WO2002018667A2

WO2002018667A2 - New amorphous fe-based alloys containing chromium

Info

Publication number: WO2002018667A2
Application number: PCT/IL2001/000825
Authority: WO
Inventors: Vladimir Manov; Yuri Tarakanov; Shlomo Kahana
Original assignee: A.M.T.P. Advanced Metal Production Ltd.
Priority date: 2000-09-01
Filing date: 2001-08-30
Publication date: 2002-03-07
Also published as: AU2001282495A1; WO2002018667A3; IL138203A0

Abstract

The present invention provides new amorphous Fe-based alloys of the formula FeaCrbNicMdBeSifPk , where M is at least one metal selected from the group consisting of Mn, Co, Mo, Nb, Hf, W, Zr, V and Al, where the sum a+b+c+d+e+f+k is essentially 100, wherein a ranges from about 40 to about 75 atomic %, b ranges from about 3 to about 14 atomic %, c ranges from zero to about 40% atomic %, d ranges from zero to about 20 atomic %, e ranges from about 5 to about 15 atomic %, f ranges from about 3 to about 10 atomic % and k ranges from about 0.1 to about 5 atomic %. The alloys of the invention are characterized by having low coercivity, high squareness, and high permeability, which implement their use in the production of markers in EAS systems or in the production of magnetic amplifiers.

Description

NEW AMORPHOUS Fe-BASED ALLOYS CONTAINING CHROMIUM

FIELD OF THE INVENTION

This invention relates to amorphous alloys for use in the production of magnetic markers or magnetic amplifiers. The present invention also relates to an electronic article surveillance system employing such a magnetic marker.

BACKGROUND OF THE INVENTION

Anti-theft systems, also called electronic article surveillance (EAS) systems, are used in a wide variety of settings, including supermarkets, shopping malls and libraries. They typically consist of one or two columns placed opposite each other near entrances and exits. Several technologies are currently on the market, but generally an electromagnetic detection field is produced between the two columns and an alarm sounds if an article with a special marker or tag is carried between the columns. When a legitimate purchase of the article is made, the marker can either be removed from the article, or converted from an activated state to a deactivated state.

One type of EAS system is known as a harmonic system. In such a system, a marker is formed of an amorphous soft magnetic ribbon and have a width of 1-2 mm and a length of 5-12 cm. When the magnetic marker passes through the electromagnetic field, it disturbs the field and causes harmonics of the frequency of an incident field. The detection system is tuned to detect certain harmonic frequencies. If such harmonic frequencies are detected, an alarm is triggered. A system of this type is disclosed, for example, in U.S. Pat. No. 4,484,184. A large number of alloy compositions are known in the amorphous metal field in general, and a large number of amorphous alloy compositions have also been proposed for use in electronic article surveillance systems.

Many alloy compositions contain relatively large amounts of cobalt, see for example the compositions described in US 5,841,348 and US 5,037,494. However, a substantial disadvantage of these alloys is their high cost because cobalt is the most expensive raw material used in the formation of the alloys.

During the last decade the sensitivity of the detecting means has increased. At the same time, it is a common goal of marker designing techniques to decrease the marker dimensions and to enhance the uniqueness of its response. However, the reduction of the marker length below 35 mm, when the marker is formed of Co-based amorphous alloys, may lead to a drastic deterioration in the marker properties, due to the effect of demagnetization field. In view of that, and in view of the high cost of cobalt, the use of Co-based amorphous alloys for the fabrication of markers is at times unfavorable.

Alloys with low cobalt content, or totally without cobalt, for use in magnetomechanical electronic article surveillance system, are described in US 6,018,296.

In recent years, switching power sources having magnetic amplifiers have been widely used. The main component of a magnetic amplifier is a saturable reactor (i.e. an inductor in which the core is saturable by turns carrying d.c.) which has to be produced from magnetic materials having good squareness and magnetization characteristics at high frequencies. However, magnetic alloys known in the art have various drawbacks which make their use in the production of magnetic amplifiers unfavorable. For example, Fe-Ni based crystalline alloys or Fe-based amorphous magnetic alloys have large core loss and thus are not suitable for use in high frequency applications, while Co-based alloys have a high cost. SUMMARY OF THE INVENTION

There is accordingly a need for alloys with low cobalt content or cobalt-free, which can serve to fabricate a short magnetic marker or a magnetic amplifier and which have, correspondingly, a significant reduced price. It is thus a major object of the present invention to provide such an alloy for use in magnetic markers employed in EAS systems. It is a further object of the present invention to provide EAS system employing such markers.

