Effect of Silver Addition on the Dielectric Properties of Barium Titanate-Based X7R Ceramics

Different amounts of silver (0.5–10 wt%) have been mixed with EIA X7R-type ceramic powders based on barium titanate. The XRD analysis indicated that no phases other than BaTiO₃ and silver were present in the doped ceramics; it further suggested that no reaction took place between BaTiO₃ and silver during calcination and sintering. SEM observation showed that the silver particles presented island distribution in the BaTiO₃ ceramic matrix. The densification and dielectric properties of the silver-doped ceramics in disk form were investigated. A large amount of silver addition (>1 wt%) was found to improve the sintered density and dielectric properties. The temperature coefficient of capacitors of the ceramics doped with 10 wt% silver still met the X7R characteristics, and the dielectric constant of the ceramics at room temperature was >6000, which is the highest dielectric constant in the BaTiO₃-based X7R system.     

Direct Writing of Dielectric Ceramics and Base Metal Electrodes

Three-dimensional structures were assembled by direct writing of colloidal gel-based inks of electro-ceramic and metal electrode compositions. The ternary mixtures of BaTiO₃, BaZrO₃, and SrTiO₃ were explored via an active mixing nozzle print strategy that allowed for rapid screening of complex oxide ceramics. In a second experiment, Ni and BaTiO₃ inks were developed from commercially available powders in the form of concentrated, aqueous colloidal gels having shear thinning with yield stress rheology. The different surface chemistry of Ni and BaTiO₃ was accommodated by adsorbing a bi-layer of oppositely charged polyelectrolytes on the BaTiO₃ particles. Alternating layers of metal and ceramic were printed to build a multilayer structure. The multilayer Ni–BaTiO₃ structures were sintered in a reducing atmosphere and the capacitance was measured. These two examples illustrate the use of discrete or graded deposition capabilities of direct writing.     

Multilayer Construction with Various Ceramic Films for Electronic Devices Fabricated by Aerosol Deposition

Aerosol deposition (AD) is applicable as a fabrication technology for microstructures comprising different materials. We used this method for electronic devices that consist of ceramic films and metal electrodes. Various ceramic thick films (5–50 μm thickness), for example, Al₂O₃, 2MgO·SiO₂, and BaTiO₃, were deposited on substrates using room-temperature aerosol deposition. The dielectric constant of BaTiO₃ was 78 at 1 MHz. Multilayer constructions with ceramic films and copper electrodes were obtained using aerosol deposition and sputtering. During deposition, photoresist film masks were applied to produce patterns of ceramic films and connections between upper and lower electrodes through the ceramic films.     

In–Situ Transmission Electron Microscopy Crystallization Studies of Sol–Gel-Derived Barium Titanate Thin Films

Barium titanate (BaTiO₃) thin films that were derived from methoxypropoxide precursors were deposited onto (100) Si, Pt/Ti/SiO₂/(100) Si, and molecular-beam-epitaxy-grown (MBE-grown) (100) BaTiO₃ on (100) Si substrates by spin coating. The crystallization behavior of the amorphous-gel films was characterized using in-situ transmission electron microscopy heating experiments, glancing-angle X-ray diffraction, and differential thermal analysis/thermogravimetric analysis. Amorphous-gel films crystallized at a temperature of ∼600°C to an intermediate nanoscale (5–10 nm) barium titanium carbonate phase, presumably BaTiO₂CO₃, that subsequently transformed to nanocrystalline (20–60 nm) BaTiO₃. Random nucleation in the bulk of the gel film was observed on all substrates. In addition, oriented growth of BaTiO₃ was concurrently observed on MBE-grown BaTiO₃ on (100) Si. High-temperature decomposition of the intermediate carbonate phase contributed to nanometer-scale residual porosity in the films. High concentrations of water of hydrolysis inhibited the formation of the intermediate carbonate phase; however, these sols precipitated and were not suitable for spin coating.     

