U.S. patent application number 09/766451 was filed with the patent office on 2001-08-02 for fabrication of superconducting wires and rods.
This patent application is currently assigned to UNIVERSITY OF HOUSTON. Invention is credited to Ponnusamy, Devamanohar, Ravi-Chandar, Krishnaswamy, Salama, Kamel.
Application Number | 20010011066 09/766451 |
Document ID | / |
Family ID | 23468924 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010011066 |
Kind Code |
A1 |
Ravi-Chandar, Krishnaswamy ;
et al. |
August 2, 2001 |
Fabrication of superconducting wires and rods
Abstract
The fabrication of superconducting wires and rods having desired
and consistent electrical and mechanical properties, in particular
those based on Yttrium Barium Copper Oxide (YBCO) and Bismuth
Strontium Calcium Copper Oxide (BSCCO), is disclosed. The first
fabrication step is to form an extrudable paste by mixing YBCO or
BSCCO superconducting powder with a set of organic additives, which
include binder, plasticizers lubricant, dispersant, and a solvent.
The following additional steps are performed on both YBCO and BSCCO
based wires or rods: (i) using a piston extruder to extrude the
superconducting wire or rod; (ii) drying the wire or rod to remove
the solvent; and (iii) subjecting the wire or rod to a binder
burn-out treatment to remove the remaining organic additives. In
addition, YBCO wires and rods also require a sintering step, while
BSCCO wires and rods also require cold isostatic pressing and heat
treatment steps. The formation of cracks in thicker YBCO and BSCCO
based rods during the drying and heating process steps is avoided
by extruding hollow rods. The additives, processing parameters, and
processing stages used to fabricate the superconducting wires and
rods help achieve desired and consistent electrical and mechanical
properties.
Inventors: |
Ravi-Chandar, Krishnaswamy;
(Sugar Land, TX) ; Ponnusamy, Devamanohar;
(Houston, TX) ; Salama, Kamel; (Houston,
TX) |
Correspondence
Address: |
Erik R. Nordstrom
FULBRIGHT & JAWORSKI L.L.P.
Suite 2400
600 Congress Avenue
Austin
TX
78701
US
|
Assignee: |
UNIVERSITY OF HOUSTON
|
Family ID: |
23468924 |
Appl. No.: |
09/766451 |
Filed: |
January 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09766451 |
Jan 19, 2001 |
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08831235 |
Apr 2, 1997 |
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6191074 |
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08831235 |
Apr 2, 1997 |
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08372615 |
Jan 13, 1995 |
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5656574 |
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Current U.S.
Class: |
505/430 ;
505/704; 505/740; 505/776 |
Current CPC
Class: |
Y10S 505/74 20130101;
H01L 39/248 20130101; Y10T 29/49014 20150115 |
Class at
Publication: |
505/430 ;
505/704; 505/740; 505/776 |
International
Class: |
H01B 012/00 |
Claims
What is claimed is:
1. An extrudable paste used in the fabrication of superconducting
wires and rods, comprising: (a) a superconducting powder; (b) ethyl
cellulose; (c) glycerol; (d) stearic acid; (e) triolein; and (f) a
solvent.
2. The extrudable paste of claim 1, wherein said superconducting
powder is YBCO or BSCCO.
3. The extrudable paste of claim 1, wherein said solvent is butyl
carbitol, ethyl alcohol, or a mixture of butyl carbitol and ethyl
alcohol.
4. The extrudable paste of claim 3, wherein said superconducting
powder is YBCO or BSCCO.
5. The extrudable paste of claim 1, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
6. The extrudable paste of claim 2, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
7. The extrudable paste of claim 3, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
8. The extrudable paste of claim 4, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
9. An extrudable paste used in the fabrication of superconducting
wire, comprising: (a) superconducting powder; (b) about 2 to 3
weight percent of ethyl cellulose; (c) about 0.7 to 0.75 weight
percent of glycerol; (d) about 0.4 to 0.5 weight percent of stearic
acid; (e) about 0.1 weight percent of triolein; and (f) about 15 to
18 weight percent of solvent.
10. The extrudable paste of claim 9, wherein said superconducting
powder is YBCO or BSCCO.
11. The extrudable paste of claim 9, wherein said solvent is butyl
carbitol, ethyl alcohol, or a mixture of butyl carbitol and ethyl
alcohol.
12. The extrudable paste of claim 11, wherein said superconducting
powder is YBCO or BSCCO.
13. The extrudable paste of claim 9, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
14. The extrudable paste of claim 10, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
15. The extrudable paste of claim 11, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
16. The extrudable paste of claim 12, wherein said superconducting
powder consists of particles having sizes of about 2 to 10
.mu.m.
17. An extrusion process for extruding superconducting wires and
rods, comprising the steps of: (a) providing a piston extruder
comprising a die barrel, an extrusion nozzle, and a ram; (b)
closing said extrusion nozzle; (c) loading extrudable paste in the
form of granules into said die barrel, said extrudable paste
comprising superconducting powder, a binder, a plasticizer, a
lubricant, a dispersant, and a solvent; (d) subjecting said die
barrel to a vacuum of about 200 millitorr; (e) compacting said
extrudable paste by using said ram to apply a compaction stress of
about 100 MPa; (f) opening said extrusion nozzle; and (g) extruding
a wire or rod through said extrusion nozzle using said ram, said
ram moving at a rate of about 0.02 to 0.1 inch/minute.
18. The extrusion process of claim 17, further comprising the step
of forming a hollow rod by attaching a mandrel to said ram.
19. The extrusion process of claim 17, wherein said granules of
extrudable paste are cubes having sides measuring about 2 to 3
mm.
20. The extrusion process of claim 18, wherein said granules of
extrudable paste are cubes having sides measuring about 2 to 3
mm.
21. The extrusion process of claim 17, wherein said dispersant is
triolein.
22. The extrusion process of claim 18, wherein said dispersant is
triolein.
23. The extrusion process of claim 19, wherein said dispersant is
triolein.
24. The extrusion process of claim 20, wherein said dispersant is
triolein.
25. A method of drying a superconducting wire or rod, comprising
the steps of: (a) providing a superconducting wire or rod
comprising at least superconducting powder, and a solvent; (b)
initially drying said wire or rod in still air at room temperature
until about 20% of said solvent is removed; and (c) subsequently
drying said wire or rod in flowing air at a temperature of about
60.degree. C. until all remaining solvent is removed.
26. The extrusion process of claim 17, further comprising the steps
of: initially drying said wire or rod in still air at room
temperature until about 20% of said solvent is removed; and
subsequently drying said wire or rod in flowing air at a
temperature of about 60.degree. C. until all remaining solvent is
removed.
27. The extrusion process of claim 26, further comprising the step
of forming a hollow rod by attaching a mandrel to said ram.
28. A method of removing organic additives from a superconducting
wire or rod, comprising the steps of: (a) providing a
superconducting wire or rod comprising at least superconducting
powder and organic additives; (b) initially heating said wire or
rod in flowing oxygen until a temperature of about 100.degree. C.
is reached; (c) subsequently heating said wire or rod in flowing
oxygen at a rate in the range of about 6 to 15.degree. C./hour
until a temperature of about 300.degree. C. is reached; and (d)
thereafter heating said wire or rod in flowing oxygen at a rate of
about 20.degree. C./hour until a temperature of about 500.degree.
C. is reached.
29. The method of claim 28, wherein in the temperature range of
about 100 to 300.degree. C., if said wire or rod has a diameter
less than about 2 mm, said wire or rod is heated, in flowing
oxygen, at a rate of about 15.degree. C./hour, while if said wire
or rod has a diameter equal to or greater than about 2 mm, said
wire or rod is heated, in flowing oxygen, at a rate of about
6.degree. C./hour.
