Patent Application
20080017279
These and other features, aspects, and advantages of the present technique will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Referring now to
Scanner 12 includes a series of associated coils for producing controlled magnetic fields, for generating radiofrequency excitation pulses, and for detecting emissions from gyromagnetic material within the patient in response to such pulses. In the diagrammatical view of
In the presently contemplated configuration, the gradient coils 26, 28 and 30 have different physical configurations adapted to their function in the imaging system 10. As will be appreciated by those skilled in the art, the coils are comprised of superconductive elements, such as wires, cables, bars or plates that are wound or cut to form a coil structure which generates a gradient field upon application of control pulses as described below. In certain embodiments, the superconducting elements include doped magnesium diboride powders having a first phase and a second phase. In certain embodiments, the first phase may include a plurality of magnesium diboride particles, which are surrounded by the second phase. As will be described in detail below, the second phase may include particles or films of carbide, boride, nitride, oxide, oxy-carbide, oxy-boride, oxy-nitride, or combinations thereof. The placement of the coils within the gradient coil assembly may be done in several different orders, but in the present embodiment, a Z-axis coil is positioned at an innermost location, and is formed generally as a solenoid-like structure which has relatively little impact on the RF magnetic field. Thus, in the illustrated embodiment, gradient coil 30 is the Z-axis solenoid coil, while coils 26 and 28 are Y-axis and X-axis coils respectively.
The coils of scanner 12 are controlled by external circuitry to generate desired fields and pulses, and to read signals from the gyromagnetic material in a controlled manner. As will be appreciated by those skilled in the art, when the material, typically bound in tissues of the patient, is subjected to the primary field, individual magnetic moments of the paramagnetic nuclei in the tissue partially align with the field. While a net magnetic moment is produced in the direction of the polarizing field, the randomly oriented components of the moment in a perpendicular plane generally cancel one another.
Gradient coils 26, 28 and 30 serve to generate precisely controlled magnetic fields, the strength of which vary over a predefined field of view, typically with positive and negative polarity. When each coil is energized with known electric current, the resulting magnetic field gradient is superimposed over the primary field and produces a desirably linear variation in the Z-axis component of the magnetic field strength across the field of view. The field varies linearly in one direction, but is homogenous in the other two.
The coils of scanner 12 are controlled by scanner control circuitry 14 to generate the desired magnetic field and radiofrequency pulses. In the diagrammatical view of
Interface between the control circuit 36 and the coils of scanner 12 is managed by amplification and control circuitry 40 and by transmission and receive interface circuitry 42. Circuitry 40 includes amplifiers for each gradient field coil to supply drive current to the field coils in response to control signals from control circuit 36. Interface circuitry 42 includes additional amplification circuitry for driving RF coil 32. Moreover, where the RF coil serves both to emit the radiofrequency excitation pulses and to receive MR signals, circuitry 42 will typically include a switching device for toggling the RF coil between active or transmitting mode, and passive or receiving mode. A power supply, denoted generally by reference numeral 34 in
System control circuitry 16 may include a wide range of devices for facilitating interface between an operator or radiologist and scanner 12 via scanner control circuitry 14. In the illustrated embodiment, for example, an operator controller 46 is provided in the form of a computer work station employing a general purpose or application-specific computer. The station also typically includes memory circuitry for storing examination pulse sequence descriptions, examination protocols, user and patient data, image data, both raw and processed, and so forth. The station may further include various interface and peripheral drivers for receiving and exchanging data with local and remote devices. In the illustrated embodiment, such devices include a conventional computer keyboard 50 and an alternative input device such as a mouse 52. A printer 54 is provided for generating hard copy output of documents and images reconstructed from the acquired data. A computer monitor 48 is provided for facilitating operator interface. In addition, system 10 may include various local and remote image access and examination control devices, represented generally by reference numeral 56 in
Turning now to
In certain embodiments, the doped magnesium diboride powders may include the plurality of magnesium diboride particles that may be represented by a chemical formula MgB2-xSx, where S represents the dopant, and B represents the chemical element boron. In certain embodiments, S may include one or more of carbon, boron, oxygen, nitrogen or combinations thereof. In one embodiment, S includes carbon, i.e., the magnesium diboride powder is carbon doped. In other embodiments, S includes boron, nitrogen or oxygen. In these embodiments, the magnesium diboride powder is boron doped, nitrogen doped or oxygen doped. Further, x represents an atomic percentage of the dopant S that substitutes boron in the magnesium diboride lattice. In some embodiments, the value of x may vary in a range of about 5 percent to about 15 percent, from about 6 percent to about 12 percent, and preferably in a range of about 8 percent to about 10 percent. Preferably, the doping S in the magnesium diboride lattice may be homogeneous. The doping of S into the lattice of magnesium diboride may result in higher upper critical field (Hc). Additionally, dispersing the second phase, such as silicon carbide particles throughout the microstructure of magnesium diboride may enhance the critical current density (Jc). As will be appreciated, high values of both Hc and Jc are required for MRI applications.
