Hussein H. Hamdeh

Magnetic properties of partially-inverted zinc ferrite aerogel powders

Fine powders of ZnFe2O4 with an average particle size of 10 nm and inversion parameter of 0.21 were synthesized by the aerogel procedure. Portions of the powders were calcined in air at 500 and 800 °C and other portions were ball-milled for 10 h. The materials were characterized by x-ray diffractometry, vibrating sample, and SQUID magnetometry, Mossbauer spectrometry, and low temperature calorimetry. Upon calcination the powders underwent significant changes in grain size, inversion parameter, and hence magnetic properties. The magnetic state of the as-produced and calcined samples is best described as disordered and highly dependent on temperature. Upon ball-milling the grain size varied widely and the inversion parameter attained a value of 0.55. The magnetic properties of the ball-milled sample are similar to those of ferrimagnetic MgFe2O4 powders having comparable grain size and inversion parameters.

Large zinc cation occupancy of octahedral sites in mechanically activated zinc ferrite powders

The cation site occupancy of a mechanically activated nanocrystalline zinc ferrite powder was determined as (Zn0.552+Fe0.183+)tet[Zr0.452+Fe1.823+]octO4 through analysis of extended x-ray absorption fine structure measurements, showing a large redistribution of cations between sites compared to normal zinc ferrite samples. The overpopulation of cations in the octahedral sites was attributed to the ascendance in importance of the ionic radii over the crystal energy and bonding coordination in determining which interstitial sites are occupied in this structurally disordered powder. Slight changes are observed in the local atomic environment about the zinc cations, but not the iron cations, with respect to the spinel structure. The presence of Fe3+ on both sites is consistent with the measured room temperature magnetic properties.

Structural analysis of unpromoted Fe-based Fischer–Tropsch catalysts using X-ray absorption spectroscopy

The structure of unpromoted precipitated Fe catalysts was determined by Mossbauer emission and X-ray absorption spectroscopies after use in the Fischer–Tropsch synthesis (FTS) reaction in well-mixed autoclave reactors for various periods of time. X-ray absorption near-edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS) analysis, and Mossbauer spectroscopy showed consistent trends in the structural evolution of these catalysts during reaction. The nearly complete formation of Fe carbides during initial activation in CO was followed by their gradual re-oxidation to form Fe3O4 with increasing time-on-stream. Fe3O4 became the only detectable Fe compound after 450 h. The observed correlation between FTS rates and Fe carbide concentration, and the unexpected re-oxidation of the catalysts as CO conversion decreased, suggest that the deactivation of Fe catalysts in FTS reactions parallels the conversion of Fe carbides to Fe3O4. It appears that the CO activation steps responsible for replenishing carbidic surface species and for removing chemisorbed oxygen are selectively inhibited by deactivation of surface sites, leading to the oxidation of Fe carbide even in the presence of a reducing reactant mixture.

Structure and magnetic properties of magnesium ferrite fine powders

Abstract Fine powders of magnesium ferrite, MgFe2O4, were produced through the sol-gel supercritical drying method, with two portions then being calcined at 773 K and 1073 K. The powder structural and magnetic properties were determined from transmission electron microscope micrographs, x-ray diffraction, Mossbauer effect spectroscopy and magnetometry measurements. The powder structure matched the MgFe2O4 spinel phase, with small amounts of α-Fe2O3 being observed in heated samples. As-produced powders were superparamagnetic at room temperature, with single magnetic domain particle behavior being observed at low temperatures, and for the 1073 K heated sample. The particle size distribution for the as-produced powder was evaluated separately from the micrographs, by fitting the magnetization data to a weighted Langevin function, and by fitting Mossbauer spectra taken at temperatures from 25 K to 298 K. Very similar particle size distributions were found from all three methods. The average particle diameter was 11 nm for the as-produced powder, and increased for heated samples. The saturation magnetization and magnetocrystalline anisotropy energy density values were both consistent with bulk values, in contrast to the large differences between particle and bulk values described for other fine particle systems.

