U.S. patent application number 14/699936 was filed with the patent office on 2016-11-03 for co-extrusion printing of filaments for superconducting wire.
The applicant listed for this patent is Energy-to-Power Solutions (e2P), Palo Alto Research Center Incoporated. Invention is credited to RANJEET RAO, CHRISTOPHER MAREK REY.
Application Number | 20160322131 14/699936 |
Document ID | / |
Family ID | 57204192 |
Filed Date | 2016-11-03 |
United States Patent
Application |
20160322131 |
Kind Code |
A1 |
RAO; RANJEET ; et
al. |
November 3, 2016 |
CO-EXTRUSION PRINTING OF FILAMENTS FOR SUPERCONDUCTING WIRE
Abstract
A structure has a substrate, and stripes of superconducting
material on the metal substrate, wherein each stripe is separated
from adjacent stripes by a gap. A method of manufacturing a
superconducting tape includes forming a slurry of superconducting
material, forming a slurry of sacrificial material, extruding the
slurries of superconducting and sacrificial materials as
interdigitated stripes onto a substrate, and removing the
sacrificial material to form superconducting filaments.
Inventors: |
RAO; RANJEET; (Redwood City,
CA) ; REY; CHRISTOPHER MAREK; (Knoxville,
TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Palo Alto Research Center Incoporated
Energy-to-Power Solutions (e2P) |
Palo Alto
Knoxville |
CA
TN |
US
US |
|
|
Family ID: |
57204192 |
Appl. No.: |
14/699936 |
Filed: |
April 29, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 39/2487 20130101;
H01L 39/141 20130101 |
International
Class: |
H01B 12/06 20060101
H01B012/06; H01B 13/00 20060101 H01B013/00 |
Claims
1. A structure, comprising: a substrate; and stripes of
superconducting material on the substrate, wherein each stripe is
separated from adjacent stripes by a gap.
2. The structure of claim 1, wherein the superconducting material
comprises magnesium diboride.
3. The structure of claim 1, wherein the substrate comprises one of
metal, metal foil, silicon-carbide, alumina or sapphire, carbon or
graphene.
4. The structure of claim 1, wherein each stripe has a width of 50
micrometers or less.
5. The structure of claim 1, further comprising a buffer layer on
the substrate between the substrate and the stripes of
superconducting material.
6. The structure of claim 5, wherein the buffer layer further
comprises a material selected to provide one of a chemical barrier,
an electrical insulator, or to match coefficients of thermal
expansion of one of the substrate or the superconducting
material.
7. The structure of claim 1, further comprising a top layer on top
of the stripes of superconducting material.
8. The structure of claim 7, wherein the top layer comprises one of
a metal, an electrical insulator, or combination of a metal and an
electrical insulator.
9. A method of manufacturing a superconducting metal tape,
comprising: forming a slurry of superconducting material; forming a
slurry of sacrificial material; extruding the slurries of
superconducting and sacrificial materials as interdigitated stripes
onto a metal substrate; and removing the sacrificial material to
form superconducting filaments.
10. The method of claim 9, further comprising forming the metal
tape into a rotor for a synchronous machine.
11. The method of claim 9, wherein forming a first slurry of
superconducting material comprises wherein mixing a superconductive
powder into an organic solvent with a binder.
12. The method of claim 11, wherein mixing a superconducting powder
into an organic solvent comprises mixing magnesium diboride powder
into butyl carbitol.
13. The method of claim 11, wherein mixing a superconducting powder
into an organic solvent comprises mixing magnesium diboride powder
and chemical dopants into butyl carbitol
14. The method of claim 9, wherein forming a second slurry of
sacrificial material comprises mixing a binder into a solvent.
15. The method of claim 14, wherein mixing a binder into a solvent
comprises dissolving a cellulose binder into an organic
solvent.
16. The method of claim 9, wherein removing the sacrificial
material comprises heating the substrate and slurries to a
temperature in the range of 700-800.degree. C.
17. The method of claim 9, further comprising removing excess
liquid from the first slurry separately from removing the
sacrificial material.
18. The method of claim 9, wherein forming a slurry of
superconducting material comprises forming two slurries of
superconducting material and extruding the slurries of
superconducting and sacrificial material comprises extruding the
two slurries of superconducting material as adjacent stripes.
19. The method of claim 18, further comprising allowing the two
slurries of superconducting material to react and form a
superconducting compound.
20. The method of claim 9, wherein forming a slurry of
superconducting material comprises mixing powders of two different
materials to form the slurry, and removing the sacrificial material
comprises sintering the slurries, which causes the powders to form
a superconducting compound.
