Preparation method and application of nanocrystalline magnetic core

Abstract

This invention discloses a method for preparing and applying a nanocrystalline magnetic core. The method for preparing a nanocrystalline magnetic core includes the following steps: (1) heating an amorphous magnetic core to a preset temperature for heat treatment while simultaneously applying a longitudinal magnetic field; cooling after heat treatment to obtain a longitudinally magnetically pretreated magnetic core; (2) subjecting the longitudinally magnetically pretreated magnetic core to stepped vacuum annealing; and cooling to obtain a nanocrystalline magnetic core. This invention utilizes a magnetic field heat treatment-stepped vacuum annealing composite heat treatment technology to regulate the microstructure, induce magnetic anisotropy rearrangement, and thus effectively regulate the magnetization mechanism, resulting in a significant increase in the magnetic permeability of the core in the low-frequency range. This meets the application requirements for high-performance nanocrystalline magnetic cores in devices such as current transformers, leakage current protection switches, common-mode inductors, and precision sensors.

Description

Technical Field

[0001] This invention relates to the field of magnetic materials technology, and in particular to a method for preparing and applying a nanocrystalline magnetic core. Background Technology

[0002] Iron-based amorphous / nanocrystalline soft magnetic alloys, with their superior characteristics of high saturation magnetic induction and low coercivity, are hailed as the "crown jewel" of soft magnetic materials and are currently the most promising metallic soft magnetic materials in terms of comprehensive performance. With the industrialization of third-generation wide-bandgap semiconductor (SiC / GaN) technology, power electronic equipment is undergoing a comprehensive leap towards higher frequencies, higher power densities, and smaller, lighter designs. This trend directly forces the evolution of strip materials used in core magnetic components towards ultra-thin designs. Reducing the strip thickness from the traditional 25-30 μm to 16 μm or even 14 μm can effectively reduce eddy current losses in the mid-to-high frequency range (tens to hundreds of kHz), enabling it to gradually replace traditional ferrites in common-mode inductors, high-frequency transformers, and EMC filters where size requirements are stringent. However, this also leads to a core technological bottleneck spanning multiple high-end manufacturing fields: the thinner the strip, the more severely its low-frequency permeability is eroded.

[0003] This physical dilemma of "thinner and lower" is particularly prominent in several key application scenarios, as these fields have a rigid requirement for materials with ultra-high permeability in the power frequency to low-frequency range of several kHz, rather than simply being a bonus. In the fields of smart grids and precision metering, electronic current transformers (CTs) in next-generation smart meters, distribution automation terminals, and high-sensitivity leakage current protection devices (RCDs) must have extremely high initial permeability (typically ≥100,000) in the power frequency and subharmonic frequency bands to ensure linear transmission accuracy and anti-saturation margin over a wide dynamic range from milliamperes to tens of amperes; low-frequency collapse of permeability will directly lead to excessive metering errors and the risk of protection malfunctions. In the fields of new energy power generation and energy storage, DC-side leakage current monitoring modules (RCMUs) and input / output common-mode chokes in photovoltaic inverters and energy storage converters (PCS), with the increase in power levels and the compression of chassis size, also rely on ultra-thin strips to provide sufficiently high permeability in the low-frequency band to maintain detection sensitivity and filtering impedance. In the field of new energy vehicles, the current sensors in on-board chargers (OBC), DC / DC converter modules, and battery management systems (BMS) need to maintain high permeability under complex operating conditions of strong DC bias and high-frequency switching to ensure compatibility between low-current resolution and high-current undersaturation. Furthermore, the precision power filtering in rail transit traction converter systems and high-end medical imaging equipment places complex and stringent requirements on "low-frequency high permeability + high saturation magnetic induction + low loss".

