Relationships between sintering conditions,microstructure and dielectric properties of lead magnesium niobate

Lead magnesium niobate, (Pb(Mg_1/3Nb_2/3)O₃; PMN) ceramics have been produced by sintering PMN powders synthesized from lead oxide (PbO) and magnesium niobate (MgNb₂O₆). As these PMN powders could be prepared in a reproducible manner, attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development and dielectric properties. A method has been developed whereby volatilisation of PbO can be minimised, thereby avoiding the formation of pyrochlore phases and maximising the perovskite yield. The optimum sintering conditions have been identified as 1275 °C for 2 h.     

Effect of Pb excess in sol–gel process on phase composition in PFN ceramics

Lead iron niobate Pb(Fe_0.5Nb_0.5)O₃ (PFN) precursors were prepared using sol–gel synthesis by mixing acetates Pb and Fe with Nb-ethylene glycol–tartarate (Pechini) complex at 80 °C and calcination of gels at 600 °C. Single pyrochlore phase with structure close to Pb₃Nb₄O₁₃ was formed in stoichiometric precursor and Pb₃Nb₄O₁₃ with small amount of perovskite phase Pb(Fe_0.5Nb_0.5)O₃ in nonstoichiometric precursor prepared with the excess of Pb in molar ratio (Pb:Fe:Nb = 1.2:0.5:0.5). Average particle sizes of PFN calcined powders were ～120 nm. The metastable pyrochlore phase was partially decomposed to perovskite phase at sintering temperature of 1150 °C for 2, 4 and 6 h. Excess of Pb caused increasing of the density (7.4 g/cm³) and content of the perovskite phase (～53 vol.%) in ceramics sintered for 4 h. In microstructures of PFN ceramics sintered at 1150 °C for different times, the bimodal grain size distribution was observed with small spherical grains of perovskite phase and larger octahedral grains, which represent the pyrochlore phase. Results of EDX analysis confirm that complex types of pyrochlore phases that differ in iron content were present in ceramics.     

Reaction sintering of lead zinc niobate–lead zirconate titanate ceramics

Lead zinc niobate (PZN)–lead zirconate titanate (PZT) ceramics were produced by the reaction-sintering process. The specimens were prepared directly from a mixture of their constituent oxides without any calcination step. When 50% PZN was added to tetragonal Pb(Zr_0.47Ti_0.53)O₃ ceramics, the densities and electrical properties were found to be optimal (ρ = 7.91 g/cm³, K = 1947 at 1 kHz and room temperature, d₃₃ = 530 pC/N, k_p = 0.61). However, the specimen containing more than 50% PZN showed reduced density and decreased electrical properties, due to the formation of pyrochlore phases. The improved densification behavior of the reaction-sintering process was attributed to the enhanced diffusion of lattice defects, which were created by differences in the ionic valence of the B-sites ions of the perovskite structure.     

Stability and single crystal growth of dielectric materials containing lead under hydrothermal conditions

Stability of lead magnesium niobate (PMN; Pb(Mg_1/3Nb_2/3)O₃), lead scandium niobate (PSN; Pb(Sc_0·5Nb_0·5)O₃), a solid solution in the PSN–lead titanate system (PSNT), and lead zirconate titanate (PZT; Pb(Zr_0·53Ti_0·47)O₃) were all examined in a platinum capsule under hydrothermal conditions. The perovskite PMN was formed from the low crystalline pyrochlore PMN at 600°C by the hydrothermal treatment in pure water. However, the perovskite was decomposed to the pyrochlore by further hydrothermal treatment in pure water for a longer time or at higher temperature. In KF solutions, single crystals of PMN, PSN and PSNT with a pyrochlore structure were grown at the top of the capsule by hydrothermal treatments at 600°C with a temperature gradient of 40°C. In the case of PZT, single crystals of the tetragonal PZT were grown, but the content of Zr in the grown crystals decreased in comparison with that of the starting material.     

Relationships between Sintering Conditions,Microstructure and Dielectric Properties of Lead Iron Niobate

Lead iron niobate, Pb(Fe_1/2Nb_1/2)O₃ (PFN) ceramics have been produced by sintering PFN powders synthesized from lead oxide (PbO) and iron niobate (FeNbO₄), with an effective method developed for minimising the level of PbO loss during sintering. Attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development and dielectric properties. The sintering temperature has been found to have a pronounced effect on the density, grain growth and dielectric properties of the sintered PFN ceramics, with maximum density and relative permittivity values obtained under sintering conditions of 1175°C for 2 h. The origin of the strong dependence of values of ε_r and tan δ on frequency is discussed. ©     

Magnetic and magnetoelectric properties of nickel ferrite–lead iron niobate relaxor composites

