanate powders synthesized using the sonochemical method contain a smaller amount of residual phases and have a larger specific surface area, smaller crystallite size (~100 nm), and smaller unit-cell parameters than do the powders prepared without ultrasonication.     

Water-based sol–gel nanocrystalline barium titanate: controlling the crystal structure and phase transformation by Ba:Ti atomic ratio

Highly stable, water-based barium titanate (BaTiO₃) sols were developed by a low cost and straightforward sol–gel process. Nanocrystalline barium titanate thin films and powders with various Ba:Ti atomic ratios were produced from the aqueous sols. The prepared sols had a narrow particle size distribution in the range 21–23 nm and they were stable over 5 months. X-ray diffraction pattern revealed that powders contained mixture of hexagonal- or perovskite-BaTiO₃ as well as a trace of Ba₂Ti₁₃O₂₂ and Ba₄Ti₂O₂₇ phases, depending on annealing temperature and Ba:Ti atomic ratio. Highly pure barium titanate with cubic perovskite structure achieved with Ba:Ti = 50:50 atomic ratio at the high temperature of 800 °C, whereas pure barium titanate with hexagonal structure obtained for the same atomic ratio at the low temperature of 500 °C. Transmission electron microscope revealed that the crystallite size of both hexagonal- and perovskite-BaTiO₃ phases reduced with increasing the Ba:Ti atomic ratio, being in the range 2–3 nm. Scanning electron microscope analysis revealed that the average grain size of barium titanate thin films decreased with an increase in the Ba:Ti atomic ratio, being in the range 28–35 nm. Moreover, based on atomic force microscope images, BaTiO₃ thin films had a columnar-like morphology with high roughness. One of the highest specific surface area reported in the literature was obtained for annealed powders at 550 °C in the range 257–353 m²g⁻¹.     

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 lattice hydroxyl on the phase transition and dielectric properties of barium titanate particles

Presence of the hydroxyls in the lattice is believed to be the major cause of the reduced tetragonality in the barium titanate ceramic powder. Commercial barium titanate that is known to be cubic in nature has been used in this study. This sub-micron powder is treated with N-Methyl-2-Pyrrolidinon (NMP) to obtain a tetragonal powder as confirmed by X-ray diffraction analysis, differential scanning calorimetry and the c/a ratio. The dielectric constant of a single particle of this NMP treated cubic powder is reported to be around 64% higher than the as-received cubic powder. To add weight to the hypothesis mentioned hitherto, simulation experiments have been performed by aging in acidic water, with a pH ∼ 3–4 and in basic water, with a pH ∼ 12–13. The as-received cubic barium titanate powder, calcined at different temperatures, has been aged in different pH conditions, acid and basic waters. Then the powder is further used for the characterization of electrical properties. The dielectric properties of the barium titante ceramic powder that is determined does depend inversely on the lattice OH content as confirmed by FT-IR spectroscopic analysis and TGA results.     

Semiconducting ceramics produced using nanocrystalline barium titanate powder

Positive temperature coefficient of resistance ceramics of composition (Ba_0.89Ca_0.08Pb_0.03)TiO₃ + Y₂O₃ + MnO + SiO₂ have been produced using barium titanate powder with an average crystallite size of 125 nm prepared by calcining barium titanyl oxalate at 900°C. The effect of firing temperature on their microstructure and electrical properties has been studied. The results demonstrate that the ceramics possess semiconducting properties starting at a firing temperature of 1205–1215°C. The room-temperature resistivity of the ceramics has a minimum at t _firing ≈ 1245–1250°C. The samples sintered at 1250–1260°C have the largest positive temperature coefficient of resistance. The highest electric strength (360 V/mm at ρ_25°C = 290 Ω cm) is offered by the thermistor materials sintered at 1260°C, which is 60–70°C below the firing temperature of analogous ceramics produced by solid-state reaction.     

