JPH076911A - Magnetic core consisting of agglomerate - Google Patents

Abstract

PURPOSE: To increase the permeability of a compression-molded magnetic core, made of a sealed powdered metal by causing the compression-molded magnetic core to contain a plurality of agglomerates, each of which is made of sheathed ferromagnetic particles, and which are substantially surrounded by a specific polymer coating material. CONSTITUTION: Each of compression-molded magnetic cores contains a plurality of agglomerates, each of which is made of sheathed ferromagnetic particles and which are substantially surrounded by a specific polymer coating material. The polymer coating material is selected from a group, consisting of polybenzimidazole and a polyimide induced from 3,4'-oxydianiline and polymethylene dianiline. The polymer coating material is less than 1 wt.% of the with respect to the entire agglomerate and has a magnetic field strength of 50 oersteds and a permeability of at least 175 G/Oe, when measured at 100 to 400 Hz. As a result, the permeability of the compression-molded magnetic core, made of a sealed powdered metal can be increased.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention generally relates to a method according to the preamble of claim 4 for enhancing the magnetic permeability of shaped bodies from coated ferromagnetic particles as described in the preamble of claim 1. More specifically, the present invention forms compression molded magnetic bodies from ferromagnetic particles coated with a polymeric material which, in turn, have enhanced magnetic permeability without significant loss of mechanical properties. In particular, it relates to a method of annealing to substantially reduce the stresses induced in the magnetic body during compression molding without significantly degrading the polymer coating material.

[0002]

The use of powdered metals, in particular iron and its alloys, for forming magnets such as soft AC cores for transformers, inductors, motors, generators and relays is known.
The advantage of using powdered metal is that, using molding operations such as compression molding, injection molding or isostatic press molding methods, without the need for additional machining and / or drilling operations, For example, it is possible to form a molded part having a complicated shape such as a magnetic core.
As a result, molded parts are often substantially usable within their environment of use as formed by the molding process.

Molded magnetic cores for AC applications generally have low iron loss (m
should have a magnetic core loss). In order to obtain low iron loss, the particles in the magnetic core must be electrically isolated from each other. Many types of insulating materials, which also act as necessary binders for molding, have been suggested by the prior art, including inorganic materials such as iron sulfate and alkali metal silicates, as well as a variety of organic polymeric materials. Has been done. It is also known to coat powdered metals with an inorganic undercoating before applying an organic topcoat. Besides providing sufficient insulation and adhesion between the metal particles during molding, the coating material enhances the fluidity and compressibility of the particles, which allows the molded product to obtain maximum density and strength. In addition, it must have the ability to provide sufficient lubrication during the molding operation.

A drawback of the prior art often arises in that the heat resistance of the insulating material used to bond the metal particles together often determines the maximum operating temperature of the magnetic core. Insulates individual metal particles from each other, thereby
It is important to maintain the bondability of the insulating material in order to give a low iron loss suitable for C applications. If the magnetic core is exposed to a temperature above the decomposition temperature of the coating material, the coating material will have less chance of encapsulating and adhering the particles, eventually destroying the magnetic core. Even if no physical destruction of the magnetic core occurs, it is believed that the magnetic field characteristics of the magnetic core are severely impaired due to the reduced insulation capacity of the coating material due to high temperatures.

Polybenzimidazole (PBI), aromatic polyamides (eg polyphthalamide (PPA)) and certain polyimides have been found to work well as coating materials for iron powder and / or iron alloy powder. . Each of these preferred materials has a use temperature limited by their heat deflection temperatures, allowing their use in high temperature applications above about 270 ° C.
As a result, these preferred polymers perform well, especially with regard to their ability to withstand relatively high service temperatures, such that the mechanical and favorable magnetic properties of the molded core do not deteriorate at elevated temperatures.

Polybenzimidazoles, aromatic polyamides (eg polyphthalamides) and preferred polyimides adhere well to the underlying iron particles and bind the iron particles together, with the potential to withstand thermal and chemical attack. Have,
Moreover, it also serves as a lubricant during the compression molding process to enhance the high density and strength of the magnetic core. The ability of the encapsulating material to also serve as a lubricant during the molding process is important because improperly low density corresponds to a reduction in the magnetic permeability of the core.

