A method for preparing anisotropic bulk ferrite permanent magnet material by low-temperature sintering

Abstract

The application discloses a method for preparing anisotropic block-shaped ferrite permanent magnet material by low-temperature sintering, which comprises the following steps: step S1, preparation of a transition solution; step S2, preparation of a cold-sintered green body; step S3, low-temperature sintering (also known as cold sintering); and step S4, stress relief annealing. The application is characterized in that ferrite permanent magnetic powder solid particles are uniformly mixed with a transition solution capable of partially dissolving or transporting magnetic particles, and the mixture is oriented and pressed into a green body under a strong magnetic field, the density of the green body is further improved through cold isostatic pressing, and finally, anisotropic block-shaped magnets are obtained through cold sintering under a large pressure and at a lower temperature relative to a conventional sintering temperature.

Description

Technical Field

[0001] This invention relates to a method for preparing anisotropic bulk permanent magnet materials, and more particularly to a method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering, belonging to the field of permanent magnet material sintering technology. Background Technology

[0002] Sintering is a crucial process in the preparation of bulk ceramics. Currently, to achieve a larger magnetic energy product in permanent magnet materials, the only option is to increase remanence and coercivity. Increasing coercivity requires smaller grains, ideally approaching a single magnetic domain; while increasing remanence is typically achieved by increasing density.

[0003] In traditional ferrite sintering methods, high density can only be achieved by increasing the sintering temperature. However, increasing the sintering temperature usually leads to grain growth, which contradicts the goal of high coercivity. In addition, the energy consumption caused by the high temperature of traditional sintering is also a major concern.

[0004] Currently, low-temperature sintering methods mainly include discharge plasma sintering and microwave sintering. These methods suffer from complex equipment and are difficult to mass-produce. Cold sintering, on the other hand, utilizes a specific transition solution and a certain pressure (generally uniaxial pressure) to achieve material densification at ultra-low temperatures (<500℃). Compared to other low-temperature sintering methods, it requires simpler equipment and is easier to put into production. It mainly utilizes the transition solution to dissolve and transport particles in high chemical potential regions under pressure, promoting their rearrangement and thus achieving densification.

[0005] Currently, cold sintering is mainly applied to some ceramic materials that are wholly or partially soluble in water. Permanent magnet materials, due to their poor water solubility and susceptibility to oxidation and corrosion, cannot be cold-sintered using water as a transition solution. This patent utilizes the reaction characteristics of a transition solution composed of alkyl alcohols as solvents and organic acids as solutes with ferrite permanent magnet powder to achieve low-temperature sintering of blocky permanent magnet materials at 250℃~500℃.

[0006] Currently, the temperature required for traditional methods to prepare bulk strontium ferrite is around 1200℃. For example, patent CN 105060870 A applied for by Zeng Dechang et al. of South China University of Technology discloses a traditional wet method for preparing sintered strontium ferrite, in which the sintering temperature mentioned is 1190℃-1290℃.

[0007] Patent CN 108147803 A, filed by Yao Rui et al. of Hunan Aerospace Magnetoelectric Co., Ltd., discloses a dry method for preparing sintered strontium ferrite, mentioning a sintering temperature of 1000℃-1300℃. Traditional sintering temperatures for strontium ferrite are much higher than the 200℃-500℃ mentioned in this patent. The cold sintering method proposed in this patent significantly reduces energy consumption.

[0008] Since its inception, the concept of cold sintering has been mostly applied to ceramic materials that are wholly or partially soluble or slightly soluble in water. For example, Zheng Mupeng et al. from Beijing University of Technology have applied for patent CN 113735580 B, which discloses a multiphase microwave dielectric ceramic and its cold sintering preparation method. The cold sintering transition solution is water, and the active substance is Li2MoO4, which is easily soluble in water.

[0009] Patent CN 112500154 B, filed by Liu Bing et al. from Hangzhou Dianzi University, discloses a method for preparing LiF-based core-shell structured microwave dielectric ceramics using a cold sintering process. The transition solution used is water, and the active ingredient is LiF, which is slightly soluble in water. Cold sintering was initially used for composite ceramic materials because some components in these materials are soluble or slightly soluble in water, a common transition solution. However, permanent magnet materials are mostly insoluble in water and easily oxidized, making it difficult to find a suitable transition solution.

