JP2007211288A - Method for producing metal magnetic particulate and metal magnetic particulate produced by using the method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To develop a new particle synthesis method with which reduction reaction using a boron hydride compound is performed in a nonaqueous (organic) solvent, and coarsening of particle sizes can be suppressed. <P>SOLUTION: A means by which, for dissolving a water soluble boron hydride compound into an organic solvent, it is temporarily mixed into an organic solvent having polarity similarly to water and is uniformized by ultrasonic wave irradiation, and is thereafter mixed into a reaction solution comprising metal ions, is used. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for producing metal magnetic fine particles having high saturation magnetization and excellent oxidation resistance.

  Magnetic fine particles uniformly dispersed in a solvent can be controlled by an external magnetic field, and are being applied and studied as carriers for magnetic functional fluids (magnetic fluids, magnetorheological fluids, etc.) or magnetic separation / drug delivery. . Conventionally, iron oxide (magnetite) having excellent chemical stability has been mainly used as a material for such magnetic fine particles. There are no major problems that require particular difficulty in producing these iron oxide fine particles, and many production examples utilizing chemical reduction reactions in aqueous solutions have been reported so far [Non-patent Document 1].

  On the other hand, from the viewpoint of application, it is desired to improve the responsiveness of magnetic fine particles to the applied magnetic field, and it is considered to use a magnetic metal (such as iron or cobalt) having a large saturation magnetization instead of iron oxide. Has been. However, these transition metal ions are difficult to reduce by ordinary chemical methods, and the generated metal particles have a problem that they are difficult to maintain stably because they are weak in an oxidizing atmosphere. Conventionally, a powerful reducing agent such as a boron hydride compound has been used as an effective means for reducing a transition metal ion which is difficult to reduce. For example, a production example of metal magnetic fine particles of Fe or Co has been reported by a reverse micelle method using a reaction in minute water droplets dispersed in an organic phase [Non-patent Documents 2-4].

On the other hand, an effective means for preventing oxidation of the generated particles is to perform the reaction in a non-aqueous solvent that does not involve oxygen or water. A typical method is a polyol method. This method synthesizes particles by using a polyhydric alcohol (such as ethylene glycol), which is a weak reducing agent, as a solvent, a reducing agent, and a surfactant. An example of preparation of the above has been reported [Non-Patent Document 5].
Masaki Abe, Nobuhiro Matsushita: Journal of the Japan Society of Applied Magnetics 27, p.721-729 (2003). “Microwave / nanobio applications of ferrite thin films and ultrafine particles prepared in aqueous solution” JP Chen et al., J. Appl. Phys. 76, p.6316-6318 (1994). “Magnetic properties of nanophase cobalt particles synthesize in inversed micelles” MP Pileni, J. Phys. Chem. B 105, p.3358-3371 (2001). “Magnetic properties of nanophase cobalt particles synthesize in inversed micelles” W. Zhou et al., J. Solid State Chem. 159, p.26-31 (2001). “Gold-coated iron (Fe @ Au) nanoparticles: Synthesis, characterization, and magnetic field-induced self-assembly” T. Hinotsu et al., J. Appl. Phys. 95, p.7477-7479 (2004). “Size and structure control of magnetic nanoparticles by using a modified polyol process”

    As mentioned above, the production of metal magnetic fine particles is desired from a practical point of view, and several methods for producing them have been developed. However, efficient production of particles having the conditions described above is not possible. It is a difficult state. The first problem is that although the reducing power of boron hydride compounds is strong, the particle synthesis reaction must be carried out in an aqueous phase because of its water solubility. Therefore, water intervenes in the reaction, which is problematic from the viewpoint of preventing oxidation of the generated particles. On the other hand, the polyol method does not require the use of water, but it effectively suppresses the fusion of the particles that are being produced because the adsorption force of the polyol molecules that also serve as a surfactant to the particle surface is weak. Cannot be produced, and the particles are likely to become coarse.

  In view of the above, it is necessary to develop a new particle synthesis method capable of performing a reduction reaction using a boron hydride compound in a non-aqueous (organic) solvent and suppressing the coarsening of the particle size.

