Iron-based high saturation induction amorphous alloy

Abstract

An iron-based amorphous alloy and magnetic core with an iron-based amorphous alloy having a chemical composition with a formula FeaBbSicCd, where 80<a≦84, 8≦b≦18, 0<c≦5 and 0<d≦3, numbers being in atomic percent, with incidental impurities, simultaneously having a value of a saturation magnetic induction exceeding 1.6 tesla, a Curie temperature of at least 300° C. and a crystallization temperature of at least 350 ° C. When cast in a ribbon form, such an amorphous metal alloy is ductile and thermally stable, and is suitable for various electric devices because of high magnetic stability at such devices' operating temperatures.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an iron-based amorphous alloy with a saturation induction exceeding 1.6 Tesla and adapted for use in magnetic devices, including transformers, motors and generators, pulse generators and compressors, magnetic switches, magnetic inductors for chokes and energy storage and sensors.

2. Description of the Related Art

Iron-based amorphous alloys have been utilized in electrical utility transformers, industrial transformers, in pulse generators and compressors based on magnetic switches and electrical chokes. In electrical utility and industrial transformers, iron-based amorphous alloys exhibit no-load or core loss which is about ¼ that of a conventional silicon-steel widely used for the same applications operated at an AC frequency of 50/60 Hz. Since these transformers are in operation 24 hours a day, the total transformer loss worldwide may be reduced considerably by using these magnetic devices. The reduced loss means less energy generation, which in turn translates into reduced CO₂ emission.

For example, according to a recent study conducted by International Energy Agency in Paris, France, an estimate for energy savings in the Organization for Economic Co-operation and Development (OECD) countries alone that would occur by replacing all existing silicon-steel based units was about 150 TWh in year 2000, which corresponds to about 75 million ton/year of CO₂ gas reduction. The transformer core materials based on the existing iron-rich amorphous alloys have saturation inductions B_s less than 1.6 Tesla. The saturation induction B_s is defined as the magnetic induction B at its magnetic saturation when a magnetic material is under excitation with an applied field H. Compared with B_s˜2 Tesla for a conventional grain-oriented silicon-steel, the lower saturation inductions of the amorphous alloys leads to an increased transformer core size. It is thus desired that the saturation induction levels of iron-based amorphous alloys be increased to levels higher than the current levels of 1.56-1.6 Tesla.

In motors and generators, a significant amount of magnetic flux or induction is lost in the air gap between rotors and stators. It is thus desirable to use a magnetic material with a saturation induction or flux density as high as possible. A higher saturation induction or flux density in these devices means a smaller size device, which is preferable.

Magnetic switches utilized in pulse generation and compression require magnetic materials with high saturation inductions, high BH squareness ratios defined as the ratios of the magnetic induction B at H=0 and B_s, low magnetic loss under AC excitation and small coercivity H_c which is defined as the field at which the magnetic induction B becomes zero, and low magnetic loss under high pulse rate excitation. Although commercially available iron-based amorphous alloys have been used for these types of applications, namely in cores of magnetic switches for particle accelerators, B_s values higher than 1.56-1.6 Tesla are desirable to achieve higher particle acceleration voltages which are directly proportional to B_s values. A lower coercivity H_c and a higher BH squarness ratio mean a lower required input energy for the magnetic switch operation. Furthermore a lower magnetic loss under AC excitation increases the overall efficiency of a pulse generation and compression circuit. Thus, clearly needed is an iron-based amorphous alloy with a saturation induction higher than B_s=1.6 Tesla, with H_c as small as possible and the squareness ratio B(H=0)/B_s as high as possible, exhibiting low AC magnetic loss. The magnetic requirements for pulse generation and compression and actual comparison among candidate magnetic materials was summarized by A. W. Molvik and A. Faltens in Physical Review Special Topics-Accelerators and Beams, Volume 5, 080401 (2002).

In a magnetic inductor used as an electrical choke and for temporary energy storage, a higher saturation induction of the core material means an increased current-carrying capability or a reduced device size for a given current-carrying limit. When these devices are operated at a high frequency, core material must exhibit low core losses. Thus, a magnetic material with a high saturation induction and a low core loss under AC excitation is preferable in these applications.

In sensor applications of a magnetic material, a high saturation induction means a high level of sensing signal, which is required for a high sensitivity in a small sensing device. Low AC magnetic losses are also necessary if a sensor device is operated at high frequencies. A magnetic material with a high saturation induction and a low AC magnetic loss is clearly needed in sensor applications.

