Materials exhibiting superconductivity above liquid nitrogen temperatures (77 K) will enable new applications of this phenomena. One of the first commercial applications of this technology will be superconducting magnets for medical imaging. However, a large number of aerospace applications of the high temperature superconducting materials have also been identified. These include magnetic suspension and balance of models in wind tunnels and resistanceless leads to anemometers. The development of superconducting wires fabricated from the ceramic materials is critical for these applications. The progress in application of a patented fiber process developed by Clemson University for the fabrication of superconducting wires is reviewed. The effect of particle size and heat treatment on the quality of materials is discussed. Recent advances made at Christopher Newport College in the development of micro-ohm resistance electrical contacts which are capable of carrying the highest reported direct current to this material is presented

Buoncristiani, A. M.

Byvik, C. E.

Caton, R.

Fain, C. C.

Lee, B. I.

Leigh, H. D.

Lewis, G.

Modi, V.

Selim, R.

Sherrill, M.

English

NASA Technical Reports Server

N93-27575
DEVELOPMENT OF SUPERCONDUCTING YBa2Cu30 x WIRES WITH
LOW RESISTANCE ELECTRICAL CONTACTS
,k *
R. Caton and R. Selim
Christopher Newport College
Newport News, Va
B. I. Lee , V. Modi , M. Sherrill ,H. D. Leigh , C. C. Fain
*A' _t*
G. Lewis , and C. H. Barron
Clemson University
Clemson, SC
A. M. Buoncristianit and C. E. Byvik
NASA Langley Research Center
Hampton, VA
SUMMARY
Materials exhibiting superconductivity above liquid nitrogen temperatures
(77 K) will enable new applications of this phenomena. One of the first
commercial applications of this technology will be superconducting magnets for
medical imaging. However, a large number of aerospace applications of the high
temperature superconducting materials have also been identified. These include
magnetic suspension and balance of models in wind tunnels and resistanceless
leads to anemometers. The development of superconducting wires fabricated
from the ceramic materials is critical for these applications. The progress in
application of a patented fiber process developed by Clemson University for the
fabrication of superconducting wires is reviewed. The effect of particle size
and heat treatment on the quality of materials will be discussed. Recent
advances made at Christopher Newport College in the development of micro-ohm
resistance electrical contacts which are capable of carrying the highest
reported direct current to this novel material is presented.
*Supported by NASA Grant NAG 1-796
**Supported by NASA Grant NAG 1-820
tNational Research Council Resident Research Associate
PRECEDING PAGE E_LANK t'_IQ"RF-_,_,_ED
343
https://ntrs.nasa.gov/search.jsp?R=19930018386 2020-03-17T05:47:59+00:00Z
INTRODUCTION
The high temperature superconductivity research program at Langley
Research Center is focused on developing technology required for the early
application of the new high temperature superconducting materials to aerospace
systems. The near term effort addresses the fabrication of superconducting
wires utilizing a process proven successful for making wires from ceramic
materials. Superconducting wires will find use in airborne and spacecraft
platforms as interconnects between electronic devices and to ground planes at
77 K(liquid nitrogen temperatures). Important to the exploitation of the effort
to develop electronic devices (detectors, etc.) is the development of low
resistance contacts which are compatible with existing electronic technology.
The long term objective will merge these efforts into the development of a
Superconducting-Insulating-Superconducting (SIS) thin film detector for 100
micron radiation for atmospheric remote sensing.
The following report summarizes the results of the research efforts on
sample preparation techniques, the development of low-resistance contacts to
the new superconducting material and the progress in forming superconducting
wires using c-shaped carbon forms.
SAMPLE PREPARATION TECHNIQUE
This section focuses on the procedure for preparing YBa2Cu30 x (YBCO)
superconducting samples using the sintering (solid-state reaction) method. The
starting materials for our samples are oxide and carbonate powders. The
purities of the starting materials are as follows: BaCO 3 - 99.997%; Y20 3 -
9=9.999%; CuO - 99.999%. The sintering method involves several steps of mixing,
grinding and heat treatment (ref. 1). Systematic studies for I_'oducing better
quality samples (capable of carrying relatively larger current densities)
resulted in the following procedure:
1) Mixing powders of BaCO3, Y203 and CuO using a mixer/mill machine.
2) Pressing pellets of 1/2" diameter using a 10 ton hydraulic press, and
