Anti-transient low temperature demagnetization permanent magnet ferrite and method of making same

Abstract

The application discloses anti-transient low-temperature demagnetization permanent magnet ferrite, taking M-type strontium ferrite as a main phase, which is composed of the following components in terms of molar percentage: main raw material: ; composite dopant: ; and the balance is inevitable impurities. 3+ The synergistic substitution of Cr 3+ significantly improves the magnetocrystalline anisotropy field of the material, and the coercive force Hcj at-40 DEG C is greater than or equal to 320 kA / m, so that the risk of transient demagnetization caused by the downward movement of the demagnetization curve inflection point at low temperature is fundamentally avoided.

Description

Technical Field

[0001] This application relates to the field of permanent magnet ferrite materials technology, and in particular to permanent magnet ferrites resistant to transient low-temperature demagnetization and methods for preparing such materials. Background Technology

[0002] Permanent magnet ferrites are widely used in various electronic and electrical devices due to their low cost and good corrosion resistance. In certain specific applications, such as cryogenic sensors, polar micromotors, and deep-sea exploration equipment, permanent magnet ferrites are required to operate stably in low-temperature environments down to -40°C.

[0003] However, existing conventional permanent magnet ferrites (such as M-type) Significant defects exist in low-temperature environments of -40℃, where the coercivity decreases drastically, causing an inflection point in the demagnetization curve. When the demagnetization field strength exceeds this inflection point under transient conditions such as startup or sudden load changes, irreversible transient demagnetization is very likely to occur, causing permanent equipment failure. Authoritative test results show that conventional ferrite magnets begin to show obvious demagnetization when the ambient temperature reaches around -40℃, and cannot meet the usage requirements of low-temperature transient conditions. Summary of the Invention

[0004] To address the aforementioned technical problems, this application provides a permanent magnet ferrite resistant to transient low-temperature demagnetization and its preparation method.

[0005] The permanent magnet ferrite resistant to transient low-temperature demagnetization is mainly composed of M-type strontium ferrite, and its raw materials are composed of the following components in molar percentage: Main raw materials: ; Composite dopant: The balance represents unavoidable impurities.

[0006] Through the precise formulation of the above components, a complete synergistic system for resisting transient demagnetization was constructed. Specifically, Al₂O₃ and Cr₂O₃ synergistically replace Fe. 3+ The material's low-temperature magnetocrystalline anisotropy field is intrinsically improved, solving the fundamental problem of low-temperature coercivity attenuation. La2O3 refines the grains, optimizes the microstructure stability, and provides a structural carrier for demagnetization resistance. The low-melting-point composite fluxing system formed by CaCO3, SiO2 and H3BO3 achieves low-temperature sintering densification, ensuring the effective implementation of the above mechanisms from a process perspective.

[0007] Preparation method of permanent magnet ferrite resistant to transient low-temperature demagnetization Includes the following steps: S1: Raw material preparation and mixing: Weigh each raw material according to the molar percentage described in claim 1, and perform wet ball milling to mix and obtain an initial slurry; S2: Pre-calcination synthesis of the main phase: After drying the initial slurry, pre-calcination is performed to generate M-type strontium ferrite main phase powder; Secondary ball milling refinement: The main phase powder is subjected to secondary wet ball milling to control the powder particle size D50 at 0.8-1.0μm; S4: Magnetic field forming: The slurry after secondary ball milling is pressed and formed under the condition of applying an external magnetic field to obtain an anisotropic green body; S5: Low-temperature sintering densification: Sintering the green body to obtain a permanent magnet ferrite green body; S6: Post-processing: Processing the sintered blank to obtain the finished product.

