Seeding of Perovskite Lead Magnesium Niobate Crystallization from Pb-Mg-Nb-EDTA Gels

The addition of BaTiO₃ seed particles (50 nm in diameter) to a Pb-Mg-Nb-EDTA solution (where EDTA represents ethylenediamine tetraacetic acid) reduces the formation temperature from 700°C to 600°C during combustion synthesis, because of epitaxial effects. The lower perovskite formation temperature eliminates the formation of pyrochlore at the surface of the precursor bed and allows for the direct crystallization of ∼98% perovskite lead magnesium niobate (PMN) during combustion synthesis. Increasing the nucleation density of the seed particles in the seeded precursor decreases the precursor volume that is associated with each perovskite nucleus and, thus, reduces the PMN particle size from 1 µm to 0.2 µm. The influence of BaTiO₃ seeding is discussed in terms of the thermodynamics of perovskite formation, the influence of BaTiO₃ seed concentration, and epitaxial nucleation.     

Formation and Stability of Ferroelectric BaTi2O5

BaTi₂O₅ (BT₂) is thermodynamically stable over a very narrow temperature range between 1220° and 1230°C: a modification to the BaO–TiO₂ phase diagram is proposed. This thermodynamic stability was shown by constructing a time–temperature transformation diagram for the decomposition of BT₂. Once formed, BT₂ appears to be stable indefinitely at 1220°–1230°C; at higher temperatures, the decomposition rate increases with temperature; at lower temperatures, the decomposition rate increases with decreasing temperature and passes through a maximum at ∼1200°C; below ∼1150°C, BT₂ has long-lived kinetic stability. Kinetic considerations show a nucleation and growth mechanism for decomposition, with a nucleation induction period that is very temperature dependent. BT₂ can be prepared by various routes, including solid-state reaction of oxides below ∼1100°C; because it is metastable at all temperatures other than 1220°–1230°C, its formation is an example of Ostwald's rule of successive reactions. Discrepancies in the literature concerning the reported stability range of BT₂ can be explained by the complex dependence on temperature and time of both its formation and decomposition, for both of which, the nucleation stage is rate limiting.     

Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.     

Aluminum Titanate Powder Synthesis via Thermal Decomposition of Transparent Gels

Aluminum titanate powder has been prepared through thermal decomposition of a transparent gel formed from a mixture of titanium butoxide and boehmite sol in acetic acid—butanol medium. The thermogravimetric curve of aluminum titanate precursor gel is characterized by a decomposition temperature extending up to 800°C while the constituent gels of boehmite and hydrous titania independently decompose at lower temperature. The removal of water entrapped in the gel structure heated at the above temperature is further made clear by infrared spectral data. The gel stays in the amorphous state up to 800°C, as revealed from XRD. The sintered aluminum titanate shows that grains with sizes above 2 μm are cracked while smaller ones are intact. When heated for a period of 2 h, the gel decomposes to powders with average particle sizes of 2.7 μm at 900°C and 6.5 μm at 1400°C.     

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.     

Activation-Induced Pyrochlore-to-Perovskite Conversion for a Lead Magnesium Niobate Precursor

One or more pyrochlore phases are often involved as transitional phases prior to the formation of the perovskite phase in lead magnesium niobate (PMN) precursors. The elimination of these pyrochlore phases requires careful control of the calcination temperature and therefore is often difficult to complete. We report on the pyrochlore-to-perovskite conversion triggered by mechanical activation in a freeze-dried PMN precursor. The resulting PMN powder exhibits a well-established crystalline perovskite structure which is stable for calcination temperatures ranging from 400° to 800°C. A refined particle size in the range of 30-50 nm was obtained.     

