GB2430682A

GB2430682A - Insulated magnetic particulate material

Info

Publication number: GB2430682A
Application number: GB0519943A
Authority: GB
Inventors: Yong Xin Pang; Simon Nicholas Hodgson; Edgar Schoeps
Original assignee: Loughborough University; Loughborough University Enterprises Ltd
Current assignee: Loughborough University; Loughborough University Enterprises Ltd
Priority date: 2005-09-30
Filing date: 2005-09-30
Publication date: 2007-04-04
Also published as: GB0519943D0

Abstract

Insulated magnetic particulate material, such as that used to fabricate magnetic core components for electrical converters and electrical machines, comprises a plurality of ferromagnetic particles and an electrically insulating layer formed on each of said ferromagnetic particles. The electrically insulating layer comprises a coating compound formed as a reaction product of the ferromagnetic particles and an acid material selected from the group consisting of (a) an acid system comprising at least two acids and (b) an acid compound formed by reacting together at least two acids. The acids used in the acid system include phosphoric, molybdic and tungstic acids. The acid compounds used can be heteropolyacids such as molybdophosphoric and tungstophosphoric acids.

Description

I

Insulated Magnetic Particulate Materials The present invention relates to insulated magnetic particulate materials, and particularly to insulated magnetic particulate materials for use in the fabrication of magnetic core components. Embodiments of the present invention are particularly concerned with dielectric coatings for magnetic particulate materials.

It is known that insulated magnetic particulate materials, often referred to as dielectromagnetic materials or soft magnetic composites, can be used to fabricate magnetic core components for electrical converters and electrical machines such as motors. * a.

Insulated magnetic particulate materials generally comprise ferromagnetic powder particles which are coated with a suitable electrically *". insulating medium, also known as a dielectric coating, to provide the requisite electrical insulation.

: * Magnetic core components fabricated using insulated magnetic I...

* a 20 particulate materials are required to demonstrate a number of characteristics in use, including low eddy current losses and low hysteresis losses, together known as core losses, and high magnetic permeability. Such fabricated magnetic core components should also demonstrate good strength properties.

It would be desirable to provide an improved magnetic particulate material which can be used to fabricate magnetic core components. In particular, it would be desirable to provide an improved magnetic particulate material which can be used to fabricate magnetic core components that are capable of being heat treated to provide stress relief and reduce hysteresis losses without resulting in an excessive increase of eddy current losses.

According to the present invention, there is provided magnetic particulate material comprising a plurality of ferromagnetic particles and an electrically insulating layer provided on each of said ferromagnetic particles, wherein the electrically insulating layer comprises a coating compound formed as a reaction product of the ferromagnetic particles and an acid material selected from the group consisting of (a) an acid system comprising at least two acids and (b) an acid compound formed by reacting together at least two acids.

The present invention also provides magnetic particulate material comprising a plurality of ferromagnetic particles and an electrically insulating layer provided on each of said ferromagnetic particles, wherein the electrically * insulating layer comprises a coating compound formed as a reaction product of the ferromagnetic particles and an acid system comprising at least two acids. I... * I I...

The present invention also provides magnetic particulate material comprising a plurality of ferromagnetic particles and an electrically insulating : . layer provided on each of said ferromagnetic particles, wherein the electrically I. insulating layer comprises a coating compound formed as a reaction product of the ferromagnetic particles and an acid compound formed by reacting together at least two acids.

The acids may comprise first and second acids and the first acid may be more reactive with the ferromagnetic particles than the second acid.

The acids may comprise first, second and third acids, and the first acid may be more reactive with the ferromagnetic particles than the second and/or third acids.

The first acid may comprise a minor component of the acids and may react with the ferromagnetic particles to provide an adhesive mechanism which may adhere the electrically insulating layer to the ferromagnetic particles.

The second and/or third acids may react with the ferromagnetic particles to provide enhanced insulation and chemical stability to the ferromagnetic particles. The second and/or third acids may be selected to reduce the oxidation and reactivity of the ferromagnetic particles.

