Preparation of Metal Ruthenates by Spray Pyrolysis

Submicrometer crystalline metal ruthenate powders with perovskite structure, MRuO₃ (M = Sr, La), and pyrochlore structure, M₂Ru₂O_{7- x} (0.5 < x < 1; M = Bi, Pb, Y, Eu, Gd, Tb, Dy, Ho, Er, Tm), were prepared by spray pyrolysis using metal nitrates and glycolates under an oxygen-gas atmosphere at temperatures up to 1100°C. Submicrometer-sized solid single crystals (SrRuO₃), submicrometer-sized hollow spheres consisting of nanocrystallites (pyrochlore rare-earth ruthenates, Bi₂Ru₂O₇, and Pb₂Ru₂O_6.5 below 1000°C), and nanometer-sized particles (Pb_2.31Ru_1.69O_6.5 and Bi-Pb-O above 1000°C) were observed. Particle formation proceeded by intraparticle reaction and intraparticle reaction followed by evaporation of volatile metal oxides to form metal oxide vapors followed by condensation and reaction to form particles. The former was observed for systems where no volatile metal oxides were formed, whereas the latter occurred for the Pb-Ru-O and Bi-Ru-O systems, where volatile metal oxides, such as Bi₂O, PbO, and RuO _x could occur. Particle morphology depended strongly on precursor properties. Submicrometer-sized single-crystal SrRuO₃ particles could be formed from the metal nitrates but not from Sr(NO₃)₂ and ruthenium glycolate, which gave hollow polycrystalline particles. In general, crystallite size could be controlled by varying precursor properties and reactor temperature, with higher temperatures giving larger crystallite sizes.     

Chromium-Doped Forsterite Nanoparticle Synthesis by Flame Spray Pyrolysis

Synthesis of chromium-doped forsterite (Mg₂SiO₄:Cr) nanoparticles by flame spray pyrolysis (FSP) was investigated. The morphology, crystalline phase, and photoluminescence of the products were evaluated. Crystalline Mg₂SiO₄:Cr nanoparticles of several 10 nm in diameter were obtained, although a small amount of the submicrometer-sized particles and the unreacted MgO phase existed. The product powder showed electron-spin resonance signals from Cr⁴⁺ and photoluminescence typical for Cr⁴⁺ in Mg₂SiO₄, suggesting that a part of the Cr⁴⁺ ions were incorporated into Si⁴⁺ sites by FSP. On the other hand, the effects of excess SiO₂ addition on the structural and optical characteristics of Mg₂SiO₄:Cr were examined. Addition of excess SiO₂ up to 20 mol% did not influence these characteristics of the products. Further addition of excess SiO₂ (60–100 mol%) enhanced the formation of the amorphous phase and resulted in the emission from Cr³⁺ in the amorphous phase in addition to an emission from Cr⁴⁺ in Mg₂SiO₄.     

Polymer-Assisted Annealing of Spray-Pyrolyzed Powders for Formation of Luminescent Particles with Submicrometer and Nanometer Sizes

Phosphor nanoparticles (e.g., SrTiO₃:Pr,Al and (Y,Gd)₃Al₅O₁₂:Ce) were prepared using a conventional spray pyrolysis method in conjunction with a polymer-assisted heat treatment (annealing). The effects of spray/annealing temperatures, polymer type, polymer concentration, and salt addition on the morphology, crystallinity, particle/crystal size, and the photoluminescence intensity of as-prepared as well as annealed phosphor particles were investigated. The results showed that the nanocrystal growth of as-prepared-sprayed submicrometer particles was improved and that the nanocrystal networks disintegrate into nanoparticles when polymer or salt solutions are added during the heat-treatment process. The additional combustion heat and gas evolution during the thermal reaction of polymer appear to be the main reasons for this. On the contrary, only compacted agglomerated particles rather than dispersed nanoparticles were found in the case of annealing without polymer/salt addition.     