Characterisation of barium titanate-silver composites, part I: Microstructure and mechanical properties

The paper presents an investigation of the influence of silver particles on the microstructure and mechanical properties of barium titanate. Barium titanate-silver composites have been prepared by ball milling precursor powder constituents; followed by drying, sieving and calcination prior to powder compaction. After sintering the green compacts, microstructural analysis was undertaken involving measurement of grain size, silver particle size, phase composition and phase content. Characterisation of mechanical strength, toughness, hardness and stiffness was also undertaken. Reaction product phases between silver and barium titanate could not be detected. The dispersed silver particles were shown to inhibit densification. Silver particles below 1 μm in size were intragranular and attached to domains. The size of the intergranular silver particles increased with silver content. An increase in silver content improved whereas strength, hardness and stiffness decreased, while toughness was unchanged.     

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.     

Synthesis of barium titanate nanopowder by a soft chemical process

A simple soft chemical method of synthesizing barium titanate nanopowders and nanorods is described here, where titanium dioxide/titanium isopropoxide was taken as a source of titanium, tartaric acid was taken as a template material, nitric acid as an oxidizing agent. The synthesized powders and rods were characterized by XRD, TG and DTA, SEM and IR spectroscopy. In this process phase pure barium titanate nanopowders and nanorods can be prepared at a temperature of 900 °C and the process is simple and cost-effective.     

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.     

Structural characteristics and dielectric properties of neodymium doped barium titanate

Neodymium (Nd) doped barium titanate powder (Ba_(1−x)Nd _x TiO₃) with x value varying from 0, 0.01, 0.03, 0.05, 0.07, 0.10 and 0.13 was prepared using the sol gel method. The powder samples were calcined at 700 °C and tetragonal phase appeared in the powders before they were sintered at 1250 °C for 3 h. The undoped samples have a polycrystalline tetragonal structure, but Nd doping into the BaTiO₃ caused phase transformation from tetragonal to cubic. The smaller grains (0.35 μm) produced with the addition of Nd is associated to the inhibition of grain growth of samples. The powders for each composition were pressed into pellets and tested as dielectric resonator antenna (DRA). It was found that on the actual antenna circuit, each sample showed a resonance frequency at X-band application and a dielectric constant value in the range of 51.25–56.89 and tangent loss was 0.039–0.045, depending on the concentration of the Nd at room temperature.     

Influence of microstructure on dielectric properties of Nd-doped barium titanate synthesized by hydrothermal method

Nd-doped barium titanates were successfully synthesized via a hydrothermal route. The as-prepared barium titanate was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transformation infrared spectroscopy (FTIR), and Vis–NIR spectroscopy respectively. The results show that pure and Nd-doped barium titanate powders have cubic perovskite structure. After sintering at a temperature of 1,250 °C for 2 h, the phase compositions of all barium titanate are tetragonal phase structure. Vis–NIR spectra well confirmed that Nd³⁺ have been doped into barium titanate. The particle diameters of Nd-doped barium titanate powders and ceramics become samller with the increase of Nd³⁺ content. When Nd/Ba molar ratio is 0.02, the dielectric loss (0.0008) of the powder measured at 1 MHz and room temperature dramatically decreases by 99 % comparing with pure barium titanate (0.083) and shows frequency independence with the frequency increasing from 40 Hz to 1 MHz. The dielectric constant and dielectric loss are 436 and 0.09 after sintering. The Nd-doped BaTiO₃ show an improvement in the dielectric quality which possess a decreased sensitivity to frequency for both the dielectric constant and dielectric loss. Such improvements are of potential importance for high energy density and low loss.     

Optimization of barium titanate nanopowder slip for tape casting

Dispersion of barium titanate nano powder of average particle size ∼30 nm in different solvent systems of tape casting (toluene–ethanol, methyl ethyl ketone–ethanol, xylene–ethanol) along with Triton x-100 or phosphate ester as dispersants has been studied using sedimentation experiments. The influence of different parameters such as type of solvent system, dispersant and concentration of dispersant on BaTiO₃ slip dispersion, viscosity and the properties of green tape were studied. The optimal concentration of dispersant was determined from the minimum slip viscosity. Xylene–ethanol with phosphate ester was found to be the best solvent and dispersant system for tape casting. Defect free, denser and smooth green tapes are formed with this system.     

