Preparation method of high-power-density high-resistivity M-type strontium ferrite

Abstract

The application discloses a high-power-density high-resistivity M-type strontium ferrite and belongs to the technical field of ferrite material preparation. The ferrite comprises a main formula formed by SrCO3, La2O3, CaCO3, Fe2O3 and Co2O3, and an additive composed of CaCO3, SiO2, H3BO3 and Sb2O3. The application is characterized in that: the powder with different pre-sintering temperatures is compounded, and the crystallization is carried out under the same sintering temperature and holding time, so that the obtained microstructure makes the green body have less porosity and more uniform structure, and the sintering density of the strontium ferrite material is remarkably improved. Meanwhile, the H3BO3-Sb2O3-CaCO3-SiO2 composite additive system is combined, the sintering driving force is connected in the whole sintering temperature range, the densification driving force in the whole sintering temperature range is improved, and the excellent performances of high density, fine grain and high resistivity are obtained.

Description

Technical Field

[0001] This invention belongs to the field of ferrite material preparation technology, and specifically relates to a method for preparing high power density, high resistivity M-type strontium ferrite material for micro-motors. Background Technology

[0002] Micromotors, as key drive components in modern industrial automation, precision instruments, and consumer electronics, are widely used in automotive electronics, aerospace, office automation, and smart homes. With the continuous expansion of application scenarios, the consumer market is placing higher demands on the performance indicators of micromotors, prompting the rapid evolution of micromotor technology towards miniaturization, lightweighting, high speed, and high output power density. Permanent magnet materials are the core component of motors, and their performance directly determines the motor's energy conversion efficiency, volume parameters, and operational stability. For high-performance motors, permanent magnets are required to provide higher magnetic flux density and stronger demagnetization resistance within a limited volume. This necessitates higher power density in the magnet, directly related to the remanent magnetic induction intensity B of the material. r Coercivity H c and the maximum energy product (BH) max The ultimate pursuit. Among many permanent magnet materials, SrFe... 12 O 19 (M-type strontium ferrite) has become the preferred material for micro-motors due to its excellent magnetic properties and outstanding cost advantages. Therefore, further optimizing and improving the overall magnetic properties of M-type strontium ferrite is of vital importance for improving the output power and operating efficiency of micro-motors, realizing the lightweight and miniaturized design of motors, and ensuring their long-term operational reliability.

[0003] However, as micromotors develop towards higher frequencies and higher speeds, the eddy current losses generated by the motor rotor in the high-speed rotating magnetic field increase exponentially. This not only reduces motor efficiency but also easily leads to irreversible demagnetization of permanent magnets due to the heat generated. Therefore, the development of ferrite materials with both high power density and high resistivity has become an urgent need in the industry. In the existing technology, patent CN108516816A discloses a preparation method that uses additives such as H3BO3, CaCO3, and SiO2 to promote liquid-phase sintering to improve density. As one of the most common composite additive systems, although it improves the sintering density of magnets to a certain extent, it still has a certain window period in sintering kinetics: low-melting-point components such as H3BO3 only play a wetting role in the early stage of sintering, while CaCO3, SiO2, and other components have higher reaction activation energies and often require the high-temperature environment at the end of sintering to generate an effective liquid phase. This means that in the middle stage of sintering, which determines the uniformity of microstructure, the traditional additive system cannot provide a sufficient liquid-phase mass transfer medium, resulting in a limited mass migration rate at this stage. The lack of mid-sintering control methods means that grains lack liquid-phase pinning or mass transfer assistance during growth, easily leading to abnormal local grain growth or residual pores. This not only limits further improvement in material density but also makes it difficult to form a continuous, dense, and controllable high-resistivity layer at grain boundaries. Simultaneously, low-melting-point oxides tend to accumulate at grain boundaries during sintering, forming continuous low-resistivity conductive channels that compromise grain boundary insulation. This results in a significant decrease in the overall resistivity of the material, making it unsuitable for high-frequency operating conditions and thus limiting its potential application in high-frequency motors.

