Sol—Gel Synthesis of Strontium-Doped Lanthanum Manganite

Strontium-doped lanthanum manganite powders were prepared using a peroxide acetate salt based solution. The stable sol was peptized by reacting ammonium hydroxide with the precursor solution. The amorphous dried gel powders exhibit a high energy level, due to their high cations coordination and small particles, to develop the perovskite phase. This crystalline phase development from powders containing monocarboxylate ligands was characterized by thermal analysis (TG, DTG, DTA), X-ray diffraction, and IR spectroscopy. The transformation from amorphous powders into a crystallized homogeneous oxycarbonate phase in a first stage corresponds to an exothermal DTA peak at 270°C. X-ray diffraction patterns and IR spectra showed similar behavior of the powders after complete organic removal, during the conversion into perovskite phase starting at approximately 630°C and achieved about 700°C and achieved about 700°C, as well as during the sintering process.     

Preparation of Nanosized Perovskite-Type LaMnO3 Powders Using the Thermal Decomposition of a Heteronuclear Complex, LaMn(dhbaen)(OH)(NO3)(H2O)4

The heteronuclear LaMn(dhbaen)(OH)(NO₃)(H₂O)₄ complex was synthesized and perovskite-type hexagonal LaMnO₃ was obtained by its thermal decomposition at approximately 700°C. The complex and its decomposition products were analyzed using simultaneous thermogravimetric and differential thermal analysis (TG/DTA), X-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy, Auger electron spectroscopy (AES), transmission electron microscopy (TEM) characterization, and specific surface area measurements. Although XRD analysis did not show the peaks of LaMnO₃ for the sample sintered at 600°C, the presence of polycrystalline LaMnO₃ together with an amorphous phase was confirmed by TEM-selected area diffraction. Particle sizes of the samples decomposed at 600° and 700°C were 20 and 50 nm, respectively. For the conventional solid-state reaction method, XRD results showed the formation of a LaMnO₃ single phase for the samples fired above 1000°C. However, AES showed that the elemental distributions of La, Mn, and O on the surface were not homogeneous even for the sample sintered at 1200°C. The thermal decomposition of the heteronuclear complex at low temperatures allows the synthesis of single-phase hexagonal LaMnO₃ powders having nanosized particles, homogeneous and free of intragranular pores, which are suitable for electroceramics applications.     

Crystalline Intermediate Phases in the Sol–Gel-Based Synthesis of La2NiO4+δ

X-ray diffraction and transmission electron microscopy were applied to investigate a sol–gel synthetic process for the mixed oxygen ion and electron conductor La₂NiO_4+δ with a K₂NiF₄ structure type. The development of the La₂NiO_4+δ is elucidated considering the influence of calcination temperatures and dwell times. Following the thermal decomposition of nitrate and organic precursors in an intermediate step, the lanthanum nickel oxide is obtained after a short dwell time above 750°C. This occurs by the transformation of an ultrafinely dispersed powder consisting of lanthanum oxycarbonate, lanthanum oxide, and nickel oxide. The pure La₂NiO_4+δ phase was obtained by similar solid-state reactions between nanocrystalline powder particles at just 950°C.     

Preparation and Characterization of High-Purity HfTiO4

The double decomposition of hafnium and titanium tetrakis tertiary amyloxides was used to produce high-purity stoichiometric submicron HfTiO₄ powders. The particle size range was 50 to 400 Å. X-ray diffraction of a 1:1 mixture, as-prepared and heated to ∼750°C, indicated that the material is 95% orthorhombic HfTiO₄. The TGA, BET, DTA, and high-temperature X-ray analysis of the as-prepared powders are also reported; the ir absorption and reflection frequencies of HfTiO₄ were determined. The homogeneity and stoichiometry of the powder were demonstrated by electron microscopy and wet chemical analysis. Mixed oxides prepared in this manner were sintered in an ambient atmosphere at 1650° to 1700°C for 4 h or Ionger into a body of nearly theoretical density and uniform microstructure.     

Preparation of High-Tc Superconducting Oxides by the Amorphous Citrate Process

The high-T_c superconducting oxide Yba₂Cu₃O_7–y has been prepared by the amorphous citrate process. A citrate-nitrate solution was prepared at room temperature and dehydrated at around 80°C to yield a solid precursor material. The precursor was fired to high temperature to form the desired compound. The decomposition mechanism was studied with thermogravimetric analysis and X-ray diffraction. Intermediate products were formed during the decomposition reactions. Nearly phase-pure Yba₂Cu₃O_7-y powders were obtained by firing the precursor in air to 900°Cfor2 h. Sintered samples were typically over 90% dense and exhibited good superconducting properties.     

