Dustin M. Clifford

Experimental evidence for the formation of CoFe2C phase with colossal magnetocrystalline-anisotropy

Attainment of magnetic order in nanoparticles at room temperature is an issue of critical importance for many different technologies. For ordinary ferromagnetic materials, a reduction in size leads to decreased magnetic anisotropy and results in superparamagnetic relaxations. If, instead, anisotropy could be enhanced at reduced particle sizes, then it would be possible to attain stable magnetic order at room temperature. Herein, we provide experimental evidence substantiating the synthesis of a cobalt iron carbide phase (CoFe2C) of nanoparticles. Structural characterization of the CoFe2C carbide phase was performed by transmission electron microscopy, electron diffraction and energy electron spectroscopy. X-ray diffraction was also performed as a complimentary analysis. Magnetic characterization of the carbide phase revealed a blocking temperature, TB, of 790 K for particles with a domain size as small as 5 ± 1 nm. The particles have magnetocrystalline anisotropy of 4.6 ± 2 × 106 J/m3, which is ten times ...

Synthesis of FeCo alloy magnetically aligned linear chains by the polyol process: structural and magnetic characterization

FeCo alloy magnetically aligned linear chains (MALCs) were synthesized by the polyol process using a unique reaction format departing from conventional bench-top reactions. FeCo MALC morphology was obtained in the presence of an external dynamic magnetic field. XRD analysis confirmed the body-centered cubic (bcc) FeCo alloy. Transverse and tangential to the incident X-ray path, XRD analyses of as-synthesized FeCo MALCs were performed further substantiating the linear chain morphology. The average chain diameter was ≈206 ± 52 nm while Scherrer analysis of (110) gave an average crystallite size of 29 nm. Chemical attachment of each aligned FeCo alloy segment was confirmed by tangential cross-sectioning by a focused ion beam (FIB). Electron diffraction confirmed the bcc FeCo phase with d-spacings of (110), (200) and (211) in good agreement with XRD. The as-synthesized FeCo alloy MALCs possessed a saturation magnetization value, Ms, of 205 emu g−1 and a coercivity, Hc, of 150 Oe at 300 K. Magnetization was recorded from 300 to 1000 K with increasing temperatures correlating with transitions from hcp Co/α1 to fcc Co/α1 at 500 K and to α1 at 600 K for Co rich portions of the MALCs (Fe32Co68). Annealed (1000 K) FeCo MALCs possessed an Ms of 212 emu g−1 with a Hc value of 120 Oe. Morphological changes of the 1000 K annealed sample were microwire formation and the secondary phase of cubic FeO. As-synthesized FeCo alloy MALCs as highly magnetic, air stable, non-tethered and high aspect ratio structures are candidates for radar absorbent materials (RAMs) when magnetically oriented in coatings or for magnetic sensing devices.

Room Temperature Synthesis of Highly Magnetic Cobalt Nanoparticles by Continuous Flow in a Microfluidic Reactor

Cobalt nanoparticles were synthesized using continuous-flow (CF) chemistry in a stainless steel microreactor for the first time at high output based on the ethanol hydrazine alkaline system (EHAS) producing a yield as high as 1 g per hour [1, 2]. Continuous-flow (CF) synthetic chemistry provides uninterrupted product formation allowing for advantages including decreased preparation time, improved product quality, and greater efficiency. This successful synthetic framework in continuous-flow of magnetic Co nanoparticles indicates feasibility for scaled-up production. The average particle size by transmission electron microscopy (TEM) of the as-synthesized cobalt was 30±10 nm, average crystallite size by Scherrer analysis (fcc phase) was 15±2 nm, and the estimated magnetic core size was 6±1 nm. Elemental surface analysis (X-ray photoelectron spectroscopy [XPS]) indicates a thin CoO surface layer. Assynthesized cobalt nanoparticles possessed a saturation magnetization (Ms) of 125±1 emu/g and coercivity (Hc) of 120±5 Oe. The actual Ms is expected to be greater since the as-synthesized cobalt mass was not weight-corrected (nonmagnetic mass: reaction by-products, solvent, etc.). Our novel high-output, continuous-flow production (>1 g/hr) of highly magnetic cobalt nanoparticles opens an avenue toward industrial-scale production of several other single element magnetic nanomaterials.

Solvothermal synthesis of Fe7C3 and Fe3C nanostructures with phase and morphology control

A phase transition, from orthorhombic Fe3C to hexagonal Fe7C3, was observed using a wet synthesis mediated by hexadecyltrimethylammonium chloride (CTAC). In this study, CTAC has been shown to control carbide phase, morphology, and size of the iron carbide nanostructures. Fe7C3 hexagonal prisms were formed with an average diameter of 960 nm, the thickness of 150 nm, and Fe3C nanostructures with an approximate size of 50 nm. Magnetic studies show ferromagnetic behavior with Ms of 126 emu/g, and Hc of 170 Oe with respect to Fe7C3 and 95 emu/g and 590 Oe with respect to Fe3C. The thermal studies using high temperature x-ray diffraction show stability of Fe7C3 up to 500 °C. Upon slow cooling, the Fe7C3 phase is recovered with an intermediate oxide phase occurring around 300 °C. This study has demonstrated a simple route in synthesizing iron carbides for an in depth magnetic study and crystal phase transition study of Fe7C3 at elevated temperatures.