The acid system or the acid compound may comprise between 0.05 wt% and 5 wt% of the ferromagnetic particles.

The acid system or the acid compound may comprise a greater percentage by weight of the second acid than the first acid. The acid system or the acid compound may comprise a greater percentage by weight of the * 15 second and third acids than the first acid. S...

S

The acid system or the acid compound may comprise between approximately I and 20 wt% of the first acid and may comprise between approximately 80 and 99 wt% of the second acid. S...

S S S...

The acid system or the acid compound may comprise between approximately I and 20 wt% of the first acid and between approximately 80 and 99 wt% of the second and third acids.

The acid system may comprise a mixture or blend of acids in which the acids have not reacted together.

The first acid may comprise phosphoric acid.

The second and/or third acids may comprise molybdic acid and/or tungstic acid.

The acid compound may comprise molybdophosphoric acid or may alternatively comprise tungstophosphoric acid.

The acid compound may comprise a heteropolyacid. The term heteropolyacid as referred to in this specification may be as defined by the IUPAC nomenclature of chemical compounds.

The heteropolyacid may comprise an acid compound defined by the general formula H3A12040P.XH2O, where A is typically a transition metal such as tungsten (W) or molybdenum (Mo).

The heteropolyacid may comprise molybdophosphoric acid which may * : be defined by the formula H3Mo12O40P.XH2O. * S..

The heteropolyacid may comprise tungstophosphoric acid which may * . be defined by the formula H3W12040P.XH2O. 5*S*

* According to the present invention, there is also provided a magnetic * S's. core component fabricated using the magnetic particulate material defined S...

above.

According to the present invention, there is further provided a method for preparing magnetic particulate material, the method comprising providing a plurality of ferromagnetic particles and coating each of the plurality of ferromagnetic particles with an acid material selected from the group consisting of (a) an acid system comprising at least two acids and (b) an acid compound formed by reacting together at least two acids, wherein the coating step results in the formation of a coating compound as a reaction product of the ferromagnetic particles and the acid system or of the ferromagnetic particles and the acid compound, the coating compound providing electrical insulation to each of the plurality of ferromagnetic particles.

According to the present invention, there is also provided a method for preparing magnetic particulate material, the method comprising providing a plurality of ferromagnetic particles and coating each of the plurality of ferromagnetic particles with an acid system comprising at least two acids to form a coating compound as a reaction product of the ferromagnetic particles and the acid system, the coating compound providing electrical insulation to each of the plurality of ferromagnetic particles.

According to the present invention, there is also provided a method for preparing magnetic particulate material, the method comprising providing a plurality of ferromagnetic particles and coating each of the plurality of ferromagnetic particles with an acid compound, formed by reacting together at : * least two acids, to form of a coating compound as a reaction product of the ferromagnetic particles and the acid compound, the coating compound providing electrical insulation to each of the plurality of ferromagnetic * : : : : particles.

Where the acid material is an acid system, the coating step may : * comprise coating the ferromagnetic particles with a first acid and thereafter * ** . **20 coating the ferromagnetic particles with a second acid.

Where the acid material is an acid system, the coating step may comprise coating the ferromagnetic particles with a first acid and thereafter coating the ferromagnetic particles with second and third acids.

Where the acid material is an acid system, the coating step may comprise coating the ferromagnetic particles with a mixture of acids. The coating step may comprise dispersing the ferromagnetic particles in a solution of the mixture of acids and a solvent.

Where the acid material is an acid compound, the coating step may comprise dispersing the plurality of ferromagnetic particles in a solution of the acid compound and a solvent.

Where the coating step comprises dispersing the ferromagnetic particles in a solution of the mixture of acids and a solvent or dispersing the plurality of ferromagnetic particles in a solution of the acid compound and a solvent, the coating step may also comprise evaporating the solvent from the solution after said step of dispersing the ferromagnetic particles in the 1 0 solution.

