Patent Application
20190288175
This application claims foreign priority under 35 U.S.C. § 119(a)-(d) to European Application No. 18 161 977.6 filed on Mar. 15, 2018, the entire contents of which are hereby incorporated into the present application by reference.
The invention relates to a tape type superconductor, extending along a longitudinal direction, comprising
Such a tape type superconductor is known, e.g., from US 2007/0191202 A1.
Superconductors can be used to carry an electric current at practically no ohmic losses, for example in order to produce high strength magnetic fields in superconducting magnet coils, or simply to transport the current from a source to a consumer. Superconductor materials have to be cooled to cryogenic temperatures in order to assume superconductivity. While low temperature superconductors (=LTS) such as NbTi are in general metallic and therefore can be prepared easily for example as drawn wires, high temperature superconductors (=HTS) such as YBCO are in general ceramic and therefore are often prepared as tape type superconductors to compensate for the brittle characteristics of the ceramic HTS.
Tape type superconductors typically comprise a substrate of preferably flexible material such as steel, covered with at least one buffer layer and a superconductor layer. Manufacturing of a HTS coated tape is for example described in EP 2 490 273 A1.
Tape type superconductors are difficult to prepare at high quality, in particular over an extended length. In particular, local defects in the superconductor layer may deteriorate the achievable critical current. When transporting AC currents, significant AC losses due to eddy currents may occur. More generally, induced superconducting closed loop shielding currents store energy in the tape type superconductor, and generate magnetic moments. The magnetization may decrease the conductor stability, i.e. the risk of a sudden loss of superconductivity (“quench”) may rise. Further, magnetization may lead to field oscillations, deteriorating measurements such as NMR measurements performed with a magnetic coil using the tape type superconductor.
In order to reduce AC losses, EP 2 144 255 A1 proposes a superconducting cable having a channel for cooling liquid and a tubular support structure, wherein high Tc superconductors are arranged in two layers using different high Tc materials, and having normal-conducting interruptions in the high Tc conductors of the outer layer.
US 2007/0191202 A1 proposes a superconducting tape with a segmented HTS layer. Disruptive strips are formed in one of the tape substrate, a buffer layer, and a superconducting layer, and create parallel discontinuities in the superconductor layer, which reduces AC losses.
In this design, the superconductor layer is formed of parallel superconducting filaments. While this works well for relatively short tape lengths, for more extended tape lengths local defects in the superconductor filaments become more probable, which can make an affected superconducting filament useless. Therefore, good critical currents are difficult to achieve for more extended tape lengths.
U.S. Pat. No. 9,786,415 B2 proposes a high temperature superconductor tape including a plurality of superconducting filaments, to improve a transverse tensile strength. The filaments may be produced by removing portions of a superconductive layer.
US 2017/0287599 A1 proposes a superconducting wire with a base material, an intermediate layer, and an oxide superconducting layer. The intermediate layer comprises non-orientation regions extending in a longitudinal direction of the base material, which lead to non-orientation regions in the oxide superconducting layer, too, which have no superconducting characteristics or a significantly lower critical current.
DE 10 2009 038 920 A1 proposes a multifilament conductor, wherein superconducting filaments are inclined with respect to a longitudinal direction of the multifilament conductor. The filaments are wound about a substrate. The multifilament conductor has decreased electric losses, and minimizes inductance of currents in external alternating fields. In one embodiment, bridges are provided between neighbouring wound filaments.
The filaments wound about the substrate are difficult to produce and come along with currents flowing in a screw like fashion, what generates magnetic fields along the screw axis, which are generally undesired. Further, using the bridges between neighbouring wound filaments may require current flow against the overall flow direction, which deteriorates the achievable magnetic field strength.
It is an object of the invention to present a tape type superconductor, with which high critical currents may be achieved in a simple way over extended tape lengths with suppressed magnetization.
This object is achieved, in accordance with one formulation of the invention, by a tape type superconductor as introduced above,
characterized in
that the tape type superconductor comprises a plurality of elongated barrier structures which are oriented in parallel with the longitudinal direction,
wherein a respective barrier structure has a length LBS in longitudinal direction,
with LBS≥0.20*WSL and LBS≤0.20*LTTS,
that the barrier structures are arranged distributed along the longitudinal direction, and that the barrier structures are located at least partially in the superconductor layer and impede a superconducting current flow in width direction across a respective barrier structure.
