Effect of Precursor and Solvent on Morphology of Zirconia Nanoparticles Produced by Combustion Aerosol Synthesis

Zirconia nanoparticles were synthesized using a flame-based system involving spray droplet combustion of different precursor solutions. The characteristics of the feed were varied by varying the precursor compound, precursor concentration, and solvent type, and by using droplets of different mean sizes. When large droplets were used, agglomerated particles were formed when an organometallic precursor was used and large cenospheric particles were produced when an inorganic precursor was used. Reduction of the droplet size to a number-mean droplet diameter of 3.2 μm resulted in the production of solid spherical particles regardless of the precursor type. When an inertial impactor was used to eliminate droplets larger than 2.3 μm, the large particles in the final product were eliminated and uniformly sized solid zirconia particles having a smaller mean size were produced. The final particle size did not vary with the concentration of the precursor, indicating that multiple ceramic particles resulted from each precursor-containing droplet.     

Synthesis of Spherical β-Silicon Carbide Particles by Ultrasonic Spray Pyrolysis

Fine agglomerate-free spherical β-SiC powder was synthesized from a dispersion of colloidal silica, saccharose, and boric acid, by means of an ultrasonic spray pyrolysis method. Droplets of 2.2 μm were formed with an aerosol generator, operated at 2.5 MHz, and carried into a reaction furnace at 900°C with argon. Spherical X-ray amorphous gel particles of 1.1 μm were obtained. β-SiC particles with a mean diameter of 0.79 μm and spherical shape resulted when the SiC gel precursor particles were heated at 1500°C in argon.     

Synthesis of (La,Sr)MnO3–YSZ Composite Particles by Spray Pyrolysis

(La_0.8Sr_0.2)_0.9MnO₃–YSZ composite particles were synthesized by spray pyrolysis. The mean particle size of the synthesized powders was about 1 pm and the particle size distribution was very narrow. The synthesized powders were composed of the perovskite (La,Sr)MnO₃ and cubic phase YSZ. Each particle synthesized consisted of uniform and well-dispersed line primary particles of (La,Sr)MnO₃ and YSZ (0.1 μm particle size).     

Preparation of Spherical SnO2 Powders by Ultrasonic Spray Pyrolysis

Spherical SnO₂ powders were prepared from SnCl₄ aqueous solution by ultrasonic spray pyrlysis. The particle size distributions normalized by mean diameters are almost identical in spite of the variation in the concentration of the source solution. The mean diameter of SnO₂ particles increases with the concentration of the source solution and it is in the range of 0.2 to 0.8 μm. Each spherical particle is composed of many primary particles and has a solid, microporous structure.     

Synthesis of Lithium-Cobalt Oxide Nanoparticles by Flame Spray Pyrolysis

Crystalline LiCoO2nanoparticles were synthesized from an aqueous solution of acetate compounds of lithium and cobalt by a flame spray pyrolysis, and characterized by TEM, XRD, and BET method. We investigated the evolution of LiCoO2nanoparticles from liquid droplets sprayed along the flame and observed disintegration of aqueous precursor droplets about 10μm into smaller fragments around 50 nm in the high temperature flame, as well as decomposition/oxidation of the precursor and coalescence/coagulation. We also examined effects of process variables such as molar concentrations of the precursors and flow rates of combustible gases on the particle size and crystal structure. The average particle diameter increased with an increase in the molar concentration of the precursor. Raising the maximum flame temperature by controlling the gas flow rates also led to an increase in the average diameter of the particles. The crystalline nanoparticles synthesized were nearly spherical, and their average primary particle diameters ranged from 11 to 35 nm.     

Synthesis of Solid, Spherical Zirconia Particles by Spray Pyrolysis

A model is used to predict solid particle formation during spray pyrolysis by correlating droplet shrinkage before salt precipitation with its relative solution saturation. For the rapid drying conditions and droplet size of ∼ 10 μm, which are characteristic of conventional spray pyrolysis, solid salt particles are formed when a droplet's initial relative solution saturation is ∼ 10⁻² and the precipitated salt is sufficiently permeable to permit evolution of the remaining solvent after precipitation. It is proposed that this concentration allows the drying droplet more time and a shorter diffusion distance in which to maintain chemical homogeneity before precipitation. Using these concepts it is demonstrated that zirconyl chloride (ZrOCI₂· 8H₂O) and zirconyl hydroxychloride (ZrO(OH)CI) are excellent zirconium salts for solid zirconia particle synthesis by spray pyrolysis.     

Effect of precursor characteristics on zirconia and ceria particle morphology in spray pyrolysis

Ultrasonic spray pyrolysis of acetate-based precursors with precisely measured precursor drop size was employed to produce ZrO₂ and CeO₂ particles. A bimodal size distribution of the product particles indicates a significant influence of the gas-to-particle conversion mechanism in addition to the conventionally accepted one-particle-per-drop mechanism. Due to the differences in solubility of the precursors, ZrO₂ particles are spherical in shape and smooth on their surfaces while the CeO₂ particles are bowl-like in shape with uneven surfaces. Spherical and monodispersed particles with a peak diameter <100 nm can be obtained by reducing the precursor concentrations to 0.01 wt.% in both the different precursor system.     