Investigations on surface composition and microstructure of sintered barium titanate

The paper describes studies on surface atomic composition, microstructure and microarea elemental distribution in sintered undoped as well as donor or acceptor doped polycrystalline barium titanate ceramics. The specimens examined are derived from barium titanate powders synthesized by two different wet chemical procedures namely oxalate precursor route and gel-to-crystallite conversion. The compositional analysis is carried out by backscattering spectrometry (BS) involving 3.05 MeV ¹⁶O(α,α)¹⁶O resonant scattering while investigations on microstructure and microarea elemental distribution are performed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The specimens prepared by either method are monophasic; however, their surface atomic composition, microstructural features as well as electrical characteristics are significantly different. The specimens obtained by oxalate precursor route generally have Ti rich surfaces and exhibit coarse to fine grained microstructure depending on the nature and extent of doping. The Mn-doped specimens exhibit appreciable O deficiency. The specimens prepared by gel to crystallite conversion, in contrast, usually have Ba enriched surfaces and exhibit fine-grained microstructure. EDS measurements show the segregation of acceptors such as Mn, Cu and Zn in the grain boundaries of oxalate precursor derived ceramics. Further, the relative atomic ratio of Ti to Ba at the sites of segregations is higher compared to other locations.     

Synthesis and characterization of nanocrystalline barium strontium titanate powders prepared by citrate precursor method

Nanocrystalline and well dispersed barium strontium titanate (BST) powders were prepared by a novel and simple citrate precursor method. This method involved direct crystallization of a white precursor from a stable solution in the citric acid (CA)–ethylene glycol (EG)–tetrabutyl titanate–M²⁺ (M = Ba, Sr) system under a specific pH value range. Subsequent heat treatment of the precursor at 850 °C led to a pure phase BST powder. TG/DTA was used to examine the decomposition behaviour of the precursor. The crystalline phase and morphology of the BST powders were investigated by XRD and TEM. It was found that the BST powders synthesized by citrate precursor process were more homogeneous and uniform than that obtained by the citrate gel method.     

Modeling of local thermal fields in semiconducting barium titanate ceramics

A mathematical model for describing local temperature fields in barium titanate ceramics with a positive temperature coefficient of resistance under the action of electric current has been constructed. The space-time temperature distribution within the grains of semiconducting barium titanate with varying microstructure and specific resistance has been studied. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 4, pp. 114–117.     

Doped barium titanate nanoparticles

We have synthesized nickel (Ni) and iron (Fe) ion doped BaTiO₃ nanoparticles through a chemical route using polyvinyl alcohol (PVA). The concentration of dopant varies from 0 to 2 mole% in the specimens. The results from X-ray diffractograms and transmission electron micrographs show that the particle diameters in the specimen lie in the range 24–40 nm. It is seen that the dielectric permittivity in doped specimens is enhanced by an order of magnitude compared to undoped barium titanate ceramics. The dielectric permittivity shows maxima at 0.3 mole% doping of Fe ion and 0.6 mole% of Ni ion. The unusual dielectric behaviour of the specimens is explained in terms of the change in crystalline structure of the specimens.     

The hydroxyl concentration and the dielectric properties of barium titanate nano-powder synthesized by water-based ambient condition sol process

Nanocrystalline barium titanate, BaTiO₃, powders have been successfully synthesized via water-based ambient condition sol (WACS) process, using barium hydroxide and titanium isopropoxide. The properties of the powder were investigated as a function of various processing parameters such as the concentration of Ba²⁺ ions ([Ba²⁺]), the concentration of base, and reaction time. The concentration of hydroxyl (OH⁻) groups in the BaTiO₃ powder was greatly affected by changing the values of each processing parameter. The dielectric constant and tetragonality of the powders which contain lattice OH⁻ more than 0.35 wt% were little affected with further increase in the lattice OH⁻ concentration. The dielectric loss was highly varied with the concentration of the OH⁻ groups, and it was increased with increasing OH⁻ concentration. Calcination treatment significantly improved the dielectric properties of the powder. With higher calcination temperature, the dielectric constant increased and the dielectric loss decreased.