Lanthanum Strontium Manganite Powders Synthesized by Gel-Casting for Solid Oxide Fuel Cell Cathode Materials

Lanthanum strontium manganite ((La_0.8Sr_0.2)_0.9MnO₃; LSM) powder was successfully synthesized by an aqueous gel-casting technique, using carbonaceous precursors. Both thermal and X-ray diffraction analysis confirmed that the gel-casting LSM powder formed a single perovskite phase at 850°C, which is 100°–150°C lower than that of the LSM powder prepared by the conventional solid-state reaction route. The significantly reduced phase formation temperature of the gel-casting LSM powder is most likely due to the homogeneously distributed and immobilized precursor particles in a polymeric network, promoting the sintering and crystallization process. The LSM electrode prepared by the gel-cast LSM powder showed good electrocatalytic activity for the O₂ reduction reaction for solid oxide fuel cells.     

Sol–Gel Synthesis of Silica-Based Oxyfluoride Glass-Ceramic Thin Films: Incorporation of Eu3+ Activators into Crystallites

Transparent oxyfluoride glass-ceramic thin films were prepared using a sol–gel method starting from rare-earth trifluoroacetate/silicon alkoxide solutions. SiO₂–LaF₃ and SiO₂–LaOF glass-ceramics were formed by heating at temperatures of 300°–500°C and 600°–900°C, respectively. Eu³⁺ activators were successfully incorporated into oxyfluoride crystals, as evidenced by their luminescent properties, such as capability of a charge-transfer (O^2––Eu³⁺) excitation, suppression of a multiphonon relaxation, and occurrence of a cross-relaxation at low Eu³⁺ concentrations. As a result, the films exhibited strong red emission by ultraviolet excitation. The incorporation supposedly originated from decomposition of the (La,Eu)-trifluoroacetates in the silica-gel matrix.     

Monodisperse Calcium Carbonate Microtablets Forming at 70°C in Prerefrigerated CaCl2–Gelatin–Urea Solutions

Calcium carbonate particles with a unique tablet shape were produced by simply aging the prerefrigerated (at 4°C for 24 h) CaCl₂–gelatin–urea solutions at 70°C for 24 h in ordinary glass media bottles. Gelatin is known to be the denatured collagen. The thermal decomposition of dissolved urea was exploited to provide the Ca²⁺ ion and gelatin-containing solutions with aqueous carbonate ions. Monodisperse CaCO₃ microtablets formed in solution had a mean particle size of 4±2.5 μm. CaCO₃ microtablets were biphasic in nature and comprised of about 93% vaterite and 7% calcite. Identical solutions used without prerefrigeration yielded only trigonal prismatic calcite crystals upon aging at 70°C for 24 h. Prerefrigeration of CaCl₂–gelatin–urea solutions was thus shown to have a remarkable effect on the particle morphology. Samples were characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, and powder X-ray diffraction.     

Fabrication and Luminescent Properties of Nd3+-Doped Lu2O3 Transparent Ceramics by Pressureless Sintering

The fabrication of transparent Nd³⁺ ion-doped Lu₂O₃ ceramics is investigated by pressureless sintering under a flowing H₂ atmosphere. The starting Nd-doped Lu₂O₃ nanocrystalline powder is synthesized by a modified coprecipitant processing using a NH₄OH+NH₄HCO₃ mixed solution as the precipitant. The thermal decomposition behavior of the precipitate precursor is studied by thermogravimetric analysis and differential thermal analysis. After calcination at 1000°C for 2 h, monodispersed Nd³⁺:Lu₂O₃ powder is obtained with a primary particle size of about 40 nm and a specific surface area of 13.7 m²/g. Green compacts, free of additives, are formed from the as-synthesized powder by dry pressing followed by cold isostatic pressing. Highly transparent Nd³⁺:Lu₂O₃ ceramics are obtained after being sintered under a dry H₂ atmosphere at 1880°C for 8 h. The linear optical transmittance of the polished transparent samples with a 1.4 mm thickness reaches 75.5% at the wavelength of 1080 nm. High-resolution transmission electron microscopy observations demonstrate a "clear" grain boundary between adjacent grains. The luminescent spectra showed that the absorption coefficient of the 3 at.% Nd-doped Lu₂O₃ ceramic at 807 nm reached 14 cm⁻¹, while the emission cross section at 1079 nm was 6.5 × 10⁻²⁰ cm².     

