Production process of permalloy CT energy-taking magnetic core suitable for small current starting

Abstract

The application relates to the technical field of magnetic materials, and particularly discloses a production process of a permalloy CT energy-taking magnetic core suitable for small-current starting, which comprises strip preparation, surface coating, spiral molding, hot pressing diffusion and gradient heat treatment. The application adopts a spiral sheet structure thin strip as a magnetic core raw material for the first time, combines a PVD coated metal plating layer of inner Mo and outer Cr, designs a Cr-Mo-Cr gradient coating and subsequent heat diffusion, forms an outer hard inner lead structure, and the structure has diffusion and progressive layers in height, solves the contradiction between the soft magnetic lead and high hardness of the existing magnetic core, realizes metallurgical bonding between strip layers through a hot pressing diffusion process, and the product has excellent mechanical strength and interlayer bonding force; through accurate alloy component design and gradient heat treatment, the product has ultrahigh magnetic performance and extremely low starting current: mu i > 60,000, H c < 0.73 A / m, starting current < 0.5 mA.

Description

Technical Field

[0001] This invention relates to the field of magnetic materials technology, and in particular to the production process of permalloy CT energy harvesting magnetic cores suitable for low-current start-up. Background Technology

[0002] In fields such as smart grids and online monitoring, it is often necessary to directly extract energy from transmission lines to power sensing devices. Current transformers (CTs) are the core components for achieving this function. However, when transmission lines are unloaded or lightly loaded, the current flowing through the lines is extremely small (as low as amperes or even milliamperes), placing extremely high demands on the magnetization capability of the CT core. Conventional silicon steel, ferrite, or ordinary permalloy cores are difficult to magnetize effectively under this weak magnetic field, resulting in low energy extraction efficiency or even failure to start.

[0003] Permalloy (an iron-nickel alloy) is considered an ideal material for low-current energy extraction due to its high permeability, low coercivity, and high saturation magnetic induction. Its magnetic properties are highly dependent on the nickel content and heat treatment process. 1J85 type permalloy with a nickel content of 80-85% exhibits extremely high initial permeability (μ). i It has the potential of >50000, but it is extremely dependent on whether internal stress can be eliminated, impurities purified and an ideal ordered crystal structure (Ni3Fe superlattice) can be formed through heat treatment.

[0004] However, the traditional manufacturing process of permalloy magnetic cores has many inherent defects, which limit their application in ultra-low current scenarios:

[0005] First, there is a contradiction between magnetic properties and mechanical strength. To obtain high initial permeability (μ... i ) and low coercivity (H c The grains must be abnormally grown and form a Gaussian texture ({100}<001>) that is conducive to magnetization through high-temperature and long-term annealing. At the same time, long-range ordering of Ni3Fe must be completed to reduce the magnetocrystalline anisotropy constant K1 and the magnetostriction coefficient λ. s However, this process leads to material softening and a significant decrease in mechanical strength, making the magnetic core prone to deformation or damage during subsequent winding, assembly, and operational vibrations. In turn, the mechanical stress will drastically degrade its magnetic properties, creating a vicious cycle.

[0006] Second, there is the issue of interlayer bonding reliability. Traditional processes use organic adhesives to bond multiple layers of alloy strips, which suffers from problems such as aging, poor thermal stability, and weak corrosion resistance. Under long-term operation or temperature changes, the adhesive is prone to failure, leading to interlayer delamination and the creation of non-magnetic gaps. This not only increases magnetic resistance and reduces effective permeability but also generates noise due to vibration and may even cause the magnetic core to crack.

[0007] Third, extremely high stress sensitivity. Permalloy with ultra-high permeability is extremely sensitive to internal stress; any minute stress can become a pinning point for the movement of magnetic domain walls, significantly increasing coercivity. The stress inevitably introduced during traditional winding, cutting, and other processing will cause the actual performance of the magnetic core to be far lower than the theoretical value, resulting in an increase in starting current.

[0008] Fourth, the function is singular and lacks gradient design. The performance requirements of the inner and outer sides of the magnetic core are different: the outer side needs high hardness and wear resistance to protect the magnetic core; the inner side needs extreme soft magnetic properties to sensitively respond to weak currents. Traditional homogeneous materials cannot simultaneously meet these two contradictory requirements.

[0009] In conclusion, developing a novel permalloy core manufacturing process that can balance ultra-high permeability, extremely low starting current, high mechanical strength, and excellent reliability has become crucial for the advancement of energy harvesting CT technology. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of the prior art and to produce a magnetic core with extremely high initial permeability, extremely low coercivity, high mechanical strength and impact and wear resistance by precisely controlling the alloy composition, innovating the gradient heat treatment system and the unique spiral composite structure design. In other words, it proposes a production process for a permalloy CT energy harvesting magnetic core suitable for low current start-up.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] The manufacturing process for permalloy CT energy harvesting cores suitable for low-current start-up includes the following steps:

[0013] S1. Strip preparation:

[0014] The alloy raw materials are processed through multiple processes such as vacuum melting, casting, hot forging, hot rolling, and cold rolling to finally roll into thin strips with a predetermined thickness of 0.05mm-0.2mm.

[0015] S2, Surface coating:

[0016] Using a roll-to-roll PVD coating equipment, a 0.5-2μm thick metal coating is applied to the surface of a thin strip. The metal coating consists of a Cr coating, a Mo coating, and a Cr coating in sequence along the width of the thin strip. The widths of the two Cr coatings are 0.4-1% of the width of the thin strip. For example, if the width of the thin strip is 300mm [corresponding to a magnetic core with an inner diameter of 600mm, a ring width of 300mm, and a height of 300mm], then the width of the Cr coating is 1.2-3mm. A high-hardness layer of a certain thickness is formed through diffusion, and the hardness near it is 250HV, while the uncoated part is only 150HV.

