Method of producing ferromagnetic rare earth-transition metal-boron powder by precipitation

Abstract: An aqueous solution containing reducing agent such as potassium borohydride or sodium borohydride is added with another solution containing salt of an iron-triads-group element and salt of a rare earth element to conduct reaction to effect reduction to the iron-triads-group metal and the rare earth metal to thereby produce fine powder of rare earth magnet composed of alloy of the iron-­ triads-group metal and the rare earth metal.

Description: The present invention relates to the method of producing fine powder of rare earth magnet. As the conventional method of producing rare earth magnet fine powder composed of alloy of iron-triads-group metal and rare earth metal, there have been known a method of making ingot of mother alloy and then crushing the same, or another method of making ribbon of mother alloy by instant quenching of molten alloy and then crushing the same. Further, chemical reaction method of producing alloy powder has been studied by Saita et al. of Tohoku University (Special Working Group in method of making amorphous metallization and application thereof. The ninth regular meeting text, 28). For making and crushing ingot or for making ribbon by instant quenching of molten alloy and crushing the same so as to produce fine powder of rare earth magnet, there has been needed high energy consumption, complicated processes and precious equipments such as a big furnace, liquid instant quenching apparatus and crushing machine, thereby causing the problem of high production cost. US-A-4 715 890 discloses a method of forming a magnetic material from a ferromagnetic powder which is precipitated by forming a solution of a reducible iron halide, a reducible rare earth halide and lithium borohydride in an aprotic solvent. This aprotic solvent is considered necessary, in order to prevent a decomposition of water. An object of the present invention is to therefore produce fine powder of rare earth magnet at reduced production cost. The solution according to the invention is given by the features of the single claim. The invention provides an advantageous simple method of adding an aqueous solution containing salt of iron-triads-group metal and salt of rare earth metal to another aqueous solution containing reducing agent such as potassium borohydride or sodium borohydride, for producing fine powder of rare earth magnet, thereby reducing the production cost and simplifying the process as compared to the conventional methods. When reducing aqueous solution of MSO₄ and RCℓ₃ by potassium borohydride, reactions concurrently occur as represented by the following formulas: 2MSO₄ + KBH₄ + 2H₂O → 2M + 2H₂ + 2H₂SO₄ + KBO₂ (1) 4MSO₄ + 2KBH₄ → 2M₂B + K₂SO₄ + 4H₂ (2) 2RCℓ₃ + KBH₄ + 2H₂O → 2R + H₂ + 6HCℓ + KBO₂ (3) 4RCℓ₃ + 3KBH₄ → R₄B₃ + 14KCℓ + 6H₂ (4) where M: iron-triads-group element (Fe, Ni or Co) and R: rare earth element. The reactions are theoritically represented by the above formulas, and actually the resulting substance is composed of R-M-B alloy according to eutectoid mechanism in manner similar to electroless plating. These reduction reactions occur instantly to suppress crystal glowth to thereby precipitate fine powder of the R-M-B alloy. Therefore, the fine powder of the R-M-B alloy can be produced directly in contrast to the conventional methods in which ingot or ribbon of the alloy is crushed. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing relation between reducing agent concentration and yield of precitptate according to the inventive method; Fig. 2 is a diagram showing reducing agent concentration and composition of precipitate according to the inventive method; Fig. 3 is a diagram showing relation between solution composition and precipitate composition according to the inventive method; Fig. 4 is a diagram showing measurement results, by X-ray diffraction device, of microstructure of precipitate according to the inventive method; Fig. 5 is a photograph, taken by scanning electron microscope, of precipitate according to the inventive method; and Fig. 6 is a diagram showing temperature dependence of saturation magnetization of rare earth magnet obtained according to the inventive method. DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the description is given for embodiments of the present invention. Embodiment 1 Fine powder of Nd-Fe-B alloy was produced by the following method. Namely, drops of aqueous solution containing FeSO₄ and NdCℓ₃ were added into aqueous solution of potassium borohydride to effect reduction reaction to precipitate fine powder of Nd-Fe-B alloy. The precipitated substance was filtered by a glass filter, then washed sequentially by distilled water, methanol and acetone, and thereofter dried in vacuum together with the glass filter. Concentration of reducing agent A 2.0 mℓ of aqueous solution containing FeSO₄ and NdCℓ₃ at mole ratio of 8:2 by concentration of 0.2 mol/ℓ was added to 2.0 mℓ of aqueous solution containing potassium borohydride at different concentrations of 0.2, 0.4, 0.8, 1.6 and 2.0 mol/ℓ to produce fine powder of Nd-Fe-B alloy in order to determine optimum range of the