Structure and magnetic properties of the oxide layers on iron ultrafine particles

5 . The γ-Fe particles, because of their paramagnetic nature, are very convenient for investigation on the attributes of iron oxide layers formed on the particle surfaces. Structures, morphologies and magnetic properties of the oxide layers covering the iron ultrafine particles have been studied using transmission electron microscopy observation, magnetic property measurement, X-ray diffraction and annealing treatment. Convincing evidences established that the iron oxide layers are not continuous and consist of very fine crystallites, and that these layers are non-ferromagnetic and have no contribution to the saturation magnetization of the iron particles. The iron oxide layers formed at room temperature was determined to be Fe₃O₄. Additionally, a brief annealing of the iron particles in air were performed to examine magnetic properties of the formed iron oxide layers and ultrafine oxide particles. Received: 30 April 1996/Accepted: 5 November 1996     

Preparation and characterization of Fe-doped TiO2 on fly ash cenospheres for photocatalytic application

Fe³⁺-doped TiO₂ film deposited on fly ash cenosphere (Fe-TiO₂/FAC) was successfully synthesized by the sol-gel method. These fresh photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analyses (TGA). The XRD results showed that Fe element can maintain metastable anatase phase of TiO₂, and effect of temperature showed rutile phase appears in 650 °C for 0.01% Fe-TiO₂/FAC. The SEM analysis revealed the Fe-TiO₂ films on the surface of a fly ash cenosphere with a thickness of 2 μm. The absorption threshold of Fe-TiO₂/FACs shifted to a longer wavelength compared to the photocatalyst without Fe³⁺-doping in the UV-vis absorption spectra. The photocatalytic activity and kinetics of Fe-TiO₂/FAC with varying the iron content and the calcination temperatures were investigated by measuring the photodegradation of methyl blue (MB) during visible light irradiation. Compared with TiO₂/FAC and Fe³⁺-doped TiO₂ powder (Fe-TiO₂), the degradation ratio using Fe-TiO₂/FAC increased by 33% and 30%, respectively, and the best calcined temperature was 450 °C and the optimum doping of Fe/Ti molar ratio was 0.01%. The Fe-TiO₂/FAC particles can float in water due to the low density of FAC in favor of phase separation to recover these photocatalyst after the reaction, and the recovery test shows that calcination contributes to regaining photocatalytic activity of Fe-TiO₂/FAC photocatalyst.     

Microstructural and magnetic characterization of dusts from a stone crushing industry in Birbhum, India

Stone dust sample collected from a stone crushing industry situated at Muhammad Bazar in Birbhum, India, is studied for its physical characterization using various techniques. Morphology and compositional analysis of the stone dust by scanning electron microscopy (SEM) reveal that the dust is an agglomeration of many tiny particles (0.32-2.12 μm), mostly having sharp edges, as well show microstructure heterogeneity. Elements present in the sample are detected by energy dispersive X-ray spectrometry (EDX). The X-ray diffraction (XRD) pattern analysis shows that the sample mainly contains minerals like anorthite, augite, esseneite and albite. An overall antiferromagnetic interaction in this sample has been indicated by the nature of the thermal dependence of magnetization. The remnant magnetization study apparently indicates two magnetic transitions at low temperatures. ⁵⁷Fe Mössbauer spectroscopy has been employed to detect different possible iron sites as well as to estimate the respective site population. In general, Mössbauer spectroscopic results corroborate the observations made through XRD analysis in general.     