However, the drawbacks associated with compression molded magnetic cores are:
Processing hardness occurs during the compression molding process, including stresses in the core that also lead to reduced permeability and possibly high iron loss. As an example, the magnetic permeability of a magnetic core formed by conventional compression molding methods typically does not exceed about 125 Gauss / Oersted (G / Oe) at about 50 Oersted field strength and about 100-400 Hz. As a result, magnetic cores compression molded from encapsulated iron particles typically have a magnetic field strength of about 50 Oersteds and about 10 Oersteds.
Permeability high enough to be useful in AC applications, such as generators, stator cores, transformers, etc., that require a permeability greater than about 175 G / Oe measured at 0-400 Hz. Does not have.

In order to reduce the undesired stresses induced during compression molding, it is necessary to anneal the core at a temperature of at least about 450 ° C. and then to cool the core without quenching it. However, polymer coatings generally cannot withstand such temperatures and decompose,
Pyrolysis tends to result in a significant reduction in strength and magnetic properties in the core.

[0009]

Accordingly, there is provided a method of enhancing the permeability of a compression molded core formed from encapsulated metal powder, wherein the coating material reduces the stress induced by the compression molding process. It is desirable to provide a method that has the potential to withstand processing temperatures that are sufficient for annealing. Furthermore, it is desirable that such a method does not result in a corresponding degradation of the mechanical and magnetic properties of the molded core as a result of decomposition and / or thermal decomposition of the coating material during annealing. In addition, such coating materials should be soluble in a suitable solvent to improve lubrication during the molding process and to obtain maximum density and strength of the as-molded compacts, and post-molding. It should be possible to bond between the metal particles.

A method of molding magnetic particles suitable for a wide range of high temperature applications in accordance with the present invention is characterized by the features set forth in the characterizing portion of claim 4. The compression-molded magnetic core according to the invention is characterized by the features described in the characterizing part of claim 1.

It is an object of the present invention to provide a method of enhancing the magnetic permeability of compression molded cores molded from encapsulated powder metal.

Such a method involves the use of a coating material for encapsulating powdered metal particles, the stress induced in the magnetic core by the compression molding process resulting from the decomposition and / or thermal decomposition of the coating material during annealing. This coating material is sufficient to anneal the compressed core from the encapsulated metal particles so that it can be mitigated without significantly degrading the mechanical and magnetic properties of the core as Being able to withstand temperature is another object of the invention.

The coating material enables the metal particles to adhere strongly to each other to allow for direct handling and use of the core after the molding process, and the metal particles to promote low iron loss in the core. It is yet another object of the present invention that such coating materials have high strength and insulation so that they are well insulated from each other.

It is a further object of the present invention that such coating materials have a high compressibility in order to facilitate the compression molding of the metal particles and thus optimize the density of the resulting magnetic core. .

Finally, it is a further object of the invention that such coating materials can be deposited on the metal particles using methods such as the fluidized bed process.

[0016]

According to a preferred embodiment of the present invention, these and other objects and advantages are achieved as follows.

According to the invention, a method of strengthening the permeability of a compression-molded magnetic core formed from encapsulated powder metal, which can withstand temperatures sufficient to anneal the magnetic core, such as iron and iron. A method is provided that includes the use of a coating material to encapsulate or coat a powder metal such as an alloy. As a result, the stresses induced by the compression molding process can be reduced without substantially degrading the mechanical and magnetic properties of the magnetic core as a result of decomposition of the coating material during annealing.

Coating materials which have been found to be able to withstand the required annealing temperatures are polybenzimidazoles (PBI) and certain polyimides (PI), which have a heat deflection temperature of at least about 400.degree.

The magnetic bodies compression molded from metal particles encapsulated by these preferred coating materials will withstand temperatures generally above about 450 ° C. for a period of time sufficient to reduce the work hardening stress induced by the compression process. It turns out to be possible. It is considered that the removal of these stresses enhances the magnetic permeability of the magnetic material. According to the present invention, the preferred coating materials do not significantly degrade or pyrolyze at these annealing temperatures, thus mitigating the loss of mechanical strength (eg strength) and / or magnetic properties (eg magnetic permeability) of the magnetic core.

Each of the preferred coating materials has about 400
Having a heat deflection temperature in excess of 0 ° C., magnetic bodies formed from metal particles coated with any of the preferred coating materials are particularly suitable for use at relatively high service temperatures. In particular, these coating materials enable the magnetic core to maintain its mechanical and magnetic properties at service temperatures up to at least the corresponding heat deflection temperature of the particular service coating material.

