EP0557689A2 - Method for manufacturing a magnetic pulse generator - Google Patents

Abstract

In order to produce a pulse generator, in the case of which a voltage pulse, which is independent of the magnetic field change, can be produced by sudden reversal of the magnetisation (Barkhausen effect) when a magnetic field is applied, an iron alloy 2 is used for one of the materials of the composite body, the additional alloying components of which alloy are selected such that a structural transformation takes place, with a volume change, at different temperatures in each case. In order to produce the braced state, heat treatment is then carried out, which includes heating above the upper transition temperature and cooling below the lower transition temperature. This results in considerably higher stresses between the materials of the composite body and hence a pulse behaviour which is considerably better than that known and which can be used for identifying constant or alternating magnetic fields. <IMAGE>

Description

The invention relates to a method for producing a pulse generator acting by sudden magnetic reversal when a magnetic field is applied, which consists of an elongated composite body made of at least two materials which have different thermal expansion behavior and are mechanically braced against one another by a heat treatment.

Such a pulse generator as a composite body is described in DE-PS 31 52 008. This composite body contains a core and a shell, the materials of which may partially or all of magnetic materials with different coercive field strength. When using two magnetic materials with different coercive field strength, an alloy in the range of 45 to 55% by weight of cobalt, 30 to 50% by weight of iron and 4 to 14% by weight (chromium + vanadium) is used for the magnetically harder material. used, while nickel is provided as the soft magnetic material. Here, by installing a material component with shape memory or by using materials with different thermal expansion coefficients by heat treatment, a certain stress state is created, which results in a sudden magnetic reversal in the tensioned soft magnetic component of the composite body when exposed to an external magnetic field.

This known composite body is in the form of an elongated magnetic switching core.

It is also known from DE-OS 29 33 337 to use a composite body consisting of nickel or unalloyed steel as the bracing component and a cobalt-vanadium-iron alloy as the magnetically active switching component. A heat treatment is carried out during manufacture. First, the wire, from which the composite body is preferably made, is heated to such an extent that a material component plastically deforms under the stresses that arise, so that these stresses are largely reduced. With subsequent cooling, the different coefficients of thermal expansion in turn cause mechanical stresses which, due to the lower temperature, no longer lead to plastic deformation and which, due to their magnetostriction, result in the sudden magnetic reversal in the magnetically active component when a specific magnetic field is applied.

An elongated composite body with a low response field strength of 1.0 Oe (about 0.8 A / cm) is also described in US Pat. No. 4,660,025. For example, an elongated, 7.6 cm long wire made of amorphous material is used here and it is stated that the length of this wire can be between 2.5 and 10 cm. Here, the internal stresses resulting from the quenching of the material during the production of the amorphous state are the cause of the magnetic jumping behavior.

In DE-OS 34 11 079 a combination of hard and soft magnetic alloys is used to produce the composite body. From DE-PS 31 52 008 it is known that the hard magnetic component serve at the same time for bracing the soft magnetic component can. This construction has the advantage that a wire with a sheath of high strength can be obtained and that relatively short wires can be provided.

By magnetizing the hard magnetic jacket of a composite body, the magnetization characteristic curve shifts, so that demagnetization zones at the edge of the strip are largely avoided by the flow in the hard magnetic envelope, with the result that a sudden magnetic reversal (Barkhausensprung) occurs in one direction , while this is missing in the case of magnetic reversal in the other direction. Much shorter switching cores can be used here, since the permanent magnet largely prevents demagnetization zones at the ends of the wire (pulse generator).

The object of the present invention is to provide a method for producing such a pulse generator which, without additional process steps between the materials of the composite body, results in significantly higher voltages and thus considerably higher voltage pulses in the event of sudden magnetic reversal of the active component. The invention further solves the problem of realizing, in addition to the improved pulse behavior, a premagnetization of the magnetically active part of the composite body with sufficient coercive field strength, without the need to provide an additional strip of permanent magnetic material.

This object is achieved in that an iron alloy is used for one of the materials, the additional alloy components of which are selected such that a microstructure transformation with volume change takes place at different temperatures, that an elongated composite body is produced from the materials and that this composite body is used as a heat treatment first heated above the upper conversion temperature and later cooled below the lower conversion temperature.

