Tubular filaments containing magnesium diboride powder

Abstract

The tubular (PIT) wire containing magnesium diboride (MgB2) powder exhibits a cross-section showing a void-Mg diboride-oxide structure, as measured by energy-dispersive X-ray spectroscopy. The oxide is located at the boundary between the voids and the magnesium diboride. The MgB2 PIT wire exhibits a high degree of superconductivity.

Description

Technical Field

[0001] This invention relates to tubular filaments containing magnesium diboride powder, and more particularly to tubular filaments containing in-situ magnesium diboride powder for use as superconducting filaments. The invention also relates to a method for manufacturing such filaments. Background Technology

[0002] WO 2017 / 064471A1 discloses a tubular filament containing in-situ magnesium diboride powder and a method for manufacturing such a filament.

[0003] The powder-infused tubular filament is manufactured using the known Powder-Infused Tube (PIT) technology. According to this technology, precursor magnesium and boron powders are packed into one or more metal tubes. The tubes are mechanically deformed through stretching, forging, rolling, and heat treatment to obtain the final tubular superconducting filament containing magnesium diboride powder.

[0004] In the non-in-situ route, the already reacted magnesium diboride powder is used as the starting material.

[0005] In the in-situ route, unreacted magnesium and boron powders, and possibly doped powders, are used as starting materials.

[0006] WO 2017 / 064471A1 discloses the use of a carrier liquid that is chemically inert to magnesium powder, boron powder, or both. The liquid does not oxidize to magnesium powder and / or boron powder. The carrier liquid, together with the powder, forms a slurry continuously fed into channels in one or more tubes. Heating is then performed under a non-oxidizing atmosphere to obtain a solid residue. By mixing the powder into an inert carrier liquid, problems such as oxidation and moisture absorption are reduced, and the powder is mixed to a better level of uniformity. Summary of the Invention

[0007] The general aspect of the present invention aims to further improve the degree of superconductivity.

[0008] A more specific aspect of the invention is to position the oxygen or oxide in the superconducting filament in a location that is less detrimental to the superconducting properties.

[0009] Another specific aspect of the invention aims to further reduce the oxygen or oxide content in superconducting wires.

[0010] Another aspect of the invention is that it allows the use of larger magnesium powders without reducing electrical conductivity.

[0011] Another aspect of the present invention is to increase the propagation speed of a superconducting filament in the normal region.

[0012] According to a first aspect of the invention, a tubular filament containing magnesium diboride powder is provided, having a cross-section showing:

[0013] - Gap or cavity,

[0014] - Magnesium diboride, and

[0015] -Oxides,

[0016] The oxide is located at the boundary between the voids and magnesium diboride.

[0017] The development of voids, magnesium diboride, and oxides can be achieved using energy-dispersive X-ray spectroscopy.

[0018] The tubular filament containing magnesium diboride powder is preferably a tubular filament containing in-situ powder.

[0019] The abundant oxygen reacts with highly reactive magnesium to form magnesium oxide. Since the majority of this magnesium oxide exists at the boundary between unavoidable voids or cavities and magnesium diboride, very little oxygen remains in magnesium diboride crystalline materials.

[0020] Therefore, magnesium diboride has strong grain boundaries because it is contaminated with oxygen to a very limited extent.

[0021] Because the current path is through magnesium diboride, a higher critical current Ic can be obtained.

[0022] The stronger grain boundaries in the magnesium diboride matrix result in improved grain connectivity and better bending properties. This means that for the same wire diameter, a smaller bending radius is possible, which is beneficial for the final coil manufacturing.

[0023] Another advantage is that the content and location of magnesium oxide are not dependent on the size of the magnesium powder. Therefore, larger magnesium powders in the range of 100 μm to 350 μm can be used as starting materials, which reduces the cost of starting materials and lowers the risk of explosion. In fact, magnesium is known to be highly reactive, and it is known that the smaller the size of the magnesium powder, the greater the risk of ignition.

[0024] Another advantage is the higher longitudinal normal region propagation speed compared to existing magnesium diboride wires. The longitudinal normal region propagation speed determines the speed at which local hot spots propagate along the wire length. A high longitudinal normal region propagation speed means a rapid expansion of the normal resistance region within the superconductor, resulting in a rapid increase in the resistance voltage measured at the end of the coil. The faster this measurement can be performed, the faster an impending quench can be detected, and the faster protection mechanisms can be triggered to safeguard the magnet from localized overheating and irreversible damage.

[0025] It is believed that a more homogeneous magnesium diboride matrix with a lower oxygen concentration will result in a higher n-value, and therefore a higher propagation velocity in the normal region. The n-value is a commonly used parameter for measuring conductor uniformity. The higher the n-value, the higher the propagation velocity in the normal region.

