This application claims benefit of the filing dates of German Patent Application Serial No. DE 10 2006 028 389.9, filed Jun. 19, 2006, and of U.S. Provisional Application Ser. No. 60/805,599, filed Jun. 23, 2006.

BACKGROUND

1. Field

Disclosed herein is a magnet core pressed using an alloy powder and a pressing additive to form a composite. Also disclosed is a method for producing a magnet core of this type.

2. Description of Related Art

The use of powder cores made from iron or alloy powder has been established for many years. Amorphous or nanocrystalline alloys, too, are increasingly used, being superior to other crystalline powders, for example in their remagnetisation properties. Compared to amorphous powders, nanocrystalline powders offer the advantage of higher thermal stability, making magnet cores made from nanocrystalline powders suitable for high operating temperatures.

The raw material for nanocrystalline powder cores typically is an amorphous strip or a strip material made nanocrystalline by heat treatment. The strip, which is usually cast in a rapid solidification process, first has to be mechanically pulverised, for example in a grinding process. It is then pressed together with an additive in a hot or cold pressing process to form composite cores. The finished pressings may then be subjected to heat treatment for turning the amorphous material into nanocrystalline material.

EP 0 302 355 B1 discloses a variety of methods for the production of nanocrystalline powders from iron-based alloys. The amorphous strip is pulverised in vibratory or ball mills.

U.S. Pat. No. 6,827,557 discloses a method for the production of amorphous or nanocrystalline powders in an atomising process. This method involves the problem that the cooling rate of the melt depends heavily on particle size and that the cooling rates required for a homogenous amorphous microstructure are often not obtainable, in particular with larger particles. This results in powder particles with a strongly varying degree of crystallisation.

The level of iron losses is an important characteristic of magnet cores. Two factors contribute to iron losses, these being frequency-dependent eddy-current losses and hysteresis losses. In applications such as storage chokes or filter chokes, for instance, iron losses at a frequency of 100 kHz and a modulation of 0.1 T are relevant. In this typical range, iron losses are dominated by hysteresis losses.

SUMMARY

The magnetic cores, inductive components, and methods disclosed herein are therefore based on the problem of specifying a magnet core made from an alloy powder with minimal hysteresis losses and therefore low iron losses.

In addition, the features disclosed herein are based on the problem of specifying a method suitable for the production of a magnet core of this type.

According to the embodiments disclosed herein, this problem is solved.

In one embodiment disclosed herein is a composite magnet core made from a powder of nanocrystalline or amorphous particles and a pressing additive, wherein the particles have first surfaces represented by the original surfaces of a nanocrystalline or amorphous strip and second surfaces represented by surfaces produced in a pulverisation process. The overwhelming majority of these second surfaces are essentially smooth cut or surfaces resulting from fracture without any plastic deformation, the proportion T of areas of plastic deformation of the second surfaces being 0â‰¦Tâ‰¦0.5.

The embodiments disclosed herein were obtained based on the perception that the characteristics of the individual powder particles, in particular their fracture or surface characteristics, significantly affect the properties of the finished magnet core. It has been found that the surfaces of particles produced by pulverisation, for example of strip material, include areas of major plastic deformation. Mechanical stresses developing in these deformed areas result in undesirably high hysteresis losses. In addition, a high energy input in the pulverisation process leads to structural damage and the formation of nuclei for crystallite.

In the pressing process, too, mechanical stresses are introduced into the magnet core, and mechanical distortion due to different coefficients of thermal expansion for the powder and the pressing additive is possible. These stresses can, however, be reduced to an insignificant level by subsequent heat treatment.

Structural damage caused by deformation at the particle surface, however, cannot be repaired. For this reason, it has to be avoided largely in advance to reduce iron losses.

The proportion T of areas of plastic deformation of the particle surfaces is expediently limited to 0â‰¦Tâ‰¦0.2.

By reducing mechanical stresses, in particular by reducing plastic deformation at the particle surfaces, cycle losses P of Pâ‰¦5 Î¼Ws/cm³, preferably Pâ‰¦3 Î¼Ws/cm³, are obtainable.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The nanocrystalline particles expediently have the alloy composition (Fe_1-aM_a)_{100-x-y-z-Î±-Î²-Î³}Cu_xSi_yB_zMâ€²_Î±Mâ€³_Î²X_Î³, wherein M is Co and/or Ni, wherein Mâ€² is at least one element from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, wherein Mâ€³ is at least one element from the group consisting of V, Cr, Mn, Al, elements of the platinum group, Sc, Y, rare earths, Au, Zn, Sn and Re, wherein X is at least one element from the group consisting of C, Ge, P, Ga, Sb, In, Be and As, and wherein a, x, y, z, ac, P and y are specified in atomic percent and meet the following conditions: 0â‰¦aâ‰¦0.5; 0.1â‰¦xâ‰¦3; 0â‰¦yâ‰¦30; 0â‰¦zâ‰¦25; 0â‰¦y+zâ‰¦35; 0.1â‰¦Î±â‰¦30; 0â‰¦Î²â‰¦10; 0â‰¦Î³â‰¦10.

