Physica B 165&166 (1990) 1383-1384North-Holland

TRANSPORT PROPERTIES OF Ag-Y123 FIBERS PREPARED USINGTHE POLYMER-METAL PRECURSOR TECHNIQUE

W. M. Tiernan and R. B. HallockLaboratory for Low Temperature Physics, Department of Physics and AstronomyUniversity of Massachusetts, Amherst, Massachusetts 01003

James C. W. Chien, B. M. Gong, S. H. Dong, and Y. S. YangDepartment of Polymer Science and EngineeringUniversity of Massachusetts, Amherst, Massachusetts 01003

We report on the fabrication and superconducting transport properties of supercon­ducting fibers with typical cross-sectional areas of 10 2 to 10 3 JLm 2 made using thepolymer-metal precursor (PMC) technique. These samples show superconducting prop­erties similar to good quality bulk polycrystalline pellets, with Jc (77K) ~ 103A/cm2

and J c ( 4K) ~ 104A/cm2 • Magnetic field measurements show that the fiber's super­conducting transport properties are dominated by weak links between grains, as in bulkpolycrystalline materials.

The PMC precursor technique (1) involves com­plexing metal nitrates or carbonates to a functionalpolymer chain with equal affinity. It offers two advan­tages over standard mix and grind techniques: 1) Thefunctional polymer chain mixes the metal ions on amolecular level to form intermediate mixed metal com­pounds prior to calcination. 2) The polymer precursorcan be produced in a fiber form which holds its generalmorphology during heat processing.

The samples reported here were made using polyamicacid of pyromellitic dian hydride and aminophynyl etherwith a polymer to metal-nitrate weight ratio of 2:1.The metal atomic ratios were 1:2:3:3 for Y, Ba, Cu,and Ag respectively, giving 33% Ag by weight. Thepolymer a.nd metal nitrates were dissolved in dimethylformamide giving a transparent solution. The PMCprecursor solution was concentrated under vacuum atroom temperature to form a spinning solution. A spin­ning machine produced approx 100 JL m diameter trans­parent green precursor fibers which were dried for aday at room temperature in air. The precursor fiberswere then hung in a vertical furnace and pyrolized inflowing nitrogen for 2 hours each at 300 C and 600C, and cooled to 300 C. The fibers were then calci­nated in flowing oxygen at 300 C and 600 C for 1 houreach, and then given a final heat treatment of 900 Cto 940 C. SEM photos reveal that the resulting fibersconsist of many small tightly coupled grains. The typ­ical grain size is found to vary with calcination tem­perature, with smaller grains ( ~ 1JLm) produced atthe lower calcination temperatures and larger grains(:s; ~ 10JLm) at the higher temperatures. The silverappears to occur predominantly at the surface of thefibers, with smaller amounts dispersed throughout thesample. The silver apparently facilitates making ele-

crical contact; the contact resistance at a typical con­tact (area ~ 2 x 1O-3cm2) was 81 mr! at 77 K and 58mr! at 4 K.

There are three samples reported on here. Sample421 is a bundle of three cylindrical fibers each with adiameter of ~ 15 JLm. Samples 483 and 486 are bothsingle fibers with rectangular cross sections of 14 JLmby 96 JLm and 12 JLm by 57 JLm, respectively.

These samples were mounted in a standard fourlead configuration with elecrical contact made using sil­ver filled epoxy. Transport measurements were madeusing a DC reversing current technique. Table 1 listsvarious normal state and superconducting properties ofthe samples. The normal state resistivity of these sam­ples is a factor of 5 to 50 times less than good qualitybulk 123 pellets, indicating that in the normal statemost of the current is shunted through the silver. Thetemperature at which R/Rn = .9 is 90 to 91.5 K, com-

TABLE 1. Summary of Ag-Y123 fiber properties.

Sample 421 483 486

p(300K) JLr!cm lOA 88.7 24.9

T(.9Rn ) K 91.5 89.9 90.9

T(.5Rn ) K 9004 89.0 89.8

Jc(4K) A/cm2 15300 12300 7100

Jc(77K) A/cm2 830 650 130

0921-4526/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

1384 W.M. Tiernan, R.B. Hallock, J.C. W. Chien, B.M. Gong, S.H. Dong, Y.S. Yang

paren with 92 K for bulk pellets. This is most likelyattributable to silver carrying most of the current whenthe sample is normal; before the superconducting tran­sition shows an appreciable drop, the Y123 resistancemust drop enough to be similar to the silver's resis­tance.

