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IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002
The Magnetic Properties of Magnetic Nanoparticles Produced by Microwave Flash Synthesis of Ferrous Alcoholic Solutions J. C. Niepce, D. Stuerga, T. Caillot, J. P. Clerk, A. Granovsky, M. Inoue, N. Perov, G. Pourroy, and A. Radkovskaya
Abstract—Microwave heating is an emerging technologythat uses the ability of some liquids and solids to transform electromagnetic energy into heat. We present the results of experimental study of magnetic and structural properties of magnetic nanoparticles fabricated by this technique. Compared with similar nanoparticles fabricated by using a conventional heating, we obtained much smaller grain size (up to 10 nm) and very stable magnetic properties. The hysteresis loops for the samples of the nonoriented assemblies of magnetite particles have a coercive force about 100 Oe with a squreness about 0.4. The superparamagnetic fraction was found in the samples. The particles distribution on the anisotropy fields has a maximum at 200 Oe. Index Terms—Iron oxide nanoparticles, magnetic measurements, microwave heating.
I. INTRODUCTION
S
INCE the mid 1980s, numerous works have been devoted to the synthesis of various compounds by using microwave heating. Microwave heating is an emerging technology that uses the ability of some liquids and solids to transform electromagnetic energy into heat. Indeed, the specificity and the essential interest of microwave heating are the increase in the heating rate induced. Several degrees per second can be reached according to the microwave power used. Consequently, reaction rate enhancements are usually close to two or three order of magnitude (where reaction rate is enhancement defined as the ratio between the conventional reaction time and the microwave reaction time). Recently, it was shown [1] that microwave heating of ferrous alcoholic solutions is a promising technique that produces hematite nanoparticles of various shapes. The main difference with the conventional heating is that it is possible to force hydrolysis of ferric solutions (Fe3+) and obtain hematite without
Manuscript received February 14, 2002. J. C. Niepce, D. Stuerga and T. Caillot are with the Laboratoire de Recherche sur la Réactivitédes Solides UMR 5613 CNRS, Université de Bourgogne, Dijon, France (e-mail:
[email protected]). J. P. Clerk is with the IUSTI, Technopole Chateau-Gombert, University of Provence, Marseille, France (e-mai:
[email protected]). A. Granovsky, N. Perov and A. Radkovskaya are with the Faculty of Physics, Lomonosov Moscow State University, Moscow, Russia (e-mail:
[email protected]). M. Inoue is with the Depatment of Electric and Electronic Engineering, Toyohashi University of Technology, Toyohashi, Japan (e-mail:
[email protected]). G. Pourroy is with the Institut de Physique et de Chimie des Matériaux de Strasbourg Strasbourg, France (e-mail:
[email protected]). Digital Object Identifier 10.1109/TMAG.2002.801963.
production of goethite. We used an original microwave system for one step flash synthesis of magnetite and iron-magnetite by disproportion of ferrous alcoholic solutions [1]. The disproportion of ferrous hydroxide Fe OH in potassium hydroxide aqueous solution has been previously observed by using conventional heating at atmospheric pressure or in hydrothermal pressure. It leads to iron-magnetite composites when KOH M l or to Fe O for lower KOH concentrations. The powders are made of octahedral shape particles of about 1 m in size in which the metal is imbricate into the oxide. The association of a metal and an oxide in a composite structure leads to new properties. Indeed, it has been shown that composites made of a Fe-Co alloy and cobalt substituted magnetite allow the production of light olefins with a low CO rate. Moreover, they are more resistant than the classical iron based catalysts. In this paper, we present the results of measurements of the magnetic properties of magnetite nanoparticles produced by this new technique. II. EXPERIMENTAL PROCEDURE A. Magnetite Nanoparticles Preparation System We used for the synthesis an original microwave reactor called the the Reacteur Autoclave MicroOnde (RAMO) system. This experimental device is constituted of a microwave applicator associated with an autoclave. By varying the position of a plunger, the resonant frequency of the cavity can be controlled, and the effective cavity power can be increased by three orders of magnitude. The microwave generator used is a continuous wave system with a power up to 2 kW (2.45 GHz). The autoclave is made with polymer materials, which are microwave transparent, chemically inert, and sufficiently strong to accommodate the pressure induced. The reactants are placed in a Teflon flask inserted within a polyetherimide flask. A fiber-optic thermometry system, a pressure transducer, and a manometer allow simultaneous measuring of the temperature and the pressure within the reactor. The system is controlled by pressure. We adjust the microwave power to allow constant pressure within the vessel. A pressure release valve incorporated permits use of this experimental device routinely and safely. Furthermore, we can introduce an inert gas such as argon within the reactor to avoid sparking risk with flammable solvents. This experimental device is able to raise the temperature from ambient to 200 C in less than 20 s (the pressure is close to 1.2 MPa and the heating rate is close to 5 /s).