  In order to dissolve the water-soluble boron hydride compound in an organic solvent, it is once mixed in an organic solvent with polarity similar to water and homogenized by ultrasonic irradiation, and then mixed in a reaction solution containing metal ions. I used the technique to do. Specifically, oleylamine or oleic acid was used as a polar organic solvent for dissolving the boron hydride compound. These solvents also function as surfactants, and have a function of effectively adsorbing to the surface of the transition metal particles and effectively suppressing the fusion of the particles.

  In the particle synthesis method used in the present invention, a metal fine particle is obtained by reducing a target metal ion in an organic solvent using a boron hydride compound (sodium borohydride, potassium borohydride, etc.) having a strong reducing power. Form. Since no water or air that causes oxidation is present during the reaction, it is possible to synthesize transition metal particles that are weak in an oxidizing atmosphere.

  Since boron hydride compounds are ionic substances, they dissociate in polar solvents and are relatively easy to dissolve. Utilizing this property, once mixed in a polar organic solvent such as oleylamine or oleic acid, homogenized by ultrasonic irradiation, and then mixed in a nonpolar organic solvent that is the main component of the reaction solution. The solubility of boron hydride compounds in nonpolar organic solvents was increased.

  A polar organic solvent such as oleylamine or oleic acid not only serves as a solvent for dissolving the reducing agent but also serves as a surfactant. Since these molecules have long alkyl chains, when adsorbed on the particle surface, they have a feature of effectively suppressing the fusion of particles due to steric hindrance.

  Using the above three methods, we succeeded in realizing the use of boron hydride compounds in organic solvents and suppressing grain coarsening.

  In the presence of water, the oxidation reaction of iron is accelerated and it is difficult to produce metallic iron particles. If the method described in the present invention is used, the above-mentioned problem is solved because the synthesis reaction is performed in an organic liquid phase excluding water.

  In addition, boron particles are mixed in the generated particles due to the boron hydride compound as a reducing agent, and iron fine particles having much better oxidation resistance than pure iron can be synthesized.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings.

FIG. 1 shows a flowchart of a specific particle synthesis procedure. The reagents used are as follows.
-Fe atom supply source: Iron (III) acetylacetonate (hereinafter referred to as Fe (acac) ₃ ; purity 98%)
・ Reducing agent: Sodium borohydride (purity 90%)
・ Surfactant: Oleic acid (purity 60%) alone, oleylamine (purity 70%) alone, or a mixture of oleic acid and oleylamine ・ Solvent: Dioctyl ether (purity 99%)

The particle synthesis method under typical conditions is described below. First, weigh Fe (acac) ₃ (0.5 mmol), surfactant (0.5-5 mmol), and dioctyl ether (20 mL) from the above reagents and fill the flask (300 mL). Here, as the surfactant, oleic acid alone, oleylamine alone, or a mixture of these two is used. Then, heat and stir the mixed solution at 150 ° C for 60 minutes with a mantle heater while replacing the inside of the flask with inert (argon) gas. This operation removes dissolved oxygen and water from the solution. Separately, add sodium borohydride to the same type of surfactant (0.5-5 mmol) as described above in a small vial, and prepare a solution uniformly dissolved by ultrasonic irradiation for about 10 minutes. To do. Using a pipette, quickly add this solution to the mixed solution in a flask heated at 150 ° C for 60 minutes. The total amount of surfactant charged into the flask through both of these operations is 1-10 mmol. After that, the temperature is rapidly raised to 290 ° C (15 ° C / min) and held at that temperature for 60 minutes to form particles. After completion of the reaction, the reaction solution was air-cooled to room temperature, ethanol and methanol dehydrated with molecular sieves were added to precipitate the particles, and only the particles were separated and extracted by centrifugation. After repeating this washing operation 2 to 3 times, the produced particles were finally dispersed in anhydrous hexane.