Magnetoelectric effect in bulk ceramic and multilayer (laminated) structures consisting of 6 nickel ferrite and 7 lead iron niobate relaxor (PFN) layers was investigated. This paper describes the synthesis and tape-casting process for ferrimagnetic Ni_0.3Zn_0.62Cu_0.08Fe₂O₄ ferrite and multiferroic relaxor Pb(Fe_0.5Nb_0.5)O₃. X-ray analysis and studies of the electrical and magnetic properties were performed for bulk and layered composites. Complex impedance and dielectric permittivity of bulk and layered composites were studied in a temperature range from ?40 to 300 °C and a frequency range of 10 Hz to 2 MHz. Magnetic hysteresis, ZFC–FC curves and dependencies of magnetization versus temperature for nickel ferrite, PFN relaxor and magnetoelectric composites were measured with a vibrating sample magnetometer (VSM) in an applied magnetic field up to 85 kOe at ?269 °C. Magnetoelectric effect at room temperature was investigated as a function of static magnetic field (0.3–6.5 kOe).     

Thermal behavior of the nonstoichiometric lithium niobate powders synthesized via a combustion method

Lithium niobate (Li_xNb_1?xO_3+δ) powders with various compositions are prepared via combustion synthesis. The thermal properties, crystal structure, and surface morphology of the as-prepared lithium niobate powders are characterized by thermogravimetric and differential thermal analyses (TG/DTA), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). When the calcination temperature reached 900 °C, the secondary phases Li₃NbO₄ and LiNb₃O₈ were observed. The lithium concentration before 900 °C was 40–43%. The lattice parameters increased slightly with decreasing concentration of lithium ions. When the calcination temperature was higher than 900 °C, the major Li_0.91NbO₃ phase and the minor LiNbO₃ phase coexisted in the nonstoichiometric lithium niobate with 43% lithium content.     

Preparation and dielectric properties of barium iron niobate by molten-salt synthesis

In this work, barium iron niobate; BaFe_0.5Nb_0.5O₃ powder has been successfully synthesized by molten salt method. The power was characterized by a variety technique. Pure phase perovskite was obtained at relative low calcination temperature of <700 °C. The powder exhibits a fine grain with a relatively uniform particle size and microstructure. In addition, BaFe_0.5Nb_0.5O₃ ceramics were prepared and its dielectric properties were investigated. The result suggested that the ceramic prepared from molten-salt synthesis may be has a better properties than that of the ceramics synthesized by a conventional method.     

Phase formation and transitions in the lead nickel niobate–lead zirconate titanate system

Despite the many previous studies of the synthesis and characterization of several perovskite ferroelectric materials which have potential applications in electronic and medical diagnostic devices, the synthesis and phase formation of the whole series in the solid solution of (1 ? x)Pb(Ni_1/3Nb_2/3)O₃–xPb(Zr_1/2Ti_1/2)O₃ (PNN–PZT) system has rarely been studied. In this work, the phase formation, morphology, particle size and chemical composition of perovskite powders in the (1 ? x)PNN–xPZT system were investigated. Powders were prepared by a modified mixed-oxide synthesis route for various chemical compositions under different calcination temperatures. It is found that the perovskite phase undergoes a pseudo-cubic to tetragonal transition as composition changes. The degree of spherical shape and agglomeration were observed to increase with increasing PZT content.     

Direct synthesis of PMN samples by spray-drying

The processing and characterisation of Pb(Mg_1/3Nb_2/3)O₃ (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900 °C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400 °C converting into an almost pure perovskite phase at 800 °C. The powder calcined at 350, 500 and 800 °C were sintered at different temperatures, ranging from 950 to 1150 °C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050 °C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350 °C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.     

Microstructures and electrical properties of lead zinc niobate–lead titanate–lead zirconate ceramics using microwave sintering

Electrical properties and microstructural characteristics of (1 − x)(0.94PbZn_1/3Nb_2/3O₃ + 0.06BaTiO₃) + xPbZr_yTi_1−yO₃ (PZN–PZ–PT) ceramics, sintered by microwave heating, were investigated using electron microscopy, energy-dispersive spectroscopy (EDS) and electrical property measurement. Experimental results imply that the microwave-sintered (MW) samples with x = 0.5 and y = 0.52 (1150 °C, 10 min) possess higher dielectric constant than the conventionally sintered (CS) specimens (1150 °C, 2 h). Microstructural investigations show that ZnO precipitated on the surfaces of specimens during a thermal process, implying that ZnO diffusion may have influenced the distribution of phases in a specimen due to an eutectic reaction of PbO and ZnO. TEM–EDS investigations show that the CS specimens exhibit pronounced elemental segregation of PbO and ZnO at the grain boundaries, but it is much less significant for MW samples. The results imply that microwave sintering not only enhances material densification markedly, but also reduces the PbO/ZnO segregation and amorphous intergranular layers effectively, and thus improve the electrical properties of PZN–PZ–PT ceramics.     

