CN115831577A - Method for preparing anisotropic massive ferrite permanent magnet material by low-temperature sintering - Google Patents
Method for preparing anisotropic massive ferrite permanent magnet material by low-temperature sintering Download PDFInfo
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Abstract
The invention discloses a method for preparing an anisotropic massive ferrite permanent magnet material by low-temperature sintering, which comprises the following steps: s1, preparing a transition solution; s2, preparing a cold-sintered green body; s3, low-temperature sintering (also called cold sintering); and S4, stress relief annealing. The invention provides a method for uniformly mixing ferrite permanent magnetic powder solid particles with a transition solution capable of partially dissolving or transporting magnetic particles, performing orientation pressing on the mixture in a strong magnetic field to obtain a green body, further improving the density through cold isostatic pressing, and finally obtaining an anisotropic block magnet through cold sintering under high pressure and at a lower temperature relative to the traditional sintering temperature.
Description
Technical Field
The invention relates to a preparation method of an anisotropic massive permanent magnet material, in particular to a method for preparing the anisotropic massive ferrite permanent magnet material by adopting low-temperature sintering, belonging to the technical field of permanent magnet material sintering.
Background
Sintering is an important process for preparing bulk ceramics. In order to obtain a larger magnetic energy product, the permanent magnet material at the present stage can only be obtained by improving the remanence and the coercive force. The improvement of the coercive force requires smaller crystal grains, ideally close to a magnetic single domain; while increasing remanence is generally achieved by increasing density.
In the conventional ferrite sintering method, high density can only be achieved by increasing the sintering temperature, which generally causes grain growth, thereby defeating the purpose of high coercivity. In addition, the energy consumption caused by the high temperature of conventional sintering is also of great concern.
The low-temperature sintering mode at the present stage mainly comprises spark plasma sintering, microwave sintering and the like, which have the defects of complicated equipment and difficult mass production, while the cold sintering realizes the densification of materials under the condition of ultralow temperature (less than 500 ℃) by utilizing specific transition solution and certain pressure (generally uniaxial pressure), and has simple equipment and easier production compared with other low-temperature sintering. The method mainly utilizes the transition solution to dissolve and transport particles in a high chemical potential area under the action of pressure so as to promote the particles to be rearranged, thereby achieving the effect of densification.
At present, cold sintering is mainly applied to some ceramic materials which are totally or partially soluble in water, and permanent magnet materials cannot be used as transition solutions to realize cold sintering due to the characteristics of being insoluble in water, easy to oxidize and easy to corrode. The method utilizes the reaction characteristic of transition solution and ferrite permanent magnet powder which are formed by taking alkyl alcohol as a solvent and organic acid as a solute, and realizes the low-temperature sintering of the massive permanent magnet material at the temperature of 250-500 ℃.
The temperature required for preparing the massive strontium ferrite by the traditional method is about 1200 ℃. For example, patent CN 105060870A of zender long et al, university of south china, discloses a method for preparing sintered strontium ferrite by a conventional wet process, wherein the sintering temperature is 1190 ℃ to 1290 ℃.
Patent CN 108147803A applied by the company yaorui et al, limited by aerospace and magnetoelectricity, of Hunan, discloses a method for preparing sintered strontium ferrite by a dry method, wherein the sintering temperature is 1000-1300 ℃. The sintering temperature of the traditional strontium ferrite is far higher than 200-500 ℃ mentioned in the patent. The cold sintering that this patent provided has reduced energy consumption greatly.
The cold sintering concept has been proposed mostly for use in ceramic materials which are wholly or partly soluble or sparingly soluble in water. For example, application patent CN 113735580B of Zhengtrichong et al of Beijing university of industry discloses a multiphase microwave dielectric ceramic and its cold sintering preparation method, in which the cold sintering transition solution is water, and the acting substance is Li which is easily dissolved in water 2 MoO 4 。
Patent CN 112500154B applied by Liu soldier of Hangzhou electronic science and technology university discloses a method for preparing LiF-based core-shell structure microwave dielectric ceramic based on cold sintering process, wherein transition solution is water, and active substance is LiF slightly soluble in water. Cold sintering was first used for composite ceramic materials because some of the materials in composite ceramic materials are soluble or slightly soluble in the common transition solution, water. However, most permanent magnetic materials are insoluble in water and are easily oxidized, so that a proper transition solution is difficult to find.