  FIG. 2 shows a transmission electron microscope (TEM) photograph of a typical Fe particle produced by the above method. It can be seen that the inside of the particle has a so-called core-shell structure, which is classified into a dark portion near the center and a thin portion near the surface. From the X-ray diffraction results, a peak of metallic iron with good crystallinity was confirmed (FIG. 3), and the core particle portion with good crystallinity described above is considered to be metallic iron, and the portion with a low surface contrast is considered to be an iron oxide coating. The results of elemental analysis by XPS and atomic absorption revealed that the particles contained about 30 at% boron. In general, iron particles with a size of about 10 nm are known to be difficult to maintain a metallic iron phase because oxidation proceeds at once in the atmosphere. The inclusion of boron in the particles and the passivated surface oxide film are considered to be the factors that improve the oxidation resistance of the particles.

  Figure 4 shows typical magnetization curves at room temperature (300 K) and low temperature (5 K). When the saturation magnetization per unit mass of iron is determined using the amount of iron determined by atomic absorption, it is about 100-140 emu / g at 5K, which is higher than normal iron oxide (about 90 emu / g). A value was obtained. Although saturation magnetization is somewhat reduced at room temperature due to the influence of superparamagnetism, saturation magnetization of 100 emu / g or more is maintained (saturation magnetization of iron oxide at room temperature is about 80 emu / g). This is proof that metallic iron with high saturation magnetization is held stably.

  FIG. 5 shows a TEM photograph of particles produced by changing the mixing ratio of oleylamine and oleic acid used as the surfactant. In the case of oleylamine alone, the produced iron particles show relatively good crystallinity, but amorphous metal iron particles are formed by mixing oleic acid (FIG. 6).

  FIG. 7 is an X-ray diffraction study of the oxidation process when the dried particles are kept in the atmosphere at room temperature. Even after two months have passed since the preparation of the sample, the X-ray diffraction pattern does not change significantly, indicating that the metal iron fine particles are held stably. On the other hand, when heating for 30 minutes at different temperatures in the atmosphere, oxidation proceeds gradually at temperatures above 200 ° C (Figure 8).

  In addition to sodium borohydride, lithium borohydride and potassium borohydride can be used as the reducing agent.

  In addition to oleylamine and oleic acid, liquid alkylamines and fatty acids with different alkyl chain lengths can be used as surfactants.

  By using nickel (II) acetylacetonate in addition to iron (III) acetylacetonate as a metal supply source, nickel fine particles can be produced.

The synthesis procedure of iron fine particles is shown. It is a transmission electron microscope (TEM) photograph of typical Fe particles. It is an X-ray diffraction result (oleylamine: oleic acid = 7: 0 mmol). It is a magnetization curve (oleylamine: oleic acid = 7: 0 mmol) of iron fine particles at room temperature (300 K) and low temperature (5 K). It is a TEM photograph of iron fine particles produced by changing the mixing ratio of oleylamine and oleic acid. It is an X-ray diffraction result of iron fine particles produced by changing the mixing ratio of oleylamine and oleic acid. This is an oxidation process of iron fine particles held in the atmosphere at room temperature (change with time in X-ray diffraction pattern) (oleylamine: oleic acid = 3.5: 0 mmol). It is an oxidation process of iron fine particles heated in the atmosphere (change of X-ray diffraction pattern with heating temperature) (oleylamine: oleic acid = 3.5: 0 mmol).

Claims (5)

A method for producing metal magnetic fine particles, comprising mixing a boron hydride compound in an organic solvent having the same polarity as water, homogenizing the mixture by ultrasonic irradiation, and then mixing the mixture in a reaction solution containing metal ions. 2. The method for producing metal magnetic fine particles according to claim 1, wherein the boron hydride compound according to claim 1 is sodium borohydride, potassium borohydride or the like. The method for producing metal magnetic fine particles according to claim 1 or 2, wherein oleylamine and / or oleic acid is used as the organic solvent having the same polarity as the water of claim 1. The oleylamine and oleic acid mixture according to claim 3 has a role as a surfactant, and by changing the mixing ratio thereof, metallic magnetic fine particles having crystalline and amorphous properties can be made separately. A method for producing metal magnetic fine particles according to any one of claims 1 to 3. Metal magnetic fine particles, wherein the size of the metal magnetic fine particles produced by the method for producing metal magnetic fine particles according to claim 1 is 30 nm or less, particularly 10 nm order.