Structure and magnetic properties of iron based nanoparticles with oxide shells

Oxide-coated iron nanoparticles with average dimensions from 6 to 75 nm have been synthesized by chemical vapor condensation. The structure of particles and their size distribution have been determined. These data are used to interpret the results of measurements of the magnetic hysteresis characteristics.     

Structural and magnetic properties of glass-ceramics containing silver and iron oxide

Glass-ceramics with a nominal composition of 25SiO₂–(50 − x)CaO–15P₂O₅–8Fe₂O₃–2ZnO–xAg (where x = 0, 2 and 4 mol%) have been prepared. Structural features of glass-ceramics have been investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Magnetic properties were studied using vibrating sample magnetometer and Mössbauer spectroscopy. Ca₃(PO₄)₂, hematite and magnetite are formed as major crystalline phases. The microstructure reveals the formation of 25–30 nm size particles. Mössbauer spectroscopy has shown the relaxation of magnetic particles. Saturation magnetization value is increased with an increase of Ag content up to 4 mol%, which has been attributed to the formation of magnetically ordered particles. The antibacterial response was found to depend on Ag ions concentration in the glass matrix and samples with 4 mol% Ag in glass matrix have shown effective antibacterial activity against Escherichia coli.     

Ultrafine magnetic particles dispersed in silica: Characterization of cobalt iron citrate precursor and magnetic properties

Ultrafine magnetic particles dispersed in a silica matrix were successfully obtained by treatment of a cross-linked cobalt iron citrate precursor, synthesized by a modified Pechini route, with 0.001 M K₂Cr₂O₇ at 130 °C. The IR and NMR spectroscopic characterization of the precursor gel containing Co²⁺ and Fe³⁺ shows that the citric acid reacts with the metallic ions by coordination, the ethylene glycol by esterification and the tetraethylorthosilicate by substitution. SQUID measurements of the composite indicate superparamagnetic behavior. The blocking temperature, from the peak of the zero-field-cooled measurements, was 3 K at 1000 Oe. The magnetic diameter calculated using Langevin's equation was 4 nm.     

Structural and magnetic properties of phase controlled iron oxide rods

Iron oxide rods are synthesized by thermal decomposition of iron (II) acetate at 700 °C. The X-ray powder diffraction measurement confirms the hematite (α-Fe₂O₃) phase of iron oxide. We observed that the Morin transition depends on applied magnetic field and shift towards lower temperature with increase in applied magnetic field. The Morin temperature at applied field of 500 and 2000 Oe is observed to be 261.1 and 260.3 K respectively. Magnetization versus magnetic field measurements confirms the antiferromagnetic behavior below Morin transition and weak ferromagnetic behavior above Morin transition.     

Preparation and electrochemical properties of carbon nanofiber composite dispersed with silver nanoparticles using polyacrylonitrile and beta-cyclodextrin

A simple one-step method was used for preparing the beta-cyclodextrin/polyacrylonitrile (PAN) nanofibers deposited with silver nanoparticles by electrospinning and followed by the reduction of the Ag+ ions. The nano-composite fibers were stabilized at 280 degrees C in air and activated at 800 degrees C for 1 h in steam/N2. The structures of nano-composite fibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier-transform infrared (FTIR) spectroscopy, and X-ray diffraction analysis (XRD). The electrochemical behaviors of the composite of carbon nano-fibers were investigated by cyclic voltammetry and charge/discharge tests.     

Synthesis and characterization of iron oxide nanoparticles dispersed in mesoporous aluminum oxide or silicon oxide

Iron oxide nanoparticles dispersed in aluminum (Al) or silicon (Si) oxides were prepared via a polymeric precursor derived from the Pechini method. The samples were characterized by thermogravimetric analysis, Fourier-transform infrared spectroscopy, X-ray diffraction, N₂ adsorption/desorption isotherms (Brunauer–Emmett–Teller, BET), M?ssbauer spectroscopy, and vibrating sample magnetometry (VSM). BET analysis shows that the samples are mesoporous materials and have a high surface area. The size of the Fe₂O₃ nanoparticles in Al₂O₃ is smaller than that in SiO₂. M?ssbauer spectra of the samples show that the Fe₂O₃ nanoparticles in Al₂O₃ are non-magnetic at room temperature but magnetic below 50 K. The FeSi samples are magnetic at both room and low temperatures. The magnetic measurements with VSM confirmed this point.     

