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.     

New Preparation Method of Low-Temperature-Sinterable Perovskite 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Powder and Its Dielectric Properties

A relaxor ferroelectric material, 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (0.9PMN-0.1PT) with a pyrochlore-free phase, was prepared by using one-step calcination in the present study. The 0.9PMN-0.1PT powder with the pure perovskite phase was prepared successfully from a mixture of the PMN precursor and the crystalline PT by heating for 2 h at temperatures greaterthan equal to750°C. The PMN precursor was synthesized by adding an aqueous Mg(NO₃)₂ solution, rather than MgO, to the alcoholic slurry of PbO and Nb₂O₅. The 0.9PMN-0.1PT powder sintered to >96% relative density via heat treatment for 2 h at temperatures of 900°-1200°C. The highest room-temperature dielectric constant (epsilon_rt) was 24700 at 1 kHz for the samples that were sintered at 1100°C; however, the samples that were sintered at 900°C still had epsilon_rt values of 22600 at 1 kHz.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Single-Calcination Synthesis of Pyrochlore Free Pb(Mg1/3Nb2/3)O3 Powders Using Particle-Coating Method

Using two-step particle-coating method, pyrochlore-free Pb(Mg_1/3Nb_2/3)O₃ (PMN) powders have been successfully synthesized by a single calcination step at a relatively lower calcined temperature of 850°C. The XRD and EDS results confirmed that the Mg–citric acid polymeric complex coatings effectively prevent direct contact between PbO and Nb₂O₅ and thus avoid the formation of pyrochlore phase. The coated powders were calcined directly without the ball-milling procedure at 850°C. The pyrochlore-free PMN powders obtained showed uniform and even grain size. The results showed that this method is an attractive method for the synthesis of PMN-based composite powders.     

Low-Temperature, Single Step, Reactive Sintering of Lead Magnesium Niobate Using Mg(OH)2-Coated Nb2O5 Powders

A low-temperature, single step, reactive sintering method for Pb(Mg_1/3Nb_2/3)O₃ (PMN) and PMN–PbTiO₃ (PMN–PT) processing was developed based on the coating of Mg(OH)₂ on Nb₂O₅. This method simplified the processing of PMN and PMN–PT to a single step of heat-treatment and decreased the sintering temperature to 1000°C. It was found that the pyrochlore phase formation reaction at 500°C reduced the particle size to 130 nm. The overlap of the pyrochlor-perovskite phase transformation between 700° and 900°C and the densification process between 800° and 1000°C improved the sintering process. These two factors were the major reasons of the low temperature sintering.     

A Polyethylene Glycol-Modified Solid-State Reaction Route to Synthesize Relaxor Ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT)

By introducing polyethylene glycol (PEG) to the conventional simultaneously mixed oxide reaction route, the 0.65Pb(Mg_1/3Nb_2/3)–0.35PbTiO₃ (0.65PMN–0.35PT) powders and ceramics with pure perovskite phase have been successfully synthesized. It is found that PEG interacts with PbO oxide in a way favoring the formation of the desired perovskite phase. As a result, pyrochlore-free 0.65PMN–0.35PT powders are synthesized at a low temperature of 850°C. The ceramics sintered at 1000°C show uniform grains with the size ranging from 1 to 3 μm. The room temperature dielectric constant is 3440. The maximum dielectric constant is 16 220 at 1 kHz. This method can be applied to the synthesis of other Pb-containing and Bi-containing ferroelectric materials, especially the relaxor-type ferroelectrics in which the pyrochlore phase is difficult to eliminate.     

Single-Calcination Synthesis Mechanism of Pure-Perovskite Pb(Ni1/3Nb2/3)O3-PbTiO3 Using a Coating Method

Pure-perovskite 0.64Pb(Ni_1/3Nb_2/3)O₃–0.36PbTiO₃ (PNN–PT) powder has been successfully synthesized by only one-step calcination using a coating method. SEM photograph shows that PNN–PT powder with the size of 2–4 μm is cubic and well dispersed. Based on X-ray diffraction analysis, solid-state reactions in the process of calcination in PbO–Nb₂O₅–TiO₂–NiO system are investigated. In comparison with conventional solid state method, the single-calcination synthesis mechanism of pyrochlore-free lead-based niobate ferroelectrics using a coating method is tentatively proposed. A typical coating structure of Ni precipitate-coated Nb₂O₅ powder facilitates the formation of perovskite PNN–PT phase at a relatively low calcination temperature, resulting in the successful synthesis of pyrochlore-free PNN–PT powder in one-step calcination at 900°C.     