[0004] However, existing technologies encounter insurmountable physical and technological barriers when dealing with 16 μm-scale ultrathin strips. When the strip thickness decreases to this scale, the dramatic increase in specific surface area significantly amplifies the proportion of free surface and roller-attached stress layers, micro-oxide scale, and surface defects introduced by the rapid quenching process in the total cross-section. These surface defects constitute dense magnetic domain wall pinning centers, severely hindering domain wall displacement under low-frequency, weak fields. Simultaneously, the 16 μm strip has extremely low heat capacity and very low tolerance for traditional crystallization annealing processes. Even small fluctuations in furnace temperature can easily lead to over-annealing (abnormal grain growth) or under-annealing (unreleased residual amorphous stress) in localized micro-regions, disrupting the uniformity of nanocrystal size and thus increasing the equivalent anisotropic field. Furthermore, the magnetostrictive effect in the thin strip state is significantly more sensitive to residual stress, and the winding stress and interlayer contact stress further suppress the improvement of magnetic permeability. The current state of the industry is that although ultrathin strips can achieve high apparent permeability at high frequencies (such as 100 kHz), 16 μm-level products using conventional processes often struggle to consistently exceed the 100,000 permeability threshold at 1 kHz (and the extended power frequency range), a critical range that determines the sensitivity of current transformers and the low-frequency impedance of filters. This forces downstream applications to compensate by increasing the core volume or reverting to using thicker strips, severely hindering the miniaturization process of high-end equipment. Therefore, developing a method specifically for the preparation and control of 16 μm-level ultrathin strips, while retaining their high-frequency, low-loss advantages, and removing the constraints of microstructure and stress on low-frequency magnetic domain movement, to achieve a breakthrough performance of stable initial permeability of 1k (i.e., ≥100,000) at 1 kHz, has become the key to overcoming the aforementioned cross-domain technological gap. Summary of the Invention

[0005] To overcome the problems existing in the prior art, this invention proposes a method for preparing and applying nanocrystalline magnetic cores. This invention achieves precise control over the microscopic magnetic structure and crystallization behavior of materials through a synergistic control process of longitudinal magnetic field pretreatment and vacuum annealing, thereby obtaining nanocrystalline magnetic cores with extremely high initial permeability, extremely low coercivity and excellent loss characteristics in the low-frequency range below 1 kHz, so as to meet the stringent requirements of high-end current transformers and power supplies for high efficiency, high power density and excellent electromagnetic compatibility performance.

[0006] The first objective of this invention is to provide a method for preparing a nanocrystalline magnetic core, comprising the following steps:

[0007] (1) Heat the amorphous magnetic core to a preset temperature for heat treatment, while applying a longitudinal magnetic field. After the heat treatment is completed, cool it to obtain a longitudinally magnetically pretreated magnetic core.

[0008] (2) The longitudinal magnetic pretreated magnetic core is subjected to stepped vacuum annealing and cooled to obtain a nanocrystalline magnetic core.

[0009] The proposed synergistic control of magnetic field pretreatment and vacuum annealing aims to systematically address the low-frequency performance degradation caused by "thinning." By using a longitudinal magnetic field pretreatment to achieve "magnetic domain pre-alignment," the previously disordered magnetic domains in the amorphous alloy undergo preliminary orientation along the magnetic field direction under the influence of the magnetic field. Simultaneously, the magnetic field induces pre-relaxation of high-energy atomic clusters within the material that are not aligned with the magnetic field direction, releasing some internal stress and providing more uniform, lower-energy nucleation sites for subsequent heterogeneous nucleation of nanocrystals. Thus, under vacuum protection, by controlling the heating rate, holding temperature, and holding time, α-Fe(Si) nanocrystalline phases are induced to precipitate uniformly and densely from the pretreated amorphous matrix.

[0010] Preferably, the composition of the amorphous magnetic core, in atomic percentage, includes the following elements: Fe 70%-83%, Si 10%-16%, B 7%-10%, Cu 0-1.5%, Nb 0-3%.

[0011] Further preferred, the composition of the amorphous magnetic core, in atomic percentage, includes the following elements: Fe 73.5%, Si 15.5%, B 7%, Cu 1%, Nb 3%.

[0012] Further preferably, the amorphous magnetic core is a thin strip amorphous magnetic core with a thickness of 10-18 μm.

[0013] Further preferred, the thickness of the amorphous magnetic core is 16 μm.

[0014] Preferably, the preset temperature in step (1) is 440℃-500℃, the heat treatment time is 30-120 min, and the current of the longitudinal magnetic field is 10-150 A. The amorphous magnetic core is wound from amorphous strip, the magnetic field direction of the magnetic field heat treatment is parallel to the width direction of the amorphous magnetic core, and the cooling method is air cooling after furnace exit.