The preferred coating materials are also sufficiently soluble, highly resistant to chemical action, have a relatively high strength and good dielectric properties. As a result, the coating material can be applied such as in fluidized bed processes and is suitable for use in applications requiring high strength and insulation in relatively hot environments. This coating material is capable of strongly adhering metal particles together to form a compact using a compression molding process. Moreover, the iron loss caused by the insulating effect of the coating material is reasonably low, which guarantees the favorable magnetic properties of the core. Finally, the coating material is sufficiently lubricious to assist in compression and densification during the compression molding process. The above properties are particularly advantageous for the production of magnetic cores that are compression molded from coated metal particles.

The preferred coating material is a relatively small amount,
That is, the above advantages can be exhibited while being present at less than about 1% by weight compared to the mass of encapsulated metal particles. Introducing the coated metal particles into a suitable molding apparatus, such as a compression molding or injection molding machine or an isostatic press,
The coated metal particles are compacted in a heated mold cavity under an appropriate high pressure to densify the coated metal particles to produce a dense, strong solid magnetic material.

The magnetic body is annealed at a temperature and for a period of time sufficient to reduce the work hardening induced by the molding process to aid in the magnetic permeability of the magnetic body and, then, preferably to produce heat-induced stress. Allow to cool at a slow enough rate to avoid. The preferred coating material can withstand the annealing process so that no significant degradation or thermal decomposition of the coating material occurs. Therefore, there is no significant reduction in the strength or AC magnetic properties of the magnetic body as a result of adverse changes in the preferred coating material. Thus, a magnetic core compression molded from metal particles encapsulated by the preferred coating materials of the present invention requires a magnetic field strength of about 50 Oersteds and a permeability of greater than about 175 G / Oe measured at 100-400 Hz.
It has a sufficiently high magnetic permeability to be useful in AC applications such as generators, stator cores, transformers and the like.

Other objects and advantages of the present invention will be better understood from the following detailed description in connection with the accompanying drawings.

FIGS. 1 and 2 are graphs illustrating the effect of various annealing temperatures on the mechanical properties of the magnetic body formed according to the present invention; FIG. 3 is a graph showing the magnetic permeability of the magnetic body formed according to the present invention. It is a graph explaining the influence which annealing has on the other hand.

The ranges set forth herein, as are well known in the art, do not significantly affect the results desired to be achieved,
It is understood that it is possible to extend slightly beyond the limits quoted in some cases.

The method of the present invention is used to coat powdered materials, and more particularly powdered iron and ferromagnetic iron alloys, such as to form magnets particularly suitable for use as AC cores used in the automotive industry. Including the use of a group of polymer coating materials to encapsulate the. The preferred polymeric coating materials are capable of withstanding high temperatures that are sufficient to anneal the core in order to reduce stresses induced during the molding process to produce a magnetic permeability assistance. It should be noted that molding other types of products is also within the scope of the invention.

According to the present invention, the preferred polymer coating materials are polybenzimidazoles (PBI) and polyamides (PI) having a heat deflection temperature of at least about 400 ° C. Such polyimides include those derived from 3-4'oxydianiline and polymethylene dianiline. Polybenzimidazole is a Hoechst Celanese Corporat
ion) (USA) under the trade name Celazole U-60
It is available at. A polyimide derived from 3-4 ′ oxydianiline and polymethylene dianiline is available under the trade name Imitech 201A from Imitech (USA).
Available from. It should be noted that although these are the preferred polymeric coating materials of the present invention, it is anticipated that other polymers with moderately high heat deflection temperatures could be used. In addition, thermosetting polymers could be used, but a suitable high temperature epoxy resin is required.

Each of the preferred coating materials has an ASTM test D entitled "Plastic Deflection Temperature Under Bending Load" in which a sheet of polymeric material is supported at three points and the deflection is measured as a function of the temperature rise of the polymeric material.
It is characterized by excellent mechanical and dielectric properties over a temperature range of at least about 400 ° C., generally measured by standardized heat deflection temperatures according to -648.

Polybenzimidazole has a heat deflection temperature of about 435 ° C. and the preferred polyimides derived from 3-4'oxydianiline and polymethylene dianiline have heat deflection temperatures of at least about 400 ° C.
It was initially thought that these substances do not maintain their binding properties at high annealing temperatures, but cause the degradation of the binding properties of the shaped bodies formed from the mass of encapsulated particles. Also, shaped bodies formed from ferromagnetic metal particles coated with the preferred polybenzimidazole withstand an annealing temperature of at least about 500 ° C. for 1 hour to reduce stress induced during the shaping process of such cores. It turned out to be possible to anneal for. In addition, shaped bodies formed from ferromagnetic particles coated with the preferred polyimides, such as polyimides derived from 3-4'oxydianiline and polymethylene dianiline, for example, have an annealing temperature of at least about 450 ° C. It is possible to endure time. Even at these relatively high temperatures, the bond between the physical and dielectric properties of the magnetic core is maintained, resulting in, if any, degradation of the magnetic properties of the compact, as measured by iron loss, for example. It hardly happens.