As illustrated in
As will be appreciated, a higher amount of superconducting material enables high current density (Ic), critical current density (Jc) and upper critical field (Hc). In an exemplary embodiment, Ic at self-field and 4 K may be in a range of about 80 A to about 1000 A. In another embodiment, Jc may be in a range of about 105 A/cm2 to about 107 A/cm2. In one embodiment, Hc may be in a range of about 10 Tesla to about 100 Tesla.
At block 78, the polymeric precursor is dissolved in a solvent to form a solution. In embodiments where the polymeric precursor is a thermoplastic, the polymeric precursor may be pre-melted into a liquid under controlled atmosphere and the magnesium diboride powder may subsequently be blended into the liquid by using techniques such as milling, high density mixing to form a uniform coating of the polymeric precursor on the magnesium diboride particles. In some embodiments, the solvent may include one or more ketones, alcohols, THF, or combinations thereof. In one embodiment, the solvent includes ethanol.
At block 80, a magnesium diboride powder is added to the solution to form a mixture. The magnesium diboride powder may be commercially available. Subsequently, at step 82, the magnesium diboride powder and the polymeric precursor solution are mixed, by milling, high density mixing for example, to form a homogeneous mixture of the powder and the precursor and to coat the polymeric precursor solution on the magnesium diboride particles.
Additionally, the amount of doping of carbon, oxygen, nitrogen, or boron may be increased in a controlled fashion by addition of their respective sources. For example, the amount of carbon doping may be increased by adding suitable amounts of carbon sources such as carbon fibers, carbon nanoparticles to the mixture. Further, titanium, lithium, yttria, aluminum, silicon, and/or zirconium may also be added to the mixture to further enhance Jc. Preferably, these additives may be in the form of nanoparticles to facilitate homogeneous dispersion of these additives throughout the bulk of the magnesium diboride powder.
Subsequently, at block 84, the solvent may be removed from the solution to form a coating of the polymeric precursor on the plurality of magnesium diboride particles. In one embodiment, the solvent may be evaporated from the solution. In certain embodiments, the evaporation of the solvent may be carried out in an inert atmosphere or vacuum, thereby preventing any undesired contaminants, such as oxygen, from entering the bulk of the magnesium diboride powder. The solvent may be removed by heating the mixture at a temperature of about 15° C. to about 100° C. Further, the mixture may be heated in an inter atmosphere or vacuum.
In alternate embodiments, the magnesium diboride particles may be coated with the polymeric precursor by employing deposition techniques such as metal organic chemical vapor deposition, reactive plasma assisted chemical vapor deposition, reactive plasma assisted physical vapor deposition, chemical vapor infiltration, or combinations thereof.
At block 86, the magnesium diboride powders coated with polymeric precursor may be subjected to sieving and/or granulation to obtain a homogeneous particle size. At block 88, the magnesium diboride powder is subjected to heat-treatment to form second phase coating, such as silicon carbide coating, on the magnesium diboride particles and to diffuse dopants, such as carbon, into the lattice of the magnesium diboride. In an exemplary embodiment, during heating, the polymeric decomposition or pyrolysis of the precursor forms silicon carbide, thereby forming silicon carbide coating on the magnesium diboride particles. In certain embodiments, the heat-treating may be carried out in an oxygen-free environment to prevent any formation of non-conducting oxides in the powder which may result in decrease of Jc. In another embodiment, the heat-treating may be carried out in an inter atmosphere or a vacuum.
Further, the heat-treatment may be carried out at a slow rate to facilitate homogeneous diffusion of dopants in the magnesium diboride lattice and homogeneous dispersion of the second phase particles in the bulk. In one embodiment, the magnesium diboride particles coated with polymeric precursor may be subjected to sintering to facilitate homogeneous dispersion of the second phase particles and homogeneous doping. In certain embodiments, the heat-treatment may be carried out at a temperature in a range of about 1400° C. to about 1900° C. In some embodiments, the heat-treatment is carried out for a period of time in a range of from about 1 hour to about 24 hours. Further, the heat-treatment may be carried out at a pressure in a range of from about 10−6 atm. to about 1 atm.
The magnesium diboride powder made by the various techniques described above may be drawn into various shapes such as wires, cables, or sheets. In one embodiment, the sheets may be encased in metal. In this embodiment, co-extrusion or swagging may be employed to produce these composite structures of magnesium diboride sheets encased in metal. The cables formed from the magnesium diboride powders of the present technique may be employed in imaging applications, such as MRI. As described above, these cables have high Ic, Jc and Kc, thereby making these cables a desirable candidate for MRI applications. Additionally, the magnesium diboride powders of the present technique may be easily drawn into mono-filament or multi-filament cables having diameters in a range of about 1 mm to 5 mm. Also, the mechanical strength of these cables may be suited for various applications. In an exemplary embodiment, the stress experienced by the wires/cables may be in a range from about 50 MPa to about 500 MPa, and the strain experienced by the wires/cables may be in a range from about −1% to about 1%. In one embodiment, the length of the wires/cables may be in a range from about 10 cm to about 106 cm.