A structural change with volume change is, for example, a change in the crystal structure through phase change e.g. from the alpha phase (body-centered cubic grid) to the gamma phase (face-centered cubic grid) or into the epsilon phase (hexagonal grid) and vice versa.

Advantageous developments of the invention are described in the subclaims.

Fig. 1

zeigt einen drahtförmigen Impulsgeber im Schnitt;

Fig. 2

stellt die Magnetisierungskurve bei kleiner Aussteuerung und nicht magnetisiertem hartmagnetischem Mantel dar;

Fig. 3

zeigt die Magnetisierungskennlinie bei voller Aussteuerung, bei der auch der Mantel des Impulsgebers nach Fig. 1 ummagnetisiert wird;

Fig. 4

zeigt die Magnetisierungskennlinie eines erheblich verkürzten Impulsgebers mit und ohne aufmagnetisiertem Mantel;

Fig. 5

zeigt den damit erzielbaren Spannungsimpuls bei Ummagnetisierung des weichmagnetischen Kerns;

Fig. 6

vergleicht den bei nicht aufmagnetisiertem Mantel erzielten Impuls mit dem bei einem amorphen Draht, der innere Verspannungen aufweist.

An embodiment is shown with reference to FIGS. 1 to 6 including the mode of operation of the pulse generator produced by the method according to the invention.

Fig. 1

shows a wire-shaped pulse generator in section;

Fig. 2

represents the magnetization curve with low modulation and non-magnetized hard magnetic jacket;

Fig. 3

shows the magnetization characteristic at full modulation, in which the jacket of the pulse generator according to Fig. 1 is remagnetized;

Fig. 4

shows the magnetization characteristic of a significantly shortened pulse generator with and without a magnetized jacket;

Fig. 5

shows the voltage pulse that can be achieved when magnetizing the soft magnetic core;

Fig. 6

compares the impulse achieved with a non-magnetized sheath with that with an amorphous wire with internal tension.

A wire is shown in FIG. 1 as a composite body, the core of which consists of a soft magnetic material 1 and the jacket of which is made of an iron alloy 2. The coercive force of the iron alloy 2 is higher than that of the soft magnetic material 1. In the exemplary embodiment, the soft magnetic material 1 consists of an alloy with 75.5 Ni 2.9 Mo 3.0 Ti 1.0 Nb rest Fe. In this alloy, the Ti and the Nb serve as a hardening additive to prevent the plastic magnetic material from being deformed too easily. This soft magnetic material has a magnetostriction greater than zero, i. H. the material expands in the direction of magnetization. For this reason, the desired jumping behavior is achieved when the soft magnetic material 1 is under tension in the finished pulse generator.

In order to achieve this tensile stress to a much greater extent than in the case of known composite bodies, the jacket is produced from an iron alloy, which undergoes structural changes at different temperatures. In the exemplary embodiment, a martensitic hardening steel with the composition 17Cr 4Ni 4Cu 0.4Nb rest iron was selected. This is a martensitic hardening commercial steel, as it is known for example under the name ARMCO 17-4PH. Please refer to the "PRODUCT DATA" brochure from Armco Steel Corporation, Baltimore, Maryland, No. S-6c. This iron alloy has - like many other known steels - structural transformation points between the so-called alpha and gamma structure. The temperature behavior is shown on page 11 of the brochure mentioned. It can be seen from this diagram that during the heating process there is initially a continuous increase in volume up to a temperature of approximately 620 ° C .; from then on the structural transformation begins, which involves a volume reduction up to a temperature of about 660 ° C. From here the volume - and thus the length of the jacket according to FIG. 1 - increases further without any further conversion or other abnormality occurring.

After this iron alloy has been heated above the upper transition temperature, the alloy can then be cooled again, which causes a continuous volume reduction in accordance with the broken line to a temperature of below 200 ° C. This is where a back transformation of the structure begins, which is used in known steels in order to harden the steel. The resulting martensite (alpha phase) means that the volume does not decrease further as it cools further, but in contrast expands here, like the dashed curve (Product Data, Armco 17-4 PH, page 11) ) in the range from 300 to 100 ° C.

