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.     

B-Site Order–Disorder Transition in Pb(Mg1/3Nb2/3)O3–Pb(Mg1/2W1/2)O3 Triggered by Mechanical Activation

B-site cation order–disorder transition induced by mechanical activation was observed in Pb(Mg_1/3Nb_2/3)O₃–Pb(Mg_1/2W_1/2)O₃ (PMN–PMW) solid solution, which was examined using both XRD diffraction and Raman spectroscopic study. The order–disorder transition is composition dependent. Mechanical activation triggers the B-site disordering, which can be steadily recovered by thermal annealing at elevated temperature, i.e., at temperatures around 600°C. Raman spectroscopy demonstrated that there existed tiny ordered microdomains in 0.4PMN·0.6PMW subjected to up to 20 h of mechanical activation, although they cannot be shown by X-ray diffraction. This is a result of the equilibrium between the mechanical destruction and temperature-facilitated recovering at the collision points during mechanical activation. It is therefore unlikely that a complete disordering can be realized in PMN–PMW by mechanical activation. The disordering in PMN–PMW triggered by mechanical activation occurs simultaneously with the refinement in crystallite size at the initial stage of mechanical activation, suggesting that the fragmentation of crystallites is responsible for the order–disorder transition at least during the initial stage of mechanical activation.     

Direct Compositional Analysis of Ordered Domains in Pure and La-Doped Pb(Mg1/3Nb2/3)O3 by Analytical Electron Microscopy Techniques

The ordered domain structures in Pb(Mg_1/3Nb_2/3)O₃(PMN) and Pb_{1– x} La _x (Mg_1/3Nb_2/3)O₃ are identified using high-resolution transmission electron microscopy (HRTEM) and nanobeam diffractometry (NBD). The chemical compositions in the ordered domains and in the disordered matrices are also acquired using energy-dispersive spectroscopy (EDS). The best matching computer-simulated HRTEM image has a Mg²⁺/Nb²⁺ ratio of return ½. There is no obvious chemical composition difference between the ordered domains and the disordered matrices. The number of the normalized total positive valence electrons remains almost constant in the ordered domains and in the disordered matrices for all the samples. The reason for the growth of the ordered domains in La-doped PMN also is discussed.     

Effect of Lead Content on the Structure and Electrical Properties of Pb((Zn1/3Nb2/3)0.5(Zr0.47Ti0.53)0.5)O3 Ceramics

The effects of lead content on the structure and electrical properties of Pb((Zn_1/3Nb_2/3)_0.5(Zr_0.47Ti_0.53)_0.5)O₃ ceramics were investigated. Specimens with various lead concentrations were prepared by the conventional oxide-mixing method. When the lead concentration was slightly less than the stoichiometric amount, a large amount of pyrochlore phase was formed along with the perovskite phase. On the other hand, excessive amounts of lead led to the formation of PbO on the surface of the specimen. These second phases were seriously detrimental to electromechanical properties. The highest piezoelectric properties were observed when an excess of 1 mol% lead was added. By optimizing the specimen composition, excellent piezoelectric and dielectric properties ( k _p= 0.7, d ₃₃= 490 pC/N, and ɛ_m= 15000) were obtained.     