In all of the above applications which are just a few representatives of magnetic applications of a material, a high saturation induction material with a low AC magnetic loss is needed. It is thus an aspect of this invention to provide such materials based on iron-based amorphous alloys which exhibit saturation magnetic induction levels exceeding 1.6 T and which are close to the upper limit of the commercially available amorphous iron-based alloys.

Attempts were made in the past to achieve an iron-based amorphous alloy with a saturation induction higher than 1.6 T. One such example is a commercially available METGLAS®2605CO alloy with a saturation induction of 1.8 T. This alloy contains 17 at. % Co and therefore too expensive to be utilized in commercial magnetic products such as transformers and motors. Other examples include amorphous Fe—B—C alloys as taught in U.S. Pat. No. 4,226,619. These alloys were found mechanically too brittle to be practically utilized. Amorphous Fe—B—Si-M alloys where M=C as taught in U.S. Pat. No. 4,437,907 were intended to achieve high saturation inductions, but were found to exhibit B_s<1.6 T.

Thus, there is a need for ductile iron-based amorphous alloys with saturation induction exceeding 1.6 T, having low AC magnetic losses and high magnetic stability at devices' operating temperatures.

SUMMARY OF THE INVENTION

In accordance with aspects of the invention, an amorphous metal alloy has a composition having a formula Fe_aB_bSi_cC_d where 80<a≦84, 8≦b≦18, 0<c≦5 and 0<d≦3, numbers being in atomic percent, with incidental impurites. When cast in a ribbon form, such an amorphous metal alloy is ductile and thermally stable, and has a saturation induction greater than 1.6 T and low AC magnetic loss. In addition, such an amorphous metal alloy is suitable for use in electric transformers, pulse generation and compression, electrical chokes, energy-storing inductors and magnetic sensors.

Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 illustrates a graphical representation with respect to coordinates of magnetic induction B and applied field H of up to 1 Oe, that compares the BH behaviors of the amorphous alloys of embodiments of the present invention with that of a commercially available iron-based amorphous alloy METGLAS®2605SA1;

FIG. 2 illustrates a graphical representation with respect to coordinates of magnetic induction B and applied field H, that depicts the first quadrant of the BH curves up to the induction level of 1.3 Tesla for the amorphous alloys of embodiments of the present invention and that of a commercially available iron-based amorphous alloy METGLAS®2605SA1; and

FIG. 3 illustrates a graphical representation with respect to coordinates of exciting power VA at 60 Hz and induction level B, that compares the exciting power of the amorphous alloys of embodiments of the present invention with that of a commercially available iron-based amorphous alloy METGLAS®2605SA1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

An amorphous alloy, in accordance with embodiments of the present invention, is characterized by a combination of high saturation induction B_s exceeding 1.6 T, low AC core loss and high thermal stability. The amorphous alloy has a chemical composition having a formula Fe_aB_bSi_cC_d where 80<a≦84, 8≦b≦18, 0<c≦5 and 0<d≦3, numbers being in atomic percent, with incidental impurities.

Iron provides high saturation magnetic induction in a material below the material's Curie temperature at which magnetic induction becomes zero. Accordingly, an amorphous alloy with a high iron content with a high saturation induction is desired. However, in an iron-rich amorphous alloy system, a material's Curie temperature decreases with the iron content. Thus, at room temperature a high concentration of iron in an amorphous alloy does not always result in a high saturation induction B_s. Thus, a chemical compositional optimization is necessary, as is set forth in accordance with embodiments of the present invention as described herein.

An alloy, in accordance with embodiments of the present invention, was readily cast into an amorphous state by using a rapid solidification method described in U.S. Pat. No. 4,142,571, the contents of which are incorporated herein by reference. The as-cast alloy is in a ribbon form and ductile. Typical examples of the magnetic and thermal properties of the amorphous alloys, in accordance with embodiments of the present invention, are given in Table I below:

TABLE I
Saturation Induction, Curie and Crystallization Temperature
of the Alloys For Embodiments of the Present Invention
	Saturation	Curie	Crystallization
	Induction	Temperature	Temperature
Composition (at. %)	(T)	(° C.)	(° C.)
Fe81B18Si1	1.65	372	452
Fe82B16Si2	1.66	354	455
Fe82B14Si2C2	1.66	355	444
Fe82B13Si5	1.64	357	457
Fe83B15Si2	1.64	336	428
Fe84B14Si2	1.63	310	409

All of these alloys have saturation inductions B_s exceeding 1.6 T, Curie temperatures exceeding 300° C. and crystallization temperatures exceeding 400° C. Since most of the magnetic devices commonly used are operated below 150° C., at which electrically insulating materials used in these devices burn or deteriorate rapidly, the amorphous alloys in accordance with embodiments of the present invention are thermally stable at the operating temperatures.