As an alternative, the particles may have the alloy composition (Fe_1-a-bCo_aNi_b)_100-x-y-zM_xB_yT_z, wherein M is at least one element from the group consisting of Nb, Ta, Zr, Hf, Ti, V and Mo, wherein T is at least one element from the group consisting of Cr, W, Ru, Rh, Pd, Os, Ir, Pt, Al, Si, Ge, C and P, and wherein a, b, x, y and z are specified in atomic percent and meet the following conditions: 0â‰¦aâ‰¦0.29; 0â‰¦bâ‰¦0.43; 4â‰¦xâ‰¦10; 3â‰¦yâ‰¦15; 0â‰¦zâ‰¦5.

The compositions listed above include alloys such as Fe_73.5Cu₁Nb₃Si_13.5B₉and the non-magnetostrictive alloy Fe_73.5Cu₁Nb₃Si_15.5B₇.

A possible alternative are amorphous particles of the alloy composition M_Î±Y_Î²Z_Î³, wherein M is at least one element from the group consisting of Fe, Ni and Co, wherein Y is at least one element from the group consisting of B, C and P, wherein Z is at least one element from the group consisting of Si, Al and Ge, and wherein Î±, Î² and Î³ are specified in atomic percent and meet the following conditions: 70â‰¦Î±â‰¦85; 5â‰¦Î²â‰¦20; 0â‰¦Î³â‰¦20. Up to 10 atomic percent of the M component may be replaced by at least one element from the group consisting of Ti, V, Cr, Mn, Cu, Zr, Nb, Mo, Ta und W and up to 10 atomic percent of the (Y+Z) component may be replaced by at least one element from the group including In, Sn, Sb und Pb. These conditions are for example met by the alloy Fe₇₆Si₁₂B₁₂.

One possible pressing additive is glass solder, and ceramic silicates and/or thermosetting resins such as epoxy resins, phenolic resins, silicone resins or polyimides may also be used.

The magnet core described herein offers the advantage of significantly reduced iron losses compared to conventional powder composite cores, which can be ascribed to a reduction of the frequency-independent proportion of the losses, i.e. the hysteresis losses. The magnet core according to the invention can be used in inductive components such as chokes for correcting the power factor (PFC chokes), in storage chokes, filter chokes or smoothing chokes.

According to the invention, a method for the production of a magnet core comprises the following steps: first, a strip or foil of a typically amorphous, soft magnetic alloy is made available. The strip of foil may, however, alternatively be nanocrystalline. The term â€œstripâ€ in this context includes fragments of strip or a roughlyâ€”i.e. without a particularly high energy inputâ€”crushed strip, for example flakes. The strip or foil is pulverised using a technique which causes a minimum of structural damage. This process is usually based on cutting and/or breaking. The aim is a pulverisation process with minimum energy input. For this purpose, the powder particles are removed from the pulverising chamber on reaching their final grain size, the dwell time t in the pulverising chamber preferably being t<60 s. The powder produced in this way is then mixed with at least one pressing additive and pressed to form a magnet core.

As a result of the short pulverisation process, the energy input into the powder particles produced, which would cause their plastic deformation, is kept to a minimum. As the strip is not pulverised by crushing or grinding, but mainly by cutting, those surfaces of the powder particles which represent new particle surfaces following pulverisation are largely smooth cut or fracture surfaces without any plastic deformation. Mechanical distortion, which would result in undesirably high hysteresis losses which cannot be reversed by heat treatment to the required degree, are in this production method avoided from the start.

Before pulverisation, the strip or foil is expediently made brittle by heat treatment, so that it can be pulverised even more easily and with a lower energy input. The amorphous strip can be converted into coarse-grained powder fractions at a temperature T_millof âˆ’195Â° C.â‰¦T_millâ‰¦20Â° C., because such low temperatures improve grindability, thus further reducing the energy input of the process.

After pressing, the magnet core is expediently subjected to a heat treatment process, whereby distortions caused by the different coefficients of thermal expansion of powder and additive or pressing stresses can be eliminated. The heat treatment of the pressed magnet core also enables its magnetic properties to be adjusted as required.

