METHOD FOR PRODUCING COMPOSITE STRUCTURE COMPRISING MAGNETIC FILLER MATERIAL EMBEDDED IN A RESIN MATRIX

Abstract

The present invention relates to a method for producing a composite structure comprising a magnetic filler material embedded in a resin matrix. So far, a balance between sufficient magnetic performance and mechanical fatigue resistance, for applications involving cyclic strains of hundreds of thousands of cycles, has not been achieved, largely due to the poor compatibility between metallic and plastic surfaces. The present invention solves this problem by embedding magnetic filler material in a resin, after the magnetic filler material has been subjected to a possible surface treatment for improving the adhesion of the magnetic filler material to the resin matrix in the composite structure. Further, a composite structure comprising magnetic filler material embedded in a resin matrix obtainable by the method as disclosed in the current specification is disclosed. Still further, the use of the composite structure in applications requiring magnetic properties and resistance to mechanical fatigue is disclosed.

Claims

1. Method for producing a composite structure comprising magnetic filler material embedded in a resin matrix, characterized in that the method comprises: providing the magnetic filler material; subjecting the magnetic filler material to a surface treatment for improving the adhesion of the magnetic filler material to the resin matrix in the composite structure, wherein the surface treatment is selected from plasma treatment and/or coating of the magnetic filler material; mixing the surface treated magnetic filler material with the resin to form a composite composition, wherein the composite composition comprises magnetic filler material in an amount of at least 10 volume-% based on the total volume of the composite composition; exposing the composite composition to vacuum to remove gas bubbles therefrom; and allowing the composite composition to cure to form the composite structure.

2. The method of claim 1, wherein allowing the composite composition to cure comprises subjecting the composite composition to exposure to a hardener, heating, irradiation with UV or visible light, or exposure to a solvent.

3. The method of claim 1, wherein the method comprises annealing the composite structure after having allowed the composite composition to cure.

4. The method of claim 1, wherein prior to allowing the composite composition to cure, the composite composition is subjected to injection molding or compression molding.

5. The method of claim 1, wherein the surface treatment is a plasma treatment performed by exposing the magnetic filler material to plasma to generate bondable chemical groups on the surface of the magnetic filler material.

6. The method of claim 1, wherein the surface treatment is the application of a coating on the magnetic filler material by immersing the magnetic filler material in a solution comprising a complexing agent or by preparing the magnetic filler material in the presence of a complexing agent.

7. The method of claim 1, wherein the coating is applied during the filler preparation by exposing the filler to the complexing agent during or after the precipitation of the magnetic particles or deposition of the magnetic coating.

8. The method of claim 7, wherein the complexing agent is selected from a group consisting of citric acid, malic acid, glutaric acid, oleic acid, and phthalocyanine, or a compound containing both a hydroxyl group and an amine group, or amines containing at least two amino groups, diaminodiphenylsulfone polyamides, polyphenols, organic acids, isocyanates, polymercaptans, or any mixtures thereof.

9. The method of claim 1, wherein the composite composition comprises magnetic filler material in an amount of at least 20 volume-%, or at least 30 volume-%, or at least 35 volume-%, or at least 40 volume-%, or at least 45 volume-%, or at least 50 volume-% based on the total volume of the composite composition.

10. The method of claim 1, wherein the composite composition comprises magnetic filler material in an amount that is 10-70 volume-%, or 30-60 volume-%, or 45-55 volume-% based on the total volume of the composite composition.

11. The method of claim 1, wherein the resin matrix comprises or consists of a thermosetting resin or a thermoplastic resin.

12. The method of claim 1, wherein the magnetic filler material is selected from a group consisting of magnetic particles, magnetic fibres, pyrolytic graphite, and any combination or mixture thereof.

13. The method of claim 12, wherein the magnetic particles are of spherical, elliptical, rod-like, plate-like, dumbbell-like, or undefined shape.

14. The method of claim 1, wherein the magnetic filler material is selected from a group consisting of hard magnetic materials and soft magnetic materials.