Comparison of the BH behaviors of the amorphous alloys in accordance with embodiments of the present invention and that of a commercially available iron-based amorphous alloy shows unexpected results. As clearly seen in FIG. 1 in which the BH loops are compared, the magnetization toward saturation is much sharper in the amorphous alloys in embodiments of the present invention than that in a commercially available amorphous iron-based alloy. The consequence of this difference is a reduced magnetic field needed to achieve a predetermined induction level in the alloys of embodiments of the present invention than the commercially available alloy as shown in FIG. 2.

In FIG. 2, the excitation level was set at 1.3 Tesla, and the fields needed to achieve this excitation level were determined for the two amorphous alloys in accordance with embodiments of the present invention and for a prior art amorphous alloy, METGLAS®2605SA1. It is clearly demonstrated that the amorphous alloys for embodiments of the present invention require much less field, and hence less exciting current to achieve a same magnetic induction compared with the commercially available alloy. This is shown in FIG. 3 where exciting power, which is a product of the exciting current of the primary winding of a transformer and the voltage at the secondary winding of the same transformer, is compared among the three amorphous alloys of FIGS. 1 and 2. It is clear that exciting power for the amorphous alloys in accordance with embodiments of the present invention is lower at any excitation level than that of a commercially available METGLAS®2605SA1 alloy. Lower exciting power in turn results in a lower core loss for the alloys in accordance with embodiments of the present invention than for the commercially available amorphous alloy, especially at high magnetic excitation levels. Typical examples of core loss at high excitation are given in Table II for the two amorphous alloys of embodiments of the present invention showing Bs=1.66 T in Table I and a commercially available amorphous alloy, METGLAS®2605SA1.

TABLE II
Core loss comparison at different induction levels between B = 1.3
and 1.5 T among the high saturation induction alloys for embodiments of
the present invention and a commercially available amorphous iron-based
alloy METGLAS ® 2605SA1.
	Core Loss at 60 Hz (W/kg)
			B =
Alloy	B = 1.3 T	B = 1.4 T	1.45 T	B = 1.5 T
Fe82B16Si2	0.24	0.29	0.33	0.38
Fe82B14Si2C2	0.25	0.29	0.32	0.34
METGLAS ® 2605SA1	0.27	0.32	0.35	n/a
n/a: cores could not be excited at this level.

As expected and seen in Table II, core loss of a commercial amorphous alloy METGLAS®2605SA1 increases rapidly above 1.45 T induction because this alloy has a saturation induction B_s=1.56 T and cannot be excited above about 1.5 Tesla. Thus, no data point for B=1.5 T is given in Table II for METGLAS®2605SA1 alloy. The amorphous alloys in accordance with embodiments of the present invention, on the other hand, show lower core loss than that of the commercially available alloy and can be excited beyond 1.45 T as indicated in Table II because these alloys have higher saturation inductions of 1.66 T.

The unexpected sharpness of the BH behavior shown in FIG. 1 for the amorphous alloys for embodiments of the present invention is suited for their use as inductors in magnetic switches for pulse generation and compression. It is clear that an amorphous alloy in accordance with embodiments of the present invention has a higher saturation induction B_s, a lower coercivity and a higher BH squareness ratio than the commercial alloy. The higher level of B_s of the alloy in accordance with embodiments of the present invention is especially suited to achieve a larger flux swing which is given by 2B_s. Values of DC coercivity, a DC BH squareness ratio and 2B_s are compared in Table III.

TABLE III
		Squareness
		Ratio
Alloy	Coercivity (Oe)	(Br/Bs)	2Bs (Tesla)
Fe82B16Si2	0.030	0.85	3.32
Fe82B14Si2C2	0.025	0.90	3.32
METGLAS ® 2605SA1	0.043	0.78	3.12

From Table III, it is clear that the amorphous alloys in accordance with embodiments of the present invention are more suited for use as core materials for pulse generation and compression than a commercially available amorphous alloy.