Preparation of Crystallized Sr1−XCaXMoO4 Solid-Solution Films by an Electrochemical Method at Room Temperature and Their Luminescence

A complete series of well-crystallized solid-solution Sr_{1– X} -Ca_XMoO₄ films has been prepared on molybdenum substrates in an electrolytic solution containing Sr²⁺ and Ca²⁺ ions by an electrochemical method at room temperature (25°C). The peak positions of excitation and emission of the Sr_1–XCa_XMoO₄ solid-solution films were independent of the Ca content: 285 ± 2 nm for excitation and 536 ± 2 nm for emission at liquid-nitrogen temperature (−196°C).     

Preparation of Transparent Conductive (ZnO)mIn2O3 Fine Powder by Gel-Combustion Reaction

A fine powder of (ZnO) _m In₂O₃ ( m =3, 4) was obtained by the self-combustion reaction, under firing at a furnace temperature of 350°C in a nitrogen atmosphere, of a gel prepared from a mixed zinc and indium nitrate aqueous solution peptized with citric acid. A (ZnO)₃In₂O₃ single-phase product was obtained when citric acid was substituted by glycine, because the combustion temperature of glycine mixture is >1260°C. The highest electrical conductivity, 0.2 S cm⁻¹, was obtained in a tin-substituted, transparent conducting (ZnO) _m In₂O₃ product, where m =3 and 4.     

Structure and Microwave Dielectric Properties of Hexagonal Ba[Ti1−x(Ni1/2W1/2)x]O3 Ceramics

Microwave dielectric ceramics with the composition of Ba[Ti_{1− x} (Ni_1/2W_1/2) _x ]O₃ ( x =0.4–0.6) were prepared by a solid-state reaction method. The evolution of the crystalline phases was investigated by X-ray powder diffraction analysis. A cubic-to-hexagonal phase transition occurred between 1000° and 1300°C. The phase transition is irreversible; thus, the hexagonal phase remains stable at room temperature. The X-ray powder diffraction data for x =0.5 were refined using the Rietveld method. It was identified as a h -BaTiO₃-type hexagonal perovksite with the space group of P 6₃/ mmc . It also reveals that random occupancy of Ti⁴⁺ and W⁶⁺ ions occurs in the B-site substructures, whereas Ni²⁺ ions exclusively occupy the octahedral site in the corner-sharing octahedron. The dielectric properties of dense-sintered ceramics were characterized at microwave frequencies. With an increase in x from 0.4 to 0.6, the Q × f value increased from 26 700 to 42 000 GHz, whereas ɛ_r decreased from 29.8 to 20.0, and τ_f from +6.5 to −9.9 ppm/°C.     

Phase Transformation in Barium Orthoferrate, BaFeO3-x

The compound BaFeO_{3- x} exists in many forms, the hexagonal phase having a wide range in oxygen content (BaFeO_2.63–2.92). The other phases have the limited compositions and the distinct structures of perovskite: triclinic I, BaFeO_2.50; triclinic II, BaFeO_2.64–2.67; rhombohedral I and II, BaFeO_2.62–2.64; and tetragonal, BaFeO_2.75–2.81. The phases contain Fe⁴⁺ ions correlating with the oxygen content. The hexagonal phase shows a continuous change in oxygen content with temperature down to BaFeO_2.63. The perovskitelike phases, however, show characteristic transformations. The triclinic I phase oxidizes to the triclinic II form at 320° to 500°C and to the hexagonal form at 720° to 915°C in oxygen. These transformations are related to oxidation-reduction of iron ions (Fe³⁺⇌ Fe⁴⁺).     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

High-Thermal-Expansion Polycrystalline Leucite Ceramic

A high-thermal-expansion ceramic consisting of leucite crystals was prepared by sintering leucite powder. The densification was promoted by adding Li₂CO₃ or Na₂CO₃. The leucite ceramic obtained had a thermal expansion coefficient of about 2.4 × 10⁻⁵/°C from room temperature to 600°C. Some of the Li⁺ and Na⁺ ions were incorporated into the leucite crystal lattice to form solid solutions, and the characteristic tetragonal-cubic inversion point shifted from ca. 600° to 650°C.     