15. The method of claim 14, wherein the hard magnetic filler material is selected from a group consisting of Nd, Nd.sub.2Fe.sub.13CoB, Nd.sub.7Fe.sub.4B.sub.3, SmCo.sub.5, Sm.sub.2Co.sub.7, Sm.sub.2Co.sub.17, Fe.sub.2O.sub.5Sr.sub.2, SrFeO.sub.2, SrFeO.sub.3, SrFe.sub.12O.sub.19, BaFe.sub.12O.sub.19 and any mixtures thereof.

16. The method of claim 14, wherein the soft magnetic material is selected from a group consisting of Fe, Co, Ni, mu-metal (such as Ni.sub.77Fe.sub.17Cu.sub.5Mo, Ni.sub.76Fe.sub.17Cu.sub.5Cr.sub.2, or Ni.sub.81Fe.sub.16Mo.sub.3), Ni.sub.4Fe, Ni.sub.3Fe, CoNi, Co.sub.2Ni.sub.3, Co.sub.3Ni.sub.2, Co.sub.4Ni, FeCo, Fe.sub.7Co.sub.3, Fe.sub.51Co.sub.49, Fe.sub.51Co.sub.47V.sub.2, Co.sub.8FeNi, NiFe.sub.2O.sub.4, CoFe.sub.2O.sub.4, MnZnFe.sub.4O.sub.8, or NiZnFe.sub.4O.sub.8.

17. The method of claim 1, wherein the composite composition comprises resin in an amount of 2.5-60 weight-% based on the total amount of the composite composition.

18. The method of claim 1, wherein the composite composition comprises up to 60% of additional additives selected from a group consisting of reinforcements, tougheners, hardeners, softeners, thermoplastic fillers to prompt self-healing, metal chelators to increase the magnetic performance of the composite, and any combinations thereof.

19. A composite structure comprising magnetic filler material embedded in a resin matrix obtainable by the method of claim 1.

20. A device with active magnetic bearings, electromagnetic actuators applying force on composite parts or as magnet-powered mobility systems including the composite structure of claim 19.

Description

LIST OF THE FIGURES

[0072] In the following, the invention is presented in detail by referring to the attached drawings, where:

[0073] FIG. 1 shows a flow chart of one embodiment of the method for producing a composite structure.

[0074] FIG. 2 shows the size distribution of 5 micron and 25 micron particles as determined by sieving.

[0075] FIG. 3 shows the cumulative size distribution of 200 micron particles as determined by sieve screen analysis.

[0076] FIG. 4 shows the relative permeability as a function of particle loading for different particle sizes.

[0077] FIG. 5 shows saturation magnetization Bmax as a function of particle loading for different particle sizes.

[0078] FIG. 6 shows hysteresis losses as a function of particle loading for different particle sizes.

[0079] FIG. 7 shows remanence as a function of particle loading for different particle sizes.

[0080] FIG. 8 shows normal coercivity as a function of particle loading for different particle sizes.

[0081] FIG. 9 shows intrinsic coercivity as a function of particle loading for different particles.

[0082] FIG. 10 shows the maximum energy product as a function of particle loading for different particle sizes.

[0083] FIG. 11 shows the specimen used for the measurement of tensile properties in Example 3.

[0084] FIGS. 12-14 present the average stress-strain curves for carbonyl iron particles in DGEBA at varying loadings.

[0085] FIG. 15 shows the average stress-strain curves for carbonyl iron particles in DGEBA (example 1.2).

[0086] FIG. 16 shows the average stress-strain curves for elastomer-containing matrix without (Ref, solid line) and with (Fe75, dashed line) 75 wt-%/30 vol-% carbonyl Fe particles.

[0087] FIG. 17 shows the average stress-strain curves for elastomer-containing matrix with varying loadings of carbonyl Fe particles (example 1.3.

[0088] FIG. 18 shows the average stress-strain curves for NdFeCoB particles in DGEBA (example 1.4).

