Geraldo Magela da Costa

Magnetic Nanoparticles for in Vivo Use: A Critical Assessment of Their Composition

Three different magnetic samples with particle sizes ranging from 10 to 30 nm were prepared by wet chemical methods. The powders were heated at 100, 150, 200, and 250 °C during 30 min under air. Ferrous and total iron contents were determined immediately after the synthesis and after the thermal treatments. All samples were characterized by X-ray diffraction, transmission and integral low-energy electron Mössbauer spectroscopy (ILEEMS) at 298 K. These samples are composed of a mixture of individual particles of maghemite and magnetite, which implies that once oxidation starts in this kind of material, it occurs throughout the entire particle volume. The existence of a maghemite/magnetite core-shell model was ruled out. A linear correlation between the average isomer shift and the magnetite content was found, allowing the estimation of the amounts of magnetite and maghemite in an unknown sample without the need of performing chemical analysis.

The occurrence of tinsleyite in the archaeological site of Santana do Riacho, Brazil

Abstract A second occurrence of tinsleyite, KAl2(PO4)2(OH)⋅2H2O, is reported. The mineral exists as a thin layer in a quartzite wall partially covered by rock paintings, and was characterized by X-ray diffraction, thermal and chemical analysis, and by Mössbauer spectroscopy. The calculated cell parameters are a = 9.58(6), b = 9.53(4), c = 9.54(6) Å, β = 103.2(4)°. Chemical analysis showed the presence of 3.9% Fe which probably replaces Al in the octahedral site. The Mössbauer spectra from room temperature down to 85 K show the existence of two Fe3+ doublets with ΔEQ1 ~ 0.57 mm/s and ΔEQ2 ~ 1.0 mm/s. At 77 K the spectrum drastically changes, suggesting that a structural transition might have occurred. The formation of tinsleyite might be due to the reaction of phosphate-rich water which runs along the fractures of the wall. The existence of tinsleyite in such a relatively large abundance indicates that this mineral might not be rare as previously thought.

Vibrational spectroscopy of the mineral meyerhofferite CaB3O3(OH)5·H2O--an assessment of the molecular structure.

Meyerhofferite is a calcium hydrated borate mineral with ideal formula: CaB3O3(OH)5·H2O and occurs as white complex acicular to crude crystals with length up to ~4 cm, in fibrous divergent, radiating aggregates or reticulated and is often found in sedimentary or lake-bed borate deposits. The Raman spectrum of meyerhofferite is dominated by intense sharp band at 880 cm(-1) assigned to the symmetric stretching mode of trigonal boron. Broad Raman bands at 1046, 1110, 1135 and 1201 cm(-1) are attributed to BOH in-plane bending modes. Raman bands in the 900-1000 cm(-1) spectral region are assigned to the antisymmetric stretching of tetrahedral boron. Distinct OH stretching Raman bands are observed at 3400, 3483 and 3608 cm(-1). The mineral meyerhofferite has a distinct Raman spectrum which is different from the spectrum of other borate minerals, making Raman spectroscopy a very useful tool for the detection of meyerhofferite in sedimentary and lake bed deposits.

Quantitative phase analysis of iron ore concentrates

The quantification of goethite, magnetite, martite and specularite in iron ores was successfully achieved by a combination of wet chemical analysis and x-ray diffraction. It was found that the intensity of the goethite (111) peak is constant for a certain sample provided that the same sample holder is used. Calibration curves with a linear behavior have been derived using the areas of the above mentioned peak and the amounts of goethite obtained by Mossbauer spectroscopy and optical microscopy. In addition, the integral width of the hematite (012) line broadens linearly as the amount of martite increases, thus allowing an estimation of the amounts of martite and specularite.

A Comparative Study of Ultra-Fine Iron Ore Tailings from Brazil

This paper presents a characterization study of ultra-fine solid particles contained in slimes of some iron mines from the Iron Quadrangle, Minas Gerais State, and from the Carajás region, Pará State, Brazil. This characterization is expected to provide a basis for the development and choice of mineral-processing techniques suitable for the tailings characteristics of each mine. Particle size separation was done by wet sieving and in a cyclosizer, whereas particle size distribution was determined by laser diffraction (Cilas). The powders were further characterized by chemical analysis, X-ray diffraction, scanning electron microscopy, and Mössbauer spectroscopy. The tailings had a significant content of iron (from 44% to 64%), mainly in the form of hematite (α-Fe2O3) and goethite (α-FeOOH). Mössbauer results showed that the amount of goethite increased as the particle size decreased. Furthermore, the hematite content was always greater in the coarser fractions (>10 μm). The phosphorus contents were high and associated with the presence of goethite in most samples. Scanning electron microscopy showed that the vast majority of the particles were not spherical and that the cyclosizer does not separate the material in the intended fractions efficiently. The particles obtained by wet sieving had much more uniform shape and size.

