Surface Decomposition of Strontium-Doped Soft PbZrO3–PbTiO3

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZT) soft piezoelectric ceramics ( d ₃₃≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO₂, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO₂ and tetragonal PZT (PbZrO₃–PbTiO₃) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.     

Phase Equilibrium in the System PbO-TiO2–ZrO2

Phase equilibrium relations in the system PbO–TiO₂–ZrO₂ were studied by quenching in the range where the PbO content is 50 mole % and more. Isotherms were examined at 1100°, 1200°, and 1300°C and tie lines were determined between the liquid and solid solution in equilibrium. The incongruent melting point of PbZrO₃ was 1570°C and the equilibrium between liquid, PbO-type solid, and PbZrO₃ is peritectic. Pb(Zr,Ti)O₃ solid solutions containing more than 14 mole % PbZrO₃ decomposed to liquid, ZrO₂, and Pb(Zr,Ti)O₃ and the decomposition temperature rises from 1340° to 1570°C with increasing PbZrO₃ content. The system PbTiO₃–PbZrO₃ should not be treated as a binary, but as a section of the ternary system.     

Thermodynamic Stability and Mechanisms of Formation and Decomposition of Perovskite Pb(Zn1/3Nb2/3)O3 Prepared by the PbO Flux Method

The mechanisms of formation and decomposition of the ferroelectric perovskite PZN [Pb(Zn_1/3Nb_2/3)O₃] were examined. The formation of perovskite PZN from the excess molten PbO environment is characterized by an inititial rapid formation of pyrochlore phase, followed by a subsequent reaction between the intermediate pyrochlore phase, ZnO, and liquid PbO to produce perovskite phase. The pure perovskite PZN crystal prepared by the PbO flux method is thermodynamically unstable over a wide range of temperature (600° to 1400°C), yielding pyrochlore phase and PbO as the decomposition products. The decomposition reaction of perovskite PZN proceeded uniformly with a spatial homogeneity throughout the specimen. The estimated activation energy of the decomposition reaction is approximatley 18 kcal/mol.     

Formation of PbZrO3 by Transformation of Cubic ZrO2 Solid Solution

Metastable ZrO₂ solid soutions containing up to 50 mol% PbO are formed at low temperatures from amorphous materials prepared by the hydrazine method: the tetragonal phase up to 30 mol% PbO and the cubic phase between 30 and 20 mol% PbO. The compound PbZrO₃ is formed at 615° to 690°C by transformation of c -ZrO₂ solid solution.     

Phase Formation and Magnetoresistance of Double Perovskite Sr2FeMoO6

Phase formation behaviors of double perovskite Sr₂FeMoO₆ (SFMO) derived from mechanical activation were investigated. Polycrystalline double perovskite SFMO of grain sizes in nanometer range were synthesized by heat treatment of the precursors derived from mechanical activation of SrO, Fe₂O₃, and MoO₃ at temperatures in the range of 600°–900°C in flowing atmosphere of H₂/Ar. Mechanical activation at room temperature led to formation of SFMO at 700°C, which is about 200°C lower than what is required in the conventional solid state reaction. The magnetization of thus-derived SFMO increases with increasing heat-treatment temperature in the range of 700°–900°C. Similarly, its magnetoresistance ratio also increases with increasing heat-treatment temperature, which is accounted for by the elimination of insulating SrMoO₄ impurity phase and enhancement in crystallinity of the double perovskite SFMO phase.     

Mechanical Activation-Assisted Synthesis of Pb(Fe2/3W1/3)O3

Perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) was prepared via a mechanical activation-assisted synthesis route from mixed oxides of PbO, Fe₂O₃, and WO₃. The mechanically activated oxide mixture, which exhibited a specific area of >10 m²/g, underwent phase conversion from nanocrystalline lead tungstate (PbWO₄) and pyrochlore (Pb₂FeWO_6.5) phases on sintering to yield perovskite PFW, although the formation of perovskite phase was not triggered by mechanical activation. When heated to 700°C, >98% perovskite phase was formed in the mechanically activated oxide mixture. The perovskite phase was sintered to a density of ∼99% of theoretical density at 870°C for 2 h. The sintered PFW exhibited a dielectric constant of 9800 at 10 kHz, which was ∼30% higher than that of the PFW derived from the oxide mixture that was not subjected to mechanical activation.     

