US2147791A - Magnetic material having low hysteresis losses - Google Patents

Description

Feb. 21, 1939. G. HOLST ET AL 2,147,791

MAGNETIC MATERIAL HAVING LOW HYSTERESIS LOSSES 7 Filed Nov. 22, 1934 0574mm 5y saw/24 mas lrweniora': GJYoZJfi, Z0. Six J1; 573061: and; Z0. G.Bwgyenr,

Patented Feb. 21, 1939 UNITED STATES PATENT OFFICE Gilles Holst, Willem Six, Jacob Louis Snoek, and

Wilhelm Gerard Burgers, Eindhoven, Netherlands, assignors to N.

V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application November 22, 1934, Serial No. 754,318 In Germany December 4, 1933 8 Claims.

The present invention relates to magnetic material and more particularly to magnetic material adapted to form the core of magnetic coils, transformer coils, etc.

In many applications, for instance, for telcphone and telegraph work, transmitters, etc., and especially for so-called Pupin coils, relays, electroacoustic devices and the like, it is highly important that the magnetic material used possesses low hysteresis losses, i. e., should have a magnetization curve which is substantially rectilinear.

Suitable magnetic materials for such purpose which show small hysteresis losses are certain nickel-iron alloys, possibly also containing additions of one or more other metals as cobalt, copper or aluminum.

It has already been proposed to reduce the hysteresis losses of such materials, and particularly of magnetic material consisting of an ironnickel alloy, by subjecting such materials during their use to external compressive or tensile forces, acting on the core formed of such material, either in the direction of the magnetic force or in a direction normal to said force. Thereby the crystals of the magnetic material are elastically deformed by such external forces, which results in the material exhibiting preferred directions of magnetization.

The present invention has for its purpose to provide magnetic materials exhibiting definite preferred directions of magnetization, which materials can be obtained in a simple manner and without the application of external mechanical tensile or compressive stress in the direction of the magnetic force or normally thereto.

According to the invention, the magnetic material employed is anisotropic and, when used as a core, the direction of the magnetizing force is so selected as to extend normally or substantially normally to the direction of the greatest permeability of the material.

The following explanation will give a better understanding of the present invention.

,It is known that a ferromagnetic body which consists of a single-crystal, entirely free from strains, exhibits three preferred directions of magnetization which, in cube-shaped space-grids, coincide with the main axes of the cube.

Assuming a Cartesian co-ordinate system established in such a single crystal with the X, Y and Z directions coinciding with the three main axes of the cubes; the single crystal will be equally well magnetizable in the positive and in the negative X, Y and Z directions.

Furthermore,.a single crystal comprises, in the same manner as any other ferromagnetic body, a certain number of areas (so-called Weiss complexes), within each of which areas the internal or molecular field has the same orientation throughout, These directions of spontaneous magnetization of the Weiss complexes coincide with the cube "edges and can be considered to lie in the positive and negative X, Y and Z directions. Since the probability that the direction of the spontaneous magnetization coincides with the positive or negative directions is the same, the action of the Weiss complexes neutralize each other towards the outside.

Thus, if a magnetic field is set up from the outside, the direction of the spontaneous magnetization orients itself in the direction of the magnetic field and this orientation, in the case of an ideal single-crystal, occurs without any hysteresis. For an explanation of this phenomenon the so-called reversible wall displacement and reversible rotation hypotheses have been offered.

As has been stated above, the preferred directions of magnetization in a single-crystal iron coincide with the main axes of the cube-shaped space-grid. In an ideal single-crystal an infinitely small external field, applied in the direction of one of the main axes, will be sufficient to cause the vector of the spontaneous magnetization to have the same direction as the field, so that the permeability would have an almost infinitely large value. However, the artificially-produced single crystals available in practice differ from the ideal single-crystal, due to the impurities always contained in same, whereby a magnetic field of finite strength is required for magnetic saturation.

. Even with such impurities, however, the artificially manufactured iron single-crystals still exhibit a very high initial permeability in the preferred directions, and, despite the impurities contained therein, hysteresis losses of these single-crystals are but small.

In view of the above, it appears that a ferromagnetic body consisting of a single crystal would be the ideal core material for choke coils, transmitters and the like. However, in many devices, such as for instance, Pupin coils, a too high initial permeability is undesirable. While the effective permeability of a magnetic core consisting of a single crystal could be reduced by providing air-gaps in the core, such a measure brings about various inherent drawbacks and diificulties. One of the drawbacks of coils having a core with airgaps is that there exists an external field so that undesirable induction effects on other coils in the neighbourhood will occur.