[0004] Furthermore, at the manufacturing process level, for example, patent CN119707473A discloses a ferrite preparation method that prepares uniform M-type strontium ferrite with uniform grain size through solid-state reaction sintering. As a traditional solid-state sintering method for single-size powder, its essence is still based on the powder sintering mechanism of a single particle size distribution. In this traditional uniform powder system, there is a highly strong coupling relationship between grain growth kinetics and densification process, making it difficult to achieve independent control of the two. Specifically, if high-temperature sintering is used to pursue high density, it is very easy to induce abnormal grain growth, resulting in grain size far exceeding the single-domain critical size, leading to a significant decrease in coercivity. Conversely, if the sintering temperature is lowered to suppress grain growth, it will lead to insufficient sintering driving force, making it difficult to achieve sufficient material filling between particles. Residual pores will generate a demagnetizing field and reduce the effective magnetic phase volume, resulting in a significant deterioration of remanence. This inherent thermodynamic contradiction makes it difficult for conventional sintering methods to achieve the synergistic optimization of high density and fine grain size, and to simultaneously improve remanence and coercivity. This severely restricts the application of high-performance ferrite materials in the design of micromotors that balance high power density and high efficiency. Summary of the Invention

[0005] The purpose of this invention is to address the problems existing in the background technology by proposing a method for preparing high power density and high resistivity M-type strontium ferrite.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A high power density, high resistivity M-type strontium ferrite comprises a main formulation and additives, wherein the main formulation comprises: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3;

[0008] The additives, in percentage of the main formulation, include: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3.

[0009] A method for preparing high power density, high resistivity M-type strontium ferrite includes the following steps:

[0010] Step 1. Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture.

[0011] Step 2. The mixture obtained in Step 1 is ball-milled for 10-14 hours to obtain primary ball-milled material;

[0012] Step 3. After drying the primary ball milling material obtained in Step 2, sieve it through a 20-40 mesh screen to obtain sieved material;

[0013] Step 4. Divide the sieved material obtained in Step 3 into 3 to 5 groups. Pre-fire the powders of the 3 to 5 groups at different pre-fire temperatures, with a pre-fire temperature range of 1200 to 1300°C. The pre-fire time for each group of powders is 1 to 3 hours. After pre-fire, each group of powders is cooled to room temperature with the furnace to obtain multiple groups of pre-fired materials with different grain sizes and phase evolution.

[0014] Step 5. The multiple groups of pre-fired materials obtained in Step 4 are graded and mixed, wherein the addition amount of each single component pre-fired material is 10 wt%~40 wt%. After thorough mixing, a composite pre-fired powder with multi-peak particle size distribution is obtained.

[0015] Step 6. Add additives to the pre-calcined powder obtained in Step 5 to obtain a mixture; the additives, according to the weight percentage of the pre-calcined powder, include: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3;

[0016] Step 7. The mixture obtained in Step 6 is ball-milled a second time for 18-22 h in a ball mill, and the particle size of the powder is controlled between 0.5 and 0.9 μm to obtain the secondary ball-milled material;

[0017] Step 8. The secondary ball milling material obtained in Step 7 is dehydrated using filter paper and gauze, and the moisture content is controlled between 15% and 30%. Then, it is pressed into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120MPa to obtain a shaped blank.

[0018] Step 9. Place the shaped blank obtained in Step 8 into a sintering furnace and sinter at a temperature of 1100~1200℃ for 0.5~1.5 h. After sintering, cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

[0019] Furthermore, in step 4, 3 to 5 groups of powders are placed in a pre-firing environment with a temperature gradient for heat preservation, and the temperature gradient difference between the pre-firing temperatures is 10 to 30°C.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. This invention combines powders with different pre-sintering temperatures, enabling them to crystallize under the same sintering temperature and holding time. Because powders with different pre-sintering temperatures possess different physicochemical activities, the highly active powder acts as a sintering driver, promoting interparticle bonding and initial densification of the green body at lower temperatures, effectively filling interparticle gaps. The less active powder, on the other hand, inhibits abnormal grain growth in the later stages of sintering, allowing for differentiated control of grain growth rates and final sizes in different regions, thus forming an optimized grain size gradient distribution within the material. This unique microstructure results in a green body with fewer pores and a more uniform microstructure, significantly improving the sintering density of the strontium ferrite material.