It is yet another object of the present invention to provide a magnetic alloy that has saturable magnetic properties suitable for application in high frequency regions when used as a saturable reactor for magnetic amplifiers.

The new alloys of the present invention are amorphous and have magnetic characteristics such as low coercivity, high squareness, and high permeability, which implement their use in the production of markers in EAS systems or in the production of magnetic amplifiers. There is thus provided according to one aspect of the invention, an amorphous Fe-based alloy of the formula Fe_aCrbNicMdB_eSifPk , where M is at least one metal selected from the group consisting of Mn, Co, Mo, Nb, Hf, W, Zr, V and

Al, where the sum a+b+c+d+e+f+k is essentially 100, wherein a ranges from about

40 to about 75 atomic %, b ranges from about 3 to about 14 atomic %, c ranges from zero to about 40% atomic %, d ranges from zero to about 20 atomic %, e ranges from about 5 to about 15 atomic %, f ranges from about 3 to about 10 atomic % and k ranges from about 0.1 to about 5 atomic %. The alloys of the invention are characterized by having substantially low coercivity (less than 10

A/m), high permeability (higher than 2x10⁴) and high squareness ratio (higher than 0.7). Preferably, b ranges from about 7.5 to about 9.5 atomic %.

It is another aspect of the present invention to provide a magnetic marker for use in EAS systems, such marker comprising at least one strip formed of the amorphous Fe-based alloy of the invention.

According to yet another aspect of the invention, there is provided an EAS system utilizing a marker mounted within an article to be detected by the system when entering an interrogation zone, such system comprising an interrogation zone, means for generating an alternating magnetic field within the interrogation zone, a magnetic field receiving coil, a signal processing unit and an alarm device.

BRIEF DESCRD?TION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. 1 illustrates a schematic block diagram of a conventional EAS system; Fig. 2 illustrates a hysteresis loop B(H) characteristic of the marker of the present invention.

Fig. 3 illustrates a hysteresis loop B(H) characteristic of a long (9 cm) marker made of a cobalt containing alloy.

Fig. 4 illustrates a hysteresis loop B(H) characteristic of a short (3 cm) marker made of a cobalt containing alloy.

Fig. 5 illustrates a device for the production of a Fe-Cr alloy of the invention.

Fig. 6 illustrates a device for the production of amorphous ribbons.

Fig. 7 block diagram of a magnetic amplifier arrangement Fig. 8 illustrates a hysteresis loop B(H) characteristic of a saturable reactor made of the alloy of the invention

DETAILED DESCRIPTION OF THE INVENTION

A typical diagram of the main components included in an EAS system is shown in Fig. 1. The generator 1 provides an alternating current of frequency f to a transmitting coil 2 which creates an alternating magnetic field in a interrogation zone 3. This magnetic field causes the re-magnetization of the marker 4 which demonstrates substantially non-linear B(H) characteristics, i.e. while re-magnetizing, it generates a magnetic field harmonically related to the frequency f. In general, as the non-linearity of the loop of the magnetic material increases, more harmonics are generated. The signal induced in the receiving coil 5 passes through the filter 6, which subtracts from the signal the component f and allows the passage of the components which occur due to the re-magnetization of the marker. Consequently, the signal at the filter output indicates the appearance of the marker in the interrogation zone thus activating the alarm device 7.

The amorphous Fe-based alloys of the invention used for marker fabrication have the following characteristics which implement their use in the production of markers in EAS systems: low coercivity, high squareness and high permeability. These new alloys have the formula Fe_aCrbNicMdB_eSifPk , where M is at least one metal selected from the group consisting of Mn, Co, Mo, Nb, Hf, W, Zr, V and Al. The sum a+b+c+d+e+f+k is essentially 100, wherein a ranges from about 40 to about 75 atomic %, b ranges from about 3 to about 14 atomic %, c ranges from zero to about 40% atomic %, d ranges from zero to about 20 atomic %, e ranges from about 5 to about 15 atomic %, f ranges from about 3 to about 10 atomic % and k ranges from about 0.1 to about 5 atomic %. Preferably, the chromium content is between about 7.5 and 9.5 atomic percent.