Low-Temperature Synthesis of Bismuth Titanate Niobate (Bi7Ti4NbO21) Nanoparticles from a Metal-organic Polymeric Precursor

This paper describes the preparation of homogeneous Bi₇Ti₄NbO₂₁ single-phase ceramic powders of ∼55 nm crystallite size, at temperatures as low as 400°–500°C using a metal citrate complex method based on the Pechini-type reaction route. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, and SEM, led to the formation of a well-defined orthorhombic Bi₇Ti₄NbO₂₁ compound with lattice parameters a = 0.544, b = 0.540, and c = 2.905 ± 0.0005 nm. Reaction takes place through an intermediate binary phase with a stoichiometry close to Bi₂₀TiO₃₂ which forms between 300° and 375°C. The metal-organic precursor synthesis method, where Bi, Ti, and Nb ions are first chelated to form metal complexes and then polymerized to give a gel, allows control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of nanosized bismuth titanate niobate (Bi₇Ti₄NbO₂₁) ceramic powders, at temperatures much lower than usually needed by conventional processing of mixed-oxide powders.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Metal Organic Resin Derived Barium Titanate: I, Formation of Barium Titanium Oxycarbonate Intermediate

Crystallization of BaTiO₃ from an X-ray amorphous, metal organic precursor was investigated by comparing samples heated in O₂, air, argon, and CO₂. It is evident that an intermediate barium titanium oxycarbonate phase forms between 500° and 620°C and that BaTiO₃ forms directly by the endothermic decomposition of this phase between 635° and 700°C. From thermodynamic calculations, thermal analysis, X-ray diffraction, and Raman spectroscopy, it is concluded that the intermediate oxycarbonate is a highly disordered, metastable, and weakly crystalline phase with a stoichiometry close to Ba₂Ti₂O₅CO₃.     

Preparation of Strontium- and Zinc-Doped LaGaO3 Powders via Precipitation in the Presence of Urea and/or Enzyme Urease

Solid oxide fuel cell powders having a composition of La_0.8Sr_0.2Ga_0.8Zn_0.2O_2.8 (LSGZ) were prepared by aqueous chemical precipitation in the presence of decomposing urea, followed by single-step calcination in air. In some synthesis experiments the decomposition of urea was catalyzed by the enzyme urease. The calcination behavior of the precursor powders was studied over the temperature range of 90–1300°C, in an air atmosphere. Characterization of the samples was performed by XRD, TG/DTA, FTIR, FESEM, EDS, and carbon analyses. Two tentative XRD patterns have been created for the hydroxycarbonate precursors and the product LSGZ ceramics, respectively.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Chemical Preparation of Pure and Strontium- and/or Magnesium-Doped Lanthanum Gallate Powders

Powder compositions of LaGaO₃, La_0.9Sr_0.1GaO_2.95, and La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 were prepared via a Pechini-type process that uses citric acid and ethylene glycol. The calcination behavior of the precursor powders of the above-mentioned phases was studied in the temperature range of 200°–1400°C in an air atmosphere. Characterization of the powder samples were performed using several processes, including X-ray diffractometry, thermogravimetry/differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy, and carbon and nitrogen analyses.     

Surface Decomposition of Strontium-Doped Soft PbZrO3–PbTiO3

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZT) soft piezoelectric ceramics ( d ₃₃≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO₂, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO₂ and tetragonal PZT (PbZrO₃–PbTiO₃) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.     

Preparation and Characterization of Submicron Hafnium Oxide

Submicron hafnium oxide powder prepared by hydrolytic decomposition of alkoxides was studied. The particle size range of this powder was 10 to 50 Å. Emission spectrographic analysis of the powder after it was calcined at 250°C for 0.5 h indicated a purity of >99.995%. Up to 320°C, the powder showed no crystallinity by X-ray analysis. The amorphous HfO₂ was isothermally aged at 5° to 10°C intervals between 200° and 500°C. X-ray diffraction patterns indicate a sharp transition from an amorphous state to the monoclinic phase at 325°C. High-temperature X-ray studies and DTA suggest nucleation and growth of small crystallites at 420°C leading to conversion to monoclinic HfO₂ at 480°C. BET surface area measurements and TGA of the powders were also conducted. A powder which transformed at 325°C to the monoclinic phase was isothermally aged below 325°C for 150 h without change.     