[0015] The longitudinal magnetic field is generated by applying an axial current, which includes the following: the longitudinal magnetic field is generated by an input current through a copper rod inserted inside the initial amorphous alloy magnetic core.

[0016] Further preferred, the preset temperature in step (1) is 460℃, the heat treatment time is 60 min, and the current of the longitudinal magnetic field is 80 A.

[0017] Preferably, the magnetic core described in step (2) undergoes a stepped annealing process under vacuum or high-purity nitrogen protection. The first stage involves heating to 300℃-350℃ and holding for 30-60 min; the second stage involves further heating to 410℃-460℃ and holding for 60-120 min; the third stage involves heating to 470℃-500℃ and holding for 60-120 min; and the fourth stage involves final heating to 540℃-570℃ and holding for 120-170 min. In step (2), under vacuum or high-purity nitrogen protection, the cooling method involves rapidly cooling the system to 200℃-300℃, followed by natural cooling after removal from the furnace.

[0018] Further preferred, the magnetic core described in step (2) is subjected to step annealing under vacuum or high-purity nitrogen protection. The first stage is heated to 350°C and held for 60 min. The second stage is heated to 430°C and held for 90 min. The third stage is heated to 480°C and held for 90 min. The fourth stage is finally heated to 555°C and held for 150 min.

[0019] This invention also protects the nanocrystalline magnetic core prepared by the aforementioned method.

[0020] This invention addresses the issue of low low-frequency permeability in existing iron-based amorphous and nanocrystalline ribbons after thinning for optimized high-frequency performance. It proposes a synergistic control of longitudinal magnetic field pretreatment and stepped vacuum annealing to meet the demands of high-end home appliance power supplies for high-permeability, low-loss magnetic components in the low-frequency range. Simultaneously, it achieves precise control of magnetic domain pre-alignment and microstructure, resulting in a finished magnetic core exhibiting excellent performance in the low-frequency range: permeability greater than 160,000 at 1 kHz (optimally reaching 188,000), and greater than 110,000 at 10 kHz, with low coercivity and low loss. This magnetic core can be widely used in devices such as current transformers, leakage current protection switches, common-mode inductors, and precision sensors. Its composite heat treatment technology is universally applicable to nanocrystalline magnetic powder cores and ribbons.

[0021] Preferably, the nanocrystalline magnetic core is an iron-based nanocrystalline magnetic core. At a frequency of 1 kHz, the permeability of the iron-based nanocrystalline magnetic core is greater than 160,000, and at a frequency of 10 kHz, the permeability of the nanocrystalline magnetic core is greater than 110,000. The permeability of the nanocrystalline magnetic core is greater than or equal to 100,000 in the low-frequency range.

[0022] This invention also protects the application of the nanocrystalline magnetic core in current transformers, leakage protection switches, common mode inductors and / or precision sensors.

[0023] Compared with existing technologies, this invention has the following advantages: This invention utilizes a magnetic field heat treatment-stepped vacuum annealing composite heat treatment technology to regulate the microstructure and induce magnetic anisotropic rearrangement, thereby effectively controlling the magnetization mechanism and avoiding the low yield problem encountered in conventional stress heat treatment. This significantly improves the permeability of the magnetic core in the low-frequency range, with a permeability greater than 160,000 at 1 kHz, meeting the application requirements for high-performance nanocrystalline magnetic cores in devices such as current transformers, leakage current protection switches, common-mode inductors, and precision sensors. Furthermore, this composite heat treatment technology is universally applicable to nanocrystalline magnetic powder cores and strips. Attached Figure Description

[0024] Figure 1 This is a comparison chart of the permeability of the nanocrystalline magnetic cores obtained in Examples 1-4 and Comparative Examples 1-3 at different temperatures after magnetic field heat treatment.

[0025] Figure 2 This is a comparison of the permeability of the nanocrystalline magnetic cores obtained in Example 1 and Comparative Examples 4-5 under different magnetic field heat treatment times.

[0026] Figure 3 This is a comparison chart of the permeability of the nanocrystalline magnetic cores obtained in Example 1 and Comparative Example 3 under conditions of magnetic field pretreatment.