21. The method of claim 9, further comprising coating the substrate
with a buffer layer prior to extruding the slurries.
22. The method of claim 9, further comprising coating the stripes
of superconducting material with a top layer.
Description
TECHNICAL FIELD
[0001] This disclosure relates to superconductors, more
particularly to co-extruded superconducting filaments or wires.
BACKGROUND
[0002] Many types of superconducting devices require
superconducting wires or filaments with sub-50 micron sizes for
operational performance reasons. Superconducting wires and
filaments with these small dimensions lead to significantly lower
AC losses and reduced error fields caused by persistent
magnetizations currents. Developing superconducting wire
architectures with these sub-50 micron sizes at relatively low cost
would benefit a wide variety of devices and applications. Some
common types of applications in the commercial electric power
industry where these types of superconducting wires and filaments
would be useful include but are not limited to: motors, generators,
transformers, fault-current limiters, AC and DC cables, among other
types of electrical power equipment.
[0003] Superconducting wires and filaments with sub-50 micrometer
sizes would also be beneficial in the fabrication of medical
devices including but not limited to: Magnetic Resonance Imaging
(MRI) magnets, and Nuclear Magnetic Resonance (NMR) magnets, among
other types of medical imaging devices. Superconducting wires and
filaments with sub-50 micrometer sizes would also be beneficial in
the fabrication of accelerator magnets for high energy physics and
fusion energy magnets. In these types of superconducting magnets,
fast ramping of currents and hence magnetic fields can lead to
excessive AC loss as well as error magnetic fields.
[0004] Small filament sizes of the superconducting wires leads to
reduced losses and reduced error fields, which simultaneously
benefit spatial magnetic field homogeneity and temporal stability.
For military type applications there exists a wide variety of
applications where sub-50 micron sized superconducting wires or
filaments would be beneficial including but not limited to:
electrical bus bars, current leads, data and power transmissions
cables, mine sweeper magnets, AC and DC cables, electromagnetic
rail guns, magnetic energy storage, among other types of military
applications.
[0005] In applications such as motors and generators, current
superconducting machines focus on `hybrid` AC synchronous machines.
In these machines, the rotor is typically superconducting but the
stator will consist of conventional non-superconducting copper
coils, which is why they are referred to as hybrid machines. While
these hybrid machines have considerable performance improvements
over their conventional non-superconducting counter-parts, fully
superconducting machines would achieve further reductions in weight
and size with improved energy efficiency. While some fully
superconducting machines exist, their manufacture remains
prohibitively complicated and expensive.
[0006] Economically viable superconducting materials typically
cannot perform adequately under the high magnetic fields and
standard AC power frequencies (50-60 Hz) required in the stator.
High speed motors and generators require even higher frequency
operation up to 400 Hz, exacerbating the problem of excessive AC
loss. Current high temperature superconducting (HTS) materials such
as first generation BSCCO (bismuth strontium calcium copper oxide)
powder-in-tube (PIT) and second generation YBCO (yttrium barium
copper oxide) have inherently high losses under AC excitation.
Other low temperature superconductors (LTS) such as NbTi (niobium
titanium) and Nb.sub.3Sn (niobium tin) are too expensive to
fabricate and operate to achieve widespread market penetration.
[0007] Magnesium diboride (MgB.sub.2) is a relatively new
superconducting material that has the potential to overcome these
limitations. MgB.sub.2 has low cost, relatively high critical
temperature (T.sub.c), and sustained performance under high
magnetic fields. The high critical temperature (T.sub.c.about.39 K)
is above well above the boiling point of costly liquid helium
(.about.4.2 K at atmospheric pressure). Unlike YBCO and BSCCO,
MgB.sub.2 does not require a high degree of grain-grain alignment,
does not show weak-link behavior, and has strong performance under
high magnetic fields.
[0008] Known methods for fabricating MgB.sub.2 cannot achieve the
fine filament sizes of less than 50 microns needed to support high
frequency operation. Volume production of MgB.sub.2 has been
accomplished using traditional metallurgical techniques based upon
ex-situ PIT or continuous tube forming and filing (CTFF) process.
This approach has significant drawbacks, namely inefficient use of
the MgB.sub.2 fiber and resulting in lower and expensive
multifilament wire fabrication. Most importantly, this method can
only achieve sub-mm filament sizes, rather than the sub-50
micrometer sizes needed.