According to a preferred embodiment of the invention, the marker is in the form of a strip. It is cut from amorphous 25-35 micron thick ribbon formed from the alloy of the invention by well known melt spinning quench technique, such as that described in US 4,142,571.

The magnetic marker in accordance with the present invention demonstrates the following characteristics: coercive force 5-lOA/m, magnetic permeability 3-5 10 and squareness ratio 0.9. The B(H) characteristic of the proposed material is shown in Fig. 2. This characteristic does not depend on the marker's length and demonstrates a sharp turn at a magnetic field of 10 A/m for a 3 cm long marker. The demagnetization factor has little effect on the characteristics of the marker, since the magnetization of the proposed material is relatively low. So the low magnetization which seems to be unfavorable for long markers turns out to be an advantage for short markers below 35 mm. The B(H) characteristic of a long (9 cm) Co-based marker is presented in

Fig. 3 which shows that the B(H) loop has a sharp turn at a magnetic field of 10

A/m. The B(H) characteristic of a short (3 cm) Co-based marker is comparatively presented in Fig. 4 which shows the effect of the demagnetization factor on the loop: it reaches the turn at much higher magnetic field, i.e. at about 30 A/m.

Magnetic amplifiers are used in switching power sources, control and automatic systems. A method of processing the magnetic cores used in the production of magnetic amplifiers comprises the following steps: heating the core to a temperature of 360-430°C and holding it at this temperature for 0.5-3 hours under magnetic field (H>800A/m), slowly cooling down (0.1-10.0°C/min) to a temperature lower than Curie temperature by 10°C and subsequent cooling to room temperature, either in the air or under inert atmosphere.

The invention will be described in more detail with the aid of the following examples.

Example 1: Preparation of the alloys and ribbon thereof will be described herein below with reference to Figures 5 and 6.

The alloys of the invention can be prepared from the following raw materials: iron with a low carbon content, pure chromium, nickel, cobalt, at least one pure metal selected from Mn, Mo, Nb, Hf, W, Zr, V and Al, ferroboron alloy, ferrophosphorus alloy and ferrosilicon alloy or pure silicon. The raw materials are charged in crucible 31 shown schematically in Fig. 5. 31 is made of a heat-resistant material, for example alumina (i.e. A1₂0₃). Alumina has a melting point temperature of about 2100°C and is chemically stable, thus being capable to hold the alloy in liquid form. Induction coil 32 is used for heating and melting the raw materials, to produce a molten, pourable alloy 20. The crucible 31 is held in the container 33. The alloy preparation is carried out under vacuum or inert atmosphere in order to prevent undesired reactions between the liquid metals and oxygen from the atmosphere. An inert gas such as argon is inserted into the container 33 through inlet 34. After heating, a molten alloy having a temperature of about 1500°C is obtained and it is kept at this temperature for 10-15 minutes before it is poured into ingot molds.

The ingots thus obtained are further processed by the single roll melt-quenching technique into amorphous ribbons, as shown schematically in Fig. 6. The alloy ingots are crushed and inserted into heating container 41. The container is heated and the alloy is brought to a temperature above its melting point by using the induction coil 42. Then the liquid alloy 43 is poured onto a rotating drum 51 made of copper. The drum may be cooled during the process. The container 41 is equipped with a cover 44 which enables, at will, to carry out the process in an inert atmosphere.

The liquid melted alloy is poured through outlet 45 onto the rotating drum 51. The liquid melt undergoes rapid cooling and solidification on contact with 51, to be transformed into a solid, thin ribbon (not shown). A knife 46 separates the solid ribbon from the drum.

The ribbons produced by the above process have a thickness of about 25-35 microns and a width of about 10 mm. However, ribbons having various dimensions may be fabricated.

Example 2

Strips of ferromagnetic Fe-Cr based alloys having different compositions (Table 1) were tested in the "Esselte METO" security system. Each marker was passed between antennas at different angles between the marker's axis and the vertical direction as this angle indicates the quality of the marker. The less the angle, the higher the quality of the marker. The results are shown in Table 1. For comparison, we presented also the results obtained for the conventional "Meto" cobalt-based marker having the same dimensions. Table 1

As seen from the above Table 1, the markers of the present invention demonstrate quality comparable with the cobalt based one and consequently may be used in anti-theft electronic article surveillance system. Furthermore, they have a lower cost in comparison to markers made of alloys which contain cobalt (at least twice a lower cost ) and the alloys have characteristics which implement their use in the formation of miniature and flexible markers.