[0010] Most materials prepared by cold sintering are currently isotropic. For example, patent CN 113277860 A, filed by Clive A. Randall et al. of Pennsylvania State University, discloses most methods for cold sintering ceramics: mixing the cold-sintered matrix material with an inorganic solvent that can dissolve it → pressurizing and heating (to evaporate the solvent) → forming a densified inorganic material. Since this method can only form isotropic inorganic materials, the magnetic properties of permanent magnet materials will be significantly reduced. Summary of the Invention

[0011] To address the shortcomings of the aforementioned technologies, this invention provides a method for preparing anisotropic bulk ferrite permanent magnet materials using low-temperature sintering, thereby obtaining anisotropic permanent magnet bulk materials with high magnetic properties at a lower temperature.

[0012] To address the technical problems of high sintering temperatures in traditional bulk ferrite material preparation and the inability to obtain anisotropic bulk magnets from existing cold-sintered materials, the present invention adopts the following technical solution: a method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering, comprising the following steps: Step S1: Preparation of the transition solution; Step S2: Preparation of cold-sintered green bodies; Step S3: Low-temperature firing (also known as cold sintering); Step S4: Stress-relief annealing.

[0013] Preferably, the specific process of step S1 is as follows: The organic acid that can partially dissolve ferrite permanent magnet powder is diluted with an organic alcohol to form a mixed solution of a certain concentration, which is the transition solution.

[0014] Preferably, in step S1: Hexagonal planar anisotropic ferrite permanent magnet powder is SrFe 12 O 19 BaFe 12 O 19 PbFe 12 O 19 The mixture of two or more of them; the organic acid is one or a mixture of several of acetic acid, organophosphonic acid, and acetic acid; the organic alcohol is an alkyl alcohol; In the transition solution, alkyl alcohols are used as solvents and organic acids as solutes, with the concentration of organic acids controlled at 10.5-15 mol / L; Organic matter in the transition solution is removed by volatilization during subsequent cold sintering or annealing processes.

[0015] Preferably, the specific process of step S2 is as follows: Ferrite permanent magnet powder with a particle size of 0.1-2.0 micrometers was added to the transition solution; The magnetic powder and the transition solution are uniformly mixed by grinding or ball milling to form a slurry with a viscosity of 2000-10000 mPa·s; Ferrite powder is thoroughly mixed with the transition solution to form a slurry. The viscosity of the slurry is controlled between 2000-10000 mPa·s. If the viscosity is too high, it will not be conducive to the dispersion of magnetic powder in the transition solution and the fluidity will be poor when filling the mold. Excessive viscosity will also be detrimental to improving the orientation degree when the magnetic powder is oriented. If the viscosity is too low, the mixture will be easily squeezed out of the mold when pressure is applied.

[0016] The slurry was then filled into a non-magnetic steel mold, which was then placed in a strong magnetic field for orientation and simultaneously pressed to obtain a density of 2.2-3.0 g / cm³. 3 Cold-sintered green bodies; The green body is then subjected to further cold isostatic pressing to increase its density to 3.0-4.0 g / cm³. 3 .

[0017] Preferably, the specific process of step S3 is as follows: The cold sintered green compact after cold isostatic pressing is placed in a steel mold with a heating device. First, a pressure of 100-250MPa is applied and the temperature is controlled at 50-150℃, so that the ferrite permanent magnet powder in the green compact is partially chemically dissolved by organic acid in the transition solution to form colloidal metal salt, while the undissolved oxides are coated on the surface of the ferrite permanent magnet powder to form a core-shell structure. Further increasing the pressure and temperature causes the organic matter in the transition solution to volatilize, and the colloidal metal salt dissolved in the transition solution to dehydrate, decompose, and crystallize into nanoparticles. These nanoparticles then come into close contact with the iron oxides on the surface of the ferrite permanent magnet powder, undergoing a sintering reaction and densifying the magnet to obtain a bulk material.

[0018] Preferably, the specific process of step S4 is as follows: The block obtained by cold sintering is placed in an annealing furnace and annealed at 700℃-1100℃ for 0.5h-4h to eliminate the internal stress during the sintering process, and finally a high-density, high-performance anisotropic bulk ferrite permanent magnet material is obtained.