[0017] S3, Spiral Molding:

[0018] A high-precision spiral winding machine is used with a high-strength graphite mandrel whose diameter is equal to the target inner diameter of the magnetic core (e.g., 600mm). A thin strip with a coating is cold-rolled and wound onto the mandrel. During the winding process, continuous radial compressive stress is applied to create a spiral spring sheet structure. This structure naturally possesses a preset elastic stress in the axial direction (height direction), which can reduce element diffusion during hot-pressing and stress damage during the bonding and molding process. The high-precision spiral winding machine is equipped with constant tension control and a photoelectric edge-aligning system. Using a high-strength graphite mandrel whose diameter is equal to the target inner diameter of the magnetic core (e.g., 600mm), a gradient-coated strip is cold-rolled and wound onto the mandrel. During the winding process, continuous radial compressive stress is applied, causing the strip to undergo plastic deformation and form a tight spiral structure. This process introduces a preset, uniform axial (height direction) elastic compressive stress into the strip. This stress is designed to partially offset the tensile stress caused by the difference in thermal expansion coefficients during subsequent hot-pressing and diffusion, greatly reducing the risk of interlayer peeling or microcracks.

[0019] S4, Hot-pressure diffusion:

[0020] The wound spiral core is placed in a hot press furnace and heated under a protective atmosphere (such as argon) or vacuum environment, with axial pressure applied. The core is then subjected to hot-press diffusion at 800-950℃, 5-20MPa, and a holding time of 1-4 hours to obtain the core blank. During this process, the pressure tightly bonds the spiral layers, increasing the overall structural density and mechanical strength. The heat causes the surface-coated metal powder to interdiffused with the permalloy matrix, forming a metallurgical bond. Through the previously designed gradient coating, a gradient composite material is ultimately formed on the core's microstructure: the outer and inner sides of the core have a higher Cr diffusion concentration, resulting in a surface layer with high hardness and good wear resistance; the inner region of the core has a higher Mo diffusion concentration, further optimizing the magnetic domain structure and improving the permeability.

[0021] S5. Gradient heat treatment: The magnetic core blank obtained by hot pressing diffusion is subjected to gradient heat treatment, including high temperature purification, medium temperature ordering stage and low temperature stabilization stage, to obtain the CT energy harvesting magnetic core finished product.

[0022] Preferably, the alloy raw material comprises the following components by weight percentage: Ni: 80-85%, Mo: 3-4%, Cr: 0.5-1.5%, Cu: 0.2-0.4%, Si: 0.2-0.5%, C≤0.02%, S≤0.005%, P≤0.005%, with the balance being Fe; Cu promotes processing, and this component is the basis for subsequently obtaining high magnetic permeability and effective diffusion.

[0023] Preferably, step S1 includes the following steps:

[0024] S101, Smelting:

[0025] Using a vacuum induction melting furnace (VIM), the prepared alloy raw materials are placed in a ceramic crucible and evacuated to a vacuum level of ≤5×10⁻⁶. - 2 Pa, purge with high-purity argon gas (Ar≥99.999%) to -0.05MPa as a protective atmosphere. Start the medium-frequency power supply and slowly heat to 1550-1600℃ to fully melt and refine the furnace charge for 20-30 minutes. Use electromagnetic stirring to ensure uniform composition, and then cast into flat ingots.

[0026] S102, Hot Forging and Hot Rolling:

[0027] The ingot is heated to 1180-1200℃ and held for 2-3 hours before being hot-forged in multiple passes to the required thickness. Then, it is hot-rolled in the same temperature range with a total deformation of ≥80%, and finally rolled into a hot-rolled plate with a thickness of 2.0-3.0 mm.

[0028] S103, Cold rolling and intermediate annealing:

[0029] Cold rolling: The hot-rolled plate is pickled (e.g., with HCl solution) to remove the oxide scale, and then subjected to initial cold rolling with a deformation of 50-60% to about 1.0 mm.

[0030] Intermediate annealing: carried out in a bell-type hydrogen-protected annealing furnace, with an annealing temperature of 1000-1050℃, a protective atmosphere of pure hydrogen (dew point ≤ -40℃), and a holding time of 3-4 hours, followed by furnace cooling to achieve complete recrystallization and eliminate work hardening.

[0031] Precision cold rolling: After 3-4 cycles of cold rolling and intermediate annealing, the strip is finally rolled to a target thickness of 0.05-0.2mm using a 20-roll precision rolling mill. The thickness tolerance is controlled within ±0.002mm. The strip width is set at 300mm according to the magnetic core design.

[0032] Final high-temperature annealing: Before winding, the cold-rolled strip needs to undergo softening annealing, which involves holding at 850°C for 2 hours under hydrogen protection and then cooling in the furnace. This step aims to eliminate cold rolling stress, reduce hardness, facilitate subsequent winding and coating, and obtain uniform equiaxed grains.

[0033] Preferably, S2, surface coating, refers to coating one or two surfaces of the thin strip, which will significantly affect the thickness and distribution of the Mo and Cr coatings after diffusion, and thus the magnetic permeability will also be different.

[0034] Preferably, the roll-to-roll PVD coating equipment in S2 is an EB-PVD roll-to-roll vacuum coating equipment, which has multiple deposition chambers. One chamber is equipped with a pure Cr target, and through a precisely controlled shield, only the two edge areas of the strip to be coated with Cr are exposed to the sputtering particle stream. The other chamber is equipped with a pure Mo target, and through a precisely controlled shield, only the two edge areas of the strip to be coated with Mo are exposed to the sputtering particle stream, thereby forming a gradient coating structure of Cr on the outside and Mo on the inside, ensuring a structure with magnetic inner surface and hard outer surface.

[0035] Preferably, the EB-PVD roll-to-roll vacuum coating equipment includes:

[0036] Unwinding system: The system continuously feeds the coiled, high-temperature annealed permalloy strip (e.g., 100-300 mm wide, 0.1 mm thick, and up to several kilometers long) into the system from outside the vacuum chamber through a vacuum-sealed feed inlet.

[0037] Pretreatment chamber: The strip first enters this chamber, which usually contains a plasma cleaning or ion equipping unit. In a high vacuum environment, the surface of the strip is bombarded with ionized plasma to thoroughly remove residual oil, oxides and adsorbed gases, exposing a clean metal surface. This is a key prerequisite for obtaining good coating adhesion.