SUMMARY
[0009] One embodiment comprises a structure having a substrate, and
stripes of superconducting material on the substrate, wherein each
stripe is separated from adjacent stripes by a gap.
[0010] Another embodiment comprises a method of manufacturing a
superconducting tape, including forming a slurry of superconducting
material, forming a slurry of sacrificial material, extruding the
slurries of superconducting and sacrificial materials as
interdigitated stripes onto a substrate, and removing the
sacrificial material to form superconducting filaments separated by
gaps.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows an embodiment of a superconducting synchronous
machine.
[0012] FIG. 2 shows a structure of superconducting stripes extruded
using a co-extrusion print head.
[0013] FIG. 3 shows a more detailed view of superconducting
stripes.
[0014] FIG. 4 shows a flowchart of a method of manufacturing a
structure having superconducting stripes.
[0015] FIGS. 5 and 6 show alternative embodiments of a structure
having superconducting stripes.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0016] As discussed above, many areas and applications would
benefit from sub-50 micron superconducting wires. These range from
the electric power industry to medical devices. For ease of
understanding, the discussion below focuses on synchronous AC
machines to provide a comparison between current hybrid machinery
and fully superconducting machinery. This discussion merely serves
as an example and is in no way intended to limit application of the
embodiments of the invention as claimed, which are directed to
sub-50 micron superconducting wire filaments and the methods of
their manufacture.
[0017] The commercial value of fully superconducting topologies for
applications such as large power plant generators, which already
operate at high efficiencies, is primarily driven by capital
efficiency during construction through reduced generator sizes.
This leads to smaller footprints and less supporting
infrastructure. Some applications like large wind turbines over 10
MW, could see dramatic reductions in size and weight that will
increase economic viability and lead to greater adoption. The
reduction in size and weight comes from superconductors that can
generate the same amount of power with less material. The below
table provides a comparison.
TABLE-US-00001 Hybrid Fully Permanent Super- Super- Machine Type
Magnet conducting conducting Power (MW) 41 41 41 RPM 720 720 720
Material rotor/stator Permanent YBCO/Cu MgB.sub.2/Nb.sub.3Sn
magnet/Cu Top Rotor(K)/Stator(K) 300/420 30/400 20-30/10 Mass (kg)
27,000 10,000 3,800 Specific torque (Nm/kg) 20 55 140 Net machine
efficiency 94% 97% 99% (@ 15% of Carnot)
[0018] As used here, the term `superconducting,` `superconductor,`
`superconductive,` etc., refers to a material that has zero
electrical resistance when cooled below a critical temperature
(T.sub.c) and a complete ejection of magnetic field lines as the
material enters the superconducting state.
[0019] The term `motor` may be used as an example of a synchronous
AC machine, with the understanding that the embodiments here apply
to generators as well. No limitation to either motors or generators
is intended, nor should it be implied. In addition, the embodiments
here may also apply to induction motors, depending upon the
selection of materials.
[0020] FIG. 1 shows an example of a synchronous machine 10. The
synchronous machine has a rotor 12 and a stator 14. One should note
that the example shown is for a mounted motor rather than a turbine
generator. The rotor 12 typically has electromagnets or other
active materials 16, and the stator has similar elements 18. In
most cases these electromagnets take the form of `windings` or
coils of copper wire or other materials that provide the magnet
poles. It is in these windings where the superconductive materials
reside.
[0021] One of the challenges lies in high frequency operation. High
frequency operations require filaments or wires that are sub-50
micrometers, and current manufacturing techniques for the filaments
can only reach the sub-millimeter range. Other challenges include
manufacturability and expense. None of the current processes can
manufacture these filaments easily and the resulting processes are
too expensive to make the filaments cost effective.
[0022] FIG. 2 shows an embodiment of a co-extrusion head depositing
interdigitated stripes of superconducting paste and a sacrificial
material. The co-extrusion head may be referred to as a
co-extrusion print head, as the process of feeding the slurries of
materials and the motion of the head is similar to printing
processes. Different embodiments of the co-extrusion head are
discussed in U.S. Pat. Nos. 9,004,001 and 9,012,090 and US Patent
Publication Nos. 20140186697, 20140186519, and 20150056432 all of
which are incorporated by reference here in their entirety.
[0023] Essentially, two or more slurries or pastes are fed into the
print head and then flowed into adjacent paths so the materials
form adjacent stripes. For ease of discussion only two materials
will be discussed. The flow of two adjacent stripes is then split
vertically and rejoined laterally to form four stripes of
alternating materials. The slurries are typically formulated so
they do not mix when they come into contact with the other
slurries. The vertically splitting and lateral joining can be
repeated several times, resulting in a final flow if interdigitated
stripes of materials, where each stripe of material forms a fine
filament.