The present invention has for its purpose to obtain a magnetic material of low hysteresis losses without at the same time obtaining unduly highinitial permeability, and is based on the realization that, by artificially suppressing one of the preferred directions of the magnetization-of a single-crystal, it is possible to also decrease the permeability in the suppressed direction.

As will be explained in more detail hereinafter, this can be eifected by means of a suitable mechanical and thermal treatment by which the ferromagnetic material, for instance an ironnickel alloy, is first brought into a state in which the structure corresponds substantially to that of a single-crystal. Subsequently the material is subjected to further mechanical treatment to obtain the desired magnetic anisotropy in which the material exhibits, instead of three, only two preferred directions of magnetization. The anisotropic property so exhibited is intimately associated with the state of the internal stress of the material brought about by the above treatments.

In order that the invention will be clearly understood and more readily carried into eflect it will be more fully explained with reference to the accompanying drawing, in which,

Figure 1 is a graph showing magnetization curves, and

Fig. 2 is a perspective view of a loading coil.

Below will be given a specific example to obtain a magnetic material. in accordance with the invention:

Iron and nickel are fused together in a furnace in the proportion of about 50% of iron and 50% of nickel. The molten composition is formed in a mould and cooled to'form a bar or rod which is formed at a temperature between 1000-1100 C. into a bar having a square cross-section whose sides have a length of 5-8 cm. The latter is rolled at a temperature between 900-1100 C. into a tape of 1-4 mm. thickness, which tape is annealed in hydrogen at a temperature between IOU-900 C. According to the invention this material of a thickness of about 1 mm. is then subjected to repeated rolling without annealing above the recrystallization temperature to decrease its thickness to about 110 microns so that the crosssection of the tape is reduced or still more. Now the band or tape is subjected to recrystallization by annealing it at about 1100 C. As a result chiefly of the reduction in cross section of 90% or more and also of the recrystallization, the examination of the band by X-rays shows that the structure of the band closely corresponds to that of a single crystal. In correspondence therewith the magnetic examination shows two pronounced preferred directions of magnetization; one falls in the direction of rolling, i. e. in the longitudinal direction of the band, and the other at right angle thereto, namely, in the direction of the width of the band. Athird preferred direction of magnetization normally to both these directions or to the surface of the hand must also be present; however because of the small thickness of the band, determination thereof entails considerable difiiculties. The initial permeability of the band in the longitudinal and width direction is about 500. The recrystallized band is subjected to cold rolling to decrease its thickness in one or more rolling steps to about 60 microns. This causes a considerable decrease in the permeability of the material in direction of rolling, l. e. in the length direction of the band, without, however, materially affecting the permeability in the direction of the width of the band. It follows that the band is rendered anisotropic. in which the ordinate axis represents the magnetization (I) and the abscissa axis represents the field strength (H). Curve I represents the magnetization of the band taken in its width direction and curve II represents the magnetization of the band taken in its rolling or longitudinal direction. From. these curves it will be seen that the permeability in the longitudinal direction is only a fraction of permeability in the width direction and is nearly constant in a wide range of field strength H. The decrease in permeability in the longitudinal or rolling direction is accompanied by a small increase of the hysteresis losses, which, however, remain within permissible limits. The following data will be of interest.

Longitudinal direction direction Coercive force 8 l. 5 Remanent magnetisation 30 950 Initial permeability 40 400 Maximum permeability 50 4000 Hysteresis value q 7-8 7-8 The above value was measured with an alternating current of 1 milliampere at a frequency of 800 cycles for a coil provided with a core having a volume of 20 c. c. As above stated, the rolling of the recrystallized band from microns down to 60 microns only decreases the permeability in the rolling or longitudinal direction of the band, and does not materially afiect the permeability in the width direction. Although it is difiicult to determine the permeability in the direction normal to the surface of the band, it appears that this permeability also remains substantially unaffected. If it is desired to increase the permeability in the longitudinal direction of the 60 micron band, this can be effected by a thermal treatment at a temperature of about 400 0. Such thermal treatment partly removes the internal stresses and causes an increase in the permeability in.the rolling direction. In the case of the above described material, the initial permeability was raised from 40 to 80 by a thermal treatment at about 420? C. for two hours without, however, substantially changing the hysteresis losses. The improved magnetic material according to our invention is especially useful for loading coils. For this purpose a band of the magnetic material is formed and spirally wound into a core upon which the windings of the loading coil are mounted. Such a coil is shown in Fig. 2 in which the core 2 is formed of a spirally wound band whose roiling or longitudinal direction-in which the permeability is a minimum-is indicated by the arrow. A winding l-I is provided on the core 2. Flow of current through the The above is illustrated in Fig. 1,