Morphology and sintering behaviour of yttria stabilised zirconia (8-YSZ) powders synthesised by spray pyrolysis

This work is focused on the synthesis of nano-crystallised yttria stabilised zirconia (YSZ) powders by the spray pyrolysis method, the aim of the study being a better understanding of the influence of the spray pyrolysis parameters on the morphology of the produced powders. Spray pyrolysed powder consists of polycrystalline particles, which are spherical. Each particle consists of nanometric primary grains. The morphology of these polycrystalline particles was characterised by scanning electron microscopy (SEM), helium pycnometry, thermogravimetric analysis (TGA) and mass spectroscopy (MS), and the results are compared. Thus, particle size, particle size distribution and particle porosity were determined and correlated to the process parameters. Finally, by dilatometric measurements, sintering curves of pellets prepared from different sets of powders were analysed in regard of their morphologies. Two main conclusions could be deduced from these studies. Firstly, the process parameters influence both internal porosity and particle size distribution of the synthesised powders. Secondly, the morphologies of the spray pyrolysed nano-powders lead to particularly high sintering activities.     

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.     

Novel Low-Temperature Synthesis of Disodium Dimolybdate by Ultrasonic Spray Pyrolysis

A new method for the synthesis of disodium dimolybdate (Na₂Mo₂O₇) in the process of ultrasonic spray pyrolysis using acidified aqueous solutions of thermodynamically stable molybdenum (VI) oxide clusters as a precursor is described. Na₂Mo₂O₇ particles were collected in alcohols (ethanol, 2-propanol, isobutyl alcohol), and ultracentrifugation was used to isolate solid material from solution. Scanning electron microscopy revealed the formation of uniform spherical Na₂Mo₂O₇ particles with an average diameter of about 0.25 μm. The X-ray diffraction analysis undoubtedly confirmed the formation of orthorhombic Na₂Mo₂O₇ for samples synthesized at a temperature as low as 300°C.     

Formation of silver iodide particles from thermodynamically stable clusters using ultrasonic spray pyrolysis

Silver iodide particles in the submicronic size domain were synthesized in the process of ultrasonic spray pyrolysis (USP) using aqueous solutions of thermodynamically stable silver iodide clusters as precursor. After the process of USP, the AgI particles were collected in water. In order to study influence of aging time on the morphological and structural properties of the AgI particles, ultra-filtration was employed to isolate solid material from solution. The scanning electron microscopy showed change from spherical to hexagonal/triangular shape and increase of average particle size of the AgI particles as a function of aging time, which is characteristic for the Ostwald ripening growth mechanism. The X-ray diffraction measurements revealed the presence of wurtzite hexagonal and zinc blende cubic AgI modifications whose abundance is also dependent on the aging time.     

TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Synthesis and sintering of Gd-doped CeO2 nanopowders prepared by ultrasonic spray pyrolysis

Ultrasonic spray pyrolysis (USP) of nitrate-based precursors was used to fabricate fine and polycrystalline Ce_0.8Gd_0.2O_1.9 (CGO) powders with controlled microstructure in a one-step process varying four spray-pyrolysis parameters: precursor solution concentration, atomizing frequency, carrier gas flow rate, and pyrolysis temperature. A design of experiment (DOE) was used to understand the independent and combined influences of these process parameters on the particle size, particle size distribution and crystallite size. In this study, smaller CGO particles with narrower particle size distribution and finer grain size showed a full densification starting at a low temperature. CGO densification was found to be strongly dependent on CGO powder microstructural properties.     

Characteristics and Formation Mechanism of BaTiO3 Powders Prepared by Twin-Fluid and Ultrasonic Spray-Pyrolysis Methods

Spherical fine (micrometer and submicrometer in size) homogeneous BaTiO₃ powders were synthesized from ethanol: water solutions of BaCl₂ and TiCl₄ using the spray-pyrolysis technique. Two different atomizers—twin-fluid and ultrasonic, with a resonant frequency of 2.5 × 10⁶ Hz—were used for mist generation. Hollow spherical particles containing a certain amount of unreacted BaCl₂ phase and having a mean particle diameter of 2.5 μm were obtained at 1173 K using a twin-fluid atomizing system. Decomposition of precursors and their transition to the cubic BaTiO₃ phase occurred, even at 973 K in the case of the ultrasonic atomizing system. For the initial droplet size of 2.2 μm and residence time of ∼60 s, spherical BaTiO₃ particles with the mean particle diameter of 0.53 μm were obtained. A BaTiO₃ formation mechanism has been proposed as a reaction between TiO₂ and BaCl₂ rather than a reaction of oxides.     