[0038] Multiple deposition chambers: This is the core area. In order to achieve gradient coating of "Cr on the outside and Mo on the inside", the production line will be designed with multiple independent vacuum deposition chambers, each equipped with different targets and shielding devices.

[0039] Chamber 1 (Cr deposition): The strip passes through the first chamber, which is equipped with a pure Cr target (such as a magnetron sputtering Cr target). Through a precisely controlled shield, only the two edge areas of the strip that need to be coated with Cr are exposed to the sputtering particle stream.

[0040] Chamber 2 (Mo deposition): The strip then enters the second chamber, which is equipped with a pure Mo target. The shield is adjusted so that sputtering deposition is performed only on the central region of the strip.

[0041] The process parameters (power, air pressure, strip conveying speed) of each chamber can be controlled independently and precisely, thereby accurately regulating the coating thickness;

[0042] Post-processing / cooling chamber: After deposition, the strip is slowly cooled in this area to avoid deformation due to excessive thermal stress.

[0043] Rewinding System: The strip with double-sided gradient coating is rewound into a roll and drawn out from the vacuum environment through a vacuum sealing device.

[0044] Central control system: The computer-integrated control system synchronously controls the vacuum, temperature, sputtering power, and most importantly, the conveying speed of the strip in all chambers, ensuring the stability and repeatability of the entire process.

[0045] Preferably, before surface coating in S2, the surface of the thin strip (1) is subjected to plasma cleaning: Ar ion source bombardment is used with parameters: Ar gas flow rate 50-100 sccm, vacuum degree 0.1-0.5 Pa, bombardment energy 500-1000 eV, and time 30-60 seconds. The micro-adsorbed layer and oxides on the surface of the strip are completely removed. This step is automatically completed in the pretreatment chamber.

[0046] Preferably, the process parameters for S2 and surface coating are: Ar working pressure 0.3-0.5 Pa, sputtering power 8-10 kW, and strip conveying speed 0.5-1.0 m / min. The thickness of the Cr and Mo coatings is precisely controlled to be 0.5-2 μm by adjusting the conveying speed. After coating inspection: the thickness is monitored by an online film thickness gauge, and after the coating is removed from the line, it is tested by a microhardness tester. The hardness of the Cr coating area is ≥250 HV, the hardness of the Mo coating area is ≈180 HV, and the hardness of the substrate is ≈150 HV.

[0047] Preferably, step S4, hot-press diffusion, includes the following steps:

[0048] S401, Core Placement:

[0049] A vacuum hot-pressing sintering furnace with a water-cooled pressurization system is used to place the wound spiral magnetic core together with the graphite core shaft into a high-strength graphite mold. The inner cavity of the mold matches the outer diameter of the magnetic core, and the upper and lower pressure heads are flat.

[0050] S402, Vacuuming and Gas Cleaning:

[0051] Evacuate the furnace cavity to ≤5×10 -3 Pa, then fill with high-purity Ar gas to -0.03 MPa, repeat 2-3 times to fully remove oxygen;

[0052] S403, Heating and Pressurization:

[0053] The temperature is increased at a rate of ≤10℃ / min. When the temperature reaches 600℃, axial pressure is applied and gradually increased to 5-20MPa.

[0054] S404, thermal insulation and pressure retention:

[0055] Hot-press diffusion is carried out at 800-950℃ for 1-4 hours. This stage is the key to diffusion and densification. The pressure enables the strip layers to achieve close contact at the atomic scale, eliminating micro gaps and improving the stacking factor. The high temperature activates Cr and Mo atoms to diffuse into the permalloy matrix (Fe-Ni), while Fe and Ni atoms also diffuse into the coating, forming a strong metallurgical bond, rather than a simple physical bond.

[0056] S405, Cooling:

[0057] After the heat preservation is completed, the furnace is cooled to below 300°C while maintaining pressure. Then the pressure is released and the furnace is cooled to room temperature to form a preliminary gradient composite material structure with high interlayer bonding strength and an overall density ≥95% of the theoretical density, avoiding magnetic permeability loss due to pores.

[0058] Preferably, the gradient heat treatment equipment is a horizontal hydrogen high-temperature annealing furnace with an atmosphere of high-purity hydrogen (H2) with a purity ≥99.999% and a dew point ≤-60℃. It is equipped with multi-zone temperature control, a high-purity gas inlet system, and a rapid cooling (quenching) device. The furnace tube is made of high-purity alumina material.

[0059] Preferably, the gradient heat treatment specifically includes the following steps:

[0060] S501, High-Temperature Purification Stage:

[0061] The temperature is increased from room temperature to 1100-1200℃ at a rate of 200℃ / h, and held at 1100-1200℃ for 3 hours. The purpose of this stage is: a. to completely eliminate all processing stress; b. to induce secondary recrystallization of the grains, resulting in abnormal growth and the formation of coarse {100}<001> Gaussian texture, which is the basis for high magnetic permeability grain orientation; c. to allow H2 to react with impurities such as C, S, and O to generate gases such as CH4, H2S, and H2O, which are then carried away by the gas flow, greatly purifying the grain boundaries.

[0062] S502, Intermediate Temperature Ordering Stage:

[0063] This is the most critical stage, involving slow cooling at a strictly controlled rate of 80℃ / h to ensure that the cooling time within the critical temperature range of 550℃ to 450℃ is no less than 1 hour. This slow cooling is to provide sufficient kinetic conditions for Ni and Fe atoms to align fully and orderly in the crystal lattice, forming a long-range ordered Ni3Fe superlattice structure. This structure can significantly reduce the magnetocrystalline anisotropy constant K1 and the magnetostriction coefficient λ. s Thus, the ultimate soft magnetic properties are obtained;

[0064] S503, Low-Temperature Stabilization and Quenching:

[0065] When the temperature is slowly reduced to 350℃, it is kept at that temperature for 1 hour to relax the tiny internal stress generated during the ordering process and stabilize the superlattice.

[0066] After the heat preservation is completed, high-purity Ar gas is immediately introduced into the furnace and forced air cooling (quenching) is performed to quickly cool the magnetic core to below 100°C. The purpose of this operation is to freeze the high-temperature disordered state and prevent the precipitation of harmful phases (such as excessive growth of FeNi3 ordered phase) or changes in order due to slow cooling below 350°C, thereby stabilizing the ultra-high magnetic properties.

