Fabrication of perovskite lead magnesium niobate

The perovskite relaxor ferroelectric lead magnesium niobate ($\text{PbMg}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Nb}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{O}{}_3$) is an important material because of its high dielectric constant and correspondingly large electrostrictive strains. However, it is difficult to prepare in polycrystalline ceramic form because of the formation of a stable pyrochlore phase. The reaction kinetics during calcining were investigated and an improved fabrication scheme was developed.     

Dielectric properties of pyrochlore lead magnesium niobate

A pyrochlore with the composition Pb_1.83Nb_1.71Mg_0.29O_6.39 was fabricated and found to belong to the anion deficient structural family of pyrochlores having space group Fd3m with a lattice parameter $\text{a}\text{= 10.5988}\text{A}\text{?}$. A room temperature dielectric constant of 130 with an anomalous peak near 20 K due to a relaxation phenomenon were as observed.     

Ordering in lead magnesium niobate solid solutions

Nonstoichiometric ordering of magnesium and niobium cations in undoped and lanthanum-doped lead magnesium niobate solid solutions has been investigated by means of high-resolution transmission electron microscopy and computer image simulation. High-resolution lattice images were obtained using different objective apertures for various microscope imaging conditions. Computer image simulations were performed for a wide range of sample thicknesses, defocusing conditions, local Mg/Nb ratios, and long-range order parameters. The simulated images demonstrated that the lattice images of the ordered region are predominantly dependent on the long-range order parameter. By a comparison of the experimental and simulated images, a range of values of the long-range order parameter 0.2S<0.8 was obtained for the ordered regions. It was also shown that the ordered structure has a (NH₄)₃FeF₆ structure, which consists of alternating magnesium- and niobium-preferred sublattices along the 111 directions.     

Stability of lead magnesium niobate under hydrothermal conditions

Low-crystalline pyrochlore powder of lead magnesium niobate was treated under various hydrothermal conditions. The perovskite phase crystallized in pure water above 580 °C, but hydrothermal treatments at high temperatures for a long time decreased the amount of the perovskite phase and increased that of the cubic pyrochlore phase. The addition of lead oxide enhanced the formation of the perovskite, but not magnesium oxide. The perovskite phase was not stable in many salt solutions even with the addition of lead oxide, but its stability was increased in solutions consisting of large anions and cations.     

Dielectric characteristics of bismuth-modified lead magnesium niobate ceramics

Aliovalent Bi was substituted for Pb in Pb(Mg_1/3Nb_2/3)O₃ with required alteration in the Mg/Nb ratio. Resultant changes in the perovskite developments, lattice parameters as well as dielectric characteristics were investigated. Powders were prepared via a two-step B-site precursor route to enhance the perovskite formation. The perovskite structure persisted up to the range of 30 mol% Bi(Mg_2/3Nb_1/3)O₃ substitution. Values of the maximum dielectric constant decreased drastically, while the dielectric maximum temperatures changed only moderately. Meanwhile, the diffuseness exponent values decreased continuously with the Bi modification.     

Low firing dielectrics based on lead magnesium niobate

A fabrication process has been developed which allows to obtain pyrochlore free lead magnesium niobate ceramics with a low sintering temperature (～ 900–1000°C). The elimination of the pyrochlore phase improves dielectric characteristics and compositions which a Curie temperature of around room temperature show high dielectric constants (15–25000).     

Dielectric constant enhancement in a silicone elastomer filled with lead magnesium niobate–lead titanate

Lead magnesium niobate–lead titanate (PMN–PT) ferroelectric powder was used to develop a particulated composite based on a silicone elastomer matrix, with improved dielectric permittivity. The filler was characterised by X-ray diffraction and scanning electron microscopy. Complex dielectric permittivity (10–10⁸ Hz) and tensile mechanical properties (elastic modulus and ultimate stress) of composites at various filler contents (up to 30% by vol.) were compared with those of the neat silicone elastomer. Both the dielectric constant and loss factor regularly increased with the filler content. The elastic modulus increased with a lower rate than that of the dielectric constant. Even though the addition of filler resulted in a detriment of both toughness, ultimate stress and elongation at break, a good stretchability was still retained, as elongation ratios greater than 3 were possible at the highest filler content. Several dielectric models were compared to the experimental data and the best match was achieved by the Bruggeman model, which can be used as a predictive rule for different volume contents of filler.     

