RU2431545C2 - Method of producing permanent magnet from strontium hexaferrite powder - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to powder metallurgy, particularly, to materials intended for production of permanent magnets. Strontium hexaferrite powder is synthesized from aqueous solutions of individual sulphates in stoichiometric ratio corresponding to final product, i.e. phase SrFe₁₂O₁₉. To increase content of phase SrFe₁₂O₁₉ additives are introduced in the following concentrations: CaCO₃-1.2%, SiO₂-0.25% and H₃BO₃-0.3% or CaCO₃-0.75%, SiO₂-0.3% and H₃BO₃-0.35%. Mixed solution is dispersed and frozen by liquid commercial nitrogen while produced granules are subjected to sublimation dewatering at 1.5 Pa and 230 K in drying start and 363 K at drying finish to obtain salt product. Salt product is subjected to thermal decomposition at 1200-1220 °C for 4 hours to produce ferrite powder by ferritising oxides formed. Impurities are extracted from powder by treatment with distilled water and ultrasound at frequency of 20=25 kHz for 20 minutes. Powder is dried at 353-373 °C to residual moisture content of not over 2 wt %.Thereafter, compaction in magnetic field is carried out and sintering at 1100-1300 °C in tunnel furnace in jog mode.

EFFECT: higher coercive force and mechanical strength.

Description

The invention relates to a technology for the production of electrical materials, in particular to a technology for the production of permanent magnets, and can be used to improve the characteristics of ferrite magnets.

A known method for the synthesis of strontium ferrite powders in vertical furnaces (RU No. 2313151) and a method for manufacturing anisotropic strontium ferrite (RU No. 2256534), which use ceramic technology for the production of magnetic powders for the manufacture of permanent magnets based on strontium ferrite, which are increasingly used in electronics, radio engineering, instrument making, medicine and other areas. The disadvantages of these analogues are the low quality of the manufactured magnets (the maximum value of the coercive force of magnets based on strontium ferrite powders obtained by ceramic technology is not more than 3 kOe) and the rapid loss of magnetic qualities over time. These disadvantages are explained by the structural properties of the initial powders, which have an average particle size of about 0.7-0.9 microns.

As a prototype can be used cryochemical technology [1], which allows to obtain materials with high homogeneity. Using this technology, magnetic semiconductors such as chalcospinels, catalysts, electrode materials, porous ceramics, solid electrolytes, composites, including dispersoids, resistive materials, powders for glass melting and growing single crystals, pharmaceuticals, piezomaterials, enzymes, chemicals, sorbents, pigments are made. However, this technology is not used for the production of magnetically rigid materials.

The objective of the proposed method is the manufacture of permanent magnets with high values of coercive force, a large margin of mechanical strength and retaining their qualities for a long time.

The proposed method allows the manufacture of permanent magnets based on cryogranulate, from which, after sublimation, blanks of the desired shape are pressed in a magnetic field.

The difference of the proposed method is the use of cryochemical technology for the production of magnetically rigid materials with a high percentage of the SrFe ₁₂ O ₁₉ phase, the use of ultrasonic treatment of an aqueous suspension of ferrite powder to disaggregate particle agglomerates and compress the workpiece in a magnetic field.

The proposed method for the production of a permanent magnet from strontium hexaferrite powder includes the following steps:

1. In distilled water GOST 6709-72, acidified with sulfuric acid to pH 1.5 ... 2.0 at room temperature, close to saturated solutions of individual sulfates are prepared in accordance with the chemical composition. Then, by rigorous dosing of individual solutions, previously subjected to analysis for the content of the basic substance, a mixed solution with a concentration of 20-22% is prepared. Water-soluble nitrates or strontium and iron sulfates are taken as starting components. The concentration of solutions is 1-2 mol / l and 0.3-0.32 mol / l for the solution. The solutions are mixed in a stoichiometric ratio for the final product (6: 1). The composition contains additives in the following concentrations of CaCO ₃ - 1.2%, SiO ₂ - 0.25% and H ₃ BO ₃ - 0.3% or CaCO ₃ - 0.75%, SiO ₂ - 0.3% and N ₃ VO ₃ - 0.35% to increase the phase content of SrFe ₁₂ O ₁₉ .

