Dielectric Properties of Fine-Grained Barium Titanate Based X7R Materials

The next generations of multilayer ceramic capacitors are expected to have active ceramic layer thicknesses of 3 μm or less. To retain a reliable ceramic device, it is advantageous to engineer a uniform and fine-grained microstructure. In this study, a dense ceramic (>96% of theoretical) with fine-grained size (<0.2 μm), and permittivity of 2800, meeting the EIA X7R temperature characteristic has been obtained using hydrothermally synthesized BaTiO₃ powder with precipitation-coated additives. The temperature coefficient of capacitance (TCC) and the dissipation factor (DF) are investigated as a function of active layer thickness and compared with that of a conventional coarse-grained dielectric of similar composition and fired density. The fine-grained ceramic maintained X7R characteristic in active layers as thin as 3 μm under 1 V_AC and 1 kHz test conditions. For equivalent active thickness, the DF of the fine-grained version was consistently lower than the coarse-grained material. These results indicate that an improved voltage coefficient is possible for finer-grained materials under otherwise equivalent conditions. The possible causes for this behavior are also discussed.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Microstructure of Barium Titanate Ceramics

The microstructure of polycrystalline barium titanate as seen in polished and etched specimens is more complicated than that in the single crystal. The greater complexity is revealed in the form of a banded structure which is considered to be due to the stress configuration arising from mutual impingement of randomly oriented grains during the cubic-tetragonal phase transformation.     

Effects of Excess Barium Ions on Aqueous Barium Titanate Tape Properties

The effects of excess free barium ions in aqueous barium titanate slip on the resulting BaTiO₃ tape properties were investigated in terms of the slip behavior, green/sintered tape density and morphology, and dielectric properties. The excess free barium ions expressed by means of the Ba/Ti ratio adversely affected most tape properties. Increase in the slip viscosity, green porosity, and agglomeration along with a decrease in mechanical properties and green/sintered density were found with the increase in the Ba/Ti ratio. However, dielectric permittivity was increased with increase in the Ba/Ti ratio. An effort was made to correlate these phenomena with Ba²⁺ leaching in water for realistic multilayer ceramic capacitor applications.     

Characterization of Barium Titanate Powders: Barium Carbonate Identification

A range of methods for the detection of barium carbonate contaminant in barium titanate powder has been assessed, namely: X-ray diffraction (XRD), scanning electron microscopy (SEM), with EDS-X-ray microanalysis, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), gas chromatography (GC) for analysis of carbon, and X-ray photoelectron spectroscopy (XPS). The most satisfactory procedure for the detection of the small amounts of BaCO₃ commonly present is FT-IR. Surface analyses by XPS show that the carbonate is present as a discrete phase and is not a surface film on barium titanate particles.     

Solution-Precipitation of Fine Powders of Barium Titanate and Strontium Titanate

Hydrolytic reactions of metal alkoxides offer a broad range of possibilities for their use in the preparation of ceramic powders. This paper reports a unique and novel process to prepare fine powders of BaTiO₃ and SrTiO₃ from stable precursor solutions by carefully controlling the pH and temperature. This simple route offers good control of stoichiometry and the powders are agglomerate-free with fine particles of size 0.06–0.1 µm and were well sintered at 1200° and 1350°C, respectively. The dielectric properties of the dense ceramics are also reported. The formation aspects of these perovskite phases are also discussed.     

Investigation of Rare-Earth Doped Barium Titanate

In this study the effect of additions of 0.0015 to 0.0030 mole fraction of rare-earth oxides on the d.-c. resistivity of sintered barium titanate was investigated. The substitution may be represented by ( X ₂O₃)_M (BaTiO₃)_1-M where X is the rare earth. The rare earths samarium, gadolinium, and holmium were introduced singly into the titanate, and the resistivity was measured as a function of temperature from −170° to +330°C. An anomalous increase near the tetragonal - cubic transition temperature at 120°C. occurred which in some cases amounted to an increase in the resistivity of 4000 times the value in the tetragonal phase. The thermoelectric power of the material changed sign at the Curie temperature. The tetragonal phase exhibited n -type behavior whereas the cubic phase was p -type. The rhombohedra1 and orthorhombic phases exhibited conduction activation energies of the order of 0.2 e.v. whereas that in the tetragonal phase was approximately 0.1 e.v.     

Peroxo-oxalate Preparation of Doped Barium Titanate

The peroxo-oxalate complexation method is a method that can be used for the preparation of doped barium titanate. In this paper we focus on BaTi_0.91Zr_0.09O₃, which can be used for discharge capacitors in lamp starters. The preparation method described here is based on the complexation and subsequent precipitation in basic environment of Ba, Ti, and Zr ions with hydrogen peroxide and oxalate. The influence of several process parameters, like precipitation temperature and pH, on powder properties is described. A single-phase perovskite crystal structure is obtained after calcination starting from a chloride precursor solution using a precipitation temperature of 40°C and a pH of 9. Because the peroxo-oxalate process starts with inexpensive chlorides and is performed in air, the peroxo-oxalate process is suitable for the commercial production of doped barium titanate.     

