Mechanochemical and magnetomechanical synthesis of hematite nanoparticles

Samples of hematite were exposed to mechanochemical activation by high energy ball milling for 0–27 h. The milling-induced changes to the structural and magnetic properties of hematite were characterized by X-ray diffraction (XRD) and Mössbauer spectroscopy. The particle size was found to decrease from 80 to 16.5 nm after 8 h of ball milling time, followed by a small increase to 19.8 nm at the end of the milling period. An overall expansion of the crystalline lattice parameters a and c with the milling time was deduced. The magnetic hyperfine field decreased with the ball milling time, from 51.46 down to 50.68 T after 27 h of grinding. Magnetite and traces of iron were observed at the longest milling time employed. The recoilless fraction (f ) was measured simultaneously using a dual Mössbauer absorber consisting of hematite and a stainless steel etalon. The f factor first decreased with the milling time due to occurrence of nanoparticles in the system, had a maximum at 12 h due to agglomerations of nanoparticles and exhibited a second maximum at 27 h, due to the appearance of magnetite in the system. More samples of hematite were subjected to magnetomechanical activation by magnetic ball milling for 52 and 134 h. A phase mixture of hematite and magnetite was observed.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

Size dependence of the magnetic and hyperfine properties of nanostructured hematite (α-Fe 2 O 3 ) powders prepared by the ball milling technique

In this work we present the study of hematite (α-Fe₂O₃) nanostructures synthesized by the ball milling technique. The structural characterization and the crystallite size estimation have been carried out using the X-ray diffraction (XRD) technique. Data analyses indicate that the hematite phase (space group, R-3C) is preserved after the milling process. As the milling time is increased, a second phase (α-Fe) appears. The mean crystallite size shows a decreasing tendency as the milling time is increased. High-resolution transmission electron microscopy (HRTEM) images show the formation of grains composed of crystallites with irregular shapes. Mössbauer spectra of milled powders carried out at 297 and 77 K are well modeled with a histogram distribution of hyperfine fields. The presence of one additional sextet which corresponds to the ∝-Fe phase is also determined in agreement with XRD data analysis. Magnetic measurements suggest the suppression of the Morin transition in the milled samples and the absence of thermal relaxation effects in agreement with the Mössbauer spectroscopy results.     

Mössbauer and X-ray study of the Fe 65 Ni 35 invar alloy obtained by mechanical alloying

Fe₆₅Ni₃₅ samples were prepared by mechanical alloying (MA) with milling times of 5, 6, 7, 10 and 11 h, using a ball mass to powder mass ratio of 20:1 and at 280 rpm. The samples were characterized by X-ray diffraction (XRD) and transmission ⁵⁷Fe Mössbauer spectrometry. The X-ray diffraction pattern showed the coexistence of one body centered cubic (BCC) and two face centered cubic (FCC1 and FCC2) structural phases. The lattice parameters of these phases did not change significantly with the milling time (2.866 Å, 3.597 Å and 3.538 Å, respectively). After 10 h of milling, the X-ray diffraction pattern showed clearly the coexistence of these three phases. Hence, Mössbauer spectrometry measurements at low temperatures from 20 to 300 K of this sample were also carried out. The Mössbauer spectra were fitted using a model with three components: the first one is a hyperfine magnetic field distributions at high fields, related to the BCC phase; the second one is a hyperfine magnetic field distribution involving low hyperfine fields related to a FCC phase rich in Ni, and the third one is a singlet related to a FCC phase rich in Fe, with paramagnetic behavior. As proposed by some authors, the last phase is related with the antitaenite phase.     

Fe-doped TiO2 prepared by mechanical alloying

The effect of different milling conditions on the formation of Fe-doped TiO₂ powders by mechanical alloying was investigated by Mössbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO₂ induce the anatase to rutile phase transformation via high pressure srilankite. Mössbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe^3?+? incorporated in the TiO₂ structure, and to the Fe^2?+? located either at the surface or the interstitial sites of TiO₂. A greater incorporation of Fe in the TiO₂ structure was observed when samples were prepared from hematite instead of α-Fe.     