Ultrafine ash aerosols from coal combustion: Characterization and health effects

Ultrafine coal fly-ash particles, defined here as those with diameters less than 0.5 μm, typically comprise less than 1% of the total fly-ash mass. These particles are formed primarily through ash vaporization, nucleation, and coagulation/condensation mechanisms, which lead to compositions notably different compared to other fine or coarse particle fractions formed by fragmentation. Whereas previous studies have focused on health effects of particulate matter with aerodynamic diameters less than 2.5 μm (PM_2.5) (including both vaporization and fragmentation modes), this paper reports results of interdisciplinary research focused on both characterization and health effects of primary ultrafine coal ash aerosols alone. Ultrafine, fine, and coarse ash particles were segregated and collected from a coal burned in a 20 kW laboratory combustor and two additional coals burned in an externally heated drop tube furnace. Extracted samples from both combustors were characterized by transmission electron microscopy (TEM), wavelength dispersive X-ray fluorescence (WD-XRF) spectroscopy, Mossbauer spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Pulmonary inflammation was characterized by albumin concentrations in mouse lung lavage fluid after instillation of collected particles in saline solutions and a single direct inhalation exposure. Results indicate that coal ultrafine ash sometimes, but not always, contains significant amounts of carbon, probably soot originating from coal tar volatiles, depending on coal type and combustion device. Surprisingly, XAFS results revealed the presence of chromium and thiophenic sulfur in the ultrafine ash particles. Although the single direct inhalation study failed to reveal significant health effects, the instillation results suggested potential lung injury, the severity of which could be correlated with the carbon (soot) content of the ultrafines. Further, this increased toxicity is consistent with theories in which the presence of carbon mediates transition metal (i.e., Fe) complexes, as revealed in this work by TEM and XAFS spectroscopy, promoting reactive oxygen species, oxidation–reduction cycling, and oxidative stress.     

Magnetic properties of iron loaded MCM-48 molecular sieves

Mesoporous molecular sieves of MCM-48 type were loaded with iron by the wet impregnation method, using Fe(III) nitrate or Fe(II) sulfate aqueous solutions as Fe sources, to obtain a magnetic porous composite. The iron loaded materials were characterized by XRD, N₂ adsorption and DRUV-vis and compared with the Si-MCM-48 host. Their magnetic properties were studied by measuring the hysteresis loops up to 1.5 T at different temperatures (5-300 K) and by magnetization vs. temperature curves following the conventional zero field cooling (ZFC) and field cooling (FC) protocols. Materials with high structure regularity and surface area are obtained, which exhibit a mixed paramagnetic and superparamagnetic behavior, arising in isolated iron ions inserted in the host framework, and in small iron oxide clusters or nanoparticles forming inside the pores, respectively. Larger hematite particles (8-13 nm) grown on the external surface provide a quite small ferromagnetic contribution to the hysteresis loop.     

Novel magnetic Fe onion-like fullerene micrometer-sized particles of narrow size distribution

Magnetic polydivinylbenzene (PDVB)/magnetite micrometer-sized particles of narrow size distribution were prepared by entrapping Fe(CO)₅ within the pores of uniform porous PDVB particles, followed by the thermal decomposition of the encapsulated Fe(CO)₅ at 300 °C in a sealed cell under inert atmosphere. Magnetic Fe onion-like fullerene micrometer-sized particles of narrow size distribution have been prepared by the thermal decomposition of the PDVB/magnetite magnetic microspheres at 1100 °C under inert atmosphere. The graphitic coating protects the elemental iron particles from oxidation and thereby preserves their very high magnetic moment for at least a year. Characterization of these unique magnetic carbon graphitic particles was also performed.     

Mössbauer study of ultrafine FeF2 particles

Ultrafine antiferromagnetic FeF₂ particles (<10 nm) were prepared by a new technique, viz, the SF₆-sensitized infrared photodecomposition of Fe(CO)₅ induced by a transversely excited atmospheric (TEA) CO₂ laser. The magnetic properties have been examined by⁵⁷Fe Mössbauer spectroscopy. At low temperatures the magnetic hyperfine field decreases faster with increasing temperature than the hyperfine field of the bulk; this behavior appears to be consistent with collective magnetic excitations. The transition between the paramagnetic and antiferromagnetic states takes place at a higher temperature and over a broader range as compared to the bulk. FeF₂ ultrafine particles are relatively sensitive to oxidation; cubic-type iron oxide is formed.     

Study on the coercivity of granular solid iron

The iron granular solid, in which ultrafine iron particles are dispersed, has been prepared with both SiO₂ and Cu matrices using the sol-gel method. The structure and morphology of these granular solid samples are investigated by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The magnetic properties are measured using a vibrating sample magnetometer with 20 kOe maximum applied field. It is found that the coereivity decreases very slightly with temperature from 80 to 300 K for these Fe–SiO₂ and Fe–Cu granular solid samples with different average size of iron particles from 50 to 300 Å. The magnetic anisotropy has been obtained from the measured magnetization curves for these granular solid samples using the law of approach to saturation, and the obtained values of the effective magnetic anisotropy are all more than 10⁶ erg/cm³, which are larger than the value of the magnetocrystalline anisotropy for bulk iron. The coercivity vs temperature for these granular solid samples has been calculated using the Kneller and Luborsky theory, in which the magnetic anisotropy values obtained from the law of approach to saturation are used. The trends of the calculated coercivity as a function of temperature are in reasonable agreement with the observations.     

