Synthesis, morphology, and formation mechanism of mullite particles produced by ultrasonic spray pyrolysis
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Synthesis, morphology, and formation mechanism of mullite particles produced by ultrasonic spray pyrolysis Dj. Jana´ckovi´c Faculty of Technology and Metallurgy, Belgrade, Yugoslavia
V. Jokanovi´c Institute for Technology of Nuclear and Other Mineral Row Materials, Belgrade, Yugoslavia
Lj. Kosti´c-Gvozdenovi´c Faculty of Technology and Metallurgy, Belgrade, Yugoslavia
ˇ Lj. Zivkovi´ c Faculty of Electronics, Niˇs, Yugoslavia
D. Uskokovi´c Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade, Yugoslavia (Received 7 August 1995; accepted 12 February 1996)
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(NO3 )3 ? 9H2 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 g –Al2 O3 (or Al, Si-spinel) from the amorphous phase and its subsequent reaction with amorphous SiO2 , 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 g –Al2 O3 between 900 and 1000 ±C and its further reaction with amorphous SiO2 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.
I. INTRODUCTION
Because of its relatively high bending strength at low and high temperatures (200–500 MPa), resistance to creeping, relatively high melting point (around 1850 ±C), and resistance to thermal shock, mullite has been widely used in the refractory materials industry.1,2 Having a relatively low dielectric constant (e 6–6.7), low thermal expansion coefficient (4–5 3 1026 ±C21 in the 25 –800 ±C temperature range), and low sintering temperature which enables its cosintering with metals, mullite and mullite glass-ceramics have been used for manufacturing substrates for integrated circuits and microchips.1,3 Optical properties of mullite glass-ceramics make it applicable to production of a window-glass material for infrared light in a wavelength range of 3–5 mm, and, with Cr31 addition, as an optical activator in lasers.1 Ultrasonic spray pyrolysis is a useful method for synthesis of powders of submicrometer spherical particles of narrow size distribution with defined chemical and phase compositions. The method integrates ultrasonic formation of aerosol droplets and their direct 1706
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 7, Jul 1996
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introduction into a reactor. In the course of pyrolysis, the droplets undergo solvent evaporation, precipitation of dissolved substance
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