SIO2 SNO2 AND SN/SB-OXIDE/SIO2 GEL-DERIVED COMPOSITES .2. THERMAL EVOLUTION AND PHASE-ANALYSIS/

The thermal behaviour of samples with nominal composition 80/20 = SiO2 /SnO2, class B, 76.8/19.2/4.0 = SiO2/SnO2/Sb2O5, class C-1, and 76.8/1 9.2/2.0/2.0 = SiO2/SnO2/Sb2O5/Sb2O3, class C-2, is studied in the inte rval 25-1050 degrees C by various instrumental methods. Results on the se classes of samples, obtained from alkoxide precursors, are compared themselves and with samples of class A obtained from Si(OEt)(4) and S nCl4. The segregation and crystallization of SnO2 occurs at 400 degree s C in the presence of microdomains of SnO2 . nH(2)O in the SiO2 gel m atrix (class A), whereas it is observed at 700 degrees C for samples B and C composed of Sn and Sb cations homogeneously dispersed in SiO2. This fact implies different mechanisms of SnO2 nucleation and growth. The crystallization of SiO2 is observed at 1200 degrees C for samples A, at 1050 degrees C for B and at 800 degrees C for C. For this latter , the presence of Sb-oxide/SiO2 reactive glass is invoked to the low-t emperature crystallization of SiO2.