Structure and Dielectric Properties of Pb(Sc2/3W1/3)O3–Pb(Zr/Ti)O3 Relaxors

The structure and dielectric properties of (1− x )Pb(Sc_2/3W_1/3)O₃–( x )Pb(Zr/Ti)O₃ ceramics have been investigated over a full substitution range. All compositions with x < 0.5 adopt a cubic perovskite structure; however, for x ≤ 0.25 a doubled cell results from a 1:1 ordered distribution of the B-site cations. The structural order in Pb(Sc_2/3W_1/3)O₃ (PSW) can be described by a random-site model with one cation site occupied by Sc³⁺ and the other by a random distribution of (Sc_1/3³⁺W_2/3⁶⁺). The ordering is destabilized in solid solutions of PSW with PbZrO₃ (PSW–PZ), but stabilized by PbTiO₃ in the (1− x )PSW–( x )PT system. The changes in order are accompanied by alterations in the dielectric response of the two systems. For PSW–PZ the temperature of the permittivity maximum ( T _ɛ,max) increases linearly with x ; however, for PSW–PT T _ɛ,max decreases in the ordered region (up to x = 0.25) and then increases rapidly as the order is lost. Similar effects were produced by modifying the degree of order of (0.75)PSW–(0.25)PT; when the order parameter was reduced from ∼1.0 to ∼0.65, T _ɛ,max increased by more than 60°C.     

Analysis of Ferroelectric Domain Switching in 0.7Pb(Mg1/3Nb2/3)O3·0.3PbTiO3: Heterogeneous Nucleation

Polarization reversal and domain dynamics were investigated in 0.7Pb(Mg_1/3Nb_2/3)O₃·0.3PbTiO₃ using a method of current transients. Investigations were performed as a function of applied electric field. The kinetics of the transients were modeled to a stretched exponential-type function.     

In-Plane Polarized 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films

Ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) thin films were deposited on ZrO₂/SiO₂/silicon substrates using a chemical-solution-deposition method. Using a thin PZT film as a seed layer for the PMN-PT films, phase-pure perovskite PMN-PT could be obtained via rapid thermal annealing at 750°C for 60 s. The electrical properties of in-plane polarized thin films were characterized using interdigitated electrode arrays on the film surface. Ferroelectric hysteresis loops are observed with much larger remanent polarizations (∼24 μC/cm²) than for through-the-thickness polarized PMN-PT thin films (10–12 μC/cm²) deposited on Pt/Ti/Si substrates. For a finger spacing of 20 μm, the piezoelectric voltage sensitivity of in–plane polarized PMN-PT thin films was ∼20 times higher than that of through-the-thickness polarized PMN-PT thin films.     

A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite

(Na_1/2La_1/2)(Mg_1/3Nb_2/3)O₃ undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (ɛ_r∼ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

Dielectric-State Analysis in Solid-Solution Pb(Yb1/2Ta1/2)O3–Pb(Lu1/2Nb1/2)O3

The structure and temperature dependence of complex lead perovskite dielectrics were investigated for the system (1 − x )Pb(Yb_1/2Ta_1/2)O₃– x Pb(Lu_1/2Nb_1/2)O₃. Superlattice reflections for the compositions 0.8 < x < 1.0 were observed by X-ray diffractometry, and the temperature-composition dielectric-state diagram was determined. In the present study, the disordered middle composition, with 0.2 < x < 0.8, showed a diffuse paraelectric–ferroelectric phase transition, whereas the ordered end compositions, with 0 ≤ x < 0.2 and 0.8 < x ≤ 1.0, revealed successive sharp paraelectric–antiferroelectric and weak antiferroelectric–ferroelectric phase transitions. The dielectric state was confirmed by examining the variation of polarization ( P ) with electric field ( E ).     

Microstructures and Microwave Dielectric Characteristics of Ca(Zn1/3Nb2/3)O3 Complex Perovskite Ceramics

Ca(Zn_1/3Nb_2/3)O₃ microwave dielectric ceramics were prepared using a solid-state reaction process, and their microwave dielectric properties were evaluated as functions of sintering and postdensification annealing conditions. The relationship between microwave dielectric properties and processing was interpreted through the variation of microstructures. The dielectric constant showed slight variation with sintering and annealing conditions, but the Q × f value increased at first and then decreased with increased sintering temperature, and annealing in oxygen indicated significant improvement in Q × f , especially for the specimens sintered at higher temperatures. The good microwave dielectric properties were obtained in the ceramics sintered at 1225°C in air for 3 h and annealed at 1100°C in oxygen for 8 h: ɛ= 34.1, Q × f = 15 890 GHz, τ_f=−48 ppm/°C.     