[0022] 2. This invention achieves a seamless connection of sintering driving force across the entire sintering temperature range by setting an H3BO3-Sb2O3-CaCO3-SiO2 composite additive system.

[0023] Initial stage: Utilizing the low-temperature melting properties of H3BO3 near 400℃, an initial liquid phase is formed before the shrinkage of M-type strontium ferrite grains. Simultaneous particle rearrangement and wetting of the magnetic powder particle surface at a lower temperature help maintain consistent orientation along the c-axis during magnetic field formation.

[0024] Mid-term: To address the kinetic window of failure of traditional fluxes in M-type strontium ferrites in the 600~900℃ range, Sb2O3 is introduced to connect the mass transfer channels between the low-temperature and high-temperature sintering stages. Utilizing its suitable melting point of 656℃ and high room temperature resistivity of over 104Ω·cm, high-resistivity grain boundaries are formed while maintaining the continuity of densification rate in the mid-term of sintering, significantly improving the density and resistivity of the material.

[0025] In the final stage: During the sintering of M-type strontium ferrite at 1000~1100°C, a stable liquid phase is generated by the eutectic reaction of the decomposition products of CaCO3 and SiO2. Residual pores are eliminated through a dissolution-precipitation mechanism, and a continuous high-resistivity glassy phase is formed together with the Sb2O3 component at the grain boundaries. The second-phase particle pinning effect restricts grain boundary migration and limits the abnormal growth of M-type strontium ferrite grains along the a-axis.

[0026] By using the above methods to enhance the densification driving force across the entire sintering temperature range, not only is efficient fluxing achieved throughout the entire time period, but abnormal grain growth is also suppressed from the source of kinetics, resulting in excellent performance of high density, fine grains, and high resistivity. Attached Figure Description

[0027] Figure 1 SEM image of the M-type strontium ferrite prepared in Comparative Example 1 of this invention;

[0028] Figure 2 The image shows a SEM image of the M-type strontium ferrite prepared in Comparative Example 2 of this invention.

[0029] Figure 3 SEM image of the M-type strontium ferrite prepared in Comparative Example 3 of this invention;

[0030] Figure 4 SEM image of the M-type strontium ferrite prepared in the embodiment of the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0032] A method for preparing high power density, high resistivity M-type strontium ferrite includes the following steps:

[0033] Step 1. Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture.

[0034] Step 2. The mixture obtained in Step 1 is ball-milled for 10-14 hours to obtain primary ball-milled material;

[0035] Step 3. After drying the primary ball milling material obtained in Step 2, sieve it through a 20-40 mesh screen to obtain sieved material;

[0036] Step 4. Divide the sieved material obtained in Step 3 into 3-5 groups, A, B, C (D, E), and place them in a pre-firing environment with a temperature gradient for heat preservation. The pre-firing temperatures are set to T1, T2, T3 (T4, T5), and satisfy 1200℃ < T1 < T2 < T3 (< T4 < T5) < 1300℃. The temperature gradient difference between adjacent groups is set to 10-30℃. Keep the pre-firing holding time of each group consistent, which is 1-3 h. After pre-firing, each group of powder is cooled to room temperature with the furnace to obtain multiple groups of pre-fired materials with different grain sizes and phase evolution degrees.

[0037] Step 5. The multiple groups of pre-calcined materials with different activities obtained in Step 4 are mixed according to a preset mass percentage of W1, W2, W3 (W4, W5), wherein the addition amount of each single component pre-calcined material is 10 wt%~40 wt%, and satisfies W1+W2+W3 (W4+W5)=100 wt%; after thorough mixing, a composite pre-calcined powder with multi-peak particle size distribution is obtained.

