High frequency low loss magnetic core and method of manufacture

Abstract

A high saturation, low loss magnetic material suitable for high frequency electrical devices, including power converters, transformers, solenoids, motors, and other such devices.

Claims

1. A low loss, high magnetic saturation magnetic material having a saturation magnetization above 1.5 T and a power density of less than 2000 kW/m.sup.3 at frequencies above 5 KHz, said material formed from a plurality of magnetic particles having an average particle size of 20-1500 μm and having a saturation magnetization above 1.5 T, said magnetic particles structured and engineered to have low coercivity of less than <10 Oersted, said particles including a thermally-stable coating, said thermally-stable coating having a melting point above 350° C. and a thickness of 5-200 nm, said coated magnetic particles consolidated by a sintering process using rapid heating and pressure to formed said low loss, high magnetic saturation magnetic material that has a density that is greater than 90% of theoretical density of said particles used to form said low loss, high magnetic saturation magnetic material.

2. The material defined in claim 1, wherein said magnetic particles are iron, iron-silicon, glassy iron, iron-nickel, and/or iron-cobalt alloy.

3. The material as defined in claim 1, wherein said coating includes a high melting point insulator material that has a melting point of greater than 800° C.

4. The material as defined in claim 1, wherein said coating includes PEEK, polyimide, polyphenyl sulfone, polysiloxane, silicone, polysilizane, phenolic, and/or a polymeric material.

5. The material as defined in claim 1, wherein said coating includes a semiconducting material.

6. The material as defined in claim 1, wherein said coating includes ferrite, silicon metal, germanium metal, glassy carbon, boron, arsenide, selenide, or other high resistivity metallic or semiconducting material.

7. The material as defined in claim 1, wherein said coating includes an inorganic material.

8. The material as defined in claim 1, wherein said coating includes one or more materials selected from the group consisting of silica, silicate, silicate glass, BN, BN nanosheets, AlN, Si.sub.3N.sub.4, TiO.sub.2, titanate, Al.sub.2O.sub.3, SiOC, SiAlON, aluminate, aluminosilicate, mica, B.sub.2O.sub.3, borosilicate, borate glass, Fe.sub.3O.sub.4, (Mn(Fe.sub.2O.sub.4), and Zn(Fe.sub.2O.sub.4).

9. The material as defined in claim 1, wherein said coated particle has at least one dimension in the 0.5-20 μm size range, and a second dimension 10-5000× greater than said first dimension.

10. The material as defined in claim 1, said particles includes a second coating, said second coating is in the form of a nanosheet.

11. The material as defined in claim 1, wherein said material is used as part of power system that is selected from the group consisting of a power converter, power supply, pulse forming network, inverter, rectifier, motor controller system, part of a transformer, choke, inductor, and filter circuit.

12. The material as defined in claim 11, wherein said power system operates at a frequency above 50 KHz.

13. The material as defined in claim 11, wherein said power system operates at a frequency above 200 KHz.

14. The material as defined in claim 11, wherein said power system operates at a frequency above 500 KHz.

15. The material as defined in claim 1, wherein said material is used as a power system which includes a wide band gap semiconductor that is selected from the group consisting of a silicon-carbon and a gallium-nitrogen power electronic component.