Novel Route to Lead-Based Ferroelectric Compounds via Tetragonal Lead(II) Oxide Intermediates

Single-phase perovskite lead-based ferroelectric powders—Pb(Mg_1/3Nb_2/3)O₃(PMN) or 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃(0.9PMN–0.1PT)—were prepared using Mg(NO₃)₂, instead of MgO or MgCO₃, via a mixed-oxide method and one-step calcination. The reaction proceeded via the formation of 3Pb(NO₃)₂·7PbO, Pb(OH)₂, tetragonal PbO, and then 2PbO–Nb₂O₅(P₂N) for PMN or 3PbO–Nb₂O₅(P₃N) for PMN–PT; a mixture of PMN and Pb₂(Mg _x Nb_1.33)O_{5.33+ x} then formed, followed finally by the formation of single-phase PMN or 0.9PMN–0.1PT. Such prepared powder showed excellent room-temperature dielectric constants—13800 for PMN or 22600 for 0.9PMN–0.1PT—by sintering at a temperature of 900°C for 2 h.     

Preparation and Characterization of Relaxor Ferroelectric 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 by a Polymerizable Complex Method

A modified polymerizable complex (PC) method for the preparation of the relaxor ferroelectric 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ (PMN–PT) ceramics has been developed using a novel water-soluble Nb precursor. The effects of Pb content and sintering temperature on the structure, morphology, composition, and electrical properties of PMN–PT powders and ceramics were investigated systematically. It was found that the modified PC method could effectively reduce the initial crystallization temperature of the perovskite phase to 500°C. For PMN–PT samples with 15% excess Pb content sintered at 600°C for 2 h, the 87% perovskite phase can be achieved, which is much higher than that in conventional solid-state reactions and other solution-based methods at the same temperature. On further increasing the sintering temperature to 1100°C, the perovskite phase content basically remains constant. This is attributed to the Pb-deficient pyrochlore phase formation. On increasing the sintering temperature to 1250°C, the dielectric constant and remnant polarization of PMN–PT ceramics significantly improved due to the larger grain sizes, enhanced density, and the decreasing pyrochlore phase. PMN–PT ceramics with a 98.5% content of the perovskite phase have been fabricated at 1250°C. It displays typical ferroelectric relaxor characteristics with a remnant polarization of 18 μC/cm², a coercive field of 9.6 kV/cm, a piezoelectric coefficient of d ₃₃=360 pC/N, and room-temperature and maximum dielectric constants of 3600 and 10 500 at 1 kHz, respectively.     

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.     

Ferroelectric 90° Domain Evaluation in Tetragonal Pb(Mg1/3Nb2/3)O3–PbTiO3 Ceramics

The domain structure of ferroelectrics changes during poling has a direct influence on the macroscopic properties of the materials. The intensity variation of the different X-ray diffraction (XRD) pattern profiles was used to identify the percentage of 90° domain reorientation in the tetragonal phase of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) ceramics after poling. The results are consistent with the change of piezoelectric properties. In addition, by using XRD patterns, a spatial distribution of polarization in a well-poled 0.62PMN–0.38PT ceramics has been determined and was found to be best described by the Cauchy function W _00l (φ)=1/(1+0.023φ²).     

Mechanochemical Synthesis of 0.9[0.6Pb(Zn1/3Nb2/3)O3·0.4Pb(Mg1/3Nb2/3)O3]·0.1PbTiO3

Lead zinc niobate–lead magnesium niobate–lead titanate (PZN–PMN–PT) ceramic powders of perovskite structure have been prepared via a mechanochemical processing route. A single-phase perovskite powder of ultrafine particles in the nanometer range was successfully synthesized when a MZN powder (columbite precursor) was mechanically activated for 10 h together with mixed lead and titanium oxides. The following steps are involved when the ternary oxide mixture is subjected to an increasing degree of mechanical activation. First, the starting materials are significantly refined in particle size as a result of the continuous deformation, fragmentation and then partially amorphized at the initial stage of mechanical activation. This is followed by the formation of perovskite nuclei and subsequent growth of these nuclei in the activated oxide matrix with increasing activation time. When calcined at various temperatures in the range of 500–800°C, pyrochlore phase was not detected by XRD phase analysis in the mechanochemically synthesized powder. Only a minor amount (∼2%) of pyrochlore phase was observed when the calcination temperature was raised to 850°C. The PZN–PMN–PT derived from the mechanochemically synthesized powder can be sintered to ∼98% relative density at a sintering temperature of 950°C. The PZN–PMN–PT sintered at 1100°C for 1 h exhibits a dielectric constant of ∼18 600 and a dielectric loss of 0.015 at the Curie temperature of 112°C when measured at a frequency of 0.1 kHz, together with a d ₃₃ value of 323 ×10⁻¹² pC/N.     

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.     

High Strain, 〈001〉 Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties

Lead magnesium niobate–lead titanate, 0.675Pb(Mg_1/3Nb_2/3)O₃–0.325PbTiO₃ (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO₃ (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d ₃₃-coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d ₃₃-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT.     

