Synthesis of yttrium-aluminum-garnet hollow microspheres by reverse-emulsion technique

Yttrium-aluminum-garnet (YAG, Y3Al5O12) hollow microspheres were synthesized by reverse-emulsion (w/o) technique starting with aqua-based precursors of oxides. The non-ionic surfactant was used as the emulsifying agent. The gel powders were calcined at 700 degrees-1200 degrees C. The synthesized powders were characterized by differential thermal analysis (DTA), thermogravimetry, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The appearance of an exothermic peak at 932 degrees C in the DTA curve revealed the crystallization of YAG, which was further confirmed by XRD and FTIR studies. SEM confirmed the formation of hollow microspheres

Plasma spraying of an indigenous yttria stabilized zirconia powder prepared by the sol-gel technique

An indigenous sol-gel derived yttria-partially stabilized zirconia (Y-PSZ) powder has been characterized and its suitability for plasma spraying applications evaluated. The powder, determined to have about 5.1% yttria content, predominantly consisted of spherical particles with an average equivalent particle diameter close to 25 mum. Furthermore, it was found that the powder did not contain any particles > 50 mum, which is considered the ideal upper size limit for spray-grade ceramic powders in order to ensure complete melting during spraying. The sol-gel produced powder exhibited good flow characteristics and the plasma sprayed coatings developed using this powder were also found to have excellent thermal shock resistance. The corresponding results obtained using an imported Y-PSZ powder are also presented for the purpose of comparison

Alkoxy-derived monodisperse silica microspheres: the role of solvents in synthesis

Monodisperse, ultrafine, spherical particles of amorphous silica were produced by the hydrolysis of tetraethyl orthosilicate under a variety of experimental conditions with methanol, ethanol, 1-propanol and 2-propanol as solvents. 1- and iso-butanol, acetone and 1, 4-dioxan produced non-spherical agglomerates under equivalent conditions. The smallest size and narrowest size range of the spherical particles were observed for the product generated from methanolic solutions. Attempts were made to correlate solvent characteristics, e.g. polarity and viscosity with the time lag of precipitation after onset of reaction, and particle size. It was indicated that under specific conditions, the nature of the solvent could also determine the nature of the polymeric aggregation in the precipitated particles

Preparation of ZrO2-CaO and ZrO2-MgO fibres by alkoxide Sol-Gel processing

Extensive studies were made on the spinnability of Zr(O-nC3H7)4-derived sols for the preparation of ZrO2-CaO and ZrO2-MgO fibres. An improved preparative route was proposed in which acetylacetone controlled the hydrolyzability of the alkoxide and 2-methoxyethanol served as a slow-drying solvent. The advantages of the method of sol preparation over the known methods were highlighted. Crystallization behaviour of the gel fibres was studied mostly up to 1300-degrees-C for ZrO2-CaO fibres (up to 20 %mol CaO) and up to 1000-degrees-C for ZrO2-MgO fibres (up to 12 %mol MgO). ZrO2-CaO fibres (7-11% mol CaO) were Found to yield only the tetragonal and cubic polymorphs of ZrO2 after heating at 1300-degrees-C/4h and furnace cooling. ZrO2-MgO fibres, on the other hand, showed exsolution of MgO above 800-degrees-C and development of monoclinic ZrO2

Ambient drying and cracking phenomena in monolithic zirconia gels

Preparation of monolithic zirconia gels from alkoxides has been described for a series of compositions in the system zirconium propoxide-2-propanol-acetylacetone-water. When large volumes of the sols were cast in glass containers, an unusual crack pattern was observed in all cases; in each sample, the crack divided the gel into a top part with a central root-like protrusion and a bottom part surrounding the protrusion. The bottom part characteristically contained more volatiles than the top. The relatively high viscosity of the alcohol and the adhesion of the gel to the glass container wall were suggested to be the major factors guiding the crack pattern

Preparation of pure and stabilizer (MgO,CeO2)-added ZrO2 powders and their crystallization behaviour

Amorphous precursor materials for pure and MgO and CeO" containing Zr02 were prepared via alkoxide hydrolysis, gel formation and solution spray drying techniques. The crystallization behaviour of such precursor powders was studied by calcining them at 700°, 1000° and noocc. It was shown that the initial product(s) of crystallization, as well as the conversion product(s) at higher temperatures could depend significantly not only on the overall mode of precursor preparation, but also on subtle differences within the same preparative technique

Intermediate-stage lon exchange in sodium zeolites: Role of cation valency

The effect of cation valency on the value of equivalent counterion supply at maximum exchange has been discussed for zeolites NaA, NaX and NaY. It has been suggested that the valency dependence of diffusion was much stronger in the zeolite than in the solution. © 1983 Taylor & Francis Group, LLC

Cadmium sulfide aggregates through reverse micelles

CdS aggregates, composed of primary nanocrystalline particles, were synthesized in cyclohexane-sorbitan monooleate (Span 80) reverse micelles at 800 +/- 1 degreesC under mechanical agitation and sonication with varying Cd2+ concentrations in the solutions. The critical micelle concentration (CMC) of the surfactant Span 80 in cyclohexane was determined from discernible breaks in the surface tension and optical (ultraviolet) absorbance versus surfactant concentration curves. The synthesized aggregates were characterized using transmission electron microscopy, X-ray diffractometry, particle-size analysis, Fourier-transform infrared spectroscopy, and thermal analyses. The generated aggregates exhibited the characteristic green emission at 520 nm. A minor decrease in emission intensity with increased aggregate size at the same wavelength was observed