The first acid may be more reactive with the ferromagnetic particles than the second and/or third acids. *0*)

The magnetic particulate material prepared in accordance with the * : : : :* method may be as defined above.

::.. : Embodiments of the present invention will now be described by way of

* example only.

* S. *

S S a...

The magnetic particulate material according to the invention comprises a plurality of ferromagnetic particles. In one embodiment, the ferromagnetic particles are iron-based, substantially spherical, particles.

In order to provide the magnetic particulate material with appropriate electrical insulation, in other words with a suitable dielectric coating, in one embodiment of the invention the ferromagnetic particles are coated with an acid material in the form of an acid compound formed by reacting together at least two acids, and desirably an acid compound in the form of a heteropolyacid in which the relative proportions of the acids are predetermined.

In a preferred implementation of this embodiment, the heteropolyacid comprises molybdophosphoric acid which is defined by the formula H3Mo12O40P.XH2O.

In order to coat the ferromagnetic particles with the molybdophosphoric acid, the ferromagnetic particles are dispersed in a molybdophosphoric acid solution comprising molybdophosphoric acid mixed with a compatible carrier solvent, such as alcohol. As the carrier solvent is evaporated, the concentration of the molybdophosphoric acid increases and it reacts with the ferromagnetic particles to form a coating compound as a reaction product of the ferromagnetic particles and the molybdophosphoric acid. This coating compound acts as an electrically insulating layer, also known as a dielectric * ** coating. S.. S

In this preferred implementation of the first embodiment, the * : : : : molybdophosphoric acid comprises a first acid in the form of approximately 3 * : * *.: wt% phosphoric acid and a second acid in the form of approximately 97 wt% molybdic acid. The addition of the molybdophosphoric is typically between 0.05 and 5 wt% of the ferromagnetic particles.

Phosphoric acid is very reactive when exposed to ferromagnetic material. When phosphoric acid and molybdic acid are reacted together to form molybdophosphoric acid, and the resultant molybdophosphoric acid is applied to the ferromagnetic particles, the reaction between the molybdophosphoric acid and the ferromagnetic material takes place much more slowly. This arises because the molybdic acid acts to reduce the reactiveness of the phosphoric acid with the ferromagnetic particles and its susceptibility to oxidation. Moreover, due to the electrically insulating properties which result from a reaction between the molybdic acid and the ferromagnetic particles, only a small amount of phosphoric acid is required, thereby reducing the amount of reaction between the phosphoric acid and the ferromagnetic material.

The use of molybdophosphoric acid in the preferred embodiment is thus highly advantageous as it enables the reaction to be more carefully controlled, resulting in the formation of a superior electrically insulating layer on the ferromagnetic particles.

Although the applicants do not wish to be bound by any theory, they believe that the phosphoric acid component of the molybdophosphoric acid reacts preferentially with the ferromagnetic particles to bind or adhere the coating compound formed as a product of a reaction between the molybdophosphoric acid and the ferromagnetic particles to the ferromagnetic particles, and that the molybdic acid component, together with any unreacted phosphoric acid of the molybdophosphoric acid, forms a coating compound which rests on the surface of the reaction product formed between the ferromagnetic particles and the phosphoric acid component. * *

These highly and unexpected advantageous effects are also provided by alternative embodiments of the invention in which the ferromagnetic : * particles are coated with an acid material in the form of an acid system * . . :: 20 comprising at least two acids. The effects are believed to arise for the same reasons discussed above.

In one alternative embodiment of the invention, the ferromagnetic particles are coated with an acid system comprising a mixture or blend of at least two acids, such as a first acid in the form of phosphoric acid and a second acid in the form of molybdic acid. The first and second acids of the mixture react with the ferromagnetic particles to form a coating compound which acts as an electrically insulating layer, also known as a dielectric coating.

In this alternative embodiment, the phosphoric and molybdic acids are mixed together before application to the ferromagnetic particles.

