Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Chemical Preparation of Lead-Containing Niobate Powders

A chemical precipitation method was developed for synthesis of typical relaxor compounds—Pb(Mg_1/3Nb_2/3)O₃ (PMN), Pb(Fe_1/2Nb_1/2)O₃ (PFN), and Pb(Sc_1/2Nb_1/2)O₃ (PSN)—from nitrate solutions. To obtain a niobium precursor compatible with the aqueous chemical routes, peroxo-niobium complex solutions were prepared by dissolving hydrated niobia precipitates in a dilute nitric acid solution with hydrogen peroxide. Powders that consisted of small particles ranging from 20 to 40 nm were successfully precipitated from the mixed nitrate solutions by hydrolysis with aqueous ammonia solutions. On calcination, these powders were highly reactive. For example, PMN precursor powder began to crystallize simultaneously to cubic pyrochlore and perovskite phases at ∼400°C and yielded ∼95% of the perovskite phase after calcination at 800°C for 1 h. PFN and PSN precursor powders calcined under similar conditions formed single perovskite phases.     

High-Pressure Synthesis of Perovskites Pb(Li1/4Nb3/4)O3 and Pb(Li1/4Ta3/4)O3

Complex perovskite-type compounds with the general formula Pb(B⁺_1/4B⁵⁺_3/4)O₃, where B⁺= Li⁺ and B⁵⁺= Nb⁵⁺ or Ta⁵⁺, were synthesized using a high-pressure technique and studied by X-ray powder diffraction. The X-ray patterns were indexed on the basis of a cubic cell with a ₀= 4.071 Å for Pb(Li_1/4 Nb_3/4)O₃ and a ₀= 4.052 Å for Pb(Li_1/4Ta_3/4)O₃. Electrical properties of the new perovskites were also studied.     

Thick Film Patterned Ceramics Using UV-Curable Pastes

Screen-printed layers of Al₂O₃, BaTiO₃, 0.90Pb(Mg_1/3Nb_2/3)-0₃–0.10PbTiO₃, Pb(Zr,Ti)O₃, ZnO, and glass alumina pastes have been patterned using photoprinting techniques. Curability of each paste by ultraviolet light and formation of holes with various diameters were studied for application as a production method for very small-sized vias. The dependence of hole diameters on various experimental parameters is presented and discussed. Wall slopes were found to depend on the transmission of the powders used in the pastes.     

Synthesizing Nanocrystalline Pb(Zn1/3Nb2/3)O3 Powders from Mixed Oxides

The attempt to synthesize a Pb(Zn_1/3Nb_2/3)O₃ (PZN) powder of perovskite structure via both traditional ceramic and chemistry-based novel processing routes over the last three decades has failed. Difficult-to-synthesize nanocrystallite PZN powders have, for the first time, been successfully prepared via a mechanochemical reaction either among PbO, ZnO, and Nb₂O₅ or between PbO and pre-reacted ZnNb₂O₆ from ZnO and Nb₂O₅ for more than 15 h in a high-energy mechanochemical reaction chamber. The resulting PZN powders exhibit a well-established perovskite structure and their crystallite sizes are in the range of 10 to 15 nm, as has been indicated from the peak broadening of X-ray diffraction and direct observation using a high-resolution transmission electron microscope.     

Preparation of Lead Zirconate Titanate (Pb(Zr0.52Ti0.48)O3) by Homogeneous Precipitation and Calcination

Preparation of phase-pure PZT (Pb(Zr_0.52Ti_0.48)O₃) powders was achieved, in the presence of urea (CH₄N₂O), by homogeneous precipitation. Aqueous solutions of PbCl₂, ZrOCl₂·8H₂O, and TiCl₄ were used as the starting materials in the synthesis of phase-pure PZT powders. Phase evolution behavior of precursor powders was studied by powder X-ray diffraction (XRD) in air, over the temperature range of 90° to 750°C. The morphology of the formed powders was studied by scanning electron microscopy (SEM). Semiquantitative chemical analyses of the samples were performed by energy-dispersive X-ray spectroscopy (EDXS).     

