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.     

Properties of barium titanate powders in relation to the heat treatment of the barium titanyl oxalate precursor

Barium titanate powders have been prepared by calcining barium titanyl oxalate precipitated by the Clabaugh and Merker processes, and their crystallization kinetics, morphology, and phase composition have been assessed. The results demonstrate that the Clabaugh process allows one to obtain powders with a low content of residual phases and tune the grain size (68–1935 nm) and crystal structure of barium titanate in wide ranges. The powders prepared through the Merker process have a narrower range of crystallite sizes (110–740 nm) and higher content of residual phases.     

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.     

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.     

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.     

Low temperature preparation of nanocrystalline solid solution of strontium barium niobate by chemical process

Sr_xBa_1-xNb₂O₆ (with x = 0.4, 0.5 and 0.6) powders have been prepared by thermolysis of aqueous precursor solutions consisting of triethanolamine (TEA), niobium tartarate and, EDTA complexes of strontium and barium ions. Complete evaporation of the precursor solution by heating at ∼ 200°C, yields in a fluffy, mesoporous carbon rich precursor material, which on calcination at 750°C/2 h has resulted in the pure SBN powders. The crystallite and average particle sizes are found to be around 15 nm and 20 nm, respectively.     

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.     

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.     

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.     

Density variation and piezoelectric properties of Ba(Ti1 − x Sn x )O3 ceramics prepared from nanocrystalline powders

Nanocrystalline powders of tin-doped barium titanate with different concentrations of tin have been synthesized by a combination of solid state reaction and high-energy ball milling. The average particle size of the milled powders as determined from TEM analysis was about 5·96 nm. Analysis of all the milled powders using X-ray diffraction method showed single phase perovskite structure. The density variation of the ceramics with sintering temperature has been studied by sintering the samples at different temperatures. Density variation results show that 1350°C is the optimum sintering temperature for tin-doped barium titanate ceramics. SEM micrographs show high density and increasing trend of grain size with increasing content of Sn. The ferroelectricity decreases with increasing concentration of Sn. The electromechanical coupling coefficient also decreases with increasing Sn content corroborating decreasing trend of ferroelectricity. The bipolar strain curves show piezoelectric properties of the prepared ceramics.     

Electrical characterization of BaTiO3 made by hydrothermal methods

The manufacture of multilayer capacitors based on barium titanate requires improved precursor powders. In this paper the dielectric properties of barium titanate ceramics made from hydrothermally precipitated material are compared with those powders obtained commercially and prepared by calcination. The greater porosity of hydrothermal powders tends to reduce their effective dielectric constant but preheating them to 950 C before sintering produced high density ceramic with higher values of _r. The dielectric losses of these materials are higher than those produced by traditional methods. Their low Curie temperatures (106–8 C) are related to the high strontium concentration (4.7 mol%) present in the starting materials. The grain sizes of these ceramics are less sensitive to sintering temperature than those made using calcined powders, with no anomalous grain growth.     

Comparative dielectric studies of nanostructured BaTiO3, CaCu3Ti4O12 and 0.5BaTiO3⋅ 0.5CaCu3Ti4O12 nano-composites synthesized by modified sol–gel and solid state methods

BaTiO₃ (BTO), CaCu₃Ti₄O₁₂ (CCTO) and 0.5BaTiO₃·0.5CaCu₃Ti₄O₁₂ (BTO–CCTO), as a new nano-composite ceramic, were successfully designed and fabricated by a semi-wet gel route and a modified solid state method. The dielectric properties of the BTO–CCTO ceramic were compared to those of the BTO and CCTO ceramics at lower sintering temperatures and durations. The X-ray diffraction analysis revealed that the BTO and CCTO ceramics form a single crystalline phase and the average crystalline sizes calculated from X-ray diffraction data were in the range of 40–65 nm. The particle sizes of the BTO, CCTO, and BTO–CCTO ceramics obtained from transmission electron microscopy images were in the ranges of 40–65 nm, 80–110 nm, and 70–95 nm, respectively. The phase composition and microstructure were studied by X-ray diffraction and scanning electron microscopy. The energy dispersive X-ray results demonstrated the purity and stoichiometry of the BTO–CCTO nano-composite. The grain sizes of the BTO, CCTO and BTO–CCTO ceramics were found to be in the ranges of 500 nm–1 μm, 4–24 μm, and 250 nm–4 μm, respectively. The AC conductivity as a function of frequency confirmed the semiconducting nature of all of the ceramics and obeyed the Jonscher's power law. The impedance spectrum measurement result showed that the CCTO ceramic possessed an exceptional grain boundary resistance, which supports the internal barrier layer capacitance (IBLC) mechanism present in this ceramic and is responsible for the high ε_r values.     