[0089] FIG. 19 shows the average stress-strain curves for elastomer-containing matrix with varying loadings of NdFeCoB particles (example 1.5).

DETAILED DESCRIPTION OF THE INVENTION

[0090] Reference will now be made in detail to various embodiments. The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

Example 1—Preparing Magnetic Composite Composition

[0091] The process applied for producing the composite structures in this example is outlined in FIG. 1.

[0092] 1.1 Carbonyl Iron Fillers in DGEBA Resin 1

[0093] 100 g of Sika Biresin CR83 was manually mixed with 30 g of Sika Biresin CH83-6 hardener (modified polyamine). The total 130 g of the resin mixture was exposed to vacuum for approx. a minute in order to get the moisture and majority of the air bubbles out. The mixture was divided into two smaller pots (65 g+65 g) to prevent the exothermic reaction from escalating. The two 65 g pots were heated in oven for 55 min at 50° C. to reach a suitable viscosity.

[0094] Carbonyl iron particles (spherical, diameter 5-9 μm as determined by sieves) were mixed with the epoxy mixture in correct amounts to attain a set of loadings specified in Table 1. Volumetric percentages refer to fractions in the cured composite and are calculated assuming a particle density of 7.9 g/cm.sup.3 and cured epoxy density of 1.2 g/cm.sup.3.

TABLE-US-00001 TABLE 1 Epoxy Particles Epoxy Particles Sample weight-% weight-% volume-% volume-% Ref 100 0 100 0 Fe10 90 10 98.4 1.6 Fe20 80 20 96.5 3.5 Fe30 70 30 94.1 5.9 Fe40 60 40 91.1 8.9 Fe50 50 50 87.2 12.8 Fe60 40 60 82.0 18.0 Fe70 30 70 74.5 25.5 Fe80 20 80 63.1 36.9

[0095] The powder-epoxy mixtures were manually mixed for 10 min to attain a visually homogenous mixture. The attained mixtures were poured to silicon molds. Possible air bubbles were removed from the bottom edge of the mold by following the edge with a thin metal needle. The extra resin was removed from the samples by sweeping the surface of the mold by a metal plate. This resulted in an equal amount of resin in each sample. The molded samples were initially cured for 24 h at room temperature in the mold, followed by final curing out of the mold at 70° C. for 5 h.

[0096] 1.2 Carbonyl Iron Fillers in DGEBA Resin 2

[0097] Sika Biresin CR144, anhydride hardener HY 917 and imidazol accelerator Huntsman DY070 were mixed in 100:88.77:1.8 wt:wt:wt ratio as follows: First the resin was exposed to vacuum to make sure that all the air was removed. The hardener was added and mixed for 25 min with a dispersion disk under vacuum. The accelerator was then added, and the vacuum mixing continued for 5-10 min. After that, carbonyl iron particles were added in alternative sizes and loadings specified in Table 2. Volumetric percentages refer to fractions in the cured composite and are calculated assuming a particle density of 7.7 g/cm3 and cured epoxy density of 1.2 g/cm3. The particle size indicated in Table 2 refers to the size of a sieve through which at least 80% of the particles can pass.

TABLE-US-00002 TABLE 2 Particle size as determined by Particle Resin Particle Resin sieves fraction fraction fraction fraction Small: Medium: Large: (wt %) (wt %) (vol %) (vol %) 5 μm 44 μm 150 μm 60 40 18.0 82.0 Sample Sample Sample SS60 SM60 SL60 70 30 25.5 74 5 Sample Sample Sample SS70 SM70 SL70 80 20 36.9 63.1 Sample Sample Sample SS80 SM80 SL80

[0098] The particle size distribution of the small particles, determined by sieves, is as follows (percentages signify the proportion of particles below the indicated diameter in μm): 10%=2.078, 20%=2.570, 30%=2.943, 40%=3.288, 50%=3.641, 60%=4.033, 70%=4.507, 80%=5.151, 90%=6.260, and 95%=7.485.