Most materials prepared by cold sintering are isotropic at present. For example, patent CN 113277860A of Clav A Landel et al, pennsylvania State university, discloses a method for cold sintering of most ceramics: mixing the cold sintered matrix with inorganic solvent for dissolving it 10145, pressurizing and heating to evaporate the solvent 10145to form densified inorganic material. Since only isotropic inorganic materials can be formed under this method, the magnetic properties are greatly compromised for permanent magnetic materials.
Disclosure of Invention
In order to solve the defects of the technology, the invention provides a method for preparing an anisotropic bulk ferrite permanent magnet material by low-temperature sintering, and the anisotropic permanent magnet bulk material with higher magnetic property can be obtained at lower temperature.
In order to solve the technical problems that the sintering temperature of the traditional preparation of the bulk ferrite material is high, the anisotropic bulk magnet cannot be obtained by the existing cold sintering material, and the like, the invention adopts the technical scheme that: a method for preparing an anisotropic massive ferrite permanent magnet material by low-temperature sintering comprises the following steps:
s1, preparing a transition solution;
s2, preparing a cold sintering green body;
s3, low-temperature sintering (also called cold sintering);
and S4, stress relief annealing.
Preferably, the specific process of step S1 is:
organic acid capable of partially chemically dissolving ferrite permanent magnetic powder is diluted by organic alcohol to form a mixed solution with a certain concentration, namely a transition solution.
Preferably, in step S1:
the hexagonal plane anisotropic ferrite permanent magnetic powder is SrFe 12 O 19 、BaFe 12 O 19 、PbFe 12 O 19 Mixtures of two or more thereof; the organic acid is one or more of acetic acid, organic phosphonic acid and acetic acid; the organic alcohol is alkyl alcohol;
in the transition solution, alkyl alcohol is used as a solvent, organic acid is used as a solute, and the concentration of the organic acid is controlled to be 10.5-15mol/L;
the organic matter in the transition solution is removed by volatilization in the subsequent cold sintering process or the annealing process.
Preferably, the specific process of step S2 is:
adding ferrite permanent magnetic powder with the particle size of 0.1-2.0 microns into the transition solution;
uniformly mixing the magnetic powder and the transition solution into slurry with the viscosity of 2000-10000mPa.s by a grinding method or a ball milling method;
the ferrite powder and the transition solution are fully mixed to form a slurry, the viscosity of the slurry is controlled to be 2000-10000mPa.s, the over-high viscosity is not favorable for the dispersion of the magnetic powder in the transition solution and the filling of the magnetic powder in a mold, the fluidity is not favorable, and the over-high viscosity is not favorable for improving the orientation degree when the magnetic powder is oriented; and the viscosity is too low, so that the mixture is easily extruded from a die when the pressure is applied.
Then the slurry is filled into a non-magnetic steel mould, and the non-magnetic steel mould is positioned in a strong magnetic field for orientation and is pressed at the same time to obtain the density of 2.2-3.0g/cm 3 Cold sintering the green compact;
further subjecting the green body to cold isostatic pressing to increase the density of the green body to 3.0-4.0g/cm 3 。
Preferably, the specific process of step S3 is:
placing the cold-sintered green body subjected to cold isostatic pressing into a steel mold with a heating device, firstly applying pressure of 100-250MPa and controlling the temperature to be 50-150 ℃ so that ferrite permanent magnetic powder in the green body is partially chemically dissolved in a transition solution by organic acid to form colloidal metal salt, and the insufficiently dissolved oxide is coated on the surface of the ferrite permanent magnetic powder to form a core-shell structure;
further increasing the pressure and temperature value to volatilize the organic matter in the transition solution, dewatering and decomposing the colloidal metal salt dissolved in the transition solution into nanometer small particles, and making the nanometer small particles closely contact with the iron oxide on the surface of the ferrite permanent magnetic powder to perform a sintering reaction and densify the magnet to obtain a block.
Preferably, the specific process of step S4 is:
and placing the block obtained by cold sintering in an annealing furnace, and annealing at the temperature of 700-1100 ℃ for 0.5-4 h to eliminate the internal stress in the sintering process, thereby finally obtaining the high-density high-performance anisotropic bulk ferrite permanent magnet material.
The key to achieving the low temperature firing reaction is that the ferrite powder reacts with the transition solution containing the organic acid, and the reaction rate affects the density and magnetic properties of the final magnet. The reaction rate is related to the size of the magnetic powder, the concentration of the transition solution, the mass ratio of ferrite to the transition solution, the reaction temperature and pressure, and the like.