Nanosized PbZrO3 Powder from Oxalate Precursor: Microwave-Aided Synthesis and Thermal Characterization

Nanosized lead zirconate (PbZrO₃) powder was synthesized from its oxalate precursor, namely lead zirconyl oxalate (LZO). LZO heated in a microwave heating system for 1 h yielded the PbZrO₃ at 600°C. The same precursor (LZO), when heated in a resistance-heated furnace at 850°C for 3 h, does not give a pure product. Thermogravimetry, differential thermal analysis, and X-ray diffraction techniques were used to characterize the precursor and optimize the conditions for microwave processing. The particle size of PbZrO₃ powder prepared at 600°C using microwave heating was measured using transmission electron microscopy (TEM). The TEM images show that the particles of PbZrO₃ are spherical in shape and that the particle size varies between 20 and 22 nm.     

Interface Reactions between Silicon Dioxide-Lead Oxide Glass and Manganese Zinc Ferrite

The interface reations between SiO₂–PbO melt and Mn-Zn ferrite were studied using electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Intermediate layers were formed at the interface between the glass and the Mn-Zn ferrite which were heated at 800° and 900°C, although those layers were not found in specimens heated at 1000°C. Using EPMA and XRD, the intermediate layers were found to be Pb₂(Mn, Fe)₂Si₂O₉ and Pb₈(Mn, Fe)Si₆O₂₁. The mechanisms of interface reactions are discussed, related to glassforming regions. It was concluded that the interface reaactions between SiO₂–PbO melt and Mn-Zn ferrite are controlled by the dissolution of Zn ions and Mn ions from the Mn-Zn ferrite.     

Synthesis of Pb(Mg1/3Nb2/3)O3 in Excess Lead Oxide by Mechanical Activation

Twenty hours of mechanical activation of mixed oxides at room temperature led to the formation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) in excess PbO. The crystallinity of the activation-derived perovskite PMN phase was further established when the activated PMN–PbO phase mixture was subjected to calcination at 800°C. Pyrochlores, such as Pb₃Nb₄O₁₃ and Pb₂Nb₂O₇, were not observed as transitional phases on mechanical activation and subsequent calcination, although 50% excess PbO was deliberately added. The perovskite PMN phase was recovered by washing off excess PbO using acetic acid solution at room temperature. It was sintered to a relative density of 98.9% of theoretical at 1200°C for 1 h and the sintered PMN exhibited a dielectric constant of ∼14 000 at 100 Hz and a Curie temperature of −11°C.     

Deposition of Pb(Zr,Ti)O3 Thin Films by Metal-Organic Chemical Vapor Deposition Using β-diketonate Precursors at Low Temperatures

Lead zirconate titanate (PZT) thin films were deposited by metal-organic chemical vapor deposition (MOCVD) using β-diketonate precursors and 0₂ at temperatures below 500°C on variously passivated Si substrates. PZT thin films could not be deposited on bare Si substrates, owing to a serious diffusion of Pb into the Si substrate during deposition. Pt/SiO₂/Si substrates could partially block the diffusion of Pb, but a direct deposition of PZT thin films on the Pt/SiO₂/Si substrates resulted in a very inhomogeneous deposition. A TiO₂ buffer layer deposited on Pt/SiO₂/Si substrates could partially suppress the diffusion of Pb and produce homogeneous thin films. However, the crystallinity of PZT thin films deposited on the TiO₂-buffered Pt/SiO₂/Si substrate was not good enough, and the films showed random growth direction. PZT thin films deposited on the PbTiO₃-buffered Pt/SiO₂/Si substrates had good crystallinity and a- and c-axis oriented growth direction. However, the PZT thin film deposited at 350°C showed fine amorphous phases at the grain boundaries, owing to the low chemical reactivities of the constituent elements at that temperature, but they could be crystallized by rapid thermal anneaiing (RTA) at 700°C. PZT thin film deposited on a 1000-å PbTiO₃,-thin-film-buffered Pt/SiO₂/Si substrate at 350°C and rapid thermally annealed at 700°C for 6 min showed a single-phase perovskite structure with a composition near the morphotropic boundary composition.     

