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.     

Structure Evolution from Pb(Mg1/3Nb2/3)O3 to La(Mg2/3Nb1/3)O3

The crystal structure of lanthanum-modified lead magnesium niobates having composition (Pb_{1− x} La _x ) (Mg_{(1+ x )/3}-Nb_{(2− x )/3})O₃ with X = 0 to 1 was investigated by X-ray powder diffraction. It was found that the fundamental reflections from perovskite structure remain in the whole range of composition. The superlattice reflections from the A(B'_1/2-B"_1/2)O₃ ordered structure are also well preserved for La content greater than 50 at.%; however, a series of extra peaks of mixing indices appears, with intensities gradually enhanced with the increase of La content. For the complete substitution of Pb by La, a splitting of some reflections can be observed in the diffraction pattern. The results indicate that the crystal structure evolves continuously with the La content, from disordered cubic perovskite of space group Pm 3 m for X = 0, to ordered cubic perovskite of space group Fm 3 m for X = 0.5, distorted cubic perovskite of space group Pa 3 for 0.5 < X < 0.9, and finally to a rhombohedral perovskite, possibly belonging to the space group R 3 , for X ≥ 0.9. In the evolution of structure, a linear reduction of the lattice constant of the perovskite cell from 4.048 to 3.964 Å was observed.     

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.     

Estimation of Phase Stability in Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 Using the Bond Valence Approach

The structure stability of perovskite-type compounds has been quantitatively estimated by applying bond valence calculations to Pb(Mg_1/3Nb_2/3)O₃ (PMN) and Pb(Zn_1/3Nb_2/3)O₃ (PZN). The bond valence calculations revealed that the bond strength between oxygen and cations in the pyrochlore-type compounds is greater than that in the perovskite PMN. It is found that the absolute value of the bond valence sum of oxygen, | V _O|, for a PZN single crystal is smallest in reported Pb-containing perovskite-type compounds, corresponding to the fact that it is impossible to synthesize PZN by solid-state reaction under atmospheric pressure. The calculated amount of additives required for stabilizing PZN under atmospheric pressure agreed well with the experimental values.     

Spray Pyrolysis Synthesis and Dielectric Properties of Pb(Mg1/3Nb2/3)O3

Spray pyrolysis was used to synthesize lead magnesium niobate (PMN) by atomizing a mixture of nitrate aqueous solutions into a high-temperature furnace. This approach allows for instant removal of solvents and decomposition of metal–salts, thereby limiting phase segregation on a nanometer scale, and lowering the transformation temperature for pyrochlore-to-perovskite phase transition. As-synthesized particles were nanocrystalline pyrochlores, with an average crystallite size ∼22 nm. More than 96% perovskite phase was obtained when as-sprayed powders were subsequently calcined at 750°C for 4 h. Sintered PMN ceramics exhibited the typical frequency-dependent dielectric properties, with a peak value of dielectric constant of 18 000, and a transition temperature at −9.6°C at 100 Hz. A series of ceramics were prepared with varied grain sizes. Increasing the grain size increased the dielectric constant, probably due to the smaller fraction of the less-polarizable grain-boundary phases.     

Powder Characteristics of Pb(Mg1/3Nb2/3)O3 Prepared by Molten Salt Synthesis

Lead magnesium niobate (Pb(Mg_1/3Nb_2/3)O₃, PMN) was prepared by a molten salt synthesis method. Powder characteristics were greatly influenced by the processing parameters, such as amount of salt, species of salt, and the presence of excess PbO. Grown in sulfate salts, PMN crystallites showed faceted morphology with a bimodal particle-size distribution in which the smaller particles were in the submicrometer range. The presence of excess PbO in the sulfate salts greatly influenced the faceted morphology and resulted in rounded PMN crystallites. The possible origin of particle morphology changes due to the processing parameters is discussed.     

Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3-La(Mg2/3Nb1/3)O3 System

A complete range of perovskite solid solutions can be formed in the (1 − x )Ba(Mg_1/3Nb_2/3)O₃- x La(Mg_2/3Nb_1/3)O₃ (BMN-LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.     

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.     

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.     

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.     

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.     