The ferromagnetic particles can be coated with the acid mixture in the same manner described above for molybdophosphoric acid, or using any suitable coating method.

In another alternative embodiment of the invention, the ferromagnetic particles are coated with an acid system comprising first and second acids by firstly coating the ferromagnetic particles with a first acid, such as phosphoric acid, and thereafter coating the ferromagnetic particles with a second acid, such as molybdic acid. The sequential coating results in a first reaction between the first acid and the ferromagnetic particles, and a thereafter a second reaction between the second acid and the ferromagnetic particles.

* .. The sequential reactions result in the formation of a coating compound which *: acts as an electrically insulating layer, also known as a dielectric coating.

* * Where phosphoric and molybdic acids are applied to the ferromagnetic particles sequentially, they may be applied by any suitable method, for example using separate dispersion steps in the manner described above or using spray-based techniques.

*: : : : 20 In order to fabricate magnetic core components using magnetic particulate material, it is usual to compact the magnetic particulate material and simultaneously, or subsequently, heat the material. This causes the electrically insulating layers of adjacent ferromagnetic particles to undergo additional reaction and bind together. In addition the use of higher temperature heat treatments, for example above approximately 400 C, induces stress relief in the compacted body reducing hysteresis losses.

During compaction, individual ferromagnetic particles may be deformed and it is possible that the electrically insulating layer on individual ferromagnetic particles may crack as a result of this deformation. With conventional magnetic particulate materials, oxidation may occur between adjacent ferromagnetic particles which are exposed due to cracks in the electrically insulating layer, and when the compacted material is heated, the electrical conductivity of iron oxides formed between adjacent exposed particles increases significantly. This is undesirable as it results in decreased resistivity and hence increased core losses when the magnetic core component is in use.

Embodiments of the present invention reduce this drawback associated with known magnetic particulate materials since the less reactive acid component of the acids, whether part of an acid system or an acid compound, reduces the susceptibility to this mechanism of decreased resistivity by inhibiting the formation of deleterious iron oxides.

The use of an acid material comprising an acid system or an acid compound, as described above, in which one of the acids is molybdic acid is * : : ::* advantageous since it assists with lubrication of the ferromagnetic particles enabling them to slide more easily during compaction when fabricating magnetic core components. This may obviate the need to mix lubricants with the coated magnetic particulate material. Nevertheless, it may still be **., ***"20 desirable to admix suitable lubricant powders, such as zinc stearate or suitable organic polymers, or lubricant fluids to enhance the lubrication of the resultant magnetic particulate material. Smaller quantities of lubricant may, however, be required due to the lubricating properties of the molybdic acid, this being generally beneficial to the resultant magnetic properties.

There is thus described a magnetic particulate material whose electrically insulating layer, also known as dielectric coating, offers significant and unexpected advantages over conventional insulated magnetic particulate materials.

Although embodiments of the invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that various modifications to the examples given may be made without departing from the scope of the present invention, as claimed.

For example, the ferromagnetic particles may be of a shape other than substantially spherical. The ferromagnetic particles may comprise pure iron or iron alloyed with other elements.

The second acid of the first and second acids of the acid system or the second acid of the first and second acids which are reacted together to form the acid compound may comprise tungstic acid instead of molybdic acid.

Where the acids comprise phosphoric acid and tungstic acid, the highly * ** advantageous effects described above with regard to phosphoric acid and molybdic acid are also obtained.

S... . ** The acid system or the acid compound may comprise more than two acids. For example, three acids may be used, such as phosphoric acid, molybdic acid and tungstic acid. S...

The acid compound may comprise tungstophosphoric acid.

The acid compound may be a compound other than a heteropolyacid.

The heteropolyacid may comprise any suitable acid defined by the formula H3A12040P.XH2O, where A is typically a transition metal such as tungsten or molybdenum.

The heteropolyacid may comprise tungstophosphoric acid which may be defined by the formula H3W12040P.XH2O.

