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TEMPERATURE EFFECTS ON THE SYNTHESIS OF SI-FERRIHYDRITE NANOPARTICLES OF VARIABLE SIZES IDENTIFIED BY MAGNETIC MEASUREMENTS
Citation:
BERQUO, T. S., S. K. BANERJEE, R. G. FORD, AND R. L. PENN. TEMPERATURE EFFECTS ON THE SYNTHESIS OF SI-FERRIHYDRITE NANOPARTICLES OF VARIABLE SIZES IDENTIFIED BY MAGNETIC MEASUREMENTS. Presented at Fall 2005 National Meeting of the American Geophysical Union, San Francisco, CA, December 05 - 09, 2005.
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Description:
Ferrihydrite is an antiferromagnetic iron oxyhydroxide formed as an ubiquitous product of natural iron diagenesis, and found in iron-containing water, soil, river sediment and oceanic crust. As such, it is a sensitive indicator or proxy of environmental change. This iron phase has a small average particle size (3-5 nm) and due to its large surface area the material has been used for surface adsorption and catalysis studies. In this work we have studied four ferrihydrite samples coprecipitated with ~15 wt% Si at different temperatures (75ºC, 50ºC, 20ºC and 7.5ºC). Static and dynamic magnetic techniques were used to elucidate the magnetic behavior of these samples. ZFC/FC magnetization curves were obtained by cooling the sample in zero field (ZFC) and then measuring the induced magnetization by stepwise increasing temperatures from 2K to 400K in a small applied field (B=5 mT). The sample was again cooled in the same small field and field cooling (FC) magnetization curves were obtained by measuring induced magnetization by stepwise increasing temperatures (2K to 400 K). Hysteresis loops were taken at different temperatures (2K, 100K, 200K and 300K) and ac susceptibility was measured with changing frequency (1-1000 Hz) in the temperature range 2-400K. Finally, Mössbauer spectra were obtained at temperatures at 4.2K and 300K. Ferrihydrite particle size seems to present a strong dependence with the synthesis temperature. The highest synthesis temperature produced a wide distribution of particle size and an unusual coarse-grained ferrihydrite with magnetic blocking temperature above 300K. With the decrease of synthesis temperature we observed a progressive decrease of particle size and the usual superparamagnetic behavior was present. At the lowest synthesis temperature we observed a low blocking temperature (33 K) and evidence of spin glass state. Transmission electron microscopy (TEM) observations and X-ray diffraction (XRD) support the magnetic interpretation showing the same dependence of synthesis temperature and particle size. Studies on Si effects (e.g., structural substitution or adsorption) on the ferrihydrite surface are in progress.