[0019] The key to achieving low-temperature sintering is the reaction between ferrite powder and a transition solution containing organic acids, and the reaction rate affects the density and magnetic properties of the final magnet. The reaction rate is related to factors such as 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 the pressure.

[0020] Smaller magnetic particle sizes result in a larger surface area in contact with the transition solution, making the cold sintering process under pressure easier and yielding a denser sample. However, the magnetic particles are more likely to overflow from the mold gaps, leading to difficulty in demolding. Therefore, ferrite powder with a size of 0.1-2 micrometers is most suitable.

[0021] When the mass ratio of ferrite powder to transition solution is fixed, the concentration of the transition solution (as a percentage of the total solution mass or molar ratio of organic acid) has a significant impact on the cold sintering process. A low concentration results in a relatively low content of organic acid, leading to a slow chemical dissolution rate of the ferrite powder, resulting in incomplete cold sintering, low density, and a high content of organic alcohol, which is difficult to remove completely through volatilization. However, an excessively high concentration leads to an excessive amount of iron oxides generated during chemical dissolution, resulting in incompletely sintered iron oxides in the magnet after cold sintering, causing a decrease in magnet performance. The molar concentration of the transition solution should be controlled between 10.5-15 mol / L.

[0022] Preferably, in step S2, the mass ratio of ferrite permanent magnet powder to transition solution during the preparation of the cold sintered green body is 2:1-7:1. When the ratio is greater than 7:1, the transition solution is too small, resulting in too few dissolved magnetic particles and insufficient material to precipitate between the particles, leading to poor densification. For example, when the ratio of solid magnetic particles to transition solution is 10:1, the density of the cold sintered product is 68%, resulting in more porosity and poor performance. If the transition solution is too small, it may also lead to cold sintering failure. When the ratio is less than 2:1, the excessive fluidity of the particles will cause them to overflow from between the upper and lower molds and the outer mold, making demolding difficult.

[0023] Preferably, in step S2, during the preparation of the cold sintered green body, the magnitude of the strong magnetic field is 1.0T-2.5T, and the magnetic field is perpendicular or horizontal to the pressure direction, with the magnetic field being preferably perpendicular to the pressure direction.

[0024] When the green billet is cold isostatically pressed, it is first vacuum sealed and then placed in a mold filled with oil medium, and pressure is applied at 200MPa-450MPa.

[0025] Pressure and temperature control are crucial parameters during cold sintering. The pressure is uniaxial, ranging from 200 MPa to 1.5 GPa. Insufficient pressure fails to provide enough kinetic energy for the precipitate in the supersaturated solution, while excessive pressure leads to premature compaction and closure of transport channels. Both situations are detrimental to product densification. A moderate temperature, ranging from 250°C to 500°C, is applied simultaneously with the pressure. Too low a temperature, similar to too low a pressure, fails to provide sufficient kinetic energy for the precipitate's migration; too high a temperature causes the transition solution to evaporate too quickly, resulting in an incomplete "dissolution-precipitation" process, also negatively impacting the final density. A final pressure and temperature value can be set, and the temperature and pressure can be applied sequentially in multiple steps from low to high, maintaining the pressure and temperature for a period of time.

[0026] Preferably, in step S3, during the cold sintering process, the final pressure and temperature values ​​are set, and the temperature and pressure are applied sequentially from low to high in multiple steps to the final values, and the pressure and temperature are maintained for a period of time.

[0027] Preferably, in step S3, the firing pressure is between 200 MPa and 1.5 GPa, the firing temperature is between 200°C and 500°C, and the firing time is between 0.5 h and 4 h.

[0028] This invention is based on the following principle: planar hexagonal ferrites are all composite oxides, such as strontium ferrite (SrFe). 12 O 19 It is a composite oxide composed of strontium oxide and iron oxide, and these two oxides have different chemical dissolution rates in organic acids.

[0029] Under certain temperature catalytic action, a metal oxide in ferrite materials is preferentially dissolved by organic acids and enters the organic matter to form colloidal metal salts, while the iron oxide that is not fully dissolved coats the surface of the ferrite powder to form a core-shell structure.

