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) . 9H(2)O and MgCl2 . 6H(2)O were used as precursors. Scanning electron micrographs have shown that particle surfaces were smooth and the mean particle diameter was 0.834 mu m. 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 degrees C, Si-O-M bonds (M = Al, Mg) were formed. The synthesis of cordierite occurred by the crystallization of mu-cordierite from the amorphous phase at 900 degrees C followed by the phase transformation of mu- into alpha-cordierite in the temperature range 1100-1200 degrees C.