Formation of Lead Zirconate-Lead Titanate Solid Solutions

Formation of the Pb(Zr,Ti)O₃ solid solution in the system PbO-TiO₂-ZrO₂ was studied by chemical analysis and X-ray powder diffrac-tometry. Only PbTiO₃ and Pb(Zr,Ti)O₃ were found as reaction products. The following three elementary reactions seemed to be reasonable for the formation of the Pb(Zr,Ti)O₃ phase: PbO + TiO₂→ PbTiO₃, PbTiO₃+ PbO + ZrO₂→Pb(Zr_1-λTi_λ)O₃, and Pb(Zr_l-λTi_λ)O₃+ PbTiO₃→ Pb(Zr_1-λ'Ti_λ')O₃ (λ<λ').     

Synthesis and Properties of Dielectric Ceramics from Metallo-Organic Precursors through the Solid-State Reaction of Mixed Gels

A perovskite structure of 0.4Pb(Mg_1/3Nb_2/3)O₃·0.3Pb(Mg_1/2W_1/2)O₃·0.3PbTiO₃ was prepared from metallo-organic precursors through the solid-state reaction of the mixed gels. Three types of mixed gels were crystallized to obtain PbTiO₃, MgNb₂O₆, and MgWO₄ powders. These powders were calcined at 900°C after mixing with a stoichiometric amount of Pb(CH₃COO)₂·3H₂O. The dielectric constant of the ceramic fired at 900°C was improved by adding an excess of 10 mol% Mg(OC₂H₅)₂, and the ceramic achieved X7T specification of the Electric Industries Association standard. The dielectric loss was reduced by adding an excess of 5 mol% Pb(CH₃COO)₂·3H₂O.     

Hydrothermal Synthesis of Spherical Perovskite Oxide Powders Using Spherical Gel Powders

Spherical perovskite oxide powders, composed of fine particulates, were prepared by using spherical gel powders under hydrothermal conditions. Spherical PbTiO₃, BaTiO₃, and SrTiO₃ powders were synthesized from spherical TiO₂ gel powders, and spherical PbZrO₃ powder from spherical ZrO₂ gel powder. Spherical Pb(Zr_0.5, Ti_0.5)O₃ and Ba(Zr_0.5,Ti_0.5)O₃ powders were prepared from spherical ZrTiO₄ gel powders. Lead acetate trihydrate, barium hydroxide octahydrate, and strontium hydroxide octahydrate were used as the sources of A-site ions in each perovskite oxide (ABO₃). The spherical TiO₂ and ZrO₂ gel powders were prepared by thermal hydrolysis of titanium tetrachloride and zirconium oxychloride, respectively, and spherical ZrTiO₄ gel powder by thermal hydrolysis of a mixture of them in alcohol-water mixed solvent. During the hydrothermal treatment, the spherical gel powders retained their spherical shape to produce spherical perovskite oxide powders, composed of nanometer-sized particulates.     

Temperature-Stable Dielectrics Based on Chemically Inhomogeneous BaTiO3

The dielectric properties and chemical homogeneity of BaTiO₃ ceramics sintered with additions of the pseudophase "CdBi₂Nb₂O₉" were investigated using SEM, TEM, STEM, and EDX. In materials showing the "X7R" dielectric temperature characteristic, the microstructure exhibits the grain core-grain shell structure. The perovskite material in the shell shows a temperature characteristic determined by mixed crystals of BaTiO₃ with the complex perovskites Ba(Bi_1/2Nb_1/2)O₃ and Ba(Cd_1/3Nb_2/3)O₃ having an approximate Curie point of -80°C. The chemical inhomogeneity emerges during a process of reactive liquid-phase sintering. Application of too-high sintering temperatures leads to uniform distributions of the additives via solid-state diffusion and to the loss of the X7R characteristic.     