Interfacial Reaction of BaTiO3 Ceramics with PbO–B2O3 Glasses

Interfacial reactions of pure, lead-, and zirconium-substituted BaTiO₃ ceramics with PbOB₂O₃ glasses were studied, with an emphasis on the effect of glass composition. Microstructures were analyzed by scanning electron microscopy and electron-probe microanalysis aided with X-ray diffractometry of powder mixtures in the system BaTiO₃PbOB₂O₃ heated at 850°C. The interfacial microstructures were divided into two types, depending on the glass composition. The first type was characterized by precipitates of TiO₂ dispersed in the glass matrix. Extended heating or limited glass volume resulted in the formation of a continuous layer of BaTi(BO₃)₂. The second type of microstructure was characterized by a lead-rich perovskite phase, which developed at the glass/ceramic interfacial region. Growth kinetics for this phase denied the diffusion-controlled mechanism. The substitution of lead in BaTiO₃ enhanced the penetration of glass into the ceramics along the grain boundaries and developed a coreshell structure.     

Core–Shell Structure Analysis of BaTiO3 Ceramics by Synchrotron X-Ray Diffraction

The shell thickness of a BaTiO₃ ceramic with a core–shell structure has been measured by means of synchrotron X-ray diffraction (XRD). BaTiO₃ ceramic is known from transmission electron microscope (TEM)/energy dispersive X-ray spectroscopy (EDS) observations to have an inhomogeneous microstructure with cores of a pure BaTiO₃ and shells doped with additives. It is also known, from XRD observations, that the BaTiO₃ cores have a tetragonal lattice structure and the shells are pseudocubic. We have estimated the shell thickness d from the full-width at half-maximum (FWHM) of the cubic ( 400 )_c peak, using Scherrer's equation. The shell thickness d _cal was also evaluated from the volume fraction of tetragonal BaTiO₃ using a spherical core–shell model. The two values thus determined agree well, confirming that the BaTiO₃ ceramic specimens have a core–shell structure. Our results show that synchrotron XRD is a simple and effective tool for quantitative analysis of the core–shell structure. It enables us to understand quantitatively the relationship between the microstructure and the dielectric properties of BaTiO₃ ceramics.     

Metal Organic Resin Derived Barium Titanate; II, Kinetics of BaTiO3Formation

A physicochemical model has been developed for the kinetics of barium titanate formation from X-ray-amorphous, metal organic precursors by relating the changes in the physical structure of the precursor particles with the degree of transformation in isothermally heated powder samples. From electron microscopy and gas adsorption, it is evident that the precursor particles consist of 20-to 60-nm crystallites and < 10-nm intraparticle pores. A Ba,Ti oxycarbonate phase forms on heating the Ba,Ti metal organic precursor, which subsequently decomposes to form BaTiO₃ It is concluded that the formation of BaTiO₃ follows the shrinking core model, and the overall transformation is rate-controlled by the diffusion of CO₂ through the nanometer-size intraparticle pores.     

Phase Equilibria in the System BaO–TiO2

The system BaO-TiO₂ was investigated using quenching, strip-furnace, and thermal techniques. Five compounds were found to exist in the system: Ba₂TiO₄, BaTiO₃, BaTi₂O₅, BaTi₃O₇, and BaTi₄O₉. Of these, only barium metatitanate (BaTiO₃) melts congruently (at 1618°C.). The dititanate melts incongruently at 1322° C. to yield BaTiO₃ and liquid; the trititanate melts at 1357°C. to yield BaTi₄O₉ and liquid; the tetra-titanate melts to TiO₂ and liquid at 1428° C. The nature of melting of the orthotitanate could not be determined accurately because of the high temperature involved and the rapid reaction with platinum. The two eutectics in the system occur between Ba₂TiO₄ and BaTiO₃ at 1563°C. and between BaTi₂O₅ and BaTi₃O₇ at 1317°C. The temperature of the cubic-hexagonal transition in barium metatitanate was determined as 1460°C. and the transition has been shown to be reversible. The transition temperature is raised sharply by the addition of a small percentage of TiO₂ although the extent of solid solution is quite limited. Some applications to the manufacture of titanate bodies and to the growth of single crystals of barium metatitanate are discussed.     