Moreover, since each of the preferred coating materials is soluble in a suitable solvent, their use in the preferred Wurster type fluid coating process known in the art as described above is possible. Specifically, polybenzimidazole is 1-methyl-2- with lithium chloride.
Although soluble in pyrrolidone, the preferred polyimides are generally soluble in N-methyl-2-pyrrolidone, it is expected that other suitable solvents are present and can be used.
However, the preferred coating materials of the present invention tend to be insoluble in solutions other than their above solvents, which renders them susceptible to chemistry in most environments, such as the environment of automotive engine components. It is virtually impervious.

The solubility of both the polybenzimidazole and the preferred polyimide in at least one solvent is advantageous from the point of view of the preferred coating and molding processes used according to the present invention. It is expected that the preferred coating materials, especially polybenzimidazole, can be used in slurry coating processes that do not require the coating material to be first dissolved in a solvent, but to obtain a more uniform coating on the powder material, It is generally preferred to use a fluidized coating process in which the preferred polymer is in solution in order to promote low core loss thereby.

According to the present invention, a magnetic body formed of ferromagnetic particles encapsulated by a preferred coating material has at least 45 to enhance the magnetic permeability of the magnetic body.
It can withstand an annealing temperature of 0 ° C. for a sufficient period of time to reduce the stress induced by the molding process. After annealing, the mechanical and magnetic properties of the preferred coating material are retained to provide sufficient adhesion between adjacent metal particles in order to maintain the desired strength and shape of the magnetic core after forming. Furthermore, the insulating ability of the preferred coating material is adequately maintained to minimize core loss of the compact.

The temperatures that can be used for ferromagnetic metal particles coated with any of the preferred coating materials exceed about 375 ° C., thus allowing their widespread use in high temperature applications. Accordingly, the iron loss characteristics (magnetic core l
oss property) is also maintained.

The preferred coating materials also have desirable flow and feed properties, are compressible and compact, which makes them very suitable for use in compression molding processes. As a result, the preferred coating materials are easily handled by conventional dispensing equipment. Furthermore, the maximum metal particle density can be obtained by the compression molding process.

The above advantages can be achieved while each of the preferred coating materials is present in an amount less than about 1% by weight relative to the total weight of the encapsulated metal particles. Most preferably, the polybenzimidazole is about 0.5 to about 1% by weight.
And the preferred polyimide is about 0.25 to
It is present in the range of about 0.75% by weight. It is envisaged that larger amounts of the preferred coating material can be used, but this may lead to a corresponding change in the physical properties of the shaped body and / or a decrease in the permeability.

The balance of the compact, about 99% by weight, is preferably about 5 to about 400 μm, more preferably about 25 to about 35, in order to obtain a magnetic core having a high magnetic permeability as described in detail below.
It consists of ferromagnetic particles with sizes in the range of 0 μm.

The preferred method of coating the ferromagnetic metal particles employs a Wurster type spray coating fluidized bed of the type known to those skilled in the art, although other methods which produce a uniform coating on the particles are also possible. The fluidized bed essentially comprises a concentric pair of upright cylindrical vessels, one inside the other. The outer container has its axial lower end closed to form a floor of the outer container only, and the inner container is suspended above this floor. The bed has apertures of various sizes through which heated air is drawn into both vessels. The apertures are sized and arranged so that the majority of the air flow rises through the inner container and then descends between the inner and outer containers. Prior to introduction into the fluidized bed, it is preferred, if not necessary, to pre-sort the metal particles by size in order to promote a substantially uniform coating thickness on the metal particles during the coating process.

At the beginning, a batch of powdered metal is placed at the bottom of the vessel and hot air is circulated at a rate sufficient to fluidize the particles. Depending on batch size and particle size, air flow rates are generally in the range of about 100 m ³ / hour to about 200 m ³ / hour. Also, the temperature of the air is in the range of about 55-80 ° C. at the beginning of the coating process, but will change due to solvent introduction and evaporation during the coating process. If the air temperature is too low, the solvent will not evaporate on contact with the metal particles, which will result in insufficient coated particles, and if the air temperature is too high, the solvent will evaporate too quickly, resulting in a uniform thickness on the particles. Formation of the film is prevented. As the coating process progresses, each particle is randomly coated an abnormally large number of times to ensure a uniform thickness of coating on the particles.