[0067] Final product performance:

[0068] The permalloy magnetic core produced using the above complete process is expected to achieve the following properties:

[0069] Initial permeability (μ) i ): >60,000 (measured at 0.1mT, 10kHz);

[0070] Coercivity (H) c <1.5A / m;

[0071] Saturation magnetic induction intensity (B s Approximately 0.75T;

[0072] Edge hardness: ≥250HV;

[0073] Starting current: <0.5mA (suitable for small current energy extraction);

[0074] Mechanical strength: Good, capable of withstanding the stress of subsequent automatic winding without performance degradation.

[0075] The process design of this invention is rigorous, the parameters are clear, and each step is closely linked, which has a high degree of repeatability and feasibility for industrial production.

[0076] Compared with the prior art, the beneficial effects of the present invention are:

[0077] 1. This invention is the first to use a thin strip with a spiral structure as the magnetic core material, and combines it with a PVD-coated inner Mo and outer Cr metal coating to design a Cr-Mo-Cr gradient coating and subsequent thermal diffusion, forming a composite material structure with gradually changing performance at the microscopic level: the Mo element inside the magnetic core diffuses to a higher concentration towards the center, further optimizing the magnetic domain structure and promoting the improvement of magnetic permeability; the Cr element on the surface of the magnetic core (inner and outer circumferences) diffuses to a higher concentration, forming a reinforcing layer with high hardness and high wear resistance, solving the contradiction between soft magnetic permeability and high hardness in existing magnetic cores, and realizing functional integration.

[0078] 2. In the spiral molding stage, this invention pre-sets a uniform axial elastic compressive stress through cold rolling. This stress can effectively counteract the tensile stress caused by the difference in thermal expansion coefficients between the substrate and coating materials during the subsequent hot pressing diffusion process, greatly reducing the risk of interlayer delamination or microcracks, and ensuring the yield and consistency of the product from the source.

[0079] 3. This invention completely abandons traditional organic adhesives and achieves metallurgical bonding between strip layers through hot-press diffusion process. The overall density is ≥95%. The product has both excellent mechanical strength and interlayer bonding strength. The interlayer bonding strength is extremely high and can withstand the stress of subsequent automatic winding without performance degradation. At the same time, the infiltration of Cr element on the surface increases the edge hardness to over 250HV, significantly enhancing wear resistance and deformation resistance.

[0080] 4. This invention achieves this through precise alloy composition design (Ni-Mo-Cr-Cu-Si): the addition of molybdenum (Mo) (3.5%) is key, as it effectively suppresses the kinetics of Ni3Fe ordering, providing a generous process window for subsequent ordering heat treatment and avoiding the deterioration of magnetic properties caused by excessive ordering;

[0081] Combined with gradient heat treatment, especially the strict control of the "intermediate-temperature ordering stage" (slow cooling at ≤80℃ / h within the 450-550℃ range), sufficient kinetic conditions were provided for Fe and Ni atoms, resulting in a long-range ordered Ni3Fe superlattice structure. This structure can greatly reduce the magnetocrystalline anisotropy constant K1 and the magnetostriction coefficient λ. s This is how to obtain ultra-high μ i and ultra-low H c The fundamental reason is that rapid cooling ultimately "freezes" this metastable superlattice structure, stabilizing its performance and thus enabling the product to achieve ultra-high magnetic properties and extremely low start-up current.

[0082] 5. In summary, this invention, through synergistic innovation in composition, structure, and process, systematically solves the long-standing technical challenges hindering the development of high-performance energy-harvesting CT magnetic cores, resulting in a product. The prepared magnetic core product has an initial permeability μ. i >60,000, coercivity H c With an efficiency of <0.73 A / m and a starting current of <0.5 mA, its overall performance far surpasses that of traditional processes. Attached Figure Description

[0083] Figure 1 The hysteresis loop diagram of the CT energy harvesting magnetic core product produced by the manufacturing process proposed in Embodiment 2 of the present invention;

[0084] Figure 2This is a schematic diagram illustrating the product structure changes during the hot-pressure diffusion process in the production process of a permalloy CT energy harvesting magnetic core suitable for low-current startup, as proposed in this invention.

[0085] In the figure: 1-thin strip, 101-Cr coating, 102-Mo coating, 2-magnetic core blank. Detailed Implementation

[0086] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0087] Example 1

[0088] Taking a magnetic core with dimensions of 600mm inner diameter, 300mm ring width, and 300mm height as an example, a manufacturing process for a permalloy CT energy harvesting magnetic core suitable for low-current startup is proposed, including the following steps:

[0089] S1. Strip preparation:

[0090] The alloy raw materials are processed through vacuum melting, casting, hot forging, hot rolling, cold rolling and intermediate annealing to finally roll into a thin strip with a predetermined thickness of 0.05 mm; the alloy raw materials include, by weight percentage: Ni: 85%, Mo: 3%, Cr: 0.5%, Cu: 0.4%, Si: 0.5%, C≤0.02%, S≤0.005%, P≤0.005%, and the balance is Fe.

[0091] S2, Surface coating:

[0092] The surface of the thin strip was subjected to plasma cleaning with the following parameters: Ar gas flow rate 50-100 sccm, vacuum degree 0.1-0.5 Pa, bombardment energy 500-1000 eV, and time 30-60 seconds.

[0093] A roll-to-roll physical vapor deposition (PVD) apparatus was used to gradient-coat metal layers on both sides of the thin strip 1 along its width direction. The metal layers were Cr coating 101, Mo coating 102 and Cr coating 101 in sequence. The width of each of the two Cr coatings 101 was 0.4% of the width of the thin strip 1, i.e., 1.2 mm. The Mo coating 102 was located in the middle region. The total thickness of the metal coating was 2 μm.

[0094] S3, Spiral Molding:

[0095] Using a high-precision spiral winding machine and a high-strength graphite mandrel, the coated thin strip 1 is subjected to continuous radial compressive stress during the winding process and cold-rolled onto the mandrel to form a spiral spring sheet structure.

