HR: 0800h
AN: GP41A-0863    [Abstracts]
TI: Temperature effects on the synthesis of Si-Ferrihydrite nanoparticles of variable sizes identified by magnetic measurements
AU: * Berquo, T S
EM: berqu013@umn.edu
AF: Department of Geology and Geophysics, Institute for Rock Magnetism, University of Minnesota, 310 Pillsbury Drive SE, Minnesota, MN 55455 United States
AU: Banerjee, S K
EM: banerjee@umn.edu
AF: Department of Geology and Geophysics, Institute for Rock Magnetism, University of Minnesota, 310 Pillsbury Drive SE, Minnesota, MN 55455 United States
AU: Ford, R
EM: ford.robert@epamail.epa.gov
AF: U. S. Environmental Protection Agency, National Risk Management Research Laboratory, 919 Kerr research Dr, Ada, OK 74820 United States
AU: Lee Penn, R
EM: penn@chem.umn.edu
AF: School of Chemistry, University of Minnesota, 207 Pleasant St SE, Minneapolis, MN 55455 United States
AB: 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 (75°C, 50°C, 20°C and 7.5°C). 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 (1-1000Hz) in the temperature range 2-400K. Finally, Mössbauer spectra were obtained at temperatures of 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 blocking temperatures (150K and 50K for synthesis temperatures of 50^oC and 20°C, respectively) and the usual superparamagnetic behavior was present. At the lowest synthesis temperature we observed a low blocking temperature (33K) and evidence of spin glass state. Transmission electron microscopy (TEM) observations and X-ray diffraction (XRD) support the magnetic interpretation showing a similar dependence of synthesis temperature and particle size. Studies on the effects of Si partitioning (e.g., structural substitution or adsorption) on the ferrihydrite surface are in progress.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005