Processing Effects for Integrated PZT: Residual Stress, Thickness, and Dielectric Properties

Processing effects on the dielectric properties of sol–gel-derived PbZrO₃–PbTiO₃ (PZT) films integrated onto Pt/Ti/SiO₂//Si substrates are reported. Sol–gel synthesis and deposition conditions were designed to produce films of varying thickness (95–500 nm) with consistent chemical composition (Pb (Zr_0.53Ti_0.47)O₃), phase content (perovskite), grain size (∼110 nm), crystallographic orientation (nominally (111) fiber textured), and measured residual stress. The Stoney method, using laser reflectance to determine wafer curvature, derived biaxial tensile stress values of 150 and 180 MPa for PZT films after a baseline correction for electrode interactions during thermal processing was employed. The PZT films were of high dielectric quality, with low losses and negligible dispersion. Calculated values of dielectric constant ( ̄ ') were found to decrease from 960 to 600 with decreasing film thickness. A series-capacitor model successfully recovered a room-temperature K ₁' for the PZT (1,170) in good agreement with bulk reports but was unable to reproduce the expected dielectric anomaly near 380°C. This discrepancy and the resulting diffuse phase transformation were attributed to the biaxial tensile stress present in the PZT films.     

Reaction Mechanisms in the Formation of Lead Zirconate Titanate Solid Solutions under Hydrothermal Conditions

Reaction mechanisms in the formation of PZT solid solution were studied under hydrothermal conditions (Pb/(Zr+Ti) = 1.0 to 1.9, Zr/Ti = 0/10 to 10/0, 1 M to 5 M KOH, 100° to 220°C, 2 h). A yellow tabular crystallite with tetragonal symmetry and Pb/Ti ∼ 2 was formed at 100° to 130°C. A PZT crystallite was formed just above 150°C. The crystallite was a mixture of Ti-rich PZT and Zr-rich PZT phases. When the temperature and KOH concentration were increased, the composition of the PZT product tended to be homogeneous. The PZT in the morphotropic phase boundary zone was formed at Zr/Ti = 5/5, 5M KOH, 220°C, 2 h. Neither PbTiO₃ nor PbZrO₃ was detected as a separate phase under the above hydrothermal conditions.     

Thermochemical Reactions Between PZT and Ag/Pd Powders: Relevance to Cofiring of Multilayer Actuators

Mixtures of Ag_{1− x} Pd _x ( x =0.2, 0.3) and doped PZT ceramic powders have been heat treated in air and the resulting phase content has been analyzed by X-ray diffraction and transmission electron microscopy. Beginning around 400°C, a phase similar to PbPdO₂ is formed on the surface of the Ag_{1− x} Pd _x particles and subsequently decomposes at temperatures <700°C. Consequently, the remaining Ag_{1− x} Pd _x powder becomes significantly silver-rich while the reaction progresses. After decomposition the Pd appears to realloy and the initial Ag_{1− x} Pd _x composition is recovered. We show that the reaction is all but eliminated in a nitrogen atmosphere. The occurrence of this reaction was also investigated in PZT multilayer actuators cofired with Ag_0.7Pd_0.3 electrodes. Transmission electron microscope analysis revealed the presence of distinct crystallites at the electrode–ceramic interface, most likely nucleated from a PbO liquid phase arising from the decomposition of PbPdO₂.     

Preparation of Nanosized Perovskite-Type LaMnO3 Powders Using the Thermal Decomposition of a Heteronuclear Complex, LaMn(dhbaen)(OH)(NO3)(H2O)4

The heteronuclear LaMn(dhbaen)(OH)(NO₃)(H₂O)₄ complex was synthesized and perovskite-type hexagonal LaMnO₃ was obtained by its thermal decomposition at approximately 700°C. The complex and its decomposition products were analyzed using simultaneous thermogravimetric and differential thermal analysis (TG/DTA), X-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy, Auger electron spectroscopy (AES), transmission electron microscopy (TEM) characterization, and specific surface area measurements. Although XRD analysis did not show the peaks of LaMnO₃ for the sample sintered at 600°C, the presence of polycrystalline LaMnO₃ together with an amorphous phase was confirmed by TEM-selected area diffraction. Particle sizes of the samples decomposed at 600° and 700°C were 20 and 50 nm, respectively. For the conventional solid-state reaction method, XRD results showed the formation of a LaMnO₃ single phase for the samples fired above 1000°C. However, AES showed that the elemental distributions of La, Mn, and O on the surface were not homogeneous even for the sample sintered at 1200°C. The thermal decomposition of the heteronuclear complex at low temperatures allows the synthesis of single-phase hexagonal LaMnO₃ powders having nanosized particles, homogeneous and free of intragranular pores, which are suitable for electroceramics applications.     

