US20020198111A1 | 2002-12-26 | |||
EP1467418A2 | 2004-10-13 | |||
US6586370B1 | 2003-07-01 | |||
US6569360B2 | 2003-05-27 | |||
US20030219911A1 | 2003-11-27 |
MGB2 SUPERCONDUCTING MULTIFILAMENTARY WIRES
AND FILMS USING NB-TI SUPERCONDUCTOR AS A TUBE
AND A SUBSTRATE Technical Field [1] This invention is related to superconducting multifilamentary wires and films. Giving a more detail of this invention, Nb-Ti superconductor are used as a tube (the tube in the Powder In Tube process) in the case of superconducting wire and Nb-Ti su¬ perconductor are used as a substrate in the case of superconducting film, Background Art [2] After superconductors were discovered, interest focused on applications for super¬ conducting wires and films. [3] The well-known Nb-Ti superconductor was discovered in 1960. The Nb-Ti super¬ conductor was developed as a multifilamentary superconducting wire which have several Cu matrices in 1970. This multifilamentary superconducting wire is elec¬ tronically stable, therefore it is typical of the applied superconductor wires. [4] Nb-Ti multifilamentary superconducting wires which are mentioned above are fabricated by the start of cold-drawing in the Cu matrix(tube). Then the specimens are heat treated in 300 - 400°C for annealing. During annealing, fine Ti phases are pre¬ cipitated. The critical current density(Jc) of multifilamentary superconducting wire is enhanced by these fine Ti phases which are pinning center. These processes( cold- drawing and annealing) are repeated to enhance Jc and change demensions. [5] However Nb-Ti multifilamentary superconducting wire are restrained in their practical applications, because of it's superconducting transition temperature (around 9K). This means that it can only be used in liquid helium state. Therefore wide use of Nb-Ti multifilamentary superconducting wires is not possible. [6] [7] Drawing 4 shows a cross section of Nb-Ti multifilamentary superconducting wire. As shown in Drawing 4, superconducting wire is generally composed of shielding metal(30) which coats the entire wire and superconducting filament(lθ) which consist of a multi-state and tube(20) which coat the superconducting filament. [8] [9] On the other hand, since the MgB2 superconductor was discovered by Akimuzu in 2001, many efforts have been made to develop practical applications for it, but the results have not been satisfactory. Various methods have been introduced for tapes and wires, but they only have one or two order lower critical current density compared to well made films. Therefore it has many problems when used to fabricate tapes and wires. MgB2 is simple intermetallic compound composed by Magnesium and Boron, but it is very brittle in bulk form. It is impossible to direct draw and roll it for wires, film and tapes. Therefore usually PIT(powder in tube) method is used to fabricate MgB2 wires like the high Tc oxide superconductor. One of the most important factors in PIT method is selecting the tube metal. [10] Because Mg has high reactivity with oxygen and other materials, and because B also has a high melting point, it is very difficult to select a suitable tube metal for a MgB2 superconducting wire. There have been many studies using Fe, Nb, Cu, stainless steel and other metals as a tube, but the results have not been satisfactory and many problems remain in practical applications. [11] In the case of films, MgB2 superconductor do not get out of the vacumm processes which are ABAD(ion beam assisted deposition), PLD(pulse laser deposition) and others. Moreover, substrates are not free from single crystalls such as A12O3. A variety of metal substrates have been studied such as Fe, Ni, stainless steel and Nb for long length film, but the results have not been satisfactory. It is difficult to select metal substrates for film because of Mg's high reactivity with oxygen and other metals, Mg's high volatility and B's high melting point. That many problems limit practical ap¬ plications that MgB 2 film has. [12] Disclosure of Invention Technical Problem [13] This invention is created to compensate for these problems which have been mentioned. This invention is relate to superconducting multifilamentary wire and films. Giving a more detail of this invention, for superconducting wire, This