A spray nozzle located in the floor below the inner chamber serves to feed one of the preferred coating materials, dissolved in a suitable solvent, into the chamber. To maximize the efficiency of the coating operation, the solution is preferably about 5 to about 15% by weight.
Of coating material, more preferably about 10% by weight
Of coating material, but with an extremely large range of solutions, suitable coating results can be obtained.

The solution is then sprayed into the fluidized bed. In the fluidized bed, the solvent evaporates leaving behind the coating material deposited on the particles. Once deposited, the encapsulated metal particles recirculate between the limited volume defined by the inner and outer vessels by the action of hot air. Each metal particle
Circulation is continued until a uniform, sufficiently thick coating of the particular coating material used is obtained, preferably depending on the respective weight percents mentioned above for each of the coating materials of the invention. Typically the coating thickness is from about 0.3 to about 4.5μ for metal particles within the preferred range of from about 5 to about 400μm.
It is within the range of m.

As mentioned above, other deposition methods can be used, so long as a substantially uniform coating is obtained on each particle.

Thereafter, the coated metal particles can be introduced into a suitable molding apparatus. For example, typical molding processes used to form magnetic cores include compression molding, injection molding and isostatic pressing, generally from about room temperature to about 370 ° C, more preferably from about 260 ° C to about 37 ° C.
Performed at a molding temperature in the range of 0 ° C., the particles are about 1
Preheat to 50 to about 175 ° C. At these temperatures, the preferred coating materials are sufficiently fluid that they flow under pressure during the molding operation, but are sufficiently viscous to adhere to the metal particles and provide a lubricating action between adjacent metal particles.

As a result, using an automated handling device,
Coated metal particles can be processed and fed through coating and molding processes to reduce cycle times. In addition, compression moldings formed by these processes, such as magnetic cores, generally allow direct handling and use of the as-molded moldings and, if necessary, machining of the moldings. As described above, it is physically strong and dense.

The coated metal particles easily flow into the mold cavity to preheat the metal particles and the mold cavity.
08.886-772.215 MPa [about 20 to about 50
Ton / square inch (tsi)], which is preferably sufficiently flowable and densified when subjected to typical molding pressures, eg, its density is greater than about 7.0 g / cm ³ To form a molded body such as. The coating and molding process can be widely varied to change the physical and magnetic properties of the molded body, as is known in the art.

The compact is then annealed to reduce the work hardening stress induced during the molding process. The preferred temperature range for the annealing process of the present invention depends in part on the particular coating material selected. Generally, an anneal temperature of about 425 to about 550 ° C. is preferred for the coating materials of the present invention, with a range of about 475 to about 550 ° C. being more preferred for moldings formed by polybenzimidazole, and preferred polyimides. About 425 to about 500 ° C. is a more preferable range for the formed body. The duration of the annealing process is typical A
For C applications, it is preferably about 0.5 to about 2 hours. While a period of about 1 hour appears to be sufficient for most applications, the optimum time period for a particular molding is highly dependent on the size and shape of the molding. Therefore, it is believed that an annealing period of less than 0.5 hours or more than 2 hours may be preferable in some circumstances.

After annealing, the compact is not quenched, preferably at a slow enough rate to avoid the formation of thermally induced stress in the compact. A suitable method is to cool the compact by natural convection in the furnace as it cools from its heating cycle.

In order to determine the preferred annealing temperature of the magnetic bodies formed from the ferromagnetic particles coated according to the present invention, the individual amounts of the ferromagnetic particles are selected according to the fluid bed process described above and by any of the preferred coating materials. Coated. Ferromagnetic particles are generally about 5 to about 300μ.
m having a particle size of m and these particles were coated with polybenzimidazole (Cerarose U-60 from Hoechst Celanese Corporation) or the preferred polyimide (Imitech 201A from Imitec). For particles encapsulated by polybenzimidazole,
Particles having a coating material deposited to a thickness sufficient to result in an amount of about 0.75% by weight of the coated ferromagnetic particles and encapsulated by the polyimide have a polyimide content of about 0.375% of the amount of the coated particles. It was coated to a thickness sufficient to reach 100%. These particular weight percentages are calculated to be the optimum amount for each polymer coating.