The alloys of embodiments of the present invention were found to have a high thermal stability as indicated by the high crystallization temperatures of Table I. A supporting evidence for the thermal stability was obtained through accelerated aging tests in which core loss and exciting power at elevated temperatures above 250° C. were monitored over several months until these values started to increase. The time period at which the property increase was recorded at each aging temperature was plotted as a function of 1/T_a, where T_a was the aging temperature on the absolute temperature scale. The plotted data are best described by the following formula:
tau∝ exp(−E_a/k_BT),

where tau is the time for an aging process to complete at temperature T, E_a is the activation energy for the aging process, and k_B is the Boltzmann constant. The data plotted on a logarithmic scale were extrapolated to the temperatures pertinent to the operating temperatures of widely used magnetic devices, such as transformers. This kind of plotting is known as an Arrhenius plot and is widely known in the industry to predict long-term thermal behavior of a material. An operating temperature of 150° C. was selected because most of the electrical insulating materials used in these magnetic devices either burn or deteriorate rapidly above about 150° C. Table IV is the result of the study, which indicates that the amorphous alloys according to embodiments of the present invention are thermally stable at 150° C. for more than 100 years.

TABLE IV
Lifetime of amorphous alloys accordinging to embodiments of the
present invention at 150° C.
Alloy       Lifetime (years)
Fe82B16Si2  450
Fe82B14S2C2 100

The following examples are presented to provide a more complete understanding of the invention. The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles and practice of the invention according to preferred embodiments are exemplary and should not be construed as limiting the scope of the invention.

EXAMPLE I

About 60 kg of the constituent metals, such as FeB, FeSi, Fe and C, were melted in an crucible and the molten metal was rapidly solidified by the method described in the U.S. Pat. No. 4,142,571. The ribbon formed had a width of about 170 mm and a thickness of about 25 μm and was tested by a conventional differential scanning calorimetry to assure its amorphous structure and determine the Curie and crystallization of the ribbon material. A conventional Archimedes' method was used to determine its mass density, which was needed for material's magnetic characterization. The ribbon was found ductile.

EXAMPLE II

The 170 mm wide ribbon was slit into 25 mm wide ribbon which was used to wind toroidally shaped magnetic cores weighing about 60 gram each. The cores were heat-treated at 300-350° C. with a DC magnetic field of about 20 Oe (1600 A/m) applied along the toroid circumference direction. A primary copper wire winding of 10 turns and a secondary winding of 10 turns were applied on the heat-treated cores for magnetic measurements.

EXAMPLE III

The magnetic characterizations of the heat-treated magnetic cores with primary and secondary copper windings of Example II were performed by using commercially available BH loop tracers with DC and AC excitation capability. AC magnetic characteristics, such as core loss, were examined by following ASTM A912-93 Standards for 50/60 Hz measurements.

EXAMPLE IV

The well-characterized cores of Example III were used for accelerated aging tests at temperatures above 250° C. During the tests, the cores were in an exciting field at 60 Hz which induced a magnetic induction of about 1 T to simulate actual transformer operations at the elevated temperatures.

Although a few embodiments and examples of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

Claims

1. An iron-based amorphous alloy comprising: a chemical composition with a formula FeaBbSicCd where 81<a≦84, 8≦b≦18, 0<c≦5 and 0<d≦3, numbers being in atomic percent, with incidental impurities, simultaneously having a value of saturation magnetic induction greater than 1.6 tesla, a Curie temperature of at least 300° C. and a crystallization temperature of at least 350° C., wherein the alloy is heat-treated to be annealed from 300° C. to 350° C.

2. The alloy of claim 1, having a formula of Fe82B14Si2C2.

3. The alloy of claim 1, having the saturation magnetic induction greater than 1.65 tesla.

4. The alloy of claim 3, having a formula of Fe82B14Si2C2.

5. The alloy of claim 1, wherein a core loss is less than or equal to 0.5 W/kg after the alloy has been annealed, when measured at 60 Hz, 1.5 tesla and at room temperature.

6. The alloy of claim 1, in which a DC squareness ratio is greater than 0.8 after the alloy has been annealed.

7. A magnetic core comprising an amorphous alloy of claim 5, the magnetic core being a magnetic core of a transformer or a electrical choke coil.

8. A magnetic core comprising an amorphous alloy of claim 6, the magnetic core being an inductor core of a magnetic switch in a pulse generator and/or compressor.

9. An article of manufacture comprising an iron-based amorphous alloy cast in a ribbon form wherein the alloy comprises: a chemical composition with a formula FeaBbSicCd where 81<a≦84, 8≦b≦18, 0<c≦5 and 0<d≦3, numbers being in atomic percent, with incidental impurities, simultaneously having a value of saturation magnetic induction greater than 1.6 tesla, a Curie temperature of at least 300° C. and a crystallization temperature of at least 350° C., wherein the alloy is heat-treated to be annealed from 300° C. to 350° C., and the article of manufacture is a magnetic switch, an electrical choke, a transformer, a motor, a generator, a magnetic inductor, or a magnetic sensor.