16. A low loss, high magnetic saturation magnetic material having a saturation magnetization above 1.5 T and a power density of less than 2000 kW/m.sup.3 at frequencies above 5 KHz; said material formed from a plurality of magnetic particles having an average particle size of 20-1500 μm and having a saturation magnetization above 1.5 T; said magnetic particles including a thermally-stable coating prior to formation of said low loss, high magnetic saturation magnetic material; said thermally-stable coating having a melting point above 350° C.; said thermally-stable coating having a thickness of 5-200 nm prior to formation of said low loss, high magnetic saturation magnetic material; said magnetic particles include one or more materials selected from the group consisting of iron, iron-silicon, glassy iron, iron-nickel, iron-cobalt-vanadium, NiTi, orthinol, silicon steel, and iron-cobalt alloy; said coating including one of more of A) polymer that includes one or more materials selected from the group consisting of polyether ether ketone, polyimide, polyphenyl sulfone, polysiloxane, silicone, polysilizane, phenolic, B) inorganic material that includes one or more materials selected from the group consisting of silica, silicate, silicate glass, BN, BN nanosheets, Si.sub.3N.sub.4, TiO.sub.2, titanate, MgO, Al.sub.2O.sub.3, SiOC, SiAlON, aluminate, aluminosilicate, mica, B.sub.2O.sub.3, borosilicate, borate glass, Fe.sub.3O.sub.4, Mn(Fe.sub.2O.sub.4), and Zn(Fe.sub.2O.sub.4), and C) metal material including one or more materials selected from the group consisting of iron, ferrite, silicon metal, germanium metal, carbon, boron, arsenide, and selenide; said low loss, high magnetic saturation magnetic material formed by a consolidation process by a sintering process that includes use of heat and pressure to cause said coating on different coated magnetic particles to flow and bond together; said low loss, high magnetic saturation magnetic material formed by said sintering process has a density that is greater than 90% of a theoretical density of said magnetic particles.

17. The low loss, high magnetic saturation magnetic material as defined in claim 16, wherein said coating includes said metal material, said metal material including one or more materials selected from the group consisting of iron, ferrite, silicon metal, germanium metal, carbon, boron, arsenide, and selenide.

18. The low loss, high magnetic saturation magnetic material as defined in claim 16, wherein said coating includes said inorganic material, said inorganic material that includes one or more materials selected from the group consisting of silica, BN, BN nanosheets, AlN, Si.sub.3N.sub.4, TiO.sub.2, titanate, MgO, Al.sub.2O.sub.3, SiOC, SiAlON, aluminate, aluminosilicate, mica, B.sub.2O.sub.3, borosilicate, borate glass, Fe.sub.3O.sub.4, Mn(Fe.sub.2O.sub.4), and Zn(Fe.sub.2O.sub.4).

19. The low loss, high magnetic saturation magnetic material as defined in claim 16, wherein a BN nanosheet is applied to said coated magnetic particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Reference may now be made to the drawings, which illustrate various embodiments that the invention may take in physical form and in certain parts and arrangements of parts wherein:

(2) FIG. 1 illustrates typical DC magnetization curves for common magnetic materials.

(3) FIG. 2 illustrates coercivities as a function of grain size for conventional and nanocrystalline soft magnetic alloys.

(4) FIG. 3 illustrates two non-limiting embodiments of this particle coating concept in accordance with the present disclosure.

(5) FIG. 4 illustrates silica-coated iron powders.

(6) FIG. 5 illustrates a test core fabricated in accordance with Example 4.

(7) FIG. 6 illustrates the B—H curves of the core of FIG. 5 measured for SiO.sub.2 coated iron powders showing saturation at over 1.5 T at room temperature, and coercivities in the <100 Oe range.

DETAILED DESCRIPTION OF THE DISCLOSURE

(8) A more complete understanding of the articles/devices, processes, and components disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments.

(9) Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

(10) The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

(11) As used in the specification and in the claims, the term “comprising” may include the embodiments “consisting of” and “consisting essentially of.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named ingredients/steps and permit the presence of other ingredients/steps. However, such description should be construed as also describing compositions or processes as “consisting of” and “consisting essentially of” the enumerated ingredients/steps, which allows the presence of only the named ingredients/steps, along with any unavoidable impurities that might result therefrom, and excludes other ingredients/steps.

(12) Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

(13) All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 grams to 10 grams” is inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate values).

(14) The terms “about” and “approximately” can be used to include any numerical value that can vary without changing the basic function of that value. When used with a range, “about” and “approximately” also disclose the range defined by the absolute values of the two endpoints, e.g. “about 2 to about 4” also discloses the range “from 2 to 4.” Generally, the terms “about” and “approximately” may refer to plus or minus 10% of the indicated number.

