A. Paesano

Synthesis and characterization of Fe-Al2O3 composites

Abstract We have synthesized composites of the type Fex(Al2O3)100−x by arc-melting compacted pellets of iron and alumina powders with starting compositions of x=0.4, 2, 3, 5, 10, 20, 40 and 60. Aluminas of two different nominal grades (99.7% and 99.99%) were used in the preparation of the composites. The samples were characterized by X-ray diffractometry, magnetization and Mossbauer spectroscopy. The results revealed the formation of the spinel FeAl2O3+z (hercynite phase) and of metallic iron partially nanostructured by this melting process. The Mossbauer subspectral areas of the formed phases were plotted against x, showing that increasing the starting iron content of the mixed powders favors the hercynite reaction. It was also observed that when the more pure alumina is used for cast, more hercynite is formed or, alternatively, less iron nanocrystallites precipitate, when compared to the less pure oxide. An arc-melted sample (x=2) was annealed at 1200°C in a hydrogen atmosphere, revealing hercynite reduction and phase separation of metallic iron.

Adsorption of Adenine, Cytosine, Thymine, and Uracil on Sulfide-Modified Montmorillonite: FT-IR, Mössbauer and EPR Spectroscopy and X-Ray Diffractometry Studies

In the present work the interactions of nucleic acid bases with and adsorption on clays were studied at two pHs (2.00, 7.00) using different techniques. As shown by Mössbauer and EPR spectroscopies and X-ray diffractometry, the most important finding of this work is that nucleic acid bases penetrate into the interlayer of the clays and oxidize Fe2+ to Fe3+, thus, this interaction cannot be regarded as a simple physical adsorption. For the two pHs the order of the adsorption of nucleic acid bases on the clays was: adenine ≈ cytosine > thymine > uracil. The adsorption of adenine and cytosine on clays increased with decreasing of the pH. For unaltered montmorillonite this result could be explained by electrostatic forces between adenine/cytosine positively charged and clay negatively charged. However for montmorillonite modified with Na2S, probably van der Waals forces also play an important role since both adenine/cytosine and clay were positively charged. FT-IR spectra showed that the interaction between nucleic acid bases and clays was through NH+ or NH2+ groups. X-ray diffractograms showed that nucleic acid bases adsorbed on clays were distributed into the interlayer surface, edge sites and external surface functional groups (aluminol, silanol) EPR spectra showed that the intensity of the line g ≈ 2 increased probably because the oxidation of Fe2+ to Fe3+ by nucleic acid bases and intensity of the line g = 4.1 increased due to the interaction of Fe3+ with nucleic acid bases. Mössbauer spectra showed a large decreased on the Fe2+ doublet area of the clays due to the reaction of nucleic acid bases with Fe2+.

Nanometric particle size and phase controlled synthesis and characterization of γ-Fe2O3 or (α + γ)-Fe2O3 by a modified sol-gel method

Fe2O3 nanoparticles with sizes ranging from 15 to 53 nm were synthesized by a modified sol-gel method. Maghemite particles as well as particles with admixture of maghemite and hematite were obtained and characterized by XRD, FTIR, UV-Vis photoacoustic and Mossbauer spectroscopy, TEM, and magnetic measurements. The size and hematite/maghemite ratio of the nanoparticles were controlled by changing the Fe:PVA (poly (vinyl alcohol)) monomeric unit ratio used in the medium reaction (1:6, 1:12, 1:18, and 1:24). The average size of the nanoparticles decreases, and the maghemite content increases with increasing PVA amount until 1:18 ratio. The maghemite and hematite nanoparticles showed cubic and hexagonal morphology, respectively. Direct band gap energy were 1.77 and 1.91 eV for A6 and A18 samples. Zero-field-cooling–field-cooling curves show that samples present superparamagnetic behavior. Maghemite-hematite phase transition and hematite Neel transition were observed near 700 K and 1015 K, respectively. Magnet...

Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system

The system (α-Fe2O3)x(α-Al2O3)1−x was subjected to 24 h of high-energy ball-milling varying its nominal concentration, x. The milled samples were structurally and magnetically characterized at room temperature by x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. Mossbauer studies were also performed in the temperature range 250–6 K. As a result of the earlier analyses, it was observed that the milling products were extremely dependent on the hematite starting concentration. In samples with low α-Fe2O3 initial concentration (i.e., x⩽0.12), the paramagnetic solid solution α-(FeΔYAl1−ΔY)2O3, the α-Fe and the FeAl2O4 phases were identified, along with alumina, which was always residual. The presence of spinel and metallic iron was attributed to the stainless-steel vial and balls abrasion. For x>0.12, the iron component was no longer present but another magnetic component, corresponding to an aluminum-substituted hematite phase, α-(Fe1−ΔWAlΔW)2O3, could be seen to increase with increasing x...

Structural, Magnetic, and Dielectric Investigations of the FeAlO3 Multiferroic Ceramics

The FeAlO3 phase presents an orthorhombic symmetry (Pna21 space group) with a double combination of hexagonal and cubic closed packing of oxygen ions. In this phase, there are four distorted different cationic sites, promoting piezoelectricity, ferrimagnetism and magnetoelectric effects at low temperatures. In this work, high-energy ball milling was employed to produce the FeAlO3 multiferroic compound. Its structural (Rietveld refinement), dielectric and magnetic properties were carefully investigated.

