Search

Monodisperse hematite (α-Fe2O3) nanoparticles were synthesized by forced hydrolysis of acidic Fe3+ solution. Rietveld analysis was applied to the X-ray powder diffraction data to refine the lattice constants and atomic positions. The lattice constants for a hexagonal unit cell were determined to be a~0.50327 and c~1.37521 nm. High resolution transmission electron microscopy was employed to study the morphology of the particles. Atomic scale micrographs and diffraction patterns from several zone axes were obtained. These reveal the high degree of crystallinity of the particles. A series of observations made on the particles by tilting them through a range of ±45° revealed the particles to be micaceous with stacking of platelets with well defined crystallographic orientations. The Morin transition in these nanoparticles was found to occur at 210 K, which is lower temperature than 263K of bulk hematite. It was ascertained from the previous Mössbauer studies that the spin orientation for nano-sized hematite particle flips from 90° to 28° with respect to the c-axis of the hexagonal structure during the Morin transition, which is in contrast to that observed in bulk hematite where spin orientation flips from 90° to 0°. [ABSTRACT FROM AUTHOR]

(Fe50Co50)97V2Nb1 nanoparticles were synthesized in an induction plasma torch and oxidized isochronally at temperatures between room temperature (RT) and 900 °C. The particles exhibited three stages of oxidation present at different temperatures. The initial oxide layer was nearly Co-free, beginning at 3 nm at RT, and Co appeared to oxidize separately from the iron. Iron cations were determined to be the mobile species during oxidation, yielding a progressively more Co-rich core as oxidation progressed, decreasing from an average diameter of 42 to 25 nm, while the oxide tripled in size. At 350 °C, the particles exhibited a less dense core, and at 900 °C the particles were observed to have changed morphology completely with some sintering, forming completely oxidized particles with an average diameter three times the as received size. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles (MNPs) possessing low Curie temperatures (TC's) offer the possibility for self-regulated heating of cancer cells, where the TC acts as an upper limit to heating to prevent damage to neighboring healthy tissue. We report the synthesis of monodisperse metastable fcc γ-phase Fe-Ni MNPs possessing tunable TC's, whose stoichiometries have been predicted from metastable extensions to an equilibrium Fe-Ni phase diagram. Metastable alloys have been predicted within the constructs of the T0 construction in the Fe-Ni eutectoid phase diagram. Fe-Ni MNPs have been synthesized via chemical reduction in Fe- and Ni-precursors with stoichiometries ranging from Fe90Ni10 to Fe70Ni30. Mn-precursors have been added to further reduce the alloy's TC. MNP morphology and structure have been confirmed by x-ray diffraction and transmission electron microscopy while magnetic properties were investigated using vibrating sample magnetometry. Fe70Ni30 MNPs were found to have a TC of 82 °C and specific magnetization of 66 emu/g. Addition of 1 wt % Mn to Fe75Ni25 reduced the TC to 78 °C, which is the lowest reported for fcc Fe-Ni alloys. We also report a model for radio frequency self-regulated heating, in which the maximum achievable temperature of water-MNP suspensions (TC<100 °C) or octyl ether-MNP suspensions (TC>100 °C) is approximately the TC of the MNPs suspended in the solution. [ABSTRACT FROM AUTHOR]

FeCo magnetic nanoparticles (MNPs) have been investigated for curing polymer epoxy composites through radio-frequency (rf) heating. The rf response of functionalized FeCo MNPs is shown to uniformly cure the epoxy without parasitic heating for potential applications in electronic packaging. The FeCo/(Co,Fe)3O4 MNPs were synthesized using a rf induction plasma torch and were ultrasonicated in diglycidyl ether of bisphenol F epoxy to form stable ferrofluids. Transmission electron microscopy studies reveal as-synthesized MNPs to have a mean diameter of 17.6 nm with a 5.2 nm standard deviation. The mean MNP diameter is reduced to 9 nm after cryomilling and causes particles to form ∼200 nm agglomerates. Ferrofluids of varying MNP concentrations, sizes, and shapes were rf heated using a precision rf coil operated at 20.0 kA/m and 267 kHz frequency. Using a 1.36 vol % ferrofluid, the epoxy composite was effectively rf cured, reaching temperatures >100 °C in ∼70 s. The results suggest that rf heating of FeCo MNPs may provide an effective method to curing epoxy composites. [ABSTRACT FROM AUTHOR]