Synthesis of Solid, Spherical CeO2 Particles Prepared by the Spray Hydrolysis Reaction Method

To avoid the formation of hollow particles during spray pyrolysis, a spray hydrolysis reaction method (SHRM) was studied. Unlike the conventional spray pyrolysis that uses metal salt as a precursor and dry air as a carrier gas, the SHRM introduces a mixture of metal salt and dimethyl oxalate (DMO) as precursors and a gas mixture of water vapor and air as the carrier gas. Spherical, solid CeO₂ particles characterized by SEM, BET, and density analysis were produced by the SHRM using Ce(NO₃)₃ and DMO as the precursors. DMO, as an internal precipitant, hydrolyzed and produced oxalic acid, which precipitated with cerium ions to form volume precipitation in the whole droplet at enough temperature and relative humidity. The volume precipitation induced by the in situ formation of oxalic acid in the whole droplet prevented Ce(NO₃)₃ nucleation at the droplet surfaces, thus avoiding the formation of hollow particles which usually occur in the conventional spray pyrolysis process. XRD and IR analysis showed that cerium oxalate was an intermediate product in the SHRM process.     

Hydrolytic Condensation of Tin(IV) Alkoxide Compounds to Form Particles with Well-Defined Morphology

The sol-gel-type condensation of tin(IV) ethoxide [Sn(OEt)₄] n (where OEt is ethoxide) under basic conditions produced spherical, submicrometer-sized tin(IV) oxide (cassiterite) particles. Transmission electron microscopy and powder X-ray diffraction data indicated that the grain size was approximately 20 to 30 Å (2 to 3 nm). The mixed-metal alkoxide compound [ZnSn(OEt)₆] was hydrolyzed under analogous conditions to give either spherical or octahedral submicrometer-sized crystalline particles of ZnSn(OH)₆ depending on the solvents used. These data demonstrated that the stoichiometry of the mixed-metal alkoxide precursor was retained during condensation. Thermal treatment of ZnSn(OH)₆ resulted in crystallization of ZnSnO₃ at approximately 676°C. At neutral pH, hydrolysis of [ZnSn(OEt)₆] resulted in formation of a high surface area (261 m²/g) amorphous powder.     

Preparation of Nickel Powders by Spray Pyrolysis of Nickel Formate

The preparation of nickel powders by the ultrasonic spray pyrolysis of Ni(HCOO)₂ was studied. Phase-pure nickel powder was obtained at as low as 350°C. HCOOH was a reducing source for nickel formation. Moreover, metallic nickel was obtained at a residence time as short as 0.1 s at 600°C. A broad range of particle morphologies, which included agglomerated nanoparticles, nonagglomerated submicrometer particles, hollow particles, and spherical dense particles, were obtained from Ni(HCOO)₂ pyrolysis and were shown to depend on the precursor solution and the operating condition.     

Volatile Metal Oxide Evaporation during Aerosol Decomposition

The role of particle evaporation during synthesis of volatile metal oxide powders (Bi₂O₃, MoO₃, PbO, and V₂O₅) by aerosol decomposition (spray pyrolysis) in a heated flow reactor was investigated. Solid particles (0.1–0.6 (Am) of predominantly β-Bi₂O₃ were formed with smooth spherical shape at all reactor temperatures (673–1173 K) employed. Solid MoO₃ particles (0.1–1.2 μm) produced at low temperatures (673–773 K) had a roughly spherical or faceted morphology and at high temperatures (873 K) had a platelike or layered structure. Solid V₂O₅ particles produced at low temperatures (573–673 K) were spherical and at high temperatures (973–1073 K) were platelike. The PbO particles were solid and spherical for all temperatures studied (773°–1073°C). Evaporative losses of up to 100% to the reactor walls were observed for all the metal oxides, due to their substantial vapor pressures. The evaporative losses were modeled by considering simultaneous particle evaporation and mass transfer of the metal oxide vapor to the reactor walls. The calculations indicated that, for most of these volatile metal oxides, evaporative losses were limited by diffusional transport of the vapor to the reactor walls and that evaporative losses occur when the vapor pressure of the oxides in the reactor is above 10^-5-10^-3 mm Hg.     