concentration of the reducing agent. Fig. 1 shows the relation between concentration of the reducing agent and yield of the precipitate. As shown in the figure, whole of Nd ions and Fe ions contained in the aqueous solution of FeSO₄ and NdCℓ₃ was entirely reduced when the concentration of the reducing agent was more than about 0.5 mol/ℓ. This concentration value is about five times as great as the theoritical value calculated according to the chemical reaction formulas. Fig. 2 shows the relation between the concentration of reducing agent and the composition of precipitate, which was measurement results by plasma luminescence spectroanalyzer. It was found that stable composition of the precipitate was not obtained in lower range of the reducing agent concentration. In view of the above fact and taking in account of degradation of the reducing agent, the concentration should be set eight to twenty times as much as the calculated value for safety. Composition of precipitate A 2.0 mℓ of aqueous solution containing by concentration of 0.2 mol/ℓ FeSO₄ and NdCℓ₃ at different mole ratios of 8:2, 4:6, 6:4 and 2:8 was added to 2.0 mℓ of aqueous solution containing potussium borohydride by concentration of 2.0 mol/ℓ to produce fine powder of Nd-Fe-B alloy. The composition of precipitate was measured by the plasma luminescence spectroanalyzer, the results of which is shown in Fig. 3. According to the results, the ratio of Nd and Fe of the precipitate corresponds to that of FeSO₄ and NdCℓ₃ in the solution. The boron amount in the precipitate increases proportionally to the Nd amount in the precipitate. Microstructure of precipitate A 2.0 mℓ of qaueous solution containing by concentration of 0.2 mol/ℓ FeSO₄ and NdCℓ₃ at mole ratio of 8:2 was added to 2 mℓ of aqueous solution containing potassium borohydride by concentration of 2.0 mol/ℓ to produce fine powder of Nd-Fe-B alloy. Microstructure of the precipitate was measured by an X-ray diffraction device, the result of which is shown in Fig. 4. In the figure, rising of the graph on left side is due to the glass filter which was utilized to filter the fine powder of Nd-Fe-B alloy. In the X-ray diffraction, any peak indicative of crystal lattice was not detected. Therefore, it was found that Nd-Fe-B alloy has amorphous microstructure. Particle diameter of precipitate A 2.0 mℓ of aqueous solution containing by concentration of 0.2 mol/ℓ FeSO₄ and NdCℓ₃ was added to 2mℓ of solution containing potassium borohydride by concentration of 2.0 mol/ℓ to produce fine powder of Nd-Fe-B alloy. Particle diameter of the precipitate was measured by a scanning electron microscope, the measurement results of which is shown in Fig. 5. The particle diameter is more or less 0.1 µm and is substantially uniform. In the above described embodiment, the fine powder of Nd-Fe-B alloy was produced such that it has Fe composition in the range of 0-95 at %, Nd composition in the range of 0-95 at % and B composition in the range of 5-65 at %, and it has particle diameter of more or less 0.1 µm. Embodiment 2 Various kinds of neodymium salt and iron salt were utilized as listed in Table 1. A 2.0 mℓ of aqueous solution containing by concentration of 0.2 mol/ℓ neodymium salt and iron salt at the mole ratio of 8:2 was added to 2.0 mℓ of aqueous solution containing potassium borohydride by concentration of 2.0 mol/ℓ to produce fine powder of Nd-Fe-B alloy. The obtained fine powder has substantially uniform particle diameter of more or less 0.1 µm , and has amorphous microstructure as confirmed by X-ray diffraction measurement results. Neodymium salts iron salts NdF₃ (dissolved into sulfuric acid and then diluted by water) FeCℓ₂ FeCℓ₃ FeSO₄·nH₂O NdI₃ Fe₂(SO₄)₃·nH₂O Nd₂(SO₄)₃·nH₂O Fe(NO₃)₂·nH₂O Nd(NO₃)₃·nH₂O Fe(NO₃)₃·nH₂O Nd₂(CH₃COO)₃·H₂O FeBr₂·nH₂O Nd₂O₃ (dissolved into diluted hydrochloric acid) FeBr₃·nH₂O FeI₂·nH₂O Fe(CH₃COO)₂·nH₂O Embodiment 3 Fine powder of R-Fe-B alloy having the composition ratio of rare earth and iron 12.5:87.5 was produced with using various salts of rare earth elements listed in Table 2. The obtained fine powder of R-Fe-B alloy was press-fromed under magnetic field, then sintered within argon gas at 1000°C for one hour and quickly cooled to the room temperature, and thereafter treated by aging process at 600°C to thereby produce tablet of R-Fe-B alloy magnet. Fig. 6 shows temperature dependence of saturation magnetization of the magnet. As described above, according to the present invention, fine powder of rare earth magnet can be easily and industrially produced without crushing ingot or ribbon material.

Claims: A method of producing ferromagnetic powder comprising the steps of: preparing an aqueous solution containing a reducing agent selected from potassium borohydride and sodium borohydride, an iron-triads-group element ion, a rare earth element ion selected from Nd ion, Pr ion, Sm ion and Y ion, and precipitating ferromagnetic powder composed of alloy of iron-triads-group metal, rare earth metal and boron.

Inventors: WATANABE SHINJI C O SEIKO INST, WATANABE, SHINJI, C/O SEIKO INSTRUMENTS INC.
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1990
Main classification: H01F1
IPC codes: H01F1, B22F9