SOFT MAGNETIC METAL-GLASS COMPOSITE MATERIAL WITH LOW EDDY CURRENT LOSSES

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SOFT MAGNETIC METAL-GLASS COMPOSITE MATERIAL WITH LOW EDDY CURRENT LOSSES

F. Kloucek, C. Schüler, U. Feller

To cite this version:

F. Kloucek, C. Schüler, U. Feller. SOFT MAGNETIC METAL-GLASS COMPOSITE MATERIAL

WITH LOW EDDY CURRENT LOSSES. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-197-

C6-202. �10.1051/jphyscol:1985634�. �jpa-00224882�

Colloque Ce, supplément au n°9, Tome 46, septembre 1985 page C6-197

SOFT MAGNETIC METAL-GLASS COMPOSITE MATERIAL WITH LOW EDDY CURRENT LOSSES

F. Kloucek, C. Sch'uler and U. Feller

Brown Boveri Research Center, CH-5405 Baden-Dattwil, Switzerland

+

Brown Boveri Central Laboratory, CH-5401 Baden, Switzerland

Résumé - On montre qu

une nouvelle méthode de métallurgie des poudres permet d'obtenir des matériaux composites magnétiques doux avec une haute induction à saturation et faible perte par courant de Foucault. A partir d' une poudre de métal magnétique doux, les particules sont enrobées d' une petite quantité de verre par un procédé sol-gel. Après compactage à température élevée, les particules métalliques sont séparées et isolées électriquement par un film mince de verre.

Abstract - A new powder metallurgical method is demonstrated which produces magnetic composite materials with high saturation induction and low eddy current losses. Starting with a soft magnetic metal powder the particles are coated at room temperature with a small amount of glass by a sol-gel process.

After compacting at elevated temperature, the metallic particles are separa- ted and electrically insulated by a thin glass film. The resulting properties are described and discussed.

I - INTRODUCTION

Magnetic cores made from iron powder are standard materials. Eddy currents are supressed by electrically insulating the iron particles from each other using a thermoplastic organic polymer /l/ or an oxide /2/. All these materials lack mecha- nical strength, due to the weakness of the polymer binder phase or oxide layer between the iron grains.

II - IRON-GLASS COMPOSITES

The basic idea for the new material described here is to coat particles of a high permeability soft magnetic powder with a thin glass layer at room temperature and consolidate the powder to theoretical density at elevated temperatures, where com- pacting stresses are reasonably low. The finished product consists of soft magnetic particles completely embedded in glass. The question arose whether this could be accomplished. In most cases the glass film would be squeezed out between the contact areas of the particles and metallic contact between the particles would occur during compaction. In other words, the resistivity of the composite material would not be so much different from iron. Some simple considerations show how this can be avoided.

Consider a stacked sample consisting of layers of metal, glas, metal, etc. under compressive load. If frictional forces at the die interface are neglected, then the glass will not be squeezed out of the stack even after extensive deformation, pro- vided the plastic flow behaviour of the glass and the metal are similar, i.e. if the plastic strain rate of the metal roughly equals the strain rate of the glass layer.

This can be achieved at elevated temperatures, where creep deformation can occur in the metal and the glass shows viscous flow behaviour. By choosing the correct com- bination of metal and glass, one can always find a set of conditions where the strain rates in both materials are equal.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985634

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COMPOSITE FABRICATION

PROCEDURES

Since eddy-current losses are expected to vary with the square of the particle dia- meter / 3 / , a powder with a small particle size should be chosen for the fabrication process. The main investigations were performed using C-type carbonyl iron powder to demonstrate that a composite material with high electrical resistivity is attain- able. Carbonyl iron was found to be most suitable, since the spherical shape of the particles gives a more even stress distribution during consolidation compared to powders with irregular shape. An even stress distribution is a prerequisite for avoiding excessive formation of metallic contact areas between the particles.

The manufacturing process is schematically shown in Fig.

In the first step the powder particles must be coated with a thin layer of glass. The literature indicates that the overall volume fraction of glass plus porosity should be less than 10 %.

Otherwise the maximum permeability is far below the desired value of at least 100.

Assuming a mean particle diameter of 5 pm an homogeneous glass coating of less than 0.2 pm has to be applied.

CARBOUIL IRON

qq

PROCESS

TEMPERATURE EWCUATlON

COYPACTION

Q

Fig. 1: Fabrication process for soft magnetic composite.

The glass serves two different purposes. It electrically insulates the metallic particles and acts as a binder for the composite. In this application the desired properties of the glass are: (1) a thermal expansion coefficient that matches iron;

(2) good adhesion to iron; and (3) a softening point at about 600°C. Guided by these requirements, the following glass composition was chosen: 53% Si02, 16% B2O3, 10% A1203, 19% Na20, 2% COO.

We developed a suitable coating process for powders by taking advantage of the sol- gel method. Sol-gel processing forms solid ceramic or glass products at room tem- perature from a liquid solution / 4 / . Starting materials can be metal alkoxides, metal salts, silica, and others. A sol of the above mentioned oxidic composition was prepared in alcoholic solution and a gel coating was applied to the iron powder by hydrolysis and polycondensation reactions of the sol. Figure 2 illustrates the difference between carbonyl powders as delivered and after application of the sol- gel process. After cleaning in alcohol and $ater, the coated powder was heated in vacuum up to 150°C to remove water and decomposable products.