treating these-pellets in a split tube furnace at 950 C for 8-12 hrs in air.
3) Regrinding the material in alcohol slurry using the mixer/mill machine;
heating the slurry to remove alcohol, and repeating step 2.
4) Regrinding the material in alcohol slurry using the mixer/mill machine,
heating the slurry to remove alcohol, and pressing the resulting powder to
form 1/2" pellets for the final 02 treatment.
344
5) Retreating the pellets in oxygen flow (0.2 Ipm in a 1" diameter quartz
tube) at 950 C for 24 hrs, cooling down the pellets to 700 C and treating
for 16 hrs, then cooling to 400 C and treating for 5 more hours.
The resulting samples are hard, black and brittle. Resistance
measurements indicate a sharp superconducting transition with an onset
temperature of 93 K and a transition width of ! K.
Figure 1 shows the resistance of two samples after air treatment. One
sample was pressed into a pellet before the air treatment while the other was
treated as compacted powder. They were subsequently given an additional air
treatment as pellets. It is clear that we get better results by pressing the
samples before the air treatment.
Figure 2 shows the effect of duration of the air treatment. All samples
were given the initial air treatment indicated on the graph followed by a 24
hour air treatment and oxygen treatment. The resistance results suggest that
treating samples for periods of 8 to 12 hrs is sufficient to produce a
transition temperature of 92 K and little improvement for samples treated for
longer periods. Similar results were obtained for the time of the second air
treatment.
Figure 3 shows the effect of hand grinding (HG) where the sample was
ground by hand using a mortar and pestle, machine grinding (MG) using a
mixer/mill machine, and machine grinding in alcohol (AG). All samples
underwent two air treatment and a final oxygen treatment. Alcohol ground
sample has a higher transition temperature.
We have found that using the results in figures 1-3 to develop the
sample preparation procedure described earlier results in dense samples with
mass density of 5.7 g/cm3 ( 90% of the theoretical value) and a relatively
larger current density that is limited only by grain effects present in
polycrystalline bulk superconductors (ref. 2). Using this procedure we obtained
critical current density Jc of 200 A/cm 2 at 77 K and Jc of 480 A/cm 2 at 20 K.
LOW-RESISTANCE CONTACTS
Since the new high Tc superconductors are ceramics, electrical contact
to these novel materials presents a challenge. While silver paste and pressure
345
contacts can be used to measure the resistive transition, their inability to
carry large currents without burning out make them unsuitable for determining
large critical currents directly or for use in high current carrying applications
(refs. 3 and 4).
Following previous work on gold contacts to sapphire (ref. 5), we form
the contacts by placing small squares of gold foil on the surface of a shaped
resistivity sample that has been cut from a fully oxygen treated pellet. The gold
squares are = 1 mm on a side (or less) and 0.1 to 0.3 mm thick (the thicker
pieces for current contacts and thinner for voltage contacts). We then bring the
sample to = 1065 °C in air in from one hal f to one hour. The gold melts and
diffuses into the sample pores. The sample is removed from the oven once it has
cooled to = 900 °C. At this point the gold bead that is formed is slightly
oxidized and firmly connected to the sample; however, the contact resistance is
high (=kohms). Since some of the oxygen is lost from the sample during this
process, retreatment in oxygen is necessary.
Restoration of the superconducting properties and good electrical
contact are achieved by the following" expose to flowing oxygen (= 0.2 Ipm) at
900 °C for 24 hr, 700 °C for 16 hr, and 400 °C for 8 hr. The slight oxide coating
remaining on the gold bead after retreatment is abraded off before soldering
wires to the bead for measurement. It is easy to solder external leads to the
beads making good electrical contact. Lead-tin solder can be used if the
temperature of the soldering iron is kept to a minimum. This relatively simple
technique promises to be of practical use in many applications of high T c
superconductors.
Figure 4 shows a macroscopic scanning electron microscope (SEM)
picture of-a typical 13ead of diameter = 1 mm with a copper wire soldered in
piac'el All measurements of resistance were made in a closed Cycle refrigerator
capable of covering the temperature range of 15 to 300 K. The samples were
mounted on a copper block with GE varnish for thermal contact and lens paper
soaked in GE varnish for electrical insulation. A calibrated platinum
thermometer (good to 0.1 K) mounted in a well directly below the sample was
used to measure the temperature. A four probe dc method was used to measure