30. The method of claim 28, wherein said flowing oxygen has a flow
rate of about 2 liters per minute for about 100 grams of said wire
or rod being heated to a temperature of about 300.degree. C., and a
flow rate of about an additional 2 liters per minute for each
additional 100 grams of said wire or rod being heated to about
300.degree. C., and wherein said flowing oxygen has a flow rate of
about 0.25 liters per minute for about 100 grams of said wire or
rod being heated at a temperature above about 300.degree. C., and a
flow rate of about an additional 0.25 liters per minute for each
additional 100 grams of said wire or rod heated about 300.degree.
C.
31. The extrusion process of claim 26, further comprising the steps
of: initially heating said wire or rod in flowing oxygen until a
temperature of about 100.degree. C. is reached; subsequently
heating said wire or rod in flowing oxygen at a rate in the range
of about 6 to 15.degree. C./hour until a temperature of about
300.degree. C. is reached; and thereafter heating said wire or rod
in flowing oxygen at a rate of about 20.degree. C./hour until a
temperature of about 500.degree. C. is reached.
32. The extrusion process of claim 31, further comprising the step
of forming a hollow rod by attaching a mandrel to said ram.
33. A process for fabricating superconducting wires and rods,
comprising the steps of: (a) providing a piston extruder comprising
a die barrel, an extrusion nozzle, and a ram; (b) closing said
extrusion nozzle; (c) loading extrudable paste in the form of
granules into said die barrel, said extrudable paste comprising
YBCO powder, a binder, a plasticizer, a lubricant, a dispersant,
and a solvent; (d) subjecting said die barrel to a vacuum of about
200 millitorr; (e) compacting said extrudable paste by using said
ram to apply a compaction stress of about 100 MPa; (f) opening said
extrusion nozzle; (g) extruding a wire or rod through said
extrusion nozzle using said ram, said ram extruding said wire or
rod at a rate of about 0.02 to 0.1 inch/minute; (h) initially
drying said wire or rod in still air at room temperature until
about 20% of said solvent is removed; (i) subsequently drying said
wire or rod in flowing air at a temperature of about 60.degree. C.
until all remaining solvent is removed; (j) initially heating said
wire or rod in flowing oxygen until a temperature of about
100.degree. C. is reached; (k) subsequently heating said wire or
rod in flowing oxygen at a rate in the range of about 6 to
15.degree. C./hour until a temperature of about 300.degree. C. is
reached; (l) thereafter heating said wire or rod in flowing oxygen
at a rate of about 20.degree. C./hour until a temperature of about
500.degree. C. is reached; and (m) sintering said wire or rod at
about 900.degree. C. in an oxygen atmosphere for about 24
hours.
34. The extrusion process of claim 33, further comprising the step
of forming a hollow rod by attaching a mandrel to said ram.
35. The process of claim 33, wherein said dispersant
is-triolein.
36. The process of claim 33, further comprising the step of
exposing said wire or rod to an oxygen atmosphere at a temperature
of about 500.degree. C. for about 24 hours.
37. A method of achieving a highly oriented, polycrystalline
microstructure in superconducting wires or rods, comprising the
steps of: (a) providing a superconducting wire or rod; (b) packing
said wire or rod in a flexible container; (c) placing said
container in a support fixture; (d) applying a first isostatic load
to said container to densify said wire or rod; (e) heating said
wire or rod, in an atmosphere of about 9% oxygen and about 91%
argon, at a temperature of about 830.degree. C. for about 72 hours;
(f) applying a second isostatic load to said container to densify
said wire or rod; and (g) heating said wire or rod, in an
atmosphere of about 9% oxygen and about 91% argon at 840.degree. C.
for about 72 hours.
38. The method of claim 37, wherein said superconducting wires or
rods comprise BSCCO.
39. The method of claim 37, wherein said first isostatic load is
about 40 Ksi, and said second isostatic load is about 60 Ksi.
40. The method of claim 38, wherein said first isostatic load is
about 40 Ksi, and said second isostatic load is about 60 Ksi.
41. The method of claim 37, wherein if said superconducting wire or
rod has a hollow center, further comprising the step of inserting a
mandrel or rod into said hollow center prior to applying an
isostatic load to said wire or rod.
42. A process for fabricating superconducting wires and rods,
comprising the steps of: (a) providing a piston extruder comprising
a die barrel, an extrusion nozzle, and a ram; (b) closing said
extrusion nozzle; (c) loading extrudable paste in the form of
granules into said die barrel, said extrudable paste comprising
BSCCO powder, a binder, a plasticizer, a lubricant, a dispersant,
and a solvent; (d) subjecting said die barrel to a vacuum of about
200 millitorr; (e) compacting said extrudable paste by using said
ram to apply a compaction stress of about 100 MPa; (f) opening said
extrusion nozzle; (g) extruding a wire or rod through said
extrusion nozzle using said ram, said ram extruding said wire or
rod at a rate of about 0.02 to 0.1 inch/minute; (h) initially
drying said wire or rod in still air at room temperature until
about 20% of said solvent is removed; (i) subsequently drying said
wire or rod in flowing air at a temperature of about 60.degree. C.
until all remaining solvent is removed; (j) initially heating said
wire or rod in flowing oxygen until a temperature of about
100.degree. C. is reached; (k) subsequently heating said wire or
rod in flowing oxygen at a rate in the range of about 6 to
15.degree. C./hour until a temperature of about 300.degree. C. is
reached; (l) thereafter heating said wire or rod in flowing oxygen
at a rate of about 20.degree. C./hour until a temperature of about
500.degree. C. is reached; (m) packing said wire or rod in a
flexible container; (n) placing said container in a support
fixture; (o) applying a first isostatic load to said container to
densify said wire or rod; (p) heating said wire or rod, in an
atmosphere of about 9% oxygen and about 91% argon, at a temperature
of about 830.degree. C. for about 72 hours; (q) applying a second
isostatic load to said container to densify said wire or rod; and
(r) heating said wire or rod, in an atmosphere of about 9% oxygen
and about 91% argon at about 840.degree. C. for about 72 hours.
43. The process of claim 42, wherein a mandrel is attached to said
ram to form a wire or rod with a hollow center, and further
comprising the step of inserting a mandrel or rod into said hollow
center prior to applying an isostatic load to said wire or rod.
44. The process of claim 42, wherein said dispersant is
triolein.
45. A superconducting wire or rod which is extruded by the steps
of: (a) providing a piston extruder comprising a die barrel, an
extrusion nozzle, and a ram; (b) closing said extrusion nozzle; (c)
loading extrudable paste in the form of granules into said die
barrel, said extrudable paste comprising superconducting powder, a
binder, a plasticizer, a lubricant, a dispersant, and a solvent;
(d) subjecting said die barrel to a vacuum of about 200 millitorr;
(e) compacting said extrudable paste By using said ram to apply a
compaction stress of about 100 MPa; (f) opening said extrusion
nozzle; and (g) extruding a wire: or rod through said extrusion
nozzle using said ram, said ram moving at a rate of about 0.02 to
0.1 inch/minute.
46. The superconducting wire or rod of claim 45, wherein said
granules of extrudable paste are cubes having sides measuring about
2 to 3 mm.
47. The superconducting wire or rod of claim 45, wherein said
dispersant is triolein.