[0008] As a preferred embodiment of the above technical solution, in S1, the ball-to-material-to-water ratio of the ball mill is 10:1:2, and the ball milling process is to first use high-energy ball milling at 800-1000 r / min for 2-3 hours, and then use low-energy ball milling at 300-400 r / min for 1-2 hours. The ball milling process employs a "high-energy + low-energy" two-stage speed-changing process; the ball-to-material-to-water ratio is 10:1:2: this ratio ensures that the ball milling media (hard steel balls) have sufficient impact and grinding energy, while the slurry has suitable fluidity, which can effectively crush and mix raw materials, and avoid the decrease in ball milling efficiency caused by excessive slurry thickness.

[0009] As a preferred embodiment of the above technical solution, in step S2, the pre-firing temperature is 1180-1220℃, and the holding time is 2-3 hours. Because CaCO3, SiO2, and H3BO3 in the formulation of this invention begin to form a small amount of liquid phase during the pre-calcination stage, the mass transfer process of the solid-phase reaction is promoted, thereby reducing the temperature required for the formation of the main phase; at this temperature, SrO decomposed from SrCO3 reacts fully with Fe2O3, while Al... 3+ Cr 3+ La 3+ Doped ions begin to enter or affect the crystal lattice.

[0010] As a preferred embodiment of the above technical solution, in step S3, the rotation speed of the secondary wet ball mill is 600-800 r / min, and the time is 4-6 h; Controlling the powder particle size to the submicron level ensures that each particle has good rotational orientation ability in the magnetic field (mostly single-domain particles), which can significantly improve the orientation degree of the green body, thereby increasing the remanence Br and magnetic energy product of the final magnet. On the other hand, powders in this particle size range have suitable sintering activity, which can achieve full densification in the subsequent low-temperature sintering process.

[0011] As a preferred embodiment of the above technical solution, in step S4, the strength of the external magnetic field is 1800-2200 kA / m, and the molding pressure is 150-200 MPa. Through the coupling effect of "strong magnetic field + pressure", a green body with a high c-axis orientation was obtained. This highly oriented structure maximizes the remanence Br of the final sintered body and improves the rectangularity of the demagnetization curve. As a result, it can more effectively resist the external demagnetization field at low temperatures, which is an important structural basis for ensuring the ability to resist transient demagnetization.

[0012] As a preferred embodiment of the above technical solution, in S5, the sintering process is as follows: first, the temperature is raised to 600-700℃ at a rate of 5-8℃ / min and held for 1 hour; then, the temperature is raised to 1160-1200℃ at a rate of 3-5℃ / min and held for 1.5-2.5 hours; finally, the temperature is lowered to room temperature at a rate of 2-3℃ / min. The low-temperature sintering process achieves high density while successfully suppressing grain growth, resulting in a fine and uniform grain structure (in synergy with the pinning effect of La2O3). This high-density and fine-grained microstructure ensures high remanence, high coercivity, and excellent mechanical properties. At the same time, the lower sintering temperature significantly reduces production costs.

[0013] In summary, the present invention has at least one of the following beneficial technical effects: 1. Through AI 3+ With Cr 3+ The synergistic substitution significantly enhances the magnetocrystalline anisotropy field of the material, making the coercivity Hcj at -40℃ ≥ 320kA / m, fundamentally avoiding the risk of transient demagnetization caused by the downward shift of the demagnetization curve inflection point at low temperatures; 2. By combining La2O3 grain refinement with magnetic field forming process, a highly oriented fine-grained structure is obtained, so that the material's magnetic properties can be reversibly changed by ≥98% in the range of -40℃ to room temperature, and the remanence Br at -40℃ is ≥0.40T; 3. The material itself has the ability to work stably in a low temperature environment of -40℃, and can be directly applied to scenarios such as polar exploration, deep-sea operations, and low temperature sensors, which greatly simplifies the system structure and reduces costs and failure rates; 4. The low-temperature pre-firing and low-temperature sintering process reduces the temperature by 40-90℃ compared to the traditional process, saving energy and reducing high-temperature defects; the raw materials are all conventional industrial raw materials, and the amount of composite dopants used is small, which is easy to mass-produce in the industrial sector. Detailed Implementation

[0014] The present application will be further described in detail below with reference to the embodiments.