A set of IV curves was measured for sample 421,the bundle of three fibers, at sample currents rangingfrom .01 rnA to 100 rnA and voltage levels from .1 JLVto 1000 JLV. This corresponds to a range of currentdensities from 3 A/cm2 to 30,000 A/cm2 and electricfield levels of 1 JLV/cm to 10,000 JLV/cm. IV curveswere determined at T = 77 K as a function of mag­netic field, with field values ranging from 0.1 Gauss to9 kGauss; and at a fixed field of ~ 0.5 Gauss for var­ious temperatures from 4.3 K to 85.7 K. See figures 1and 2.

J c as a function of field was determined from theIV curves of figure 1 using a.1 JLV (1 JLV/cm) cri­terion. J c was found to be very sensitive to smallfields in the range of 1 Gauss to 30 Gauss. Similarbehavior has been observed in bulk pellets and beenattributed to weak links between Y123 grains. Thissensitivity to small fields is strong evidence that thesllperconducting transport properties of these fibers isdominated by weak links between grains, as is the casefor bulk pellets. The shape of the Jc(H) curve hasbeen explained as being roughly of the form sinx/xseen in a single Josephson junction with the oscillationswashed out due to variations in junction size and ori­entation(2,3). Interpreting our data in this way givesan effective Josephson junction area of ~ 3JLm2 • Thetypical grain size in this fiber as determined by SEM is2 to 4 JLm.

J c as a function of temperature at H ~ 0.5 Gausswas determined from the IV curves shown in figure 2.Jc(T) falls roughly linearly between 4 K and 77K andthen goes to zero more slowly as T approaches Tc ' in amanner consistent with (1 - T /Tc) 2. Since J c is lim­ited by Josephson junction weak links between grains,Jc(T) should correspond to J c for a typical weak link.Jc(T) for this sample is inconsistent with that for anAmbegaokar- Baratoff SIS Josephson jnnction, which isfairly constant over a large temperature range and fallssharply to 0 near T c with a 1 - T /Tc dependence.This data could be consistent with an SNS JJ or withthe SIS JJ behavior for high Tc superconductors pre­dicted by Deutscher and Meul1er(4). In both of thesecases the limiting behavior of J c near Tc is Jc(T) ~

(1 - T/Tc )2 (5).

This work was supported in part by Research TrustFunds administered by the University and to a lim­ited extent by the National Science Foundation throughDMR 88-20517.

REFERENCES(1) .1. C. W. Chien,H. M. Gong,.1. M. Madsen, and R.

B. Hallock, Phys. Rev. B 38, 11853 (1988).(2) J. F. Kwak, E. L. Venturini, P. J. Nigrey, and D.

S. Ginley, Phys. Rev. B 37, 9749 (1988).(3) R. L. Peterson and J. W. Ekin, Phys. Rev. B 37,

9848 (1988).(4) D. Deutscher and K. A. Meuller, Phys. Rev. Lett.

59, 1745 (1987).(5) R. Gross, P. Chaudhari, D. Dimas, A. Gupta, and

G. Koren, Phys. Rev. Lett. 64, 228 (1990).

1000

"'

10 100(rnA)

.. .. ........ . ... .~ .... . .. .-... I 4 ...

,/ .:":.:....'" ...f .. ..:. .:/ : • e:

.f .. : =!" "t· .. ....· ".-..· " .· ::

" .:

".

1

I

'"

,,'

."

II

ooo-

o0.1

,.....

oo-

->

>­::to--

100101(rnA)

0.1o

0.01-

oo-

>

->-::to--

ooo-

FIG. 1. V vs. I at 77 K for magnetic field strengths of90UO (left), 1000, 15U, 38, 28, 20,13,9,6,4,2, and 0.1(right) Gauss.

FIG. 2. V vs. I at H ~ 0.5 Gauss at temperatures of85.7 (left), 76.7,60.7,40.8,21.6, and 4.3 K (right).