48. A hollow superconducting wire or rod which is extruded by the
steps of: (a) providing a piston extruder comprising a die barrel,
an extrusion nozzle, and a ram having a mandrel attached; (b)
closing said extrusion nozzle; (c) loading extrudable paste in the
form of granules into said die barrel, said extrudable paste
comprising superconducting powder, a binder, a plasticizer, a
lubricant, a dispersant, and a solvent; (d) subjecting said die
barrel to a vacuum of about 200 millitorr; (e) compacting said
extrudable paste by using said ram to apply a compaction stress of
about 100 MPa; (f) opening said extrusion nozzle; and (g) extruding
a hollow wire or rod through said extrusion nozzle using said ram,
said ram moving at a rate of about 0.02 to 0.1 inch/minute.
49. The hollow superconducting wire or rod of claim 48, wherein
said granules of extrudable paste are cubes having sides measuring
about 2 to 3 mm.
50. The hollow superconducting wire or rod of claim 48, wherein
said dispersant is triolein.
51. A superconducting wire or rod which is fabricated by the steps
of: (a) providing a piston extruder comprising a die barrel, an
extrusion nozzle, and a ram; (b) closing said extrusion nozzle; (c)
loading extrudable paste in the form of granules into said die
barrel, said extrudable paste comprising YBCO powder, a binder, a
plasticizer, a lubricant, a dispersant, and a solvent; (d)
subjecting said die barrel to a vacuum of about 200 millitorr; (e)
compacting said extrudable paste by using said ram to apply a
compaction stress of about 100 MPa; (f) opening said extrusion
nozzle; (g) extruding a wire or rod through said extrusion nozzle
using said ram, said ram extruding said wire or rod at a rate of
about 0.02 to 0.1 inch/minute; (h) initially drying said wire or
rod in still air at room temperature until about 20% of said
solvent is removed; (i) subsequently drying said wire or rod in
flowing air at a temperature of about 60.degree. C. until all
remaining solvent is removed; (j) initially heating said wire or
rod in flowing oxygen until a temperature of about 100.degree. C.
is reached; (k) subsequently heating said wire or rod in flowing
oxygen at a rate in the range of about 6 to 15.degree. C./hour
until a temperature of about 300.degree. C. is reached; (l)
thereafter heating said wire or rod in flowing oxygen at a rate of
about 20.degree. C./hour until a temperature of about 500.degree.
C. is reached; and (m) sintering said wire or rod at about
900.degree. C. in an oxygen atmosphere for about 24 hours.
52. The superconducting wire or rod of claim 51, wherein said wire
or rod is fabricated in a hollow form by attaching a mandrel to
said ram.
53. The superconducting wire or rod of claim 51, wherein said
dispersant is triolein.
54. The superconducting wire or rod of claim 51, further comprising
the fabrication step of exposing said wire or rod to an oxygen
atmosphere at a temperature of about 500.degree. C. for about 24
hours.
55. A superconducting wire or rod which is fabricated by the steps
of: (a) providing a piston extruder comprising a die barrel, an
extrusion nozzle, and a ram; (b) closing said extrusion nozzle; (c)
loading extrudable paste in the form of granules into said die
barrel, said extrudable paste comprising BSCCO powder, a binder, a
plasticizer, a lubricant, a dispersant, and a solvent;, (d)
subjecting said die barrel to a vacuum of about 200 millitorr; (e)
compacting said extrudable paste by using said ram to apply a
compaction stress of about 100 MPa; (f) opening said extrusion
nozzle; (g) extruding a wire or rod through said extrusion nozzle
using said ram, said ram extruding said wire or rod at a rate of
about 0.02 to 0.1 inch/minute; (h) initially drying said wire or
rod in still air at room temperature until about 20% of said
solvent is removed; (i) subsequently drying said wire or rod in
flowing air at a temperature of about 60.degree. C. until all
remaining solvent is removed; (j) initially heating said wire or
rod in flowing oxygen until a temperature of about 100.degree. C.
is reached; (k) subsequently heating said wire or rod in flowing
oxygen at a rate in the range of about 6 to 15.degree. C./hour
until a temperature of about 300.degree. C. is reached; (l)
thereafter heating said wire or rod in flowing oxygen at a rate of
about 20.degree. C./hour until a temperature of about 500.degree.
C. is reached; (m) packing said wire or rod in a flexible
container; (n) placing said container in a support fixture; (o)
applying a first isostatic load to said container to density said
wire or rod; (p) heating said wire or rod, in an atmosphere of
about 9% oxygen and about 91% argon, at a temperature of about
830.degree. C. for about 72 hours; (q) applying a second isostatic
load to said container to density said wire or rod; and (r) heating
said wire or rod, in an atmosphere of about 9% oxygen and about 91%
argon at about 840.degree. C. for about 72 hours.
56. The superconducting wire or rod of claim 55, wherein said wire
or rod is fabricated with a hollow center by attaching a mandrel to
said ram, and further comprising the step of inserting a mandrel or
rod into said hollow center prior to applying an isostatic load to
said wire or rod.
57. The superconducting wire or rod of claim 55, wherein said
dispersant is triolein.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to the fabrication of
superconducting wires and rods. More particularly, the present
invention primarily relates to the fabrication of superconducting
wires and rods made from Yttrium Barium Copper Oxide and Bismuth
Strontium Calcium Copper Oxide.
[0003] 2. Brief Description of the Related Technology
[0004] High T.sub.c superconductivity was first discovered in
Yttrium Barium Copper Oxide (YBCO) in 1987. Since then, much
progress has been made in discovering other compounds and in
understanding the fundamental properties of these materials. At the
same time, significant effort has been directed towards identifying
the potential applications of these new materials, and developing
the processing conditions for fabrication of useful forms of these
materials, such as wires and rods.
[0005] Some of the large scale applications of superconducting
wires are in power generation and transmission, high field magnets,
magnetic energy storage, and current leads for low T.sub.c
superconducting magnets and magnetic energy storage systems. These
materials are also used in other applications like electronic
devices and magnetometers.
[0006] Conventionally, wires of ductile materials are made by
extrusion or by rolling and drawing processes. These processes
cannot be directly applied to the high T.sub.c superconductors,
which are ceramic materials and not capable of plastic deformation.
Several variations of these processes are being developed, however,
to adapt them to ceramic materials. These include the
powder-in-tube process used to fabricate metal-clad composites and
the plastic extrusion process used to fabricate bare wires and
rods. Other processes like surface coating techniques are also
being pursued. See S. Jin, R. C. Sherwood, R. B. Van Dover, T. H.
Tiefel and D. W. Johnson, Jr., Appl. Phys. Lett., 51, 203 (1987);
M. T. Lanagan, R. B. Poeppel, J. P. Singh, D. I. Dos Santos, J. K.
Lumpp, U. Balachandran, J. T. Dusek and K. C. Goretta, J. Less
Common Metals, 149, 305 (1989); S. R. Su, M. O'Connor, M. Levinson
and P. G. Rossoni, Physica C, 178, 81 (1991); L. D. Woolf, W. A.
Raggio, F. E. Elsner, M. V. Fisher, R. B. Stephens, T. L. Figueroa,
C. H. Shearer, J. D. Rose, K. M. Schaubel, R. A. Olstad, T. Ohkawa,
D. M. Duggan, M. DiMartino and R. L. Fagaly, Appl. Phys. Lett., 58,
534 (1991); and H. Shimizu, H. Kumakura and K. Togano, Jpn. J.
Appl. Phys., 27, L414 (1988), which are incorporated herein by
reference.
[0007] The powder-in-tube process uses a preform composed of a
nominal ductile metal tube packed with fine superconducting powder.
A wire is rolled, drawn, or both rolled and drawn in successive
stages until the required final dimensions are achieved. The
resulting composite superconducting wire is easily wound into coil
shapes. When the radius of the resulting superconducting wire is
small, the wire is first wound into a coil and then the coil is
heat treated to sinter the powder.
[0008] In the case of the plastic extrusion process,
superconducting powder is mixed with a set of organics to prepare a
paste, which is then extruded to form the desired cross sections.