the resistance of the sample and the contact resistance (see inset of figure 5).
Each data point represents an average of twenty readings with the current
reversed Bach time to eliminate thermal offsets (with a precision of =0.1%).
Figu-res_5 an_=_-d6 show typical_resiStance vs temperature runs using gold bead
contacts andsilver epoxied leads bonded to sputtered gold films. Each figure
346
displays the resistance of the sample only (curve a) and the resistance of the
sample plus two current contacts (curve b)after adjusting for the different
length of sample between the current and voltage leads. Our best sample
contacts show resistances of < 50 #_ below the superconducting transition and
< 18 mR at room temperature. We can only report an upper limit since the
resistance we measure includes the solder and gold bead. Since the area of the
beads is typically 0.01 cm2 the surface resistivities are < 0.5 p._cm2 and 180
#_cm2 respectively. Figure 6 shows that the contact resistances for epoxied
leads bonded to sputtered gold films is considerably higher: -- 0.2 _ per contact
at 77 K. The gold bead contacts are clearly superior.
Poor contact hampers direct measurement of critical current densities.
With silver paint contacts we were typically obtaining critical current
densities up to 30 A/cm2 which did not vary much with temperature. Feeling
that these were spurious due to the local heating effects of the high resistance
of the silver paste contacts ( especially once they burn out), we made
measurements on similar samples with the gold bead contacts. We obtained 200
A/cm2 at 77 K and > 480A/cm2 at 20 K. A current of 7.5 ampere was passed
through the sample while making the critical current measurement at 20 K in
vacuum with the sample bonded to a copper block. If the contacts can perform
that well in vacuum, they should present no problem in high current carrying
applications where the devices will most likely be immersed in liquid nitrogen.
The strength of the contact was determined by measuring the force
necessary to cause shear failure of a contact of given area. The results varied
among the samples" so we report the range, the average value, and the standard
deviation of the shear strength. The gold beads ranged from 5 to 23 MPa with an
average of 13 .+..6 MPa; the epoxy ranged from 4 to 11 MPa with an average of 8 +
3 MPa; and the silver paste ranged from 0.6 to 1.7 MPa with an average of 1.2 .+..
.8 MPa. The gold bead contacts were the strongest, having a shear strength about
1/20 that of iron while that of the silver paste was about the same as concrete.
In some cases the sample failed before the gold bead contact.
The contacts have the clear advantage of being reasonably robust making
it possible for the leads attached to the contacts to withstand mechanical
stress without need for support.
347
DEVELOPMENT OF SUPERCONDUCTING WIRE
Clemson University has recently demonstrated a technique to
produce ceramic fibers using its C-shape carbon fibers (reference 6). In this
technique, ceramic fibers are formed by filling c-troughs of the carbon fibers
(CF) (fig. 7) with a ceramic precursor sol or a fine slurry followed by pyrolyzing
the CF. This technique was thought to be equally applicable in fabricating
superconducting fibers from the new ceramic oxide superconductor.
Since the discovery of the new ceramic high Tc superconductor, Clemson
University has also developed a simple way (refs. 7 and 8) to produce high quality
YBCO powder. Moreover, it has developed a method to produce a YBCO material
by way. 0!a homogeneous solution called a sol-gel technique. Thus, it is the
objective of this study to develop methods to obtain strong and flexible fibers
or wires which are superconducting above the temperature of the boiling point
of liquid nitrogen.
The C-shape of CF were produced by the procedure described in
reference 6_ Two kinds of C-shape CF were used. One is heat treated at 240 C
for crosslinking--and the other one is carbonized at 1500 C. YBCO sol was
prepared by dissolving alkoxides of yttrium and barium and copper
cyclohexanebutyrate in an organic solvent in a nitrogen atmosphere under
refluxing conditions: A YBCO powder dispersion (b) was prepared in the
following steps. The carbonized CF were pretreated to improve wetting by YBCO
A Or B. The pretreatments included soaking the CF in concentrated HNO3,
heating in air @400 C, or soaking in ethanolic 3-aminopropyltriethoxysilane.
Bundles of theunCarbonized CF and the surface treated carbonized CF Were
dipped into YBCO A or B. The wet fibers were dried in air followed by heat
treatments in air, oxygen or nitrogen at 500 C, 600 C or 900 C. To prevent CF
from oXidizing, the YBCO coated fibers were heat treated in nitrogen gas at 900
C fo]10wed by oxygen annealing at 400 C. In some cases, silver oxide powder