[0099] The particle size distribution of the medium particles, determined by sieves, is as follows (percentages signify the proportion of particles below the indicated diameter in μm): 10%=15.69, 20%=21.26, 30%=25.34, 40%=28.75, 50%=31.95, 60%=35.20, 70%=38.92, 80%=43.69, 90%=51.71, and 95%=60.35.

[0100] After adding the particles, the vacuum mixing was continued for 10-20 min. The obtained dispersions were molded into plates in glass molds. The samples were placed in an oven preheated to 60° C. and cured according to the following program: [0101] 1. Heating from 60 to 90° C. at a ramp of 0.3° C./min, [0102] 2. 4 h at 90° C., [0103] 3. Heating from 90 to 105 deg C. at a ramp of 1° C./min, [0104] 4. 2 h at 105° C., [0105] 5. Heating from 105 to 140 at a ramp of 1° C./min, and [0106] 6. 4 h at 140° C.

[0107] Once the samples had cured, the sample shapes needed for testing were cut out of the cured plate.

[0108] 1.3 Carbonyl Iron Fillers in Elastomer-Containing Resin

[0109] West System G/flex 650-8 components A and B were manually mixed in a 1:1 vol/vol (1.2:2 w/w) ratio. The resin mixture was exposed to vacuum for approx. a minute or not. (Both vacuumed and non-vacuumed samples were tested and no significant difference found in mechanical performance.)

[0110] Carbonyl iron particles (spherical, diameter <10 μm as determined by sieves) were mixed with the epoxy mixture in correct amounts to attain a set of loadings specified in Table 3. Volumetric percentages refer to fractions in the cured composite and are calculated assuming a particle density of 7.8 g/cm.sup.3 and cured epoxy density of 1.1 g/cm.sup.3.

TABLE-US-00003 TABLE 3 Epoxy Particles Epoxy Particles Sample weight-% weight-% volume-% volume-% Ref 100 0 100 0 Fe75 25 75 70 30 Fe82.5 17.5 82.5 60 40 Fe87.5 12.5 87.5 50 50

[0111] The powder-epoxy mixtures were manually mixed until attaining a visually homogeneous mixture. The ensuing mixtures were manually pushed to silicon molds. The molded samples were cured for 24 h at room temperature in the mold.

[0112] 1.4 NdFeCoB Fillers in in DGEBA Resin

[0113] Sika Biresin CR144, anhydride hardener HY 917 and imidazol accelerator Huntsman DY070 were mixed with a dispersion disk under vacuum. NdFeCoB (26.4% Nd, 67.6% Fe, 5% Co, 1% B) plate-like particles were added in alternative sizes and loadings specified in Table 4 and the dispersion mixing under vacuum was continued. Volumetric percentages refer to fractions in the cured composite and are calculated assuming a particle density of 7.6 g/cm.sup.3 and cured epoxy density of 1.2 g/cm.sup.3.

TABLE-US-00004 TABLE 4 Particle size Particle Resin Particle Resin Medium-large fraction fraction fraction fraction Small Medium epoxy-coated Large (wt %) (wt %) (vol %) (vol %) (D50 = 5 μm) (D50 = 25 μm) (D50 = <250 μm) (D50 = <420 μm) 60 40 18.5 81.5 Sample Sample Sample Sample HS60 HM60 HE60 HL60 70 30 26.1 73.9 Sample Sample Sample Sample HS70 HM70 HE70 HL70 80 20 37.7 62.3 Sample Sample Sample Sample HS80 HM80 HE80 HL80 82.5 17.5 42.5 57.5 Sample Sample Sample Sample HS82.5 HM82.5 HE82.5 HL82.5

[0114] The particle size distribution of the small and medium-sized particles, determined by sieves, are shown in FIG. 2 (from manufacturer).

[0115] The particle size distribution of the large particles, determined by sieves, are shown in FIG. 3 (from manufacturer).