[0024] In FIG. 2, the co-extrusion head 30 receives two different
materials through ports 32 and 34. It must be understood that the
head may have more ports to receive more materials or to receive
addition slurries of the same materials. The splitting and
rejoining of the flows occur inside the co-extrusion head, the
details of which are beyond the scope of this disclosure. The head
deposits the resulting flow of interdigitated stripes on a
substrate. In this particular embodiment, the substrate consists of
a thin metal substrate like a metal foil, but other types of
substrates may be used provided they can withstand the further
processing.
[0025] In the embodiment of FIG. 2, one of the materials is a
superconducting paste of magnesium diboride (MgB.sub.2) and the
other material is a sacrificial material. The superconducting
material forms stripes such as 24 and the sacrificial material
forms stripes such as 22.
[0026] After deposition of the slurries, the substrate and slurries
undergoes heating to remove the excess liquid. The sacrificial
material also undergoes removal, which may occur during heating or
during a different process such as cleaning with a solvent, etc.
FIG. 3 shows the resulting filaments of superconducting material 24
on the substrate 20. The gaps between the stripes are the regions
from which the sacrificial material has been removed, leaving the
substrate exposed. Each stripe of superconducting material has a
width of less than 50 micrometers (.mu.m). The thin metal foil with
the superconducting filaments may be referred to as superconducting
tape, as the foil has high enough flexibility that it can wrap
around objects or be formed into winding structures like tape.
[0027] FIG. 4 shows a flowchart of an embodiment of a method of
manufacturing the superconducting filaments. At 40, the
superconducting material is formed into a first slurry. In one
embodiment, the superconducting material takes the form of
MgB.sub.2 powder. Mixing the powder into an organic solvent forms a
slurry or paste of the material. The slurry may contain other
components such as a binder and chemical dopants such as carbon,
silicon carbide, among other chemical dopants. Chemical dopants
have been shown to enhance the flux pinning in the MgB.sub.2
superconducting wires or filaments. Increasing the flux pinning
force in superconductors enhances the current carrying capacity of
the wire in the presence of external magnetic fields. The organic
solvent may consist of many different types of solvents such as
butyl carbitol or toluene. At 42, a sacrificial material is formed
into a second slurry. In one embodiment, this may consist of a
cellulose binder mixed into an organic solvent or binder.
[0028] As will be discussed below, one or more layers may reside on
the substrate prior to the deposition of the slurries. These
optional layers will be deposited onto the substrate by one of many
possible processes including sputtering, slot coating, vapor
deposition, etc., prior to the extrusion of the slurries.
[0029] One embodiment of the process is to print a stripe of
MgB.sub.2 slurry, supported by stripes of sacrificial vehicle on
both sides. This will be referred to as an "ex-situ" process,
because the MgB.sub.2 is synthesized outside of the printhead, then
ground up into particles, then turned into ink. In another
embodiment of an in-situ process the process takes magnesium
particles and boron particles, mix them together in the correct
proportion, make an ink out of the mixture, and prints the same
structure. Then, during the sintering process there is a reaction
that turns the magnesium and boron powders into MgB.sub.2. The
"in-situ" process forms the MgB.sub.2 after deposition.
[0030] A third embodiment is to print three materials at once, such
that each line is (Sacrificial ink).parallel.(Mg
slurry).parallel.(B Slurry).parallel.(Sacrificial ink). Then,
during the reaction process, the Mg diffuses into the B side to
form the compound MgB.sub.2. This is also an in-situ process, but
may require a different printhead than that shown in FIG. 2, as it
would need a third port and path for a third fluid.
[0031] However the slurries are formed, they are extruded onto a
substrate at 44. One must note that other slurries may be used in
addition to the two slurries. Formation of three or more stripes of
materials may provide wider separation between the stripes of
superconducting material, for example, or serve other functions.
The additional slurries may consist of a different or the same
sacrificial material.
[0032] Once the slurries have been deposited, the sacrificial
material is removed at 46. In one embodiment, the removing of the
sacrificial material takes the form of heating the substrate to a
temperature in the range of 700-800.degree. C. In this embodiment,
this serves to remove the sacrificial material and its slurry, to
remove excess liquid from the superconducting slurry, and sinter
the superconductor particles, causing the superconducting material
to become more dense and solid.