In order to produce a magnet core of maximum homogeneity with defined properties, the powder is expediently subjected to a separation or grading process following pulverisation. Different size fractions of powder particles are then processes separately.

Example 1

In one embodiment of the method described herein, a strip was produced from an Fe_73.5Cu₁Nb₃Si_13.5B₉alloy in a quick solidification process, followed by thermal embrittlement and pulverisation with minimum energy input, largely by cutting action. For comparison, a strip produced in the same way was pulverised by conventional methods. The fracture surfaces or particle surfaces of the powder particles produced according to the minimum energy input process described herein showed virtually no plastic deformation, while the conventionally produced powder particles exhibited major deformation. Both powders were graded, and identical fractions were mixed with 5 percent by weight of glass solder as a pressing additive. In a uniaxial hot pressing process, the mixtures were pressed to form powder cores at a temperature of 500Â° C. and a pressure of 500 MPa. The cycle losses of the magnet cores produced by these processes were then determined. The cycle losses correspond to the hysteresis losses during a complete magnetisation cycle. Cycle losses are determined by dividing the losses through frequency and by forming limit values for vanishing frequencies. Cycle losses depend on maximum modulation, but no longer on remagnetisation frequency.

Cycle losses following the pressing process were approximately 16 Î¼Ws/cm³for conventionally produced magnet cores and approximately 15.8 Î¼Ws/cm³for magnet cores produced according to the invention.

After pressing, the magnet cores were subjected to one hour's heat treatment at 520Â° C. to effect a nanocrystallisation of the powder particles. Following this, the cycle losses were once again determined. They were approximately 5.5 Î¼Ws/cm³for conventionally produced magnet cores and approximately 2 Î¼Ws/cm³for magnet cores produced according to the minimum energy input process described herein. During the heat treatment process, the stresses induced by pressing into the magnet core are therefore largely eliminated, and at the same time, the heat treatment effects the nanocrystallisation of originally amorphous structures and thus the adjustment of good magnetic properties. Following this, the hysteresis losses of the finished nanocrystalline powder cores are virtually exclusively determined by the characteristics of the fracture or particle surfaces.

Example 2

In a further embodiment of the method described herein, a strip was likewise produced from an Fe_73.5Cu₁Nb₃Si_13.5B₉alloy in a quick solidification process, followed by thermal embrittlement and pulverisation with minimum energy input, largely by cutting action, in less than 60 s. For comparison, a strip produced in the same way was pulverised with high energy input and a duration of more than 600 s. Once again, the fracture surfaces or particle surfaces of the powder particles produced according to the minimum energy input process showed virtually no plastic deformation, while the conventionally produced powder particles exhibited major deformation.

As in the first example, the powders were graded and pressed together with glass solder to form magnet cores. After a heat treatment process as described above, the cycle losses of the magnet cores were determined. Magnet cores produced from different size fractions of powder particles were investigated separately in order to take account of the effect of particle size. For particles with a diameter of 200-300 Î¼m, the cycle losses of the magnet cores produced according to the minimum energy input process amounted to 2.3 Î¼Ws/cm³and for comparable cores produced by conventional means to 4.3 Î¼Ws/cm³.

For particles with a diameter of 300-500 Î¼m, the cycle losses of the magnet cores produced according to the minimum energy input process amounted to 2.0 Î¼Ws/cm³and for comparable cores produced by conventional means to 3.2 Î¼Ws/cm³. For particles with a diameter of 500-710 Î¼m, the cycle losses of the magnet cores produced according to the minimum energy input process amounted to 1.7 Î¼Ws/cm³and for comparable cores produced by conventional means to 2.3 Î¼Ws/cm³.

Example 3

In a further embodiment of the method described herein, a strip was likewise produced from an Fe₇₆Si₁₂B₁₂alloy in a quick solidification process, followed by thermal embrittlement and pulverisation with minimum energy input, largely by cutting action, in less than 60 s to produce particles with a diameter of 200-300 Î¼m.

As in the first and second examples, the powders were graded and pressed together with glass solder at a temperature of 420Â° C. to form magnet cores. Cycle losses were determined after a two-hour heat treatment process at 440Â° C. For particles with a diameter of 200-300 Î¼m, the cycle losses of the magnet cores produced according to the minimum energy input process amounted to 4 Î¼Ws/cm³at a modulation of 0.1 T.

These examples show clearly that the cycle or hysteresis losses of powder cores are strongly affected by the characteristics of the fracture or particle surfaces and that the plastic deformation of these surfaces causes higher hysteresis losses.

The examples and embodiments described herein are provided to illustrate various embodiments of the invention, and are not limiting of the appended claims.