(10-50'_V/O) was added to YBCO B to furnish oxygen in situ for firing in nitrogen
gas and the resulting silver to increase ductility of the fibers. Cotton threads
in various lengths were impregnated with YBCO A or B followed by direct heat
treatments at 910 C in air, or the impregnated threads were coated with silver
paint prior to heat treatments. Resistance vs temperature curves were obtained
by using AC four-point probe and silver paste and indium pads.
Figure 8 shows an SEM micrograph of the YBCO powder used in the slurry
B. It shows submTcron size of particles after 8 hrs. ball-milling in ethanol.
348
|
|
J_
Figure 9 shows the superconducting transition temperature of 94 K
where the resistance is zero. As shown in figure 10, a wafer derived from YBCO
sol A after firing in air has no transition above 77 K. This is thought to be the
incorrect stoichiometry of the sol. The composition of the sol is currently
being determined. Figure 11 shows the CF after coated with YCO A or B with or
without silver oxide followed by heat treatments in nitrogen at 940 C for 2
hours and oxygen annealing at 400 C for 8 hours. The YBCO containing CF were
too brittle and showed little strength. These poor mechanical properties should
be caused by the reactions between carbOn and YBCO during nitrogen firing
and/or the oxygen annealing at 400 C. It is conceivable that carbon provides a
reducing condition during the firing in nitrogen gas. In that case, oxygen
annealing should have no effect in restoration of the superconductivity of the
fibers.
Figure 12 shows wires produced by impregnating a cotton thread and
coated with silver paint followed by firing in air for 6 hours at 910 C. The
wires are flexible and mechanically moderately strong. However, the surface is
not so uniform resulting from the all manual process. Figure 13 shows a
resistance versus temperature curve indicating the wires are superconducting
above 77 K. The low resistance throughout the temperature region is due to the
silver conductivity. It clearly shows the superconducting transition above the
liquid nitrogen temperature. The wire showed good ductility originating from
the silver. Refinement of the technique to produce more uniform surface of the
wire is underway.
ACKNOWLEDGMENTS
We wish to acknowledge the efforts of Raquel Caton and Robert Harvey
for preparing samples.
REFERENCES
1. Xiao-Dong Chen, Sang Young Lee, John P. Golben, Sung-lk Lee, R, McMichael, Yi
Song, Tae W. Noh, and J.R. Gaines, Rev. Sci. Instrum. 58, 1565 (1987).
2. D.S. Ginley, E.L. Venturini, J.F. Kwak, R.J. Baughman, B. Morosin, and J.E.
Schirber, Phys. Rev. B 36, 829 (1987).
. Caton, R.; Selim, R.; Buoncristiani, A.M.; and Byvik, C.E.: "Rugged Low-
Resistance Contacts to the High Tc Superconductor YBa2Cu3Ox." Meeting
of the American Physical Society, March 1988.
349
° Caton, R.; Selim, R.; Buoncristiani, A. M.; and Byvik, C. E.: "Rugged Low-
Resistance Contacts to YBa2Cu3Ox," Applied Physics Letters,
Vol. 52, March 21, 1988, pp. 1014-1016.
6. C. Varmazis, R. Viswanatnan, and R. Caton, Rev. Sci. lnstrum. 49, 549 (1978).
7. N. Mack, C. C. Fain, T. D. Taylor, J. Cogburn, and D. D. Edie, "Piggybacks: A
Unique Process for Forming Ceramic Fibers and Ceramic/Carbon
Bicomponent Fibers," NASA Conf. Pub. 2482, Metal Matrix, Carbon, and
Ceramic Matrix Composites, 1987, J. D. Buckley, ed, p.169.
. B. I. Lee, V. Modi, and _M. Sherrii[_:"YBCO Supe-rconductor Vqa N it-ra_tes,,,::
Accepted for Presentation at 90th Am. Ceramic Soc. Annual Meeting, May
1-5, 1988, Cincinnati, OH.
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351
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352
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6. Resistance vs temperature for the epoxy to metal film contacts. Curve a is
the resistance of the sample only. Curve b includes the resistance of two
contacts.
353
Figure 7. A C-shape carbon fiber derived from a petroleum pitch (left), the
C-trough filled by a silica precursor sol (right).
354
Figure 8. A scanning electron micrograph of YBCO powder after ball milling.
m_
30
20 ¸
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/
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directly from nitrates of yttrium, barium and copper.
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355
Figure 11. CF before dipping in YBCO Sol (left) and after dipping and firing
(right).
Figure i2. Superconducting wires produced by impregnation of cotton
threads.
356
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Temp.,°K
Figure 13. Resistance as a function of temperature for wires in figure 12.
357



Development of superconducting YBa2Cu3O(x) wires with low resistance electrical contacts

Development of superconducting YBa2Cu3O(x) wires with low resistance electrical contacts

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