[0015] This application relates to a permanent magnet ferrite resistant to transient low-temperature demagnetization, with M-type strontium ferrite as the main phase, and its raw materials are composed of the following molar percentage components: Main raw materials: Composite dopant: The balance represents unavoidable impurities. Al2O3+Cr2O3 is responsible for fundamentally solving the problem of insufficient coercivity at low temperatures and providing the core capability to resist demagnetization; La2O3 is responsible for building a stable microstructure, eliminating internal defects that may cause demagnetization, and protecting the stable performance of the core capability; CaCO3+SiO2+H3BO3 is responsible for completing densification at the optimal temperature, avoiding the destruction of the effects of the former two by high temperature, and providing additional gains through grain boundary optimization. By precisely matching composite dopants and synergistically controlling process parameters throughout the entire process, the intrinsic anti-demagnetization capability and microstructure stability of M-type strontium ferrite are fundamentally improved, thereby ensuring that it can maintain stable magnetic properties under extreme cold and transient conditions.

[0016] A method for preparing permanent magnet ferrite resistant to transient low-temperature demagnetization includes the following steps: S1: Raw material preparation and mixing: Weigh each raw material by molar percentage, perform wet ball milling and mixing to obtain the initial slurry; It is essential to ensure that all components, especially the composite dopants used in small quantities but with crucial effects, achieve atomic-level uniform distribution at both the macroscopic and microscopic scales. This is the foundation for the full progress of subsequent solid-state reactions and the synergistic effect of each dopant element. The ball milling process employs a "high-energy + low-energy" two-stage speed-changing process; the ball-to-material-to-water ratio is 10:1:2: this ratio ensures that the ball milling media (hard steel balls) have sufficient impact and grinding energy, while the slurry has suitable fluidity, which can effectively crush and mix raw materials, and avoid the decrease in ball milling efficiency caused by excessive slurry thickness. High-energy ball milling (800-1000 r / min, 2-3 h): This stage utilizes the high impact energy of the steel balls under high-speed rotation to rapidly crush and initially mix the raw material powder (especially coarser SrCO3, Al2O3, etc.). High-energy impact can also introduce lattice defects and stress inside the particles, increasing the activity of the powder and providing more nucleation sites for the solid-phase reaction in the subsequent pre-calcination. Low-energy ball milling (300-400 r / min, 1-2 h): After the high-energy ball milling achieves crushing, it is transferred to the low-energy ball mill, which mainly plays a role in "homogenization". By using gentler grinding and stirring, the broken component powders are further evenly dispersed, preventing agglomeration and ensuring the chemical homogeneity of the final slurry. This variable-speed ball milling process can obtain an initial slurry with extremely uniform component distribution and high activity, creating conditions for the synthesis of a single, high-purity M-type main phase at a slightly lower pre-calcination temperature, and avoiding local component segregation and impurity phase formation caused by uneven mixing.

[0017] S2: Pre-calcination synthesis of the main phase: After drying the initial slurry, pre-calcination is performed to generate M-type strontium ferrite main phase powder; Through solid-state reaction, the mixed raw material powder is transformed into a main phase powder with an M-type magnetoplumbourine crystal structure; The temperature range of 1180-1220℃ is 30-50℃ lower than that of the traditional process (around 1250℃). This is because CaCO3, SiO2, and H3BO3 in the formula of this invention begin to form a small amount of liquid phase during the pre-calcination stage, which promotes the mass transfer process of the solid-phase reaction, thereby reducing the temperature required for the formation of the main phase. At this temperature, SrO decomposed from SrCO3 reacts fully with Fe2O3, while Al... 3+ Cr 3+ La 3+ Doped ions begin to enter or affect the crystal lattice; Using a rotary kiln allows materials to be heated dynamically and evenly, avoiding local overheating and ensuring the uniformity of the main phase composition and structure; it also controls the cooling rate to avoid internal stress caused by rapid cooling or freezing of lattice defects, thus ensuring the crystal integrity of the main phase powder. Using a lower pre-calcination temperature than traditional processes can significantly suppress abnormal grain growth during pre-calcination, resulting in pre-calcined powder with high activity and moderate grain size, laying the foundation for subsequent secondary ball milling and final sintering to obtain a fine-grained structure; at the same time, it reduces energy consumption.