Mössbauer spectroscopy study of mechanically alloyed Fe2O3–(Al, Co and WC) systems

Mössbauer spectroscopy revealed that a central hyperfine interaction doublet and an additional sextet characterized the appearance of new phases in the mechanically alloyed Fe₂O₃–Al and Fe₂O₃–Co systems. In the Fe₂O₃–Al system, the intensity of the central super paramagnetic doublet which represents small particles of iron, increased with increasing milling time from 5 to 30 h of mechanical alloying. The magnetic sextet characterizing hematite vanished in the room temperature Mössbauer spectra of samples produced after 25 h of mechanically alloying the 50% Fe₂O₃ and 50% Al system. In general XRD peak broadening was observed as a result of extensive material structural distortion and formation of small particles. Fe, Al₂O₃ and mixed iron–aluminium oxide phases were identified in the XRD patterns with a small persistence of the iron oxide up to 20 h of mechanically alloying the 1:1 system Al–Fe₂O₃. In the 50% Co–50% Fe₂O₃ system, a 55% abundant new phase CoFe₂O₄ was observed, from the Mössbauer spectra of the system. The presence of this new phase was confirmed by the XRD analysis. The high energy ball milling of WC–Fe₂O₃ revealed a more effective grinding compared to hematite alone. The hematite particles were reduced to nanosized particles.     

Mechanosynthesis of nanocrystalline MgFe2O4—neutron diffraction and Mössbauer spectroscopy

The evolution of nanocrystalline n-MgFe₂O₄ by high-energy milling a mixture of MgO and α-Fe₂O₃ for periods of between 0 h and 12 h has been investigated by neutron diffraction in addition to previous Mössbauer, XRD and HRTEM measurements. Complete transformation of the milled products to n-MgFe₂O₄ only occurs on milling to ～8 h even though the average particle size decreases to <?～10 nm after milling for 2 h. The applied field Mössbauer spectra of n-MgFe2O4 can be well described by two subspectra representing core and shell regions with different cation distributions and spin canting angles. The neutron pattern of nanocrystalline MgFe₂O₄ is described well by two components comprising nanoparticles of core and shell dimensions ～7(1) nm and ～0.7(1) nm, respectively, in support of the Mössbauer core-shell model.     

Magnetic and Mössbauer studies of fucan-coated magnetite nanoparticles for application on antitumoral activity

Fucan-coated magnetite (Fe₃O₄) nanoparticles were synthesized by the co-precipitation method and studied by Mössbauer spectroscopy and magnetic measurements. The sizes of the nanoparticles were 8–9 nm. Magnetization measurements and Mössbauer spectroscopy at 300 K revealed superparamagnetic behavior. The magnetic moment of the Fe₃O₄ is partly screened by the Fucan coating aggregation. When the magnetite nanoparticles are capped with oleic acid or fucan, reduced particle-particle interaction is observed by Mössbauer and TEM studies. The antitumoral activity of the fucan-coated nanoparticles were tested in Sarcoma 180, showing an effective reduction of the tumor size.     

Magnetic Study of Nanocrystalline Ferrites and the Effect of Swift Heavy Ion Irradiation

100 MeV Si⁺⁷ irradiation induced modifications in the structural and magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ nanoparticles have been studied by using X-ray diffraction, Mössbauer spectroscopy and a SQUID magnetometer. The X-ray diffraction patterns indicate the presence of single-phase cubic spinel structure of the samples. The particle size was estimated from the broadened (311) X-ray diffraction peak using the well-known Scherrer equation. The milling process reduced the average particle size to the nanometer range. After irradiation a slight increase in the particle size was observed. With the room temperature Mössbauer spectroscopy, superparamagnetic relaxation effects were observed in the pristine as well as in the irradiated samples. No appreciable changes were observed in the room temperature Mössbauer spectra after ion irradiation. Mössbauer spectroscopy performed on a 12 h milled pristine sample (6 nm) confirmed the transition to a magnetically ordered state for temperatures less than 140 K. All the samples showed well-defined magnetic ordering at 5 K, whereas, at room temperature they were in a superparamagnetic state. From the magnetization studies performed on the irradiated samples, it was concluded that the saturation magnetization was enhanced. This was explained on the basis of SHI irradiation induced modifications in surface states of the nanoparticles.     