The inventive tape type superconductor permits high electrical currents to be transported along the longitudinal direction (or tape direction) superconductively. In particular, the barrier structures extending in parallel to the longitudinal direction do not hinder the transport of the electrical current in this longitudinal direction in the superconductor layer. Further note that no currents turned around the substrate tape are established.
Further, less energy is stored in the form of magnetization. The tape type superconductor is more stable against quenches, and also field oscillations (that might deteriorate NMR measurements, for example) are reduced. The reduction in magnetization is achieved in spite of the limited length of the barrier structures in longitudinal direction and their arrangement distributed in longitudinal direction.
In order to have a good protection against shielding currents, the invention proposes a minimum length LBS of the barrier structures as compared to the (overall) superconductor layer width WSL, with LBS≥0.20 WSL.
In accordance with the invention, the barrier structures are significantly shorter than the length LTTS of the tape type superconductor, with LBS≤0.20 LTTS. This allows a circumvention of possible local defects in the superconductor layer. The superconducting current may, if needed, flow in width direction (“transverse flow”) in front of and behind a barrier structure, and thus switch to an area in width direction not affected by a local defect in the superconductor layer. The switching of the superconductive current in width direction is easy and does not require any effective back currents, since the barrier structures extend in parallel with the longitudinal direction.
The electric current flow in width direction may be controlled by a non-linear conductivity of intermediate regions of superconductor layer material which precede and follow barrier structures in width direction; the intermediate regions typically form narrow gaps between each two barrier structures which are at the same position in width direction and which are subsequent in longitudinal direction. The intermediate regions form superconductive links between areas of the superconductor layer which are adjacent in width direction. These superconductive links are relatively weak, though. The superconductive links may easily be overloaded with shielding currents which are responsible for a coupling of said adjacent areas. Therefore, in high magnetic fields, the intermediate regions tend to be at normally conducting state, and due to this normally conducting state, shielding currents responsible for coupling of said adjacent areas are suppressed. However, if a non-superconductive defect is present in one of said adjacent areas, the coupling shielding currents are at a lower level in the vicinity of the defect. Therefore, the neighboring intermediate regions may carry additional current helping in bypassing the defect. This behavior results in a self-adjusting cross-coupling between said adjacent areas of the superconductor film, and may allow for a further increase in the usable length of the inventive tape-type superconductor.
Further, the barrier structures are simple to produce, in particular over long lengths, since they are in parallel with the extension direction. In the most simple case, the tape type superconductor may be treated during a winding the tape in order to establish the barrier structures, with a tool or tools located at a constant position in width direction.
Note that the substrate tape of an inventive tape type superconductor is in general only covered with a superconductor layer on one of its flat sides. Typically, there is also a metallic coating covering the superconductor layer (and also the barrier structures). The substrate tape is typically polished before depositing a buffer layer and the superconductor layer.
Note that typically LBS≤(10−3)*LTTS or even LBS≤(10−5)*LTTS, and further typically LTTS≥10 m or even LTTS≥100 m. The width WSL of the superconducting layer (and also the width of the tape type superconductor in general) is typically from 1.5 mm to 2 cm, and often from 2.5 mm to 1.0 cm. A typical length LBS is from 2.0 mm to 2.5 cm, and often from 3.0 mm to 1.5 mm. In general, the barrier structures are also arranged distributed in width direction. The arrangement of barrier structures is preferably non-periodic along the longitudinal direction (or tape direction). However, a periodic arrangement of the barrier structures is also possible, in particular with a period P much longer than the (average) length LBS, such as P≥5*LBS.
Typically, all barrier structures have a uniform length LBS. However, it is also possible to have a distribution of barrier structure lengths in the tape type superconductor. Structures having dimensions not covered by the defined requirements (see above) are not considered as barrier structures in the sense of the present invention.
In a preferred embodiment of the inventive tape type superconductor, a respective barrier structure stretches across the complete height HSL of a the superconductor layer in a height direction, with the height direction being perpendicular to the longitudinal direction and being perpendicular to the substrate tape flat side. This provides a maximum impediment for a superconducting current in width direction at the barrier structure, and thus optimum protection against undesired shielding currents or magnetization, and is simple to produce.