Processing and Electrical Properties in Lead-Based (Pb(Mg1/3Nb2/3)O3, Pb(Yb1/2Nb1/2)O3, PbTiO3) Systems

Lead-based ferroelectric (FE) ceramics exhibit superior electromechanical properties; therefore, there has been an increased focus on developing new lead-based FE materials with high Curie temperature ( T _c) and enhanced properties. The aim of this study was to investigate new compositions in the Pb(Mg_1/3Nb_2/3)O₃–Pb(Yb_1/2Nb_1/2)O₃–PbTiO₃ ( PMN–PYbN–PT) system to enhance the electromechanical properties while increasing the T _c and lowering the sintering temperature. The 0.575[0.5PMN–0.5PYbN]–0.425PT composition at PMN/PYbN (50/50) mole ratio were prepared by reactive sintering PMNT and PYbNT powder mixtures at 950°–1200°C for 4 h. PMNT and PYbNT powders were calcined via the columbite method. Samples were prepared by cold isostatic pressing at 80 MPa. Dense and fully perovskite 0.575[0.5PMN–0.5PYbN]–0.425PT ceramics were fabricated at 975°C for 4 h, and these samples displayed a remnant polarization ( P _r) of 32 μ C/cm², coercive field ( E _c) of 17 kV/cm, and a piezoelectric charge coefficient ( d ₃₃) of 475 pC/N. It is proposed that this ternary system can be tailored for various applications.     

Dielectric Characteristics of Bi- and Ti-Substituted Pb(Mg1/3Nb2/3)O3

Pb[Mg_1/3Nb_2/3]O₃ was gradually substituted by Bi[Mg_2/3Nb_1/3]O₃ (BiMN) up to 30 mol%, with an overall modification by a constant fraction of PbTiO₃ (10 mol%). Monophasic perovskite powders could be prepared via the B-site precursor route. Ceramic samples of the system showed a typical relaxor behavior of frequency-dependent dielectric dispersion. Values of the maximum dielectric constant decreased substantially with increasing BiMN concentration, whereas corresponding temperatures changed only moderately.     

Preparation of Monodispersed Y-Ba-Cu-O Superconductor Particles via Sol—Gel Methods

The high-T _c superconducting material YBa₂Cu₃O_{7- x} was synthesized using a modified amorphous citrate or oxalate process. Phase-pure superconducting powders were obtained by firing the citrate precursor to 900°C and the oxalate precursor to 950°C inflowing O₂. The preparation is highly reproducible and leads to powders with excellent homogeneity and sinterability. The resulting material was examined by transmission electron microscopy, thermogravimetry, and X-ray diffraction. Particle sizes for the superconductors obtained varied from 75 to 3300 Å (7.5 to 330 nm).     

Synthesis of 0.4Pb(Mg1/3Nb2/3)O3–0.3Pb(Mg1/2W1/2)O3–0.3PbTiO3 Ceramics by Metal Alkoxide Method, and Its Dielectric Properties

0.4Pb(Mg_1/3Nb_2/3)O₃–0.3Pb(Mg_1/2W_1/2)O₃–0.3PbTiO₃+ x MgO ( x = 0 to 0.04) were prepared by a metal alkoxide method. The percent of perovskite phase of the calcined powders increased with increased calcination temperatures. About 89% of perovskite phase was obtained at 1050°C. The dielectric constant of the pellets fired at 1100°C was increased by the addition of 10 wt% excess Mg(OC₂H₅)₂ and had a maximum value of 7532 at 1 kHz.     

Rapid Formation of the 110 K Phase in BI-Pb-Sr-Ca-Cu-O through Freeze-Drying Powder Processing

Three techniques for processing Bi-Pb-Sr-Ca-Cu-O (BPSCCO) powders were investigated: dry-mixing, sol-gel formation, and freeze-drying. It was found that sintering for 120 h at 850°C is required to form nearly single-phase (Bi,Pb)₂Sr₂Ca₂Cu₃O_10-y by dry-mixing, whereas sintering for 30 h at 840°C was sufficient to form the 110 K (2223) phase when freeze-drying was used. The sol-gel route was found to be intermediate in efficiency between these two techniques. Freeze-drying provided highly reactive, intimately mixed, and carbon-free precursors. The presence of carbonates in the uncalcined powders was the major cause of phase segregation and sluggishness of the 110 K phase formation.     