Insight of arsenic transformation behavior during high-arsenic coal combustion

The release of arsenic vapors (As³⁺) during high-arsenic coal combustion not only raises serious environmental concerns, but also causes catalyst deactivation in selective catalytic reduction (SCR) systems. To illuminate the mechanisms involved in the transformation of arsenic vapors towards less troublesome arsenates (As⁵⁺) during coal combustion, the accessory minerals in the high-arsenic coal were identified and the association relationship of these compounds with arsenic in fly ash was estimated. The results showed that Si/Al were the main inorganic elements in high-arsenic coal while the content of Ca was quite low. Ca was mostly transformed into sulfates during coal combustion and the effect of Ca on the arsenic transformation was limited. Al/Fe played a more significant role in arsenic speciation transformation and arsenic in the fly ash was predominantly bound with Al/Fe-oxides as arsenates. It was further confirmed that Al in kaolin/metakaolin showed good capacity on arsenic capture. In addition, few arsenic vapors were captured through the physical adsorption mechanism and the large fraction of As³⁺ in some fine particles was mostly attributed to the chemical reactions between arsenic vapors and Al-compounds. Meanwhile, a certain amount of arsenic vapors were converted into As₂O₅(s) under the influence of SCR catalyst and then carried by flue gas to participate in fly ash. Besides, part of arsenic distributed in the fly ash was through the stabilization of ash matrix in high temperature conditions. The transformation of arsenic from vapors towards particulate arsenic favored arsenic emission control by particulate matter control devices.     

Growth,structure and thermal reduction of MOCVD-deposited Fe films on Al2O3 (0 0 0 1) substrates

Fe films with strong preferred orientation were prepared on Al₂O₃ (0 0 0 1) substrates by a new two-step method using low-pressure metal-organic chemical vapor deposition (LP-MOCVD) method. X-ray diffraction (XRD) and a vibrating sample magnetometer were employed to characterize the structure and magnetic properties of the Fe films before and after thermal reduction, which was performed in hydrogen flow at 723–1023 K. XRD patterns indicate that the films changed into α-Fe (bcc) mono-phase from a mixture of α-Fe₂O₃ and/or Fe (bcc).     

Interparticle interaction effects on magnetic behaviors of hematite (α-Fe2O3) nanoparticles

The interparticle magnetic interactions of hematite (α-Fe₂O₃) nanoparticles were investigated by temperature and magnetic field dependent magnetization curves. The synthesis were done in two steps; milling metallic iron (Fe) powders in pure water (H₂O), known as mechanical milling technique, and annealing at 600 °C. The crystal and molecular structure of prepared samples were determined by X-ray powder diffraction (XRD) spectra and Fourier transform infrared (FTIR) spectra results. The average particle sizes and the size distributions were figured out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The magnetic behaviors of α-Fe₂O₃ nanoparticles were analyzed with a vibrating sample magnetometer (VSM). As a result of the analysis, it was observed that the prepared α-Fe₂O₃ nanoparticles did not perform a sharp Morin transition (the characteristic transition of α-Fe₂O₃) due to lack of unique particle size distribution. However, the transition can be observed in the wide temperature range as “a continuously transition”. Additionally, the effect of interparticle interaction on magnetic behavior was determined from the magnetization versus applied field (σ(M)) curves for 26±2 nm particles, dispersed in sodium oxalate matrix under ratios of 200:1, 300:1, 500:1 and 1000:1. The interparticle interaction fields, recorded at 5 K to avoid the thermal interactions, were found as ∼1082 Oe for 26±2 nm particles.     

Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe³⁺ nature of the iron atom in ZnO matrix.     