Particularly preferred is an embodiment wherein the barrier structures are non-superconducting or exhibit a critical current density jcBS in width direction which is less than 1/100 of a critical current density jcSL in width direction of a superconducting material of the superconductor layer. This makes sure that superconductive shielding currents across the barrier structures can be excluded or at least kept significantly weaker than a (regular) longitudinal superconductive current.
In an advantageous embodiment, the barrier structures are spaces filled with a non-superconducting material of a different chemical composition as compared to the superconducting material of the superconductor layer, in particular wherein the spaces are filled with a non-superconducting metal. Establishing a different chemical structure in the spaces is a simple and highly reliable measure for implementing a barrier structure. Typically, the spaces are first made by removing material from a (closed) superconductor layer, and then the resultant gaps are filled with the non-superconducting material. Metals are particularly simple to use for the latter purpose. Alternatively, a chemical composition can be changed locally e.g. by ion bombardment. This embodiment typically includes a post treatment of a (continuous) superconductor layer.
In an alternative embodiment, the barrier structures have the same chemical composition as the superconducting material of the superconductor layer, but exhibit deviations from the phase composition and/or exhibit disturbances in the crystalline structure as compared to the superconducting material of the superconductor layer. The barrier structures are typically established by treating the substrate tape or a buffer layer at locations where barrier structures are desired (“disturbance pattern”), before depositing the superconductor layer. The superconducting material only assumes the superconducting phase away from the disturbance pattern. then no post treatment of a superconductor layer is needed. However, it is also possible to alter the chemical (or elemental) composition e.g. by a local heat treatment.
In a preferred embodiment, at least 80%, preferably at least 90%, of the length LTTS of the tape type superconductor is overlapped by barrier structures. In this way, a high level of protection against undesired shielding currents and magnetization in the superconductor layer can be achieved.
Particularly preferred is a further development of this embodiment, wherein 100% of the length LTTS of the tape type superconductor is overlapped by barrier structures. This establishes an even better protection against undesired shielding currents and magnetization.
In a preferred embodiment, at least 80%, preferably at least 90%, of the length LTTS of the tape type superconductor is overlapped by at least n barrier structures which are sequent in width direction, with n≥2. Preferably, 100% of the length LTTS of the tape type superconductor is overlapped by at least n barrier structures which are sequent in width direction, with n≥2. With more overlapping barrier structures, a finer limitation of spaces in width direction for shielding currents may be achieved, what helps to reduce undesired magnetization further.
In a particularly preferred embodiment for an average barrier density ABD, which is defined as a local barrier density of the tape type superconductor averaged along the complete length LTTS, with the local barrier density being the number of barrier structures intersected by a cross section of the tape type superconductor perpendicular to the longitudinal direction at a local position in longitudinal direction, the following applies
ABD≥0.80,
preferably ABD≥1.0,
most preferably ABD≥2.0. With a high average barrier density, a high level of protection against shielding currents or magnetization, respectively, can be achieved. Note that often ABD>2.0 also applies. Note further that often ABD≤4 applies.
A preferred further development of this embodiment provides that
In another advantageous further development, the barrier structures are arranged distributed over at least m different positions in width direction,
with m>2*ABD or m>3*ABD,
in particular wherein on average over the length LTTS, the barrier structures are basically equally distributed over the at least m different positions in width direction. In this case, the barrier structures may be put at a variety of different positions, in particular more different positions than necessary for achieving the given average barrier density ABD. In this way, the barrier structure distribution may be more versatile, and in particular random patterns may be established easily, which are less prone to quenches and undesired magnetic field components than regular or periodic patterns. Preferably, the at least m different positions are basically equally distributed in width direction.
In a preferred embodiment, 0.25*WSL≤LBS and/or LBS≤25*WSL,
preferably with 0.5*WSL≤LBS and/or LBS≤12.5*WSL. Often also LBS≤5*WSL applies. With these parameters again, the dimensions of barrier structures are typically adequate for a good protection against undesired magnetization, and for not being prone to local defects in the superconductor layer.