The smaller the size of the magnetic particles, the larger the specific surface area in contact with the transition solution, the easier the cold sintering process under pressure, and the higher the density of the obtained sample, but the easier the magnetic particles overflow from the gap of the mold, resulting in difficult mold release. It is most suitable to use ferrite powder of 0.1-2 μm.
When the mass ratio of the ferrite powder to the transition solution is determined, the concentration of the transition solution (in terms of the mass or molar ratio of the organic acid to the total solution) has an important influence on the cold sintering process. The concentration is small, the relative content of organic acid is low, the chemical dissolution speed of ferrite magnetic powder is slow, the subsequent cold sintering process is insufficient, the density is not high, the content of organic alcohol is high, and the organic alcohol is difficult to completely remove through volatilization. However, if the concentration is too high, the content of iron oxide generated by chemical dissolution is too high, and the iron oxide which is not completely sintered is doped in the magnet after cold sintering, so that the performance of the magnet is reduced. The molar concentration of the transition solution is controlled to be 10.5-15mol/L.
Preferably, in step S2, the mass ratio of the ferrite permanent magnet powder to the transition solution in the preparation process of the cold-sintered green compact is 2. When the ratio is more than 7:1, too few dissolved magnetic particles and insufficient substances are precipitated among the particles, resulting in poor densification effect, for example, when the solid magnetic particles: transition solution =10:1, the density of the cold sintering product is 68%, more pores are generated, the performance is poorer, and the cold sintering failure can be caused if the transition solution is less; when the ratio is less than 2:1, too good flowability of the particles leads to overflow of the particles from between the upper and lower molds and the outer mold, resulting in difficulty in mold release.
Preferably, in step S2, during the preparation of the cold-sintered green compact, the magnitude of the strong magnetic field is 1.0T-2.5T, the direction of the magnetic field is perpendicular to or horizontal to the pressure direction, and the direction of the magnetic field and the pressure is preferably selected to be perpendicular.
During green body cold isostatic pressing, the green body is subjected to vacuum plastic packaging, then is placed in a mold filled with an oil medium, and the applied pressure is 200MPa-450MPa.
Pressure and temperature control during cold firing are important indicators. The pressure is uniaxial and can range from 200MPa to 1.5GPa, too low a pressure providing insufficient kinetic for the migration of the material from the precipitate of supersaturated solution, and too high a pressure providing premature compaction closing of the channels transporting the material. Both of these conditions are detrimental to product densification. The pressure is applied at a temperature not high, which may be from 250 ℃ to 500 ℃. When the temperature is too low, as well as the pressure, it does not provide sufficient mass transfer kinetics for the precipitate, and when the temperature is too high, the cold sintering transition solution evaporates too quickly, so that the "dissolution-precipitation" process does not proceed completely and the final density is adversely affected. Final pressure and temperature values may be set, the temperature and pressure applied sequentially in multiple steps from low to high to final values, and the pressure and temperature maintained for a period of time.
Preferably, in step S3, during the cold sintering and firing process, final pressure and temperature values are set, the temperature and pressure are sequentially applied to the final value from low to high in multiple steps, and the pressure and temperature are maintained for a period of time.
Preferably, in step S3, the sintering pressure is 200MPa-1.5GPa, the sintering temperature is 200-500 ℃, and the sintering time is 0.5-4 h.
The invention is based on the following principle: the planar hexaferrite is a composite oxide, such as strontium ferrite (SrFe) 12 O 19 ) Is a composite oxide composed of strontium oxide and iron oxide, and the chemical dissolution rates of the two oxides in organic acid are different.
Under the catalytic action of a certain temperature, one metal oxide in the ferrite material is preferentially chemically dissolved by an organic acid and enters an organic matter to form a colloidal metal salt, and the iron oxide which is not fully dissolved is coated on the surface of the ferrite powder to form a core-shell structure.
When the temperature is further increased, the organic matter volatilizes, the colloidal metal salt dissolved in the organic matter is dehydrated, decomposed and crystallized into metal oxide nano small particles, and under the action of large pressure, the small crystallized particles are closely contacted with the iron oxide coated on the surface of the powder and generate a sintering reaction to further form the composite oxide. The firing temperature for forming the composite oxide is greatly reduced due to the action of large pressure and fine particles. The principle schematic diagram of the process is shown in fig. 1.
The invention aims to provide a method for uniformly mixing ferrite permanent magnetic powder solid particles with a transition solution capable of partially dissolving or transporting magnetic particles, performing orientation pressing on the mixture in a strong magnetic field to form a green body, further improving the density through cold isostatic pressing, and finally obtaining an anisotropic block magnet through cold sintering at high pressure and lower temperature relative to the traditional sintering temperature.