The coated powder was compacted in a protective atmosphere at temperatures ranging

Fig. 2: SEM-micrographs of carbonyl iron particles.

from

to

in a hydraulic press. Pressing stresses were up to

MPa.

Under these experimentally found conditions theoretical density and high electrical resistivity was achieved. After compaction, the composite material is easily machinable.

Fig. 3: SEM-micrograph showing iron grains embedded in glass.

Figure 3 shows an SEM micrograph of the composite material. The metallic grains are completely embedded in glass, with most of the glass accumulated at the grain boundary corners. Metallographic examinations and measured density indicate that the samples are porosity free.

The electrical resistivity was measured on disc specimens using van der Pauw's

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method /5/. AC and DC magnetic properties were determined on small ring samples and comparisons were made with iron cores without glass. The mechanical strength was de- monstrated using the four point bend test of specimens with a span-to-depth ratio of 7.

IV - ^{MAGNETIC AND} MECHANICAL PROPERTIES

The magnetic properties of test samples of the iron-glass composites are shown in Table 1. The volume-fraction of glass ranges from 3 to 12%. Sample E 10 con- sists of iron powder without any glass and was consolidated under the same con- ditions as the other samples.

Table 1: Magnetic properties of iron-glass composite samples.

p/+,: Relative electrical resistivity pmax: Maximum permeability

Hc: Coercive force V

Specific losses at B

0.2 T/50 Hz PS: Saturation induction fT: Limiting frequency

p : Initial permeability Sample

E 10 E 62 E 63 E 64

IRON-GLASS METALS

-

-

A Corovac EF 606

-

-

A Bosch

-

Corovoc OF1

-

-

Fe 3% Si

I I I I Fe

I R O N (VOL%l

p/pFe

1.3 5900 22 123

Fig. 4: Relative resistivity vs volume fraction of iron for pure iron, iron silicon alloy, iron-glass composite, iron-plastic composite and iron core.

Glass (Vol.%)

0 12 3 6

Hc (A/cm)

1.8 7.1 6.4 6.2

(T)

2.13 1.97 1.98 1.96

pi

220 45 80 70

IJmax

1570 72 190 145

"s

&/kg)

1.00 1.13 0.86 0.95

f~

(Hz)

5 . 1 0 2

>> 105

1,5 lo4

1 . 1 0 ~

Fig.

Comparison of saturation induction for different materials.

Due to the insulating glass, the electrical resistance compared to pure iron or silicon-iron alloys increases up to more than 4 orders of magnitude. A comparison of relative resistivity vs the volume fraction of iron, the balance being residual porosity and insulating material, is given in Fig. 4 for different materials. The 3 triangular symbols on the right represent novel iron-plastic composite materials manufactured by Vacuumschmelze / 6 / (trade name Corovac) and Bosch /l/. One can easily see that the iron-glas composite compares favourably with these materials.

The iron core, consisting of carbonyl iron and a binder pressed at room temperature (a technique well-known for at least

years), shows the largest resistivity value at the expense of having a low iron content. This is important since saturation- induction depends directly on the volume-fraction of iron as illustrated in Fig. 5.

I

0 3 6 9 12

GLASS ( V O L % )

Pig.

Permeability versus volume fraction of glass.

As shown in Fig.

maximum and initial permeability depend strongly on volume-

fraction of the insulating material. Both properties increase with decreasing glass

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content, which gives a reduction of the effective "built-in-air-gaps" between the iron particles. For this reason the glass content should be kept below 3%, if high permeability is demanded. Further work has to be done to improve resistivity in this composition range, since the useful frequency range is limited by eddy current losses. Table 1 indicates that the limiting frequency can only be extended to more than 100

if relative resistivity is increased by a factor of 104 which is the case for sample E 62, containing 12% glass.

Fig. 7: Bend strength of different iron powder materials compared to the tensile strength of a silicon iron alloy.

Compared with other powder-based soft magnetic materials the mechanical strength of the iron-glass composite is considerably improved. Bend strength testing at room temperature yielded values between 290 and 540 MPa, whereas the flexural strength of other composites does not exceed 120 MPa as shown in Fig. 7. These values suggest applications in highly stressed rotating parts. In Fig. 7 the tensile strength of a conventional Fe-3% Si alloy is also included. The iron-glass composite experienced no macroscopic plastic deformation up to rupture, and the failure was completely brittle.

V

CONCLUSIONS

A new fabrication process for soft magnetic composite materials with high satura- tion induction and low eddy current losses is presented. The above data reveal several features of particular interest. Permeability can be adjusted by the volume fraction of the insulating glass film at the grain boundaries. This makes the ma- terial suitable for applications in systems with air gaps, e.g. small electric motors and reactors in thyristor circuits. In the case of magnetic stray fields, the isotropic electrical properties of the composite are advantageous. Parts with rather complicated shapes can be produced using powder metallurgical methods plus machining. Further development work is expected to improve the properties of the new magnetic materials still further.

REFERENCES

/l/ A. Walter, F.J. Esper, W. Gohl, 3. Schweikhardt, J. of Magnetism and Magnetic Materials 15-18 (1980) 1441-1442.

/2/ N.M. Pavlik, J. Sefko, US-Patent 4 158 561 (1979).

/3/ R.S. Tebble, D.J. Craik, Magnetic Materials (1969) /4/ H. Dislich, Glastechnische Berichte 44 (1971) 1-8.

/5/ L.J. van der Pauw, Philips Research Report 13 (1958) 1-9.

/6/ Vacuumschmelze, product brochure "Corovac" (1982).