[0038] Step 6. Add additives to the pre-calcined powder obtained in Step 5 to obtain a mixture; the additives, according to the weight percentage of the pre-calcined powder, include: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3;

[0039] Step 7. The mixture obtained in Step 6 is ball-milled a second time for 18-22 h in a ball mill, and the particle size of the powder is controlled between 0.5 and 0.9 μm to obtain the secondary ball-milled material;

[0040] Step 8. The secondary ball milling material obtained in Step 7 is dehydrated using filter paper and gauze, and the moisture content is controlled between 15% and 30%. Then, it is pressed into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120MPa to obtain a shaped blank.

[0041] Step 9. Place the shaped blank obtained in Step 8 into a sintering furnace and sinter at a temperature of 1100~1200℃ for 0.5~1.5 h. After sintering, cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

[0042] Comparative Example 1

[0043] A method for preparing M-type strontium ferrite includes the following steps:

[0044] Step 1: Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture.

[0045] Step 2: The mixture obtained in Step 1 is ball-milled for 10-14 hours to obtain primary ball-milled material;

[0046] Step 3: After drying the primary ball milling material obtained in Step 2, sieve it through a 20-40 mesh screen to obtain sieved material;

[0047] Step 4: The sieved material obtained in Step 3 is pre-fired at a pre-fired temperature of 1200~1300℃ and a pre-fired time of 1~3h. After the pre-fired material is completed, it is cooled down with the furnace to obtain the pre-fired material.

[0048] Step 5: Add additives to the pre-calcined material obtained in Step 4. The additives include, by weight percentage: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, and 0.1~1.1 wt% H3BO3, to obtain a mixture.

[0049] Step 6: The mixture obtained in Step 5 is ball-milled a second time for 20 hours in a ball mill, and the particle size of the powder is controlled between 0.5 and 0.9 μm to obtain the secondary ball-milled material;

[0050] Step 7: Dehydrate the secondary ball milling material obtained in Step 6 using filter paper and gauze, controlling the moisture content to be between 15% and 30%. Then, press it into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120MPa to obtain a shaped blank.

[0051] Step 8: Place the shaped blank obtained in Step 7 into a sintering furnace and sinter it at a temperature of 1100~1200℃ for 0.5~1.5h. Cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

[0052] Figure 1 The image shows the SEM image of the M-type strontium ferrite prepared in Comparative Example 1. The grains are uniform in size and have a typical hexagonal plate shape. Compared with the microstructure of the tightly packed grains without obvious pores in the examples, the microstructure of the sample in Comparative Example 1 is loose, and the enrichment of pores limits the densification of the sample.

[0053] Comparative Example 2

[0054] A method for preparing M-type strontium ferrite includes the following steps:

[0055] Step 1: Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture.

[0056] Step 2: The mixture obtained in Step 1 is ball-milled for 10-14 hours to obtain primary ball-milled material;

[0057] Step 3: After drying the primary ball milling material obtained in Step 2, sieve it through a 20-40 mesh screen to obtain sieved material;

[0058] Step 4: Divide the sieved material obtained in Step 3 into four groups, A, B, C, and D. Place each group in a pre-firing environment with a temperature gradient and maintain the temperature at T1, T2, T3, and T4, satisfying the condition 1200℃ < T1 < T2 < T3 < T4 < 1300℃. The temperature gradient difference between adjacent groups is set to 10~30℃. Keep the pre-firing holding time consistent for each group, set to 1~3 hours. After pre-firing, cool each group of material to room temperature with the furnace to obtain four groups of pre-fired materials with different grain sizes and phase evolution degrees.

[0059] Step 5: The four groups of pre-calcined materials with different activities obtained in Step 4 are mixed according to the preset mass percentages W1, W2, W3, and W4, wherein the addition amount of each single component pre-calcined material is 10 wt%~40 wt%, and the condition W1+W2+W3+W4=100 wt% is met; after thorough mixing, a composite pre-calcined powder with multi-peak particle size distribution is obtained.