According to the invention, the cold sintering raw material (the reaction mixture of the permanent magnet powder and the transition solution) is firstly subjected to magnetic field orientation through a strong magnetic field, and then is put into a cold isostatic press for pressing, so that the density is improved and the orientation is maintained, and through the way, the cold sintering can obtain the anisotropic massive permanent magnet material, and the performance is more excellent compared with that of the isotropic massive permanent magnet material. Compared with the traditional high-temperature sintering, the cold sintering of the invention reduces the sintering temperature, the reduction of the sintering temperature not only saves energy, but also prevents the crystal grains from growing at low temperature, and the smaller crystal grains mean higher coercive force, thereby obtaining better magnetic performance.
Drawings
Fig. 1 is a schematic diagram of the cold sintering molding step of strontium ferrite in a transition solution composed of ethyl acetate.
Detailed Description
The present invention will be described in further detail with reference to specific examples.
Example 1
Mixing acetic acid and ethanol to form a transition solution, wherein the transition solution takes acetic acid as a solute and ethanol as a solvent, the concentrations of the acetic acid in the transition solution are respectively 10mol/L, 12mol/L, 14mol/L and 16mol/L, and then SrFe 12 O 19 The mass ratio of the magnetic powder to transition solutions with different acetic acid concentrations is 4:1 is mixed and placed in Al 2 O 3 Grind in a mortar for two minutes to mix well.
And respectively putting the samples into a non-magnetic steel die, placing the non-magnetic steel die in a 1.2T strong magnetic field, performing orientation pressing, wherein the direction of the pressure is vertical to that of the magnetic field, then putting the orientation pressed green body into a cold isostatic press, further improving the density under the pressure of 500KN, and keeping the orientation degree.
And (3) placing the obtained cold isostatic pressed green bodies into a heating mould for low-temperature sintering, applying uniaxial pressure of 1GPa, raising the temperature to 450 ℃ at the rate of 23 ℃ per minute, and keeping the temperature and pressure. The low-temperature firing process lasts for a total of 3 hours.
Table 1 shows SrFe 12 O 19 And mixing the magnetic powder with transition solutions with different acetic acid concentrations, and performing low-temperature sintering on the mixture to obtain the performance data of the sample.
TABLE 1
As can be seen from Table 1, the concentration of acetic acid in the transition solution increased from 10mol/L to 16mol/L. The density of the obtained cold-sintered sample tends to increase, because the concentration of acetic acid increases, and the generated nano-particles are more, which is beneficial to sintering and increasing the density.
But residual magnetic induction Br, intrinsic coercivity Hcj and maximum energy product (BH) max Several magnetic performance indexes have the tendency of rising first and then falling, because the concentration of acetic acid in the transition solution is too low, the chemical dissolution speed of the ferrite magnetic powder is slow, and the subsequent cold sintering process is insufficient and the density is not high. However, the excessive concentration of acetic acid in the transition solution and the excessive content of iron oxide generated by chemical dissolution may result in doping iron oxide (alpha-Fe) which is not completely burnt in the magnet after cold sintering 2 O 3 ) Resulting in a reduction in magnet performance. The molar concentration of the transition solution is controlled to be 10.5-15mol/L.
Then SrFe 12 O 19 And mixing the magnetic powder with transition solutions with different acetic acid concentrations, and annealing the sample subjected to low-temperature sintering at 800 ℃ for 3h.
Table 2 shows the density data obtained by the volume method and the magnetic property data obtained by the permanent magnet tester for the annealed samples.
TABLE 2
Comparing table 1 and table 2, the change rule of the relationship between the density and magnetic property of the obtained product and the acetic acid concentration of the transition solution after annealing the sample under the corresponding conditions is the same as that before annealing, but the density and magnetic property of the product after annealing under the same concentration are further improved, because the internal stress generated in the firing process is eliminated after annealing the sample. Compared with the traditional method, the low-temperature sintering temperature and the annealing temperature in the steps of the method are far lower than the traditional sintering temperature (more than or equal to 1000 ℃), but the performances of the product obtained by the method, such as remanence, coercive force, maximum magnetic energy product and the like, can be compared favorably with the products on the market, and particularly when the concentration of acetic acid in the filtering solution is 14mol/L, the comprehensive performance index is very excellent.
The above embodiments are not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make variations, modifications, additions or substitutions within the technical scope of the present invention.