Synthesis of Phase-Pure Pb(ZnxMg1–x)1/3Nb2/3O3 up to x= 0.7 from a Single Mixture via a Soft-Mechanochemical Route

Phase-pure perovskite Pb(Zn _x Mg_{1– x} )_1/3Nb_2/3O₃ solid solution (PZ _x M_{1– x} N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)₂, Nb₂O₅, and 2ZnCO₃·3Zn(OH)₂·H₂O at 1100°C for 1 h. Percent perovskite ( f _P) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f _P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×10⁴ and 1.7×10⁴ for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed.     

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.     

Kinetics of {001} Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 Single Crystals Grown by Seeded Polycrystal Conversion

The kinetics of {001}-oriented Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) single crystals grown by seeded polycrystal conversion were systematically quantified as a function of excess PbO liquid phase. The coarsening behavior of the corresponding matrix grains was similarly quantified. Single-crystal seed plates were embedded in a matrix of PMN-35PT with varying amounts of liquid phase (PbO) content in the range of 0 to 5 vol% and annealed at 1150°C for 0–10 h. Apparent maxima in the growth rates were observed at a PbO content of ∼3 vol% for both the single crystal and matrix grains. In both cases, the growth data were found to most closely follow cubic growth kinetics. Implications regarding the effect of PbO volume fraction on the matrix and single-crystal growth mechanisms are discussed.     

Preparation of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 Thin Films Via a Polyvinylpyrrolidone-Assisted Aqueous Sol–Gel Process and Dielectric Properties

Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) thin films were prepared by spin coating using aqueous solutions of metal salts containing polyvinylpyrrolidone, where niobium oxide layers and lead—magnesium–titanium oxide layers were laminated on Pt(111)/TiO _x /SiO₂/Si(100) substrates and fired at 750° or 800°C. 250 ± 20 nm thick 0.7PMN–0.3PT thin films of a single-phase perovskite could be prepared, and the film fired at 750°C had dielectric constants and dielectric loss of 1900 ± 350 and 0.13 ± 0.03, respectively, exhibiting polarization-electric field hysteresis with a remanent polarization of 5.1 μC/cm² and a coercive field of 21 kV/cm.     

Sequential Combination of Constituent Oxides in the Synthesis of Pb(Fe1/2Nb1/2)O3by Mechanical Activation

Pb(Fe_1/2Nb_1/2)O₃(PFN) has been successfully synthesized via a novel mechanical activation of mixed oxides and columbite precursor consisting of lead oxide and FeNbO₄. A nanocrystalline perovskite phase 5–15 nm in crystallite size was formed after 30 h of mechanical activation at room temperature for both types of starting materials. However, the nanocrystalline PFN phase derived from the mixed oxides of PbO, Fe₂O₃, and Nb₂O₅is unstable, and develops pyrochlore phases when calcined at 500°–900°C, while no pyrochlore phase is observed for the material derived from the columbite precursor consisting of PbO and FeNbO₅. Different sintering behavior and dielectric properties were also observed between the two types of PFN. These differences are accounted for by the compositional inhomogeneity in the material derived from the mixed oxides, as was revealed by Raman spectroscopic studies. This suggests that mechanical activation is analogous to thermal activation, where the phase development is strongly dependent on the sequence of combining the constituent oxides.     

Mechanism of Formation of Perovskite Phase and Dielectric Properties of Pb(Zn,Mg)1/3Nb2/3O3 Ceramics Prepared by Columbite Precursor Routes

The mechanism of formation of the perovskite phase and the dielectric properties of Pb(Zn,Mg)_1/3Nb_2/3O₃ (PZMN) ceramics were examined using two different types of columbite precursors, (Mg,Zn)Nb₂O₆ (MZN) and MgNb₂O₆+ ZnNb₂O₆ (MN + ZN). The formation of perovskite phase in the PbO + MN + ZN system is characterized by an initial rapid formation of Mg-rich perovskite phase, followed by a sluggish formation of Zn-rich perovskite phase. On the other hand, due to the formation of pyrochlore phase of mixed divalent cations Pb_{2– x} (Zn,Mg)_yNb_2−yO_{7− x −3y/2}, the pyrochlore/perovskite transformation in the PbO + MZN system proceeded uniformly with a spatial homogeneity. Further analysis suggested that the formation of perovskite phase is a diffusion-controlled process. The degree of diffuseness of the rhombohedral/cubic phase transition (DPT) is higher in the PbO + MN + ZN system than in the PbO + MZN specimen for T > T _max (temperature of the dielectric permittivity maximum), indicating a broadened compositional distribution of the B-site cations in the PbO + MN + ZN system.