A Novel Spray-Pyrolysis Technique to Produce Nanocrystalline Lanthanum Strontium Manganite Powder

Nanocrystalline, single phase, and highly homogeneous La_0.84Sr_0.16MnO₃ (LSM) powder was prepared by a unique spray-pyrolysis process for solid oxide fuel cell applications. Atomization of a citrate–nitrate precursor solution consisting of La³⁺, Sr²⁺, and Mn²⁺ ions in the molar ratio 0.84:0.16:1.0, which can initiate a controlled exothermic anionic oxidation-reduction reaction leading to a self-propagating auto-ignition (self-ignition) reaction within individual droplets led to the conversion of the precursor to their corresponding single-phase LSM powder. Characterization of the as-sprayed and calcined products by X-ray powder diffraction, thermal analysis, and microstructural analysis confirmed the formation of nanocrystalline single-phase LSM powder by this process.     

In Situ Synthesis of Yttria-Stabilized Tetragonal Zirconia Polycrystal Powder Containing Dispersed Titanium Carbide by Selective Carbonization

Reaction thermodynamics was utilized to analyze the selective carbonization conditions of ZrO₂-TiO₂-Y₂O₃ ultrafine powder and to define the temperature range of the selective carbonization. The ZrO₂-TiO₂-Y₂O₃ powder was prepared by coprecipitation from a solution containing Zr⁴⁺, Ti⁴⁺, and Y³⁺. The powder was selectively carbonized at 1350°, 1450°, 1550°, and 1650°C, respectively, for 2 h under argon atmosphere with sucrose as the carbon source. The resulting product was analyzed by X-ray diffraction. The experimental results indicated that the ZrO₂-TiO₂-Y₂O₃ powder could be selectively carbonized in situ at 1450°C to form a carbonized powder which was composed of TiC and t -ZrO₂. It was also found that zirconium carbide could be formed at 1550°C, which is much lower than the carbonization temperature of ZrO₂ defined by thermodynamics.     

Metal Nitrate-Urea Decomposition Route for Y-Ba-Cu-O Powder

Y-Ba-Cu-O powder, having desirable composition, particle size, compaction, and sintering properties, has been prepared by a novel combustion process involving metal nitrate-urea decomposition. Single-phase Y-Ba-Cu-O is obtained by reacting the mixture of yttrium, barium, and copper nitrates in 123 stoichiometry with urea at 900°C for a period of 1 h. Following grinding in acetone the powder possessed an average particle size of 1 μm, a surface area of 42.2 m²/g, a bulk density of 2.7 g/cm³, and could be sintered to 92% theoretical density at 930°C with the resulting material having a T _c of 90 K.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.     

YBCO Oxalate Coprecipitation in Alcoholic Solutions

A process for synthesis of ultrafine YBa₂Cu₃O_7–x powder by oxalate coprecipitation from nearly saturated solutions of the metal acetates and a 2-propanol solution of oxalic acid was developed. The coprecipitation was complete within 5 min in an ice bath at 0–2°C. The final stoichiometry was Y:Ba:Cu = 1:1.994:2.991, while the particle size and surface area in the homogeneous coprecipitated powder were 0.1–0.2 pm and 24.9 m².g⁻¹, respectively. Because of the uniformity and particle size of the coprecipitated material, reactive YBCO powder with a surface area of 1.7 m².g⁻¹ can be obtained at 780°C in about 12 h.     

Transparent Cr4+-Doped YAG Ceramics for Tunable Lasers

Transparent Cr⁴⁺:YAG (Y₃Al_SO₁₂) ceramics doped with Ca and Mg as counterions and SiO₂ as a sintering aid were fabricated by a solid-state reaction method using high-purity powders of Al₂O₃, Y₂O₃, and Cr₂O₃. The mixed powder compacts were sintered at 1750°C for 10 h in oxygen, or 1750°C for 10 h under vacuum, and then annealed at 1400°C for 10 h in oxygen. Cr-doped YAG ceramics sintered in oxygen had a brown color and characteristic absorption by Cr⁴⁺ ions, whereas these YAG ceramics sintered under different conditions (vacuum + oxygen) had a green color and absorption at ∼590 and 430 nm by Cr³⁺ ions. The absorption behavior of YAG ceramics sintered in oxygen was almost equivalent to that of Cr⁴⁺:YAG single crystals fabricated by the Czochralski method.     