[0096] S4, Hot-pressure diffusion:

[0097] The wound spiral magnetic core and the graphite core shaft are placed together into a graphite mold, and a vacuum is drawn to ≤5×10. -3 After Pa and gas washing, the temperature is increased at ≤10℃ / min, and the pressure is applied at 600℃ and increased to 5MPa. The temperature and pressure are held at 800℃ for 14h to carry out hot-press diffusion. The diffusion depth of Cr is about 10μm and the diffusion depth of Mo is about 5μm. Then the pressure is held and cooled to below 300℃ and then depressurized and cooled to room temperature to obtain magnetic core blank 2, so that the strip layers can achieve metallurgical bonding and form a gradient composite material structure.

[0098] S5, Gradient heat treatment:

[0099] The hot-pressed diffusion magnetic core blank 2 is then subjected to gradient heat treatment:

[0100] High-temperature purification stage: Under a pure hydrogen atmosphere, the temperature is increased to 1100℃ at a rate of 200℃ / h and held for 3 hours;

[0101] Medium-temperature ordering stage: slow cooling at 80℃ / h to ensure that the cooling time in the 550-450℃ range is not less than 1 hour;

[0102] Low-temperature stabilization and quenching stage: Cool to 350℃ and hold for 1 hour, then fill with high-purity Ar gas and force air cooling to below 100℃, finally obtaining the CT energy harvesting magnetic core product.

[0103] Example 2

[0104] A manufacturing process for a permalloy CT energy harvesting core suitable for low-current start-up includes the following steps:

[0105] S1. Strip preparation:

[0106] The alloy raw materials are processed through vacuum melting, casting, hot forging, hot rolling, cold rolling and intermediate annealing to finally roll into thin strips with a predetermined thickness of 0.05mm-0.2mm; the alloy raw materials include, by weight percentage: Ni: 83%, Mo: 3.5%, Cr: 1%, Cu: 0.3%, Si: 0.4%, C≤0.02%, S≤0.005%, P≤0.005%, with the balance being Fe.

[0107] S2, Surface coating:

[0108] The surface of the thin strip was subjected to plasma cleaning with the following parameters: Ar gas flow rate 50-100 sccm, vacuum degree 0.1-0.5 Pa, bombardment energy 500-1000 eV, and time 30-60 seconds.

[0109] A roll-to-roll physical vapor deposition (PVD) apparatus was used to gradient-coat metal layers on both sides of the thin strip 1 along its width direction. The metal layers were Cr coating 101, Mo coating 102 and Cr coating 101 in sequence. The width of each of the two Cr coatings 101 was 0.7% of the width of the thin strip 1, i.e., 2.1 mm. The Mo coating 102 was located in the middle region. The total thickness of the metal coating was 1.2 μm.

[0110] S3, Spiral Molding:

[0111] Using a high-precision spiral winding machine and a high-strength graphite mandrel, the coated thin strip 1 is subjected to continuous radial compressive stress during the winding process and cold-rolled onto the mandrel to form a spiral spring sheet structure.

[0112] S4, Hot-pressure diffusion:

[0113] The wound spiral magnetic core and the graphite core shaft are placed together into a graphite mold, and a vacuum is drawn to ≤5×10. -3 After Pa and gas washing, the temperature is increased at ≤10℃ / min, and the pressure is applied at 600℃ and increased to 15MPa. The temperature and pressure are held at 870℃ for 2.5h for hot-press diffusion. The diffusion depth of Cr is about 10μm and the diffusion depth of Mo is about 5μm. Then the pressure is held and cooled to below 300℃ and then the pressure is released and cooled to room temperature to obtain magnetic core blank 2, so that the metallurgical bonding between the strip layers is realized and a gradient composite material structure is formed.

[0114] S5, Gradient heat treatment:

[0115] The hot-pressed diffusion magnetic core blank 2 is then subjected to gradient heat treatment:

[0116] High-temperature purification stage: The temperature is increased to 1150℃ at a rate of 200℃ / h and held for 3 hours in a pure hydrogen atmosphere;

[0117] Medium-temperature ordering stage: slow cooling at 80℃ / h to ensure that the cooling time in the 550-450℃ range is not less than 1 hour;

[0118] Low-temperature stabilization and quenching stage: Cool to 350℃ and hold for 1 hour, then fill with high-purity Ar gas and force air cooling to below 100℃, finally obtaining the CT energy harvesting magnetic core product.

[0119] The specific product tests are as follows: B s =1.03T, B r =0.47T, H c =0.73A / m, H s =59A / m, μ i >60000, starting current <0.5mA.

[0120] Example 3

[0121] A manufacturing process for a permalloy CT energy harvesting core suitable for low-current start-up includes the following steps:

[0122] S1. Strip preparation:

[0123] The alloy raw materials are processed through vacuum melting, casting, hot forging, hot rolling, cold rolling and intermediate annealing to finally roll into thin strips with a predetermined thickness of 0.05mm-0.2mm; the alloy raw materials include, by weight percentage: Ni: 80%, Mo: 4%, Cr: 1.5%, Cu: 0.2%, Si: 0.2%, C≤0.02%, S≤0.005%, P≤0.005%, with the balance being Fe.

[0124] S2, Surface coating:

[0125] The surface of the thin strip was subjected to plasma cleaning with the following parameters: Ar gas flow rate 50-100 sccm, vacuum degree 0.1-0.5 Pa, bombardment energy 500-1000 eV, and time 30-60 seconds.

[0126] A roll-to-roll physical vapor deposition (PVD) system was used to gradient-coat metal layers on both sides of the thin strip 1 along its width direction. The metal layers were Cr coating 101, Mo coating 102 and Cr coating 101 in sequence. The width of each of the two Cr coatings 101 was 1% of the width of the thin strip 1, i.e., 3 mm. The Mo coating 102 was located in the middle region. The total thickness of the metal coating was 0.5 μm.

[0127] S3, Spiral Molding:

[0128] Using a high-precision spiral winding machine and a high-strength graphite mandrel, the coated thin strip 1 is subjected to continuous radial compressive stress during the winding process and cold-rolled onto the mandrel to form a spiral spring sheet structure.