Relaxor Behavior of the 0.9BaTiO3–0.1La(Mg1/2Ti1/2)O3 Solid Solution

Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO₃–0.1La(Mg_1/2Ti_1/2) O₃ (0.9BT–0.1LMT) has been studied in the temperature range of 12–550 K. In comparison with pure BT, in 0.9BT–0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12–150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole–Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La³⁺/Ba²⁺ and Mg²⁺/Ti⁴⁺ substitutions in the relaxor behavior formation is discussed.     

Short-Range Ordering in Pb(B'1/3B2/3)O3-Type Relaxor Ferroelectrics

The formation of nonstoichiometric 1:1 ordered nanodomains, which promote B-site microcompositional fluctuation, is known to be one of the most-efficient ways of enhancing diffuse phase transition (DPT) characteristics. A new theory regarding the 1:1 short-range ordering has been developed to account for the observed inability to grow ordered domains beyond the nanometer scale. The dispersion entropy associated with the formation of negatively charged ultrafine domains and the mode of the counter-ion distribution at the domain/matrix interface both have a significant role in controlling the equilibrium ordered domain size.     

Morphotropic Phase Boundary in the Pb(Zn1/3Nb2/3O3)-BaTiO3-PbTiO3 System

The morphotropic phase boundary (MPB) in the relaxor ferroelectric system Pb(Zn_1/3Nb_2/3O₃)-BaTiO₃-PbTiO₃ (PZN-BT-PT) with 15 mol% BT was investigated through dielectric permittivity and high-temperature X-ray diffraction measurements. It was revealed that MPB is a broad composition region extending from 12 to 18 mol% PT, within which the temperatures of the permittivity maximum are close to the ending temperatures for the phase transformation from coexisting rhombohedral and tetragonal phases to cubic phase on heating. When the specimen is cooled, the starting temperatures for the rhombohedral-to-tetragonal phase transition increase with increasing PT content. The large thermal hysteresis observed by X-ray diffraction is caused by the phase transformation between rhombohedral and tetragonal phases. On cooling, the MPB curves toward the PT-rich side, so that ceramics within this composition range undergoe successive phase transitions from cubic to rhombohedral and from rhombohedral to tetragonal phase. The diffuseness of the paraelectric-to-ferroelectric phase transition is remarkably decreased by the addition of PT. The enhanced dielectric permittivity peak values for the MPB compositions are correlated with the reduced lattice distortion and phase coexistence.     

Unique Dielectric Behavior of 0.6Pb(Ni1/2W1/2)O3·0.4PbTiO3 Derived from Mechanical Activation

0.6Pb(Ni_1/2W_1/2)O₃·0.4PbTiO₃(0.6PNW·0.4PT) of complex perovskite structure is successfully synthesized by mechanical activation of mixed oxide composition, followed by sintering at 950°C. It exhibits a considerably stable temperature dependence of dielectric constant over the wide temperature range of −120° to 20°C, although there occurs a dielectric peak at around 74°C. Raman spectroscopic studies show the coexistence of tetragonal and pseudocubic perovskite phases on sintering at 950°C, which are attributed to the inhomogeneous distribution of PbTiO₃ arising from mechanical activation. The dielectric behavior can be fine tuned by thermal annealing at 750°C, leading to phase redistribution in PNW-PT.     

Synthesis and Dielectric Properties of Solution Sol-Gel-Derived 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 Ceramics

A solution sol-gel method has been developed to prepare 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN-0.1PT) ceramics. During the processing the gel first converted to cubic pyrochlore phase at a calcination temperature of 600°C followed by the formation of pure perovskite phase at 775°C. The ceramics sintered at 1250°C for 4 h showed ≈98% of the theoretical density. The room-temperature dielectric constant of the pellets sintered at 1250°C showed a maximum value of 25035 at 1 kHz. Sintering studies at different temperatures revealed that the dielectric constant increased with increasing grain size in these ceramics.     

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.