(15) Percentages of elements should be assumed to be percent by weight of the stated element, unless expressly stated otherwise.

(16) The high saturation, low loss magnetic materials in accordance with the present disclosure can be nano-engineered iron-cobalt alloys that are formed into ferrite-type microstructures with insulating grain boundaries by using particle coating and spark plasma sintering, The iron-cobalt alloys can optionally include metals such a vanadium, nickel, etc. As can be appreciated, other metal alloys can be used (e.g., iron, iron-silicon alloys, glassy iron alloy, iron-nickel alloy, etc.) for the magnetic material. Specifically, the addition of nanometer/submicron-layer insulator coatings onto particles which retain their structure through consolidation using field assisting sintering techniques can be used to create artificially engineered magnetic structures. These designer nanostructures are useful for the production of solid soft magnetic cores for wide band gap power semiconductor conversion applications.

(17) The particles in accordance with the present disclosure enable the production of magnetic materials capable of operating at a frequency greater than 200 kHz at an ambient operating temperature of up to 200° C. or more, with an allowable temperature rise of 50-100° C. (and all values and ranges therebetween).

(18) The resultant magnetic materials in accordance with the present disclosure have the following properties: operating frequency of 200 kHz-5 MHz (and all values and ranges therebetween) (low eddy current losses at 0.2-5 MHz); 200° C.+ operating temperatures; >1.8 T saturation at 200° C.; and 100+ permeability.

(19) Magnetic alloys (e.g., iron-cobalt alloys, etc.) in accordance with the present disclosure are modified to increase their frequency capability in solid cores by creating a ferrite-like microstructure of insulating grain boundaries using particle nano-engineering technology.

(20) Two complimentary techniques can be used to create the desired magnetic materials in accordance with the present disclosure.

(21) Technique A

(22) Insulating dielectric coatings are applied to 0.5-1500 μm (and all values and ranges therebetween), and typically 2-50 μm, high magnetic saturation metal powders (e.g., iron-cobalt alloys, iron, iron-silicon alloys, iron-nickel alloys, etc.) using a) fluidized bed chemical vapor deposition, b) preceramic-polymer coating and pyrolysis, atomic or molecular layer deposition, or c) sol-gel/solution techniques. The insulator-coated particles are consolidated into the magnetic material using spark plasma or other field-assisted sintering processing combining rapid heating and pressure to fabricate fully dense cores having high saturation, low coercivity, and low eddy current losses at 0.2-5 MHz, and the magnetic material has a theoretical density of greater than 80%, typically greater than 90%, and more typically greater than 95%.

(23) Technique B

(24) The high saturation magnetic cores formed from magnetic powders (e.g., iron-cobalt alloy powders, etc.) can be nanostructured using rapid solidification or mechanical alloying techniques to refine their grain structure to 5-50 nm in size so as to reduce coercivity. The metal powder generally is 1-200 μm (and all values and ranges therebetween), and typically 2-100 μm. The metal powder is coated and then bonded together by any number of techniques (e.g., melt atomization, rapid solidification (such as melt-spinning), blended elemental, etc.). The size of the formed metallic core that is formed from the metal powder is generally 0.5-150 μm (and all values and ranges therebetween), and typically 1-20 μm. The use of processing aids to maintain overall particle size while refining the microstructure to very fine sizes may optionally be required to stabilize the microstructure during processing. To improve the particle properties, particle shapes (flakes, fibers, ribbon) can optionally be used with a short and a long dimension.