[0033] These two processes, removal of the excess liquid from the
superconducting slurry and the removal of the sacrificial material
may involve two processes. The removal of the excess liquid may
result from heating, drying or pressing the superconducting slurry
as a separate process from the removal of the sacrificial material.
The sacrificial material may be removed with a solvent or some type
of mechanical process that does not affect the stripes of
superconducting material. However, the heating process performs
both of these task at one time and is more efficient.
[0034] In one embodiment the process dries and sinters the film in
two processes. If, for instance, the solvent was butyl carbitol
(diethylene glycol butyl ether), which has a boiling point of
230.degree. C., the process would heat the printed film up to
.about.150.degree. C. to let the film evaporate. A later process
then sinters the film at 700-800.degree. C. In an industrial
setting, this may be done in one process in a conveyer-belt style
oven, and this long oven would have a number of different heating
zones, the first two of which would be longer and lower temperature
to give the film time to dry before densification.
[0035] Another consideration in forming the superconducting wires
is coating or passivation, such as for heat management. The
filaments produced by the above process will be flat, and after
deposition and drying/sintering, there may be a need to coat them
with some sort of metal. The materials of the superconducting
filaments may microquench under the high magnetic fields typical
inside superconducting machines. In order to avoid this, a layer of
metal, such as copper, may be deposited over the filaments.
Deposition may occur by one of many processes, including
sputtering.
[0036] FIGS. 5 and 6 show alternative embodiments of the
superconducting filament tapes or structures. As shown in FIG. 5,
the structure has a metal substrate 20, buffer layer or layers 50
on the metal substrate, and stripes of superconducting material 24
on the buffer layer or layers, wherein each stripe is separated
from adjacent stripes by a gap 52. In this embodiment, the buffer
layer or layers 50 provides many useful and beneficial functions
including but not limited to a chemical barrier layer to prevent
poisoning during heat treatment, better coefficient of thermal
expansion matching between the metal substrate and the
superconducting stripes, a means for chemically doping the
MgB.sub.2 for improved performance, electrical insulation, etc.
[0037] FIG. 6 shows a structure having a metal substrate 20, and
stripes of superconducting material 24 on the metal substrate,
wherein each stripe is separated from adjacent stripes by a gap. In
this embodiment, an additional layer 54 of metallic material such
as copper, silver, aluminum, gold, nickel, tin, alloys and mixtures
thereof, etc. is placed on top of the superconducting filaments to
provide an electric and thermal stabilizer. This layer may also
consist of an electrical insulator, discussed below, as well as a
combination of the two.
[0038] Alternatively, the structure has a metal substrate with a
buffer layer as in FIG. 5, but in this embodiment the buffer layer
consists of an electrically insulating buffer layer on the metal
substrate, and the stripes of superconducting material reside on
the electrically insulating buffer layer. In this embodiment, an
additional layer of material resides on top of the superconducting
filaments as shown in FIG. 6, but the layer consists of an
electrically insulating material to provide electrical isolation
between the superconducting filaments.
[0039] Other modifications exist, including use of a non-metallic
substrate such as SiC (silicon-carbide), carbon (C), graphene,
alumina, sapphire, etc., and stripes of superconducting material on
the non-metallic substrate, wherein each stripe is separated from
adjacent stripes by a gap.
[0040] In this manner, filaments of superconducting material are
formed on a thin substrate and the filaments have a width of less
than 50 micrometers. This makes them suitable for high frequency
operation. Referring back to FIG. 1, the resulting tape substrate
can be used to form the windings used on both a stator and a rotor
of a synchronous machine, resulting in a highly efficient, fully
superconducting machine. The manufacture of the superconducting
filaments is relatively simple and much less expensive than the
current state of the art manufacturing processes.
[0041] It should be noted that either the electrically conducting
or electrically non-conducting substrates with the co-extruded
superconducting filaments described in this disclosure can be
bundled together to comprise a superconducting cable. The advantage
of bundling multiple superconducting tapes in parallel is for
enhanced current carrying capacity when compared with a single
superconducting tape. Furthermore, it may be advantageous to twist
and transpose these bundled superconducting tapes for further
reductions in AC loss of the superconducting cable.
[0042] It will be appreciated that variants of the above-disclosed
and other features and functions, or alternatives thereof, may be
combined into many other different systems or applications. Various
presently unforeseen or unanticipated alternatives, modifications,
variations, or improvements therein may be subsequently made by
those skilled in the art which are also intended to be encompassed
by the following claims.
* * * * *