High-Energy Density Capacitors Utilizing 0.7 BaTiO3–0.3 BiScO3 Ceramics

A high, temperature-stable dielectric constant (∼1000 from 0° to 300°C) coupled with a high electrical resistivity (∼10¹²Ω·cm at 250°C) make 0.7 BaTiO₃–0.3 BiScO₃ ceramics an attractive candidate for high-energy density capacitors operating at elevated temperatures. Single dielectric layer capacitors were prepared to confirm the feasibility of BaTiO₃–BiScO₃ for this application. It was found that an energy density of about 6.1 J/cm³ at a field of 73 kV/mm could be achieved at room temperature, which is superior to typical commercial X7R capacitors. Moreover, the high-energy density values were retained to 300°C. This suggests that BaTiO₃–BiScO₃ ceramics have some advantages compared with conventional capacitor materials for high-temperature energy storage, and with further improvements in microstructure and composition, could provide realistic solutions for power electronic capacitors.     

Effect of Heating Rates during Sintering on the Electrical Properties of Ultra-Thin Ni–BaTiO3 Multilayer Ceramic Capacitors

Microstructural control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the present day challenges for increasing capacitive volumetric efficiency and high voltage dielectric properties. The present paper continues a series of investigations aimed at engineering the stability of ultra-thin Ni layers in base-metal electrode MLCCs. A kinetic approach based on the control of sintering profiles is found to not only prevent Ni electrode discontinuities, but also to significantly improve the interfacial electrical properties. Increasing sintering heating rates from 200 to 3000°C/h leads to a decrease in its temperature dependence of capacitance. Faster heating rates also reduce the BaTiO₃ grain size, which is beneficial to the reliability of multilayer capacitors. A direct correlation between heating rates, the thickness of an interfacial (Ni, Ba, and Ti) alloy reaction layer and the interfacial contact resistance has been observed. The decrease in the alloy layer thickness at high heating rates leads to an increased effective Schottky barrier height between the dielectric and electrode toward its theoretical value of 1.25 eV for pure Ni–BaTiO₃ interfaces.     

Dielectric Properties of (Pb1–xXx) (Zr0.7Ti0.3)O3 (X = Ca, Sr, Ba) Ceramics

The dielectric properties of (Pb_{1– x} X_x) (Zr_0.7Ti_0.3)O₃ (X = Ca, Sr, Ba) ceramics (abbreviated PXZT) were investigated for applications to multilayer ceramic capacitors (MLCs) with dielectric layers thinner than 10 μm. The dissipation factors for MLCs with 5-μm-thick dielectric layers were estimated from those for 100-μm-thick disk specimens measured at an oscillation voltage of 20 V_rms. Those for PCZT and PSZT were less than 1.0% when the oscillation voltage was 20 V_rms, while those for conventional BaTiO₃-based dielectric ceramics were greater than 2.5% at 20 V_rms. According to polarization–electric field hysteresis measurements, PCZT and PSZT revealed linear and double hysteresis loops, respectively, while PBZT and BaTiO₃ indicated typical ferroelectric hysteresis loops. The differences in the dissipation factors for the dielectric compositions are attributed to hysteresis in the polarization–electric field loops. These results suggest that PCZT and PSZT are promising dielectric ceramics for MLCs with dielectric layers thinner than 10 μm.     

Defect Chemistry and Microstructure of Hydrothermal Barium Titanate

Hydrothermal powders of BaTiO₃ and (Ba,Ca)(Ti,Zr)O₃ contain large amounts of protons in the oxygen sublattice. The proton defects are compensated by vacancies on metal sites. When the powder is annealed, water is released and the point defects disappear in the temperature range of 100°–600°C. Metal and oxygen vacancies combine to small nanometer-sized intragranular pores. At temperatures of >800°C, the intragranular pores migrate to the grain boundaries and disappear. In multilayer ceramic capacitors that have been prepared from hydrothermal powders, the intragranular pores are preferentially collected at the inner electrodes, which results in "bloating," cracks, and delamination.     

