Compositions – Magnetic – Free metal or alloy containing
Reexamination Certificate
1999-05-14
2001-02-27
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Free metal or alloy containing
C252S062530, C252S062540, C148S104000, C148S105000, C264S611000, C264S612000
Reexamination Certificate
active
06193903
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to AC electromagnetic cores formed by powder metallurgy. More particularly, this invention relates to ferromagnetic particles coated with a ceramic layer that, when the particles are compression molded to form a net-shaped magnetic article, enables the article to be annealed at high temperatures to improve magnetic properties, including low frequency output.
BACKGROUND OF THE INVENTION
The use of powder metallurgy (P/M), and particularly iron and iron alloy powders, is known for forming magnets, including soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression molding, injection molding and sintering techniques, can be used to form intricate molded part configurations, such as magnetic cores, without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for use immediately after the forming operation.
Molded magnetic cores for AC applications generally should have low magnetic core losses, which requires that the individual metal particles within the magnetic core be electrically insulated from each other to provide eddy current protection, while also achieving an acceptable level of permeability. Numerous types of insulating materials have been suggested by the prior art, many of which also serve as a binder that adheres the particles together. Examples of such materials include inorganic materials such as iron phosphate, alkali metal silicates, and organic polymeric materials. In addition to providing adequate insulation and adhesion between the metal particles upon molding, insulating materials are often selected for their ability to provide sufficient lubrication during the forming operation to enhance the flowability and compressibility of the particles, and therefore enable the particles to attain maximum density and strength, particularly when compression molded at high pressures.
In view of the above considerations, plastics have been widely used as insulating materials for AC magnetic cores. However, the permeability of magnetic articles formed with plastic insulating materials is not sufficiently high for many AC applications, and core losses are often high at low frequencies (e.g., 50 Hz and less), resulting in low outputs at low rpms. Increased permeability and lower hysteresis losses can be achieved by annealing the core to relieve the detrimental effects on magnetic characteristics caused by cold working during compression molding. However, relieving substantially all stresses in a work-hardened core formed of ferromagnetic materials often requires maintaining the core at a temperature of at least 600° C. for a length of time that depends on the degree of work hardening in the core, followed by slow cooling. Plastic materials currently available are unable to withstand these temperatures, and degrade and pyrolyze during annealing. The ability of the insulating material to encapsulate and adhere the particles will also degrade if the core is annealed at lower temperatures that exceed the heat deflection temperature of the insulating material. Even if physical destruction of the core does not occur, the magnetic field characteristics of the core will likely be severely impaired because of the degradation of the insulating capability of the material.
In view of the above, it can be appreciated that, because the insulating material must remain within an AC magnetic core to achieve low core losses, the ability to anneal a core is limited by the heat resistant properties of the insulating material. Maximum operating temperatures of AC magnetic cores are similarly limited by the insulating material. Therefore, it would be desirable to provide a coating for powdered metals that has the ability to withstand high processing and operating temperatures, so that P/M magnetic cores molded from such particles exhibit desirable mechanical and magnetic properties that do not deteriorate at high temperatures.
SUMMARY OF THE INVENTION
According to the present invention, methods are provided for producing and processing ceramic-coated powdered ferromagnetic materials, particularly iron and its alloys, which when used to form a magnetic article, maintain the mechanical and magnetic properties of the article at high temperatures, such as during annealing of the article to relieve stresses induced during the forming operation.
The ceramic coating materials of this invention can generally be metal oxides, nitrides, carbides, ferrites, silicates and phosphates, and are present as an encapsulating layer on each ferromagnetic particle. The particles are then compacted using any suitable technique to form a solid magnetic article, which can then be fully annealed without concern for degrading the ceramic encapsulating layer. Thereafter, the magnetic article is ready for use, though in some circumstances it may be desirable to impregnate the article with a polymeric material to increase the strength and corrosion resistance of the article.
The invention encompasses several techniques for forming the ceramic encapsulating layer. According to one embodiment, the ferromagnetic particles are oxidized. For example, iron-based ferromagnetic particles are oxidized under controlled conditions to yield an encapsulating layer that consists essentially of iron oxides. In another embodiment, the ceramic material is applied in powder form, and preferably combined with a polymeric material such that the encapsulating layer initially comprises a mixture of the polymeric and ceramic material. Annealing is then performed under conditions that decompose the polymeric material and cause the ceramic material to flow and encapsulate the ferromagnetic particles. In yet another embodiment, the encapsulating layer is formed by applying a coating of an organometallic compound to the ferromagnetic particles, and then heating the particles to convert the organometallic compound to a ceramic material. With each of these embodiments, the encapsulating layer can be overcoated with a polymeric coating that serves as a lubricant during forming of the article, and is then decomposed during annealing.
In view of the above, it can be appreciated that this invention provides a magnetic article formed of compacted and annealed ferromagnetic particles, with each particle being encapsulated with an insulating layer of ceramic material. With the ceramic insulating layer, the particles and an article formed from the particles can be fully annealed and exposed to high temperatures without degrading the insulating effect of the insulating layer, such that the mechanical and magnetic properties of the article do not deteriorate. Ceramic insulating layers of this invention have also been shown to produce articles having significantly higher permeability and lower hysteresis losses as compared to those with polymer insulating layers, while maintaining the strength, density and eddy current protection necessary for demanding AC applications, particularly at lower frequencies, e.g., 50 Hz and less.
REFERENCES:
patent: 4177089 (1979-12-01), Bankson
patent: 4919734 (1990-04-01), Ochiai et al.
patent: 5116437 (1992-05-01), Yamamoto et al.
patent: 5183631 (1993-02-01), Kugimiya et al.
patent: 5982073 (1999-11-01), Lashmore et al.
patent: 6051324 (2000-04-01), Moorhead et al.
Gay David Earl
Score David Allen
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Koslow C. Melissa
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