[0026] Preferably, more than 60%, most preferably more than 70%, 80%, 90%, or 95% of the oxides of the present invention are located at the boundary between the voids and magnesium diboride. Due to the high reactivity of magnesium, the majority of these oxides are magnesium oxide. Magnesium oxide is primarily MgO.

[0027] Even when applying the slurry method of WO2017 / 064471A1, oxides cannot be avoided. Magnesium is one of the most reactive elements to oxidation. Therefore, oxidation occurs on the surfaces of magnesium and boron powders, forming magnesium oxide (mainly MgO) and boron oxide (B2O3). Magnesium powder or boron powder, or both, along with possible dopants such as silicon carbide, is added to a first carrier liquid that is chemically inert to at least one powder. One way to further reduce oxides in the final PIT filament is to wash the slurry after forming it with the first carrier liquid and at least one powder. This washing step removes most of the magnesium oxide and / or boron oxide and / or oxides formed with the dopant. A second carrier liquid, which may be equivalent to or different from the first carrier liquid, is then added to the remaining residue to form a second slurry. As a result, the total amount of oxygen and oxides in the second slurry is less than that in the first slurry. However, magnesium oxide is still present in the second slurry. Magnesium oxide is present in close proximity to metallic magnesium particles. Both are relatively large compared to the size of the boron particles. When the heat treatment begins to form magnesium diboride later in this process, the heated metallic magnesium has almost no problem infiltrating the boron powder to react and form magnesium diboride. Most of the oxide remains in the boundary region between the voids and the magnesium diboride matrix. A small amount of oxide is able to penetrate into the magnesium diboride matrix and affect the intergranular connectivity of the magnesium diboride particles.

[0028] In addition to or as an alternative to washing, controlled chemical reactions with oxides in the slurry can reduce the oxygen content in the final PIT filament, for example, by using solvents.

[0029] In a particularly preferred embodiment, as a result of the washing step, the oxygen content of the tubular filament containing magnesium diboride powder is less than 5% by weight (wt%), for example less than 4 wt%, preferably less than 3 wt%, and most preferably less than 2.5 wt%. The oxygen content can be determined by scanning electron microscopy and energy-dispersive X-ray diffraction on a statistically significant portion of the cross-section of the superconducting material, without considering the area of ​​voids and cavities, and therefore the measurement or calculation is performed outside the area of ​​voids or cavities.

[0030] In another specific and preferred embodiment of the invention, a multifilamentary system is provided, consisting of two or more tubular filaments containing in-situ magnesium diboride powder. These filaments may be bundled together, or preferably wound together.

[0031] According to a second aspect of the present invention, a method for manufacturing a tubular filament containing in-situ magnesium diboride powder is provided. The method includes the following steps:

[0032] a) Mix magnesium powder and / or boron powder in a first carrier liquid to produce a first slurry;

[0033] b) Wash the first slurry, thereby washing away magnesium oxide and / or boron oxide, leaving magnesium powder, boron powder residue and remaining oxides;

[0034] c) Add a second carrier fluid to the residue to produce a second slurry;

[0035] d) Add the second slurry into the pre-formed metal sheath;

[0036] e) Close the pre-formed metal sheath to form a tube.

[0037] Preferably, step c) is performed without drying the residue.

[0038] Preferably, the second carrier liquid is equivalent to the first carrier liquid. Attached Figure Description

[0039] Figure 1 The first step in manufacturing a single tubular filament containing in-situ magnesium diboride powder is illustrated schematically.

[0040] Figure 2 The following steps for manufacturing polyfilaments are illustrated schematically.

[0041] Figure 3a The scanned region of the prior art non-in-situ MgB2 wire is shown. Figure 3b It shows Figure 3a Energy dispersive X-ray image of oxygen in non-in-situ MgB2 filaments in the scanning region of the prior art.

[0042] Figure 4a The scanning area of ​​the in-situ MgB2 wire in the prior art is shown. Figure 4b It shows Figure 4a Energy dispersive X-ray image of oxygen in situ of MgB2 filament in the scanning region of the prior art.

[0043] Figure 5a The scanning area of ​​the in-situ MgB2 wire of the present invention is shown. Figure 5b Shown in Figure 5a Energy dispersive X-ray image of oxygen in the in-situ MgB2 filament of the present invention in the scanning region. Detailed Implementation

[0044] The first step in manufacturing in-situ MgB2 PIT wire involves the preparation of the slurry.