Praseodymium-Doped Photo-Luminescent Strontium Indate Nanoparticles by Ultrasonic Spray Pyrolysis

Submicrometer spherical particles of praseodymium (Pr)-doped strontium indate were synthesized by spray pyrolysis using ultrasonic nebulization. The morphology, crystalline structure, particle size, and size distribution were studied by X-ray diffraction, scanning, and transmission electron microscopies. The spherical agglomerates were found to be made up of very small nanocrystallites. Upon excitation by UV radiation, sharp luminescence emission bands due to Pr were observed in the blue, green, and red regions in these Pr-doped samples.     

Size-dependent luminescent properties of hollow and dense BaMgAl<Subscript>10</Subscript>O<Subscript>17</Subscript>: Eu blue phosphor particles prepared by spray pyrolysis

Hollow and dense BaMgAl10O17: Eu₂₊ (BAM) phosphor particles were synthesized by a spray pyrolysis process and their luminescent properties were investigated under vacuum ultraviolet (VUV) excitation as varying the average particle size. The dependence of the luminescent intensity on the particle size was greatly influenced by the morphology of BAM particles. For the BAM particles with a hollow structure, the luminescent intensity linearly increased with increasing the particle size. However, no significant change in the luminescent intensity was observed for dense particles as the particle size changed. Also, all dense BAM particles had higher photoluminescence intensity than that of the hollow ones regardless of the particle size. The luminescent intensity of BAM phosphor particles prepared by spray pyrolysis was found to have a linear relationship with the crystallite size. Therefore, it was concluded that suppressing the formation of a hollow structure and increasing the crystallite size are needed to obtain high luminous BAM phosphor particles with a spherical shape and fine size of less than 1 Μm. On the basis of penetration depth of VUV, a simple relation equation between the particle size and the luminescent intensity was derived and correlated with experimental results in order to interpret the luminescent behavior of BAM phosphor as the particle size changes.     

Dense Alumina–Zirconia Coatings Using the Solution Precursor Plasma Spray Process

For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al₂O₃–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al₂O₃–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.     

Study of gradual porous metallic membranes obtained by powder sedimentation

In our preliminary studies the possibility of obtaining sintered porous materials with gradual structure by sedimentation of metallic powder was demonstrated. In this paper we continued our studies on the influencing factors of the sedimentation of metallic powders. Irregular and spherical nickel particles were used having a grain size in the 2–90 μm range measured by the laser scattering particle size analyzer. The particles with irregular shape and larger diameter can sediment faster than the spherical and the smaller diameter particles. The sedimentation rate is also influenced by the sedimentation medium and the quantity of the dispersant agent. Deviations from Stokes law was observed in the case of the irregular particles. The gradual structure is influenced by the sintering regime and the powders characteristics too. The obtained structures were characterized by scanning electron microscopy and mercury porosimetry. The permeability and the filtration fineness were also determined.     

Formation of Lead Zirconate Titanate Powders by Spray Pyrolysis

Twin-fluid atomization spray pyrolysis (SP) has been investigated for the production of lead zirconate titanate (PZT) powders, using aqueous solutions of lead acetate and zirconium and titanium alkoxide precursor reagents. The particle size distribution of the PZT powder showed a d ₅₀ value of 0.3 μm, but with a small fraction of relatively large particles, several micrometers in size. Most particles were spherical but many of the largest particles, in the size range ca. 1–5 μm, were irregular. It was demonstrated that the morphology of the final PZT powder was controlled by decomposition processes occurring during the initial drying stages, at ≤200°C. A pyrochlore or fluorite-type intermediate crystalline phase was present in the final powders, but when the maximum reactor temperature was raised, and/or when the levels of excess lead in the starting solutions were increased, the proportion of the desired perovskite phase increased. However, at the highest process temperatures studied, ∼900°C, small crystallites of another phase formed on the surface of the PZT particles; these were probably lead oxide carbonate particles. Overall, a starting solution composition containing around 5 mol% excess Pb, and a maximum reactor temperature of 800°C, were selected as offering the most suitable conditions for producing PZT (52/48) powder, with minimal secondary phases(s). Preliminary densification studies showed that the powders could be sintered at 1150°–1200°C to give pellets of 95%–96% theoretical density.     

Synthesis of Uniform Titanium and 1:1 Strontium-Titanium Carboxyhydrosols by Controlled Hydrolysis of Alkoxymetal Carboxylate Precursors

Uniform inorganic carboxyhydrosols containing titanium or strontium and titanium cations were precipitated from isopropyl alcohol solutions by the controlled hydrolysis of metal alkoxycarboxylate precursors. The hydrolysis of various triisopropoxytitanium carboxylate compounds yielded the capability to control particle size. Spherical particles of carboxyhydrosols were prepared in mean sizes from 0.4 to 2.9 μm. Controlled hydrolysis of quintaisopropoxystrontium titanium octanoate resulted in slightly agglomerated, uniform 1-μm spheres in a solvent medium in which controlled precipitation of uniform powders was not previously possible. Conversion of the hydrous oxide carboxylate precipitate to the oxide via calcination or hydrothermal treatment was possible.