Low Temperature Preparation of Ferroelectric Relaxor Composite Thick Films

In this work, the fabrication and characterization of a composite films comprising classical ferroelectric (PbTiO₃, PT) and relaxor (PbMg_1/3Nb_2/3O₃, PMN) material is reported. Thick films consisting of ferroelectric and relaxor phases in the thickness range of 2–10 μm are fabricated on Pt-coated Si substrates at a temperature of ≤550°C via a modified sol–gel route. The phase purity of the composite films was determined by X-ray powder diffraction pattern. The morphology and composition homogeneity were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and by X-ray mapping method, respectively. High dielectric permittivity (ɛ≈1100) and low loss values (tan δ≈0.03) at 1 kHz and room temperature were measured on 2-μm-thick composite films for a particular composition of 83 mol% of a PT phase and 17 mol% of a PMN phase, with associated remanent polarization of P ≈31.6 μC/cm² and coercive field of E _c ≅166 kV/cm. Piezo-response force microscopy analysis of the composite film showed that the film is piezoelectric and switchable. The temperature dependence of the effective dielectric permittivity and loss of the films at different frequencies was studied in the temperature range −50° to 200°C. In the temperature range used for applications (−50° to +100°C) the composite shows quite low temperature coefficient of capacitance (TCC=100%( C _T− C _−50°)/ C _−50°C))=18%, much lower than both PMN thick film and PT film in the same temperature range. This composition is therefore promising for low TCC applications.     

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.     

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.     

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.     

Optical Spectra of Nickel-Bearing Silicate Gels Prepared by the Geopolymer Technique, with Special Reference to the Low-Temperature Formation of Liebenbergite (Ni2SiO4)

A nickel-bearing silicate gel was prepared from sodium disilicate and nickel nitrate solutions at room temperature. Then, the gel was heat-treated in air up to 800°C at intervals of 200°C. The products were completely amorphous after heating at ≤600°C. However, liebenbergite and cristobalite were identified as products of the crystallization of the gel after heating at 800°C. The polycondensation process of the gel was discussed based on X-ray diffractometry, thermogravimetry-differential thermal analysis, and optical spectroscopy, to clarify the easy crystallization of the gel to liebenbergite and cristobalite. The present technique is very useful for the low-temperature preparation of liebenbergite, which has potential application in the decomposition catalysis of chlorofluorocarbons, in connection with the problem with the ozone layer.     

Low-Temperature Route to Lead Magnesium Niobate

Lead magnesium sniobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), was prepared by a sol-gel technique using alkoxide precursors. The hydrolysis-condensation mechanism leads to a translucent gel which is dried under hypercritical conditions to avoid collapse of the porous texture. After drying, the aerogel exhibits a B-deficient pyrochlore structure which progressively inserts magnesium oxide when the temperature increases. Near 700°C, this pyrochlore phase completely transforms into the PMN-perovskite phase. Above 1000°C, the loss of lead oxide leads to the destabilization of the PMN-perovskite with formation of an A-deficient pyrochlore phase.     

Properties of the Solid Electrolyte Gadolinia-Doped Ceria Prepared by Thermal Decomposition of Mixed Cerium-Gadolinium Oxalate

Fine-grained powder of the mixed oxide (CeO₂)_0.9(Gd₂O₃)_0.1, which is an ionic conductor for oxygen ions, was prepared by coprecipitation of the corresponding oxalates followed by calcination. The powder was used to prepare pellets sintered at a relatively low temperature of 1000°C compared with the usual sintering temperature of 1700° to 1800°C. The size of the powder grains was determined from BET surface area (S_BET) measurements. The effect of precipitation conditions and calcination temperature on S_bet was examined. The largest surface area measured was 88 m²/g. Decomposition of the oxalate powder was followed using an optical dilatometer. The decomposition was indicated by a large shrinkage and it was completed below 300°C (for a heating rate of 3.3°C/min). The formation of the oxide was verified by X–ray diffraction analysis. It shows that the product of decomposition is the oxide and that decomposition can be carried to completion at 250°C if the heating lasts for 1 h. The pellets had a density of 83% of theoretical, small grains (0.5 μm), and a conductivity which, at 900°C, is two–thirds of the conductivity of dense samples obtained from the same raw material, but calcined and fired at much higher temperatures.     

Synthesis of Nanocrystalline Lanthanum Strontium Manganite Powder by the Urea–Formaldehyde Polymer Gel Combustion Route

Nanocrystalline lanthanum strontium manganite (LSM) powder has been synthesized by combustion of a transparent gel obtained by the polymerization of methylol urea and urea in a solution containing La³⁺, Sr²⁺, and Mn²⁺ (LSM ions). Chemistry of the transparent urea–formaldehyde (UF) polymer gel formation and structure of the gel have been proposed such that the LSM ions act in between the growing UF polymer chains by interacting through NH, OH, and CO groups by co-ordination and prevent polymer self-assembly through inter-chain hydrogen bonding as evidenced from infrared spectrum. Thermally stable structures formed by the decomposition of UF polymer below 300°C undergo combustion in the presence of nitrate oxidant in a temperature range from 350°–450°C. A perovskite LSM phase has been formed by self-sustained combustion of the dried gel initiated with little kerosene. The powder obtained after deagglomeration and calcination at 600°C for 2 h has a D ₅₀ value of 0.19 μm, and the particles are aggregates of crystallites 10–25 nm in size.     