Some of the advantages of this process over the powder-in-tube
process are the following: (i) there is no problem of degradation
or contamination from other material; (ii) oxygen annealing is
comparatively easy, since the surface is exposed; (iii) the
monolithic nature of the wire makes it useful for applications like
current leads, were metallic cladding is undesirable; and (iv) this
process can be applied to the fabrication of different sections and
large cross-section area rods.
[0009] The plastic extrusion process consists of several stages
like paste preparation, extrusion, drying, binder burn-out,
sintering and oxygenation. Each one of these stages involves
several parameters which affect the quality of the final product
and its microstructure. The plastic extrusion process itself is
used in the fabrication of certain conventional ceramic shapes.
However, in prior attempts at applying the plastic extrusion
process to the fabrication of superconducting wires and rods, the
additives and processing parameters used have not achieved wires
and rods having desired and consistent electrical/superconducting
and mechanical properties.
[0010] The present invention provides the additives, processing
parameters, and processing stages necessary to achieve desired and
consistent electrical/superconducting and mechanical properties.
For example, the present invention: (i) uses polymeric additives
which form an extrudable paste but do not degrade the final
properties of superconductor, (ii) optimizes the amount of each of
the additives; (iii) optimizes extrusion parameters like compaction
load and extrusion rate; and (iv) provides for the removal of
solvent first and other polymeric additives later through proper
heat treatment and sintering parameters.
SUMMARY OF THE INVENTION
[0011] The present invention relates to the fabrication of
superconducting wires and rods having desired and consistent
electrical and mechanical properties. The fabrication of
superconducting wires and rods based on Yttrium Barium Copper Oxide
(YBCO) and Bismuth Strontium Calcium Copper Oxide (BSCCO) is the
primary focus. The present invention employs a plastic
extrusion-type process.
[0012] The fabrication process for YBCO wires and rods has several
process steps in common with the fabrication process for BSCCO
wires and rods. The first step in the fabrication of YBCO and BSCCO
superconducting wires and rods is the preparation of an extrudable
paste. Extrudable paste is formed by mixing superconducting powder,
comprising YBCO or BSCCO, with a set of organic additives, which
include binder, plasticizer, lubricant, dispersant, and a solvent.
Preferably, the binder is ethyl cellulose, the plasticizer is
glycerol, the lubricant is stearic acid, and the dispersant is
triolein. The solvent may be butyl carbitol, ethyl alcohol or
various mixtures of both.
[0013] The next step in the fabrication process is to use a piston
extruder to extrude the superconducting wire or rod. A suitable
piston extruder comprises a die, barrel, port for connecting a
vacuum pump, and a ram. The die includes a finishing tube and
extrusion nozzle. The dimensions of the piston extruder are
designed to promote slip flow and avoid laminations in the
extrusion.
[0014] The use of the piston extruder involves three main steps. In
the first main step, the extrusion nozzle is first closed, or
plugged with a plug, and the extrudable paste is loaded in the
barrel in the form of small, loose granules. The granules are
preferably made by cutting the paste into small cubes having sides
of about 2 to 3 mm. The barrel, filled with the granules, is then
connected to a vacuum pump through the port, and is subjected to a
vacuum of about 200 millitorr. The vacuum removes air in the
extrudable paste and thereby reduces the defects and improves the
extrusion behavior of the wires and rods. The second main step is
to compact the paste by using the ram to apply a compaction stress
of about 100 MPa. The third and final main step is to open or
unplug the nozzle and then move the ram at a constant rate of about
0.02 to 0.1 inch per minute to exude the wire or rod.
[0015] The next step in the process of fabricating YBCO and BSCCO
wires and rods is to dry the extruded wire or rod to remove the
solvent. Since this drying results in shrinkage, the drying
conditions are chosen so as to avoid cracking of the wire or rod.
The extruded wire or rod is first held in still air at room
temperature until about 20% of the solvent is lost. The wire or rod
is then transferred to a convection dryer, where it is exposed to
flowing warm air at 60.degree. C. The time periods for drying
depend upon the diameter of the wire or rod.
[0016] The next fabrication step involves subjecting the dried wire
or rod to a binder burn-out treatment for removing the remaining
organic additives. The dried wire or rod is first heated in a
furnace to a temperature of about 100.degree. C. Then, in the
temperature range of 100.degree. C. to 300.degree. C., wires are
heated in flowing oxygen at a rate of about 15.degree. C. per hour,
while rods are heated in flowing oxygen at a rate of about
6.degree. C. per hour. Under the terminology of the present
invention, a "wire" has a diameter of less than about 2 mm, while a
"rod" has a diameter equal to or greater than about 2 mm. Above
300.degree. C., the heating rate is increased to about 20.degree.
C. per hour, and heating in flowing oxygen continues until a
temperature of about 500.degree. C. is reached.
[0017] The oxygen flow rate in the binder burn-out process depends
upon the furnace tube diameter, the temperature, and the amount of
wire or rod being processed. For example, 100 grams of extruded
wire, heated in a furnace tube of diameter 7.5 cm, requires an
oxygen flow rate of at least 2 liters per minute as the wire is
heated to 300.degree. C. Above 300.degree. C., this rate is reduced
to 0.25 liters per minute. Each additional 100 grams requires about
an additional 2 liters per minute, in the temperature range of
about 100.degree. C. to 300.degree. C., and about an additional
0.25 liters per minute above 300.degree. C. In general, the oxygen
flow rate is increased as the furnace tube diameter or the amount
of wire or rod being heated is increased.
[0018] Thicker rods, having diameters 10 mm and above, may have
poor properties due to the formation of cracks during the drying
and heating steps. The formation of cracks in thicker rods is
completely avoided by the extrusion of hollow rods. This is
accomplished by attaching a mandrel to the ram of the piston
extruder.
[0019] In the case of YBCO based superconducting wires and rods, an
additional and final process step is to subject the wire or rod to
sintering at about 900.degree. C. in an oxygen atmosphere for about
24 hours. This results in a YBCO wire or rod having a fine
microstructure with a uniform grain size of 5 .mu.m, and a density
of 90-95% of theoretical density.
[0020] The final fabrication steps of BSCCO superconducting wires
and rods involves cold isostatic pressing and heat treatment steps.
In cold isostatic pressing, the BSCCO wire or rod is subjected to a
quasi-isostatic load in such a way that the wire or rod experiences
maximum load in the radial direction. This is accomplished by using
a loading fixture. The BSCCO wire or rod (after binder burn-out) is
packed inside a latex tube, and stoppers are used as closures to
seal both ends of the tube. A disk is then attached to each
stopper. Next, this assembly is placed in a support structure or
frame. Then, when an isostatic load is applied, the support
structure bears the axial load while the wire or rod experiences a
mostly radial load. The isostatic load applied is preferably about
40 ksi. In the case of hollow rods, a mandrel or rod should be
inserted into the hollow space prior to applying an isostatic
load.
[0021] After the cold isostatic pressing step, the BSCCO wire or
rod is subjected to a heat treatment step. The wire or rod is
heated for about 72 hours in a furnace atmosphere of about 9%
oxygen and about 91% argon at a temperature of about 830.degree.
C.
[0022] The BSCCO wire or rod is then subjected to a second cold
isostatic pressing step. The wire or rod is packed again in the
latex tube and placed in the support fixture described above, and
an isostatic load of 60 ksi is applied.
[0023] The final processing step for BSCCO wires and rods is a
second heat treatment. The second heat treatment is done at about
840.degree. C. in an atmosphere of about 9% oxygen and about 91%
argon for about 72 hours.