0018-9464/02$17.00 © 2002 IEEE
NIEPCE et al.: MAGNETIC PROPERTIES OF MAGNETIC NANOPARTICLES
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B. Operating Conditions H O All the chemical products: ferrous chloride (FeCl Prolabo, Normapur), sodium ethoxide (EtONa, Aldrich, 96%), and ethanol (Prolabo, Normapur) were reagent grade used without further purification. The solution of sodium ethoxide in ethanol was prepared at room temperature and poured into the reactor. In order to avoid oxidation, ferrous chloride was dissolved in ethanol just before its use (orange solution). This solution was rapidly poured into the solution of sodium ethoxide. A green precipitate appears. The reactor was quickly sealed, and argon pressure was introduced (0.4 MPa). Treatments were performed with RAMO. The treatment was decomposed in two steps. During the first step, the microwave power (1 kW) was applied until the pressure reaches a threshold value of 1 MPa. This pressure corresponds to a temperature close to 160 C. During the second step, this pressure threshold was kept by monitoring the microwave power. The heating rate is close to 10 C/s. Heating times were between 5 s and 30 min. After this treatment, powders were centrifuged and washed with distilled water in order to eliminate sodium and chloride ions. Finally, they were dried at room temperature. X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDXS) were used to study the microstructure, to determine individual grain compositions, and to identify phases [1]. The samples were prepared by depositing the powder dispersed into ethanol onto a holey carbon-coated 200 mesh copper grid. Compared with similar nanoparticles fabricated by using a conventional heating, we obtained much lower grain size (up to 10 nm) and very stable properties.
Fig. 1. Hysteresis loops for the powder sample (solid line) and the film sample (dot line).
C. Magnetic Measurements The magnetic measurements were performed with a vibrating sample anisometer, which is a modification of the vibrating sample magnetometer for the determination of the magnetic moment orientation [2]. Two kinds of samples were prepared: powder, capsulated into the Teflon box with a size 8 8 0.3 mm, and a thin film made on the ceramic substrate from the mixture of powder and a polymeric glue BF-2. The film polymerization was performed in the magnetic field (10 kOe) at the room temperature. The film sample had an 8–mm diameter and a thickness of about 0.1 mm. To obtain the particles distribution on the anisotropy fields (the relative amount of the particles with corresponding magnetic anisotropy field), the following procedure was used. The sample was saturated in the magnetic field of 8 kOe. Then, the magnetic field was switched off, the sample was rotated to the angle of 15 , and the remanent magnetic moment orientaOe tion was measured. After that, the magnetic field was applied and switched off again. The new orientation of the remanent magnetic moment was measured and as a result the was determined. The change of change of the orientation the magnetic moment orientation occurred because of after the sample rotation the angle between the magnetic moments of some of the particles and the external magnetic field became , and these particles were remagnetized larger than if their anisotropy fields were less than the applied magnetic
Fig. 2. Hysteresis loop of the powder sample in the magnetic field up to 8 kOe.
field value. The next step was performed with the magnetic field , and the corresponding angle was detervalue mined. The measurements were repeated until the orientation of the magnetic moment became parallel to the magnetic field. was used to calculate the particles disThe dependence tribution on the anisotropy fields. III. RESULTS AND DISCUSSION The hysteresis loops for both samples made of hematite particles had a coercive force about 130 Oe, and a ratio of the remato the saturation one was about nence magnetization (see Fig. 1). There were no saturation of the magnetization up to the magnetic field about 8 kOe (see Fig. 2). Therefore, we suppose that the large quantity of superparamagnetic fraction exists in our samples as follows from the shape of the hysteresis loops at high fields. The particles distribution on the anisotropy fields exhibits some interesting peculiarities (see Fig. 3). First, its maximum
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by traditional thermal processes. Further work is required to improve the characterization of the nanocomposites produced, especially magnetic properties. Consequently, the microwave heating appears to provide an efficient source of energy in producing magnetite nanoparticles. The current interest in the use of microwaves is therefore associated with the attainment of new or improved properties as much as for purely economic benefit. This interest has occurred with the steadily growing applications of nanostructured materials. However, in many cases, applications of nanostructured materials cannot be fully exploited because of the restrictions imposed by the absence of suitable processing routes [3]. The RAMO system is a batch system. It could be easily transposed to a continuous process with an industrial scale (several hundred kilograms per second). The possibility of improvement of the magnetic properties is under investigation. Fig. 3.
Distribution on the anisotropy field for the film sample.
is in the rather narrow range around 200 Oe. Second, there is a wide plateau from 500 to 1300 Oe. It confirms that particles with large anisotropy exist in the powder. IV. CONCLUSION The magnetite nanoparticles produced by microwave heating have smaller overall size compared with the particles made
REFERENCES [1] T. Caillot, D. Stuerga, and G. Pourroy, “Microwave flash synthesis of iron and magnetic particles by disproportion of ferrous alcoholic solutions,” J. Mater. Sci., 2002, to be published. [2] N. Perov and A. Radkovskaya, “A vibrating sample anisometer,” in Proc. 6th Int. Workshop 1-D 2-D Magn. Meas. Testing, 2002, pp. 104–108. [3] G. Herzer, “Nanocrystalline soft magnetic materials,” in Nanomagn., A. Hernando, Ed. Boston, MA: Kluwer, 1993, pp. 111–126.