We report the oxidation products and qualitative rates for polydisperse FeCo magnetic nanoparticles (MNPs) synthesized using an induction plasma torch. X-ray diffraction (XRD) and TEM showed MNPs to have a thin ferrite shell. Nanopowders were isochronally annealed to promote oxidation and XRD was used to follow the evolution of the FeCo core and the Fe3O4 and FeO oxide shells. Isothermal anneals were used to follow oxidation kinetics at 350 and 500 °C. High resolution transmission electron microscopy (HRTEM) revealed faceted morphologies terminated at (100) and (110) FeCo faces with (110)FeCo∥(111)oxide and (100)FeCo∥(100)oxide, and [010]FeCo∥[011]oxide orientation relationships between the FeCo core and oxide shell. We show HRTEM images of MNP chaining and compare the rf heating of samples of aqueous ferrofluids similarly loaded with as synthesized and oxidized MNPs. [ABSTRACT FROM AUTHOR]

Magnetic nanoparticles (MNP) offer promise for local hyperthermia, thermoablative cancer therapy and microwave curing of polymers. Rosensweig's theory predicts that particle size dependence on RF magnetic heating of ferrofluids is chiefly determined by magnetic moment, magnetic anisotropy, and the viscosity of the fluid. Since relaxation times are thermally activated and material parameters can have strong T dependences, heating rates peak at a certain temperature. We extend the model to include the T dependence of the magnetization and anisotropy using mean field theory and literature reported T dependences of selected fluids considered for biomedical applications. We model materials with Curie temperatures near room temperature for which the magnetic properties are strongly T dependent to address the problem of self-regulated heating of ferrofluids. [ABSTRACT FROM AUTHOR]

Nanocrystalline (NC) ferrite powders have been synthesized using a 50 kW-3 MHz rf thermal plasma torch for high-frequency soft magnet applications. A mixed powder of Ni and Fe (Ni:Fe = 1:2), a NiFe permalloy powder with additional Fe powder (Ni:Fe = 1:2), and a NiFe permalloy powder (Ni:Fe = 1:1) were used as precursors for synthesis. Airflow into the reactor chamber was the source of oxygen for oxide formation. XRD patterns clearly show that the precursor powders were transformed into NC ferrite particles with an average particle size of 20-30 nm. SEM and TEM studies indicated that NC ferrite particles had well-defined polygonal growth forms with some exhibiting (111) faceting and many with truncated octahedral and truncated cubic shapes. The Ni content in the ferrite particles was observed to increase in going from mixed Ni and Fe to mixed permalloy and iron and finally to only permalloy starting precursor. The plasma-torch synthesized ferrite materials using exclusively the NiFe permalloy precursor had 40%-48% Ni content in the Ni-ferrite particle, differing from the NiFe[sub 2]O[sub 4] ideal stoichiometry. EXAFS was used to probe the cation coordination in low Ni magnetite species. The coercivity and Neel temperature of the high Ni content ferrite sample were 58 Oe and ∼590°C, respectively. [ABSTRACT FROM AUTHOR]

Presents a study which discussed the magnetic properties of neodymium (Nd)-iron (Fe)-boron (B)-carbon nanoparticles. Characteristics of bulk Nd-Fe-B magnets; Measurement of the monodomain particles of Nd-Fe-B-C; Determination of nanoparticle morphology.

A correction to the article "Tuning the Curie temperature in γ-FeNi nanoparticles for magnetocaloric applications by controlling the oxidation kinetics," by Huseyin Ucar and colleagues published in a 2013 issue of the "Journal of Applied Physics" is presented.

Magnetic nanoparticle (MNP)-based solder composites can facilitate solder reflow in AC magnetic fields. We compare power loss in a solder composite sample with MNP loading of different particle size distribution. Scaling of solder composite samples shows that, for smaller samples, power loss is not sufficient to realize solder reflow temperature. Additional heating can be derived from eddy current losses in Cu planes in the substrate board. Eddy current heating in Cu sheets of various thickness were evaluated to quantify their effect for low-temperature electronic assembly. [ABSTRACT FROM AUTHOR]