[0024] One potential application for wires and rods fabricated by
the present invention is use as current leads in low T.sub.c
superconducting equipment. Current leads made from the
superconducting wires and rods reduce resistance heating and heat
conduction, thereby cutting down the superconducting equipment's
consumption of liquid helium. The superconducting leads can be made
as single, large area wires or a rod comprising a multifiliment
composite of wires.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] A better understanding of the present invention can be
obtained when the following detailed description of the preferred
embodiment is considered in conjunction with the following
drawings, in which:
[0026] FIG. 1 is a table of additives for superconducting wires and
rods fabricated in accordance with the present invention;
[0027] FIGS. 2a-c are cross-sectional drawings of a piston extruder
as used in accordance with the present invention in the fabrication
of superconducting wires and rods;
[0028] FIG. 3a is a cross-sectional drawing illustrating the
dimensions of a piston extruder used in accordance with the present
invention;
[0029] FIG. 3b is a table of the dimensions of a piston extruder
used in accordance with the present invention;
[0030] FIG. 4 is a table of drying temperatures and times for
superconducting wires and rods fabricated in accordance with the
present invention;
[0031] FIG. 5 is a graph illustrating the percentage of additives
lost during heating of superconducting wires and rods fabricated in
accordance with the present invention;
[0032] FIG. 6 is a cross-section of a piston extruder used to
fabricate hollow superconducting wires and rods in accordance with
the present invention;
[0033] FIGS. 7a and 7b are cross-sectional drawings of an assembly
as used for cold isostatic pressing in the fabrication of
superconducting wires and rods in accordance with the present
invention;
[0034] FIG. 8 is a resistivity versus temperature graph for
superconducting wires and rods fabricated in accordance with the
present invention;
[0035] FIG. 9 is a critical current density versus diameter curve
for superconducting wires and rods fabricated in accordance with
the present invention;
[0036] FIGS. 10 and 11 are tables of electrical properties versus
dimensions for superconducting wires and rods fabricated in
accordance with the present invention;
[0037] FIG. 12 is a theoretical density and grain size versus
sintering temperature graph for superconducting wires and rods
fabricated in accordance with the present invention;
[0038] FIG. 13 is a critical current density versus sintering
temperature graph for superconducting wires and rods fabricated in
accordance with the present invention; and
[0039] FIG. 14 is a fracture strength versus sintering temperature
graph for superconducting wires and rods fabricated in accordance
with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0040] I. PLASTIC EXTRUSION PROCESS STEPS FOR YBCO AND BSCCO BASED
SUPERCONDUCTING WIRES AND RODS
[0041] The present invention is primarily concerned with the
fabrication of wires and rods of YBCO and Bismuth Strontium Caicium
Copper Oxide (BSCCO). In the case of BSCCO, there are several
superconducting phases. The most common of these is
Bi.sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.10 (2223 phase), which has a
T.sub.c of 110K, and Bi.sub.2Sr.sub.2CaCu.sub.2- O.sub.8 (2212
phase), which has a T.sub.c of 85K. The 2223 phase is preferred
over the 2212 phase in bulk processing since: (a) the 2223 phase
has a higher T.sub.c; and (b) a highly oriented polycrystalline
microstructure of 2223 can be obtained by a combination of hot or
cold deformation and heat treatment.
[0042] A. Process Steps Common to Both YBCO and BSCCO.
[0043] The following plastic extrusion process steps ((i)-(v)) are
common to both YBCO and BSCCO based wires and rods. In addition to
steps (i)-(v), superconducting wires and rods made from YBCO
compounds require a sintering process step, while superconducting
wires and rods made from BSCCO compounds require cold isostatic
pressing and heat treatment steps. These additional processing
steps are also discussed below.
[0044] (i) Additives.
[0045] The first step in the fabrication of superconducting wires
and rods by plastic extrusion is the preparation of an extrudable
paste. This is accomplished by mixing superconducting powder,
comprising YBCO or BSCCO, with a set of organic additives, which
include binder, plasticizer, lubricant, dispersant and a solvent.
See J. S. Reed, Principles of Ceramic Processing, John Wiley, 1988;
and D. Ponnusamy and K. Ravi-Chandar, Extrusion of Superconducting
Wires of YBa.sub.2Cu.sub.3O.sub.7-x, J. Mater. Res.; Vol. 8, No. 2
(February 1993), which are incorporated herein by reference.
[0046] The binder consists of polymer molecules that are adsorbed,
and bridge between ceramic particles to provide interparticle
adhesion. Ethyl cellulose is a suitable binder due to its
formability, bond strength, and above all, low non-combustible
residues.
[0047] Plasticizers consist of small polymer molecules which, when
distributed among the larger binder molecules, cause them to pack
less densely, thereby increasing the flexibility of the binder.
Preferably, glycerol is used as the plasticizer since its is
compatible with ethyl cellulose.
[0048] Lubricants and dispersants are added to modify the
rheological characteristics of the extrudable paste, to avoid the
formation of agglomerates, and to reduce the friction with the die
walls of a piston extruder (discussed below). Stearic acid, which
is a very common additive in ceramic processing, serves all of
these functions and is a suitable lubricant.
[0049] Dispersants are substances that locate themselves at a
solid-liquid interface or other type of interface and alter the
characteristics of the interface. The addition of a very small
amount of dispersant improves the wettability of a ceramic particle
surface, which causes each particle to be coated uniformly with the
polymer additives. The presence of a dispersant during the mixing
process also avoids the formation of agglomerates. Triolein is a
suitable dispersant for the paste of the present invention.
Triolein, which is more commonly known as glyceryl trioleate, is a
commercially and technically important dispersant used in a wide
range of industries, including ceramic processing.
[0050] The solvent acts as the medium into which the organic
additives and the ceramic powder are placed. Butyl carbitol, ethyl
alcohol and various mixtures of both may be used as solvents.
[0051] The superconducting powder is mixed with the organic
additives in a mortar and pestle by applying shear stresses which
are essential to break down the agglomerates. The approximate
percentage of the various additives is set forth in the table of
FIG. 1. The amounts of the various additives were determined by
empirical methods developed by Onoda and by experimentation. See G.
Y. Onoda, L. L. Hench, "The Rheology of Organic Binder Solutions"
in Ceramic Processing Before Firing, edited by G. Y. Onoda (John
Wiley, 1978), which is incorporated herein by reference.
[0052] (ii) Die Design.
[0053] A piston extruder 10 is used for the extrusion of the
superconducting wires and rods. See FIGS. 2a-2c and 3a. The piston
extruder 10 comprises die 12, barrel 14, port 16 (not shown in FIG.
3a) for connecting a vacuum pump, and ram 18 (not shown in FIG.
3a). The die 12 includes a finishing tube 20, and extrusion nozzle
22. The design of piston extruder 10 is based upon the basic
mechanics of plastic extrusion and is not discussed in detail here.
The design is governed by the consistency and the rheological
properties of the paste, such as shear strength and viscosity, and
is not material specific. The dimensions of piston extruder 10,
mainly the reduction ratio (D/d), the die angle (.theta.), and the
aspect ratio of the nozzle (l/d), are all chosen so as to promote
slip flow and avoid laminations in the extrusion. See FIGS. 3a and
3b. The table in FIG. 3b sets forth some examples of suitable
numerical values for the reduction ratio, die angle, and aspect
ratio of the nozzle.
[0054] (iii) Extrusion Process.
[0055] The extrusion process is done in piston extruder 10 in three
steps, as shown in FIGS. 2a-2c. As illustrated in FIG. 2a, the
extrusion nozzle 22 of piston extruder 10 is first closed, or
plugged with plug 24, and the paste is loaded in barrel 14 in the
form of small; loose granules 26. The granules are preferably made
by cutting the paste into small cubes having sides of about 2 to 3
mm. The barrel 14, filled with granules 26, is connected to a
vacuum pump (not shown) through port 16 and is subjected to a
vacuum of about 200 millitorr. The vacuum step is important because
pastes and slurries contain pockets of trapped air which if not
removed might result in pores or cracks on the final product.
