Junming Li

Investigation into loss in ferrofluid magnetization

Ferrofluids containing γ-Fe2O3/Ni2O3 nanoparticles (not chemically treated) were synthesized using water and mixed water–glycerol as carrier liquid and the ferrofluid viscosity was modified by varying the glycerol content in the carrier liquid. The apparent magnetization of the ferrofluids decreased with increasing glycerol content. The loss in magnetization is described by the ratio of effective magnetic volume fraction to physical volume fraction of nanoparticles in the ferrofluids as a characteristic parameter. We ascribe the loss to the formation of “dead aggregates” having a ring-like structure of closed magnetic flux rather than to any chemical reaction. Such dead aggregates exist in zero magnetic field and do not contribute to the magnetization in the low or high field regime, so that the effective magnetic volume fraction in the ferrofluids decrease. An increase in carrier liquid viscosity is similar to a weakening of the thermal effect, so the number of dead aggregates increases and the magnetiza...

Preparation and Characterization of a Magnetic Binary System Comprise of γ-Fe2O3 and α-FeO(OH) Nanoparticles

Magnetic nanoparticles were prepared from a low crystalline ferrihydrite (Fe5O7(OH)·4H2O) precursor by a chemically-induced transformation method using mixed FeCl2/NaOH solution. The products obtained were characterized using a vibrating sample magnetometer, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. Experimental results show that while the concentration of NaOH was 0.17 mol/L, and the FeCl2 concentration was increased from 0.09 mol/L to 0.42 mol/L in the mixed solution, the as-prepared products were binary nanoparticle systems comprised of ferrimagnetic γ-Fe2O3 sphere-type particles and antiferromagnetic α-FeO(OH) rod-type particles. The resulting particles were highly crystalline. The volume ratio of γ-Fe2O3 and α-FeO(OH) particles was estimated from magnetization data, which showed that the volume of γ-Fe2O3 particles decreased and α-FeO(OH) particles increased with increasing FeCl2 concentration. Such nanoparticle systems could be suitable for synthesis of binary ferrofluids, which have different behavior to conventional ferrofluids.

Transformation of ferrihydrite to goethite and then to maghemite in a mixed FeCl2/NaOH solution

Ferrihydrite (Fe5O7(OH)·4H2O) is poorly crystalline hydrated ferric oxyhydroxide. Using a treatment in a mixed FeCl2/NaOH solution, we attempted to prepare magnetic nanoscale particles from ferrihydrite. During the treatment, the concentration of FeCl2 was fixed at 0.1 M, and the concentration of NaOH was varied from 0.09 to 0.26 M. The as-prepared products were characterized using X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. The experimental results showed that under the FeCl2/NaOH solution treatment, the ferrihydrite transformed into nanoscale γ-Fe2O3 via an α-FeO(OH) intermediate phase. The as-prepared products contained rod-type α-FeO(OH), sphere-type γ-Fe2O3, and flake-type γ-Fe2O3 particles. A mechanism for a coupling of the aggregation and the transition was proposed to interpret the formation of highly crystalline nanoscale particles from the poorly crystalline smaller particles.

Characterization of Co-Modified γ-Fe2O3 Based Composite Nanoparticles

During the synthesis of γ-Fe2O3 nanoparticles using a chemically-induced transition in a FeCl2 solution, Co-surface modification was attempted by adding Co (NO3)2 and NaOH to the solution. The magnetization behaviors, morphologies, crystal structure, and chemical compositions of the as-prepared samples were characterized using vibrating sample magnetometry, transmission electron microscopy, X-ray diffractometry, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The as-prepared particles consisted of γ-Fe2O3/CoFe2O4 composite crystallite and a CoCl2·6H2O coating. The molar, mass and volume ratios of the phases were estimated from the characterization results for each sample. The Co-modified γ-Fe2O3 nanoparticles’ anisotropic constant is approximately 1.48×10-1 J/cm3. Their coercivity depends on the size of composite crystallites, which is based on the γ-Fe2O3/CoFe2O4 content rather than the Co content.

The extrinsic hysteresis behavior of dilute binary ferrofluids

Abstract.We report on the magnetization behavior of dilute binary ferrofluids based on

Preparation and Characterization of γ-Fe2O3-ZnFe2O4 Composite Nanoparticles

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Characterization and magnetism of Co-modified γ-Fe2O3 core–shell nanoparticles by enhancement using NaOH

- Fe2O3/Ni2O3 composite nanoparticles (A particles), with diameter about 11nm, and ferrihydrite ( Fe5O7(OH)·4H2O nanoparticles (B particles), with diameter about 6 nm. The results show that for the binary ferrofluids with A-particle volume fraction

Modification using additional NaOH for the preparation of γ-Fe 2 O 3 nanoparticles by chemically induced transition

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Preparation of γ-Fe2O3/ZnFe2O4 nanoparticles by enhancement of surface modification with NaOH

= 0.2 % and B-particle volume fractions

Formation of highly crystalline maghemite nanoparticles from ferrihydrite in the liquid phase

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