Structural and magnetic properties of Fe–Ni mecanosynthesized alloys

Fe_100???x Ni _x samples with x?=?22.5, 30.0 and 40.0 at.% Ni were prepared by mechanical alloying (MA) with milling times of 10, 24, 48 and 72 h, a ball mass to powder mass (BM/PM) ratio of 20:1 and rotation velocity of 280 rev/min. Then the samples were sintered at 1,000°C and characterized by X-ray diffraction (XRD) and transmission Mössbauer spectrometry (TMS). From the refinement of the X ray patterns we found in this composition range two crystalline phases, one body centered cubic (BCC), one face centered cubic (FCC) and some samples show FeO and Fe₃O₄ phases. The obtained grain size of the samples shows their nanostructured character. Mössbauer spectra were fitted using a model with two hyperfine magnetic field distributions (HMFDs), and a narrow singlet. One hyperfine field distribution corresponds to the ferromagnetic BCC grains, the other to the ferromagnetic FCC grains (Taenite), and the narrow singlet to the paramagnetic FCC grains (antitaenite). Some samples shows a paramagnetic doublet which corresponds to FeO and two sextets corresponding to the ferrimagnetic Fe₃O₄ phase. In this fit model we used a texture correction in order to take into account the interaction between the particles with flake shape and the Mössbauer $\upgamma$ -rays.     

Synthesis and characterization of the xZnO-(1−x)α-Fe2O3 nanoparticles system

The xZnO-(1−x)α-Fe₂O₃ nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mössbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mössbauer spectrum for 12 h of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mössbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe₂O₄. This doublet clearly dominates the Mössbauer spectrum for x=0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.     

Magnetic and Structural Properties of the Mechanically Alloyed Nd2(Fe100 − x Nb x )14B System

In this work we report the magnetic and structural properties obtained by Mössbauer spectrometry, Vibrating Sample Magnetometer and X-ray diffraction of milled powders with initial composition Nd₂(Fe_{100 ? x} Nb _x )₁₄B with x = 0 and x = 4. The mixtures were ball milled for different times up to 240 h. Structural and microstructural parameters were derived from high statistics X-ray patterns and discussed as a function of milling time. The Mössbauer spectra of the samples were fitted by means of a sextet and an hyperfine field distribution, associated to a pure iron phase (α-Fe) and a disordered iron-based phase, respectively. The α-Fe grain size decreases from 50 nm for 6 h up to 5 nm for 240 h milling time. The Vibrating Sample Magnetometer results allow to conclude that these samples behave as soft ferromagnets.     

Study of NiFe2O4 nanoparticles using Mössbauer spectroscopy with a high velocity resolution

The nanocrystalline NiFe₂O₄ particles prepared by solution combustion synthesis technique using different fuels such as ethylene-diamine-tetra-acetic acid (NA sample) and urea (NB sample) were studied using magnetic measurement and ⁵⁷Fe Mössbauer spectroscopy with a high velocity resolution. The temperature dependence of magnetization is different for the two samples. Mössbauer spectra demonstrate the necessity to use more than two magnetic sextets, usually used to fit the NiFe₂O₄ nanoparticles spectra. Evaluation of the different local microenvironments for Fe in both tetrahedral (A) and octahedral (B) sites, caused by different Ni^2?+? occupation of octahedral sites, demonstrates at least five different local microenvironments for both A and B sites. Therefore, the Mössbauer spectra were fitted by using ten magnetic sextets which are related to the spread ⁵⁷Fe location in octahedral and tetrahedral sites.     

A Mössbauer investigation of zirconium distribution and diffusion in ball milled nanocrystalline iron powders

Ball milled nanocrystalline iron with minor zirconium additions was examined using ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction. Powder samples were synthesized using 0, 5, and 10 wt.% zirconium additions and milled at room temperature for periods up to 24 h. Progressive decrease in grain size as determined by X-ray diffraction was observed as a function of milling time. Mössbauer spectroscopy indicates increased iron-zirconium coordination with increased milling time. After milling, the powder samples were then heat treated in an inert atmosphere of argon at up to 925 K for various times up to 25 min. Analysis of X-ray peak line width (FWHM) was used to characterize grain size and grain growth kinetics as a function of heat treatment, milling time, and alloy content and reveal an increasingly finer post-heated structure in the alloy samples containing more zirconium. Mössbauer measurements were made and suggest Zr is steadily distributed into the Fe lattice with milling and rapidly diffuses to the grain boundaries with heat treatment. The impurity-rich grain boundaries appear to considerably stabilize the refined structure.     