Synthesis of γ-(Al1-xFex)2O3 solid solutions from oxinate precursors and formation of carbon nanotubes from the solid solutions using methane or ethylene as carbon source

This work reports for the first time the synthesis of ?-(Al1-xFex)2O3 solid solutions with a high specific surface area (200-230 m2/g) by the decomposition of metal oxinate [(Al1-xFex)(C9H6ON)3] and investigated the potential of these materials as catalysts for the synthesis of carbon nanotubes by catalytic chemical vapor deposition using methane or ethylene as carbon the source. The nanocomposite powders prepared by reduction in H2-CH4 contain carbon nanotubes (CNTs), which are mostly double-walled but also contain a fair amount of undesirable carbon nanofibers, hollow carbon particles, and metal particles covered by carbon layers. Moreover, abundant metallic particles are observed to cover the surfaces of the matrix grains. By contrast, the nanocomposite powders prepared by reduction in N2-C2H4 are not fully reduced, and the CNTs are much more abundant and homogeneous. However, they are multiwalled CNTs with a significant proportion of defects. The powders were studied by several techniques including Mossbauer spectroscopy and electron microscopy.

COPRECIPITATION OF ALUMINUM GOETHITE AND AMORPHOUS Al-HYDROXY-SULFATE USING UREA AND CHARACTERIZATION OF THE THERMAL DECOMPOSITION PRODUCTS

Aluminum substitution is a common phenomenon in environmental iron oxides and oxyhydroxides, affecting the color, magnetic character, surface features, etc. Several methods for preparing Al-substituted iron oxyhydroxides can be found in the literature, resulting in samples with particular properties. In the present study, the synthesis of aluminum-substituted goethites, AlxFe1-xOOH with 0 ⩽ x ⩽ 0.15, by homogeneous precipitation and the transformation to aluminum-substituted hematites, (AlxFe1-x)O3, are presented. The goethite samples were produced at 90°C from solutions of urea and iron and aluminum nitrates in the presence of ammonium sulfate (GU series). Although attempts were made to incorporate up to 33 mole% of Al into the goethite, only ~15 mole% was found to be within the structure, due to the final pH, ~7, of the synthesis. Another feature of these goethites was a lateral alignment of the tabular particles. By heating batches of the GU samples at 400°C and 800°C, two series of Al-hematites were obtained, denoted here as the HX400 and HX800 samples, respectively. X-ray diffraction, thermal analysis, Karl-Fischer titration, transmission electron microscopy, and Mössbauer spectroscopy were used to characterize the samples. The X-ray patterns showed the samples to be pure iron phases, with particle sizes of ~10 nm for the GU and HX400 samples, and of ~70 nm for the HX800 samples. An inversion in the intensities of the (104) and (110) diffraction peaks of hematite was observed to be dependent on the aluminum substitution and was explained by small particle sizes, shape anisotropy, and the presence of nanopores. The cell parameters of both GU and HX samples showed a small decrease with increasing aluminum substitution up to x ≈ 0.15. The amount of adsorbed sulfate, presumably as an aluminum hydroxy sulfate gel, increased with aluminum substitution in all GU and HX samples, reaching a maximum of ~6.5 wt.% for the highest substitution. Heating at 100°C did not remove all of the adsorbed water, and significantly higher temperatures were required to achieve complete removal. Mossbauer spectra at 295 K and 80 K are typical for small-particle goethite and hematite, and revealed that Al-for-Fe substitution in all samples seems to be limited to ~15 mol.%.Aluminum substitution is a common phenomenon in environmental iron oxides and oxyhydroxides, affecting the color, magnetic character, surface features, etc. Several methods for preparing Al-substituted iron oxyhydroxides can be found in the literature, resulting in samples with particular properties. In the present study, the synthesis of aluminum-substituted goethites, AlxFe1-xOOH with 0 ⩽ x ⩽ 0.15, by homogeneous precipitation and the transformation to aluminum-substituted hematites, (AlxFe1-x)O3, are presented. The goethite samples were produced at 90°C from solutions of urea and iron and aluminum nitrates in the presence of ammonium sulfate (GU series). Although attempts were made to incorporate up to 33 mole% of Al into the goethite, only ~15 mole% was found to be within the structure, due to the final pH, ~7, of the synthesis. Another feature of these goethites was a lateral alignment of the tabular particles. By heating batches of the GU samples at 400°C and 800°C, two series of Al-hematites were obtained, denoted here as the HX400 and HX800 samples, respectively. X-ray diffraction, thermal analysis, Karl-Fischer titration, transmission electron microscopy, and Mössbauer spectroscopy were used to characterize the samples. The X-ray patterns showed the samples to be pure iron phases, with particle sizes of ~10 nm for the GU and HX400 samples, and of ~70 nm for the HX800 samples. An inversion in the intensities of the (104) and (110) diffraction peaks of hematite was observed to be dependent on the aluminum substitution and was explained by small particle sizes, shape anisotropy, and the presence of nanopores. The cell parameters of both GU and HX samples showed a small decrease with increasing aluminum substitution up to x ≈ 0.15. The amount of adsorbed sulfate, presumably as an aluminum hydroxy sulfate gel, increased with aluminum substitution in all GU and HX samples, reaching a maximum of ~6.5 wt.% for the highest substitution. Heating at 100°C did not remove all of the adsorbed water, and significantly higher temperatures were required to achieve complete removal. Mossbauer spectra at 295 K and 80 K are typical for small-particle goethite and hematite, and revealed that Al-for-Fe substitution in all samples seems to be limited to ~15 mol.%.