[0018] Secondary ball milling refinement: The main phase powder is subjected to secondary wet ball milling to control the powder particle size D50 at 0.8-1.0μm; Crushing the pre-sintered main phase powder to a specific particle size range is the key to achieving high grain orientation during subsequent magnetic field forming and high density during final sintering. Medium-speed ball milling (600-800 r / min, 4-6 h) was used to pulverize the pre-calcined material into powder mainly composed of single-crystal or polycrystalline particles; the particle size D50 was controlled at 0.8-1.0 μm: this is an optimized key parameter that is highly consistent with the fine-grained design target of La2O3; If the particle size is less than 0.8μm, the powder is too fine, the surface energy is too high, and it is easy to agglomerate. In addition, the rotational resistance of the particles increases during magnetic field forming, which is not conducive to orientation. At the same time, the shrinkage rate is too large during sintering, which can easily lead to deformation and cracking. If the particle size is greater than 1.0 μm, it may contain multiple grains. During orientation, it is impossible to align the easy magnetization axis of each grain with the magnetic field direction, which reduces the degree of orientation. At the same time, the coarse powder has low sintering activity and it is difficult to obtain high density in subsequent low-temperature sintering. Controlling the powder particle size to the submicron level ensures that each particle has good rotational orientation ability in the magnetic field (mostly single-domain particles), which can significantly improve the orientation degree of the green body, thereby increasing the remanence Br and magnetic energy product of the final magnet; on the other hand, powders in this particle size range have suitable sintering activity, which can achieve full densification in the subsequent low-temperature sintering process. S4: Magnetic field forming: The slurry after secondary ball milling is pressed and formed under the condition of applying an external magnetic field to obtain an anisotropic green body; For anisotropic permanent magnet ferrite, an external magnetic field must be applied to orient the grains with uniaxial magnetocrystalline anisotropy along their easy magnetization axis (c-axis) in order to maximize the magnetic properties of the material. The magnetic field strength is in the range of 1800-2200 kA / m. This strong magnetic field is sufficient to overcome the friction and thermal motion between powder particles and drive the easy magnetization axis of each particle to rotate and align along the direction of the external magnetic field. If the magnetic field is too low, the orientation will be incomplete; if it is too high, the equipment requirements will be demanding and uneconomical. Applying pressure (150-200 MPa) while applying a magnetic field compacts the oriented particles, fixing their orientation and forming a green body with a certain strength. If the pressure is too low, the green body density will be insufficient and it will be easily damaged. If the pressure is too high, it may destroy the orientation of the formed particles or cause damage to the mold. Through the coupling effect of "strong magnetic field + pressure", a green body with a high c-axis orientation was obtained. This highly oriented structure maximizes the remanence Br of the final sintered body and improves the rectangularity of the demagnetization curve. As a result, it can more effectively resist the external demagnetization field at low temperatures, which is an important structural basis for ensuring the ability to resist transient demagnetization. S5: Low-temperature sintering densification: Sintering the green blank to obtain a permanent magnet ferrite blank; Transforming green bodies into sintered bodies with high density, high mechanical strength and ultimate magnetic properties while maintaining fine grain structure and high orientation; First stage (removal of binder): Heat to 600-700℃ at a rate of 5-8℃ / min and hold for 1 hour; slowly remove moisture, molding additives (if any), and any residual CO3 from the green body. 2- Wait; slow heating is to prevent rapid gas escape that could cause cracking in the green body; The second stage (densification sintering): the temperature is increased to 1160-1200℃ at 3-5℃ / min and held for 1.5-2.5h; the design advantages of the CaCO3-SiO2-H3BO3 composite fluxing system are fully utilized. Due to the presence of the fluxing system in the formulation, a suitable amount of liquid phase is formed in this temperature range, realizing liquid phase sintering; the mass transfer rate is fast, and the densification process can be completed at a relatively low temperature. Controlling the heating rate (3-5℃ / min) helps the liquid phase to form and distribute uniformly, avoids local overheating that leads to abnormal grain growth, and maintains the fine-grained structure brought by La2O3; A heat preservation time of 1.5-2.5 hours ensures that the densification process is fully completed, so that the density reaches more than 95% of the theoretical density; Cool slowly to room temperature at a rate of 2-3℃ / min; Slow cooling can: ① Eliminate thermal stress: Prevent microcracks in the magnet caused by thermal stress concentration, which can become the source of demagnetization; ② Stabilize grain boundary phase: Make the distribution of the solidified grain boundary phase during the cooling process more uniform and the structure more stable; ③ Maintain magnetic properties: Avoid rapid cooling that could disrupt the magnetic domain structure and ensure the stability of the final magnetic properties; The low-temperature sintering process achieves high density while successfully suppressing grain growth, resulting in a fine and uniform grain structure (in synergy with the pinning effect of La2O3). This high-density and fine-grained microstructure ensures high remanence, high coercivity, and excellent mechanical properties. At the same time, the lower sintering temperature significantly reduces production costs. S6: Post-processing: The sintered blank is processed to obtain the finished product. The sintered blank is then machined through cutting, grinding, and other mechanical processes to obtain precise dimensions and surface finish that meet the requirements of the final application. The surface of the finished product is then galvanized or nickel-plated to form a dense protective film that effectively isolates it from external corrosive media (such as humid air, salt spray, etc.), significantly improving the long-term reliability of the magnet in harsh environments and extending its service life. Specific Implementation (I) Example 1 The ferrite for low-temperature permanent magnets resistant to transient low-temperature demagnetization has the following raw material molar percentages: Fe2O3 62%, SrCO3 10%, CaCO3 2.5%, Al2O3 2%, La2O3 1%, Cr2O3 0.8%, SiO2 0.5%, H3BO3 0.3%, with the balance being impurities.