Shape- and size-selective electrochemical synthesis of dispersed silver(I) oxide colloids

Silver(I) oxide (Ag2O) micro- and nanoparticles were electrochemically synthesized by anodizing a sacrificial silver wire in a basic aqueous sulfate solution. Ag2O particles were released from the silver electrode surface during synthesis producing a visible sol "stream". The composition of these particles was established using selected area electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. The shape of Ag2O crystallites could be adjusted using the potential of the silver wire generator electrode. The generation of a dispersed Ag2O sol and the observed shape selectivity are both explained by a two-step mechanism involving the anodic dissolution of silver metal, Ag0 --> Ag+(aq) + 1e-, followed by the precipitation of Ag2O particles, 2Ag+ + 2OH- --> Ag2O(s) + H2O. Within 100 mV of the voltage threshold for particle growth, cubic particles with a depression in each face ("hopper crystals") were produced. The application of more positive voltages resulted in the generation of 8-fold symmetric "flower"-shaped particles formed as a consequence of fast growth in the <111> crystallographic direction. The diameter of flower particles was adjustable from 250 nm to 1.8 microm using the growth duration at constant potential.     

Synthesis of phase pure iron oxide polymorphs thin films and their enhanced magnetic properties

The α-Fe₂O₃ thin film was prepared on liquid–vapor interface at room temperature by a facile and cost effective method, which was converted to Fe₃O₄ and γ-Fe₂O₃ films by reduction and oxidation process. The morphological and structural characterizations reveal the average crystallites size in α-Fe₂O₃, Fe₃O₄ and γ-Fe₂O₃ films 12.8, 9.2 and 19 nm with rms roughness 4.35, 4.60 and 8.21 nm, respectively. From magnetic measurements, the α-Fe₂O₃ thin film shows a room temperature super-paramagnetic behavior with saturation magnetization 18 emu/cm³, while Fe₃O₄ and γ-Fe₂O₃ thin films exhibit ferrimagnetic behavior with saturation magnetization values 414.5 and 148 emu/cm³, respectively. A significantly higher value of saturation magnetization is observed in α-Fe₂O₃ film, which is trusted due to the uncompensated surface spins in the film. The converted Fe₃O₄ film also shows enhanced saturation magnetization due to the reduction in antiphase boundaries, whereas the magnetization in γ-Fe₂O₃ film decreases comparatively. The magnetic property of the γ-Fe₂O₃ is explained on the basis of the Fe³⁺ ions vacancy at the octahedral position in its structure.     

Preparation and magnetic properties of fibrous gamma iron oxide nanoparticles via a nonaqueous medium

Fibrous shape γ-Fe₂O₃ nanoparticles (the length of ∼850 nm; the width of ∼5 nm) have been prepared using lauryl alcohol as a nonaqueous medium. The resultant products were investigated by IR, TG–DTA, XRD, TEM and magnetization measurements. For the preparation of pure γ-Fe₂O₃ nanoparticles, the suitable condition of the molar ratio of lauryl alcohol to iron nitrate is determined to be 1 : 2 and the appropriate temperature is in the range of 300–400 °C. The magnetization measurements reveal that the obtained γ-Fe₂O₃ particles possess better magnetic properties for application in magnetic recording. It can be concluded that lauryl alcohol plays an important role not only in controlling the dimension, shape of the products, but also in helping the increase of magnetic properties.     

Tin oxide nanocluster hydrogen and ammonia sensors

We have prepared sensitive hydrogen and ammonia sensors from thin films of tin nanoclusters with diameters between 3 and 10?nm. By baking the samples at 200?°C in ambient air the clusters were oxidized, resulting in very stable films of tin oxide clusters with similar diameters to the original Sn clusters. By monitoring the electrical resistance, it is shown that the cluster films are highly responsive to hydrogen and ammonia at relatively low temperatures, thereby making them attractive for commercial applications in which low power consumption is required. Doping of the films by depositing Pd on top of the clusters resulted in much improved sensor response and response times. It is shown that optimal sensor properties are achieved for very thin cluster films (a few monolayers of clusters).     

