Lj. Kostic-Gvozdenovic

Synthesis of mullite nanostructured spherical powder by ultrasonic spray pyrolysis

In this paper the synthesis of nanostructured spherical particles of mullite powders by ultrasonic spray pyrolysis is presented. The mullite crystallization and the nanostructure development during heating were examined by infrared spectroscopy, X-ray diffraction, scanning and transmission electron microscope analysis. Comparative analysis of experimentally determined and theoretically calculated particle size distribution, obtained on the basis of three-dimensional (3D) model of spherical/ellipsoidal waves generated by incident ultrasonic field, confirmed that the process of aerosol/powder particle synthesis can be regarded as deterministic process.

Synthesis, morphology, and formation mechanism of mullite particles produced by ultrasonic spray pyrolysis

Submicrometer spherical particles of mullite powder were synthesized by ultrasonic spray pyrolysis of emulsion and solutions, using tetra-ethyl-orthosilicate (TEOS) or silicic-acid and Al(NO 3 ) 3 · 9H 2 O as initial compounds. Crystallization of mullite phase was determined by differential thermal (DT), thermogravimetric (TG), infrared (IR), and x-ray analyses. The synthesis of mullite from TEOS emulsion occurs by crystallization of γ–Al 2 O 3 (or Al, Si-spinel) from the amorphous phase and its subsequent reaction with amorphous SiO 2 , as well as by crystallization of pseudotetragonal mullite below 1000 °C and its subsequent phase transformation into orthorhombic mullite. In the powders produced from silicic acid solutions, synthesis of mullite occurs only by crystallization of γ–Al 2 O 3 between 900 and 1000 °C and its further reaction with amorphous SiO 2 between 1100 and 1200 °C. Particle formation mechanism depended directly on the initial emulsion or solution preparation, i.e., on the phase separation in the emulsion and on the silicic-acid crosslinking conditions.

Synthesis and formation mechanism of submicrometre spherical cordierite powders by ultrasonic spray pyrolysis

Cordierite powders containing very pure submicrometre spherical particles have been synthesized by the ultrasonic spray pyrolysis. Aqueous solutions of silicic acid, Al(NO3)3·9H2O and MgCl2·6H2O were used as precursors. Scanning electron micrographs have shown that particle surfaces were smooth and the mean particle diameter was 0.834 μm. For the estimation of chemical and phase composition and phase transformation temperatures, differential thermal analysis, thermogravimetric analysis, X-ray diffraction, energy dispersive spectroscopy and infrared analysis have been applied. It was found that during spray pyrolysis, the condensation of silicic acid mostly occurred while aluminium and magnesium ion remained incorported between Si–O–Si chains. By subsequent heating to over 800°C, Si–O–M bonds (M=Al, Mg) were formed. The synthesis of cordierite occurred by the crystallization of μ-cordierite from the amorphous phase at 900°C followed by the phase transformation of μ- into α-cordierite in the temperature range 1100–1200°C.

Influence of synthesis parameters on the structure of boehmite sol particles

Abstract Influence of the type and amount of acid, used for aluminium hydroxide peptization, on the crystallinity and specific surface area of the dispersed phase of boehmite sols as well as the pH of sols, was investigated. The ratio n (HCl)/ n (Al(OH) 3 )=0.1 was found to be optimum for aluminium hydroxide peptization. Higher acid to aluminium hydroxide ratio leads to a decrease in specific surface area and crystallinity of the solid phase as well as in pH of sols. Increase in counter ion (Cl − ) concentration causes a shift in d values of (020) crystallographic plane, from the value corresponding to boehmite to that for pseudoboehmite, and a decrease in specific surface area of the sol solid phase. Using HNO 3 as a peptizing agent, greater crystallinity and higher specific surface area values of the solids were registered.

Surface Properties of HAp Particles Obtained by Hydrothermal Decomposition of Urea and Calcium-EDTA Chelates

In this paper surface properties of calcium-hydroxyapatite, synthes ized by modified hydrothermal reaction of urea and Ca(EDTA) 2– in phosphate solutions, as well as the dependence of calcium hydroxyapatite dispersion stability on pH were determined. T he specific surface area of calcium-hydroxyapatite was experimentally found to be 67 m /g. The point of zero charge (PZC) of indifferent KCl electrolyte of 6.8 ± 0.1 was determined by the batch equilibration method. From the dependence of surface charge density on pH and electrolyte concentrati on, the intrinsic equilibrium constant of surface groups p a2 K was determined to be 8.3 ± 0.1, while p int a1 K , calculated from the p int a2 K and point of zero charge values, is 5.3 ± 0.1. Hy droxyapatite suspension stability was investigated, in the pH range from 7 to12, it was r evealed that the most stable dispersion at pH 10. Introduction Hydroxyapatite is one of the main constituent compo unds of bones and teeth. Knowledge of the hydroxyapatite surface chemistry is very important from the point of view not only of bone tissue formation but also of HAP solubility, due to its in teraction with the surrounding fluid. Solubility of apatites is directly connected with various illness es uch as decalcination of bones (osteoporosis) and dental caries[1-2]. Behavior of the aqueous dispersion of hydroxyapatit e depends on the interfacial chemistry of the solid phase and dispersive medium. Surface charge o f hydroxyapatite in aqueous systems plays one of the main roles in determination of its colloidal and sorption properties [3]. The studies of the point of zero charge (pH PZC) and surface charge characteristics of apatites in dicate that H + and OH are potential-determinig ions and that the surface charge development at the apatite-water interface could be described by amphoteric dissociation react ions of surface functional groups [4,5]. In this paper, behavior and surface properties such as specific surface area, presence of particular surface groups, point of zero charge, surface charg e density, and intrinsic equilibrium constants of carbonate HAP obtained by hydrothermal decompositio n of urea and Ca-chelate, were reported. Methods and Materials Synthesis of carbonate hydroxyapatite was performed by a modified method of Fujishiro et al.[6-8], and was described earlier [9]. Specific surface area of HAP was determined by the B.E.T. (Brunauer – Emett – Teller) method, the sample was previously degassed in vacuum for 2 hour s at 150°C. The point of zero charge of HAP was determined by t he batch equilibration method described by one of the authors[10]. The adopted technique is as follows: KCl of different ionic strengths, 0.1 and 0.01 mol/ dm, was used as an electrolyte. First set of measurements included 12 samples with 25 cm 3 KCl of 0.1 mol/dm ionic strength. The initial pH (pHi) values, from 3 to 10, were adjusted by adding sm all volumes of 0.1 mol/dm 3 HCl or KOH to Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 437-440 doi:10.4028/www.scientific.net/KEM.240-242.437

Synthesis of nanostructured mullite from xerogel and aerogel obtained by the non-hydrolytic sol-gel method

Abstract Synthesis and nanostructure evolution of mullite (3Al2O3.2SiO2) from xero- and aerogels, obtained by the non-hydrolytic sol-gel process are described. Mullite crystallization and gel nanostructure evolution after heat treatment were studied by differential-thermal, x-ray diffraction, infrared and thermomicroscopic analysis as well as by scanning electron microscopy. Crystallization of mullite from both gels starts at 900 °C and further increase in temperature increases the size of crystallites formed from 32 to 100 nm and 12 to 20 nm in the case of xero- and aerogels, respectively. Accelerated sintering of aerogels in the temperature range 900 to 1000° C is a consequence of the viscous flow of the amorphous SiO2 phase.