Physical and Dielectric Properties of (1–x)PbZrO3·xBaTiO3 Thin Films Prepared by Chemical Solution Deposition

Physical and dielectric properties of (1– x )PbZrO₃· x BaTiO₃ thin films prepared by a chemical coating process have been investigated as a function of BaTiO₃ ( x ) content (0≤ x ≤0.2). Changing the molar ratio between propylene glycol and water prior to the deposition optimized the chemical precursors. (1– x )PbZrO₃- x BaTiO₃ thin films that contained a majority of perovskite phase, but also contained large amounts of other phases, were fabricated. These films could withstand fields of 250 kV/cm at 1 kHz. The microstructure of the thin films was found to depend on the BaTiO₃ content. The phase transition from antiferroelectric to ferroelectric was gradually induced as the BaTiO₃ content increased. A maximum dielectric constant of ∼809 was obtained at the composition of x = 0.1. A maximum dielectric constant of ∼809 was obtained at the composition of x = 0.1. A thin film at the low-field antiferroelectric-ferroelectric phase boundary with x = 0.05 exhibited the highest P _sat and P _r values. The maximum values of these were 45 and 31 μC/cm², respectively.     

A Novel Reaction Path to BaTiO3 by the Oxidation of a Solid Metallic Precursor

Multilayer capacitors with thin, dielectric BaTiO₃ layers can possess a relatively high capacitance per unit volume. A solid metallic precursor method has recently been developed for preparing thin BaTiO₃/noble metal laminates. In the present paper, the phase and microstructural evolution of Ba-Ti metallic precursors were examined after oxidation at 300° to 900°C in pure oxygen at 1 atm pressure. Barium peroxide, BaO₂, formed rapidly during oxidation at 300°C, with titanium largely remaining as unoxidized particles in the peroxide matrix. Between 375° and 500°C, the solidstate reaction of barium peroxide with metallic titanium yielded barium orthotitanate, Ba₂TiO₄. Further exposure to temperatures between 500° and 900°C resulted in the oxidation of residual metallic titanium. The oxidized titanium then reacted with the orthotitanate matrix to form barium metatitanate, BaTiO₃. The rate of formation of BaTiO₃ was found to be strongly dependent on the degree of milling of the Ba-Ti precursors and on the heating rate between 300° and 500°C.     

Weakly Coupled Relaxor Behavior of BaTiO3–BiScO3 Ceramics

The structural and dielectric properties of (1− x )BaTiO₃– x BiScO₃ ( x =0–0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the symmetry of the phases, the associated dielectric properties, and the differences in the roles of Bi₂O₃ and BiScO₃ substitutions in a BaTiO₃ solid solution. The solubility limit for BiScO₃ into the BaTiO₃ perovskite structure was determined to be about x =0.4. A systematic structural change from the ferroelectric tetragonal phase to a pseudo-cubic one was observed at about x =0.05–0.075 at room temperature. Dielectric measurements revealed a gradual change from proper ferroelectric behavior in pure BaTiO₃ to highly diffusive and dispersive relaxor-like characteristics from 10 to 40 mol% BiScO₃. Several of the compositions showed high relative permittivities with low-temperature coefficients of capacitance over a wide range of temperature. Quantification of the relaxation behavior was obtained through the Vogel–Fulcher model, which yielded an activation energy of 0.2–0.3 eV. The attempt characteristic frequency was 10¹³ Hz and the freezing temperature, T _f, ranged from −177° to −93°C as a function of composition. The high coercive fields, low remanent polarization, and high activation energies suggest that in the BiScO₃–BaTiO₃ solid solutions, the polarization in nanopolar regions is weakly coupled from region to region, limiting the ability to obtain long-range dipole ordering in these relaxors under field-cooled conditions.     

