Method of Improving Magnetic Devices by Applying AC or Pulsed Current

The present invention relates to a method for improving the magnetic properties of a magnetic material, and more particularly relates to a method for improving the magnetic properties of ferromagnetic amorphous alloys by applying AC current or pulsed current.

Ferromagnetic amorphous alloys have been widely used in many magnetic applications such as distribution transformers, DC power supplies, motors, current amplifiers, magnetic shielding, etc. Fe-base amorphous alloys will produce an annealing embrittlement after the conventional furnace annealing. This is a serious problem in a certain applications.

In the past, efforts have been made to find new magnetic materials suitable for many applications with better magnetic properties such as higher magnetic induction (Bm), lower coercivity (Hc), and therefore low core loss when the transformer core is made of such materials. For ferromagnetic materials used in the past for the manufacture of transformer cores, it is very difficult to change their magnetic properties in operation.

It is therefore the main object of the present invention to provide a method for improving the magnetic properties of the ferromagnetic amorphous alloys.

An important feature of the present invention is the step of applying an AC current or pulsed current to the ferromagnetic amorphous alloys during the magnetization of the alloys to increase the maximum value of the magnetic induction (Bm) and decrease the minimum value of the coercivity (Hc).

The AC current is originated from an AC power supply and fed into the specimen of the ferromagnetic materials by directly connecting to a pair of electrodes thereof. It is believed that the current passing the ferromagnetic material causes the domain wall in the material to shift in responsive to the current density and frequency. Therefore, the soft magnetic properties of the ferromagnetic materials are improved. The method of the present invention further comprises a step of applying an AC current or pulsed current to a specimen of alloy which has been treated by AC Joule heating or pulsed high current heating process. This amorphous alloy will not have annealing embrittlement during annealing process. The AC Joule heating or pulsed high current processes for improving the magnetic properties and annealing embrittlement of the alloy is invented by the same inventors of this subject invention and is detailed in co-pending application Ser. No. 338,895, now abandoned.

The applied AC current or pulsed current has a frequency ranged from 50 to 50K Hz, a current density of 10 to 500 A/cm.sup.2 and a wave form of sine wave, triangular wave or square wave.

Accordingly, the method of improving the magnetic properties of ferromagnetic amorphous alloys of the present invention comprises a first step of providing a ferromagnetic amorphous alloy specimen in a magnetizing field, a second step of applying an AC current or pulsed current passing through said specimen, and a third step of detecting and recording the magnetic induction and coercivity of said specimen during magnetization and demagnetization process.

Those and other advantages, objects and features of the method according to the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of the system for measuring B-H loop of a straight specimen according to the method of the present invention.

FIG. 2 is a schematic diagram of the system for measuring B-H loop of a toroidal specimen according to the method of the present invention.

FIG. 3 is a perspective view of an ferromagnetic amorphous alloy ribbon showing its magnetic domain structure.

FIG. 4 is a chart showing variation of magnetic induction and coercivity of a Fe.sub.78 B.sub.13 Si.sub.9 straight specimen with a 60 Hz sine wave current passing thereon.

FIG. 5 is a chart showing variation of magnetic induction and coercivity of a Fe.sub.78 B.sub.13 Si.sub.9 straight specimen carrying AC current with different frequencies.

FIG. 6 is a chart for the B-H loop of a Fe.sub.78 B.sub.13 Si.sub.9 straight specimen as-cast after, AC Joule heating and by applying AC current therethrough.

The method for improving the magnetic properties of ferromagnetic amorphous alloys by applying high AC current or pulsed current is carried out and will become apparent in the following procedure.

1. Specimens

Ferromagnetic amorphous ribbons with different compositions, especially for Fe and Ni base amorphous ribbons. Also it is suitable for all crystalline material.

Specimen shape -straight long ribbon

toroid core wound by a long ribbon

C-type, E-type or rectangular type core

In our experiments, specimen of composition Fe.sub.78 B.sub.13 Si.sub.9 were made into straight and toroidal shapes.

2. Measuring the magnetic properties with AC current or pulsed current passing through the specimen

A. Straight specimen

The straight specimen was put in the center of a uniform magnetic field (H) produced by a long solenoid coil which was connected to a DC bipolar power supply of a function generator. Both ends of the straight amorphous ribbon were clamped by two square copper plates which were connected to the output terminals of an AC power supply which is capable of producing a search coil (S) combined with a compensating coil (C) was connected to a fluxmeter (or integrator) to measure the magnetic flux density (B) of the specimen. By connecting the terminals of the applied magnetic field (H) and magnetic flux density (B) to a X-Y recorder, the B-H hysteresis loop was obtained. (FIG. 1)

B. Toroidal specimen

The toroidal specimen was made by winding a long amorphous ribbon coated with insulation materials. The two ends of the long ribbon were connected to the output terminals of the AC power supply. The toroidal core was wound by two coils, the primary coil (N.sub.1) was connected to a DC bipolar power supply or a function generator to produce the applied magnetic field (H), a and the second coil (N.sub.2) was connected to a fluxmeter (or integrator) to measure the magnetic flux density (B). Then, by connecting the terminals of H and B to a X-Y recorder, the B-H hysteresis loop was obtained. (FIG. 2)

3. Conditions of the applied AC current through the specimen.

frequency range: 50 Hz.about.50KHz

wave form: sine wave, triangular wave and square wave

current density: J=10 A/cm.sup.2 .about.5.times.10.sup.2 A/cm.sup.2

Transverse field induced by AC current or pulsed current.