For comparison, the ferromagnetic particles were loaded with Amoco Performance Products (Amoco Performance Products).
ts, Inc.) (USA) under the trade name Amodel AD-1
It was also coated with the polyphthalamide obtained at 000. Polyphthalamide is disclosed as the preferred coating for the formation of shaped magnetic bodies suitable for high temperature applications. About 0.75% by weight of the ferromagnetic material coated with this polyphthalamide
Was adhered so that

Next, the lateral breaking bar of each of the specific coating materials is formed into a molding pressure of 772.215 MPa (about 50 ts).
Formed by room temperature compression molding in i). This transverse break bar is approximately 31.75 mm (1.25 inches) long, 12.7 mm (0.5 inches) wide and 9.52 thick.
It was 5 mm (0.375 inch). The polyimide coated lateral break bar has a density of about 7.4 g / cm ³ , and the polybenzimidazole coated lateral break bar is about 7.5 g.
It had a density of / cm ³ .

After formation, test bars for each coating material were placed at about 180 ° C, 290 ° C, 400 ° C, 450 ° C and 510 ° C.
It was selectively annealed at a temperature of ° C and one sample of each coating material was left unannealed for comparison. The sample annealing time was about 1 hour. A strength test is then carried out to determine the load at 0.2% offset and the load at rupture of the test bar as ASTM test B5 titled "Lateral breaking strength of metal powder sintered test piece".
28-83A.

The results of the tensile tests are shown in Table I below and in FIGS. In Table I and FIGS. 1 and 2, "PI" is used to represent the results corresponding to the polyimide coated lateral break bars and "PBI" is used to represent the results corresponding to the polybenzimidazole coated lateral break bars. Use "PPA"
Is used to represent the result corresponding to a polyphthalamide coated lateral break bar. The lack of data to describe means that the coating material decomposed to some extent untestable.

Table I PPA PI PBI Maximum Post-Baking Load MPa (psi) 12.79 (1855) 20.42 (2962) 15.93 (2310) 0.2% Offset Load MPa (psi) 10.63 (1542) 20.15 (2923) 15.04 (2181) ) 180 ℃ maximum annealing load MPa (psi) 38.18 (5537) 15.57 (2258) 20.07 (2911) 0.2% offset load MPa (psi) 36.71 (5325) 15.44 (2240) 18.96 (2750) 290 ℃ annealing maximum fracture load MPa (psi) 41.12 (5964) 63.52 (9213) 70.70 (11270) 0.2% Offset load MPa (psi) 39.44 (5721) 45.11 (6542) 52.54 (7621) 400 ℃ Annealed maximum breaking load MPa (psi) 29.49 (4277) 86.81 (12590) 98.04 (14220) 0.2% Offset load MPa (psi) 23.12 (3353) 53.08 (7698) 59.58 (8642) 450 ℃ Annealed maximum breaking load MPa (psi) −− 90.98 (13170) 105.90 (15360) 0.2% offset Load MPa (psi) −−51.89 (7526) 67.49 (9788) 510 ° C Annealing maximum breaking load MPa (psi) −− 68.48 (9932) 106.18 (15400) 0.2% Offset load MPa (psi) −− 46.55 (6751) 67.09 (9730) above Data illustrates that the preferred coating materials of the present invention has a suitable strength after annealing at a specific temperature. In fact, the test bar required a heavier load to break when the annealing temperature was increased, and polybenzimidazole (PBI) required the heaviest load to break at elevated temperatures up to 510 ° C (including 510 ° C). .
Specimens using the preferred polyimide as the coating material show some degradation of mechanical properties at about 510 ° C, which results in a more optimal annealing temperature of about 450 ° C.
Proved to be close to. In contrast, a test piece using polyphthalamide as the coating material has about 180
It exhibits the lowest mechanical properties at annealing temperatures above 0 ° C., and pyrolysis degrades these specimens too much, resulting in about 4
It could no longer be tested at annealing temperatures above 00 ° C.

The preferred polyimides derived from polybenzimidazole materials and 3-4'oxydianiline and polymethylenedianiline are such that at their respective preferred annealing temperatures, the stresses present in the compact are reduced. It is believed that it softens sufficiently and that the preferred polyimides are also probably imidized at relatively high temperatures, resulting in stronger, stress-free moldings.

The mechanical strength data was used to indicate the degree of polymer degradation. The results show that the mechanical properties were enhanced after annealing these ferromagnets at temperatures up to at least about 450 ° C. To evaluate the effect of annealing on the magnetic properties of magnetic materials formed from the preferred coating materials, using polybenzimidazole as the coating material, approximately 772.215 MPa.
Donut type test samples were formed by compression molding at about 290 ° C. with a molding pressure of (50 tsi). The toroidal test sample had an outer diameter of about 50.8 mm (2 inches), an inner diameter of about 43.18 mm (1.7 inches), and a transverse thickness of about 6.35 mm (0.25 inches).
This donut-shaped sample was annealed at various temperatures for about 1 hour. Additional samples were not annealed for comparison. A polybenzimidazole coating was deposited so that the polybenzimidazole comprised about 0.75% by weight of the coated ferromagnetic particles.

