Effect of mechanochemical treatment on the crystallization behaviour of diphasic mullite gel

The effect of mechanochemical treatment on the structure and crystallization behaviour of diphasic mullite gel was investigated. The structural differences caused by mechanochemical treatment in diphasic gel were characterized by Al-27 and Si-29 MAS-NMR, XRD, DTA-TG and FT-IR. These investigations show that the crystallization process in this system is affected by the structural changes caused by grinding. Grinding for 20h changed a diphasic system containing discrete octahedral Al-O units (8 ppm) and silica units (-111 ppm) to a more homogeneous system containing tetrahedral, "pentahedral" (30 ppm) and octahedral alumina, and a strong Si-29 resonance at -83 ppm (approximate to 70% of the total Si) associated with aluminosilicate units. The ground gel converts to mullite via the spinel-phase at 1150-1200 degrees C. Mechanochemical treatment thus converts the diphasic type II gel precursor into a more homogeneous type III gel precursor, as reflected by its thermal behaviour. (C) 1998 Elsevier Science Limited and Techna S.r.l. All rights reserved.