According to the invention, this behavior is used to produce a pulse generator that achieves a particularly high mechanical tensioning of the component of a composite body, which is to undergo sudden magnetic reversal (Barkhausensprung) in a specific magnetic field. For this purpose, the composite body 3 in the exemplary embodiment according to FIG. 1 is heated to a temperature above 750 ° C. and then cooled below 100 ° C. The consequence of this is that the soft magnetic material 1 and the iron alloy 2 initially expand approximately uniformly (depending on their coefficient of thermal expansion). When the upper transition temperature of the iron alloy is reached, the soft magnetic material tries to expand while the iron alloy shrinks or expands less. This creates a compressive stress in the soft magnetic material 1 and a tensile stress in the iron alloy 2. At the high temperature after However, this has the consequence that the mechanically much softer material of the core plastically deforms or recrystallizes, while this is at least not the case with iron alloy 2 to the same extent. It can therefore be assumed that the stresses are equalized during the heat treatment, so that at the beginning of the cooling there are no tensile or compressive stresses between the core and the jacket.

When cooling, both the volume of the soft magnetic material 1 and that of the iron alloy 2 initially decrease continuously to a temperature below 300 ° C. As with known composite bodies - depending on the different coefficients of thermal expansion of the materials for the core and jacket - certain mechanical stresses arise which are used in known pulse generators to pretension the magnetically active material, but are not essential here, although they can have a supporting effect.

If the range between 300 and 100 ° C is run through during the cooling process, the martensitic transformation of the iron alloy 2 causes it to suddenly try to expand strongly while the core made of soft magnetic material 1 wants to continue shrinking. The consequence of this is that a considerable tensile stress acts on the core and a corresponding compressive stress acts on the jacket. The mechanical hardness of the core made of a soft magnetic material 1 is now chosen so that at this relatively low temperature there is no significant plastic deformation, so that high elastic tensile stresses act in the core. In connection with the positive magnetostriction of the soft magnetic material 1, these cause a much faster, suddenly occurring remagnetization certain magnetic field values than is the case with less prestressed composite bodies in known pulse generators.

Instead of the steel with martensitic transformation chosen as an example in FIG. 1, all other iron alloys which undergo a corresponding transformation can also be used. For example, "RADEX-RUNDSCHAU" 1972, H. 3/4 from page 212 onwards describes "An extra-strong maraging steel with 250 kp / mm² tensile strength". The word "maraging" means "martensitic aging hardening" and indicates that these structural changes were used in the known prior art to harden the material in order to obtain particularly strong steels for mechanical applications. On page 216 in Figure 9 of this reference, the temperature profile of one of the steels described is shown and shows that here too the structural changes lead to an increase in volume after cooling with sufficient cooling between 200 and 130 ° C, which tensions positive magnetostrictive ones soft magnetic materials can be used in a pulse generator.

In order to take advantage of the change in volume during structural transformation of iron alloys to brace a soft magnetic material, it is not absolutely necessary to choose alloys that do not show any further decrease in volume when cooled and at a relatively low temperature, but even show an increase in volume in a certain temperature range. It is sufficient if the normal decrease in volume when cooling changes during the structural change. After cooling below the lower transition temperature, subsequent heating becomes below the upper transformation temperature does not result in a structural change, so that the mechanical stresses generated by the structural transformation are retained.

In addition, compressive stresses can be generated in a soft magnetic material if an iron alloy is used for the bracing, the volume of which decreases when it cools below the lower transformation temperature. This is known, for example, in the case of austenitic manganese steels in which there is no gamma-alpha conversion, but a gamma-epsilon conversion. This conversion behavior is described, for example, in the Zeitschrift für Metallkunde Volume 56, 1965 Issue 3, from page 165. Figure 3 on page 167 of this magazine shows the change in length in an iron alloy, which essentially contains 16.4% Mn in addition to iron. The composition is given on page 166, left column. From Figure 3 it can be seen that when heating (arrow to the top right) there is again a continuous increase in volume or length, which increases during the conversion between approximately 220 and 280 ° C.

If one uses a composite body with this material to produce a pulse generator, the heat treatment would in turn heat the composite material above this transition temperature to such an extent that tension compensation again takes place through plastic deformation or through recrystallization. Cooling would then cause the material to contract much more during the reverse conversion between 100 and 20 ° C than is the case with magnetic material 1, so that here - since the iron alloy shrinks more than the soft magnetic material - this soft magnetic material 1 gets under compressive stress. The iron alloys described here can therefore be used if you have a soft magnetic Wants to use material with negative magnetostriction to produce a pulse generator with sudden magnetic reversal for a given magnetic field.