[0027] Figure 4 The graph shows a comparison of the coercivity of the nanocrystalline magnetic cores obtained in Examples 1-4 and Comparative Examples 1-5 under different temperatures of magnetic field heat treatment. Detailed Implementation

[0028] The following embodiments are further illustrations of the present invention, but not limitations thereof.

[0029] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention. Unless otherwise specified, the experimental materials and reagents used herein are commercially available products conventionally available in this technical field.

[0030] Amorphous ribbons can be manufactured in-house or purchased commercially. The Fe used in the following examples or comparative examples... 73.5 Si 15.5 B7Cu1Nb3 is an amorphous ribbon made by conventional process. The method of making it can include the following steps: (1) component ratio; (2) placing the prepared raw materials in a melting furnace, heating under vacuum (20 Pa) to 1450℃-1550℃ to melt and homogenize the alloy, and obtain the master alloy melt; (3) then spinning and winding the ribbon to obtain the corresponding amorphous ribbon with a thickness of 10-18 μm.

[0031] Example 1

[0032] A method for preparing an iron-based nanocrystalline magnetic core includes the following steps:

[0033] (1) Magnetic field heat treatment: Fe was selected 73.5 Si 15.5 A 16 μm amorphous ribbon of B7Cu1Nb3 was wound into an amorphous magnetic core. This core was then placed in a horizontal longitudinal magnetic furnace and heated to a preset temperature of 460℃ at a heating rate of 10℃ / min, while a longitudinal magnetic field was applied. The preset magnetic field strength, expressed as current, was 80 A. The core was held at this temperature and magnetic field strength for 60 min. After holding, the core was removed from the furnace and air-cooled to obtain the pretreated magnetic core.

[0034] (2) Stepped vacuum annealing: The pretreated magnetic core was placed in a vacuum annealing furnace (vacuum degree of -0.1 MPa). In the first stage, the temperature was raised to 350℃ and held for 60 min. In the second stage, the temperature was raised to 430℃ and held for 90 min. In the third stage, the temperature was raised to 480℃ and held for 90 min. In the fourth stage, the temperature was finally raised to 555℃ and held for 150 min. Then, the temperature was rapidly lowered to 300℃ and cooled after being removed from the furnace to obtain the iron-based nanocrystalline magnetic core.

[0035] Example 2

[0036] Same as Example 1, except that the temperature of magnetic field pretreatment in step (1) is 440°C.

[0037] Example 3

[0038] Similar to Example 1, except that the temperature of magnetic field pretreatment in step (1) is 480°C.

[0039] Example 4

[0040] Same as in Example 1, except that the temperature of magnetic field pretreatment in step (1) is 500°C.

[0041] Example 5

[0042] Similar to Example 1, except that: in step (1), the heat treatment time is 30 min and the current of the longitudinal magnetic field is 10 A; in step (2), the magnetic core is subjected to step annealing under the protection of high-purity nitrogen. In the first stage, the temperature is heated to 240°C and held for 65 min. In the second stage, the temperature is further increased to 410°C and held for 35 min. In the third stage, the temperature is increased to 490°C and held for 80 min. In the fourth stage, the temperature is finally increased to 550°C and held for 150 min.

[0043] Example 6

[0044] Similar to Example 1, except that: in step (1), the heat treatment time is 120 min and the current of the longitudinal magnetic field is 90 A; in step (2), the magnetic core is subjected to step annealing under the protection of high-purity nitrogen. In the first stage, the temperature is heated to 260°C and held for 55 min. In the second stage, the temperature is further increased to 430°C and held for 25 min. In the third stage, the temperature is increased to 470°C and held for 100 min. In the fourth stage, the temperature is finally increased to 570°C and held for 130 min.

[0045] Comparative Example 1

[0046] Same as Example 1, except that the temperature of magnetic field pretreatment in step (1) is 400°C.

[0047] Comparative Example 2

[0048] Same as in Example 1, except that the temperature of magnetic field pretreatment in step (1) is 420°C.

[0049] Comparative Example 3

[0050] Similar to Example 1, except that in step (1), the pretreatment is performed without a magnetic field, the pretreatment temperature is 460°C, and the holding time is 60 min.

[0051] Comparative Example 4

[0052] Same as Example 1, except that the heat preservation time for magnetic field pretreatment in step (1) is 30 min. Other steps remain unchanged.