Subjecting the paste within extruder 10 to a vacuum should remove
the air and reduce these defects and also significantly improve the
extrusion behavior.
[0056] As illustrated in FIG. 2b, the paste, with the air removed,
is then compacted to form a homogeneous dense paste by using ram 18
to apply a compaction stress of about 100 MPa. The compacted paste
is identified in FIG. 2b as reference numeral 28.
[0057] As illustrated in FIG. 2c, after compaction, the nozzle 22
is opened or unplugged and the ram 18 pushes at a constant rate of
about 0.02 to 0.1 inch per minute to extrude a wire or rod 30. The
exact rate depends upon the wire or rod diameter and the reduction
ratio (D/d). The diameter of the extruded wire or rod is the same
as the diameter (d) of the nozzle 22.
[0058] (iv) Drying.
[0059] Drying is the process of removing the solvent from the
extruded wire or rod. The drying results in shrinkage of the wire
or rod. Drying conditions are chosen so as to avoid cracking due to
this shrinkage. During an initial period, the extruded wire or rod
is held in still air at room temperature. During this initial
period, the rate of removal of solvent increases until about 20% of
the solvent is lost. The wire or rod is then transferred to a
convection drier, where it is exposed to flowing warm air at
60.degree. C. During this period, the rate of solvent removal
remains constant. The time periods for drying depend upon the
diameter of the wire or rod. Examples of drying time periods are
shown in the table of FIG. 4.
[0060] (v) Binder Burn-Out.
[0061] Next, the dried wire or rod is subjected to a binder
burn-out treatment for removing the remaining organic additives. A
wire or rod with the organic additives removed is referred to as a
green compact. The weight loss versus temperature relationship for
ethyl cellulose is shown in FIG. 5.
[0062] The dried wire or rod is first heated in a furnace to a
temperature of about 100.degree. C. The bulk of the removal then
takes place in a narrow band between about 100.degree. C. and
300.degree. C., though weight loss continues until higher
temperatures. See FIG. 5. Within this range of temperatures, wires
are heated at a rate of about 15.degree. C. per hour, while rods
are heated at a rate of about 6.degree. C. per hour. Under the
terminology of the present invention, a "wire" has a diameter of
less than about 2 mm, while a "rod" has a diameter equal to or
greater than about 2 mm. Above 300.degree. C., the heating rate is
increased to about 20.degree. C. per hour for both wires and rods,
and the heating continues until a temperature of about 500.degree.
C. is reached.
[0063] The wires and rods are heated in flowing oxygen. The oxygen
flow rate depends upon the furnace tube diameter, the temperature,
and the amount of wire or rod being processed. For example, 100
grams of extruded wire, heated in a furnace tube of diameter 7.5
cm, requires an oxygen flow rate of at least 2 liters per minute as
the wire is heated to 300.degree. C. Above 300.degree. C., this
rate is reduced to a much smaller value of about 0.25 liters per
minute. Each additional 100 grams requires about an additional 2
liters per minute, in the temperature range of about 100.degree. C.
to 300.degree. C., and about an additional 0.25 liters per minute
above 300.degree. C. In general, the oxygen flow rate is increased
as the furnace tube diameter or the amount of wire or rod being
heated is increased.
[0064] In spite of the extended drying periods and slow heating
rates, thicker rods, having diameters 10 mm and above; may have
poor properties due to the formation of cracks. The formation of
cracks in thicker rods can be completely avoided by the extrusion
of hollow rods. This is accomplished by attaching a mandrel 32 to
the ram 18 of piston extruder 10, as shown in FIG. 6.
[0065] B. Additional Processing Step for YBCO.
[0066] In the case of YBCO based superconducting wires and rods, an
additional and final process step is to subject the green compact
to sintering. The material and sintering process variables, such as
particle size and distribution, sintering temperature, time, and
atmosphere are controlled to develop different microstructures in
the wire or rod. For example, the initial YBCO superconducting
powder typically consists of uniformly fine particles of size 2-5
.mu.m. After processing steps (i)-(v) are performed on this YBCO
powder, the resulting green compact is subjected to sintering at
about 900.degree. C. in an oxygen atmosphere for about 24 hours.
This results in the wire or rod having a fine microstructure with a
uniform grain size of 5 .mu.m, and a density of 90-95% of
theoretical density. Higher densities can be achieved by sintering
at higher temperatures, but this is accompanied by grain growth,
resulting in a coarse microstructure.
[0067] An initial superconducting powder with particle size greater
than 10 .mu.m requires higher sintering temperature for proper
densification, and therefore the resulting sintered wire or rod has
coarse grains. The preferred particle size for the YBCO powder is
in the range of about 2-5 .mu.m.
[0068] The superconducting YBCO powder usually contains small
amounts of low melting impurities like
BaCuO.sub.2,Y.sub.2BaCuO.sub.5 and CuO, and the liquid phases of
these impurities cause excessive grain growth. The large grains are
accompanied by cracks and segregation of impurities which cause the
value of the critical current (J.sub.c) to drop across a segment
containing a large grain. Therefore, a uniformly fine
microstructure is desirable for good superconducting and mechanical
properties. Shortening the sintering time to less than 24 hours
results in lower density, while increasing the sintering time does
not have any significant effect and is a waste of furnace time and
energy. The wires and rods sintered in oxygen have a coarser
microstructure compared to the wires and rods sintered in air, but
have lower J.sub.c values. This is because sintering in oxygen
removes residual carbon and carbon dioxide more efficiently,
leading to a cleaner grain boundaries.
[0069] C. Additional Processing Steps for BSCCO.
[0070] The fabrication of BSCCO wires and rods is similar to that
of YBCO wires and rods. However, one difference is the amount of
additives. The initial powder, in the case of BSCCO, has a
plate-like morphology compared to the granular nature of the YBCO
particles. To achieve the Theological characteristics most suitable
for plastic extrusion, the amount of additives, in particular the
amount of solvent, is increased. The different amounts of the
various additives is shown in the table of FIG. 1. As indicated
above, process steps (i)-(v) are common to both YBCO and BSCCO
compounds.
[0071] The major factors influencing the properties of BSCCO wires
and rods are the orientation of grains, the composition of the
phases present, and the mass density. The sequence of reactions, by
which the precursor chemicals react to form the superconducting
2223 phase, and their reaction kinetics make it possible to achieve
a highly oriented, polycrystalline microstructure by appropriate
mechanical loading and heat treatment steps discussed below. See H.
Kumakura, K. Togano and H. Maeda, J. of Appl. Phys., 67 (1990),
which is incorporated herein by reference.
[0072] Cold Isostatic Pressing-1
[0073] Cold isostatic pressing (CIP) is normally done for
densification of powder compacts. The powder is packed in an
air-tight flexible can or container, like a rubber bag, and this is
placed in a high pressure chamber. The chamber is pressurized by a
liquid, usually water, and this causes uniform packing of the
powder.
[0074] This cold isostatic pressing technique can be used to
increase the density of BSCCO wires and rods. The extrusion process
induces a certain amount of orientation in the BSCCO wires and
rods, particularly on the surface, but this does not significantly
affect the properties. With cold isostatic pressing, the BSCCO
wires and rods are subjected to a quasi-isostatic load in such a
way that the wire or rod experiences maximum load in the radial
direction. This is accomplished by using a loading fixture 36 as
shown in FIG. 7b. See also FIG. 7a. The extruded wire or rod (after
binder burn-out) 38 is packed inside a latex tube 40, and stoppers
42, preferably made of neoprene, are used as closures to seal both
ends of the tube. A disk 44, preferably made of steel, is attached
to each stopper 42. This assembly is then placed in a support
structure or frame 46. When an isostatic load is applied, the
support structure 46 bears the axial load, and the wire or rod 38
experiences a mostly radial load. The isostatic load applied is
preferably about 40 ksi. FIG. 7a illustrates fixture 36 without
support frame 46 and represents the radial load applied to the wire
or rod by the plurality of arrows pointing towards tube 40.