invention uses Nb-Ti superconductor as a tube(the tube in the Powder In Tube process) and MgB2 is used as a filaments. For superconducting film, Nb-Ti superconductors are used as a substrate and MgB2 is deposited on the Nb-Ti superconductor. The purpose of this invention is to provide MgB2 superconducting multifilamentary wire and film using Nb-Ti superconductor as the tube and substrate. [14] Therefore, it can be said that this invention have the object that MgB 2 super¬ conducting multifilamentary wires and films which superconducting critical tempe rature is 39K by using Nb-Ti superconductor as a tube and substrate, are supplied. [15] Technical Solution [16] To achieve the purpose which is mentioned above, the major points of this invention follow; [17] In superconducting wires, Nb-Ti superconductors are used as a tube and MgB2 is used as the filament in the Nb-Ti tube. [18] In superconducting film, Nb-Ti superconductors are used as a substrate and MgB2 is deposited on the Nb-Ti substrate. [19] This configuration makes it desirable to include Cu based alloy as the shielding metal. And This configuration also makes it desirable to include the barrier of Nb etc between the shielding metal and the NbTi superconductor as well as between NbTi su¬ perconductor and MgB2 superconductor Advantageous Effects [20] By using the already commercialized Nb-Ti superconductor as the tubes and the substrate, this invention takes advantages of suppling superconducting wires and films that show superconducting property of NbTi+MgB2 below 9K which is Nb-Ti super¬ conducting transition temperature and show MgB2 superconducting property from 9K to 39K [21] Brief Description of the Drawings [22] Drawings 1, 2, 3 are cross-sections of MgB 2 multifilamentary superconducting wire using Nb-Ti superconductor as a tube. They are composed of a superconducting Nb-Ti tube(200), superconducting MgB2 filament(lOO) and a copper alloy shielding metal(300). In addition, a barrier of Nb etc(400) is between shielding metal(300) and Nb-Ti tube(200), and also between the MgB2 filament(lOO) and Nb-Ti tube(200). [23] Drawing 4 is a cross-section of Nb-Ti multifilamentary superconducting wire. They are composed of a superconducting Nb-Ti filament(lθ), a tube(20) of coating thd filament, and shielding metal(30) coating the entire wire [24] Drawings 5 - 11 are cross sections of a variety of films using Nb-Ti superconductor as a substrate [25] Best Mode for Carrying Out the Invention [26] We consider in company with drawings [27] MgB2 multifilamentary superconducting wire using Nb-Ti superconductor as a tube ; [28] Drawings 1, 2 and 3 show a cross section of a MgB2 multifilamentary wire which use Nb-Ti superconductor as a tube according to this invention. As shown, this invention is composed of a Nb-Ti tube(200), MgB2 filament(lOO) and a Cu based shielding metal(300). This invention can add a barrier(Nb etc,400) between the MgB2(100) and the NbTi tube(200), and between the NbTi tube(200) and shielding metal(300). [29] Genernally superconducting wires, which are shown in drawing 4, are composed of superconducting filaments(lθ), and a tube(20), which coats the filaments, and the metal shielding(30), which coats the entire external wire. In this invention, the super¬ conducting filaments(lOO) are composed of MgB2, the tube(200) is composed of Nb- Ti superconductor and the metal shielding(300) is composed of a copper-based alloy. MgB2 is a superconductor with a critical temperature of 39K, and performs best at critical temperature among the nonoxide superconductor. Nb-Ti is wellknown metal superconductor with a critical temperature of 9K. Since the drawing processes for Nb- Ti superconducting wires are well known and are well developed, Nb-Ti super¬ conducting wire is widely used. [30] In this invention, Nb-Ti superconductor, which has been used as a filament, is used as a tube(200), the filament(lOO), which is inserted into the tube, is MgB2. [31] MgB2 which is inserted into the Nb-Ti tube is composed of MgB2 powder, Boron powder, Mg+B powder, MgB2+X powder, Mg+B+X powder and a compound of powders which are changed to MgB2 during heat treatment. X indicates an element or a compound which enhances flux pinning effect or reduces the porosity of MgB2. It can be Nb, Ta, Ti, Fe, W