[0116] The obtained dispersions were molded into plates in glass molds. The samples were cured as in Example 1.1. Once the samples had cured, the sample shapes needed for testing were cut out of the cured plate.

[0117] Samples were similarly prepared also out of a corresponding medium-large NdFeCoB with an epoxy coating (coated by manufacturer). Its particle size is specified in the table below. Essentially, most of the particles were between sizes 89-250 μm.

TABLE-US-00005 TABLE 5 Screen Analysis Specification Total > 60 Mesh (250 × 250 μm opening)  <2 wt-% Total > 170 Mesh (89 × 89 μm opening) >65 wt-% Total < 270 Mesh (53 × 53 μm opening) <15 wt-% Total < 325 Mesh (43 × 43 μm opening)  <8 wt-%

[0118] 1.5 NdFeCoB Fillers in Elastomer-Containing Resin

[0119] West System G/flex 650-8 components A and B were manually mixed in a 1:1 vol/vol (1.2:2 w/w) ratio. NdFeCoB (26.4% Nd, 67.6% Fe, 5% Co, 1% B) particles (plate-like, diameter <420 μm as determined by sieves) were mixed with the epoxy mixture in correct amounts to attain a set of loadings specified in Table 6. Volumetric percentages refer to fractions in the cured composite and are calculated assuming a particle density of 7.6 g/cm.sup.3 and cured epoxy density of 1.1 g/cm.sup.3.

TABLE-US-00006 TABLE 6 Epoxy Particles Epoxy Particles Sample weight-% weight-% volume-% volume-% Ref 100 0 100 0 Nd75 25 75 70 30 Nd82.5 17.5 82.5 60 40 Nd87.5 12.5 87.5 50 50

[0120] The powder-epoxy mixtures were manually mixed until attaining a visually homogeneous mixture. The ensuing mixtures were manually pushed to silicon molds. The molded samples were cured for 24 h at room temperature in the mold.

[0121] 1.6 Plasma-Treated Carbonyl Iron Fillers in Elastomer-Containing Resin

[0122] 75-90 g batch of carbonyl iron particles (spherical, diameter <10 μm as determined by sieves) was treated for 3 min with air plasma on a petri dish followed by mixing. The treatment and mixing were repeated 5 times to attain a total treatment time of 15 min. Three batches were combined (total weight 251.52 g) and mixed with 36 g pre-mixed G/Flex epoxy mixture (components A and B in a 1:1 vol/vol or 1.2:2 w/w ratio), to attain a particle-epoxy ratio of 1:1 vol/vol or 87.5:12.5 w/w. The particle-epoxy mixture was manually mixed until attaining a visually homogenous mixture. The ensuing mixtures were manually pushed to silicon molds. The molded samples were cured for 24 h at room temperature in the mold.

[0123] 1.7 Carbonyl Nickel Fillers in DGEBA Resin

[0124] Sika Biresin CR131 was manually mixed with Sika Biresin CH135-8 hardener in a 100:21 w/w (100:26 v/v) ratio. Carbonyl nickel powder (spherical, porous particle, diameter <5 μm as determined by sieves) was manually mixed with the epoxy mixture in a 50:50 v/v powder:epoxy ratio. The epoxy-particle mixture was poured into a silicon mould. The samples were cured in the mould, first at room temperature overnight and then at 140° C. for 8 hours.

[0125] 1.8 Fe.sub.3O.sub.4 Fillers in DGEBA Resin

[0126] Magnetite (Fe.sub.3O.sub.4) particles were prepared by coprecipitation of Fe.sup.2+ and Fe.sup.3+ as follows: 12.9 g of anhydrous FeCl.sub.2 (102 mmol) and 35.25 g (217 mmol) of anhydrous FeCl.sub.3 were dissolved in 600 ml deionized water in order to attain an approx. 1:2 molar ratio of Fe.sup.2+ and Fe.sup.3+. The solution was stirred for 45 min at 80° C. Subsequently, 150 ml of 25% ammonia aqueous solution was added under vigorous stirring. Stirring at 80° C. was continued for another 30 min. Finally, 15 g of citric acid mixed with 30 ml water was added to the solution and stirring was continued for 90 min at 95° C.