[0060] Step 6: Add additives to the pre-calcined powder obtained in step 5. The additives include, by weight percentage: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, and 0.1~1.1 wt% H3BO3.

[0061] Step 7: The mixture obtained in Step 6 is ball-milled a second time for 18-22 h in a ball mill, and the particle size is controlled between 0.5 and 0.9 μm to obtain the secondary ball-milled material;

[0062] Step 8: The secondary ball milling material obtained in Step 7 is dehydrated using filter paper and gauze, and the moisture content is controlled between 15% and 30%. Then, it is pressed into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120MPa to obtain a shaped blank.

[0063] Step 9: Place the shaped blank obtained in Step 8 into a sintering furnace and sinter it at a temperature of 1100~1200℃ for 0.5~1.5h. Cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

[0064] Figure 2 The image shows a SEM image of the M-type strontium ferrite prepared in Comparative Example 2. The grain size exhibits a gradient distribution, with fine grains filling the spaces between large grains, reducing porosity while increasing material density.

[0065] Comparative Example 3

[0066] A method for preparing M-type strontium ferrite includes the following steps:

[0067] Step 1: Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture.

[0068] Step 2: The mixture obtained in Step 1 is ball-milled for 10-14 hours to obtain primary ball-milled material;

[0069] Step 3: After drying the primary ball milling material obtained in Step 2, sieve it through a 20-40 mesh screen to obtain sieved material;

[0070] Step 4: The sieved material obtained in Step 3 is pre-fired at a pre-fired temperature of 1200~1300℃ and a pre-fired time of 1~3h. After the pre-fired material is completed, it is cooled down with the furnace to obtain the pre-fired material.

[0071] Step 5: Add additives to the pre-calcined material obtained in Step 4. The additives include, by weight percentage: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3, to obtain a mixture.

[0072] Step 6: The mixture obtained in Step 5 is ball-milled a second time for 18-22 hours, and the particle size is controlled between 0.5 and 0.9 μm to obtain the secondary ball-milled material;

[0073] Step 7: Dehydrate the secondary ball milling material obtained in Step 6 using filter paper and gauze, controlling the moisture content to be between 15% and 30%. Then, press it into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120MPa to obtain a shaped blank.

[0074] Step 8: Place the shaped blank obtained in Step 7 into a sintering furnace and sinter it at a temperature of 1100~1200℃ for 0.5~1.5h. Cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

[0075] Figure 3 The image shows a SEM image of the M-type strontium ferrite prepared in Comparative Example 3. The grains are uniform in size and exhibit a typical hexagonal lamellar shape. Compared to Comparative Example 1, its porosity is significantly reduced and its density is significantly increased. This is attributed to the introduction of various sintering aids, which optimized the sintering kinetics throughout the entire temperature range, especially promoting liquid-phase mass transfer between grains during the mid-sintering stage, allowing pores to be fully expelled.

[0076] Example

[0077] A method for preparing high power density, high resistivity M-type strontium ferrite includes the following steps:

[0078] Step 1: Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture.

[0079] Step 2: The mixture obtained in Step 1 is ball-milled for 10-14 hours to obtain primary ball-milled material;

[0080] Step 3: After drying the primary ball milling material obtained in Step 2, sieve it through a 20-40 mesh screen to obtain sieved material;

[0081] Step 4: Divide the sieved material obtained in Step 3 into four groups, A, B, C, and D. Place each group in a pre-firing environment with a temperature gradient and maintain the temperature at T1, T2, T3, and T4, satisfying the condition 1200℃ < T1 < T2 < T3 < T4 < 1300℃. The temperature gradient difference between adjacent groups is set to 10~30℃. Keep the pre-firing holding time consistent for each group, set to 1~3 hours. After pre-firing, cool each group of material to room temperature with the furnace to obtain four groups of pre-fired materials with different grain sizes and phase evolution degrees.

