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Your search keyword '"Vandenberghe, R. E."' showing total 186 results
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186 results on '"Vandenberghe, R. E."'

44. Magnetic anomalies and canting effects in nanocrystalline spinel copper ferrites CuxFe3-xO4.

Author

Stewart, S. J., Mercader, R. C., Vandenberghe, R. E., Cernicchiaro, G., and Scorzelli, R. B.

Subjects
MAGNETIC anomalies, GEOMAGNETISM, MAGNETIC measurements, MAGNETIZATION, MAGNETISM, GEOPHYSICS
Abstract

The magnetic behavior of nanocrystalline copper spinel ferrites CuxFe3-xO4 (0.42<=x<=1) exhibiting different degrees of crystallinity, have been investigated by means of in-field Mössbauer and dc magnetic measurements. The samples were obtained by a coprecipitation method and were subsequently high-energy ball milled for different times, which induced a progressive improvement of the ferrite crystallinity and cation redistribution. Their magnetism shows some peculiar features; at 5 K, the virgin magnetization curves remain outside the main hysteresis loop for applied fields of ≈1 T and display an S-shaped form. Below 60 K, the field-cooled hysteresis loops are shifted toward the negative field direction. In addition, in the same temperature range, the field-cooled magnetization of the samples with poorest crystallinity decreases sharply due to the spin freezing at the surface. The in-field Mössbauer spectra at 4.2 K show that the magnetic moments of the Fe3+ ions are in a canted spin state for both the tetrahedral and octahedral sites of the spinel structure, and evidence a decrease of the average spin canting with the degree of crystallinity. After calcination at 950 °C, the coprecipitated sample no longer displays the magnetic anomalous and canting features. We explain our results as due to the partially disordered surface within the frame of the core-shell model rather than to the effect of cation redistribution. [ABSTRACT FROM AUTHOR]

Published
2005
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45. X-ray diffraction and Mo¨ssbauer characterization of an Fe/SiO[sub 2] catalyst for the synthesis of carbon nanotubes.

Author

Coquay, P., Vandenberghe, R. E., De Grave, E., Fonseca, A., Piedigrosso, P., and Nagy, J. B.

Subjects
*CATALYSTS, *IRON, *SILICA, *NANOTUBES
Abstract

A catalyst powder was prepared by adsorption precipitation of iron acetate on a SiO[sub 2] support at a controlled pH. The selective reduction of this powder in a N[sub 2]/C[sub 2]H[sub 2] atmosphere at 700 °C generates multiwall carbon nanotubes. The study by x-ray diffraction and Mössbauer spectroscopy of the catalyst and reduced powders reveals that hematite particles with a 8-20 nm size range are involved in the formation of these carbon nanotubes and end up as Fe[sub 3]C after the catalysis process. Moreover, coalescence upon reduction produces large particles responsible for the formation of a minor proportion of large tubes of the order of 100 nm in diameter. [ABSTRACT FROM AUTHOR]

Published
2002
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