[0075] A mandrel or rod should be inserted into the hollow space of
a hollow BSCCO rod prior to applying an isostatic load to such rod.
The inserted mandrel or rod should substantially fill the hollow
space. This measure helps prevent damage to hollow rods when the
isostatic load is applied.
[0076] Heat Treatment-1
[0077] The precursor superconducting powder used in the fabrication
of BSCCO wires and rods is a mixture of Bismuth Oxide, Lead Oxide,
Strontium Carbonate, Calcium Carbonate and Copper Oxide powders.
The precursor powder is available from a company called Seattle
Specialty Ceramics. The powders are mixed in the stoichiometric
ratio needed to yield the 2223 phase upon a heat treatment of the
BSCCO wire or rod (after the cold isostatic pressing step discussed
above).
[0078] The reactions leading to the formation of the 2223 phase are
as follows:
[0079] (a) Bi.sub.2O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3, CuO - - -
2201, 2212, CuO, Ca.sub.2PbO.sub.4, (Sr, Ca) Cuprates
[0080] (b) Ca.sub.2PbO.sub.4, 2201 - - - Liquid Phase (rich in Ca,
Cu)
[0081] (c) 2212, Liquid Phase, (Sr, Ca) Cuprates - - - 2223
[0082] See S. S. Oh and K. Osamura, J. of Mat. Sci., 26 (1991),
which is incorporated herein by reference.
[0083] The role of lead is to enhance the dissolution of the 2212
phase and the strontium-calcium cuprates in the liquid, thereby
promoting the formation of the 2223 phase. The cold isostatic
pressing step discussed above causes the intermediate liquid phase
to spread perpendicular to the packing direction, and this leads to
alignment of the 2223 phase.
[0084] The formation of the 2223 phase in BSCCO wires and rods by
heat treatment is greatly influenced by the furnace atmosphere and
temperature. It has been shown by several researchers that a
reduced oxygen environment helps the formation of the 2223 phase.
See U. Endo, S. Koyama and T. Kawai, Jap. J. of Appl. Phys., 27
(1988), which is incorporated herein by reference.
[0085] In the present invention, the preferred furnace atmosphere
used in the heat treatment of the BSCCO wire or rod is about 9%
oxygen and about 91% argon. The processing temperature depends upon
the furnace atmosphere. For the 9% oxygen atmosphere, heating at
about 830.degree. C. results in an almost complete 2223 phase.
Small amounts of copper oxide and strontium-calcium cuprates are
always present. However, they do not have a major influence on the
final properties. The preferred processing temperature has a very
narrow window and a deviation of 5.degree. C. will lead to the
presence of other phases like 2212, thereby affecting the overall
properties of the sample. See S. S. Oh and K. Osamura, Supercond.
Sci. Tech., 4 (1991), which is incorporated herein by reference.
Processing times ranging from 70 to 150 hours have been reported in
various publications for the complete formation of the 2223 phase.
In the present invention, the sample is preferably processed for
about 72 hours. It has been determined through x-ray diffraction
studies that this results in an at least 90% 2223 phase.
[0086] Cold Isostatic Pressing-2
[0087] One of the major problems at this stage of processing is
that the density of the specimen decreases with the formation of
2223. The extruded wire or rod (after binder burn-out) 38 has a
density of 3 gm/cc. After the cold isostatic pressing, this
increases to 4.5 gm/cc. But the formation of 2223 phase during the
heat treatment results in a drastic decrease in the mass density to
approximately 2.3 gm/cc. See S. S. Oh and K. Osamura, Supercond.
Sci. Tech., 4 (1991), which is incorporated herein by reference.
Thus, in spite of being made up of the 2223 phase, the wire or rod
has very poor mechanical and superconducting characteristics and
requires further densification by a second cold isostatic pressing
step. The wire or rod 38 is packed again in the latex tube 40,
placed in the support fixture 46 described above, and subjected to
an isostatic load of 60 ksi. This increases the mass density to 5.2
gm/cc (about 80% theoretical density). This loading also improves
the alignment of the 2223 platelets formed during the heating
treatment. However, an undesirable effect of the cold isostatic
pressing is the introduction of fine compaction cracks.
[0088] Heat Treatment-2
[0089] A second heat treatment is done at about 840.degree. C. in
an atmosphere of about 9% oxygen and 91% argon for about 72 hours.
This step results in healing of cracks formed during the second
isostatic pressing, growth of the 2223 grains mostly in the
preferred direction, and also transformation of the remaining
constituents into the 2223 phase.
[0090] II. CHARACTERISTICS OF YBCO WIRES AND RODS FABRICATED BY THE
PRESENT INVENTION
[0091] A. Electrical Properties.
[0092] Though the plastic extrusion process requires the addition
of organics to the YBCO powder during the early processing stages,
this does not affect the electrical properties of the resulting
wire or rod. This can be seen from the resistivity versus
temperature curve in FIG. 8 for YBCO wires and rods. Transition to
the superconducting state starts at 91.degree. C. and the
transition width is 2.degree. C., which is the normal value for a
good YBCO superconducting product.
[0093] The J.sub.c values of YBCO wires and rods of various
diameters is shown in FIG. 9. A significant increase in the J.sub.c
value is observed with a decrease in wire/rod diameter from 8.0 mm
to 0.38 mm. The reason for this behavior is the self-field effect
due to the current. See I. Bransky and J. Bransky, J. Appl. Phys.,
66, 5510 (1989), which is incorporated herein by reference.
[0094] The measured J.sub.c values remain constant over large
lengths of wires or rods. In some specimens, small drops in the
J.sub.c values are caused by the presence of defects, such as
cracks or pores, but in the absence of defective segments, the
J.sub.c values are uniform over long lengths.
[0095] An extruded wire or rod of the present invention is flexible
before drying and can be wound to form coils or other desired
shapes. For example, a 150 cm long wire of diameter 0.75 mm was
made by folding the wire over a 30 cm long quartz plate immediately
after extrusion. This wire is free of surface defects like cracks
or pores over its entire length, and the J.sub.c value of this wire
is uniform and above 1100 A/cm.sup.2.
[0096] The J.sub.c values of YBCO rods is shown in the table of
FIG. 10. In FIG. 10, the rods with inner diameter values are
hollow. Hollow rods have a higher J.sub.c value than solid rods of
the same diameter, and the properties of the hollow rods are also
more consistent.
[0097] B. Mechanical Properties.
[0098] The fracture strength and fracture toughness are two of the
critical properties that govern the mechanical integrity of
superconducting wires and rods during operation. See M. K. Ihm, B.
R. Powell and R. L. Bloink, J. Mat. Sci., 25, 1664 (1990); and F.
Yeh and K. W. White, J. Appl. Phys., 70, 4989 (1991), which are
incorporated herein by reference. The flexural-strength or the
modulus of rupture affords a convenient basis for comparing the
mechanical properties of ceramic materials. Three-point bend tests
were done in accordance with ASTM Standard, F417-78, "Standard Test
Method For Electronic Ceramics." The fracture toughness was
measured on straight notched specimens cut from thick rods. The
fine grained wires and rods, which have the best electrical
properties, are also found to possess the best mechanical
characteristics. The modulus of rupture of these fine grained YBCO
wires and rods is about 160 MPa, and the fracture toughness is in
the range of 1.0 to 1.3 MPa{square root}{square root over (m)}.