[0030] As the temperature further increases, organic matter volatilizes, and colloidal metal salts dissolved in the organic matter dehydrate, decompose, and crystallize into metal oxide nanoparticles. Under high pressure, these crystalline particles come into close contact with the iron oxide coating on the powder surface and undergo a sintering reaction to further form composite oxides. Due to the high pressure and the small particle size, the sintering temperature for the formation of composite oxides is significantly reduced. A schematic diagram of the process is shown below. Figure 1 As shown.

[0031] The present invention provides a method for uniformly mixing ferrite permanent magnet powder solid particles with a transitional solution that can partially dissolve or transport magnetic particles, orienting and pressing them into green blanks under a strong magnetic field, further increasing the density through cold isostatic pressing, and finally obtaining anisotropic bulk magnets by cold sintering at high pressure and a lower temperature than conventional sintering temperature.

[0032] This invention first uses a strong magnetic field to orient the cold-sintering raw material (a reaction mixture of permanent magnet powder and a transition solution), then presses it in a cold isostatic press to increase density and maintain orientation. This method allows for the production of anisotropic bulk permanent magnet materials through cold sintering, resulting in superior performance compared to isotropic bulk permanent magnet materials. Furthermore, the cold sintering process of this invention lowers the sintering temperature compared to traditional high-temperature sintering. Lowering the sintering temperature not only saves energy but also inhibits grain growth. Smaller grains mean higher coercivity, thus resulting in better magnetic properties. Attached Figure Description

[0033] Figure 1 This is a schematic diagram illustrating the steps of cold sintering strontium ferrite in a transition solution composed of acetic acid and ethanol. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to specific embodiments.

[0035] Example 1 Acetic acid and ethanol were mixed to form a transition solution, with acetic acid as the solute and ethanol as the solvent. The concentrations of acetic acid in the transition solution were 10 mol / L, 12 mol / L, 14 mol / L, and 16 mol / L, respectively. Then, SrFe... 12 O 19 The magnetic powder was mixed with transition solutions of different acetic acid concentrations at a mass ratio of 4:1 and then ground in an Al2O3 mortar for two minutes to ensure uniform mixing.

[0036] The above samples were placed into non-magnetic steel molds and placed in a 1.2T strong magnetic field for orientation pressing. The pressure and magnetic field direction were perpendicular. The orientation-pressed green billets were then placed in a cold isostatic press and subjected to a pressure of 500KN to further increase the density and maintain the degree of orientation.

[0037] The cold isostatically pressed green bodies were placed in a heated mold for low-temperature firing. A uniaxial pressure of 1 GPa was applied, and the temperature was raised to 450°C at a rate of 23°C per minute, and then held at that temperature and pressure. The low-temperature firing process lasted for a total of 3 hours.

[0038] Table 1 shows SrFe 12 O 19 Performance data of samples obtained by mixing magnetic powder with transition solutions of different acetic acid concentrations and sintering at low temperature.

[0039] Table 1 As shown in Table 1, the density of the cold-sintered samples increased as the concentration of acetic acid in the transition solution increased from 10 mol / L to 16 mol / L. This is because the increased acetic acid concentration resulted in more nanoparticles, which is beneficial for sintering and increasing density.

[0040] However, the remanent magnetic induction Br, intrinsic coercivity Hcj, and maximum energy product (BH) are also important factors. max Several magnetic performance indicators showed a trend of first increasing and then decreasing. This is because the acetic acid concentration in the transition solution was too low, resulting in a slow chemical dissolution rate of the ferrite magnetic powder, leading to insufficient subsequent cold sintering and low density. However, if the acetic acid concentration in the transition solution was too high, the amount of iron oxides generated by chemical dissolution would be excessive, resulting in incomplete doping of iron oxides (α-Fe2O3) in the magnet after cold sintering, thus degrading the magnet's performance. The molar concentration of the transition solution was controlled at 10.5-15 mol / L.

[0041] Then SrFe 12 O 19 The magnetic powder was mixed with transition solutions of different acetic acid concentrations and the samples were annealed at 800℃ for 3 hours after being sintered at low temperature.

[0042] Table 2 shows the density of the annealed samples obtained by the volumetric method and the magnetic properties obtained by the permanent magnet tester.