Homogeneous Fabrication and Densification of Cordierite–Zirconia Composites by a Mixed Colloidal Processing Route

Based on the electrokinetic properties of aqueous silica, boehmite, and ZrO₂ dispersions, cordierite-ZrO₂ composites were fabricated by a mixed colloidal processing route. The fabricated composite was characterized by a dense and homogeneous microstructure and by a uniform spatial distribution of submicrometer-sized tetragonal ZrO₂ particles throughout the matrix. Increasing ZrO₂ content enhanced densification and resulted in a full density composite at 20 wt% ZrO₂. Fracture toughness was also increased with increasing ZrO₂ content. The enhanced toughening was partly attributed to the martensitic transformation of the dispersed tetragonal ZrO₂ particles in a cordierite matrix. The formation of zircon was suppressed by suitably adjusting the heating schedule during sintering.     

Sodium Carbonate Flux Effects on the Luminescence Characteristics of (Y0.5Gd0.5)2O3:Eu Phosphor Particles Prepared by Spray Pyrolysis

Na₂CO₃ flux was introduced in the preparation of phosphor particles by spray pyrolysis to improve the photoluminescence (PL) characteristics of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles. The phosphor particles directly prepared by spray pyrolysis at 1300°C from solutions with 20 wt% Na₂CO₃ flux had the highest PL intensity, which corresponded to 130% of that of particles prepared from solution without flux. On the other hand, the maximum PL intensity of the annealed particles, which were as-prepared at 900°C and posttreated at 1200°C for 3 h, was obtained from a solution with 5 wt% Na₂CO₃ flux. The maximum PL intensity of particles directly prepared by spray pyrolysis without posttreatment was 86% of that of posttreated phosphor particles. Na₂CO₃ flux was also important in control of morphology of (Y_0.5Gd_0.5)₂O₃:Eu phosphor particles.     

Effects of Ethanol Addition and Ba/Ti Ratios on Preparation of Barium Titanate Nanocrystals Via a Spray Pyrolysis Method

BaTiO₃ nanocrystals were synthesized by using a low-pressure spray pyrolysis (SP) method. A new strategy was proposed to improve nanocrystal formation and phase evolution by ethanol addition and Ba/Ti molar ratio variation, respectively. A mixture of submicron and nanoparticles was found with SP of precursors without ethanol addition, while only nanoparticles could be obtained by ethanol addition. BaTiO₃ crystal growth was improved by increasing the Ba/Ti ratio. In comparison, SP of the same precursor under atmospheric conditions resulted in only submicron particles with smooth surfaces and irregular morphology.     

Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO₂, TiO₂, and ZrO₂ thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag⁺ and Au³⁺ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H₂ atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO₂ film at 600°C and the TiO₂ and ZrO₂ films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO₂ and ZrO₂ films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO₂ and ZrO₂ films may hinder the growth of Ag and Au particles.     

Spark Plasma Sintering of Alumina

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al₂O₃ powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al₂O₃ could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m^1/2 were found for the compacts containing submicrometer-sized Al₂O₃ grains.     

Preparation of Monolithic Porous Titania-Silica Ceramics

Monolithic porous ceramics composed of TiO₂ (67 mol%) and SiO₂ (33 mol%) were prepared via casting a melt in the Na₂O-CaO-TiO₂-P₂O₅-SiO₂ system and subsequent acid leaching of the resultant ceramics which are constituted of NaCaPO₄, TiO₂, and amorphous SiO₂. The median pore diameter and specific surface area of the resulting porous ceramics are approximately 1 μm and 40 m², respectively. Amorphous silica surrounds the submicrometer-sized particles of TiO₂ acting as a binder and retaining monolithic forms. No significant shrinkage in the pore size occurred upon heating up to 1000°C.     

Bismuth Oxide Nanoparticles by Flame Spray Pyrolysis

Bismuth oxide nanostructured particles were made via the flame spray pyrolysis (FSP) of bismuth nitrate that had been dissolved in a solution of ethanol/nitric acid or in acetic acid. These self-sustaining spray flames produced tetragonal β-Bi₂O₃. The use of ethanol/nitric acid solutions resulted in a mixture of hollow, shell-like, and solid nanograined particles. The particle homogeneity was improved as the content of acetic acid in the precursor solution increased. Solid bismuth oxide nanoparticles were prepared consistent with percolation theory, accounting for the specific volume of the product and the precursor. Using pure acetic acid as the solvent, the effect of FSP variables on spray flame and product powder characteristics was investigated. The specific surface area of the Bi₂O₃ particles could be controlled over a range of 20–80 m²/g by the liquid feed and oxygen gas flow rates for powder production rates of 6–46 g/h.     