Phase Transition of Rare-Earth Aluminates (RE4Al2O9) and Rare-Earth Gallates (RE4Ga2O9)

Lattice parameters of RE₄Al₂O₉ (RE = Y, Sin, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) prepared at 1600–1800°C and those of RE₄Ga₂O₉ (RE = La, Pr, Nd, Sm, Eu, and Gd) prepared at 1400–1600°C were refined by Rietveld analysis for the X-ray powder diffraction patterns. The parameters increased linearly with the ionic radius of the trivalent rare-earth elements ( r _RE). High-temperature differential calorimetry and dilatometry revealed that both RE₄Al₂O, and RE₄Ga₂O, have reversible phase transitions with volume shrinkages of 0.5–0.7% on heating and thermal hystereses. The transition temperatures (7_tr) decreased from 1300°C (Yb) to 1044°C (Sm) for RE₄A1₂O₉, except for Y₄Al₂O₉ ( T_tr = 1377°C), and from 1417°C (Gd) to 1271°C (La) for RE₄Ga₂O, with increasing ionic radius of the rare-earth elements. These transition temperatures were plotted on a curve against the ionic radius ratio of Al³⁺ or Gd³⁺ and RE³⁺ ( r _A1Ga/r_RE) except for Y₄Al₂O₉.     

Formation of Calcium Lanthanum Sulfide by Carbonate Coprecipitation and CS2 Sulfurization

Pure calcium lanthanum sulfide (CLS) powders with various La/Ca ratios were prepared by a carbonate coprecipitation method and CS₂ sulfurization. Four coprecipitation processes were investigated by conventional titration, and a suitable means for coprecipitating CLS was found. By that process, a cation solution containing La³⁺ and Ca²⁺ ions was added to the precipitation medium (an anion solution of ammonium carbonate) with stirring. Precursors with various La/Ca ratios were synthesized and then sulfurized for 3 h using CS₂ as a sulfurizing agent. In the range of La/Ca ratios from 2 to 5, the intermediate phase of sulfide powder was LaS₂, and the CLS formation temperature could be increased by increasing the La/Ca ratio. For powder with high La/Ca ratios (La/Ca = 10, 15), and LaS₂intermediate phase formed at low temperature (700°C), a (β-La₂S₃ minor phase at 800°C, and a pure CLS phase after 950°C treatment. The resulting pure CLS powder was spherical, with slight necking in the submicrometer range.     

Structural Characterization and Luminescent Properties of a Red Phosphor Series: Y2−xEux(MoO4)3 (x=0.4–2.0)

A series of rare earth molybdates, Y_{2− x} Eu _x (MoO₄)₃ for x =0.4, 0.8, 1.2, 1.6 and 2.0 were prepared by solid-state method and their crystal structures, photo luminescent characteristics were investigated. The powders are mainly studied for their red light emission efficiency under near UV excitation. The crystal structures of the powders were found to depend on annealing temperature and the yttrium concentration. Mixtures of monoclinic ( C 2 /c ) and orthorhombic ( Pba 2, Pbna ) structures were formed in varying proportions depending on the value of x and annealing temperatures (700°–800°C). The luminescence behavior depended on the resultant composition of the crystal phase and the Eu³⁺ concentration. The excitation spectra showed the characteristic and broad O→Mo charge transfer (CT) band of the MoO₄ tetrahedra and the sharp intra-configurational 4 f –4 f transitions of Eu³⁺ in the host lattice. The integrated emission ratio (⁵D₀→⁷F₂/⁵D₀→⁷F₁) of Eu³⁺ depends on the annealing temperature and reveals that the local site symmetry of Eu³⁺ ions decreases with increasing concentration of Eu³⁺. The emission spectra obtained by exciting at 396 nm, gave highest red emission intensity for Y_0.4Eu_1.6(MoO₄)₃ annealed at 700°C/6 h among this series of samples.     

Nanostructured Dense ZrO2 Thin Films from Nanoparticles Obtained by Emulsion Precipitation

Nonagglomerated spherical ZrO₂ particles of 5–8 nm size were made by emulsion precipitation. Their crystallization and film-forming characteristics were investigated and compared with nanosized ZrO₂ powders obtained by sol–gel precipitation. High-temperature X-ray diffraction indicated that the emulsion-derived particles are amorphous and crystallize at 500°C into tetragonal zirconia, which is stable up to 1000°C. Crystallite growth from 5–20 nm occurred between 500°–900°C. Films of 6–75 nm thickness were made by spreading, spin coating, and controlled deposition techniques and annealed at 500°–600°C. The occurrence of t -ZrO₂ in the emulsion-precipitated powder is explained by the low degree of agglomeration and the corresponding low coarsening on heating to 500°–800°C, whereas the agglomerated state of the sol–gel precipitate powder favors the occurrence of the monoclinic form of zirconia under similar conditions.