A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3

ɛ-Fe2O3 was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y3Fe5O12) in a silica matrix at 300-C followed by sintering in air at 1,000-C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mossbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K

Effect of Fe-olivine on the tar content during biomass gasification in a dual fluidized bed

8 figures, 5 tablesThe Fe/olivine catalyst effectiveness regarding tar primary reduction during biomass gasification in dual fluidized beds has been investigated. The use of Fe/olivine instead olivine leads to an important decrease in the amount of produced tar, which was reduced by up to 65% at 850. °C, naphthalene being the most stable molecule. It has been found that Fe/olivine materials have a double effect on tar destruction. On the one hand, they act as a catalyst for tar and hydrocarbon reforming. On the other hand, they can act as an oxygen carrier that transfers oxygen from the combustor to the gasifier, and part of the oxygen is used to burn volatile compounds. The catalyst was fairly stable because the result was confirmed during 48. h of continuous operation. The Fe/olivine material characterization (X-ray diffraction, Mössbauer spectroscopy, temperature programmed reduction and oxidation, etc.) revealed that the catalyst structure was maintained despite the large number of oxidizing-reducing cycles. The carbon that formed on the catalyst surface was low and easily oxidized in the combustion zone. Therefore, the inexpensive and non-toxic Fe/olivine catalyst is a material suitable for use as primary catalyst in a fluidized bed gasification of biomass.Authors would like to thank the European Commission for its financial support (EC Project UNIQUE N°211517-ENERGY FP7–2008/2011). http://www.uniqueproject.eu Authors also express our gratitude to Daniel Schwartz for his help in the improvement of English of this paper.Peer Reviewe

Magnetic Study of SiO2/γ-Fe2O3 Nanocomposites Prepared by the SOL-GEL Method

Nanocomposites of γ-Fe2O3 in a silica matrix were prepared by the sol-gel method using tetramethylorthosilicate (TMOS) as a precursor of silica and introducing iron as Fe(NO3)3 with Fe/Si ratios of 2, 5, 10 and 20%. The obtained gels were calcinated at temperatures between 600°C and 1000°C. Magnetic measurements performed with a vibrating magnetometer showed that superparamagnetic γ-Fe2O3 nanoparticles began to form at 700°C as confirmed by X-rays and Mössbauer spectroscopy. Superparamagnetic magnetization curves were fitted by a Langevin function considering a log-normal particle size distribution. This allowed to determine the saturation magnetization and the particle size distribution of the samples. The latter results were in good agreement with observations made by Transmission Electronic Microscope. For all studied concentrations, the γ-Fe2O3 particles size increased with the calcination temperature up to a maximal average diameter of 40 Å. This maximum was reached at 900°C for the highest concentrations and l000°C for the lowest. Calcinations at higher temperatures led to the degradation of the γ-Fe2O3 phase with creation of defects in the particles and formation of α-Fe2O3. This phenomenon was accompanied by a decrease in the saturation magnetization and the presence of an hysteresis in the magnetization curve

Clinopyroxene from an alkali pyroxenite xenolith, Louená-Oberwiesenthal Volcanic Centre, Bohemian Massif: crystal chemistry and structure

Alkali pyroxenite (and ijolite) xenoliths occur in the Tertiary Loučná-Oberwiesenthal Volcanic Centre associated with the Ohře/Eger Rift. The alkali pyroxenite xenoliths represent fragments of an intracrustal complex with Sr-Nd isotope ratios consistent with mantle sources of HIMU-affinity. The crystal structure of diopside from an alkali pyroxenite xenolith with the formula (...) and the lattice parameters a = 9.773(2), b = 8.886(2), c = 5.308(1) [] and = 105.89(3) [°] was refined to an R-value of 0.025 for 1174 reflections. The mean interatomic distances are: within theMe1-O6 octahedron , within theMe2-O8 polyhedron . The last value reflects the occupation of this atomic position by significant amounts of Fe2+ and Ti4+. The enlargement determined for the bond length to 1.657 is in accordance with the site population for this position: (Si1.69Al0.31). The molar ratio Fe2+/Fe3+ determined by Mössbauer spectroscopy is equal to 0.786. The AlIV deficiency in T-sites of clinopyroxene of rims is negligible (up to 0.019 a.p.f.u.) restricted to sporadic local electronmicroprobe analyses. The presence of Fe3+ in the T-position of Si- and Al-poor clinopyroxenes was not confirmed by X-ray structural analyses because of its low quantity. Nevertheless, the Mössbauer spectroscopy measurements (isomer shift of 0.36 mm/s) imply that Fe3+ is present only in the Me1-O6 positions

Structural and Magnetic Properties of Nanosized La0.8Ca0.2Mn1−xFexO3 Particles (0 ≤ x ≤ 0.2) Prepared by Sol–Gel Method

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Structural disorder versus spin canting in monodisperse maghemite nanocrystals

Monodisperse maghemite nanoparticles with diameter ranging from 7 to 20 nm were examined by the In-field Mössbauer Spectroscopy (IFMS) in varying external magnetic field up to 6 T. Surprisingly, the small-sized particles (7 nm) exhibit nearly no spin canting in contrast to the larger particles with lower surface-to-volume ratio. We demonstrate that the observed phenomenon is originated by lower relative crystallinity of the larger particles with different internal structure. Hence, the persistence of the 2nd and 5th absorption lines in the IFMS cannot be unambiguously assigned to the surface spins.This work was supported by European Commission (MULTIFUN, No. 262943) and MSMT Project No. 7E12057. Experiments were performed in MLTL (http://mltl.eu/), which was supported within the program of Czech Research Infrastructures (Project No. LM2011025)

Simple and fast preparation of pure maghemite nanopowders through sol-gel self-combustion

Pure maghemite nanopowders made up of nanocrystals with average size of 19 nm was prepared by a simple sol-gel self combustion process. The gel pH and the primer temperature turned out the key parameters for the obtaining of the maghemite phase, that often is accompanied by the most thermodynamically stable hematite. Pure maghemite was achieved only with a gel pH value of 7 and with a primer temperature between 290 and 325 °C. XRD and IR pointed out the formation of maghemite with tetragonal structure and HRTEM indicated the high degree of crystallinity of the powder. Mossbauer measurements allowed to confirm the presence of maghemite phase with Fe(octa):Fe(tetra) ratio of 1.62 which is very close to the theoretical value and the presence spin canting strongly dependent on applied magnetic field. This picture is confirmed by dc magnetic measurements