The iron behaviour in aluminosilicate gel-derived materials

Electron spin resonance (ESR) measurements have been carried out on a 10Fe(2)O(3). 10Al(2)O(3). 80SiO(2) gel heat-treated at different temperatures in air and under reducing conditions. ESR spectra were obtained at 300, 50 and 5 K. The "effective" g value (g = h nu/beta H), linewidth (Delta H-pp) and ESR amplitude (A) depend on heat-treatment temperature of the gel-derived samples. ESR spectra exhibit different magnetic characteristics as a function of heat-treatment temperature and atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM) and a.c. susceptibility (chi(a.c.)) analyses were used to better understand the ESR results. The results show that in the samples heat-treated in air, up to 700 degrees C, Fe3+ ions are incorporated in the glass network in tetrahedral and/or octahedral co-ordinations. In the samples heat-treated between 250 and 700 degrees C was not detected, by ESR, the presence of iron oxide aggregates. However, the formation of hematite particles was observed by XRD and SEM. The presence of iron oxide aggregates was detected (by ESR) in the samples heat-treated at temperatures higher than 700 degrees C. These aggregates are formed, at 1200 and 1300 degrees C, by hematite and magnetite particles as proved by XRD. The ESR spectra and a.c. susceptibility, of the samples heat-treated at 250 degrees C (under reducing conditions), show a behaviour characteristic of small magnetite particles presence. The sample heat-treated at 500 degrees C (under reducing conditios) contains magnetite particles (XRD). In the ESR spectra of the sample heat-treated at 1000 degrees C, under reducing conditions, was not detected any resonance signal.