(25) For both of the above two techniques, various processes can be used to apply the one or more coatings to the magnetic particles, including: 1) preceramic-polymer coating and pyrolysis processes to form a coating such as, but not limited to Si.sub.3N.sub.4, SiOC, BN, etc. coatings; 2) chemical vapor deposition processes to form a coating such as, but not limited to MgO, AlN, SiO.sub.2, BN, etc., coatings; 3) sol-gel processes to form a coating such as, but not limited to AlO.sub.3, SiO.sub.2, SiO.sub.2—B.sub.2O.sub.3, etc., coatings; and 4) molecular/atomic layer deposition processes to form a coating such as, but not limited to AlN, Al.sub.2O.sub.3, SiO.sub.2, SiO.sub.2—B.sub.2O.sub.3, etc., coatings. The one or more coatings provide high electrical resistance for reducing eddy current losses at higher frequencies. The one or more coatings may include high resistance semiconductor coatings which facilitate rapid heating during spark plasma or field-assisted sintering. Such high resistance semiconductors coatings include silicon, boron, germanium, gallium-arsenic, gallium-nitrogen, diamond-like carbon, high resistance carbon, resistive oxides, and/or other semiconductor materials.

(26) The one or more coatings on the magnetic particles are applied at thicknesses of 1 nm to 1 μm (and all values and ranges therebetween), and typically about 5-100 nm. The coating generally constitutes less than 10 vol. % of the total volume of the coated particle, and typically less than 5 vol. % of the total volume of the coated particle, and more typically less than 2.5 vol. % of the total volume of the coated particle. Generally, the coating constitutes at least 0.05 vol. % of the total volume of the coated particle. When the coating is less than 20 nm thick, exchange coupling and easier magnetization can be achieved between isolated domains.

(27) In one non-limiting embodiment, the magnetic particles are formed of iron-cobalt alloy; however, other metal alloys can be used. The metal alloys used to form the magnetic particles generally have a very high magnetic saturation. Generally, the magnetic saturation of the metal alloys is above 1.8, typically above 2.0 T (e.g., 2.01-2.3 T, etc.). The metal alloys also have very high permittivity. Low coercivities of the metal alloy can also be obtained. B2 ordered and/or disordered BCC structures can optionally be used to improve saturation and coercivity of the magnetic particles. Two non-limiting embodiments of this particle coating concept in accordance with the present disclosure are illustrated in FIG. 3.

(28) Processing sequences include:

(29) A. Use of CVD, sol-gel, and polymer pyrolysis conditions to apply 3-100 nm thick coating of SiO.sub.2, MgO, Si, BN, or other insulator coatings onto 20-100 μm metal powder particles or cores;

(30) B. Optionally use mechanical and/or melt processing techniques to produce nanostructured, flake, ribbon, and equiaxed particles; and

(31) C. Use spark plasma sintering or other types of sintering processes to produce fully dense magnetic cores while retaining the engineered particle structures.

(32) These materials, microstructures, techniques and processes produce a magnetic material with high resistance and high saturation magnetic cores which can operate at 200° C.+ at wide band gap semiconductor frequencies, while also providing high thermal conductivity (above 50 W/m-s, and typically above 100-150 W/m-s), and having robust mechanical properties (above 5 Ksi, typically above 10 Ksi, and more typically above 20 Ksi tensile and compressive strength).

(33) The magnetic material microstructure formed in accordance with the present disclosure creates the following advantages: using micron-sized, isolated particles suspended in an insulator virtually eliminates classical eddy current losses which, when combined with low hysteresis and anomalous (domain wall motion) eddy current losses, results in a very low loss material; exceptionally high magnetic saturations and permittivity of metallic materials can be combined with the high resistivities/low losses of ferrite or oxide materials; the ability to control microstructure through powder shape control and pressing conditions further enhances the magnetic and/or mechanical properties; permittivity in the 100s and into the 1000s are achievable in a highly insulating structure—a greater than 2 order of magnitude performance increase; and saturation above 1.8 T is achievable when keeping the insulator volume percent below 5 vol. %, in a mechanically durable system.

(34) The coated particles in accordance with the present disclosure can be used to form a core that has retention of high saturation value by minimizing the amount of insulator volume needed while maintaining high insulation value and low eddy current losses at high frequencies, and maintaining low coercivity/low hysteresis losses by enabling thermal annealing and removing other impediments to domain wall motion within individual magnetic grains.