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.     

Processing and dielectric properties of (Pb,Bi)(Mg,Nb,Ti)O3 ceramics

Powders of the Pb(Mg_1/3Nb_2/3)O₃–Bi(Mg_2/3Nb_1/3)O₃ (PMN–BMN) system with PbTiO₃ (PT) substitution levels of 20 and 30 mol% were prepared by a B-site precursor method. Phase development as well as dielectric properties were examined. Two major phases, i.e., MgNb₂O₆ and [(Mg_1/3Nb_2/3)_1/2Ti_1/2]O₂ (with small fractions of Mg₄Nb₂O₉), developed in the B-site precursor compositions, whereas only monophasic perovskite formed after the addition of PbO/Bi₂O₃. Maximum dielectric constant values of the two systems decreased rapidly with increasing BMN concentration, but corresponding temperatures were lowest at intermediate compositions.     

Phase formation,microstructure and physical properties of lead iron scandium niobate

Lead iron scandium niobate, Pb(Fe_1/4Sc_1/4Nb_1/2)O₃ (PFScN), ceramics with pure perovskite phase were produced by conventional solid-state reaction method via a B-site oxide mixing route. Attention is focused on synthesis conditions, where sintering temperature exhibits a pronounced effect on phase formation, density, microstructure and physical properties of the sintered PFScN ceramics. Large bulk density (>95% of the theoretical density) and optimized dielectric properties can be obtained under sintering conditions of 1140–1180 °C for 2 h. Although synthesis conditions have been tailored to optimization, strong frequency dispersion of the dielectric behavior is observed in the sintered PFScN ceramics, which is considered correlating with the possible existence of residual semi-conductive FeNbO₄ and/or ScNbO₄ compounds and the thermally activated space-charge polarization resulted from the partial reduction of Fe³⁺ to Fe²⁺ ions during sintering.     

The microstructure and characteristics of 0.875PZT–0.125PMN ceramics with addition of Pb-based flux

The paper tries to prepare dense piezoceramics by way of reactive liquid phase sintering. Technique concerning a low-temperature sinterable process is developed by incorporating 4PbO.B₂O₃. The host system is a perovskite type piezoceramics, 0.875Pb(Ti,Zr)O₃–0.125Pb(Mg_1/3Nb_2/3)O₃. It is clear that PbO deficiency of PMN-based relaxor can result in an excessive amount of pyrochlore phase which causes poor densification and greatly degraded dielectric properties. Additives, such as the Pb-based flux, 4PbO·B₂O₃, that increase the amount of PbO also reduce the fraction of pyrochlore phase of PMN-based. If small amounts of 4PbO·B₂O₃ glass powder are added to the calcined 0.875PZT–0.125PMN ceramics, the liquid pase is formed during sintering. Hence, the piezoelectric and dielectric properties are enhanced and the sintering temperature can be reduced. Grain growth in ceramics with sintering time and amounts of 4PbO·B₂O₃ dopants was also studied. The grain growth was analyzed from the kinetic grain growth equation: Rⁿ=k×t.     

Structure and enhanced properties of perovskite ferroelectric PNN–PZN–PMN–PZ–PT ceramics by Ni and Mg doping

Perovskite ferroelectric oxide ceramics of 0.1Pb(Ni_1/3Nb_2/3)O₃–0.35Pb(Zn_1/3Nb_2/3)O₃–0.15Pb(Mg_1/3Nb_2/3)O₃–0.1PbZrO₃–0.3PbTiO₃ (0.10PNN–0.35PZN–0.15PMN–0.10PZ–0.30PT) with excess MgO and NiO were investigated in this work. The effects of the excess MgO and NiO doping on the ceramic structure, density, dielectric, ferroelectric and piezoelectric properties were studied. The chemical states of nickel were examined using X-ray photoelectron spectroscopy (XPS). Both XPS experimental results and theoretical analyses on the basis of ionic packing indicated that the excess valence-two ions substituted the A-sites in the ABO₃ perovskite structure. By completely eliminating the pyrochlore phase and enhancing densification with the excess NiO and MgO, improved piezoelectric coefficient d₃₃ up to 459 pC/N, higher ferroelectric remnant polarization and dielectric constant were demonstrated when sintered at temperature as low as 850–950 °C.     