Fischer–Tropsch Synthesis: Changes in Phase and Activity During Use

Four iron catalysts (unpromoted, K-promoted, Si-promoted and K,Si-promoted) were activated and subjected to common Fischer–Tropsch synthesis conditions. At increasing times on stream, samples were withdrawn from the continuously stirred tank reactor in the reactor wax while keeping the sample blanketed with an inert gas. Mössbauer spectra were recorded for various samples and the iron phases of the catalyst were compared to the catalytic activity. A simple model based on bulk composition of the catalyst is not related to the catalytic activity during the course of the run.

Low temperature heat capacities of Ti3Al1.1C1.8, Ti4AlN3, and Ti3SiC2

For the binary Ti–Al system, an ordering transformation in Ti3Al has been shown to result in a significant lowering of the electronic heat-capacity coefficient, γ, by removing electrons from conducting states. When γ is normalized to a per Ti atom basis, the same tendency is found in low temperature calorimetric studies of the conducting ternary carbides Ti3Al1.1C1.8, Ti4AlN3, and Ti3SiC2 reported herein. As a consequence of C- or N-induced covalent-like bond formation, the Debye temperatures in these ternaries are in excess of 700 K.

Mössbauer studies of melt-spun Pr2Fe14B ribbons

Melt-spun ribbons of tetragonal Pr2Fe14B exhibit favorable hard magnet characteristics. Technically relevant materials based on this compound, however, generally contain a certain amount of soft magnetic α-Fe or Fe3−xBx for remanence enhancement through exchange coupling. The nominal off-stoichiometric compositions lead to metallurgical complications, which are not easily resolvable by standard phase identification techniques such as x-ray diffraction and thermal magnetic analysis. As a viable alternative, 57Fe-Mossbauer spectroscopy can be used to delineate individual Fe sites. To provide a basis for such an approach, this report gives Mossbauer parameters including hyperfine magnetic field, isomer shift, and quadrupole splitting as obtained from a single-phase melt-spun Pr2Fe14B ribbon.

Ferrimagnetic zinc ferrite fine powders

Partially inverted fine powders of zinc ferrite (ZnFe/sub 2/O/sub 4/) were produced by chemical aerogel synthesis. These powders showed magnetic ordering at temperatures to 250 K. Upon ball milling, a much higher inversion parameter was attained, yielding ferrimagnetic powders having small remanence and coercive fields at room temperature. Due to the small mean particle size, superparamagnetic behavior was seen at temperatures above 25 K. Powder properties were characterized by X-ray diffraction, electron microscopy, calorimetry, magnetometry, and Mossbauer effect spectroscopy techniques.

Mössbauer spectrometry study of magnesioferrite particles

Fine particles of ferrimagnetic MgFe2O4 were synthesized through a sol‐gel supercritical drying procedure, then cold‐pressed and heated at 500 and 800 °C. These materials were characterized by Mossbauer spectrometry in the temperature range 25–298 K, and in the presence of a small applied magnetic field. The Mossbauer measurements show that heat treatment results in the formation of a small percentage of α‐Fe2O3, an increase in the mean particle size and hyperfine magnetic field, and a decrease in the inversion parameter of the magnesioferrite. An average anisotropy constant equivalent to the bulk magnetocrystalline anisotropy is estimated for particles with diameter D=14 nm.

Fischer-Tropsch synthesis: morphology, phase transformation, and carbon-layer growth of iron-based catalysts

The morphological, phase transformations and carbon‐layer growth for unpromoted and K‐promoted iron catalysts were investigated over time during Fischer–Tropsch synthesis. Catalysts were activated in CO for 24 h, which transformed hematite into a mixture containing 93 % iron carbide and 7 % magnetite for the unpromoted catalyst and 81 % iron carbide and 19 % magnetite for the K‐promoted catalyst. Initially, the activated catalysts had high CO conversions (≈85 %); however, the conversions decreased to approximately 30 % after approximately 280 h of synthesis time. For the unpromoted catalyst, the amount of iron carbide gradually decreased over time while the corresponding magnetite phase increased. However, for the K‐promoted one, only one iron carbide phase (χ‐Fe5C2) gradually decreased, while the other (