It is advantageous if the lower transition temperature is below approximately 600 ° C., since this ensures that the stresses introduced are not reduced by relaxation processes or plastic deformation.

It is also possible to use iron alloys where the lower transition temperature is below room temperature. In order to produce a composite body with good tension using such a material, it must be cooled below this transition temperature at least briefly. If the material then warms up again to room temperature, but does not reach the upper transition temperature, the bracing is maintained, since it behaves similarly to the material of the braced soft magnetic material when the temperature changes.

Such alloys are described in the magazine "METALLURGICAL REVIEWS" 126 from page 115. The diagram in Fig. 4 on page 118 shows that the lower transformation temperature in the case of an iron alloy with 29.7% Ni and 6% Al after aging annealing at 700 ° C is initially below the room temperature depending on the time of this annealing. From the figure mentioned, however, it can be seen that if the treatment time is sufficiently long, for example at 700 ° C., the lower transition temperature is also above room temperature.

In the example mentioned at the beginning with high tension of the soft magnetic material 1, a very good, extremely rectangular magnetization curve is then achieved, as shown in FIG. 2. Here's on the ordinate like Induction is usually shown and the field strength on the abscissa is in the range of ± 0.8 A / cm. In this modulation range, the magnetization of the iron alloy 2 remains essentially unchanged; and the magnetization jump of the soft magnetic material 1 is triggered at approximately ± 0.2 A / cm.

A corresponding magnetization curve is shown in FIG. 3. Here the field strength control was changed between ± 80 A / cm. A field strength that is sufficient to completely re-magnetize the iron alloy used here as a jacket. Here you can see the jump in induction at about the field strength 0, which is caused by the sudden magnetic reversal of the pre-stressed soft magnetic material 1 and you can see that the iron alloy used to brace the soft magnetic material 1 has a coercive force of 39 A / cm, like this The dashed curve in FIG. 3 shows the hysteresis loop of the iron alloy under compressive stress. This dashed curve was determined by parallel displacement of the measured curve of the composite body.

A comparison with the product brochure "PRODUCT DATA ARMCO 17-4 PH" on page 12 shows that the iron alloy used here in the example normally has a coercive field strength of ± 20 Oe = ± 16 A / cm. This substantial increase in the coercive field strength of the iron alloy compared to the value usually measured on the material is probably due to the briefly high heating of the material in connection with the compressive stresses which it experiences as part of the composite body in response to the tensile stress of the soft magnetic material. This shows another essential advantage of using iron alloys in connection with a heat treatment, which changes the structure with volume change to brace the exploits soft magnetic material, since an additional permanent magnet does not now have to be provided to generate sufficient premagnetization of the composite body.

This additional bias is advantageous and necessary if you want to use short wires as a pulse generator. With relatively short wires, the own demagnetizing field is particularly noticeable, as described in detail in DE-OS 34 11 079. In the case of the composite body according to FIG. 1, the length selected when measuring the hysteresis loops according to FIGS. 2 and 3 was therefore shortened from 90 mm to 20 mm, and the hysteresis loop was again measured. This is shown in FIG. 4. It can be seen from the dashed curve (measurement with demagnetized sheath made of iron alloy 2) that the rectangular curve shown in FIG. 2 is somewhat sheared by the edge effects. So there is no sudden magnetic reversal of the core.

If, however, the iron alloy is magnetized, the solid curve in FIG. 4 is obtained, which is shifted horizontally on the one hand by the influence of the magnetic field of the iron alloy 2 and which on the other hand shows that when it passes through in one direction, the entire soft magnetic material 1 is suddenly magnetized again takes place because when the hysteresis loops pass in this direction, the wire ends of the soft magnetic material maintain their magnetization direction under the influence of the magnetic field of the iron alloy 2 until the external magnetic field forces the sudden magnetic reversal of the soft magnetic material 1.