Structural, Microstructural and Magnetic Properties of the High-Energy Ball Milled BiFeO3 and BiFe0.95 Mn0.05O3 Ferroelectromagnetic Compounds

Multiferroic BiFeO3 ceramic powders were synthesized through high-energy ball milling. The structural, microstructural and magnetic properties of the obtained samples were carefully determined. The obtained samples were very fractured and strained due to the employed milling conditions, motivating the agglomeration of the nanopowders. The unit cell, Mössbauer and magnetic parameters presented considerable alterations in comparison with those for bulk materials, as a consequence of mechanosynthesis. The modification of BiFeO3 with 5 mol% of Mn ions do not induces significant microstructural or structural alterations in as-milled or annealed samples. However, an intense magnetization decreasing was observed.

Magnetic and Reflection Loss Characteristics of Terbium Substituted Cobalt Ferrite Nanoparticles/Functionalized Multi-Walled Carbon Nanotube

Terbium substituted cobalt ferrite dot array was formed onto surface of multi-walled carbon nanotube by hetero-coagulation. X-ray diffraction (XRD), and scanning electron microscope (SEM) were used to demonstrate the successful attachment of ferrite nanoparticles to M WCNTs. Mossbauer spectroscopy results confirm the preference of terbium for tetrahedral sites. Vibrating sample magnetometer (VSM) confirms the relatively strong dependence of saturation of magnetization and coercivity with the volume percentage of MWCNTs. The complex permittivity, permeability, and reflection loss properties of the ferrite and nanocomposites were studied separately. Reflection loss of nanocomposites is abruptly enhanced in compare to that of ferrite and carbon nanotubes. Reflection loss evaluations indicated that the nanocomposites display a great potential application as wide-band electromagnetic wave absorbers.

Structural and Mössbauer characterization of the ball-milledFex(Al2O3)100−x system

Metal-oxide composites were synthesized by high-energy ball milling of metallic iron (α-Fe) and alumina (α-Al2O3) powders, varying the starting relative concentration and the milling time. The samples were characterized by scanning electron microscopy, x-ray diffraction, and Mossbauer spectroscopy. The results revealed the formation of a FeAl2O3+W spinel phase (hercynite) and of iron (super)paramagnetic nanoprecipitates, in addition to residual magnetic iron and alumina. We also observed that the relative amounts of nanoprecipitates and hercynite for isochronally milled samples were correlated with the sample nominal concentration x, with the precursor iron being relatively more converted in those phases for low x values. Particularly for x=10 milled sample, the relative amounts of the (super)paramagnetic and spinel phases were observed to increase linearly with the milling time. An x=20∕24h milled sample was annealed in H2 atmosphere and revealed the reduction of hercynite, with iron phase separation.

Phase formation in Zr-Fe multilayers: Effect of irradiation

We have conducted a detailed in situ study of phase formation in Zr–Fe metallic multilayers using irradiation and thermal annealing. Metallic multilayers with near equiatomic and Fe-rich overall compositions and with repetition thicknesses ranging from 7.4 to 33 nm were either irradiated with 300 keV Kr ions at various temperatures (from 17 to 623 K) or thermally annealed at 773 K while being observed in situ. The kinetics of multilayer reaction were monitored by following the diffraction patterns. For near equiatomic samples, irradiation causes complete amorphization. The dose to amorphization increases in proportion to the square of the wavelength, indicating a process controlled by atomic transport. Amorphization was also achieved by 900 keV electron irradiation at 25 K showing that displacement cascades are not required. The critical dose to amorphization was independent of temperature below room temperature and decreased above room temperature. The activation energy for this second process is 0.17 eV...

Synthesis of goethite in solutions of artificial seawater and amino acids: a prebiotic chemistry study

This study investigated the synthesis of goethite under conditions resembling those of the prebiotic Earth. The artificial seawater used contains all the major elements as well as amino acids (α-Ala, β-Ala, Gly, Cys, AIB) that could be found on the prebiotic Earth. The spectroscopic methods (FT-IR, EPR, Raman), scanning electron microscopy (SEM) and X-ray diffraction showed that in any condition Gly and Cys favoured the formation of goethite, artificial seawater plus β-Ala and distilled water plus AIB favoured the formation of hematite and for the other synthesis a mixture of goethite and hematite were obtained. Thus in general no protein amino acids (β-Ala, AIB) favoured the formation of hematite. As shown by surface enhanced Raman spectroscopy (SERS) spectra the interaction between Cys and Fe 3+ of goethite is very complex, involving decomposition of Cys producing sulphur, as well as interaction of carboxylic group with Fe 3+ . SERS spectra also showed that amino/CN and C-CH 3 groups of α-Ala are interacting with Fe 3+ of goethite. For the other samples the shifting of several bands was observed. However, it was not possible to say which amino acid groups are interacting with Fe 3+ . The pH at point of zero charge of goethites increased with artificial seawater and decreased with amino acids. SEM images showed when only goethite was synthesized the images of the samples were acicular and when only hematite was synthesized the images of the samples were spherical. SEM images for the synthesis of goethite with Cys were spherical crystal aggregates with radiating acicular crystals. The highest resonance line intensities were obtained for the samples where only hematite was obtained. Electron paramagnetic resonance (EPR) and Mossbauer spectra showed for the synthesis of goethite with artificial seawater an isomorphic substitution of iron by seawater cations. Mossbauer spectra also showed that for the synthesis goethite in distilled water plus Gly only goethite was synthesized and in artificial seawater plus Cys a doublet due to interaction of iron with artificial seawater/Cys was observed. It should be pointed out that EPR spectroscopy did not show the interaction of iron with artificial seawater/Cys.