[0129] S4, Hot-pressure diffusion:

[0130] The wound spiral magnetic core and the graphite core shaft are placed together into a graphite mold, and a vacuum is drawn to ≤5×10. -3 After Pa and gas washing, the temperature is increased at ≤10℃ / min, and the pressure is applied at 600℃ and increased to 20MPa. The temperature and pressure are held at 950℃ for 1h to carry out hot-press diffusion. The diffusion depth of Cr is about 10μm and the diffusion depth of Mo is about 5μm. Then the pressure is held and cooled to below 300℃ and then the pressure is released and cooled to room temperature to obtain magnetic core blank 2, so that the metallurgical bonding between the strip layers is realized and a gradient composite material structure is formed.

[0131] S5, Gradient heat treatment:

[0132] The hot-pressed diffusion magnetic core blank 2 is then subjected to gradient heat treatment:

[0133] High-temperature purification stage: Under a pure hydrogen atmosphere, the temperature is increased to 1100-1200℃ at a rate of 200℃ / h and held for 3 hours;

[0134] Medium-temperature ordering stage: slow cooling at 80℃ / h to ensure that the cooling time in the 550-450℃ range is not less than 1 hour;

[0135] Low-temperature stabilization and quenching stage: Cool to 350℃ and hold for 1 hour, then fill with high-purity Ar gas and force air cooling to below 100℃, finally obtaining the CT energy harvesting magnetic core product.

[0136] Example 4

[0137] The process is basically the same as in Example 2, except that the surface coating is replaced by single-sided coating of thin strip 1.

[0138] Comparative Example 1

[0139] The process is the same as in Example 2, except that the alloy raw materials, by weight percentage, comprise: Ni: 83%, Cr: 1%, Cu: 0.3%, Si: 0.4%, C≤0.02%, S≤0.005%, P≤0.005%, with the balance being Fe. The magnetic permeability is somewhat reduced.

[0140] Comparative Example 2

[0141] The process is the same as in Example 2, except that the alloy raw materials, by weight percentage, include: Ni: 83%, Mo: 3.5%, Cu: 0.3%, Si: 0.4%, C≤0.02%, S≤0.005%, P≤0.005%, with the balance being Fe. The surface hardness is significantly reduced.

[0142] Comparative Example 3

[0143] The process is basically the same as that in Example 2, except that S2 and surface coating are omitted; and the magnetic permeability and surface hardness are reduced.

[0144] Comparative Example 4

[0145] The process is basically the same as that in Example 2, except that the process of holding at 870°C for 2.5 hours during hot-press diffusion in step S4 is omitted.

[0146] Comparative Example 5

[0147] The process is basically the same as in Example 2, except that the intermediate temperature ordering stage of S5 and gradient heat treatment is cancelled, and the cooling rate is adjusted to 150℃ / h.

[0148] Comparative Example 6

[0149] The process is basically the same as that in Example 2, except that the heat preservation process of S5, which involves cooling to 350°C and holding for 1 hour in gradient heat treatment, is omitted.

[0150] Comparative Example 7

[0151] Using the same alloy raw material formula as in Example 2, a ring-shaped magnetic core was cast and then processed through S4 and S5 in Example 2 to obtain the magnetic core product.

[0152] Performance testing:

[0153] The performance of the magnetic core products in each embodiment and comparative example was tested using the following standard test methods:

[0154] 1) Magnetic property test (B) s B r H c ,μ i ):

[0155] Equipment: The toroidal method is used in conjunction with a BH analyzer (such as the PM-6000 series manufactured by Iwasaki Corporation of Japan).

[0156] Sample preparation: The finished magnetic core is evenly wound with a fine wire (such as Φ0.2mm enameled wire) a certain number of turns (such as 100 turns).

[0157] Test conditions: Conducted at room temperature. Saturation magnetic induction (B s ) and remanence (B r Coercivity (H) is typically measured after applying a sufficiently large magnetizing force at a power frequency of 50Hz or 60Hz until magnetic saturation. c The initial permeability (μ) was measured by the reverse magnetization field. i It is typically measured at a low field (0.1mT) and a specific frequency (10kHz) to accurately evaluate performance under low current.

[0158] 2) Starting current test:

[0159] Test Circuit: Construct an experimental circuit simulating CT power extraction. Connect an adjustable precision AC current source and an ammeter in series on the primary side, and connect a standard load resistor (10kΩ) on the secondary side. Use a high-precision voltmeter to measure the voltage across the load resistor.

[0160] Test method: Slowly increase the primary side current from 0. When the secondary side output voltage reaches the preset effective value (1V), record the primary side current value at this time, which is the starting current. The smaller this value, the higher the sensitivity of the magnetic core to weak current.

[0161] 3) Mechanical property testing (edge ​​hardness):

[0162] Equipment: Vickers Microhardness Tester.

[0163] Methods: Metallographic samples were prepared by sampling, mounting, polishing, and etching the inner and outer circumferential edges of the finished magnetic core. Multiple points were selected in the sample edge region, and small loads (such as 50 gf or 100 gf) were applied to measure the Vickers hardness (HV), and the average value was taken.

[0164] The data is shown in Table 1 below:

[0165] Table 1. Effects of different formulations and processes on core performance

[0166]

[0167] Data Analysis:

[0168] Example 1 exhibits good performance, with low-temperature long-term diffusion and sufficient binding, but the element diffusion distance is relatively short, the gradient is obvious, the purification temperature is slightly lower, and the grain growth and purification effect are slightly inferior to Example 2.

[0169] Example 2 exhibits optimal overall performance, with the composition, coating, hot pressing, and heat treatment parameters achieving the best balance, forming a perfect gradient structure and a highly ordered superlattice, thus realizing a combination of ultra-high magnetic permeability and hardness.

[0170] Example 3 employs high-temperature, high-pressure, short-time diffusion, which is highly efficient but may result in greater interlayer stress. High Cr and high Mo content tend to ensure strength and hardness, while the magnetic permeability may be slightly lower than in Example 2.