Claims (10)
1. A method for preparing an anisotropic massive ferrite permanent magnet material by low-temperature sintering is characterized by comprising the following steps: the method comprises the following steps:
s1, preparing a transition solution;
s2, preparing a cold-sintered green body;
s3, low-temperature sintering;
and S4, stress relief annealing.
2. The method for preparing anisotropic bulk ferrite permanent magnet material by low-temperature firing according to claim 1, wherein: the specific process of the step S1 is as follows:
organic acid capable of partially chemically dissolving ferrite permanent magnetic powder is diluted by organic alcohol to form a mixed solution with a certain concentration, namely a transition solution.
3. The method for preparing anisotropic bulk ferrite permanent magnet material by low-temperature firing according to claim 2, wherein: in the step S1:
the ferrite permanent magnetic powder is SrFe 12 O 19 、BaFe 12 O 19 、PbFe 12 O 19 Mixtures of two or more thereof; the organic acid is one or more of acetic acid, organic phosphonic acid and acetic acid; the organic alcohol is alkyl alcohol;
in the transition solution, alkyl alcohol is used as a solvent, organic acid is used as a solute, and the concentration of the organic acid is controlled to be 10.5-15mol/L;
the organic matter in the transition solution is removed by volatilization in the subsequent cold sintering process or annealing process.
4. The method for preparing anisotropic bulk ferrite permanent magnet material by low temperature firing according to claim 1, wherein: the specific process of the step S2 is as follows:
adding ferrite permanent magnetic powder with the particle size of 0.1-2.0 microns into the transition solution;
uniformly mixing the magnetic powder and the transition solution into slurry with the viscosity of 2000-10000mPa.s by a grinding method or a ball milling method;
then the slurry is filled into a non-magnetic steel mould, and the non-magnetic steel mould is placed in a strong magnetic field for orientation and is pressed at the same time to obtain the density of 2.2-3.0g/cm 3 Cold sintering the green compact;
further subjecting the green body to cold isostatic pressing to increase the density of the green body to 3.0-4.0g/cm 3 。
5. The method for preparing anisotropic bulk ferrite permanent magnet material by low temperature firing according to claim 1, wherein: the specific process of the step S3 is as follows:
placing the cold-sintered green body subjected to cold isostatic pressing into a steel mold with a heating device, firstly applying pressure of 100-250MPa and controlling the temperature to be 50-150 ℃ so that ferrite permanent magnetic powder in the green body is partially chemically dissolved in a transition solution by organic acid to form colloidal metal salt, and the insufficiently dissolved oxide is coated on the surface of the ferrite permanent magnetic powder to form a core-shell structure;
further increasing the pressure and temperature value to volatilize the organic matter in the transition solution, dewatering and decomposing the colloidal metal salt dissolved in the transition solution into nanometer small particles, and making the nanometer small particles closely contact with the iron oxide on the surface of the ferrite permanent magnetic powder to perform a sintering reaction and densify the magnet to obtain a block.
6. The method for preparing anisotropic bulk ferrite permanent magnet material by low temperature firing according to claim 1, wherein: the specific process of the step S4 is as follows:
and placing the block obtained by cold sintering in an annealing furnace, and annealing at the temperature of 700-1100 ℃ for 0.5-4 h to eliminate the internal stress in the sintering process, thereby finally obtaining the high-density high-performance anisotropic bulk ferrite permanent magnet material.
7. The method for preparing anisotropic bulk ferrite permanent magnet material by low-temperature firing according to claim 4, wherein: in the step S2, the mass ratio of ferrite permanent magnet powder to transition solution in the preparation process of the cold-sintered green body is 2-7.
8. The method for preparing anisotropic bulk ferrite permanent magnet material by low temperature firing according to claim 7, wherein: in the step S2, in the preparation process of the cold-sintered green body, the size of the strong magnetic field is 1.0T-2.5T, the direction of the magnetic field is vertical or horizontal to the pressure direction, and the direction of the magnetic field and the pressure is preferably selected to be vertical.
9. The method for preparing anisotropic bulk ferrite permanent magnet material by low temperature firing according to claim 5, wherein: in the step S3, in the cold sintering and sintering process, final pressure values and temperature values are set, the temperature and the pressure are sequentially applied to the final values from low to high according to multiple steps, and pressure maintaining and heat preservation are carried out for a period of time.
10. The method for preparing anisotropic bulk ferrite permanent magnet material by low temperature firing according to claim 9, wherein: in the step S3, the sintering pressure is 200MPa to 1.5GPa, the sintering temperature is 200 ℃ to 500 ℃, and the sintering time is 0.5h to 4 h.
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