Sol–Gel Combustion Synthesis of Nanocrystalline YAG Powder from Metal-Organic Precursors

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) nanocrystalline powders were synthesized by a novel sol–gel combustion process. Yttrium acetate and aluminum sec-butoxide were used as the precursors and triethanolamine was used as the chelating agent and fuel. Thermal and crystallization behaviors of the YAG precursor powders were investigated by thermal gravimetric differential thermal analysis (DTA), Fourier transform infrared spectrum, and X-ray diffraction. The combustion-synthesized powders are amorphous and transform to a pure YAG crystalline phase at 900°C. The crystallization activation energy of amorphous YAG precursor was investigated by variable heating rate DTA. The calculated activation energy is 58.9 KJ/mol. The average crystalline size of heat-treated YAG powders at 900°C is ∼20 nm.     

A Chemical Route to BiNbO4 Ceramics

The liquid phase sintering of fine BiNbO₄ powders allows to obtain dense ceramics with excellent microwave dielectric properties (ɛ=44–46; Q × f =16,500–21,600 GHz) at T ≥700°C. The thermal decomposition of freeze-dried precursors results in the crystallization of a metastable β'-BiNbO₄ polymorph that transforms into a stable orthorhombic α-modification at T ≥700°C. The dependence of sinterability on the powder synthesis temperature shows the maximum at 600°C, corresponding to the formation of crystalline BiNbO₄ powders with a grain size 80–100 nm. Sintering temperature reduction to 700°C prevents the deterioration of silver contacts during co-firing with BiNbO₄ ceramics. In situ scanning electron microscopy observation of the morphological evolution during sintering shows that the intense shrinkage soon after the appearance of a CuO–V₂O₅ eutectics-based liquid phase is accompanied by complete transformation of the ensemble of primary BiNbO₄ particles.     

Solid Solubility and Polymorphism in the System Sr2SiO4-Sr2GeO4-Ba2GeO4-Ba2SiO4

The reciprocal salt pair Sr₂SiO₄-Sr₂GeO₄-Ba₂GeO₄-Ba₂SiO₄ was investigated using X-ray powder diffraction and DTA. Unlimited solubility in the low-K₂SO₄ structure type (α') occurs throughout the system above 85°C. The nonlinear changes of some lattice constants of the solid solutions are discussed. A stable monoclinic low-temperature (β) form of Sr₂SiO₄ was found which converts reversibly to the high-temperature α'-modification at 85°C. The enthalpy of the β-α' transition is 51 cal/mol. In the reciprocal salt pair the β-form solid solutions occur in a very narrow region below 85°C.     

Effect of Calcination Conditions and Excess Alkali Carbonate on the Phase Formation and Particle Morphology of Na0.5K0.5NbO3 Powders

Sodium-potassium niobate [Na_0.5K_0.5NbO₃] powders were prepared following the conventional mixed oxide method. An orthorhombic XRD pattern, consistent with single-phase Na_0.5K_0.5NbO₃, was obtained after calcination at 900°C for 6 h. Introducing 5 mol% excess Na₂CO₃ and K₂CO₃ into the starting mixture allowed milder calcination conditions to be used, for example 800°C for 2 h. Primary particles in 5 mol% excess samples were cuboid, with maximum sizes of ∼2.5 μm. Equiaxed 0.3–0.4-μm particles were formed for non-excess powders, and also for powders prepared with 1 and 3 mol% excess alkali carbonates. The results suggest liquid formation during calcination of the excess 5-mol% starting powders.     

High-Temperature Characteristics of Transition Al2O3 Powder with Ultrafine Spherical Particles

Ultrafine transition Al₂O₃ powder with spherical particles was prepared by an arc-discharge method. High-temperature characteristics were found to be superior to those of commercial A1₂O₃ powders by DTA, specific surface-area measurements, XRD, and TEM. The powder studied transformed to α-phase at about 1335°C. After heat treatment at 1260°C for 1 h, the specific surface area of the powder decreased from 25.0 to 17.3 m²/g. Some particles were able to retain the transition phase even after 106 h at 1160°C.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.