Preparation of Perovskite Pb(Mg1/3Nb2/3)03 Using Pb3Nb208 and MgO

The synthesis of perovskite Pb(Mg_1/3Nb_2/3)O₃ from an equimolar mixture of Pb₃Nb₂0₈ and MgO was studied by solid-state reaction techniques. An addition of 1 wt% excess MgO to the stoichiometric composition enhances the formation of the cubic perovskite phase. The absence of free PbO in the initial starting materials minimizes the volatilization loss during firing, thereby reducing the possibility of any compositional change and resulting in a substantial improvement of the perovskite phase purity over the conventional mixed-oxide processing.     

Perovskite Phase Formation in the Relaxor System [Pb(Fe1/2Nb1/2)O3]1-x–[Pb(Zn1/3Nb2/3)O3]x

Phase formation and dielectric properties of the compositions in the system [Pb(Fe_1/2Nb_1/2)O₃]₁_ _x –[Pb(Zn_1/3Nb_2/3)O₃] _x were investigated as possible materials for multilayer ceramic capacitors. The formation of the phase with perovskite structure and dielectric properties of ceramics at room temperature in the entire composition range are presented. The undesirable pyrochlore phase can be suppressed up to x = 0.6 by adopting calcination of B-site oxides, followed by reaction with PbO. Compositions in the single-phase range can be sintered at less than 1000°C.     

Preparation of Cubic Perovskites A(B2/5W3/5)O3 (A=Ba or Sr, B=Na or Li)

Pure cubic perovskites Ba(Na_2/5W_3/5)O₃, Sr(Na_2/5W_3/5)l5)O₃, and Sr(Li_2/5W_3/5)O₃ were prepared by solid-state reaction at 600° to 650°C in air, by starting with oxides or carbonates of the various elements. The cubic forms have an ordered arrangement of the B cations in the ABO₃ structure, and the lattice constants are a =0.8324 nm [Ba(Na_2/5W_3/5)O₃], 0.8136 nm Sr(Na_2/5W_3/5)O₃], and 0.7958 nm [Sr(Li_2/5W_3/5)O₃]     

Synthesis of Nanosized Titanium-Chromium Oxynitride Powders by Ammonolysis of Coprecipitated TiO2/Cr2O3 Precursors

Interstitial titanium-chromium oxynitrides in the solid solution series Ti_{1− z} Cr _z (O _x N _y ) ( z = 0.2, 0.4, 0.5, 0.6, 0.8) have been obtained by ammonolysis of the TiO₂/Cr₂O₃ precursors resulting from the coprecipitation method. The precursors and the resulting oxynitrides were characterized by auger electron spectroscopy, X-ray diffraction analysis, electron probe microanalysis, transmission electron microscopy, and BET surface area techniques. Compounds in the Ti_{1− z} Cr _z (O _x N _y ) series are prepared as single phases by nitridation at 1073 K for 8 h. The as-synthesized oxynitride powders contain only Ti_{1− z} Cr _z (O _x N _y ) with cubic structure and the particle size is in the nanometer scale.     

Single Crystals of Pb(Mg1/3Nb2/3)O3—35 mol% PbTiO3 from Polycrystalline Precursors

A potentially more cost-efficient method of growing single-crystal relaxor-based ferroelectric materials has been investigated. Seed single crystals of Pb(Mg_1/3Nb_2/3)O₃(PMN)—;35 mol% PbTiO₃(PT) were embedded within polycrystalline powders and annealed at temperatures from 900° to 1200°C. The boundary of the single crystal migrated through the polycrystal matrix under the influence of grain boundary curvature; growth distances of several millimeters were observed, verifying the feasibility of the approach. The grown single crystals exhibited macroscopic cubic growth morphologies with (100) faces. Strain levels as high as 0.68% under an electric field of 30 kV/cm were observed in initial measurements.