IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001 2651
Magnetic and Optical Properties of Fe
3
O
4
Nanoparticle Ferrofluids Prepared by Coprecipitation
Technique
K. T. Wu, P. C. Kuo, Y. D. Yao, and E. H. Tsai
Abstract—Nanometer size Fe3O4particles were fabricated by chemical coprecipitation technique. The particle shape and size are affected by the PH value of the reactive solutions. The ferrofluids were fabricated with the Fe3O4 particles as magnetic particles, ammonium oleate as surfactant, and de-ionized water as solvent. Optical transmission of Fe3O4nanoparticle ferrofluids was inves-tigated as a function of incident optical wavelengths between 450 and 750 nm and applied magnetic fields up to 150 Oe. In general, for samples with lower PH value, the particles are small and ag-glomerate together, however, for samples with higher PH value, the particles are larger and distributed uniformly. Samples precip-itated with higher PH value show larger variation of the transmit-tance(1 ). This can be understood by our transmission electron microscope and X-ray diffraction pattern studies. The behavior of agglomeration for samples with lower PH value relates to the smaller variation of the optical transmission to the magnetic field. Index Terms—Chemical coprecipitation, Fe3O4, nanoparticle ferrofluid.
I. INTRODUCTION
I
N RECENT years, research associated with magnetic fluids have received wide attention, and show a rich variety and complexity in physical phenomena [1]–[9]. Ferrofluids have been extensively applied to audio voice coil-damping, inertia-damping apparatuses, bearings, stepping motors, and vacuum seals etc. [10]. There are many chemical methods to prepare magnetic fluid. In this investigation, we successfully fabricated nanometer size Fe O ferrofluids by a chemical coprecipitation technique, and report the magnetic and optical properties of this fluid.II. EXPERIMENTS
Fe O nanoparticles were prepared by a chemical coprecip-itation technique. The starting materials were FeCl , FeCl , and NaOH. The FeCl and FeCl solutions were prepared by adding FeCl 4H O and FeCl 7H O, into de-ionized water and stirring to complete dissolution. The NaOH solution is prepared by dissolving NaOH into de-ionized water. These solutions prepared with various concentrations were mixed
Manuscript received October 27, 2000.
This work was supported in part by the Sapintia Education Foundation. K. T. Wu and E. H. Tsai are with the Department of Physics, Fu Jen University, Taipei 242, Taiwan.
P. C. Kuo is with the Institute of Mat. Sci. and Eng., National Taiwan Univ., Taipei 107, Taiwan.
Y. D. Yao is with the Institute of Physics, Academia Sinica, Taipei 115, Taiwan (e-mail: [email protected]).
Publisher Item Identifier S 0018-9464(01)07201-6.
together by stirring. The reaction temperature is kept at 70 C and the reaction time is one hour [11]. The final PH values of these mixed solution were varied between 12 and 14. The chemical reaction can be expressed as:
FeCl FeCl NaOH Fe O NaCl H O
The NaCl is separated from the precipitant of this reaction by washing and centrifuging it with de-ionized water several times leaving Fe O . The Fe O nanoparticle ferrofluids were fab-ricated using nanometer size Fe O particles as magnetic par-ticles, ammonium oleate as surfactant, and de-ionized water as solvent. The weight ratio of (magnetic particle : ammonium oleate : de-ionized water) is 2.0 : 0.6 : 97.4.
The crystal structure of the particles is investigated by x-ray diffraction. The particle shape, size, and morphology are ob-served by a transmission electron microscope (TEM). The op-tical transmission of the ferrofluids has been investigated as a function of incident optical wavelengths between 450 and 750 nm and applied magnetic fields up to 150 Oe. The mag-netic field is parallel to the incident light.