Reactive Multicomponent Powder Mixtures Prepared by Microencapsulation: Pb(Mg1/3Nb2/3)O3 Synthesis

Microencapsulation of ceramic powders using metalloorganic stearate soaps was investigated as an economical means to increase solid-state reactivity of multicomponent mixtures. The specific system investigated was lead magnesium niobate (PMN); however, the process may be applicable to a wide range of other compositions. The physical and chemical characteristics of the unfired powder mixtures and reactivity during subsequent calcination were studied as a function of batch composition and mixing method. Batch composition was varied by molar substitution of magnesium stearate for magnesium carbonate. Mixing method was investigated by comparing a dry-mixing technique developed for particle coating (mechanofusion) with conventional wet ball milling. Both mixing processes resulted in surface coating of the ceramic particles by the stearate soap. In addition, the mechanofusion process produced densely packed spherical granules of coated particles (multicored microcapsules) in the 50- to 200-μm range. Solid-state reactivity was measured in terms of perovskite phase yield, increased yields being indicative of a more reactive mixture. The highest perovskite yields (95 to 98 vol%) were achieved at 100 mol% substitution of magnesium stearate for magnesium carbonate, independent of mixing method. However, when magnesium stearate was only partially substituted for magnesium carbonate, the mechanofusion process produced consistently higher perovskite yields than did ball milling. Compared to conventional mixed-oxide processing, the increased reactivity of the microencapsulated mixtures can be attributed to higher chemical activity of the metallo-organic precursor, finer scale of mixing achieved by particle coating, and a further reduction in segregation scale due to the dense intragranule packing of multicore microcapsules.     

Phase Transition and Ferroelectric Characteristics of Pb[(Mg1/3Nb2/3)1-xTix]O3 Ceramics Modified with La(Mg2/3Nb1/3)O3

The effects of 0–5 mol% addition of La(Mg_2/3Nb_1/3)O₃ (LMN) on the phase transition and ferroelectric behaviors of Pb[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (PMNT) ceramics with compositions near the morphotropic phase boundary (MPB) were studied. An evolution of structure from rhombohedral to tetragonal was found with increasing PbTiO₃ (PT) content across the MPB (at ∼32.5 mol% PT), and a coexistence of both rhombohedral and tetragonal phases was also found at the MPB. The dual-phase field extended toward the lower PT content side of the MPB, and, moreover, the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroelectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In accordance with the structural evolution, the change of remanent polarization ( P _r) and coercive field ( E _c) also became gradually indistinct, and both P _r and E _c were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor ( k _p) in a range around 0.55–0.60, but the mechanical quality factor ( Q _m) was significantly reduced for the La-modified PMNT. The piezoelectric coefficient ( d ₃₃), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d ₃₃∼ 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q _m.     

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 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.     

Solid-State Epitaxial Effects in Structurally Diphasic Xerogel of Pb(Mg1/3Nb2/3)O3

Lead magnesium niobate (PMN), Pb(Mg_1/3Nb_2/3)O₃, with perovskite structure has been prepared using structurally diphasic PMN gels. The diphasic gels were made using various concentrations of perovskite PMN seeds. The unseeded gel calcined at 775°C for 2 h gave ∼98% of perovskite PMN phase. The use of 1% PMN perovskite seed not only led to a pure perovskite phase but also lowered the crystallization temperature of these gels by about 75°C. These results show that isostructural seeding helps to lower the crystallization temperature of perovskite PMN phase.     

Structure and Dielectric Properties of Materials in the Solid Solution System Pb(Mg1/3Nb2/33)O3:Pb(W1/2Mg1/2)O3

Solid solutions between the relaxor ferroelectric Pb₃MgNb₂O₉ (PMN) and the ordered antiferroelectric Pb₂MgWO₆ (PMW) were studied. X-ray diffraction shows that the superstructure reflections characteristic of the doubling of the perovskite subcell evident in pure PMW begin to appear in compositions containing more than 20 mol% PMW. Dielectric measurements, however, show that the diffuse transition behavior characteristic of PMN persists up to compositions Containing 80 mol% PMW. Results are discussed on the basis of present models for ferroelectric relaxor behavior.