The sample is then tested and tested from 100 Hz
Permeability (μ) when exposed to 400 Hz AC current
Was measured. The best test results were obtained with samples annealed at 450 ° C or 510 ° C and the data for these samples are shown in Figure 3. The data obtained from these samples demonstrated that there was a substantial increase in permeability in the annealed samples as compared to the unannealed samples. Such results are consistent with the view that the compression molding process work hardens the compact and correspondingly reduces its permeability. By annealing the test sample, the stress resulting from work hardening was reduced and the permeability increased, as the results in Figure 3 show. With respect to certain magnetic materials formed from ferromagnetic particles encapsulated by the preferred coating materials of the present invention, such that the magnetic materials can be used in demanding AC applications,
The annealing method of the present invention provides about 210 G / Oe measured at 50 Oersted field strength and 100-400 Hz.
It is expected that the magnetic permeability can be increased more easily.

To further evaluate the magnetic properties of magnetic bodies formed from the preferred coating materials, doughnut-shaped test samples of each preferred coating material were re-formed by the compression molding method described above. Polybenzimidazole was deposited to make up about 0.75% by weight of its respective coated particles and the preferred polyimide was deposited to make up about 0.375% by weight of its respective coated particles. The polyimide coated sample has a density of about 7.4 g / cm ³ and the polybenzimidazole coated sample is about 7.5 g.
It had a density of / cm ³ .

Samples of each preferred coating material were then annealed according to the best results obtained in the mechanical tests above. Samples formed from ferromagnetic particles encapsulated by polybenzimidazole were annealed at about 510 ° C for about 1 hour. A sample formed from ferromagnetic particles encapsulated by the preferred polyimide has about 450
Annealed at ℃ for about 1 hour.

The sample is then tested for DC current, 1
Measures magnetic field strength at Oersted (Hmax), magnetic flux density at Gauss (Bmax), total iron loss at Watts / pound (Pcm) and permeability (μ) when exposed to 00 Hz AC current and 400 Hz AC current did. The results of these tests are shown in Table II below. In this table, each coating material is identified as in Table I above.

Table II DC current PI PBI Hmax (Oe) 160 159 Bmax (G) 16,000 16,800 Permeability (μ) 100 106 Frequency = 100Hz Hmax (Oe) 59.1 75.9 Bmax (G) 12 , 520 15,100 Magnetic permeability (μ) 212 198 Iron loss (W / lb) 23.0 39.8 Frequency = 400 Hz Hmax (Oe) 58.9 78.6 Bmax (G) 12,520 15,100 Magnetic permeability (Μ) 213 192 Iron Loss (W / lb) 120 326 The above data, together with the results shown in FIG. 3, demonstrate that annealing of the samples results in useful magnetic cores with enhanced permeability. The polyimide (PI) sample showed lower iron loss than the polybenzimidazole (PBI) sample, but the polybenzimidazole (PBI) sample was obtained by obtaining a uniform coating with individual ferromagnetic particles or by using a lower molding pressure. It is considered that an improvement was made over PBI). A possible explanation for a polyimide sample with low iron loss is that the preferred polyimide is partially imidized during molding and almost completely imidized during annealing.

From the above, a significant advantage of the present invention is that it provides a group of polymer coatings for powder metal encapsulation that can withstand temperatures sufficient to anneal compression molded cores from coated metal particles. It will be apparent to those skilled in the art. As a result, stresses induced by work hardening of the magnetic core during the compression molding process are mitigated without significantly degrading the mechanical properties of the magnetic core as a result of decomposition and / or thermal decomposition of the coating material during annealing. be able to. Furthermore, it is clear that significant improvements in magnetic properties, and more particularly in permeability, can be achieved with the preferred coating materials according to the invention. As a result,
A magnetic core made in accordance with the present invention has a magnetic field strength of 50 Oersteds and about 175 measured at 100-400 Hz.
It has a significantly higher magnetic permeability, on the order of about 210 G / Oe, so that it is useful in AC applications such as generators, stator cores, transformers, etc. that require a magnetic permeability that exceeds G / Oe. be able to.