Preferred is further an embodiment wherein the barrier structures have an aspect ratio ARBS=LBS/WBS, with ARBS≥10, preferably ARBS≥20, with WBS: width of a respective barrier structure in width direction. Typically there is also ARBS≤500, in particular ARBS≤100. Note that WBS is typically about 25 μm through 250 μm. These dimensions are both easy to produce and offer a high protection against undesired increased shielding currents or magnetization, respectively.
In a preferred embodiment, the superconducting material of the superconductor layer is a high temperature superconductor, in particular REBCO or BiSCCO or MgB2. On the inventive tape type superconductor, in particular with a flexible substrate tape, these brittle materials may be handled safely and used for typical applications such as superconducting coils, for example as magnets in NMR (nuclear magnetic resonance) apparatus.
Also preferred is an embodiment wherein the substrate tape is made of metal, in particular stainless steel or Hastelloy. Metal substrate tapes are safe to handle, in particular for winding coils. Alternatively, the substrate tape can be made, for example, of a ceramic material. Note that in general, the substrate tape (and the tape type superconductor as a whole) is preferably flexible.
Also within the scope of the present invention is a method for producing an inventive tape type superconductor, wherein the barrier structures are spaces filled with a non-superconducting material of a different chemical composition as compared to the superconducting material of the superconductor layer, characterized in that
Alternatively, in accordance with the invention, there is a method for producing an inventive tape type superconductor wherein the barrier structures are spaces filled with a non-superconducting material of a different chemical composition as compared to the superconducting material of the superconductor layer, characterized in that
Further within the scope of the present invention is a method for producing an inventive tape type superconductor, wherein the barrier structures have the same chemical composition as the superconducting material of the superconductor layer, but exhibit deviations from the phase composition and/or exhibit disturbances in the crystalline structure as compared to the superconducting material of the superconductor layer, characterized in that
Alternatively, in accordance with the invention, there is a method for producing an inventive tape type superconductor, wherein the barrier structures have the same chemical composition as the superconducting material of the superconductor layer, but exhibit deviations from the phase composition and/or exhibit disturbances in the crystalline structure as compared to the superconducting material of the superconductor layer, characterized in that
Further advantages can be extracted from the description and the enclosed drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any combination. The embodiments mentioned are not to be understood as exhaustive enumeration but rather have exemplary character for the description of the invention.
The invention is shown in the drawing.
It should be noted that the figures are schematic in nature, and some features may be shown in an exaggerated or understated way, in order to show particular features of an inventive tape type superconductor or an inventive production method more clearly.
The tape type superconductor 1 comprises a substrate tape 2, which is flexible so it can be wound for example into a solenoid type coil, further at least one buffer layer 3 deposited on a flat side 8 of the substrate tape 2, and a superconductor layer 4 deposited on top of the at least one buffer layer 3. Typically, the superconductor layer 4 is further covered with a metallic protection layer or shunt layer (not shown), for example made of a noble metal such as silver or made of copper. The superconductor layer 4 is made of a superconducting material, typically a high temperature superconductor material of ceramic type such as YBCO.
Further, the tape type superconductor 1 includes a plurality of barrier structures 5 extending in parallel (within the manufacturing accuracy) to the longitudinal direction LD. The barrier structures 5 extend over the complete height HSL of the superconductor layer 4 in a height direction HD (which runs perpendicular to the flat side 8). The barrier structures 5 are filled with a material that is non-superconducting, such as a metal, or filled with a material with significantly worse superconducting characteristics as compared to the superconducting material of the superconductor layer 4, for example with a critical current density lower by a factor of more than 100 (at the same temperature and magnetic field strength during operation). Note that preferably, the material of the barrier structures 5 is normally conductive, with an electrical conductivity corresponding to the conductivity of copper or better (at operating temperature, such as at 4.2 K). The barrier structures 5 are separate from each other, such that in general, each barrier structure 5 is surrounded by superconducting material of the superconductor layer 4 in width direction WD and longitudinal direction LD (with the exception of end faces of barrier structures 5 at an end of the tape type superconductor 1, see here right end in
The tape type superconductor 1 is intended for transporting an electric current superconductingly along the longitudinal direction LD.