[0127] The particles were washed by centrifugation or dialysis as follows:

[0128] Centrifugation: the sample was first centrifuged once, and the supernatant discarded. The precipitate was mixed with deionized water and stirred for 30 min before centrifuging again. The supernatant was discarded, and the precipitate dried in ambient air.

[0129] Dialysis: Excess water was decanted off the sample. Subsequently, it was placed in a dialysis tube and dialyzed against deionized water until reaching a pH of 7. After removing the sample from the tube, it was dried in ambient air.

[0130] The magnetite particles were embedded in epoxy as follows:

[0131] Sika Biresin CR131 was manually mixed with Sika Biresin CH135-8 hardener in a 100:21 w/w (100:26 v/v) ratio. Magnetite particles washed by dialysis were added to attain a 50:50 wt:wt particle:epoxy ratio. The epoxy-particle mixture was poured into a silicon mould. The samples were cured in the mould at 140° C. for 8 hours.

[0132] 1.9 Magnetite-Coated Carbon Fibre Fillers in Elastomer-Containing Resin

[0133] Magnetite electrodeposition on carbon fibre was conducted as follows:

[0134] 256 mg of recycled carbon fibre veil (grammage 30 g/m.sup.2) was cut into 8 pieces of approximately 52 mm×18 mm. The pieces were stacked and connected to the negative pole of a voltage source, while a steel electrode was connected to the positive pole. Both the electrodes were immersed in an aqueous solution of 0.5 M FeCl.sub.2 and 0.5 M FeCl.sub.3 (pH 1). A voltage of 1.2 V was applied for 2 h.

[0135] After the electrodeposition, the pieces weighed 587 mg, implying a 129% increase in mass through addition of 331 mg of magnetite. Part of this was cut off when forming the pieces suitable for embedding, leaving approximately 300 mg of magnetite in the veils that were embedded in elastomer-containing resin as follows:

[0136] West System G/flex 650-8 components A and B were manually mixed in a 1.75:1 vol/vol (2.1:1 w/w) ratio. The magnetite-coated carbon fibre veil pieces were dipped in the mixture and manually pressed into a mould and cured overnight at room temperature.

Example 2—Magnetic Properties of Composite Composition

[0137] The magnetic properties of the magnetic composite structures prepared according to Examples 1.2 and 1.4 were tested on specimens shaped as plates of 10 mm×10 mm×1 mm, using the Permagraph method. Firstly, the sample is magnetized in an external field. Secondly, the sample is exposed to an opposing external field and its demagnetization measured as a function of the opposing field strength. For soft magnetic composites (example 1.2), a full hysteresis loop was measured, fully inversing the external field followed by returning it to the original one.

[0138] The relative magnetic permeability of the soft magnetic composites (example 1.2) was estimated as the approximate maximum slope of the magnetic flux density (B)—magnetic field strength (H) curve. The slope was approximated by dividing the magnetic flux density at a field strength of 100 kA/m with 100 kA/m and the permeability of vacuum.

[0139] The estimated relative permeability as a function of particle loading in the figure below. As seen in FIG. 4, the permeability increased as a function of particle loading, without being much influenced by the particle size. The most permeable samples were obtained with small particles at 70 and 80 weight-% loadings as well as the large particle at 80 weight-% loadings (relative permeability ˜4).

[0140] A similar trend was observed in saturation magnetization, which was determined as the largest magnetic flux density attained during the test. The results of the saturation magnetization tests are presented in FIG. 5 and FIG. 6. Note that in FIG. 5 and FIG. 6, data points overlap for small and medium particle at 60 wt-%; for medium and large particle at 70 wt-%; and for small and large particle at 80 wt-%.