[0082] Step 5: The four groups of pre-calcined materials with different activities obtained in Step 4 are mixed according to the preset mass percentages W1, W2, W3, and W4, wherein the addition amount of each single component pre-calcined material is 10 wt%~40 wt%, and the condition W1+W2+W3+W4=100 wt% is met; after thorough mixing, a composite pre-calcined powder with multi-peak particle size distribution is obtained.

[0083] Step 6: Add additives to the pre-calcined powder obtained in step 5. The additives include, by weight percentage: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3.

[0084] Step 7: The mixture obtained in Step 6 is ball-milled a second time for 18-22 hours, and the particle size is controlled between 0.5 and 0.9 μm to obtain the secondary ball-milled material;

[0085] Step 8: The secondary ball milling material obtained in Step 7 is dehydrated using filter paper and gauze, and the moisture content is controlled between 15% and 30%. Then, it is pressed into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120MPa to obtain a shaped blank.

[0086] Step 9: Place the shaped blank obtained in Step 8 into a sintering furnace and sinter it at a temperature of 1100~1200℃ for 0.5~1.5h. Cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

[0087] Figure 4 The SEM image of the M-type strontium ferrite prepared in the example shows a continuous gradient change in grain size. The grains are small in the edge region and significantly larger in the central region. The grains are closely packed, the intergranular gaps are significantly reduced, and the density is high.

[0088] The difference between the examples and Comparative Example 1 lies in the following: Firstly, Comparative Example 1 did not employ a multi-temperature gradient pre-sintering powder composite process, resulting in the inability to form a gradient distribution of grain size within the material. This hindered the cascade optimization of the densification process, leading to a lower density in the sintered body. Secondly, the additive system in Comparative Example 1 lacked the component Sb₂O₃, resulting in a lack of effective liquid-phase bridging mechanisms and mass transfer channels during the mid-sintering process. This prevented the comparative example from simultaneously improving density and resistivity using liquid-phase assisted sintering, making it difficult to replicate the superior overall performance of the examples.

[0089] The difference between the Example and Comparative Example 2 is that Sb2O3 was not introduced into the additive system of Comparative Example 2, making it difficult to achieve the synergistic improvement of liquid-phase assisted densification and high resistivity characteristics during the mid-sintering process.

[0090] The difference between the example and Comparative Example 3 is that Comparative Example 3 did not use a composite process of powders with different pre-firing temperatures, and failed to obtain a high-density structure by forming a gradient distribution of grain size.

[0091] Table 1. Magnetic properties, density, and room temperature resistivity test results for each embodiment and comparative example.

[0092] project <![CDATA[B r (mT)]]> <![CDATA[H cb (kA / m)]]> <![CDATA[H cj (kA / m)]]> <![CDATA[(BH) max (kJ / m 3 )]]> <![CDATA[d (g / cm 3 )]]> ρ (kΩ·m) Comparative Example 1 418 304 366 31.6 4.94 0.46 Comparative Example 2 437 313 379 36.2 5.09 0.48 Comparative Example 3 430 308 372 33.7 5.02 0.82 Example 442 323 384 37.1 5.14 0.88

[0093] Table 1 lists the performance data of Comparative Examples 1-3 and the Example in terms of magnetic properties, density, and room temperature resistivity. The test data show that, limited by the lower sintering density, the remanence and maximum energy product of Comparative Example 1 are difficult to improve. Furthermore, the room temperature resistivity of Comparative Example 1 is only 0.46 kΩ·m, a low level that prevents it from meeting the performance requirements for high resistivity and low eddy current losses in magnetic steel materials for micro-motors.