[0020] The preparation method is as follows: S1: Raw material batching and mixing: Weigh each raw material according to the above proportions, put them into the ball mill, with a ball-to-material-to-water ratio of 10:1:2, use hard steel balls, first ball mill at 900 r / min for 2.5 h, then ball mill at 350 r / min for 1.5 h to obtain a uniform initial slurry; S2: Pre-calcination synthesis: After filtering and drying the initial slurry, it is placed in a rotary kiln and pre-calcined at 1200℃ for 2.5h. It is then cooled to room temperature with the furnace to obtain M-type strontium ferrite main phase powder. Secondary ball milling: The main phase powder is fed into a ball mill with a ball-to-material-to-water ratio of 10:1:2 and ball milled at 700 r / min for 5 h to make the powder D50 = 0.9 μm; S4: Magnetic field molding: The slurry is concentrated to a solid content of 70%, and then pressed into shape by applying a magnetic field of 2000 kA / m and a pressure of 180 MPa to obtain an anisotropic green body; S5: Low-temperature sintering: Place the green billet in a sintering furnace, heat it to 650℃ at 6℃ / min and hold it for 1 hour, then heat it to 1180℃ at 4℃ / min and hold it for 2 hours, and finally cool it to room temperature at 2.5℃ / min. S6: Post-processing: Cut and grind to the specified dimensions, then galvanize the surface to obtain the finished product.

[0021] (II) Example 2 The ferrite for low-temperature permanent magnets resistant to transient low-temperature demagnetization has the following raw material molar percentages: Fe2O3 60%, SrCO3 11%, CaCO3 3%, Al2O3 1.5%, La2O3 1.2%, Cr2O3 0.5%, SiO2 0.6%, H3BO3 0.2%, with the balance being impurities.