Mechanism study of selenium retention by iron minerals during coal combustion

The interactions between selenium vapors and coal accessory Ca/Fe-minerals favor selenium emission control by transferring selenium into fly ash during coal combustion. Considering the complicated effects of iron transformation on selenium retention, iron species in fly ashes from seven coal-fired power plants were distinguished and the associations between selenium and iron minerals were assessed. Iron oxides (including Fe₃O₄, γ-Fe₂O₃ and ɑ-Fe₂O₃) were determined as the main form of iron minerals in fly ash. The adsorption of selenium vapors by different iron oxides was conducted at temperatures ranging from 300 to 900 °C and the species of captured selenium were identified. Furthermore, reaction sites on the surfaces of fresh and reacted iron oxides were compared to investigate the mechanism regarding selenium adsorption over these iron oxides, which were further clarified through density functional theory study. The results showed that iron oxides were surely to play a significant role in selenium retention mainly through chemisorption and the reactions probably occurred at temperatures below 900 °C. At 300 °C, ɑ-Fe₂O₃ had better selenium adsorption performance than Fe₃O₄/γ-Fe₂O₃. Regardless of iron species, Fe atoms on iron oxides participated in the selenium adsorption by forming a Se–O–Fe Structure. With temperature increasing, selenium adsorption by Fe atoms was suppressed, which caused a drop off in selenium capture capacity of Fe₃O₄ and ɑ-Fe₂O₃. Differently, increasing temperature promoted selenium adsorption over γ-Fe₂O₃, which owned a high selenium adsorption capacity even at 700 °C. Further analysis confirmed that the presence of O₂/H₂O(g) in the flue gas contributed to the formation of oxygen vacancies on the surface of γ-Fe₂O₃ at high temperatures and facilitated selenium vapors to react with Fe atoms.     

Studies of magnetite nanoparticles synthesized by thermal decomposition of iron (III) acetylacetonate in tri(ethylene glycol)

In this paper, water-soluble magnetite nanoparticles have been directly synthesized by thermal decomposition of iron (III) acetylacetonate, Fe(acac)₃ in tri(ethyleneglycol). Size and morphology of the nanoparticles are determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements while the crystal structure is identified using X-ray diffraction (XRD). Surface charge and surface coating of the nanoparticles are recognized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectra (XPS) and zeta potential measurements. Magnetic properties are determined using vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID) measurements. The results show that as-prepared magnetite nanoparticles are relatively monodisperse, single crystalline and superparamagnetic in nature with the blocking temperature at around 100 K. The magnetite nanoparticles are found to be highly soluble in water due to steric and electrostatic interactions between the particles arising by the surface adsorbed tri(ethyleneglycol) molecules and associated positive charges, respectively. Cytotoxicity studies on human cervical (SiHa), mouse melanoma (B16F10) and mouse primary fibroblast cells demonstrate that up to a dose of 80 μg/ml, the magnetic nanoparticles are nontoxic to the cells. Specific absorption rate (SAR) value has been calculated to be 885 and 539 W/gm for samples with the iron concentration of 1 and 0.5 mg/ml, respectively. The high SAR value upon exposure to 20 MHz radiofrequency signifies the applicability of as-prepared magnetite nanoparticles for a feasible magnetic hyperthermia treatment.     

Study of polydiethylsiloxane-based ferrofluid with excellent frost resistance property

The polydiethylsiloxane-based ferrofluid was prepared by dispersing finely divided magnetic Fe₃O₄ particles which are modified with oleoyl sarcosine and lauroyl sarcosine. The optimized experiment parameters including molar ratio of surfactant to Fe₃O₄ (1:5), temperature (80 °C), stirring rate (300 RPM), the surfactant content of lauroyl sarcosine (0 to 33 mol%) and the modification time (25 min) were obtained by the orthogonal test. The magnetic liquid was characterized by a transmission electron microscope (TEM), infrared (IR) spectrometer, X-ray diffractometer (XRD), thermogravimetry (TG), vibrating sample magnetometer (VSM) and differential scanning calorimetry (DSC). It is indicated that the surfactant is mainly bonded to the surface of Fe₃O₄ nanoparticles through covalent bond between carboxylate (COO⁻) and Fe atom. The modified magnetic particles are equally dispersed into the carrier and remain stable below −12 °C over 4 months. The ferrofluids exhibit excellent frost resistance property and distinctly reduced temperature coefficient of viscosity compared with polydimethylsiloxane-based ferrofluids and hydrocarbon-based ferrofluids, respectively. The saturation magnetization could reach up to 27.7 emu/g.     