Dependence of Apatite Formation on Silica Gel on Its Structure: Effect of Heat Treatment

The prerequisite for glasses and glass-ceramics to bond to living bone is the formation of biologically active bonelike apatite on their surfaces in the body. Our previous study showed that a silica gel prepared by hydrolysis and polycon- densation of tetraethoxysilane in aqueous solution containing poly(ethy1ene glycol) induces apatite nucleation on its surface in a simulated body fluid. In the present study, the effects of heat treatment of silica gel on its catalytic effects in apatite nucleation was investigated in a simulated body fluid. I t was found that apatite forms on the surfaces of silica gels heat-treated below 8OO°C, but not on those heat-treated above 900°C. The volume of nanometer-range pores in the gel remarkably decreased by heat treatment above 900°C. The concentration of silanol groups in the silica gels gradually decreased with increasing heat treatment temperature. The rate of silica dissolution from the gel into the simulated body fluid decreased remarkably by heat treatment above 900°C. This suggested that a special type of silanol group which is formed by soaking the gel treated below 800°C into the simulated body fluid is respon sible for apatite nucleation.     

Nitridation of Amorphous Silica with Ammonia

The reaction between amorphous silica and ammonia in the temperature range 200° to 1230°C has been investigated. The reaction process was monitored with respect to the nitrogen content of the reaction product, the specific surface area of the amorphous nitrided silica, and the decomposition of ammonia. A surface reaction was observed at temperatures between 300° and 500°C, but in agreement with other studies bulk reaction only occurred above 800°C, reaching its maximum rate at about 1000°C. It is suggested that the decomposition of ammonia, which also becomes important above 800°C, is essential for the bulk nitridation reaction. At temperatures above 1050°C the nitridation yield decreases, until gas-phase reaction between SiO( g ) and N₂ or NH₃ becomes dominant at 1230°C, leading to the formation of α-Si₃N₄.     

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.     

Chemical Reactions During the Thermal Processing of Borazene Polymers

A class of borazene polymers was developed which consists of a two-dimensional array of six-membered borazene rings with the borons of adjacent borazene rings separated by -NH- groups. Pyrolysis of these polymers above ∼1000°C leads to crystalline graphite-like boron nitride ( h -BN). The thermal chemistry of thin films of one polymer deposited on KOH-eched aluminum was examined by thermal decomposition mass spectroscopy (TDMS) and thermal gravimetric analysis (TGA), and the gas evolution chemistry was found to be essentially complete at temperatures less than 400°C. All products desorb with the same temperature profile and the major desorbing species are NH₃ and N₂, consistent with a loss of excess nitrogen and hydrogen in the polymer, and HCl from decomposition of byproducts of the synthesis step. Since the formation of ordered crystalline h -BN films requires heating to temperatures of the order of 1000°C, whereas the gas evolution chemistry is complete by roughly 400°C, it is concluded that gas evolution chemical processes are not rate limiting in BN ceramic production.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

Processing and Characterization of Microemulsion-Derived Lead Magnesium Niobate

Lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), with 3 wt% excess PbO content has been successfully prepared via a microemulsion processing technique. By stepwise hydrolysis using aqueous ammonia as the precipitant, hydroxide precursor was obtained from nitrate solutions dispersed in the nanosized aqueous domains of a microemulsion consisting of cyclohexane, mixed poly(oxyethylene)₅ nonyl phenol ether (NP5) and poly(oxyethylene)₉ nonyl phenol ether (NP9), and an aqueous phase. Upon calcination of the microemulsion-derived precursor at 780°C, an ultrafine perovskite PMN powder with less than 5% pyrochlore phase was obtained. The percentage of perovskite phase increases with increasing calcination temperature, reaching ∼98% at 900°C. The resulting PMN powder exhibits a near-spherical particle morphology although particle agglomerates of ∼0.3 µm in average diameter occur when they are calcined at 850°C. When sintered at 1150°C, the microemulsion-derived PMN showed a maximum relative density of ∼95.6% theoretical density, which gives a maximum dielectric constant of ∼11 000 at 1 kHz and a Curie temperature of -6°C.