winding l| sets up in the core 2 a magnetic field whose direction is indicated by the arrow and coincides with the direction of minimum permeability of the core. Although we have described our invention in connection with specific examples, we do not wish to be limited' thereto. The same results are obtained with nickel iron alloys of other composition than 50% of iron and 50% of nickel. Such alloys may also include other metals such as cobalt, copper, and aluminium, which can be added without disturbing the regular orientation of the nickel iron crystals.

What we claim is:

1. The process of providing a magnetic material of low hysteresis losses comprising the steps of cold rolling a band consisting of substantially equal parts of nickel and iron from a thickness of about 1 mm. to a thickness of about 110 microns, recrystallizing said band by heat treatment at a temperature of abolt 1100 C., and cold rolling said band down to microns.

2. The process of providing a magnetic material of low hysteresis losses comprising the steps of coldrolling a band consisting of substantially equal parts of nickel and iron from a thickness of about 1 mm. to a thickness of about 110 microns, recrystallizing said band by heat treatment at a temperature of about 1100 0., cold rolling said band down to 60 microns,and partly removing the internal stresses from said band by heat treatment at a temperature of about 400 C.

3. The process of producing a. magnetic material of substantially constant permeability and low hysteresis losses comprising the steps of, cold-rolling a band of a nickel-iron alloy in the middle percentage range to reduce its thickness at least 90%, heat-treating the band above the recrystallizing temperature, said cold-rolling and heat-treating bringing most of the crystals into substantially the same orientation and producing a preferred direction of magnetization coinciding with the rolling direction, and subsequently cold- Y rolling said band to internally stress the same and to suppress said preferred direction of magnetization.

4. The process of producing a. magnetic material of substantially constant permeability and low hysteresis losses comprising the steps of, coldrolling a nickel-iron alloy band of substantially equal parts of nickel and iron to reduce its thickness at least 90%, heat-treating the band above the recrystallizing temperature of the alloy, said cold-rolling and heat-treating bringing most of the crystals into substantially the same orientation with two cube edges lying in the band-plane, one edge extending in the rolling direction and the other edge normal thereto, and subsequently cold-rolling the band to suppress the preferred direction of magnetization extending in the rolling direction.

5. The process of producing a magnetic material of substantially constant permeability and low hysteresis losses comprising the steps of, cold-rolling a band of nickel-iron alloy in the middle percentage ranges to reduce its thickness at least 90%, heat-treating the band above the rccrystallizing temperature, said cold-rolling and heating bringing most of the crystals into substantially the same orientation and producing a preferred direction of magnetization coinciding with the rolling direction, cold-rolling the band to internally stress the same and to suppress said preferred direction of magnetization, and heattreating the band to partly remove the internal stresses.

6. The process of producing a. magnetic material of substantially constant permeability and low hysteresis losses comprising the steps of, coldrolling a nickel-iron alloy band of substantially equal parts of nickel and iron to reduce its thickness at least 90%, heat-treating the band above the recrystallizing temperature of the alloy, said cold-rolling and heat-treating bringing most of the crystals into substantially the same orientation with two cube edges of each crystal lying in the band plane, one edge extending in the direction of rolling and other edge normal thereto,

cold-rolling the band to internally stress the same and to suppress the preferred direction of magnetization extending in the rolling direction, and heat-treating the band to remove part of the internal stresses.

'7. The process of producing a. homogeneous magnetic material of substantially constant permeability and low hysteresis losses, comprising the steps of cold-working without intermediate annealing a band of a nickel-iron alloy in the middle percentage range to reduce its thickness at least of the order of 90%, heat-treating the material above the recrystallizing temperature, said cold-working and heat-treating bringing most of the crystals into substantially the same orientation and producing preferred directions of magnetization, and subsequently cold-working the material to internally stress the same and to suppress one of the preferred directions of magnetization.

8. A homogeneous magnetic-core material having substantially constant permeability and low hysteresis losses produced in accordance with the process specified in claim '7.

GILES HOLST. WILLEM SIX.

. JACOB LOUIS SNOEK.

WIm-IEIM GERARD BURGERS.

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