On short range ordering in the perovskite lead magnesium niobate

The structural ordering characteristics of lead magnesium niobate (PMN) and solid solutions (PMN-PT) of PMN with lead titanate have been investigated using transmission electron microscopy (TEM). It is proposed that short range, non-stoichiometric 11 ordering of the Mg and Nb cations on an F-centred superlattice generates space charges which dominate the kinetics of the ordering process and inhibit the development of long-range order. Furthermore, it is demonstrated that by introducing off-valent La³⁺ ions on to the A-site sublattice, the local charges can be at least partially-compensated and an increase in the extent of structural ordering is consequently observed.     

Critical slowing down in lead magnesium niobate–zirconate ceramic

Dielectric relaxation behaviour of (1 − x)PMN − xPZ, for x = 0.10, 0.30 and 0.40 have been studied. The nature of relaxational behaviour was found to change with PZ concentration. A crossover from a static freezing to critical slowing down like behaviour is observed with increase in Zr⁴⁺ concentration. We have used Glazounov and Tangastev criterion to distinguish freezing and critical slowing down like behaviour.     

Dielectric behavior and Raman spectra of lanthanum-doped lead magnesium niobate ceramics

Lanthanum-doped lead magnesium niobate (PLMN) ceramics were fabricated by columbite route. The effects of La-addition on dielectric behavior and Raman spectra were investigated. The trivalent La³⁺-doping showed inhibiting effects on grain growth process. The average grain size decreased and pyrochlore phase was detected along with the perovskite phase above 2 mol% La-addition. Temperature dependence of permittivity measured at 1 kHz was investigated. Due to La³⁺-doping, the maximum of dielectric constant decreased and the temperature of the maximum permittivity moved to lower temperature. An oval-form hysteresis loop was observed on La-doped (3 mol%) lead magnesium niobate (PMN) sample, which revealed that La-doped PMN owned partially normal ferroelectric feature. Raman spectra showed a shift of the peak around 270 cm⁻¹, which identified a distortion of the center symmetric ordered structure and enforced B-site compositional fluctuation. The shift of A_1g mode (around 780 cm⁻¹) indicated that degree of chemical order at B-site increased and PLMN contained 1:1 B-site ordered nanoregion (clusters) with Fm-3m symmetry.     

Reaction sintering and characterization of lead magnesium niobate relaxor ferroelectric ceramics

The formation and densification of Pb(Mg_1/3Nb_2/3)O₃ ceramics prepared by reaction sintering have been investigated. Two kinds of lead sources. PbO and Pb₃O₄ are used as the starting materials for Pb(Mg_1/3Nb_2/3)O₃. During heating processes, the specimens first expand while the pyrochlore phase is formed. At elevated temperatures, the formation and rapid sintering of Pb(Mg_1/3\Nb_2/3)O₃ occur simultaneously. The starting materials of the Pb₃\O₄ system exhibit better reactivity and sinterability than those of the PbO system. With Pb₃\O₄ as the starting material, monophasic Pb(Mg_1/3\Nb_2/3)O₃ ceramics with high sintering density are successfully achieved by reaction sintering at as low as 900 °C. While for the PbO system, pure perovskite phase could not be synthesized because of the existence of residual pyrochlore phase, and the ceramics obtained have low sintering density. The dielectric permittivity of the Pb(Mg_1/3Nb_2/3)O₃ ceramics obtained in the Pb₃O₄ system is higher than that in the PbO system. This is attributed to the formation of pure perovskite phase and high sintering density in the former system.     

Preparation and characterization of submicron-cerium oxide by hypergravity coprecipitation method

Submicron-cerium oxide particles were synthesized by applying hypergravity using ammonium bicarbonate (precipitant) and cerium nitrate hexahydrate (precursor). The influence of the concentration, pH, dispersant loading, flow rate, rotation speed of the hypergravity rotating bed, calcination temperature, and time on the cerium oxide particle size were examined by zeta potential, solid–liquid contact angle, thermo-gravimetry–differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The cerium oxide particles were highly dispersed, with an average particle size of 200 nm. Calcining at 650 °C for 1.5 h afforded the smallest particles. The crystallite size and fraction crystallinity increased with prolonged calcination. Crystalline cerium dioxide grew along three crystal planes, forming a complete face-centered cube, affording high hardness and activity of the polishing powder. The optimal conditions were: cerium nitrate concentration: 0.7 mol/L, cerium nitrate/ammonium bicarbonate molar ratio: 1:3, dispersant mass fraction: 3%, cerium nitrate initial pH: 4–5; the precursor solution was adjusted to pH 9 with ammonia water. Hypergravity coprecipitation with 1.5 h calcination afforded submicron-sized cerium oxide with a uniform size distribution using ammonium bicarbonate in an industrially viable process. The simple and low-cost manufacturing process may enable the development of hypergravity-assisted chemical synthesis technology.     