Iron and aluminium based mixed hydroxides: A novel sorbent for fluoride removal from aqueous solutions

In our previous studies, iron and aluminium based mixed hydroxides were prepared in different molar ratios and fluoride removal efficiencies were evaluated. It was found that the Fe/Al sample with 1:1 molar ratio exhibited maximum adsorption capacity for fluoride. In the present work detailed studies were carried out to understand the effect of fluoride concentration on kinetics, adsorbent dose and competing anion concentrations. Characterisation studies on the adsorbent by XRD, TGA, SEM-EDX, TEM and FT-IR analysis before and after fluoride adsorption were carried out to understand the adsorption mechanism. The particles were irregular in shape, <0.5 μm in size, highly porous and showed specific surface area of 268 m² g⁻¹. XRD and FT-IR studies revealed significant changes after fluoride adsorption and showed formation of new complexes on adsorbent surface. Applicability of different sorption kinetic models was studied. The surface sites are heterogeneous in nature and followed heterogeneous site binding model. The presence of phosphate, sulphate and arsenate showed adverse effect on fluoride removal efficiency of Fe/Al adsorbent. The efficiency of material towards ground water samples treatment was tested with and without adjusting pH, and the results are discussed.     

In situ synthesis and characterization of LiNi0.5La0.08Fe1.92O4-polyaniline core-shell nanocomposites

Core-shell-structured LiNi_0.5La_0.08Fe_1.92O₄-polyaniline (PANI) nanocomposites with magnetic behavior were synthesized by in situ polymerization of aniline in the presence of LiNi_0.5La_0.08Fe_1.92O₄ nanoparticles. The structure, morphology and magnetic properties of samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), UV-vis absorption, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) technique. The results of spectroanalysis indicated that there was interaction between PANI chains and ferrite particles. TEM study showed that LiNi_0.5La_0.08Fe_1.92O₄-PANI nanocomposites presented a core-shell structure with a magnetic core of 30-50 nm diameter and an amorphous shell of 10-20 nm thickness. The nanocomposites under applied magnetic field exhibited the hysteresis loops of the ferromagnetic nature. The saturation magnetization and coercivity of nanocomposites decreased with decreasing content of LiNi_0.5La_0.08Fe_1.92O₄. The polymerization mechanism and bonding interaction in the nanocomposites have been discussed.     

Mössbauer Studies of Thermal Power Plant Coal and Fly Ash

Iron-57 Mössbauer spectroscopic studies were carried out at room temperature on samples of coal, slag (bottom ash) and mechanical ash collected from Bhatinda (India) thermal power plant. Hyperfine parameters such as isomer shift, quadrupole splitting and total internal magnetic field of ⁵⁷Fe nuclei were used to characterize various iron-bearing minerals. The observed parameters indicate the presence of pyrite, siderite and ankerite in coal sample while magnetic fractions of mechanical ash and slag samples show the formation of hematite and Al-substituted magnesio-ferrite. The non-magnetic fraction of slag ash shows the dominance of Fe²⁺ phases while that of mechanical ash demonstrates the formation of both Fe²⁺ and Fe³⁺ phases. These findings are compared with Mössbauer and magnetic susceptibility studies on fly ash samples of Panipat (India) thermal power plant reported earlier.     

Iron Fine Particles Produced by Laser Evaporation and Their Chemical Reactions

Laser-ablated iron atoms and fine particles were isolated in low-temperature Ar matrices, and their chemistry was studied by means of Mössbauer spectroscopy. Reaction of Fe with O₂ produces Fe-O, O-Fe-O, and Fe-O₂ as isolated in Ar matrices. On annealing the sample, unstable species are converted to more stable species in the matrix. Similarly, Fe-O and O-Fe-O were obtained from the reaction of iron atoms with N₂O. Furthermore, Fe-N was found to be produced by the reactions of laser-ablated Fe with N₂.     

Mössbauer study of Fe-oxide surface layers formed on small Fe particles

A Mössbauer study has been made of the iron-oxide formed on the surface of ultrafine Fe particles (about 293 Å in diameter) prepared by an aerosol method. Particular attention has been paid to the morphology of the oxide layers and to the magnetic structure. X-ray analyses indicate that the oxide layer is a mixture of Fe₃O₄ and γ-Fe₂O₃, and is composed of divided fine crystallites. Further a large non-collinearity in the spin structure of the oxide layer is found that is the likely origin of the low saturation magnetization observed for this type of system.