Preparation and Properties of ε-Fe3N-Based Magnetic Fluid
© to the authors 2008
Received: 17 May 2008
Accepted: 11 July 2008
Published: 25 July 2008
In this work, ε-Fe3N nanoparticles and ε-Fe3N-based magnetic fluid were synthesized by chemical reaction of iron carbonyl and ammonia gas. The size of ε-Fe3N nanoparticles was tested by TEM and XRD. Stable ε-Fe3N-based magnetic fluid was prepared by controlling the proper ratio of carrier liquid and surfactant. The saturation magnetization of stable ε-Fe3N-based magnetic fluid was calculated according to the volume fraction of the particles in the fluid. The result shows that both the calculated and measured magnetizations increase by increasing the particle concentration. With the increasing concentration of the ε-Fe3N particles, the measured value of the magnetic fluid magnetization gradually departs from the calculated magnetization, which was caused by agglomeration affects due to large volume fraction and large particle size.
Magnetic fluids or ferrofluids, are comprised of magnetic nanoparticles stabilized by coating surfactants and dispersed in various media, most notably hydrocarbons, esters or water [1–5]. The stability of magnetic fluid depends upon a balance between repulsive and attractive interactions among nanoparticles . Besides the thermal motion, the steric and electrostatic repulsive interactions are against Van der Waals and dipolar attractive interactions . The magnetic properties of magnetic fluids depend strongly on the size of the particles and the concentration of the magnetic material in the dispersion. In the presence of a magnetic field, the magnetic moment of the particles will try to align with the magnetic field direction leading to a macroscopic magnetization of the fluid. When the external field is removed, the particles quickly randomize the directions of their magnetic moment and the fluid loses its magnetism.
The study and application of magnetic fluids were invented in the mid-1960s, involving the multidisciplinary sciences such as chemistry, fluid mechanics, and magnetism. With modern advances in understanding nanoscale systems, current research focuses on synthesis, characterization, and functionalization of magnetic fluids. Iron oxide fluids in hydrophobic media are now used industrially for rotating shaft seals, loudspeaker coils, and in various magnetically promoted separations .
However, the magnetic properties of iron oxide-based fluids are not sufficient for a number of purposes. Many efforts have been devoted to make magnetic fluid with higher magnetization. A variety of techniques have been developed to fabricate magnetic fluid using metal particles such as spark erosion  and vacuum evaporation . One of the major difficulties encountered is that the magnetization of the metal particles decays with time, due to the lack of oxidization resistance in the ambient environment. According to Nakatani et al. , the iron-nitride compounds are ferromagnetic with higher magnetization than iron oxide (Fe3O4) and are chemically stable. They are metallic with a close packed lattice of iron atoms whose interstices are occupied by nitrogen atoms. It is considered that there exists covalent bonding between the iron atoms and the nitrogen atoms. Thus, fine particles of ferromagnetic iron-nitrides have a potential application to magnetic fluids that need a high magnetization and stability against oxidation.
In this article, single-phase ε-Fe3N-based magnetic fluid and nanometer powders were analyzed. In the magnetic fluids, the nanosized ε-Fe3N powders coated by surfactants were dispersed in carrier liquid. The assays performed for the preparation, stability, and evaluation of the magnetic fluids are discussed in the following sections.
The surfactant content in five magnetic samples (FN001–FN005)
Carrier (PAO4) mL
Surfactant (succinamide) mL
X’Pert PRO (Panalytical) X-ray diffractometer was used to analyze the phase composition of magnetic particles. The diffraction was performed with Cokα1(λ = 1.7889 Å) and the ray was filtered by the graphite. The experimental parameters used were: 40 mA, 35 kV, continuous scan, scan speed 2°/min. Particle sizes of iron-nitride coated with surfactant were measured with a 2,100 fx transmission electron microscope (TEM) operated at 200 keV. TEM samples were prepared by dispersing the particles in alcohol using ultrasonic excitation, and then transferring the nanoparticles on the carbon films supported by copper grids. The density of the magnetic fluid was measured using a picnometer at 20 ± 1 °C. The sedimentation stability of the magnetic fluid was evaluated by changing the ratio of the synthetic oil (PAO4) and surfactant. After obtaining stable magnetic fluid, the magnetic properties of the magnetic powders and the magnetic fluids were measured immediately after preparation using a LDJ9500 vibrating sample magnetometer (VSM). Samples were contained in a small glass cup with internal dimensions of 2 × 2 × 2 mm3, which were sealed by gluing a small cover glass over the open end. Then the glass cup was put in the magnetic uniform area of the pole. The hysteresis curve was recorded per 1.5 s. All measurements were performed at room temperature. The relationship between the calculated/measured saturation magnetization and particle content is also discussed.
Results and Discussion
Phase Structure and Particle Size
The Stability of the Magnetic Fluids
The stability is one of the most important properties for magnetic fluid and it will strongly affect the service life. The well-known magnetite magnetic fluids have a good stability. Here the stability of ε-Fe3N-based magnetic fluid was investigated.
In this part, five ε-Fe3N magnetic fluid samples were prepared and all the reaction conditions were the same except the content of the surfactant in the carrier liquid given in Table 1.
where S% expresses the suspension percentage of the magnetic particles in fluid.
In this part, five stable ε-Fe3N magnetic fluids with different particle content were prepared using 80-mL synthetic oil (PAO4) and 20-mL surfactant. Pure ε-Fe3N particles were also prepared using synthetic oil (PAO4) only.
where σsand σ are the magnetization of the dispersed particles and magnetic fluid, respectively.
In this work, the size of ε-Fe3N nanoparticles was tested by TEM and XRD. Compared with the results, both of the methods are suitable to particle size testing within the experimental error. The amount of surfactant in the fluid seriously affects the stability of magnetic fluid. Stable ε-Fe3N-based magnetic fluid was prepared by controlling the ratio of carrier liquid and the optimized volume ratio for synthetic oil (PAO4) and surfactant was 4:1. Both the measured and calculated magnetizations increase with increasing particle fraction in the fluid. With the increasing concentration of the ε-Fe3N particles, the measured values gradually depart from the calculated magnetization, which is caused by the volume fraction and size of the particle agglomeration.
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