Pyrolysis of Titanium-Metal-Filled Poly(siloxane) Preceramic Polymers: Effect of Atmosphere on Pyrolysis Product Chemistry

Pyrolysis of unfilled as well as Ti-metal-filled poly(siloxane) preceramic polymers, heated at 1350^deg;C in N₂, Ar, or NH₃, was evaluated in terms of the effect of atmosphere on pyrolysis product chemistry. Pyrolysis of the unfilled poly(siloxane) in N₂ or Ar resulted in the formation of a metastable, amorphous SiO_xC_y phase with a residual turbostratic carbon phase, with the evolution of CH₄ and H₂ as the main pyrolysis products. However, pyrolysis of the unfilled poly (siloxane) in NH₃ resulted in a partial substitution of nitrogen from the atmosphere for network carbon to form a binary mixture of amorphous SiO_xC_y and SiO_xN_y phases. Pyrolysis of the Ti-filled poly(siloxane) in NH₃ resulted in the reaction of the Ti particles with the atmosphere to form nearly stoichiometric TiN. In Ar, the Ti particles react with either gaseous hydrocarbon, i.e., CH₄, or the carbon pyrolysis products of the poly(siloxane), to form slightly nonstoichiometric TiC along with a partial reduction of the SiO_xC_y matrix. Finally, in N₂, the Ti particles react with both carbon from the poly(siloxane) and nitrogen from the atmosphere to form a solid solution of TiC and TiN.     

AI2O3 Coatings as Diffusion Barriers Deposited from Particulate-Containing Sol-Gel Solutions

A procedure for the formation of A1₂O₃ coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi₂ and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A1₂O₃ particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A1₂O₃ particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A1₂O₃ and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially.     

Reactive Hot Pressing of Alumina-Silicon Carbide Nanocomposites

A dense alumina-silicon carbide (Al₂O₃–SiC) nanocomposite was synthesized in situ from the reaction of mullite, aluminum, and carbon by reactive hot pressing (RHP). Transmission electron microscopy investigation showed that in situ -formed, nanometer-sized SiC particles were mainly entrapped in the matrix grains, whereas submicrometer-sized particles were located at the grain boundaries or triple points of the Al₂O₃. In addition, no amorphous phase was observed at the interfaces of the Al₂O₃ and SiC grains, which indicated strong direct bonding. Fracture-surface analysis by scanning electron microscopy revealed an intrafracture mode. The bending strength of the nanocomposite RHP-treated at 1800°C was 795 ± 160 MPa, and the fracture toughness, measured by the indentation method, was 3.1 MPa·m^1/2.     

Surface Characterization of Ultrafine ZrO2-SiO2 Powders by Electrophoretic Mobility Measurements

Electrophoretic mobilities of ultrafine ZrO₂-SiO₂ powders prepared by a modified spray pyrolysis technique with an ultra-high-temperature plasma were measured to characterize their surfaces. The surfaces of as-prepared ZrO₂ particles are apparently composed of zirconium hydroxides, which when heated result in a surface which is hydrophobic or slow to hydrate. The surfaces of as-prepared and heated ZrO₂-SiO₂ particles appeared to consist predominantly of SiO₂.     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.     

XPS Investigation of Si3N4/SiC Nanocomposites Prepared Using a Commercial Polymer

We have synthesized Si₃N₄/SiC nanocomposites using a commercial polymer. Formation behavior of SiC nanoparticles has been studied using X-ray photoelectron spectroscopy (XPS). Based on the XPS results, we concluded that SiC nanoparticles were formed by a chemical reaction of Si provided from Si₃N₄ with free carbon obtained by pyrolysis of a polymer around 1300°C. The presence of CN _x compounds in samples processed in an Ar atmosphere supported our explanation.