Mixed Oxide Capacitor of CuO—BaTiO3 as a New Type CO2 Gas Sensor

An oxide capacitor consisting of BaTiO₃ and an oxide is studied as a new type CO₂ sensor based on capacitance change. Sensitivity to CO₂, as well as the optimum operating temperature, was strongly dependent on the particular oxide mixed with BaTiO₃. Among the elements investigated in this study, CuO–BaTiO₃ exhibited the highest sensitivity to CO₂. In particular, the CuO–BaTiO₃ mixed oxide at the equimolar composition is highly sensitive to CO₂. The optimum operating temperature and frequency for CuO–BaTiO₃ are 729 K and 100 Hz, respectively, and the 80% response time to 2% CO₂ is within 25 s. The equimolar mixture of CuO and BaTiO₃ can measure the CO₂ concentration from 100 to 60 000 ppm. Carbonation of oxide seems to play a key role for the detection of CO₂ on these mixed oxide capacitors. The optimum operating temperature of these mixed oxide capacitors for CO₂ detection, therefore, correlates with the decomposition temperature of the carbonate corresponding to the oxide mixed with BaTiO₃. The capacitance increase of CuO–BaTiO₃ upon exposure to CO₂ seems to result from the elevated height of the potential barrier at the grain boundary between CuO and BaTiO₃. Carbonation of CuO in the element seems to bring about the elevation in the height of the potential barrier.     

Synthesis of Ultrafine and Spherical Barium Titanate Powders Using a Titania Nano-Sol

A novel synthetic method for the preparation of spherical, homogeneous, and ultrafine barium titanate (BaTiO₃) powders is described. An aqueous titania nano-sol was prepared by peptizing coarse aggregate of hydrous titania with nitric acid. BaTiO₃ powders could be synthesized through a simple reflux method using the titania nano-sol and barium hydroxide. As decreasing the titanium concentration, the particle size of the resulting spherical BaTiO₃ powder was increased from 40 to 130 nm and the porosity also increased. It was revealed that the smaller as-prepared BaTiO₃ powder was less porous and became more tetragonal with less intragranular pores after annealing. With this method, a highly tetragonal BaTiO₃ powder ( c / a ∼1.008) with a particle size of 120.0 nm was successfully prepared and would be very suitable for the thinner dielectrics in higher capacitance multilayer ceramic capacitors.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Templated Grain Growth of Barium Titanate Single Crystals

BaTiO₃ single crystals were grown via templated grain growth (TGG), which is a process in which a single-crystal "template" is placed in contact with a sintered polycrystalline matrix and then heated to migrate the single-crystal boundary into the matrix. Millimeter-sized, stoichiometric single crystals of BaTiO₃ were produced by heating polycrystalline matrix with a relative density of 97% and a Ba/Ti ratio of <1.00, which was bonded to a BaTiO₃ single crystal, at temperatures above the eutectic temperature. Growth rates of 590–790, 180–350, and 42–59 μm/h were observed for {111}-, {100}-, and {110}-oriented single-crystal templates, respectively. Lower-surface-energy facets were formed for {111}- and {100}-oriented templates, whereas {110} crystals maintained a {110} growth front, which indicated that this plane orientation was the lowest-energy surface in this system. SrTiO₃ also was shown to be a suitable substrate for TGG of BaTiO₃.     

Structure–Property Relationship in BT-Based Dielectrics for Ni-MLCC: Modification of Grain Boundary

Structure–property relationship in BaTiO₃ (BT)-based dielectrics for multi-layer ceramic capacitors with nickel internal electrode was investigated using samples having various HoO_3/2 concentrations by measuring temperature characteristics of capacitance, voltage–current characteristics, lifetime at highly accelerated life test, high-resolution analytical electron microscope, and frequency response at elevated temperature and ultra-low frequency. It was concluded that the addition of Ho affected the shell and grain boundary (GB) characteristics. Incorporation of Ho into BT perovskite lattice and the change in GB characteristics along with the doped concentration of HoO_3/2 were discussed to better understand the role of doped Ho₂O₃.