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.     

Synthesis and characterization of (Na0.85K0.15)0.5Bi0.5TiO3 ceramics by different methods

The (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (BNKT) powders were synthesized by solid-state method, sol-gel method and stearic acid method. Microstructure, piezoelectric and dielectric properties of the ceramics were investigated. Attempts had been made to understand the reaction processes by using thermo gravimetric (TG) and differential scanning calorimetry (DSC). The BNKT powders have a perovskite structure with average crystallite sizes of 168 nm, 85 nm and 79 nm, corresponding to the solid-state method, the sol-gel method and the stearic acid method, respectively. The ceramics derived from the powder synthesized by sol-gel method presents the most homogeneous microstructure and largest grain size (5-7 μm). The effects of average crystallite size on microstructures and electric properties of the BNKT ceramics were investigated. Both the piezoelectric properties and dielectric properties were enhanced with the increase of grain size.     

Size Effect of Barium Titanate and Computer-Aided Design of Multilayered Ceramic Capacitors

The size effect of BaTiO₃ (BTO) is the most important issue to design multilayer ceramic capacitors (MLCCs) with high capacitance. In the size effect of BTO particles, the size dependence of dielectric permittivity related with the complex structure in BTO nano-particles. The grain size dependence of dielectric permittivity in BTO ceramics was due to the domain wall contribution. The core-shell structure played an important role in the size effect of dielectric layers in X7R-MLCCs. Computer simulation technique was developed to predict the limit of capacitance density of MLCCs produced by the current technology. Dielectric properties of MLCCs with different particle size of BTO were measured, and the data were analyzed using B-SPLINE fitting to predict dielectric permittivity at arbitrary temperatures and AC-fields. The dielectric properties of barium titanate grains smaller than 100 nm were predicted using least squares fitting of the B-SPLINE coefficients. It was found from the simulation that the use of barium titanate grains smaller than 80 nm did not give an advantage to increase the capacitance density as well as temperature stability of the MLCCs. The maximum capacitance was predicted for the 1608 (mm) chip size.     

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.     

The effects of CuO-doping on dielectric and piezoelectric properties of Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–Ba(Zr,Ti)O<Subscript>3</Subscript> lead-free ceramics

CuO-doped lead-free ceramics based on bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) and barium zirconate titanate (Ba(Zr_0.07Ti_0.93)O₃, BZT) were prepared via a multi-step solid-state reaction process. The BNT–BZT with CuO dopant ceramics sintered at 1150–1180 °C for 2 h in air showed a pure perovskite structure. SEM images reveal that a small amount of CuO (<2 mol%) play a significant role on the microstructure to improve its sintering attributes, while it will degrade when the dopant is added beyond 2 mol%. The dielectric and piezoelectric properties of CuO-doped BNT–BZT ceramics were evaluated. At room temperature, the sample doped with 2 mol% CuO shows quite good properties such as a high piezoelectric constant (d ₃₃ ∼156.5 pC/N) and a high electromechanical coupling factor (k _t ∼52%). The depolarization temperature increased dramatically and the maximum permittivity temperature decreased slightly.     

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.     

BaTiO3-based positive temperature coefficient of resistivity ceramics with low resistivities prepared by the oxalate method

Positive temperature coefficient of resistivity (PTCR) ceramics with low resistivities at room temperature were obtained by using oxalate-derived barium titanate powders. The average room-temperature resistivity of the PTCR ceramics was 4 cm, and the magnitude of their PTCR jump was around four orders with a voltage proof of more than 50 Vmm^–1. These PTCR properties are significantly influenced by the calcination temperature of the starting materials and by the resultant properties of the ceramic bodies. The microstructure of such-PTCR ceramics with a low room-temperature resistivity produced in this study was found to be rather heterogeneous. Complex impedance measurements revealed that the resistivity of the present PTCR materials was determined predominantly by the grain-boundary resistance even at room temperature.