Sol-gel processing and properties of lead magnesium niobate powders and thin layers

Lead magnesium niobate powders and thin layers were formed from an alkoxide-based solution by sol-gel methods. The solution was synthesized by reacting a magnesium-niobium alkoxide solution with a lead acetate-based precursor solution. The effects of gelation conditions on the properties of the dried gel, and on the organic decomposition behaviour and crystalline phase development in gel-derived powders are reported. Gels prepared with greater molar ratios of water to alkoxide (31) had the largest surface areas (130 m²2 g^–1) and required the lowest temperature (320 ° C) for organic removal. The perovskite phase first appeared at temperatures near 700 ° C, and developed at a faster rate in gels prepared with higher water contents. Approximately 95% developed after 1 h at 700 ° C, or 5 min at 775 ° C. Dielectric thin layers were prepared on platinum-coated silicon substrates by a multilayered spin-casting method. The perovskite phase formed most readily in the thin layers by a fast-firing treatment at 800 ° C. Microstructures and electrical properties are reported for the integrated thin layer dielectrics. Ferroelectric hysteresis loops were observed.     

The low and high frequency dielectric relaxations in lead magnesium niobate ceramics

The dielectric behaviour of lead magnesium niobate (PMN) was studied, and in particular its dependences on temperature and frequency. Two different relaxations were observed in PMN, one at high frequencies and one at low frequencies. The latter was detected only below T_m, the temperature at which ′_r is maximum at low frequency. Possible mechanisms for this behaviour are discussed. At T_m, the two relaxations are partially overlapped due to both a minimum in f_r high frequency and a mazimum in f_r low frequency.     

The formation and phase stability of lead magnesium niobate in the presence of a molten flux

The formation of perovskite Pb(Mg_1/3Nb_2/3)O₃ (PMN) by the molten salt synthesis method using sulphate flux has been investigated as a function of calcination temperature and mole ratio between Li₂SO₄ and Na₂SO₄. A 97% perovskite phase was synthesized at 750° C for 30 min with 0.635Li₂SO₄-0.365Na₂SO₄ flux without any sub-products affecting the formation reaction of the PMN phase. The percentage of the perovskite phase was influenced by changes in the Li₂SO₄/Na₂SO₄ mole ratio at a given temperature. The pyrochlore phase present was identified as Pb₃Nb₄O₁₃ (P₃N₂) when 0.635Li₂SO₄-0.365Na₂SO₄ flux was used. The results for other lead-based ferroelectrics are also discussed.     

Dielectric properties of lead indium niobate ceramics synthesized by conventional solid state reaction method

Pyrochlore free lead indium niobate ceramics are successfully prepared using wolframite precursor by conventional solid state reaction method in air atmosphere, by adding an excess amount of MgO in PbO-InNbO₄ mixture. The dielectric properties of lead indium niobate ceramic studied as a function of both temperature and frequency indicate relaxor ferroelectric behavior with maximum dielectric constant of 4310 at 40 ^οC for 1 kHz. Lowering of transition temperature and enhancement of dielectric constant at room temperature, compared to earlier reports, may be due to the diffusion of magnesium ion into the lead indium niobate. The saturation polarization P_s, measured at room temperature, is found to be 22.5 μC/cm² for 40 kV/cm.     

Preparation and magnetic properties of barium ferrite fineparticles by the coprecipitation salt-catalysis method

The authors describe catalytic action by NaCl in the reaction between α-Fe₂O₃ and BaCO₃ and the development of a new process for manufacturing barium ferrite fine particles. Pure α-FeOOH and reagent-grade BaCO₃ are mixed and heated at temperatures from 300 to 800°C with and without NaCl. The BaCO₃ and α-Fe₂O₃ powders which are formed during heating react and change to barium ferrite at temperatures 100°C lower than NaCl than without it. A hematite defect structure is maintained up to a relatively high temperature due to the presence of the salt, which promotes the formation of barium ferrite. The manufacturing process involves mixing an aqueous solution of metallic chlorides with a mixture of NaOH and Na ₂CO₃. This alkaline coprecipitate slurry is then neutralized by HCl solution, filtered, and dried. The dried powder, containing salts produced by the reaction, is heated at 840°C. After the salts are dissolved in water, barium ferrite fine particles are obtained