The ferromagnetic particles may be coated with the acid system or the acid compound by methods other than those described above.

Any suitable lubricant powders or lubricant fluids may be mixed with the magnetic particulate material.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance, it should be understood that the Applicants claim protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings, whether or not particular emphasis has been placed thereon. I. S *

IS I * IS. SI.. I... I...

S I.e.. * . * S *I.. *..S

Claims

Claims 1. Magnetic particulate material comprising a plurality of

ferromagnetic particles and an electrically insulating layer provided on each of said ferromagnetic particles, wherein the electrically insulating layer comprises a coating compound formed as a reaction product of the ferromagnetic particles and an acid material selected from the group consisting of (a) an acid system comprising at least two acids and (b) an acid compound formed by reacting together at least two acids.
2. Magnetic particulate material comprising a plurality of ferromagnetic particles and an electrically insulating layer provided on each of said * : :* ferromagnetic particles, wherein the electrically insulating layer comprises a * **. coating compound formed as a reaction product of the ferromagnetic particles and an acid system comprising at least two acids.

* S *S

S I...

:
3. Magnetic particulate material comprising a plurality of ferromagnetic * particles and an electrically insulating layer provided on each of said ferromagnetic particles, wherein the electrically insulating layer comprises a coating compound formed as a product of a reaction between the ferromagnetic particles and an acid compound formed by reacting together at least two acids.
4. Magnetic particulate material according to any of the preceding claims, wherein the acids comprise first and second acids, and the first acid is more reactive with the ferromagnetic particles than the second acid.
5. Magnetic particulate material according to any of the preceding claims, wherein the acids comprise first, second and third acids, and the first acid is more reactive with the ferromagnetic particles than the second and/or third acids.
6. Magnetic particulate material according to claim 4 or claim 5, wherein the first acid comprises a minor component of the acids.
7. Magnetic particulate material according to any of claims 4 to 6, wherein the first acid reacts with the ferromagnetic particles to provide an adhesive mechanism which adheres the electrically insulating layer to the ferromagnetic particles.
8. Magnetic particulate material according to any of claims 4 to 7, wherein the second and/or third acids react with the ferromagnetic particles to provide enhanced insulation and chemical stability to the ferromagnetic particles.
9. Magnetic particulate material according to any of claims 4 to 8, wherein the second and/or third acids are selected to reduce the oxidation and S..

reactivity of the ferromagnetic particles. S.. S...

:
10. Magnetic particulate material according to any of the preceding claims, * wherein the acid system or the acid compound comprises between 0.05 wt% and 5 wt% of the ferromagnetic particles.
11. Magnetic particulate material according to any of claims 4 to 10, wherein the acid system or the acid compound comprise a greater percentage by weight of the second acid than the first acid.
12. Magnetic particulate material according to any of claims 5 to 10, wherein the acid system or the acid compound comprise a greater percentage by weight of the second and third acids than the first acid.
13. Magnetic particulate material according to any of claims 4 to 12, wherein the acid system or the acid compound comprise between approximately I and 20 wt% of the first acid and between approximately 80 and 99 wt% of the second acid.
14. Magnetic particulate material according to any of claims 5 to 12, wherein the acid system or the acid compound comprise between approximately I and 20 wt% of the first acid and between approximately 80 and 99 wt% of the second and third acids.
15. Magnetic particulate material according to any of the preceding claims, wherein the acid system comprises a mixture of acids.
16. Magnetic particulate material according to any of claims 4 to 15, wherein the first acid comprises phosphoric acid.
17. Magnetic particulate material according to any of claims 4 to 16, wherein the second acid comprises molybdic acid ortungstic acid. * ..*
18. Magnetic particulate material according to any of claims 5 to 17, wherein the third acid comprises molybdic acid or tungstic acid. * .
19. Magnetic particulate material according to any of the preceding claims, wherein the acid compound comprises molybdophosphoric acid.
20. Magnetic particulate material according to any of claims I to 18, wherein the acid compound comprises tungstophosphoric acid.
21. Magnetic particulate material according to any of the preceding claims, wherein the acid compound comprises a heteropolyacid.
22. Magnetic particulate material according to claim 21, wherein the heteropolyacid comprises an acid compound defined by the general formula H3A12040P.XH2O, where A is a transition metal.
23. Magnetic particulate material according to claim 22, wherein the transition metal is tungsten (W) or molybdenum (Mo).
24. Magnetic particulate material according to any of claims 21 to 23, wherein the heteropolyacid comprises molybdophosphoric acid.
25. Magnetic particulate material according to claim 24, wherein the molybdophosphoric acid is defined by the formula H3Mo12O40P.XH2O.
26. Magnetic particulate material according to any of claims 21 to 23, wherein the heteropolyacid comprises tungstophosphoric acid.
27. Magnetic particulate material according to claim 26, wherein the * **. tungstophosphoric acid is defined by the formula H3W12040P.XH2O. I.. * *..
28. Magnetic particulate material substantially as hereinbefore described.