Synthesis of Highly Oriented Lead Zirconate–Lead Titanate Film Using Metallo-organics

Crack-free Pb(Zr,Ti)O₃ (PZT) thin films with preferred orientation were prepared successfully on MgO (100), SrTiO₃ (100), and Pt/Ti/SiO₂/Si substrates from metal alkoxide solutions. Calcination of precursor films in a H₂O─-O₂ gas mixture was found to be effective not only for low-temperature crystallization of perovskite PZT, but also for obtaining the preferred orientation of PZT films. Single-phase PZT films with high preferred orientation were synthesized on MgO (100) and Pt/Ti/SiO₂/Si substrates at 550° and 600°C for 2 h, respectively. The PZT film on the Pt/Ti/SiO₂/Si substrate showed a permittivity of 520, tan δ of 0.03, a remanent polarization of 24 μC/cm², and a coercive field of 54 kV/cm.     

Solubility Limit of La in the Lead Zirconate-Titanate System

The disappearing-phase method was used to determine the extent of the solid-solution region of the PLZT system for conventionally sintered ceramics prepared at 1100° and 1300°C in a PbO atmosphere provided by pbZrO₃. A decrease in the firing temperature from 1300° to 1100°C lowers the solubility limit by 5 to 8 at.% La. Beyond the limits of solubility, additional La forms La₂Zr₂O₇ and/or La₂Ti₂O₇. The limit determined by the disappearing-phase method (1300°C firing) is compared to values determined by the parametric method. The Curie temperature is stabilized at 5 at.% La for modified PbZrO₃ (1300°C firing).     

Effect of Excess PbO on the Sintering Characteristics and Dielectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-Based Ceramics

Additions of excess PbO to the perovskite Pb[(Mg_1/3Nb_2/3)_0.92Ti_0.08]O₃ solid solution enhanced the formation of a liquid phase at 840°C, which served as a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30°C and dissipation factors less than 1%. Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase.     

Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics via a Soft Mechanochemical Route

Single-phase perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO₂, Mg(OH)₂, and Nb₂O₅ was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO₃ packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.     

Crystallization of Sol–Gel-Derived Barium Strontium Titanate Thin Films

Microstructural development of thin-film barium strontium titanate (Ba _x Sr_{1– x} TiO₃) as a function of strontium concentration and thermal treatment were studied, using transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Thin films, ∼250 nm thick, were spin-coated onto Pt/Ti/SiO₂/Si substrates, using methoxypropoxide alkoxide precursors, and crystallized by heat-treating at 700°C. All films had the cubic perovskite structure, and their lattice parameters varied linearly with strontium content. Films with higher strontium concentrations had a larger average grain size. In situ TEM heating experiments, combined with differential thermal analysis/thermogravimetric analysis results, suggest that the gel films crystallize as an intermediate carbonate phase, Ba _x Sr_{1– x} TiO₂CO₃ (with a solid solution range from x = 1 to x = 0). Before decomposition at 600°C, this carbonate phase inhibits the formation of the desired perovskite phase.     

Formation Reaction of ZrO2 Scatterers in ZrF4-Based Fluoride Fibers

This paper clarifies the formation reaction of ZrO₂ crystals which appear as extrinsic scatterers in fluoride fibers. EPMA analysis indicates that BaO exists at grain boundaries of BaF₂ purified by sublimation. BaO reacts with ZrF₄ to form ZrO₂ at 600°C during a glass-melting process. The ZrO₂ formation reaction is influenced by H₂O. Ba(OH)₂, which is formed by the reaction between BaO and water vapor, melts at 370° to 420°C and reacts with ZrF₄ to form ZrO₂ at 450° to 520°C. When low-oxide-content BaF₂ is used for fiber preparation, scatterers significantly decrease.