(35) The coated particles in accordance with the present disclosure can be used to form a core that enables high saturation, above 1.5 T, and typically above 1.8 or 2 Tesla saturation values. Iron-cobalt-vanadium and Fe—Co-2V alloys (e.g., Fe, 40-49 wt. % Co, 1-10 wt. % V), such as Hiperco, from Carpenter® Technology, supermendur, or other ferromagnetic materials can be used to form the coated particles of the present disclosure that have a saturation magnetization above 1.8 Tesla, typically above 2 Tesla or even 2.3 Tesla. The coated particles of the present disclosure can be produced by grinding, atomization, or other method to particles having dimensions ranging from 0.5-500 μm. For frequencies of 20-500 KHz, particles with at least one dimension in the 20-200 μm range are desirable. The core ferromagnetic material should constitute at least 75 vol. % of the final coated particle, and typically at least 80 vol. %, more typically at least 90 vol. %, and even more typically at least 95 vol. %. To achieve such volume percent of the core, micron- and nanoscale-dielectric insulator coatings on the individual grains are used. Binders such as an epoxy or polymeric (non-magnetic) binder are generally not used.

(36) To reduce hysteresis losses, the magnetic particles can be nanostructured to produce a nanocrystalline material with low coercivity. The nanostructured material may be produced by rapidly solidifying the metal alloy from a melt, or by milling a metal alloy particle (optionally using a dispersion aid to stabilize the resultant structure) to create a metal alloy particle having grain sizes of less than 100 nm, and typically in the 5-30 nm size range. Alternatively, the metal alloy particles can be annealed to create single-crystal/single-domain particles, which can be aligned (easy axis) with the magnetizing field to minimize coercivity.

(37) To minimize eddy current and hysteresis losses, the particles may also be shaped into flakes to enable high packing density and enable lower coercivity in the field direction. Such particles may be 0.5-10 μm in width, with 20-1000 μm in length.

(38) Crushed, rapidly solidified (melt spun) metal ribbon, as well as ball-milled metal powders (to create flakes) are processes that can be used to form high saturation materials from materials such as iron-cobalt-vanadium, orthinol, silicon steel, and metallic glasses (2605SC).

(39) More ductile materials favor ball milling to create flakes, while brittle materials may be best processed using melt spinning followed by grinding into flakes.

(40) To minimize eddy current losses, the primary magnetic particles are coated with a thermally stable (to allow consolidation and annealing) insulator or semiconducting material. The coating on the magnetic particle can be an inorganic material, such as silica, silicate, silicate glass, BN, BN nanosheets, AlN, Si.sub.3N.sub.4, TiO.sub.2, titanate, Al.sub.2O.sub.3, SiOC, SiAlON, aluminate, aluminosilicate, mica, B.sub.2O.sub.3, borosilicate, borate glass, ferromagnetic insulator (Fe.sub.3O.sub.4, (Mn(FeO.sub.4), Zn(Fe.sub.2O.sub.4)) or other dielectric oxide or nitride or other inorganic dielectric insulating material. Semiconductor coatings are also effective, such as silicon, boron, germanium, antimony, selenium, tellurium, gallium-arsenic, silicon-carbon, sulfides, selenides, and other high resistance materials. These coatings can be applied in 1-100 nm, and typically 10-30 nm thickness using techniques such as preceramic polymer addition, chemical vapor deposition, atomic layer deposition, physical vapor deposition, solution or sol-gel coating, or other techniques. In addition, a nanosheet, for example, BN nanosheet (BNNs) can be applied to the coated particles to further increase the resistance of the magnetic material, protect the insulator during the sintering process, and allow for slippage of the particles during consolidation.

(41) In order to maintain the integrity of the coating during the sintering process, a lower glass transition temperature (Tg) will allow the coating to flow and deform during compaction without breaching during sintering process, then protect the integrity of the coating. The coating can be doped with glass modifiers (boron, sodium, and/or calcium additions) to lower the Tg. In addition, the amorphous insulator with creep or plastic deformation can also be created to protect the coating integrity during the sintering process.