2. Using a spray gun to produce polydispersed droplets, dispersion and freezing of the solution with liquid technical nitrogen is carried out according to GOST 9293-74. Frozen solution is unloaded in baking sheets.

3. Then, at a pressure of 1.5 Pa and a temperature from 230 K (beginning of the process) to 363 K (at the end of drying), freeze-drying of cryogranules is carried out. The temperature regime is changed according to a predetermined program, which allows avoiding macromelting of cryogranulate. The salt product from the sublimator is sealed.

4. In conveyor-type electric furnaces provided with an exhaust gas absorption and utilization system, at a temperature of 1200-1220 ° C for 4 hours, the salt mass is thermally decomposed, followed by ferritisation of the resulting oxides.

5. By treating the ferrite powder with distilled water at a 3-4-fold excess and a temperature close to boiling, water-soluble impurities (in particular, sulfates and chlorides of alkali and alkaline-earth metals) are removed. In this case, an aqueous suspension of ferrite powder in order to disaggregate the agglomerates of particles formed in the previous stages is subjected to ultrasonic treatment at a frequency of 20-25 kHz for 20 minutes.

6. In conventional furnaces at temperatures of 353-373 ° C, the ferrite powder is dried to a residual moisture content of not more than 2 wt.%.

7. The magnet is pressed by a dry method in a magnetic field.

8. In a continuous tunnel kiln with a pushing mode of 25 minutes at a temperature of 1100–1300 ° C, the powder is sintered and acquired with the necessary properties.

The particle size of the powder with a phase content of SrFe ₁₂ O ₁₉ 98 ± 1.4% obtained in stages 1-6 of the proposed method is 0.43-0.63 μm. The coercive force of the obtained magnets is 4.5 kOe.

An increase in the coercive force is achieved due to a change in the crystal structure of the samples, namely, the presence of nonmagnetic phases distributed in the intergrain space and a decrease in grain size.

Thus, the proposed method allows the manufacture of permanent magnets with high values of coercive force, a large percentage of the SrFe ₁₂ O ₁₉ phase, a large margin of mechanical strength and retaining their qualities for a long time.

List of references

1. Yu.D. Tretyakov. Low-temperature processes in chemistry and technology // Soros Educational Journal No. 4, 1996.

Claims (1)

A method for the production of a permanent magnet from strontium hexaferrite powder, including the synthesis of strontium hexaferrite powder by preparing aqueous solutions of individual sulfates, in accordance with the chemical composition, mixing them in a stoichiometric ratio corresponding to the final product - SrFeH ₁₂ O ₁₉ phase, with the introduction of SrFe phase to increase the content ₁₂ O ₁₉ additives in the following concentrations of CaCO ₃ - 1.2%, SiO ₂ - 0.25% and H ₃ BO ₃ - 0.3% or CaCO ₃ - 0.75%, SiO ₂ - 0.3% and H ₃ BO ₃ - 0.35%, and dispersing the mixed solution freeze liquid techni nitrogen nitrogen to obtain cryogranules, freeze-drying of cryogranules at a pressure of 1.5 Pa and a temperature from 230 K at the beginning of drying to 363 K at the end of drying, which varies according to a predetermined program, avoiding macromelting of cryogranules, with the formation of a salt product, thermal decomposition of the salt product at 1200-1220 ° C for 4 hours and subsequent ferritization of the formed oxides with the formation of ferrite powder, removal of water-soluble impurities from the ferrite powder by treatment with distilled water at a temperature re, close to the boiling point, and subsequent ultrasonic treatment with a frequency of 20-25 kHz for 20 min to disaggregate the particle agglomerates, dry the ferrite powder at 353-373 ° C to a residual moisture content of not more than 2 wt.%, subsequent pressing in a magnetic field and sintering at 1100-1300 ° C in a tunnel furnace with a pushing mode of 25 minutes