Humidity Response Characteristics of Barium Titanate

Humidity response characteristics of BaTiO₃ doped with lanthanum were examined using complex impedance measurements. A sample with relative density of 71% showed a nearly log-linear increase of conductivity with humidity at 118 Hz. The average capacitance of bulk changed little with humidity; however, the resistance showed a gradual decrease. The equivalent circuit explaining such an observation was presented.     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Particle Size Control of Barium Titanate Prepared from Barium Titanyl Oxalate

Barium titanates formed from decomposition of barium titanyl oxalates (BT-oxalates) exhibit the same powder state as that of the Bt-oxalates, so that controlling the particle size and shape of the starting BT-oxalates is necessary. In the present study, BT-oxalates were precipitated from aqueous solution under various aging conditions: aging fluids, temperature, and time. The particle size of the spherical BT-oxalates was controlled at 0.4 μm by aging at 25°C for 3 h, and the diameter of the barium titanate formed was similar to that of the BT-oxalates.     

Synthesis of Single-Crystal Barium Titanium Isopropoxide Complex to Form Barium Titanate

Single-crystal barium titanate double alkoxide complex with a composition presumed to be BaTi(OCH(CH₃)₂)₆·xC₆H₆ was successfully synthesized in this work. The crystal was converted to BaTiO₃ at 100°C. The preparation of the Oxides via single crystals of metal alkoxides has some advantages over the well-known sol-gel method in that it results in oxides with well-defined and homogeneous compositions at the atomic level and in fine particle sizes, smaller than 50 nm.     

Crystallographic Properties of Hydrothermal Barium Titanate Crystallites

The crystallographic properties and defects of hydrothermally grown barium titanate crystallites are reported. When a precursor with a Ba:Ti molar ratio of 1.0 was used, cubic barium titanate crystallites were formed. However, when the precursor Ba:Ti molar ratio was 3.0, the product was tetragonal. The defect concentration (including OH defects and barium vacancies) was higher in cubic crystallites than in tetragonal crystallites. Because of the defects in the crystallites, the displacement of Ba²⁺ cations and the deformation of the Ti-O₆ octahedrons was suppressed; thus, no phase transition from cubic to tetragonal occurred at the Curie point. When the defects were eliminated by thermal treatment, the normal crystallographic properties were obtained. A high reaction temperature and a high Ba:Ti molar ratio in the precursor were beneficial to limiting defect dormation in the crystallites; thus, tetragonal crystallites were obtained.     

A Microstructure Study of Barium Titanate Ceramics

Polishing and etching techniques suitable for the metallographic examination of polycrystalline barium titanate ceramics are described. Interpretation of the photomicrographs reveals information which may be correlated with the electromechanical properties.     

XPS Analysis of Submicrometer Barium Titanate Powder

A commercial submicrometer BaTiO₃ powder was analyzed using X-ray photoelectron spectroscopy. The analysis revealed the powder surfaces to be covered with a layer of physisorbed H₂O and chemisorbed –OH ions. A BaCO₃ residual not detected with XPS was shown to be present in the powder using X-ray diffraction, suggesting that the carbonate takes the form of discrete particles rather than of a continuous surface layer. A relaxed surface phase detected in previous XPS analyses of bulk BaTiO₃ was also shown to be present. Depth profiling revealed the powder surfaces to be Ti-rich, confirming the presence of a phase, or phases, to stoichiometrically balance the barium carbonate.     

Incorporation of Yttrium in Barium Titanate Ceramics

In this work, X-ray diffractometry and scanning electron microscopy techniques were used to study the incorporation of yttrium in BaTiO₃, with the following nominal compositions: (Ba_{1- x} Y _x )TiO₃(0.015 ≤ to x ≤ to 0.08), (Ba_{1- x} Y _x )Ti_{1- x /4}-(□_Ti) _{x /4}O₃(0.005 ≤ to x ≤ to 0.1), and Ba(Ti_{1- y} Y _y )O_3-delta(0.028 ≤ to y ≤ to 0.258). The phase assemblage and the lattice parameters indicated a slight solubility (∼1.5 at.%) of yttrium at the Ba sites at 1440°C but a high solubility (∼12.2 at.%) of yttrium at the Ti sites at 1515°C. When BaTiO₃was heavily doped with yttrium at the Ti sites (a yttrium concentration ( y ) of <0.059), the crystallographic structure was tetragonal, whereas for y greater than equal to 0.059, the crystallographic structure was cubic.     

钛酸钡纳米粉体表面包覆方法的研究

钛酸钡纳米粉体的表面包覆不仅解决了粉体的团聚和分散问题，而且赋予粉体诸多的优异性能，因此粉体的表面包覆技术受到诸多关注并且已取得了一定的研充进展。文章主要综述了近年来钛酸钡纳米粉体表面包覆方法，以及包覆后对钛酸钡纳米粉体的表面性质乃至粉体的相结构和性质的改善作用。