At block 92, the ends of the metal tube are sealed. Subsequently, at block 94, the metal tube is deformed to increase the length and to reduce the cross-sectional area. The wire may be further flattened to a tape or film if desired. In one embodiment, the metal tube may be deformed by employing processes, such as extrusion, forging, rolling, swaging, drawing or combinations thereof. Following the deformation process, the wire, tape or film may be heat treated to improve the superconducting properties and/or the mechanical properties. In one embodiment, the wire may be heat treated at a temperature of greater than or equal to about 600° C. for a time period of greater than or equal to about 1 hour.
The wires may be advantageously formed into other similar electrically conducting structures, such as flattened tapes and wound multi-wire cables. Applications for superconducting wires are found in electromagnetic devices such as superconducting magnets, motors, transformers, and generators. Such electromagnetic devices may in turn be incorporated into larger systems, such as, for example, a magnetic resonance imaging system.
In accordance with another exemplary embodiment, once the superconducting wire 98 is formed, as described above, the wire may be twisted to reduce the AC loss through the wire. As will be appreciated, twisting the wire generally reduces the flux linkage through the wire which may reduce the AC loss through the wire. The number of twists formed in the superconducting wire 98 may vary depending on the desired effect. This aspect may be quantified by referring to the “pitch” of the twisted superconducting wire 98. As used herein, the “pitch” of the twisted superconducting wire 98 refers to the length of wire traversed to complete one full rotation (twist) of the wire. For example, if the superconducting wire 98 is twisted, such that the wire is rotated one full twist over a length of wire equal to 50 mm, the superconducting wire 98 is said to have a “pitch” equal to 50 mm. In one exemplary embodiment, the pitch may be in the range of approximately 20 mm to approximately 200 mm. Twisting may be particularly advantageous in low frequency applications, such as those below 200 Hz, for example.
Following the manufacturing of the doped magnesium diboride superconducting wire, such as the wire 98, the wire may be welded or diffusion bonded to produce a continuous length of superconducting wire having a length at least equal to the sum of the lengths of each of the superconducting wires that are welded together. Diffusion bonding is a solid phase process achieved via atomic migration with no macro-deformation of the portions of the superconducting wire to be bonded.
The joining is generally carried out using at least one source of energy, such as, the energy provided by a beam of light, wherein the energy provided by the source is directed at those portions of the superconducting composition that are to be joined together. The interaction of the energy with the superconducting composition facilitates the heating of the composition, and this increase in the temperature may be advantageously utilized to facilitate the joining of the superconducting composition. Preferable methods of joining are electron beam welding, laser welding, ultrasonic welding, plasma arc welding, resistance welding and the like.
In yet another embodiment, the overlapping section comprising the exposed ends of the superconducting filaments along with a filler material comprising a doped magnesium diboride powder, a magnesium diboride powder, or a combination of magnesium powder and boron powder are resistively heated. The heating promotes a chemical reaction between the magnesium and the boron to produce magnesium diboride. The magnesium diboride may be used to facilitate the joining of the superconducting wire.
In one embodiment, the joining generally occurs at a temperature of about 650 to about 1000° C. It is generally desirable to perform the joining in a manner so as to obtain a “bridge superconducting cross section” between the first end of the first superconducting wire and the second end of the second superconducting wire. When the bridge superconducting cross section is less than the superconducting cross section on the filament or the tape, the bridge superconducting cross section limits the current carrying capacity in the connected superconducting elements. Therefore, the bridge superconducting cross section is, preferably, at least as large as the superconducting cross section on the filament or the tapes.
The current carrying capacity of a formed joint can be tested by soldering voltage probes to the superconducting filament tape on both sides of the weld. The joint is cooled below the critical temperature of the superconductor and increasing amounts of current are passed through the weld while the voltage change between the probes is monitored. The current at which a sufficient voltage change is detected, e.g., about 0.02 microvolts, is the critical current. If the current carrying capacity in the weld is less than the current carrying capacity in the filament and/or tape, the number of bridges or size of the bridges can be increased in the joint to form a larger bridge superconducting cross section.
As stated above, these methods of joining may be effectively used to create extended sections of the superconducting wire that may be advantageously used in electrically conducting structures, including, but not limited to, flattened tapes, laminated wires formed from multiple wires, and wound multi-wire cables. Applications for superconducting wires are found in electromagnetic devices such as, but not limited to, superconducting magnets for motors, transformers, and generators. Such electromagnetic devices may in turn be incorporated into larger systems, such as, for example, a magnetic resonance imaging system.
Although the magnesium diboride powders of the present technique are described with respect to MRI applications, as will be appreciated, the magnesium diboride powders as disclosed above may be employed in several other techniques, such as power generation, generators, motors, fault current limiters, or any other superconducting applications.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