Example 3

A magnetic amplifier is a device in which the saturation properties of magnetic material are utilized to modulate an exciting alternating current, using an applied signal as bias. The signal output when rectified becomes a magnification of the input signal, as shown in Fig. 7. Magnetic amplifiers are used in switching power sources, control and automatic systems.

Magnetic cores (or a saturable reactor) were produced by winding an amorphous alloy ribbon prepared according to the procedure in Example 1. The cores had the following dimensions: outside diameter 18 mm; inside diameter 13 mm; height 12 mm. The cores were annealed in longitudinal magnetic field

(H=2000 A/m) at 390°C for 40 minutes. The resulting cores were tested at 60 kHz and 100 kHz and the results are shown in Fig.8 and Table 2. As seen from Table 2 the cores obtained by using a ribbon made of the alloys of the invention have excellent squareness ratio (Br/Bs) and low coercivity (He) and therefore low core loss.

Table 2

* He - coercivity

**Bs - saturation induction; Br - remanent induction

10 Those skilled in the art will readily appreciate that various modifications and changes can be applied to the preferred embodiments of the present invention as hereinbefore exemplified, without departing from its scope defined in and by the appended claims.

Claims

CLAIMS:

1. An amorphous Fe-based alloy of the formula Fe_aCrbNicMdB_eSifPk, where M is at least one metal selected from the group consisting of Mn, Co, Mo, Nb, Hf, W, Zr, V and Al, where the sum a+b+c+d+e+f+k is essentially 100, wherein a ranges from about 40 to about 75 atomic %, b ranges from about 3 to about 14 atomic %, c ranges from zero to about 40% atomic %, d ranges from zero to about 20 atomic %, e ranges from about 5 to about 15 atomic %, f ranges from about 3 to about 10 atomic % and k ranges from about 0.1 to about 5 atomic %, said alloy having substantially low coercivity, high permeability and high squareness ratio.

2. An alloy according to claim 1 having a coercivity lower than 10 A/m, permeability higher than 2x10 and squareness ratio higher than 0.7.

3. An alloy according to claim 1 wherein b ranges between about 7.5 to about 9.5 atomic %.

4. An alloy according to claim 1 , wherein M is Mo. 5. An alloy according to claim 1 having the formula Fe ₇Cr9.₅Ni24MoιBι₄Si₄Po.

5.

6. A magnetic marker for use in an electronic article surveillance system, the marker comprising at least one strip formed of the alloy of claim 1.

7. A magnetic marker for use in an electronic article surveillance system, the marker comprising at least one strip formed of the alloy of claim 5.

8. An electronic article surveillance system utilizing a marker mounted within an article to be detected by the system when entering an interrogation zone, the system comprising an interrogation zone, means for generating an alternating magnetic field within said interrogation zone, a magnetic field receiving coil, a signal processing unit and an alarm device, wherein said marker comprises at least one strip formed of an amorphous Fe-based alloy of the formula Fe_aCrbNicMdB_eSifP_k , where M is at least one metal selected from the group consisting of Mn, Co, Mo, Nb, Hf, W, Zr, V and Al, where the sum a+b+c+d+e+f+k is essentially 100, wherein a ranges from about 40 to about 75 atomic %, b ranges from about 7 to about 14 atomic %, c ranges firom zero to about 40% atomic %, d ranges from zero to about 20 atomic %, e ranges from about 5 to about 15 atomic %, f ranges from about 3 to about 10 atomic % and k ranges from about 0.1 to about 5 atomic %, said alloy having substantially low coercivity, high permeability and high squareness ratio.

9. A saturable reactor formed of the alloy of claim 1.

10. Magnetic amplifier comprising a saturable reactor made of the alloy of claim 1.

11. A method of processing magnetic cores used in the production of a magnetic amplifier according to claim 10, comprising heating the cores to a temperature above 300°C under magnetic field for a period of between about 30 minutes to about 3 hours, and slowly cooling down to room temperature at a predetermined rate.