[0141] The hysteresis losses were calculated as the area inside the hysteresis loop. As seen in FIG. 7, the losses decreased with increasing particle loading, apart from an exception with small particle at 70 weight-% loading with remarkably high losses. The lowest losses occurred with a small particle at a loading of 80 weight-%.

[0142] In conclusion, the magnetically best-performing soft magnetic composites (highest permeability, saturation magnetization and lowest hysteresis losses) where attained by using small (80% of particles <5 μm) or large (150 μm) particle with a loading of 80 weight-%.

[0143] The remanence (i.e. residual magnetism) for hard magnetic composites (example 1.4) is presented in FIG. 8. Remanence expresses how much magnetic flux remains in the material after complete removal of the external field and can thus be considered as a measure of the ability of the magnetic material to retain its magnetization. As seen in the figure below, the remanence tends to increase with particle size and loading.

[0144] A similar trend was observed in normal coercivity. This implied that when the magnetized samples were exposed to an increasing external field opposing their magnetization, the more and the larger particles were included, the larger opposing field was needed to eliminate the total net magnetic field (i.e., superposition of the external field and the sample's own field) The normal coercivity of the samples is presented in FIG. 9.

[0145] In contrast, the intrinsic coercivity was discovered to be close to independent of particle size and loading, close to 800 kA/m for each sample, apart from a slight drop at a loading of 80 weight-%. This implies that all the samples had close to equal capability to resist demagnetization, see FIG. 10.

[0146] Maximum energy product (i.e., the maximum value of the product of magnetic flux density B, in T, and magnetic field strength H, in kA/m) also increased with particle size and loading, as seen in FIG. 11.

[0147] In conclusion, the magnetically best-performing hard magnetic composites (highest remanence, normal coercivity and maximum energy product) where attained by using large (up to 420 μm) particles with a loading of 80-82.5 weight-% or medium (median 25 μm) particles with a loading of 82.5 weight-%. For the particle with epoxy coating (end of example 1.4) with a loading of 80 weight-%, the magnetic properties were similar to the best-performing uncoated particles: remanence 0.429 T, normal coercivity 309 kA/m, intrinsic coercivity 788 kA/m and maximum energy product 33.2 kJ/m.sup.3.

Example 3—Mechanical Properties of Composite Composition

[0148] The tensile strength of the magnetic composite structures prepared according to Example 1 were tested on specimens shaped according to specimen type 1A (for examples 1.1, 1.3 and 1.5) or type 1B (for examples 1.2 and 1.4) in EN ISO 527-2 Plastics. Determination of tensile properties. Part 2: Test conditions for moulding and extrusion plastics. Only the gauge length Lo and the initial distance between grips L differed from the standard, as specified in Table 7. Specimen dimensions are shown in FIG. 12 and table 7 below.

TABLE-US-00007 TABLE 7 Length in Length in Symbol mm, examples mm, examples in 1.1, 1.3 1.2 and FIG. 12 Dimension and 1.5 1.4 l.sub.3 Overall length 170 150 l.sub.1 Length of narrow parallel- 80 60 sided portion r Radius 24 60 l.sub.2 Distance between broad 109.3 108 parallel-sided portions b.sub.2 Width at ends 20 20 b.sub.1 Width at narrow portion 10 10 h Thickness 4 4 L.sub.0 Gauge length 90 50 L Initial distance between grips 90 110

[0149] Tensile tests were carried out at a temperature of 24° C. and relative humidity of 50%. The gauge length was 90 mm, test speed 1 mm/min and load cell 5 kN (for examples 1.1, 1.3 and 1.5) or 10 N (for examples 1.2 and 1.4). The stress applied on pulling the sample was measured as a function of the tensile strain. (EN ISO 527-2 Type 1B)