A new multiferroic ternary solid solution system: NdFeO3–Pb(Fe1/2Nb1/2)O3–PbTiO3

Ceramic materials were sintered from powders of the NdFeO₃–Pb(Fe_1/2Nb_1/2)O₃–PbTiO₃ (NF–PFN–PT) ternary system synthesized by the conventional solid reaction method and their multiferroic properties investigated. The structure, electric and magnetic properties of the ternary system have been investigated. The introduction of Pb(Fe_1/2Nb_1/2)O₃ into the NdFeO₃–PbTiO₃ binary system can effectively increase its electric properties. The ternary system exhibits enhanced piezoelectric property with optimal piezoelectric constants d₃₃=143 pC/N, reduced coercive fields E_C=5.78 kV/cm and remnant polarization P_r=12.8 μC/cm² for 0.10NF–0.56PFN–0.34PT, near tetragonal phase region. The Curie temperature (T_C) of the NdFeO₃–Pb(Fe_1/2Nb_1/2)O₃–PbTiO₃ ceramics varies in the range from 108.7 °C to 67.9 °C. The magnetic hysteresis loops show that the ternary system is paramagnetic originating from canting of paramagnetic sublattices in NF–PFN–PT, due to the rare earth ions Nd³⁺ influencing on the exchange interaction between Fe³⁺ ions at the octahedral sites.     

Ferroelectric thin films obtained by pulsed laser deposition

We review our significant results concerning pulsed laser deposition (PLD) of some ferroelectric compounds: (i) lead magnesium niobate Pb(Mg_1/3Nb_2/3)O₃ (PMN); (ii) lead magnesium niobate–lead titanate Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT), with variable PT contents; (iii) La-doped lead zirconate titanate (Pb_1 − xLa_x)(Zr_0.65Ti_0.33)O₃ (PLZT); and (iv) Nb-doped lead zirconate titanate Pb_0.988(Zr_0.52Ti_0.48)_0.976Nb_0.024O₃ (PNZT). A parametric study has been performed in order to evidence the influence of the deposition parameters (laser wavelength, laser fluence, oxygen pressure, substrate type and temperature, RF power discharge addition, etc.) on the film properties and to identify the best growing conditions. Techniques including atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), secondary ions mass spectroscopy (SIMS), transmission electron microscopy (TEM), electrical and ferroelectric hysteresis measurements have been used for layer characterization.     

Seeding effects on the mechanochemical synthesis of 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3

A systematic investigation of the seeding effects on the mechanochemical synthesis of lead magnesium niobate – lead titanate 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (PMN–10PT), one of the most studied relaxor-ferroelectric material for electrocaloric applications, is reported. The perovskite crystallisation process was followed by X-ray diffraction using the Rietveld refinement method and transmission electron microscopy. Compared to the mixed-oxides case which requires 143 h of high-energy milling, the milling time needed to obtain a phase-pure PMN–10PT perovskite using PT seeds is reduced almost twice. The presence of PT seeds leads to faster transitions from the amorphous to pyrochlore and to perovskite phases compared to the mixed-oxides case. A sintering study demonstrates, for the first time, that a second, metastable, pyrochlore phase is taking part in the processes of perovskite formation. The PMN–10PT ceramic prepared from the PT-seeded powder exhibits electrocaloric properties comparable to reported values for PMN–10 PT prepared from oxides.     

Low-temperature sintering of 12Pb(Ni1/3Sb2/3)O3–40PbZrO3–48PbTiO3 with V2O5 and excess PbO additives

Low-temperature sintering of 12Pb(Ni_1/3Sb_2/3)O₃–40PbZrO₃–48PbTiO₃ (12PNS–40PZ–48PT) calcined powders with V₂O₅ and excess PbO additives has been investigated. Adding 0.20 to 0.40 wt.% V₂O₅ and 1.0 wt.% excess PbO to 12PNS–40PZ–48PT calcined powders and sintering at 950 °C for 4 h, the sintered samples only contain the perovskite structure. The calcined powders are doped with 3.0 wt.% excess PbO and 0.20 to 1.0 wt.% V₂O₅ and sintered at 950 °C for 4 h, the coexistence of both tetragonal and rhombohedral phases with the minor phase of pyrochlore is observed. During the calcined powders contain 1.0 wt.% excess PbO and are sintered at 950 to 975 °C for 2 h, the bulk density decreases with V₂O₅ addition greater than 0.6 wt.%. When the calcined powders with 3.0 wt.% excess PbO are sintered at 900 to 975 °C for 2 h, the bulk density decreases with added V₂O₅ content increased. The values of the planar coupling coefficient (K_p) approach the maxima, namely, 0.51 obtained for the compacts containing 0.40 wt.% V₂O₅ and 1.0 wt.% excess PbO and sintered at 950 °C. As the calcined powders are added with 3.0 wt.% excess PbO and 0.80 wt.% V₂O₅ and sintered at 975 °C for 2 h, the maximum Q_m value 1100 is obtained.