[0053] Comparative Example 5

[0054] Similar to Example 1, except that the heat preservation time for magnetic field pretreatment in step (1) is 90 min.

[0055] Examples 1-4 and Comparative Examples 1-5 of this invention systematically compare the effects of temperature, time, and magnetic field during magnetic field heat treatment on the performance of the magnetic core. Specific differences are shown in Table 1. The effective permeability μ of the magnetic cores in Examples 1-4 and Comparative Examples 1-5 is... e All values ​​were obtained using the inductance L conversion formula. The inductance L was measured using an LCR meter (Wayne Kerr 6030) within the frequency range of 1-10 kHz. Effective permeability μ... e Conversion formula: μ e = (L×L) e ) / (N 2 ×A e ×μ0), where L is the core inductance, μ e L is the effective permeability of the magnetic core. e A is the magnetic circuit length of the magnetic core. edenoted as the effective cross-sectional area of ​​the magnetic core, N as the number of coil turns (test condition: 1 turn), and μ0 as the free permeability.

[0056] As shown in Table 1, under different temperatures and holding times in magnetic field heat treatment, the magnetic core at 460℃ for 60min achieved the best comprehensive magnetic properties and exhibited the highest permeability in the low-frequency range, with μe exceeding 180,000.

[0057] Table 1. Effects of different magnetic field heat treatment parameters on core performance

[0058]

[0059] Depend on Figure 1-4 As shown in Table 1, compared with Comparative Examples 1-5, the present invention successfully improved the low-frequency permeability of the thin strip by performing magnetic field pretreatment within a specific temperature range. In the above Examples 1-4, the effective permeability of the nanocrystalline magnetic core is 150,000-190,000 at a frequency of 1 kHz; and above 110,000 at a frequency of 10 kHz.

[0060] The above description of the embodiments is only for the purpose of helping to understand the technical solution and core idea of ​​the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a nanocrystalline magnetic core, characterized in that, Includes the following steps: (1) Heat the amorphous magnetic core to a preset temperature for heat treatment, while applying a longitudinal magnetic field. After the heat treatment is completed, cool it to obtain a longitudinally magnetically pretreated magnetic core. (2) The longitudinal magnetic pretreated magnetic core is subjected to stepped vacuum annealing and cooled to obtain a nanocrystalline magnetic core.

2. The method according to claim 1, characterized in that, The composition of the amorphous magnetic core, in atomic percentage, includes the following elements: Fe 70%-83%, Si 10%-16%, B 7%-10%, Cu 0-1.5%, Nb 0-3%.

3. The preparation method according to claim 1, characterized in that, The preset temperature in step (1) is 440℃-500℃, the heat treatment time is 30-120 min, and the current of the longitudinal magnetic field is 10-150 A.

4. The preparation method according to claim 3, characterized in that, The preset temperature in step (1) is 460℃, the heat treatment time is 60 min, and the current of the longitudinal magnetic field is 80 A.

5. The preparation method according to claim 1, characterized in that, The magnetic core described in step (2) is subjected to step annealing under vacuum or high-purity nitrogen protection. The first stage is heated to 300℃-350℃ and held for 30-60 min. The second stage is heated to 410℃-460℃ and held for 60-120 min. The third stage is heated to 470℃-500℃ and held for 60-120 min. The fourth stage is finally heated to 540℃-570℃ and held for 120-170 min.

6. The preparation method according to claim 5, characterized in that, The magnetic core described in step (2) is subjected to step annealing under vacuum or high-purity nitrogen protection. The first stage is heated to 350°C and held for 60 min. The second stage is heated to 430°C and held for 90 min. The third stage is heated to 480°C and held for 90 min. The fourth stage is finally heated to 555°C and held for 150 min.

7. The nanocrystalline magnetic core prepared by the preparation method according to any one of claims 1-6.

8. The nanocrystalline magnetic core according to claim 7, characterized in that, The nanocrystalline magnetic core is an iron-based nanocrystalline magnetic core. At a frequency of 1 kHz, the permeability of the iron-based nanocrystalline magnetic core is greater than 160,000, and at a frequency of 10 kHz, the permeability of the nanocrystalline magnetic core is greater than 110,000.

9. The application of the nanocrystalline magnetic core according to claim 7 or 8 in current transformers, leakage protection switches, common mode inductors and / or precision sensors.

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