Patent Application
20060191602
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, less than 1.6 Tesla. The saturation induction B5 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 Bs˜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 Bs, low magnetic loss under AC excitation and small coercivity Hc 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, Bs values higher than 1.56-1.6 Tesla are desirable to achieve higher particle acceleration voltages which are directly proportional to Bs values. A lower coercivity Hc and a higher BH squareness 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 Bs=1.6 Tesla, with Hc as small as possible and the squareness ratio B(H=0)/Bs 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. Melvin and A. Flattens 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 Bs<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.
In accordance with aspects of the invention, an amorphous metal alloy has a composition having a formula FeaBbSicCd where 81<a≦84, 10≦b≦18, 0<c≦5 and 0<d<1.5, numbers being in atomic percent, with incidental impurities. 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.
According to a first aspect of the present invention, an iron-based amorphous alloy is provided wherein the alloy has a chemical composition with a formula FeaBbSicCd where 81<a≦84, 10≦b≦18, 0<c≦5 and 0<d<1.5, numbers being in atomic percent, with incidental impurities, and simultaneously has 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 400° C.
According to a second aspect of the present invention, the alloy is represented by a formula of Fe81.7B16.0Si2.0C0.3, Fe82.0B16.0Si1.0C1.0, Fe82.0B14.0Si3.0C1.0, Fe82.0B13.5Si4.0C0.5, Fe82.0B13.0Si4.0C1.0, Fe82.6B15.5Si1.6C0.3, Fe83.0B13.0Si3.0 C1.0 or Fe84.0B13.0Si2.0 C1.0.
According to a third aspect of the present invention, the saturation magnetic induction of the alloy is greater than 1.65 tesla.
According to a fourth aspect of the present invention, the alloy is represented by a formula of Fe81.7B16.0Si2.0C0.3, Fe82.0B16.0Si1.0C1.0, Fe82.0B14.0Si3.0C1.0, Fe82.0B13.5Si4.0C0.5, or Fe83.0B13.0Si3.0C1.0.
According to a fifth aspect of the present invention, the alloy is heat-treated by annealing at temperatures between 300° C. and 350° C.
According to a sixth aspect of the present invention, the alloy is utilized in a magnetic core and has a core loss 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.
According to a seventh aspect of the present invention, a DC squareness ratio of the alloy is greater than 0.8 after the alloy has been annealed.
According to an eighth aspect of the present invention, a magnetic core that includes a heat-treated iron-based amorphous alloy is provided, wherein the alloy is represented by a chemical composition with a formula FeaBbSicCd where 81<a≦84, 10≦b≦18, 0<c≦5 and 0<d<1.5, numbers being in atomic percent, with incidental impurities, and simultaneously has 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 400° C., wherein the alloy has been annealed at temperatures between 300° C. and 350° C., 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, and wherein the magnetic core is a magnetic core of a transformer or a electrical choke coil.
According to a ninth aspect of the present invention, a magnetic core that includes a heat-treated iron-based amorphous alloy is provided, wherein the alloy is represented by a chemical composition with a formula FeaBbSicCd where 81<a≦84, 10≦b≦18, 0<c≦5 and 0<d<1.5, numbers being in atomic percent, with incidental impurities, and simultaneously has 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 400° C., wherein the alloy has been annealed at temperatures between 300° C. and 350° C., wherein a DC squareness ratio is greater than 0.8 after the alloy has been annealed, and wherein the magnetic core is an inductor core of a magnetic switch in a pulse generator and/or compressor.
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.
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:
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, exceeding 1.6 T, low AC core loss and high thermal stability. The amorphous alloy has a chemical composition having a formula FeaBbSicCd where 81<a≦84, 10≦b≦18, 0<c≦5 and 0<d<1.5, 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 Bs. 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:
All of these alloys have saturation inductions Bs 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
In
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 Bs=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 alloy in accordance with embodiments of the present invention, on the other hand, shows lower core loss than that of the commercially available alloy and can be excited beyond 1.45 T as indicated in Table II because this alloy has a saturation induction of 1.65 T, which is higher than the saturation induction of 1.56 T of the commercial amorphous alloy.
The unexpected sharpness of the BH behavior shown in
From Table III, it is clear that the amorphous alloy in accordance with embodiments of the present invention is 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/Ta, where Ta was the aging temperature on the absolute temperature scale. The plotted data are best described by the following formula:
tau∝exp (−Ea/kBT),
where tau is the time for an aging process to complete at temperature T. Ea is the activation energy for the aging process, and kB 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 an amorphous alloy according to embodiments of the present invention are thermally stable at 150° C. for much more than 100 years.
In order to find optimal annealing conditions for the amorphous alloys according to embodiments of the present invention, annealing temperature and time were changed as described in Example II.
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.
About 60 kg of the constituent metals, such as FeB, FeSi, Fe and C, were melted in a 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 temperature and the crystallization temperature of the ribbon material. A conventional Archimedes' method was used to determine the mass density, which was needed for material's magnetic characterization. The ribbon was found to be ductile.
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 300-370° C. for one hour in a DC magnetic field of 30 Oe (2400 A/m), applied along the toroids' circumference direction for the alloys of embodiments of the present invention and at 360° C.-400° C. for two hours in a DC magnetic field of 30 Oe (2400 A/m) applied along the toroids' circumference direction for the commercially available METGLAS®2605SA1 alloy. 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. In addition, ribbon strips of a dimension of 230 mm in length and 85 mm in width were cut from amorphous alloys of embodiments of the present invention and from the commercially available METGLAS®2605SA1 alloy and were heat-treated at temperatures between 300° C. and 370° C. for the amorphous alloy of embodiments of the present invention and between 360° C. and 400° C. for the commercially available alloy, both with a DC magnetic field of about 30 Oe (2400 A/m) applied along the strips' length direction.
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/A912M-04 Standards for 50/60 Hz measurements. The magnetic properties such as AC core loss of the annealed straight strips of Example II with length of 230 mm and width of 85 mm were tested by following ASTM A 932/A932M-01 Standards.
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.
This application is a continuation-in-part Application of, and claims priority benefit under 35 U.S.C. §120 to, U.S. application Ser. No. 11/059,567 filed Feb. 17, 2005, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11059567 | Feb 2005 | US |
| Child | 11320744 | Dec 2005 | US |