[0045] The first slurry is prepared by mixing B powder and / or Mg powder in a first carrier liquid. As previously mentioned, small Mg powders are not required. Mg powders with a size of 100 μm to 350 μm are perfectly suitable. The B powder can be in the nanometer to micrometer size. Doped powders such as SiC can be added. Examples of suitable first carrier liquids are liquid hydrocarbons, ethanol, acetone, methyl acetate, and ethyl acetate. Examples of suitable second carrier liquids are alcohols and acetone.

[0046] The first slurry is then washed. The washing process removes not only the fluid material but also a large portion of the oxides of the present invention, such as MgO and / or B₂O₃. Residue of B and Mg powders remains, along with possible dopant powders and reduced amounts of oxides.

[0047] After the washing step, a second carrier liquid is added to the residue to form a second slurry. This second carrier liquid may be different from the first carrier liquid, but is preferably the same.

[0048] Now for reference Figure 1 It schematically illustrates the first step in manufacturing a single in-situ MgB2 PIT filament.

[0049] Step 100 involves unfolding a metal or bimetallic plate 102. This plate can be Cu, Ni, Nb, Ti, Fe, stainless steel, Cu-Ni, Monel, Ag-Mg, and Nb-Ti, or any combination thereof. Preferably, a bimetallic plate of two metals or two plates of different metals are used. One of the two metals is more conductive than the other. A preferred example is a combination of copper and steel.

[0050] Step 104 involves preforming, for example, a U-shaped metal plate 102 using a preforming element 106. The U-shape is adapted to receive a second slurry containing various powders.

[0051] Step 108 is to deliver a second slurry containing powder. This can be done in one step, where all powders B, Mg, and any doped powders are added to the deformable plate 102 through a nozzle 110. Alternatively, the various powders in the separate second slurry can be added individually through nozzles 110, 112, and 114.

[0052] Step 116 involves pretreating the powder using heater 118.

[0053] Step 120 involves sealing the metal plate 102 using a preformer 122 and a welding operation to form a closed metal tube.

[0054] Step 124 involves reducing the cross-section of the tube to form a single filament. This reduction can be achieved by roller 126 or by a series of dies.

[0055] Step 128 is a quality control step.

[0056] The result of the first series of manufacturing steps 100, 104, 108, 116, 120, 124 and 128 is a single PIT filament 130 with unreacted powder B and Mg and possibly doped powder on the inside of the metal plate.

[0057] Figure 2 The various steps involved in manufacturing polyfilaments are shown.

[0058] A single PIT filament 130 forms the main starting product for manufacturing multifilaments.

[0059] In step 200, various individual PIT wires 130 are placed adjacent to each other and adjacent to copper or aluminum wires 202 that are to be unwound and wound.

[0060] Step 204 is the winding process, in which various PIT wires 130 are wound around copper or aluminum wires 202 to form a wound structure.

[0061] In step 208, elongated insulating material 210 is wrapped or woven around the winding structure to form a solidified multifilament 212.

[0062] Subsequently, preferably during the winding and wrapping or weaving process, the consolidated polyfilaments 212 are heat-treated to cause Mg to react with B and form MgB2.

[0063] In the final step 214, the polyfilament 212 is impregnated with resin 216.

[0064] Energy-dispersive X-ray spectroscopy has been applied to several samples of MgB2 PIT superconducting wires from two existing technologies, as well as several samples of the MgB2 PIT superconducting wires of this invention.

[0065] All samples were cut using an argon-ion plasma polishing machine (JEOL type, JSM 09010). After creating the net cross-section, the samples were placed on a sample holder for analysis and stored in a vacuum chamber until the analysis began. This was done to avoid oxidation by air.

[0066] Several analyses were performed on a JEOL 7200F equipped with an Oxford X-max EDX detector from Oxford Instruments, which has an 80 mm... 2 The window is shown. The software used is Oxford Aztec version 3.3.

[0067] All samples were measured at a processing time of 5 minutes, with a fixed count / spectral number of 500,000. All elements except B, O, and Mg were removed from the spectra by elimination in Oxford Aztec software.

[0068] The accelerating voltage used in this method is 5kV or 15kV.

[0069] Results are expressed as a weight percentage (wt%). Measurements have an accuracy of 0.1 wt%.

[0070] Existing technology wire 1: Non-in-situ MgB2 PIT wire

[0071] Figure 3a The scanned area of ​​the non-in-situ MgB2 PIT wire is shown. Figure 3b It shows Figure 3a Energy dispersive X-ray image of oxygen in existing non-in-situ MgB2 filaments in the scanning region. Figure 3b The white dots in the image correspond to oxygen.

[0072] Oxygen is uniformly distributed across the entire cross-section, and therefore mainly exists within the MgB2 matrix, while voids or cavities are widely distributed across the entire cross-section. Measurement of pure material that does not include a certain number of cavities is impossible.