Magnetic and Mössbauer spectroscopic studies of NiZn ferrite nanoparticles synthesized by a combustion method

The properties of nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ synthesized by an auto-combustion method have been investigated by magnetic measurements and Mössbauer spectroscopy. The as-synthesized single phase nanosized ferrite powder is annealed at different temperatures in the range 673–1,273 K to obtain nanoparticles of different sizes. The powders are characterized by powder X-ray diffraction, vibrating sample magnetometer, transmission electron microscopy and Mössbauer spectroscopy. The as-synthesized powder with average particle size of ~9 nm is superparamagnetic. Magnetic transition temperature increases up to 665 K for the nanosized powder as compared to the transition temperature of 548 K for the bulk ferrite. This has been confirmed as due to the abnormal cation distribution, as evidenced from room temperature Mössbauer spectroscopic studies.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Magnetic and Mössbauer study of ε-Fe<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>N (2 < <Emphasis Type="Italic">y</Emphasis> < 3) nanoparticles

ε-Fe _y N (2 < y < 3) nanoparticles were prepared by the nitridation of spindle type hematite nanoparticles using ammonia at a temperature of 550 °C. Chemical analysis for the elements brings out the composition as y = 2.56. These nanoparticles show ferromagnetic behaviour even though it has higher nitrogen content than the stoichiometric Fe₃N. The quenching of the particles from higher temperature and the higher nitrogen content introduces disorder in the system unlike the ordered ε-Fe₃N. Mössbauer spectroscopy at lower temperature clearly shows the existence of two sextets corresponds to the two types of iron environments. The presence of more than one site excludes perfect ordering. This is consistent with the peak widths which result from variations in the distributions of next nearest interstitial neighbours. Existence of two different Debye temperatures for two iron sites shows that it varies according to the number and type of neighbouring atoms.     

Local structural order in nanostructured hematite

Nanostructured α-Fe₂O₃ powders were prepared by high-energy ball milling. The milling process spans grinding times from 30 min to 24 h. The as-milled samples were characterized by means of ⁵⁷Fe Mössbauer spectrometry, Rietveld analysis of X-ray diffraction data and particle size analysis. The obtained results evidence the presence of disordered hematite characterized by a hyperfine field distribution with a well-behaved dependence on the mean crystallite size for which the mean hyperfine field decreases asymptotically as the grain size decreases. A new relationship is proposed in order to describe such behavior. Finally the presence of superparamagnetic grains, the occurrence of a partial topotactic phase transformation into a spinel phase and tool induced contamination are also presented and discussed.     

Synthesis and characterization of uncoated and gold-coated magnetite nanoparticles

We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticles. Structural characterizations, carried out using X-ray diffraction, confirm the formation of magnetite phase with a mean size of ~7 and ~8 nm for the uncoated and gold covered magnetite nanoparticles, respectively. The value of the gold coated Fe₃O₄ nanoparticles is consistent with the mean physical size determined from transmission electron microscopy images. Mössbauer spectra at room temperature are consistent with the thermal relaxation of magnetic moments mediated by particle-particle interactions. The 77 K Mössbauer spectra are modeled with four sextets. Those sextets are assigned to the signal of iron ions occupying the tetrahedral and octahedral sites in the core and shell parts of the particle. The room-temperature saturation magnetization value determined for the uncoated Fe₃O₄ nanoparticles is roughly ~60 emu/g and suggests the occurrence of surface effects such as magnetic disorder or the partial surface oxidation. These surface effects are reduced in the gold-coated Fe₃O₄ nanoparticles. Zero-field–cooled and field-cooled curves of both samples show irreversibilities which are consistent with a superparamagnetic behavior of interacting nanoparticles.     

Magnetic properties of γ-Fe2O3 nanoparticles encapsulated in surface-treated polymer spheres

Mössbauer and magnetic characterization of polymer-dispersed γ-Fe₂O₃ nanoparticles treated under different chemical processes are reported in this work. X-ray powder diffraction analysis provides a mean particle size of D ~ 8.0 nm. Whereas Mössbauer spectroscopy data suggest the presence of only Fe^3?+? ions, magnetization measurements indicate the occurrence of a freezing phenomenon in agreement with the thermal evolution of Mössbauer spectra. A core–shell model was used to determine a magnetically disordered layer (shell) of d ~ 1.0 nm covering a region of collinear magnetic moments (core). The chemical treatments with H₂O₂ and Na₂S₂O₈ modify notoriously the magnetic response of the polymer-dispersed nanoparticles.