[0099] III. CHARACTERISTICS OF BSCCO
[0100] The plastic extrusion process for BSCCO superconducting
powder results in partially aligned, polycrystalline wires and rods
with an almost 100% BSCCO 2223 phase. The mass density of these
rods is 5.25 grams/cc. The average critical current density of
these rods is about 650 amperes/cm.sup.2 and depends on the
diameter of the wire or rod. Examples of the typical dimensions of
BSCCO rods fabricated in accordance with the present invention, and
their current capacities, are shown in the table of FIG. 11.
[0101] IV. EXAMPLE OF A YBCO ROD FABRICATED IN ACCORDANCE WITH THE
PRESENT INVENTION
[0102] The various steps in the fabrication of a polycrystalline
YBCO rod of diameter 0.8 cm and length 30 cm are described
below.
[0103] Step 1: Extrusion Die
[0104] An extruded wire or rod undergoes considerable shrinkage
during the various stages of processing. Drying of the extruded
wire or rod results in a shrinkage of 5% and sintering causes a
reduction of 16%. The final sintered diameter is close to 80% of
the extrusion nozzle diameter. Hence, an extrusion nozzle with a
diameter of 1.0 cm is used to fabricate a rod with a final diameter
of 0.8 cm.
[0105] Step 2: Paste Preparation
[0106] An extruded wire or rod shrinks in the axial direction also,
and therefore to achieve a final rod length of 30 cm, the initial
extruded length before drying should be approximately 38 cm. The
amount of paste required for this length is about 130 grams. During
extrusion, about 70 grams of paste remains in the tapered portion
of the piston extruder die and cannot be extruded. Thus, the total
quantity of paste required for fabricating the rod is about 200
grams. The polymeric additives (set forth in the table of FIG. 1)
are first mixed with about half the desired amount of the solvent,
and mixed to completely dissolve all the additives and form a
clear, viscous gel. The superconducting powder is then added and
thoroughly mixed with this gel. The remaining solvent is added in
small amounts and mixed to form an extrudable paste.
[0107] Step 3: Extrusion
[0108] The piston extruder is assembled with the 1.0 cm extrusion
nozzle, and the nozzle is plugged with an air-tight plug. See FIGS.
2a-c. The paste is cut into small pieces and placed in the barrel.
The ram is placed in the barrel and forms a vacuum tight chamber.
The barrel is connected to a vacuum pump and the chamber is
subjected to a vacuum of 200 millitorr. The ram is then pushed down
and a compaction load of 100 MPa is applied Next, the nozzle plug
is removed and the ram is moved down at a constant rate of 0.2 inch
per minute for extruding the rod. The extruded YBCO rod is about 38
cm long.
[0109] Step 4: Drying
[0110] The extruded YBCO rod is placed on a smooth and rigid base
(e.g., a glass plate) and placed in a still air enclosure at room
temperature for twelve days. The rod is then transferred to a
convection drier, where warm air at 60.degree. C. gently flows over
the rod. The rod is held there for thirty days during which time
most of the solvent evaporates.
[0111] Step 5: Binder Burn-Out and Sintering
[0112] The dried rod is placed in a long alumina boat and loaded in
a 7.5 cm diameter tube furnace. A continuous flow of oxygen at a
rate of 2 liters per minute (at atmospheric pressure) is maintained
and the furnace is heated at a rate of 6.degree. C. per hour until
a temperature of 300.degree. C. is reached. Beyond this
temperature, the oxygen flow rate is reduced to 0.25 liters per
minute, and the heating rate is increased to 20.degree. C. per hour
until a temperature of 500.degree. C. is reached to complete the
binder burn-out process. The heating rate can be further increased
to 40.degree. C. per hour until the temperature reaches 900.degree.
C. At the temperature of 900.degree. C., the sample is held for 24
hours for sintering and it is then cooled to room temperature at
the rate of 30.degree. C. per hour.
[0113] Step 6: Oxygen Annealing
[0114] The YBCO material in the rod is in a tetragonal,
non-superconducting phase at temperatures above 600.degree. C.
After sintering, and during cooling down to room temperature, the
rod absorbs oxygen from the atmosphere and this transforms the rod
to an orthorhombic superconducting phase. In order to ensure that
this transformation is complete, the material can be exposed to an
oxygen atmosphere at 500.degree. C. for a period of 24 hours. This
is either done while cooling after sintering or as an additional
processing step.
[0115] These processing steps produce a YBCO rod with a 0.8 cm
diameter and a 30 cm length, and with good superconducting and
mechanical properties. The rod is cut to the required length on a
diamond saw.
[0116] V. EXPERIMENTS CONDUCTED TO DETERMINE THE PROCESSING
PARAMETERS
[0117] Several experiments were conducted to determine the
processing parameters. FIGS. 12-14 show the effect of the initial
powder and sintering temperature on the grain size, mass density
and critical current density, and are examples of some of the
testing conducted to determine the processing parameters.
[0118] FIG. 12 illustrates the effect of sintering temperature on
grain size and specimen density for fine and course initial
powders. As illustrated, at any temperature, fine powder has better
density than coarse powder. Increasing the sintering temperature of
fine powder causes the density to increase, but it can be seen that
above 900.degree. C., the grain size becomes coarser. This results
in a drop in the critical current density, as shown in FIG. 13. As
is also illustrated, the critical current density of fine powder is
higher at all temperatures. From these experiments, it was
concluded that the most suitable microstructure can be achieved by
sintering fine powder at a temperature of about 900.degree. C.
[0119] FIG. 14 illustrates how fracture strengths of wires and rods
are affected by different processing conditions.
[0120] VI. APPLICATIONS OF SUPERCONDUCTING WIRES AND RODS
FABRICATED BY THE PRESENT INVENTION
[0121] One of the potential applications for wires and rods
fabricated by the present invention is use as current leads in low
T.sub.c superconducting equipment. Superconducting electromagnets
made from Low T.sub.c superconductors, such as Niobium-Tin and
Niobium-Titanium alloys, are currently being used in various
equipment including magnetic resonance imagers, magnetic energy
storage, and particle accelerators. These superconductors are
operated at a liquid helium temperature of 4.2 K. Presently,
metallic leads are used for feeding current from room temperature
to these low temperature superconductors. The operating currents of
these magnets range from a few hundred to a few thousand amperes.
The metallic leads increase helium consumption because they conduct
heat and cause ohmic resistance heating. Use of the superconductor
wires and rods of the present invention as leads will reduce this
resistance heating, and due to their low thermal conductivity, also
reduce the heat conduction, thereby cutting down the liquid helium
consumption.
[0122] These superconducting leads can be made as single, large
area wires or as a rod comprising a multifilament composite of
wires. Since smaller wires have a higher J.sub.c value, a
multifilament conductor made up of several wires is likely to carry
higher currents than a single large diameter superconducting rod. A
multifilament conductor having an overall critical current of 185 A
has been constructed-by stacking twenty wires, each having a
critical current of 10 A.
[0123] Current leads made from the superconducting wires or rods
can also be used in smaller scale applications such as in cryogenic
experiments, scanners in spacecrafts, and low current magnets.
[0124] The present invention, therefore, is well adapted to carry
out the objects and attain the ends and advantages mentioned herein
as well as other ends and advantages made apparent from the
disclosure. While preferred embodiments of the invention have been
described for the purpose of disclosure, numerous changes and
modifications to those embodiments described herein will be readily
apparent to those skilled in the art and are encompassed within the
spirit of the invention and the scope of the following claims.
* * * * *