[0022] The preparation method is basically the same as in Example 1, except that the following parameters are adjusted: pre-calcination temperature 1220℃, holding time 3h; secondary ball milling speed 800r / min, time 4h, powder D50=0.8μm; magnetic field forming magnetic field strength 2200kA / m, pressure 200MPa; sintering heating rate: first stage heating at 7℃ / min to 700℃, second stage heating at 5℃ / min to 1200℃, holding time 2.5h.

[0023] The low-temperature permanent magnet ferrite products prepared in Examples 1 and 2 were subjected to low-temperature magnetic performance tests at -40℃ and transient demagnetization tests, and were compared with conventional M-type strontium ferrite (without the composite dopant of this invention, sintering temperature >1250℃). The test results are shown in the table below: Test results show that the low-temperature permanent magnet ferrite prepared by this invention has high coercivity and high remanence in a low-temperature environment of -40℃, good magnetic reversibility, and no demagnetization phenomenon under transient load change conditions. It can completely solve the problem of transient demagnetization that easily occurs in the existing technology in a low-temperature environment of -40℃. In contrast, conventional ferrites show obvious transient demagnetization under the same conditions, and their performance drops significantly.

[0024] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it.

Claims

1. A permanent magnet ferrite resistant to transient low-temperature demagnetization, with M-type strontium ferrite as the main phase, characterized in that, Its raw materials consist of the following molar percentage components Composition: Main ingredients: Composite dopant: The balance represents unavoidable impurities.

2. A method for preparing permanent magnet ferrite resistant to transient low-temperature demagnetization, used to prepare the permanent magnet ferrite according to claim 1, characterized in that, Includes the following steps: S1: Raw material preparation and mixing: Weigh each raw material according to the molar percentage described in claim 1, and perform wet ball milling to mix and obtain an initial slurry; S2: Pre-calcination synthesis of the main phase: After drying the initial slurry, pre-calcination is performed to generate M-type strontium ferrite main phase powder; Secondary ball milling refinement: The main phase powder is subjected to secondary wet ball milling to control the powder particle size D50 at 0.8-1.0μm; S4: Magnetic field forming: The slurry after secondary ball milling is pressed and formed under the condition of applying an external magnetic field to obtain an anisotropic green body; S5: Low-temperature sintering densification: Sintering the green body to obtain a permanent magnet ferrite green body; S6: Post-processing: Processing the sintered blank to obtain the finished product.

3. The method for preparing the transient low-temperature demagnetization ferrite permanent magnet according to claim 2, characterized in that: In S1, the ball-to-material-to-water ratio in the ball mill is 10:1:2, and the ball milling process is to first ball mill at a high energy rate of 800-1000 r / min for 2-3 hours, and then ball mill at a low energy rate of 300-400 r / min for 1-2 hours.

4. The method for preparing the transient low-temperature demagnetization ferrite permanent magnet according to claim 2, characterized in that: In S2, the pre-firing temperature is 1180-1220℃, and the holding time is 2-3h.

5. The method for preparing the transient low-temperature demagnetization ferrite permanent magnet according to claim 2, characterized in that: In S3, the rotation speed of the secondary wet ball mill is 600-800 r / min, and the time is 4-6 h.

6. The method for preparing the transient low-temperature demagnetization ferrite permanent magnet according to claim 2, characterized in that: In S4, the strength of the external magnetic field is 1800-2200 kA / m, and the molding pressure is 150-200 MPa.

7. The method for preparing the transient low-temperature demagnetization ferrite permanent magnet according to claim 2, characterized in that: In S5, the sintering process is as follows: first, the temperature is raised to 600-700℃ at a rate of 5-8℃ / min and held for 1 hour; then, the temperature is raised to 1160-1200℃ at a rate of 3-5℃ / min and held for 1.5-2.5 hours; finally, the temperature is lowered to room temperature at a rate of 2-3℃ / min.

Images