* ...*
29. A magnetic core component fabricated using magnetic particulate material according to any of the preceding claims.
30. A method for preparing magnetic particulate material, the method comprising providing a plurality of ferromagnetic particles and coating each of the plurality of ferromagnetic particles with an acid material selected from the group consisting of (a) an acid system comprising at least two acids and (b) an acid compound formed by reacting together at least two acids, wherein the coating step results in the formation of a coating compound as a reaction product of the ferromagnetic particles and the acid system or of the ferromagnetic particles and the acid compound, the coating compound providing electrical insulation to each of the plurality of ferromagnetic particles.
31. A method according to claim 30, wherein where the acid material is an acid system, the coating step comprises coating the ferromagnetic particles with a first acid and thereafter coating the ferromagnetic particles with a second acid.
32. A method according to claim 30 or claim 31, wherein where the acid material is an acid system, the coating step comprises coating the ferromagnetic particles with a first acid and thereafter coating the ferromagnetic particles with second and third acids.
33. A method according to claim 30, wherein where the acid material is an acid system, the coating step comprises coating the ferromagnetic particles with a mixture of the acids. * *SS
34. A method according to claim 33, wherein the coating step includes the step of dispersing the ferromagnetic particles in a solution of the mixture of the acids and a solvent.

II
35. A method according to claim 30, wherein where the acid material is an acid compound, the coating step includes the step of dispersing the plurality of ferromagnetic particles in a solution of the acid compound and a solvent.
36. A method according to claim 34 or claim 35, wherein where the coating step includes the step of dispersing the ferromagnetic particles in a solution of a mixture of the acids and a solvent or dispersing the plurality of ferromagnetic particles in a solution of an acid compound and a solvent, the coating step also includes the step evaporating the solvent from the solution after said step of dispersing the ferromagnetic particles in the solution.
37. A method according to any of claims 31 to 36, wherein the first acid is more reactive with the ferromagnetic particles than the second and/or third acids.
38. A method for preparing magnetic particulate material, the method comprising providing a plurality of ferromagnetic particles and coating each of the plurality of ferromagnetic particles with an acid system comprising at least two acids to form a coating compound as a reaction product of the ferromagnetic particles and the acid system, the coating compound providing electrical insulation to each of the plurality of ferromagnetic particles.
39. A method for preparing magnetic particulate material, the method comprising providing a plurality of ferromagnetic particles and coating each of the plurality of ferromagnetic particles with an acid compound, formed by reacting together at least two acids, to form of a coating compound as a reaction product of the ferromagnetic particles and the acid compound, the :::: coating compound providing electrical insulation to each of the plurality of ferromagnetic particles. s'.
40. A method according to any of claims 30 to 39, wherein the magnetic particulate material prepared by the method is as defined in any of claims I to 28. * **
41. A method for preparing magnetic particulate material substantially as hereinbefore described.
42. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.