Temperature Coat of Preferred Coating Materials
rature capability is also thermally
-hostile) Advantageous for magnetic cores used in the environment.
The preferred coating material imparts mechanical properties to the core, including strength and density, resulting in strong adhesion of the metal particles together.

Furthermore, the high temperature resistance of the preferred coating materials includes the potential to electrically insulate the metal particles from each other, resulting in iron loss (an important magnetic property for AC applications) that is acceptable for many applications. You can The preferred coating materials are highly resistant to a wide range of chemicals, which makes these cores chemically-hosted, such as in the engine compartment of an automobile.
ile) Make it suitable for use in the environment. Moreover, such coating materials are sufficiently lubricious to allow the formation of dense molding compounds at typical molding temperatures.

Although the present invention has been described with respect to its preferred embodiments, it may be modified, for example, by using other thermoplastic polymers having a heat deflection temperature of at least about 400 ° C., or by changing processing parameters such as use temperature and pressure. Other formats by, for example, or by using other suitable powdered materials, such as other magnetic or magnetizable materials, or by using specific materials and methods for use in alternative applications. It is clear that can be adopted. Therefore, the scope of the present invention is limited only by the claims.

[Brief description of drawings]

FIG. 1 is a graph illustrating the effect of different annealing temperatures on the mechanical properties of magnetic materials formed according to the present invention.

FIG. 2 is a graph illustrating the effect of various annealing temperatures on the mechanical properties of magnetic materials formed according to the present invention.

FIG. 3 is a graph illustrating the effect of annealing on the magnetic permeability of a magnetic body formed according to the present invention.

Claims (12)

[Claims] 1. A compression molded core comprising an agglomerate of a plurality of coated ferromagnetic particles, each substantially surrounded by a polymeric coating material, the polymeric coating material comprising polybenzimidazole and 3,4. 'Be selected from the group consisting of polyimides derived from oxydianiline and polymethylene dianiline; the polymeric coating material comprises substantially less than 1% by weight relative to the total mass of the agglomerates; and 50 Oersted field strength and at least 175 measured at 100-400 Hz
A magnetic core having a magnetic permeability of G / Oe. 2. The polymer coating material is polybenzimidazole, which is 0.5 relative to the total mass of the aggregate.
The magnetic core according to claim 1, wherein the magnetic core comprises -1% by weight. 3. The polymer coating material is a polyimide derived from 3,4′oxydianiline and polymethylene dianiline, the polymer coating material being 0.25 to 0 relative to the total mass of the agglomerates. A magnetic core according to claim 1, comprising less than 0.75% by weight. 4. Depositing a substantially uniform encapsulation layer of a polymeric coating material on each of a plurality of ferromagnetic particles, comprising substantially less than 1% by weight of the polymeric coating material and having a size of 5 to 400 μm. Forming a plurality of ferromagnetic particles; compressing the coated particles in a mold cavity at a temperature and pressure sufficient to bond and bond the coated particles with the polymeric coating material. A polymer coating material suitable for a wide range of high temperature applications, including the step of forming a magnetic body, wherein the polymer coating material is selected from the group consisting of polybenzimidazole having a heat deflection temperature of at least 400 ° C. and a polyimide. To enhance the magnetic permeability of the magnetic material without significantly degrading or thermally decomposing the polymer coating material. Said method characterized in that the annealed the magnetic body at a temperature and duration are sufficient to alleviate the stress generated by the compression process. 5. The method of claim 4, wherein the polyimide is derived from 3,4'oxydianiline and polymethylenedianiline. 6. The method of claim 4, wherein the polymer coating material is polybenzimidazole and comprises 0.5 to 1 wt% relative to the total of the plurality of coated particles after the deposition step. 7. The method of claim 4, wherein the polymer coating material is a polyimide and comprises 0.25 to less than 0.75 wt% relative to the total of the plurality of coated particles after the deposition step. 8. The method according to claim 4, wherein the magnetic material is compression molded at a temperature within the range of room temperature to 370 ° C. 9. The method of claim 4, wherein the polymer coating material is deposited on the ferromagnetic particles using a fluidized bed spraying method. 10. The polymer coating material is polybenzimidazole, and the magnetic material is 475 ° C. to 550 ° C.
The method according to claim 4, wherein annealing is performed at the temperature of. 11. The method of claim 4, wherein the polymer coating material is polyimide and the magnetic material is annealed at a temperature of 425 ° C to 500 ° C. 12. The magnetic material has a magnetic field strength of 50 Oersteds and at least 17 measured at 100 to 400 Hz.
The method of claim 4 having a magnetic permeability of 5 G / Oe.