etc. Finally as they are heat treated at 600-950°C in an inert gas or vacuum, they become superconducting MgB2 multifilamentary wire . [32] According to the processes which are mentioned above, the drawing processes used for superconducting Nb-Ti multifilamentary wire which are known well and developed well can be entirely used to make this product, but the critical temperature can be as high as 39K. In addition, this wires can have a 400-a barrier(Nb etc) between MgB2(100) and NbTi tube(200) and 400-b barrier(Nb etc) between NbTi tube(200) and shielding metal(300). [33] [34] MgB2 multifilamentary superconducting film using Nb-Ti superconductor as a substrate; [35] Drawings 5-11 show the cross sections of MgB2 superconducting film using Nb-Ti superconductor as a substrate according to this invention. As shown, this invention is composed of a Nb-Ti superconducting substrate, MgB2, which is deposited on the substrate, and shielding material. A barrier(Nb etc) can be added between the substrate and MgB2, and between the substrate and shielding material. [36] As shown in drawing 6, superconducting films are generally composed of a buffer layer to reduce reactivity and enhance alignment between the substrate and deposited superconducting material. In this invention MgB2 is the deposited superconducting material, Nb-Ti is the substrate and YSZ and A12O3 etc are the buffer layers. [37] In this invention, the Nb-Ti superconductor, which has been used as a filament in the wire,is used as a substrate, and MgB2 is the superconducting material which is deposited on the substrate. [38] The deposited MgB2 is composed of one or more in MgB2 powders, Boron powders, Mg+B powders, MgB2+X powders, Mg+B+X powders and a compound of powders which are changed to MgB2 during heat treatment. X is an element or a compound which enhances flux pinning effect or reduces the porosity of MgB2 such as Nb, Ta, Ti, Fe, W, etc. [39] All deposition methods are available for depositing MgB2, such as PLD(Pulser Laser Deposition), ED(Electro Deposition), CVD(Chemical Vapor Deposition), EPD(Electro-Phoretic Deposition), MB, IBAD, sol gel method, etc. The resulting MgB2 superconducting films are heat treated at 600-950°C in a Mg atmosphere, in a vacuum or inert gas. [40] As shown in drawing 10, the supeconducting films can have a barrier(Nb etc) added between the substrate and MgB2, and between the substrate and shielding material. [41] In addition, the buffer layer can be more than two layers thick to enhance the flux pinning effect and the alignment of MgB2. The flux pinning effect can be enhanced by forming point defects and columnar defects on the buffer layer. It is desirable that wires and films to be irradiated by heavy ions so that columnar defects are formed Mode for the Invention [42] There are several methods for producing MgB2 multifilamentary superconducting wire. One will be discussed. MgB2 multifilamentary superconducting wire are started by making hole of the Nb-Ti superconductor rod in length direction. Then the Mg and B powders that have previously been mixed well and have a mole ratio of 1 :2 are poured into the rod through the hole. To make long wire, it is necessary to repeat the drawing and annealing processes. The wire is then cut into equal length and stacked. These are bundled together in the copper alloy metal shielding and drawing and annealing processes are repeated until the desired dimensions are achieved. Finally, they are heat treated at 600-950°C in an inert gas atmosphere. This is how the MgB2 multifilamentary superconducting wire using NbTi superconductor as a tube, shown in drawing 1 was produced. [43] [44] The case of film; [45] At first, NbTi superconducting plate handled properly for the substrate. Borons are deposited on the substrate by using any of established deposition processes which are mentioned above. Mg is deposited on the boron with mole ratio of 1 :2. Nb is deposited on the Mg and B in order to prevent a reaction between MgB2 and the copper based metal shielding . Finally a copper based metal shielding is deposited on the entire film. It is heat treated at 600-950°C in an inert gas atmosphere. This is how the MgB2 super¬ conducting film using NbTi superconductor as a substrate, shown in drawing 10 was produced.