[0094] Thanks to the composite process of powders with different pre-sintering temperatures, the embodiments shown significantly better density and magnetic properties than comparative examples 1-3. SEM microstructure clearly reveals the mechanism of performance improvement—the gradient distribution of grain size effectively optimizes the microstructure. The effective filling of the gaps between large particles by small particles significantly eliminates porosity within the sample, providing a solid structural foundation for improved magnetic properties while drastically reducing porosity. Furthermore, the introduction of Sb₂O₃ into the additive system significantly improves the room temperature resistivity of the material, reaching 0.88 kΩ·m. This is mainly attributed to the high impedance characteristics of Sb₂O₃, which allows it to form a high-resistivity layer at the grain boundaries during sintering. Simultaneously, the liquid phase generated by the decomposition of Sb₂O₃ in the middle of sintering effectively promotes interparticle mass transport and the liquid phase sintering process, thereby increasing the material's density and further synergistically enhancing its magnetic properties. In the embodiment, the remanent magnetic induction is 442 mT, the magnetic coercivity is 323 kA / m, the intrinsic coercivity is 384 kA / m, and the maximum energy product is 37.1 kJ / m. 3 The density is 5.14 g / cm³. 3 The resistivity is 0.88 kΩ·m. Therefore, the ferrite prepared by this invention possesses excellent magnetic properties, high density, and high resistivity, which can meet the application requirements of micromotors developing towards high power density and high efficiency.

[0095] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A high power density, high resistivity M-type strontium ferrite, characterized in that, It includes the main formulation and additives, wherein the main formulation includes: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3; The additives, in percentage of the main formulation, include: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3.

2. A method for preparing high power density, high resistivity M-type strontium ferrite, characterized in that, Includes the following steps: Step 1. Using SrCO3, La2O3, CaCO3, Fe2O3, and Co2O3 as raw materials, weigh them according to the following ratio: 2.2~8.2 mol% SrCO3, 2.6~5.6 mol% La2O3, 0.6~1.8 mol% CaCO3, 82.8~86.8 mol% Fe2O3, and 0.5~2.5 mol% Co2O3, to obtain a mixture. Step 2. The mixture obtained in Step 1 is ball-milled once to obtain primary ball-milled material; Step 3. Dry the primary ball milling material obtained in Step 2, and sieve it to obtain sieved material; Step 4. Divide the sieved material obtained in Step 3 into 3 to 5 groups, and pre-fire them at different pre-fire temperatures, with a pre-fire temperature range of 1200 to 1300℃. The pre-fire time for each group of powder is 1 to 3 hours. After pre-fire, cool the furnace to room temperature to obtain multiple groups of pre-fired materials. Step 5. The multiple groups of pre-fired materials obtained in Step 4 are graded and mixed, wherein the addition amount of each single component pre-fired material is 10wt%~40wt%, to obtain composite pre-fired powder; Step 6. Add additives to the pre-calcined powder obtained in Step 5 to obtain a mixture; the additives, according to the weight percentage of the pre-calcined powder, include: 0.5~1.5 wt% CaCO3, 0.2~2.2 wt% SiO2, 0.1~1.1 wt% H3BO3, and 0.1~1.1 wt% Sb2O3; Step 7. The mixture obtained in Step 6 is subjected to secondary ball milling to obtain secondary ball milled material; Step 8. Press the secondary ball milling material obtained in Step 7 into a molded blank; Step 9. Place the shaped blank obtained in Step 8 into a sintering furnace and sinter at a temperature of 1100~1200℃ for 0.5~1.5h. After sintering, cool it to room temperature with the furnace to obtain the M-type strontium ferrite.

3. The method for preparing high power density, high resistivity M-type strontium ferrite according to claim 2, characterized in that, The ball milling time in step 2 is 10-14 h, and the ball milling time in step 7 is 18-22 h, with the powder particle size controlled between 0.5 and 0.9 μm.

4. The method for preparing high power density, high resistivity M-type strontium ferrite according to claim 2, characterized in that, In step 8, the secondary ball milling material obtained in step 7 is dehydrated using filter paper and gauze to control the moisture content between 15% and 30%. Then, it is pressed into shape under a magnetic field forming press with a forming magnetic field strength of 1.2 to 1.8T and a forming pressure of 80 to 120 MPa to obtain a shaped blank.

5. The method for preparing high power density, high resistivity M-type strontium ferrite according to claim 2, characterized in that, In step 4, 3 to 5 groups of powders are placed in a pre-firing environment with a temperature gradient for heat preservation, and the temperature gradient difference between the pre-firing temperatures is 10 to 30°C.

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