[0150] Averaged stress-strain curves for soft magnetic composites with DGEBA 1 (example 1.1) are presented in FIGS. 13-15. The plateaus at strains below 0.7% are artefacts due to slippage and not to be considered as part of the curves as seen in these figures, the maximum tensile strain is ˜4% or higher for particle loadings up to 50 weight-% (13 volume-%). At 60 weight-%, the max. strain drops to ˜2% at 60-70 weight-% (18-25.5 volume-%) and further down to ˜1% at 80 weight-% (37 volume-%). As seen in example 2, a composite with nearly similarly sized soft magnetic particle and a loading of 60 weight-% has a relative magnetic permeability of only half of that of a similar composite with 70-80 weight-% loading. Moreover, the saturation magnetization at 60 weight-% was ˜20% lower than at 70-80 weight-%. As such, it is preferable to use a loading of 70-80 weight-% (25.5-37 volume-%).

[0151] The averaged stress-strain curves of the magnetically best-performing soft magnetic composites with DGEBA 2 (example 1.2) are summarized in FIG. 16. In the figure, sample codes follow Table 2; i.e., S70 and S80 refer to small particle (80% of particles <5 μm) at 70 and 80 wt-% loadings, respectively, and L80 to large particle (150 μm) at 80 wt.-% loading.

[0152] As seen in FIG. 16, increasing the particle loading makes the composite stiffer and more brittle. This effect is increased further by enlarging the particle size. Therefore, in order to balance the magnetic properties with mechanical ones, it is preferable to use a small particle. For the small particle, both loadings of 70 and 80 weight-% are satisfactory. However, 80 weight-% (37 volume-%) provides lower magnetic hysteresis losses (see example 2), whereas 70 weight-% (25.5 volume-%) provides more ductility (higher tensile strength maximum tensile strain).

[0153] Stress-strain curves for soft magnetic composites with elastomer (example 1.3) are presented in FIGS. 16 and 17. FIG. 17 presents the average stress-strain curves for elastomer-containing matrix without (Ref, solid line) and with (Fe75, dashed line) 75 wt-%/30 vol-% carbonyl Fe particles. FIG. 18 presents the average stress-strain curves for elastomer-containing matrix with varying loadings of carbonyl Fe particles (example 1.3): 75 wt-%/30 vol-% dashed line; 82.5 wt-%/40 vol-% dotted line; 87.5 wt-%/50 vol-% solid line. As seen in FIG. 18, the matrix is highly elastic, and adding magnetic particles in it drops its maximum tensile strain below 8% of the reference sample without particles. When comparing different magnetic particle loadings (FIG. 18), the amount of particles is seen to have little influence on the ductility. Based on the magnetic performance of samples with a different matrix but similar or lower particle loadings (example 2), the samples are all expected to have a satisfactory magnetic performance.

[0154] FIG. 19 shows the average stress-strain curves for the magnetically best-performing hard magnetic composites (example 1.4). In FIG. 19, sample codes follow Table 4; i.e., HL80 and HE80 refer to a 80 wt-% loading of large (<420 μm) and medium-large (<250 μm) epoxy-coated particles, respectively, whereas HM82.5 refers a 82.5 wt-% loading of medium (D50=25 μm) particles.

[0155] FIG. 19 shows that, even though the large NdFeCoB particle at a loading of 80 weight-% (37.7 volume-%) performs magnetically well, it makes the composite mechanically brittle (HL80 in FIG. 19). The brittleness can be decreased by using a smaller particle with an epoxy coating (HE80 in FIG. 19). Similar improvement can also be attained when decreasing the particle size further (HM82.5 in FIG. 19).

[0156] The average stress-strain curves for hard magnetic composites with elastomer (example 1.5) are presented in FIG. 20. In FIG. 20, sample codes follow Table 6. As seen in FIG. 20, when increasing the loading from 30 volume-% (75 weight-%) to 40 volume-% (82.5 weight-%) to 50 volume-% (87.5 weight-%), the max. strain decreases from 2.4% to 1.8% to 1%.

[0157] It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

[0158] The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method, a composite structure, or a use, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to ‘an’ item refers to one or more of those items. The term “comprising” is used in this specification to mean including the feature(s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.