Except in the vicinity of the ribbon edges, the magnetic field produced by applying a current I through a rectangular specimen is essentially transverse and varies linearly with distance from the ribbon midplane. FIG. 3 shows the cross section of amorphous ribbon and its possible magnetic domain structure.

4. Examples of improvement on the various kinds of ferromagnetic amorphous alloys resulted from the method of the invention by applying AC current passing through the specimen made of ferromagnetic materials.

Specimen: straight shape (15.24 cm.times.3.05 mm.times.25 .mu.m)

Composition: Fe.sub.78 B.sub.13 Si.sub.9

Reference magnetic properties of as-cast specimen:

When applied magnetic field: Hm=.+-.0.296 Oe

a. magnetic induction: Bm.sub.o =7.16 KG

b. coercive force: Hc.sub.o =0.074 Oe

Effects of magnetic properties under AC current passing through the specimen:

A. Dependence of AC current density

When a 60 Hz sine wave current passing through the specimen with different current density J=0.about.3.34.times.10.sup.2 A/cm.sup.2 (I=0.about.250 mA), the variations of the magnetic induction and coercivity of the specimen are shown in FIG. 4. The magnetic inductions under different current densities are almost the same which is a little higher than the value of as-cast specimen. However, the coercivity of the specimen decreases significantly as the current density increases. The decrease is slower after the current density is higher than 1.5.times.10.sup.2 A/cm.sup.2. When the current density is 3.34.times.10.sup.2 A/cm.sup.2, the coercivity will be lower than one half value of the as-cast specimen.

B. Frequency dependence

When the specimen was carrying the same AC current (current density J=1.6.times.10.sup.2 A/cm.sup.2) with different frequency (50 Hz.about.20 KHz), the variations of magnetic induction and coercivity of the specimen are shown in the FIG. 5. Also, the magnetic inductions are almost the same and a little higher than the value of as-cast specimen. The values of coercivity ratio are around 0.5 and the minimum values of coercivity are between the frequency range 100 Hz.about.1 KHz.

C. Wave form dependence

The wave form used in the AC current passing through the specimen may be sine wave, triangular wave and square wave. Under the same peak-peak current, the effect of improving the magnetic properties by square wave is the best, and the effects by sine wave and by triangular wave are almost the same. For 300 Hz current passing through the specimen, the variations of magnetic induction and coercivity when applied magnetic field is Hm=.+-.0.296 Oe are list as follows:

Specimen: toroidal specimen

Composition: Fe.sub.78 B.sub.13 Si.sub.9

A 5-layer amorphous core with diameter 3.8 cm was wound by a 60 cm long ribbon (width 7.5 cm, thickness 25 .mu.m, and weight 6.623 g)

Reference magnetic properties of as-cast specimen:

When applied magnetic field in measuring B-H loop is Hm=.+-.0.15 Oe

a. magnetic induction Bm=6.71 KG

b. coercivity Hc.sub.o =0.073 Oe

Applying 60 Hz sine wave through the core, the improved magnetic induction and coercivity of the specimen are list follows:

Specimen: straight shape (15 cm.times.3.05 mm.times.25 .mu.m)

Composition: Fe.sub.78 B.sub.13 Si.sub.9

A. As-cast specimen

When applied magnetic field in measuring B-H loop is Hm=.+-.0.292 Oe

magnetic induction Bm=7.07 KG

coercive force Hc=0.075 Oe

B. After AC Joule heating

Conditions of AC Joule heating:

frequency f=60 Hz

current density J=3.0.times.10.sup.3 A/cm.sup.2

heating time t.sub.h =50 sec

applied field Hp=100 Oe

When applied magnetic field in measuring B-H loop is Hm=.+-.0.292 Oe

magnetic induction Bm=9.70 KG

coercivity Hc=0.04 Oe

And, fracture strain .epsilon..sub.f =1 (ductility)

C. Passing AC current through the specimen after AC Joule heating

Conditions of AC current

frequency: f=300 Hz

wave form: square

current density: 1.6.times.10.sup.2 A/cm.sup.2

When applied magnetic field in measuring B-H loop is Hm=.+-.0.292 Oe

magnetic induction Bm=9.89 KG

coercivity Hc=0.017 Oe

The dc B-H loops of the specimen as-cast, after AC Joule heating and AC current passing through the specimen are shown in FIG. 6.

Although the method of the present invention has been described by way of preferred embodiments, it is to be noted that changes are still possible for those skilled in the art without departing from the spirit of the invention.