(42) To limit the hysteresis loss caused by residual stress, a lower modulus/stiffness insulator, such as boron-nitrogen, can be used. The coating being optionally plastically deformable at 0.6-0.9 Tm (of the ferromagnetic material) is desirable. The coating generally has a melting temperature that is significantly above that of the ferromagnetic particle; however, a lower melting temperature material such as a glass may be used. The higher melting material of the coating withstands the increased temperatures at the interfaces during resistive heating.

(43) A high process rate can be used to consolidate the engineered magnetic particles together to retain their engineered structure. One non-limiting method for forming a core material from the coated magnetic particles is spark plasma sintering. Spark plasma sintering, in which an electric field is imposed to heat the sample while simultaneously applying pressure, normally using a graphite die, can be used for the rapid consolidation technique. By shortening the total processing time at elevated temperatures to 60 minutes or less, generally less than 20 or 30 minutes, and typically to less than 10 minutes, diffusional processes are inhibited/prevented, thereby enabling retention of fine (nano) structures engineered into the nanostructured magnetic particles. The rapid consolidation process involves heating the coated magnetic particles very quickly, such as by electrical heating using DC, pulsed, AC, or RF frequencies to a temperature between 0.5-0.9 Tm, and normally to between 0.75-0.85 Tm (melting temperature) of the metallic phase. The coated magnetic particles are generally heated to a temperature where deformation is able to be accomplished at relatively low stress (generally below 120 MPa, and normally below 60-80 MPa, and even as low as 20-40 MPa) to enable the particles to be deformed into a space-filling array within the strength limits of graphite dies. Higher stresses may be imposed using carbide (normally tungsten-carbon but may be silicon-carbon) die materials.

(44) A non-limiting processing sequence in accordance with the present disclosure is to fill a die (generally graphite) with the engineered magnetic powders (which may be preformed to assist in loading or to structure the material, such as a graded or laminated structure having several layers or gradations of composition). A pressure is applied to the preform/powder bed, such as by hydraulically compressing the upper and/or lower die. The material can be heated by the application of an electric field/current, receiving heating from the graphite die, and/or self-heating through internal resistance. The heating is applied until a specified temperature is met. The heating profile can include one or more intermediate holds to allow for removal of gasses, conversion of coating systems (such as from preceramic polymers), reaction of particle surfaces (e.g., oxidation of an intermediate coating, reduction or cleaning, or other surface-gas interaction or reaction), after which pressure and/or temperature may be further be increased to the final set point. At the final set point, the material is held for a sufficient time to maximize the deformation (space filling) of the particles, and to enable bonding of the particles into a solid material.

(45) The consolidated magnetic material is then removed from the die after cooling sufficiently. The processing time is normally less than one hour, generally less than thirty minutes, and often less than 5 or 10 minutes per cycle.

(46) After consolidation, the magnetic material can be annealed to remove stresses at an intermediate temperature. Annealing can also include field annealing to align the easy magnetization axis with the field and to control the domain structure, followed by cooling to “lock-in” the desired magnetic domain structure.

EXAMPLE 1

(47) Iron-cobalt-vanadium powder (iron—48% Co—2% vanadium) with a particle size of −270/+400 mesh is coated with 20 nm of SiO.sub.2 using a sol-gel coating process. The coated iron-cobalt-vanadium powders were consolidated to form a magnetic core by using spark plasma sintering in a graphite die, at a pressure of 50 MPa and a peak temperature of 1100° C. for 20 minutes. The formed magnetic core had a saturation magnetization of 2.1 T, and a high resistivity.

EXAMPLE 2

(48) Iron-cobalt-vanadium powder having a particle size of −230/+325 mesh was mechanically alloyed to create a nanostructured microstructure by attrition milling using 10 mm steel balls in a union process 01HD mill for 20 hours, with the addition of 2 wt. % of nano-Y.sub.2O.sub.3 to stabilize the microstructure. The resultant particles were coated with 20 nm of silica using a sol-gel process and consolidated as in Example 1 using spark plasma sintering.