[0043] Table 2 Comparing Tables 1 and 2, the changes in density and magnetic properties of the samples after annealing under the corresponding conditions are the same as before annealing, but at the same concentration, both density and magnetic properties are further improved after annealing. This is because annealing eliminates the internal stress generated during the firing process. Moreover, compared with traditional methods, the low-temperature firing temperature and annealing temperature in this method are much lower than the traditional sintering temperature (≥1000℃), but the remanence, coercivity, and maximum energy product of the products obtained by this method are comparable to those of products on the market. In particular, when the acetic acid concentration of the filtration solution is 14 mol / L, its comprehensive performance indicators are excellent.

[0044] The above embodiments are not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the technical solution of the present invention are also within the protection scope of the present invention.

Claims

1. A method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering, characterized in that: Includes the following steps: Step S1, Preparation of transition solution: The organic acid that can partially dissolve the ferrite permanent magnet powder is diluted with an organic alcohol to form a mixed solution of a certain concentration, which is the transition solution. The organic alcohol is an alkyl alcohol. The transition solution uses alkyl alcohol as solvent and organic acid as solute. The concentration of organic acid is controlled at 10.5-15 mol / L. Step S2, Preparation of cold sintered green body: Ferrite permanent magnet powder with a particle size of 0.1-2.0 micrometers is added to the transition solution; The magnetic powder and the transition solution are uniformly mixed to a specific viscosity using grinding or ball milling methods. 2000-10000 mPa.s of Slurry; The slurry was then filled into a non-magnetic steel mold, which was then placed in a strong magnetic field for orientation and simultaneously pressed to obtain a density of 2.2-3.0 g / cm³. 3 Cold-sintered green bodies; The green body is then subjected to further cold isostatic pressing to increase its density to 3.0-4.0 g / cm³. 3 ; Step S3: Low-temperature firing; Step S4: Stress-relief annealing.

2. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 1, characterized in that: In step S1: Ferrite permanent magnet powder is SrFe 12 O 19 BaFe 12 O 19 PbFe 12 O 19 The mixture of two or more of them; the organic acid is one or a mixture of several of acetic acid, organophosphonic acid, and acetic acid; the organic alcohol is an alkyl alcohol; Organic matter in the transition solution is removed by volatilization during subsequent cold sintering or annealing processes.

3. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 1, characterized in that: The specific process of step S3 is as follows: The cold sintered green compact after cold isostatic pressing is placed in a steel mold with a heating device. First, a pressure of 100-250MPa is applied and the temperature is controlled at 50-150℃, so that the ferrite permanent magnet powder in the green compact is partially chemically dissolved by organic acid in the transition solution to form colloidal metal salt, while the undissolved oxides are coated on the surface of the ferrite permanent magnet powder to form a core-shell structure. Further increasing the pressure and temperature causes the organic matter in the transition solution to volatilize, and the colloidal metal salt dissolved in the transition solution to dehydrate, decompose, and crystallize into nanoparticles. These nanoparticles then come into close contact with the iron oxides on the surface of the ferrite permanent magnet powder, undergoing a sintering reaction and densifying the magnet to obtain a bulk material.

4. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 1, characterized in that: The specific process of step S4 is as follows: The block obtained by cold sintering is placed in an annealing furnace and annealed at 700℃-1100℃ for 0.5h-4h to eliminate the internal stress during the sintering process, and finally a high-density, high-performance anisotropic bulk ferrite permanent magnet material is obtained.

5. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 1, characterized in that: In step S2, the mass ratio of ferrite permanent magnet powder to transition solution during the cold sintering green body preparation process is 2:1-7:

1.

6. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 5, characterized in that: In step S2, during the preparation of the cold sintered green body, the magnitude of the strong magnetic field is 1.0T-2.5T, and the magnetic field is perpendicular or horizontal to the pressure direction.

7. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 3, characterized in that: In step S3, during the cold sintering process, the final pressure and temperature values ​​are set, and the temperature and pressure are applied sequentially from low to high in multiple steps to the final values, and the pressure and temperature are maintained for a period of time.

8. The method for preparing anisotropic bulk ferrite permanent magnet materials by low-temperature sintering according to claim 7, characterized in that: In step S3, the firing pressure is between 200 MPa and 1.5 GPa, the firing temperature is between 200°C and 500°C, and the firing time is between 0.5 h and 4 h.

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