[0171] Example 4 uses single-sided coating, which results in a halving of the diffusion source, a weakening of the gradient effect and performance improvement, and poorer interlayer bonding and overall performance compared to Example 2 with double-sided coating.

[0172] Comparative Example 1: Without Mo, magnetic properties deteriorate. The lack of Mo to regulate the ordering dynamics leads to excessively high or disordered order in Ni3Fe, resulting in increased magnetocrystalline anisotropy and coercivity, and decreased permeability.

[0173] Comparative Example 2: Without Cr, mechanical properties deteriorate. The lack of Cr as a reinforcing phase prevents the formation of a high-hardness surface layer, resulting in loss of wear resistance and deformation resistance, completely negating the surface protection advantages of the gradient design.

[0174] Comparative Example 3: No coating, performance degraded across the board. No coating means no diffusion source; hot pressing only achieves mechanical bonding, resulting in microscopic gaps that pin magnetic domain walls, leading to H... c Increase, μ i It decreased, and there was no increase in hardness.

[0175] Comparative Example 4, without the heat preservation and pressure holding process, exhibits a catastrophic performance degradation. The interlayer lacks diffuse bonding, resulting in numerous voids that severely hinder the magnetization process, causing a sharp increase in coercivity and a dramatic drop in permeability and saturation magnetic induction.

[0176] Comparative Example 5: Removing the ordered slow cooling and replacing it with rapid cooling resulted in a catastrophic decrease in magnetic properties. Rapid cooling prevented the formation of the Ni3Fe superlattice, leading to extremely strong magnetocrystalline anisotropy. The material lost its soft magnetic properties, becoming hard and brittle, making it unsuitable for low-current energy extraction.

[0177] Comparative Example 6: Eliminating the 350℃ stabilization stage resulted in slight performance degradation and poor stability. Insufficiently eliminated internal stress became additional magnetic domain pinning points, leading to increased coercivity and permeability dispersion, resulting in poorer product consistency.

[0178] Comparative Example 7 eliminated the spiral structure, thus eliminating the coating process. Its overall performance was slightly better than that of Comparative Examples 3-5 with spiral blade structures, but its overall mechanical properties were significantly reduced, making it prone to damage.

[0179] The following mechanism can be derived from the comparison of the above data:

[0180] 1) Composition (Ni 83%, Mo 3.5%, Cr 1%): The Ni content ensures the foundation for the formation of the Fe-Ni main phase; Mo can effectively suppress the ordering rate of Ni3Fe, providing a window for subsequent ordering heat treatment, while increasing resistivity and reducing eddy current loss; Cr is the key element for forming a high surface hardness layer.

[0181] 2) Coating and hot-press diffusion (1.2μ) m (870℃ / 15MPa / 2.5h): This parameter ensures that Cr and Mo atoms can fully diffuse into the matrix, forming a strong metallurgical bond and a clear compositional gradient, while preventing excessive diffusion that would lead to homogenization of properties. Pressure densifies the interlayer, eliminates non-magnetic gaps, and improves effective magnetic permeability.

[0182] 3) Gradient heat treatment (1150℃ purification + slow cooling for ordering): High-temperature hydrogen purification at 1150℃ thoroughly eliminates impurities, purifies grain boundaries, and promotes abnormal grain growth to form a favorable {100}<001> Gaussian texture for magnetization. Subsequent slow cooling in the 450-550℃ range (approximately 80℃ / h) is crucial for forming a long-range ordered Ni3Fe superlattice structure. This structure significantly reduces the magnetocrystalline anisotropy constant K1 and the magnetostriction coefficient λ. s This is how to obtain extremely high μ i and extremely low H c The fundamental reason is that the final rapid cooling (quenching) freezes and preserves this highly ordered metastable structure.

[0183] Through a systematic comparative analysis of the above embodiments and comparative examples, the following conclusions can be drawn:

[0184] Composition is fundamental: the synergistic alloying design of Ni-Mo-Cr-Cu-Si is the cornerstone for achieving high performance, in which the roles of Mo and Cr are irreplaceable, respectively dominating the core magnetic properties and surface mechanical properties.

[0185] Gradient coating is the core of innovation: the innovative Cr-Mo-Cr gradient coating design, realized through PVD process, is the key means to achieve the contradictory goal of "internal magnetism and external hardness".

[0186] Hot-press diffusion is the guarantee: combined with the spiral structure design, it realizes interlayer metallurgical bonding and structural densification, completely solves the reliability problem of traditional adhesive bonding methods, and provides conditions for element diffusion.

[0187] Gradient heat treatment is key: especially the medium-temperature ordered slow cooling process, which is crucial for activating the extreme soft magnetic properties (ultra-high μ) of permalloy. i Ultra-low H c This is a crucial step. Any deviation from this process will lead to a catastrophic decline in performance.

[0188] The process exemplified by Example 2, through precise design and synergistic effect of multiple steps and parameters, successfully fabricated a CT energy harvesting core that simultaneously satisfies ultra-high permeability, extremely low starting current, high mechanical strength and excellent reliability, thus solving the contradiction that traditional processes cannot achieve.