In the example shown, the barrier structures 5 have a uniform length LBS in longitudinal direction LD, the overall tape type superconductor 1 has a length LTTS in longitudinal direction LD, and the superconductor layer 4 has a constant width WSL (which is here identical to a width of the tape type superconductor 1 in general) in width direction WD. The barrier structures 5 are arranged subsequent in longitudinal direction LD, and are all arranged at the same position 6a (m=1) in width direction, such that the position 6a is in the middle of the tape type superconductor 1 with respect to the width direction WD. Between each two neighboring barrier structures 5 in the sequence, there is an intermediate region 7 belonging to the superconductive layer 4, and therefore with the superconductive characteristics of the superconducting material of the superconductor layer 4. At the intermediate regions 7, a superconducting current may flow between an (in
In the example shown, the following roughly applies:
a) LBS=0.20*LTTS; note that typically LTTS is much longer than shown in the example, so often LBS≤0.001*LTTS, for example;
b) LBS=0.92*WSL;
c) LIR=0.17*LBS; note that this means here that about 86% of the entire length LTTS is overlapped by barrier structures 5, and an average barrier density ABD is about 0.86 in this case.
The barrier structures 5 separate the superconductor layer 4 into an (in
In the following, further embodiments of inventive tape type superconductors 1 are explained, and only the major differences with respect to the embodiment shown in
In this embodiment, barrier structures 5 of uniform length LBS are located at two positions 6a, 6b (m=2) in width direction WD; note that in general, embodiments wherein the barrier structures 5 are distributed over a plurality of positions in width direction WD (i.e. m≥2) are generally preferred, so shielding currents may be more limited in space in width direction WD, in order to achieve a lower magnetization. At each position 6a, 6b, barrier structures 5 are arranged subsequent in longitudinal direction LD, with intermediate regions 7 of uniform length LIR between barrier structures 5 neighboring in longitudinal direction LD.
The intermediate regions 7 of positions 6a and 6b are displaced in longitudinal direction such that they do not mutually overlap. Seen the other way, the barrier structures 5 of positions 6a and 6b are displaced in longitudinal direction such that they do mutually overlap, here at both ends. As a result, all of the length LTTS, i.e. 100%, are overlapped by at least one barrier structure 5.
In general it is preferred that
LIR≥0.25*WSL/(m+1) and/or LIR≤4*WSL/(m+1),
preferably LIR≥0.5*WSL/(m+1) and/or LIR≤2*WSL/(m+1), with m: number of positions in width direction over which the barrier structures 5 are distributed. Often LIR≤WSL is also preferred, and often LIR≤WSL/4 or even LIR≤WSL/10 also applies.
The positions 6a, 6b are basically equally distributed over the width WSL of the superconductor layer 4 or of the tape type superconductor 1, respectively. The barrier structures 5 have an aspect ratio ARBS=LBS/WBS, with WBS being the width of the barrier structure 5 in width direction WD, and with here approximately ARBS=14; note that in general, aspect ratios ARBS of 10 or more, or even 20 or more are preferred.
In the example shown, the following approximately applies:
a) LBS=2.38*WSL;
b) LIR=0.11*LBS; since the intermediate regions 7 do not overlap, this results in an average barrier density ABD of [3*0.11*2+(1.11−3*0.11)*3]/(1.11)=2.70.
In this embodiment, the barrier structures 5 are arranged periodically with respect to the longitudinal direction LD, here with a period P corresponding to the entirety of one barrier structure 5 and one adjacent intermediate region 7, and here with approximately P=2.63*WSL.
In the fourth embodiment, the barrier structures 5 have a variable length LBS. However, intermediate regions 7 between barrier structures subsequent in longitudinal direction at the same position 6a-6c have a uniform length LIR in longitudinal direction LD.
In the example shown, the barrier structure 5a has the shortest length LBSshort, for which applies here approximately LBSshort=1.0*WSL, and the barrier structure 5b has the longest length LBSlong; for which applies here approximately LBSlong=2.9*WSL. The lengths LBS of all barrier structures 5 are randomly distributed between LBSshort and LBSlong, and said barrier structures 5 are randomly arranged at the positions 6a-6c in random sequences.
However, as a border condition, an overlap of intermediate regions 7 should not be allowed for neighboring positions 6a-6c, and preferably should not be allowed for any positions 6a-6c (as shown here). Please note that in case of long enough (average) lengths LBS as compared to the length LIR, for example for (average) LBS≥50*LIR, an overlap of intermediate regions 7 for neighboring positions 6a-6c in random arrangements becomes so rare that it does not need to be considered any more.