Cation distribution,structure and magnetic properties of lithium manganese iron oxide spinel solid solutions

Single phase cubic spinel compounds Li_xMn_1+xFe_2−2xO₄ (x = 0, …, 1) were obtained by thermal decomposition of freeze-dried formate solutions of appropriate composition. The samples were characterized by X-ray powder diffraction and Rietveld refinement, XANES, ⁵⁷Fe Mössbauer spectroscopy and magnetization measurements. The combination of these methods provides useful conclusions concerning the structure, cation distribution and properties of the spinel solid solutions. The Li_xMn_1+xFe_2−2xO₄ samples contain Mn(II) and Mn(III) or Mn(III) and Mn(IV) for x < 0.5 or x > 0.5, respectively. With the increase of x the portion of Li ions occupying tetrahedral sites increases and becomes 100% at about x = 4/7. In spite of the preferred occupation of octahedral sites by manganese(III), the experimental results can only be explained by a partial occupation also of tetrahedral sites by Mn(III). An increase of M_S with the increase of x (expected for a preferred substitution of magnetic ions in tetrahedral sites by non-magnetic Li ions) is not observed. It should be prevented by the decreasing cooperative coupling effects due to the reduction of the iron content.     

A surfactant dispersed SWCNT-polystyrene composite characterized for electrical and mechanical properties

Single wall carbon nanotubes (SWCNTs) were dispersed in polystyrene (PS) at 0.1, 0.2, 0.3 and 1.0 wt.% (weight percent) concentrations using a surfactant assisted method. The resulting nanocomposites were characterized for their electrical conductivity, mechanical strength and fracture toughness properties. Results show a significant improvement in electrical conductivity with electrical percolation occurring by 0.2 wt.% SWCNT loading and the SWCNT-PS nanocomposite fully conductive at 1.0 wt.%. Three-point bend tests showed a decline in flexural strength and break strain with the addition of 0.1 wt.% SWCNTs. Improvements in the flexural modulus, strength and break strain with increasing SWCNT wt.% content followed The fracture toughness of the SWCNT-PS nanocomposites, in terms of the critical stress-intensity factor K_IC, was reduced relative to the neat material. From optical and high resolution scanning electron microscopy the presence of the carbon nanotubes is shown to have an adverse effect on the crazing mechanism in this PS material, resulting in a deterioration of the mechanical properties that depend on this mechanism.     

Thermally induced changes in the magnetic properties of iron oxide nanoparticles under reducing and oxidizing conditions

Application of iron oxide nanoparticles in the fields of water purification, biomedicine or chemistry often requires controlled magnetic properties that can be modified by changing temperature and redox conditions. Therefore, this work investigates the changes in the magnetic properties of iron oxide nanoparticles in the FeOOH − Fe₂O₃ − Fe₃O₄ system (i.e. hematite, goethite, lepidocrocite, maghemite and magnetite) at heating under reducing and oxidizing conditions. The results show that heat treatment of hematite and goethite in the presence of a reducing agent (5% starch) leads to their conversion into high magnetic magnetite. The starting temperature of transformation is approximately 350 °C for both samples. The magnetization increases to 86 Am²/kg for hematite reduced at 700 °C and to 88 Am²/kg for goethite reduced at 900 °C. An intense reaction occurs within the first 10 min and then the conversion process decelerates. Thermal treatment of lepidocrocite under both oxidizing and reducing conditions leads to an increase in magnetization due to the formation of maghemite and magnetite, respectively. Regardless of the redox conditions, the formation of magnetic phase begins at a temperature of 250 °C and is associated with the formation of maghemite from lepidocrocite. Under oxidizing conditions, the magnetization begins to decrease at 350 °C, which is associated with the conversion of maghemite to hematite. On the contrary, under reducing conditions, the magnetization of lepidocrocite increases up to 900 °C, which is associated with the formation of magnetite. Maximum values of magnetization are 36 Am²/kg for maghemite obtained at 350 °C, and 88 Am²/kg for magnetite obtained at 900 °C from lepidocrocite. With the help of conventional heating, the magnetic properties of IONs can be altered by phase transformations in the FeOOH − Fe₂O₃ − Fe₃O₄ system. Temperature and redox conditions are the two most important factors controlling the transformation pathways and the magnetic properties of the resulting IONs.     