5, the voltage is now plotted on the ordinate and the time in microseconds on the abscissa. Here a composite wire with a length of 20 mm was surrounded by a winding with 1000 turns. The remagnetization was carried out by an alternating current at 50 Hz in a separate excitation coil, which was set so that the field strength along the composite wire was 5 A / cm. It can be seen that a voltage pulse of approximately 0.95 V can be achieved, which, however, occurs only in every second half-wave due to the asymmetry of the hysteresis loop in the case of magnetized iron alloy.

Fig. 6 now shows the voltage pulse of the composite body according to Fig. 1 with a diameter of 0.2 mm and a length of 90 mm in a coil of 1500 turns and also 90 mm in length after heating the composite body for 6 seconds to 1100 ° C and subsequent cooling. In this state, the composite body can be operated with a small modulation of, for example, 0.8 A / cm, since the core has a low coercive force of approximately 0.1 A / cm. The impulse achieved here with magnetized iron alloy 2 is compared in FIG. 6 with the amorphous wire according to US Pat. No. 4,660,025. Curve 4 shows the voltage pulse of the amorphous wire and curve 5 shows the voltage pulse which results from the pulse generator produced according to the invention.

Although in the exemplary embodiment the iron alloy is used as a sheath and the soft magnetic material as the core of a wire, other materials can be used by plating etc., as is known. Flat, elongated composite bodies are obtained particularly advantageously by rolling the finished wire before the heat treatment. Using the iron alloy as a sheath offers the advantage of having a solid surface receives. In principle, however, it is also possible to use the iron alloy as the core of a wire or as the middle layer of a flat composite body.

In the event that an even higher coercive field strength of the iron alloy or a further improvement in strength is desired, the finished composite wire can be annealed for at least 10 minutes at a temperature of 360 to 750 ° C. after the heat treatment according to the invention. With the improvement in strength of the iron alloy achieved in this way, a further increasing coercive field strength is then obtained. In addition to the strength-increasing additives contained in the soft magnetic material 1 of the exemplary embodiment, the elements Nb, Ti, Al, Cu, Be, Mo, V, Zr, Si, Cr, Mn can be used to increase the strength and / or to improve the corrosion resistance advantageously add to the iron alloy without its properties-reversible structural changes at different temperatures being significantly affected with volume changes.

Since the composite body is only required to be heated for a short time, the entire wire or the strip from which the composite body is produced does not necessarily have to be subjected to the stationary heat treatment; heating can also be carried out as continuous annealing or by passing electrical current through it.

Claims (10)

Process for producing a pulse generator which acts by sudden magnetic reversal when a magnetic field is applied and which consists of an elongated composite body made of at least two materials which have different thermal expansion behavior and are mechanically braced against one another by heat treatment, characterized in that an iron alloy (2) is used for one of the materials. is used, the additional alloy components of which are selected so that at different temperatures a structural change with volume change takes place, that an elongated composite body (3) is produced from the materials and that this composite body is heated as a heat treatment only above the upper transition temperature and later below the lower one Cooling temperature is cooled. Process according to Claim 1, characterized in that an iron-based alloy is used, the lower transition temperature of which is below 600 ° C. A method according to claim 1, characterized in that a martensitic hardening steel is used as the iron alloy, which expands during the structural transformation upon cooling. A method according to claim 1, characterized in that the elongate composite body is produced by pulling a wire core with the sheath surrounding the core. A method according to claim 4, characterized in that for the production of the composite body by pulling the wire core from soft magnetic material (1) and the sheath from the iron alloy (2). Method according to Claim 4, characterized in that the heat treatment is carried out by briefly heating the composite body to a temperature which is so far above the upper transition temperature of the iron alloy (2) that the internal stresses are reduced by recrystallization of the soft magnetic material (1). A method according to claim 4, characterized in that the heat treatment of the wire is carried out in the form of a continuous annealing. A method according to claim 4, characterized in that the heat treatment of the wire is carried out by brief heating by passing electrical current through it. Process according to claim 4, characterized in that the finished composite wire is annealed for at least 10 minutes in order to increase the strength of the iron alloy in connection with an increase in the coercive field strength at a temperature between 360 and 750 ° C. A method according to claim 1, characterized in that an iron alloy (2) is used which, to increase the strength and / or the corrosion resistance, additives such as Nb, Ti, Al, Be, Cu, Mo, V, Zr, Si, Cr, Mn contains.

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