Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2001-07-23
2004-06-01
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C336S234000
Reexamination Certificate
active
06744342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to bulk magnetic components; and more particularly, to a generally three-dimensional high performance bulk metal magnetic component for large electronic devices such as magnetic resonance imaging systems, television and video systems, and electron and ion beam systems.
2. Description of the Prior Art
Certain steel alloys have long been used in magnetic devices in numerous technological applications. The most commonly used of these steels are low-carbon alloys and alloys with up to 3-3.5 weight percent silicon, often referred to as electrical steels and silicon steels, respectively. (As is conventional in the silicon steel art, the content of Si and other elemental additions recited herein is to be understood as a weight percentage unless otherwise specified.) These alloys find widespread use in electric motors, transformers, actuation devices, relays, and the like. Although steels are generally inexpensive, they are often unsuitable for demanding requirements. Among their most significant limitations are their core losses and their magnetostrictions. The low carbon steels are generally the least expensive alloys used for magnetic devices; they are widely available in commerce as unoriented sheet in thicknesses as low as 350 &mgr;m (0.014″). However, their core losses are high enough to preclude their use in most applications requiring high efficiency or excitation frequencies greater than line frequency (50-60 Hz). Somewhat lower losses are exhibited by the silicon-containing alloys. They are produced in vast quantities either as non-oriented or oriented sheets in thicknesses as low as 125-175 &mgr;m (0.005-0.007″). Oriented sheets have marked crystallographic texture that results in a substantial difference in their magnetic properties for excitation in different directions within the sheet. Oriented sheets are thus most suited for applications wherein flux is predominantly along a defined single direction including transformers and segmented components. Non-oriented materials are best suited to applications wherein the flux direction is not constant during operation, e.g. motor stators.
In addition to steels, other high induction, crystalline materials are known for use in certain magnetic applications, including Fe—Si—Al alloys like Sendust, Fe—Co alloys, and Fe—Ni alloys. In each of these alloy families, small additions of other elements may be added for the sake of metallurgical processing or enhancement of soft magnetic properties.
Magnetic resonance imaging (MRI) has become an important, non-invasive diagnostic tool in modern medicine. An MRI system typically comprises a magnetic field generating device. A number of such field generating devices employ either permanent magnets or electromagnets as a source of magnetomotive force. Frequently the field generating device further comprises a pair of magnetic pole faces defining a gap with the volume to be imaged contained within this gap.
The earliest magnetic pole pieces were made from solid magnetic material such as carbon steel or high purity iron, often known in the art as Armco iron. They have excellent DC properties but very high core loss in the presence of AC fields because of macroscopic eddy currents. Some improvement has been gained by forming a pole piece of laminated conventional steels.
U.S. Pat. No. 4,672,346 teaches a pole face having a solid structure and comprising a plate-like mass formed from a magnetic material such as carbon steel. U.S. Pat. No. 4,818,966 teaches that the magnetic flux generated from the pole pieces of a magnetic field generating device can be concentrated in the gap therebetween by making the peripheral portion of the pole pieces from laminated magnetic plates. U.S. Pat. No. 4,827,235 discloses a pole piece having large saturation magnetization, soft magnetism, and a specific resistance of 20 &mgr;&OHgr;-cm or more. Soft magnetic materials including permalloy, silicon steel, amorphous magnetic alloy, ferrite, and magnetic composite material are taught for use therein.
U.S. Pat. No. 5,124,651 teaches a nuclear magnetic resonance scanner with a primary field magnet assembly. The assembly includes ferromagnetic upper and lower pole pieces. Each pole piece comprises a plurality of narrow, elongated ferromagnetic rods aligned with their long axes parallel to the polar direction of the respective pole piece. The rods are preferably made of a magnetically permeable alloy such as 1008 steel, soft iron, or the like. The rods are transversely electrically separated from one another by an electrically non-conductive medium, limiting eddy current generation in the plane of the faces of the poles of the field assembly. U.S. Pat. No. 5,283,544, issued Feb. 1, 1994, to Sakurai et al. discloses a magnetic field generating device used for MRI. The devices include a pair of magnetic pole pieces which may comprise a plurality of block-shaped magnetic pole piece members formed by laminating a plurality of non-oriented silicon steel sheets.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved pole pieces. This is so because these pole pieces are essential for improving the imaging capability and quality of MRI systems. Although steel alloys are widely available, they have still been considered unsuitable for use in bulk magnetic components such as the tiles of poleface magnets for advanced magnetic resonance imaging systems (MRI), largely because of their high core losses under AC excitation.
It has also been known in the magnetic materials art that certain advantages might potentially be obtained by using silicon steels with considerably higher silicon content than the typical 3-3.5%. That limit is set by fundamental metallurgical constraints. An alloy with silicon content of greater than about 2.5% is said to have a closed &ggr; loop. That is, upon cooling an alloy with lower than 2.5% Si from high temperature, there is a series of successive allotropic transformations of the alloy from the body-centered cubic (bcc) &dgr; crystallographic phase to the face-centered cubic (fcc) &ggr; phase and finally to the room-temperature bcc &agr; phase. Instead, at higher Si the alloy remains bcc throughout. This allows a careful interplay of rolling operations and controlled grain growth essential for producing thin-gage, low core loss sheet stock. However, above about 4-4.5% Si there is another difficulty, namely the formation of DO
3
and B
2
phases which are characterized by superlattice ordering. The presence of the ordered DO
3
and B
2
phases results in brittleness, precluding normal rolling operations.
It has been recognized that alloys with 6-7% Si have certain attractive electromagnetic characteristics. The increased solute content increases the alloy's electrical resistivity, tending to improve the eddy current component of core loss. At about 6.5%, the magnetostriction of the alloy is nearly zero, reducing the susceptibility of the component to degradation of its magnetic properties by internally or externally imposed stresses. However, processing difficulties have meant that high silicon iron alloys are still not widely recognized or applied.
Several non-conventional methods have recently been taught for producing sheets of high silicon content Fe-base alloys. First, rapid solidification processing has been used to form directly thin strip material with high Si content. U.S. Pat. No. 4,265,682 to Tsuya et al. discloses a high silicon steel strip consisting of 4-10 weight % of Si and the remainder being substantially Fe and incidental impurities. The strip is produced by rapidly cooling a melt to form a microstructure comprising very fine crystal grains with substantially no ordered lattice. U.S. Pat. Nos. 4,865,657 and 4,990,197, each to Das et al., disclose heat treatment of a rapidly quenched Fe—Si containing 6-7 weight % Si to promote and control grain orientation and an order-disorder reaction.
Another method for producing sheets of high silicon
Decristofaro Nicholas J.
Fish Gordon E.
Barrera Ramon M.
Buff Ernest D.
Ernest D. Buff and Associates, LLC
Fish Gordon E.
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