Phase composition and magnetic properties of iron oxide nanoparticles obtained by impulse electric discharge in water

The chemical composition and the magnetic properties of iron oxide nanoparticles obtained by impulse electric discharge in water are investigated. The phase composition of the Fe₃O₄, Fe₂O₃, and Fe nanoparticles is determined. By means of the nuclear magnatiec resonance (NMR) technique, the magnetic moments of the nanoparticles are determined. The magnetic moment of the spherical nanoparticles equals to 2.39 × 10^–19 A m², and that of the cubical ones is 4.56 × 10^–19 A m².     

Relationships between magnetic tape properties and magnetic oxide concentration

A series of tapes were coated in which the concentration of the magnetic ferric oxide was essentially the sole variable. Magnetic and electroacoustic properties were evaluated as a function of magnetic oxide concentration. The intrinsic coercivity was found to increase slightly with decreasing oxide concentration but not to the extent predicted by the simple packing factor law. Biasing current changed in approximately the same ratio as coercivity. The sensitivity was directly proportional to the remanent flux density but the saturation output was proportional to the remanent flux. The linear range of the recording process increased appreciably with increasing oxide concentration. Frequency response decreased very slightly. Signal-to-noise ratio increased with increasing magnetic oxide concentration. Particle interactions had a strong effect upon the signal-to-print-through ratio. A decrease in magnetic oxide concentration to 50 percent of normal increased this ratio by 4 dB although coercivity was increased only slightly more than 5 percent.     

Synthesis and magnetic properties of Fe5C2 by reaction of iron oxide and carbon monoxide

The formation behaviour and magnetic properties of Fe₅C₂ were investigated. Iron oxides, as starting material, were carburetted with carbon monoxide gas by heat treatment. Fe₅C₂ was formed as a single phase in the reaction temperature range 350 to 375° C. The amount of carbon monoxide gas was controlled by mixing with nitrogen gas to prevent free carbon deposition by the disproportionation of carbon monoxide. Synthesized Fe₅C₂ particles were identified as a single phase by XRD and the weight change measurement of reactants. The saturation magnetization of Fe₅C₂ was about 100e.m.u.g^–1, regardless of the reaction temperature. The coercive force decreased from 800 to 400 Oe with increasing reaction temperature.     

Droplet orientation and optical properties of polymer dispersed liquid crystal composite films

The electro-optic and thermo-optic properties of polymer dispersed liquid crystal (PDLC) films have been investigated. The effects of applied voltage and temperature on liquid crystal droplet morphology and its transmission characteristics were studied. Threshold voltage (V_th) and optical transmission increases with increasing temperature. It may be due to the reduction in effective voltage drop across the liquid crystal droplets in the composite matrix. The liquid crystal droplet size was found to vary in the range of 5–28 μm.     

Abrasive properties of aluminum iron oxide nanoparticles

Aluminum iron oxide nanoparticles have been prepared by heat-treating ammonium hydroxycarbonate complexes with the general formula NH₄Al₂Fe(OH)₅(CO₃) · nH₂O and have been characterized by X-ray diffraction, IR spectroscopy, differential thermal analysis, scanning electron microscopy, and particle size analysis. The mixed oxide α-Al_{2 − x} Fe _x O₃ with x = 0.30−0.37 prepared from hydroxy complexes ensures surface roughness values R _a = 0.005−0.02 μm in polishing of the ShKh15 quenched steel with an austenite/martensite structure and offers high abrasion rate owing to its enhanced tribochemical activity and the presence of particles in the size range 1–10 nm.