A random arrangement of barrier structures 5 along the tape type superconductor 1, as shown for example in
In this embodiment, the barrier structures 5 have a uniform length LBS in longitudinal direction LD and are distributed equally over five positions 6a-6e (i.e. m=5) in width direction WD, with said positions 6a-6e also being equally distributed along the width direction WD.
In the example shown, each barrier structure 5 has an overlap with two other barrier structures 5a, 5b, with each of the other barrier structures 5a, 5b overlapping with half of the length of said barrier structure 5 at the end and at the front, respectively. As a result, an average barrier density ABD=2 is established.
Along the longitudinal direction LD, for a given barrier structure 5, the position 6a-6e at which the next overlapping barrier structure 5b is located is randomly chosen from the positions which are unequal to the positions of said barrier structure 5 and the previous barrier structure 5a. For example, for said barrier structure 5 marked in
As a consequence of the random arrangement of barrier structures 5, barrier structures 5 at the same width position 6a-6e and subsequent in longitudinal direction LD are separated by intermediate regions 7, with the intermediate regions 7 having random extensions in longitudinal direction.
When the number m of available positions 6a-6e is relatively big as compared to ABD, for example with m>2*ABD or with m≥(ABD+2), and here with m=2.5*ABD or m=ABD+3, respectively, a particularly large variety of possible (random) arrangements of the barrier structures 5 is available. In this case, congeneric behavior and self-enforcing effects are even less likely.
In the illustrated example, approximately LBS=0.67*WSL applies; note that for relatively big m as compared to ABD, relatively short lengths LBS of the barrier structures 5 are preferred, for example with LBS≤2*WSL/(ABS+1).
The method starts with a substrate tape 2, for example a steel substrate or a Hastelloy substrate, polished at its surface 2a of the flat side 8, see
Then at locations intended for barrier structures, a laser beam 60 originating from a laser device 61 is applied, compare
Please note that in
In the second variant of a method for producing a tape type superconductor shown in
On a polished surface 2a of a substrate tape 2, see
Note that in the tape type superconductor 1, the elemental composition of the material of the superconductor layer 4 and the barrier structure 5 are identical here, but the disturbance pattern 81 causes a different phase composition and/or a different crystallinity, resulting in different characteristics with respect to superconductivity.
It should be noted that instead of scratching (or otherwise disturbing) the surface 3a of the buffer layer 3, also the polished surface 2a of the substrate tape 2 may be scratched (or otherwise disturbed). The buffer layer (or layers) deposited on top can carry on this disturbance pattern to the surface 3a of the buffer layer 3 then, also resulting in a superconductor layer 4 and barrier structures 5 upon material deposition.
In the fourth variant of a method for producing a tape type superconductor shown in
In the illustrated variant, the elemental composition of the originally superconducting material of the superconductor layer 4 does not change upon the heat treatment. However, in another variant, very strong heating may lead to a thermolysis, with elements evaporating into the surrounding; in this case the elemental composition will change in the space 65 or the barrier structure 5 as compared to the superconductor layer 4.
In summary, the invention proposes a tape type superconductor with a plurality of barrier structures within its superconductor layer. The barrier structures are much shorter than the total length of the tape type superconductor, and the barrier structures are arranged subsequent in longitudinal direction, to which they are parallel. At a particular position in width direction, numerous barrier structures, typically 10 or more, often 100 or more, are arranged subsequently in longitudinal direction, but separated from each other by superconducting intermediate regions. The barrier structures are arranged at at least one position in width direction, but there may be a plurality of positions in width direction over which the barrier structures are distributed. The barrier structures may be distributed in a pattern periodic in longitudinal direction, or may be arranged in a random pattern. The barrier structures are non-superconducting or worse superconducting as compared to the superconductor layer. The separated barrier structures allow for a decoupling of regions in the superconductor layer, but all regions of the superconductor layer are still interconnected superconductingly. This reduces unwanted induced magnetization, without a substantial reduction of the critical current. Inventive tape type superconductors may be used in spools, magnet coils, in particular for NMR magnets, for motors or generators, transformers, fault current limiters or cables, for example.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18161977.6 | Mar 2018 | EP | regional |