III. RESULTS ANDDISCUSSION
The quality of the precipitated Fe O nanoparticles is mostly affected by the reaction temperature and the PH value of the reactive solution. According to [11], the reaction temperature affects the particle size, phases, and reaction time. For temper-atures below roughly 60 C, the final precipitates will be dom-inated by either -FeOOH or -FeOOH. The particle shape of -FeOOH is long and rodlike, and its color is yellow; for -FeOOH particle, its shape is lath and vein-like, and its color is orange. For temperatures above roughly 60 C, the final pre-cipitates are dominated by Fe O , its shape is granular and its color is black. In this study, the reaction temperature is always kept at 70 C in order to get good quality of Fe O particles. The x-ray diffraction pattern of the final precipitates is shown in Fig. 1. From this figure it is clear, single phase Fe O for all the final precipitates.
We find that samples with PH 13.9, the particles are smaller and agglomerated, however, for samples with PH 13.9, the particles will become spherical and larger but still in the nanometer range with uniform distribution. The satu-ration magnetization of these Fe O nanoparticles is roughly 52 emu/g. Fig. 2 shows the transmission electron microscopy (TEM) image of the Fe O nanoparticles for samples with 0018–9464/01$10.00 © 2001 IEEE
2652 IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001
Fig. 1. X-ray diffraction patterns of the final precipitates.
(a)
(b)
Fig. 2. TEM image of Fe O nanoparticles for samples with (a) PH= 12.49 and (b) PH= 13.98.
(a) PH 12.49, and (b) PH 13.98. It shows clearly that sam-ples with higher PH value exhibit better distribution and with particles in spherical shape. Optical transmission of the Fe O nanoparticle ferrofluid sealed in a 4 cm 1 cm rectangular glass cell with a thickness of 100 um has been investigated as a function of incident optical wavelengths between 450 and 750 nm and applied magnetic fields up to 150 Oe. In general, the optical transmission of a Fe O ferrofluid increases with
(a)
(b)
Fig. 3. 1T versus wavelength for samples with (a) PH = 12.49 and (b) PH
= 13.98.
increasing wavelength of incident visible light between 450 and 750 nm.
Fig. 3 shows the variation of the transmittance as a func-tion of wavelength with an applied magnetic field of 1.6, 2.4, 3.6, 5.7, 7.1, 13.2, 19.0, and 150 Oe for samples with (a) PH 12.49 and (b) PH 13.98.
In general, samples precipitated with higher PH value show larger variation of the transmittance . Our experimental data show that is almost the same for applied magnetic field increasing from 23 to 150 Oe. In Fig. 3(a), varies from roughly 0.5 to 2.7% with increasing incident wave length from 450 to 750 nm for samples with PH value of 12.49 under mag-netic field of 150 Oe. From Fig. 3(b), varies from roughly 6.3 to 10.4% with increasing incident wave length from 450 to 750 nm for samples with PH value of 13.98 under magnetic field of 150 Oe. This can be understood by our X-ray diffraction pat-tern and transmission electron microscope studies; i.e., the be-havior of agglomeration for samples with lower PH value relates to the smaller variation of the optical transmission to the applied magnetic field.
The variations of the transmittance of the samples as a func-tion of the applied field with incident wavelength at 600 nm are depicted in Fig. 4. The increases rapidly for samples with PH 13.9. This is related to the homogeneous separation and spherical shape of the nanometer size magnetic particles. How-ever, the variation of is very small for samples with PH 13.9. Below roughly 20 Oe, increases with increasing ap-plied magnetic field; but, is almost the same for magnetic
WU et al.: MAGNETIC AND OPTICAL PROPERTIES OF Fe O NANOPARTICLE FERROFLUIDS 2653
Fig. 4. The variation of the transmittance of the samples as a function of the applied field with incident wavelength at 600 nm.
fields between roughly 20 and 150 Oe, so that it is not plotted in Fig. 4.
In summary, nanometer size Fe O ferrofluids have been suc-cessfully fabricated by chemical coprecipitation technique. Op-tical transmission of Fe O nanoparticle ferrofluids have been investigated as a function of incident optical wavelengths be-tween 450 and 750 nm and applied magnetic fields up to 150 Oe.
For samples with lower PH value, the particles are small and ag-glomerate together, however, for samples with higher PH value, the particles are larger and distributed quite uniformly. Sam-ples precipitated with higher PH value show larger variation of the transmittance . These magnetic and optical properties of nanoparticle ferrofluids may present some potential applica-tions, such as magnetic induced optical sensors.
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