[0073] Nine different samples were measured using different magnifications (2000x or 10000x) and different accelerating voltages (5kV or 15kV). The nine samples exhibited the following range:

[0074] -B: 51.0wt% - 52.2wt%

[0075] -O: 6.8wt% - 8.7wt%

[0076] - Mg: 40.5wt% - 42.2wt%

[0077] Existing technology wire 2: In-situ MgB2 PIT wire

[0078] Figure 4a The scanning area of ​​the prior art in-situ MgB2 wire is shown. Figure 4b It shows Figure 4a Energy dispersive X-ray image of oxygen in situ of MgB2 filament in the scanning region of the prior art.

[0079] Figure 4b The white dots in the diagram correspond to oxygen. Oxygen is distributed in areas where oxygen is present and areas where oxygen is insufficient, but the oxygen concentration cannot be correlated with cavities or voids.

[0080] Unlike the existing technology of wire 1, measurements can be taken here with or without cavities.

[0081] Including the cavity and using an accelerating voltage of 15kV and a magnification of 2000x or 10000x, the following ranges were found in the four samples:

[0082] -B: 40.9wt% - 50.3wt%

[0083] -O: 13.8wt% - 23.4wt%

[0084] - Mg: 29.4 wt% - 38.3 wt%

[0085] These broad ranges confirm the uneven distribution of oxygen.

[0086] After eliminating the cavity and using an accelerating voltage of 5kV or 15kV, the following range was found:

[0087] -B: 29.9wt% - 57.3wt%

[0088] -O: 1.6wt% - 34.1wt%

[0089] - Mg: 33.8 wt% - 42.6 wt%

[0090] Similarly, these wide ranges confirm the uneven distribution of oxygen.

[0091] In-situ MgB2 PIT wire of the present invention

[0092] Figure 5a The scanning area of ​​the in-situ MgB2 wire of the present invention is shown. Figure 5b It shows in Figure 5a Energy dispersive X-ray image of oxygen in the in-situ MgB2 filament of the present invention in the scanning region. Figure 5b The white spots correspond to oxygen. Oxygen is concentrated at the boundary between the MgB2 matrix and the cavity.

[0093] Including the cavity and using accelerating voltages of 5kV or 15kV and a magnification of 2000x, the following ranges were found in four samples:

[0094] -B: 52.8wt% - 54.4wt%

[0095] -O: 6.2wt% - 8.4wt%

[0096] - Mg: 37.2 wt% - 40.2 wt%

[0097] These ranges are narrower compared to existing in-situ MgB2 PIT wires.

[0098] Excluding the cavity and using accelerating voltages of 5kV or 15kV and varying amplification factors, the following range was observed:

[0099] -B: 56.0wt% - 58.1wt%

[0100] -O: 0.9wt% - 2.3wt%

[0101] - Mg: 41.0 wt% - 42.3 wt%

[0102] The range here is also quite narrow. The very low oxygen content (always below 2.5 wt%) is significant and illustrates the advantages of the invention.

[0103] The MgB2 PIT wire according to the present invention can be used in superconductors. The superconductor is preferably used in superconducting magnets of magnetic resonance imaging equipment. The superconductor according to the present invention can also be applied to magnetic levitation vehicles, superconducting electromagnetic propulsion ships, nuclear fusion reactors, superconducting generators, accelerators, electron microscopes, energy storage devices, and power cables.

Claims

1. A tubular filament containing magnesium diboride powder, having a cross-section showing: -gap, - Magnesium diboride, and -Oxides, The tubular filament is a tubular filament containing in-situ magnesium diboride powder; in, The oxide is located at the boundary between the voids and magnesium diboride; When measured on a cross-section excluding voids, the amount of oxygen is less than 3 wt%.

2. The filament according to claim 1, wherein, More than 60% of the existing oxides are located at the boundary between the voids and magnesium diboride.

3. The filament according to claim 1 or 2, wherein, The oxide is magnesium oxide.

4. The filament according to claim 3, wherein, Magnesium oxide is MgO.

5. A multifilament comprising two or more filaments according to any one of the preceding claims.

6. A method for manufacturing a filament according to any one of claims 1 to 4, the method comprising the following steps: a) Mixing magnesium powder and / or boron powder in a first carrier liquid to produce a first slurry; b) Wash the first slurry, thereby washing away magnesium oxide and / or boron oxide, leaving a residue of magnesium powder and / or boron powder and the remaining oxides; c) Add a second carrier fluid to the residue to produce a second slurry; d) Add the second slurry to the preformed metal sheath; e) Close the preformed metal sheath to form a tube.

7. The method according to claim 6, wherein, The first carrier liquid is equivalent to the second carrier liquid.

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