[0189] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A manufacturing process for permalloy CT energy harvesting cores suitable for low-current start-up, characterized in that, Includes the following steps: S1. Strip preparation: The alloy raw materials are subjected to multiple processes including vacuum melting, casting, hot forging, hot rolling, and cold rolling to finally roll into a thin strip with a predetermined thickness of 0.05mm-0.2mm (1); the alloy raw materials include the following weight percentage components: Ni: 80-85%, Mo: 3-4%, Cr: 0.5-1.5%, Cu: 0.2-0.4%, Si: 0.2-0.5%, C≤0.02%, S≤0.005%, P≤0.005%, and the balance is Fe; S2, Surface coating: A 0.5-2 μm thick metal coating is applied to the surface of the thin strip (1) using a roll-to-roll PVD coating equipment. The metal coating consists of a Cr coating (101), a Mo coating (102), and a Cr coating (101) in sequence along the width direction of the thin strip (1). The widths of the two Cr coatings (101) are 0.4-1% of the width of the thin strip (1). S3, Spiral Molding: A high-precision spiral winding machine is used, and a high-strength graphite mandrel with a diameter equal to the inner diameter of the magnetic core is used. The coated thin strip (1) is cold-rolled and wound onto the mandrel. During the winding process, continuous radial compressive stress is applied to form a spiral spring sheet structure. S4, Hot-pressure diffusion: The spiral sheet magnetic core is placed in a hot press furnace, and under a protective atmosphere or vacuum environment, axial pressure is applied and heated. The hot pressing diffusion is carried out at a temperature of 800-950℃, a pressure of 5-20MPa, and a holding time of 1-4h to obtain the magnetic core blank (2). S5. Gradient heat treatment: The magnetic core blank (2) obtained by hot pressing diffusion is subjected to gradient heat treatment to obtain the CT energy harvesting magnetic core finished product. The gradient heat treatment specifically includes the following steps: S501, High-Temperature Purification Stage: The temperature was increased from room temperature to 1100-1200℃ at a rate of 200℃ / h, and then held at 1100-1200℃ for 3 hours. S502, Intermediate Temperature Ordering Stage: Slowly cool at a strictly controlled rate of 80℃ / h to ensure that the cooling time in the critical temperature range of 550℃ to 450℃ is no less than 1 hour. S503, Low-Temperature Stabilization and Quenching: When the temperature slowly drops to 350℃, keep it at that temperature for 1 hour. After the heat preservation is completed, high-purity Ar gas is immediately introduced into the furnace and forced air cooling is performed to quickly cool the magnetic core to below 100°C.

2. The manufacturing process for the permalloy CT energy harvesting core suitable for low-current start-up as described in claim 1, characterized in that, S1 includes the following steps: S101, Smelting: A vacuum induction melting furnace is used. The prepared alloy raw materials are placed in a ceramic crucible and the vacuum is evacuated to ≤5×10. -2 Pa, fill with high-purity argon gas to -0.05MPa as a protective atmosphere; start the medium frequency power supply, slowly heat to 1550-1600℃ to fully melt and refine the furnace charge for 20-30 minutes, and use electromagnetic stirring to ensure uniform composition, then cast into flat ingots; S102, Hot Forging and Hot Rolling: The ingot is heated to 1180-1200℃ and held for 2-3 hours before being hot-forged in multiple passes to the required thickness. Then, it is hot-rolled in the same temperature range with a total deformation of ≥80%, and finally rolled into a hot-rolled plate with a thickness of 2.0-3.0 mm. S103, Cold rolling and intermediate annealing: Cold rolling: The hot-rolled plate is pickled to remove the oxide scale, and then subjected to initial cold rolling with a deformation of 50-60% to about 1.0 mm. Intermediate annealing: carried out in a bell-type hydrogen-protected annealing furnace at a temperature of 1000-1050℃, with pure hydrogen as the protective atmosphere, for 3-4 hours, followed by furnace cooling to achieve complete recrystallization and eliminate work hardening. Precision cold rolling: After 3-4 cycles of cold rolling and intermediate annealing, the strip is finally rolled to a target thickness of 0.05-0.2mm using a 20-roll precision rolling mill. The thickness tolerance is controlled within ±0.002mm. The strip width is set to 100-500mm according to the magnetic core design. Final high-temperature annealing: Before winding, the cold-rolled strip needs to be softened and annealed. Under hydrogen protection, it is held at 850°C for 2 hours and then cooled in the furnace.

3. The manufacturing process for the permalloy CT energy harvesting core suitable for low-current start-up as described in claim 1, characterized in that, S2, surface coating, refers to coating one or both surfaces of the thin strip (1).

4. The manufacturing process for a permalloy CT energy harvesting core suitable for low-current start-up as described in claim 1, characterized in that, The roll-to-roll PVD coating equipment in S2 is an EB-PVD roll-to-roll vacuum coating equipment, which has multiple deposition chambers. One chamber is equipped with a pure Cr target, and through a precisely controlled shield, only the two edge areas of the strip to be coated with Cr are exposed to the sputtering particle stream. The other chamber is equipped with a pure Mo target, and through a precisely controlled shield, only the two edge areas of the strip to be coated with Mo are exposed to the sputtering particle stream.

5. The manufacturing process for a permalloy CT energy harvesting core suitable for low-current start-up according to claim 1, characterized in that, Before S2 and surface coating, plasma cleaning is performed on the surface of the thin strip (1): Ar ion source is used for bombardment, with parameters: Ar gas flow rate 50-100 sccm, vacuum degree 0.1-0.5 Pa, bombardment energy 500-1000 eV, and time 30-60 seconds.

6. The manufacturing process for a permalloy CT energy harvesting core suitable for low-current start-up according to claim 1, characterized in that, The process parameters for S2 and surface coating are: Ar working pressure 0.3-0.5Pa, sputtering power 8-10kW, and strip conveying speed 0.5-1.0m / min; the thickness of Cr coating (101) and Mo coating (102) is precisely controlled to be 0.5-2μm by adjusting the conveying speed.

7. The manufacturing process for a permalloy CT energy harvesting core suitable for low-current start-up according to claim 1, characterized in that, S4, hot-press diffusion, includes the following steps: S401, Core Placement: A vacuum hot-pressing sintering furnace with a water-cooled pressurization system is used to place the wound spiral magnetic core together with the graphite core shaft into a high-strength graphite mold. The inner cavity of the mold matches the outer diameter of the magnetic core, and the upper and lower pressure heads are flat. S402, Vacuuming and Gas Cleaning: Evacuate the furnace cavity to ≤5×10 -3 Pa, then fill with high-purity Ar gas to -0.03 MPa, repeat 2-3 times to fully remove oxygen; S403, Heating and Pressurization: The temperature is increased at a rate of ≤10℃ / min. When the temperature reaches 600℃, axial pressure is applied and gradually increased to 5-20MPa. S404, thermal insulation and pressure retention: Perform hot-press diffusion by holding the temperature and pressure at 800-950℃ for 1-4 hours; S405, Cooling: After the heat preservation is completed, the furnace is cooled to below 300°C while maintaining pressure, then the pressure is released and the furnace continues to cool to room temperature.

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