EXAMPLE 3

(49) In order to further increase the resistance of the composite as well as protect the insulator during sintering process, and allow for slippage of the particles during consolidation, boron nitride nanosheets (BNNS's) applied to the coated particles of Examples 1, 2 and 4-6 to improve the resistance and protection layer during sintering process by electrophoretic (controlled surface/isoelectronic charge) coating onto the insulator-coated magnetic cores. A zeta potential analyzer was used to determine charge density and isoelectronic points, and a titration as used to coat the BNNS onto the magnetic core particles. The effect of the coating on the resistance of the particles (Fe, SiO.sub.2/Fe, BNNs/SiO.sub.2/Fe) was tested under 300 MPa for five minutes. The resistance of the compressed samples was tested directly through multi-meter. By coating SiO.sub.2 thin films onto the surface, the resistance can be increase for thousand times. By further coating BNNSs onto SiO.sub.2/Fe powder, the resistance was improved by 106 times. It was demonstrated that the resistance of the particle can be highly increased by coating insulator SiO.sub.2 and further BNNs coating on surface.

EXAMPLE 4

(50) In order to maintain the integrity of the coating during the sintering process, lower glass transition temperature (Tg) allows the coating to flow and deform during compaction without breaching during sintering process, then protect the integrity of the coating. The silica Tg is related to the composition, such as Na.sub.2O, MgO, and Al.sub.2O.sub.3. By combining a different oxide or element into the silica, the Tg can be decreased. The addition of trimethylborate and sodium hydroxide to the silica sols produce modified silicates with controllable melting points. B.sub.2O.sub.3 is a highly viscous glass; SiO.sub.2—B.sub.2O.sub.3—NaO glasses have viscosities that can be controlled at consolidation temperatures, allowing them to flow and conform to the particles. During CVD, atomic deposition and sol-gel method process, doping SiO.sub.2 with different elements (for example, sodium, magnesium, aluminum) was used to low the glass transition temperature. The process of modifying the coating can be used in Examples 1-3, 5 and 6.

EXAMPLE 5

(51) −270/+400 mesh Fe—Co—V particles were coated with 150-200 nm of silicon metal using the decomposition of silane in a fluidized bed CVD reactor at 650° C. The resultant silica-coated magnetic particles were consolidated using spark plasma sintering at 1250° C. and 60 MPa force to form a high resistivity magnetic core.

EXAMPLE 6

(52) 30-50 μm iron powder was coated with 100 nm of SiO.sub.2 using the decomposition of tetraethylorthosilicate (TEOS) at 600° C. in a fluidized bed reactor. The coated powders were consolidated at 950° C. using a powder forging rapid consolidation technique consisting of heating rapidly in an external preheated furnace, and then consolidating at very high pressures using mechanical compaction in a powder bed (80 tons/sq. inch) to form a magnetic core.

(53) FIG. 4 illustrates silica-coated iron powders. A test core fabricated in accordance with Example 4 is illustrated in FIG. 5. FIG. 6 illustrates the B—H curves of the core of FIG. 5 measured for SiO.sub.2-coated iron powders showing saturation at over 1.5 T at room temperature and coercivities in the <100 Oe range.

(54) It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the constructions set forth without departing from the spirit and scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. The disclosure has been described with reference to preferred and alternate embodiments. Modifications and alterations will become apparent to those skilled in the art upon reading and understanding the detailed discussion of the disclosure provided herein. This disclosure is intended to include all such modifications and alterations insofar as they come within the scope of the present disclosure. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the disclosure herein described and all statements of the scope of the disclosure, which, as a matter of language, might be said to fall there between. The disclosure has been described with reference to the preferred embodiments. These and other modifications of the preferred embodiments as well as other embodiments of the disclosure will be obvious from the disclosure herein, whereby the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims.