Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO2Fe2O3 system (A: 5 wt% Fe2O3, B: 10 wt% Fe2O3, C: 20 wt% Fe2O3, D: 40 wt% Fe2O3). The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40-1000°C. Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect. In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C. The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe3+ and Fe2+ oxidation states was also determined, together with the attribution of the probable coordination states for Fe3+ ions. Complex magnetic structure appeared in samples treated above 800°C. © 1982.
Gel-glass transformation in the SiO2-Fe2O3 system
GUGLIELMI, MASSIMO;PRINCIPI, GIOVANNI
1982
Abstract
Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO2Fe2O3 system (A: 5 wt% Fe2O3, B: 10 wt% Fe2O3, C: 20 wt% Fe2O3, D: 40 wt% Fe2O3). The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40-1000°C. Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect. In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C. The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe3+ and Fe2+ oxidation states was also determined, together with the attribution of the probable coordination states for Fe3+ ions. Complex magnetic structure appeared in samples treated above 800°C. © 1982.
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