Mössbauer spectroscopic study of natural eosphorite, [(Mn,Fe)AlPO4(OH)2H2O]

Abstract A 57Fe Mössbauer spectroscopic study of natural eosphorite, (Mn,Fe2+)AlPO4(OH)2H2O, is reported. The Mössbauer spectra were collected at temperatures between 295 and 4.2 K. At temperatures exceeding 30 K, the Mössbauer spectra consist of a somewhat broadened quadrupole doublet with a splitting of 1.73 mm/s at 295 K. From the spectrum recorded at 250 K in an external magnetic field of 60 kOe, it is derived that the sign of the principal component of the electric field gradient (EFG) is negative and that the EFG’s asymmetry parameter is large (~0.5). From these findings, it is concluded that the octahedral coordination of the ferrous cations exhibits in first order a trigonal compression with a further lowering from axial symmetry due to an additional distortion. The spectrum collected at 4.2 K shows the existence of magnetic ordering. It was analyzed in terms of the full hyperfine interaction Hamiltonian, yielding a magnetic hyperfine field value of 146 kOe and EFG characteristics that are fully in line with those obtained from the external-field spectrum. In the temperature range from 10 to 30 K, the spectra indicate the occurrence of relaxation effects. They could be satisfactorily described in terms of the Blume-Tjon (BT) model for electronic relaxation, assuming a superposition of a slow- and a fast-relaxation component. The observed temperature dependence of the isomer shift is adequately described by the Debye model for the lattice vibrations. The characteristic Mössbauer temperature was found as (360 ± 20) K and the zero-Kelvin intrinsic isomer shift as (1.480 ± 0.005) mm/s. The variation of the quadrupole splitting with temperature is explained by the thermal population of the Fe2+ electronic states within the T2g orbital triplet, which is split by the trigonal crystal field in a singlet ground state and an upper doublet state, the latter being further split into two orbital singlets by an additional distortion. The energies of these latter excited states with respect to the ground state are calculated to be 280 ± 20 and 970 ± 50 cm-1, respectively, based on a point-charge calculation of the 5D level scheme. This calculation confirms the sign of the Vzz component of the electric field gradient being negative as it is determined from the external magnetic field measurement and from the magnetic spectrum acquired at 4.2 K. Iron phosphates are widely spread minerals in the Earth’s crust and are expected to occur in soils and rocks on Mars as well. Mössbauer spectroscopy as complementary source of information to the results of other analytical techniques, could therefore be useful to identify and characterize the environmental Fe-bearing phosphates.

Association between phosphorus and iron oxides in manganese ores

Abstract Four manganese ore samples were subjected to sequential extractions with hydrogen peroxide and dithionite-citrate-bicarbonate. The first attack was meant to remove all manganese-bearing phases, whereas the second treatment was done to remove any residual manganese and the iron oxyhydroxides. The ores and the residues of both treatments were characterized by chemical analyses, X-ray diffraction, optical microscopy, and Mössbauer spectroscopy. The combination of a hydrogen peroxide extraction followed by the well-known dithionite-citrate-bicarbonate extraction allowed to establish of how phosphorus is present in these ores. No correlation between phosphorus and the manganese phases was found in any sample. In one sample, phosphorus was found as crandalite, an aluminum phosphate. On the other hand, the association between phosphorus and goethite was clearly observed in the other samples.

Síntese e caracterização de mulita utilizando sílica obtida da casca de arroz

Mullite, composed of 1,4Al2O3.SiO2 was synthesized using commercial α-alumina (Alpha Cesar Company) and silica obtained from the calcinations of rice husk. This silica exhibits a high specific surface area, high purity and small average particle diameter. Silica (26.2 wt.%) and α-alumina powders (71.8 wt. %) were mixed and homogenized using an alumina orbital disc mill before and after the calcinations at 1100oC for 4 h in an air atmosphere. Then, the powders were pressed uniaxially at a pressure of 1200 MPa. The green compacts were sintered at 1650oC, for 4 h, in air. The density of compacted sintering samples was determined using the hydrostatic method. The chemical and structural properties were observed using ICP (Inductively Coupled Plasma), XRD (X-ray diffraction) and 29Si MAS (Magic Angle Spinning) NMR (Nuclear Magnetic Resonance) and the microstructural characterization performed using SEM (Scanning Electron Micrography). The results show that it is possible to synthesize mullite ceramic of high-purity and with little glassy phase from the α-alumina and silica obtained from the rice husk that is purified and calcinated.