REFERENCE

NBS

PUBLICATIONS NAT-L INST OF STun..n.

I.C.

A 1 1 1 0

2

2M7ST3

NBSIR 86-3044 A11inOO/l7cno

OF SUPERCONDUCTORS FOR DOE

FUSION APPLICATIONS

J.W. Ekin

J. Moreland

J. C. Brauch

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

March 1986

4C

100

• U56

86-3044

1986

KBS

RESEARCH

REFORMATION CENTER NBSIR 86-3044

ELECTROMECHANICAL PROPERTIES

OF SUPERCONDUCTORS FOR DOE

FUSION APPLICATIONS

J. W. Ekin

J. Moreland

J. C. Brauch

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

March 1986

Prepared for Department of Energy

Office of Fusion Energy

Washington, DC 20545

(Interagency Agreement #DE-AI01 -84ER52113)

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

PREFACE

This is an interim report covering the work performed in the first year

and a half of a three-year contract (3/4 staff year per year). The objective

of the research is to obtain data on superconductor performance under mechanical load; these data are needed for the mechanical design of superconducting

magnets for DOE fusion energy systems. A further aim is to measure and

understand the electromechanical properties of promising new superconductor

materials with strong application potential.

Testing is proceeding along two lines. First, a new measurement system

has been built to measure critical current as a function of transverse compressive strain. Preliminary tests of the apparatus have been completed, and a new adaptation for measuring single superconductor strands has been constructed.

Engineering data on the effect of this transverse component of strain have not

been measured previously, although it has a significant role in present magnet

designs

.

Second, new superconducting material testing has been performed at very

high magnetic fields using an apparatus for measuring axial strain effects in

fields up to 24 T. Results have been obtained on several experimental superconductors, including the first strain-effect data on a Chevrel phase superconductor, PbMo,S Q , which has strong application potential above 20 T, and several

0 o new Nb^Sn conductors specially fabricated for economical operation at higher

fields (14 to 18 T)

.

In addition to these two main thrusts, two ancillary projects have been

started. A new diagonostic tool for probing the microstructure of practical

superconductors has been developed using electron tunneling, and thermal

contraction data have been obtained on several new candidate materials for

superconductor sheathing and strengthening. The latter should eventually

result in significantly less electrical degradation of the critical current in

reinforced superconductors.

The first year's efforts have resulted in three publications:

1. "Electromechanical and Metallurgical Properties of Liquid-infiltration

Nb-Ta/Sn Multifilamentary Superconductor," by J. W. Ekin and M. Hong, Appl.

Phys. Letters 45_, 297 (1984).

2. "Effect of Uniaxial Strain on the Critical Current and Critical Field of

Chevrel Phase PbMo,S_ Superconductors," by J. W. Ekin, T. Yamashita, and K. Hamasaki, IEEE Trans. Magn. MAG-21, 474 (1985).

iii
3. "Electron Tunneling Into Superconducting Filaments Using Mechanically

Adjustable Barriers," by John Moreland and J. W. Ekin, Appl. Phys. Letters 47,

175 (1985).

Next year's objectives include the first direct measurements of the

effect of transverse compression on the critical current of Nb^Sn and design

of an apparatus to measure uniaxial tension and transverse-compression effects

simultaneously. The electromechanical properties of several new superconducting materials and processes will also be evaluated; these include modified

PbMo,S 0 superconductors, Nb 0 Sn with new additions, V Ga, hot-isostatic-pressure bo J J treatment, powder metallurgy and in-situ processes, and modifications to the

internal-tin and jelly-roll Nb^Sn conductors. A fundamental study of the

origin of the strain effect will also be made.

This report is divided into five sections. Electromechanical-property

data are presented in the first section for several experimental (short-length)

superconductors, and in the second section for several commercial Nb^Sn

conductors that are present candidates for fusion applications. The transversepressure test apparatus, thermal-contraction data, and tunneling diagnostic

probe are treated in Secs. 3, 4, and 5, respectively. References are given at

the end of each section. The appendix contains a trip report covering superconductor-technology developments in Japan and two publications on relevant work

performed under an earlier contract. The latter two treat electromechanical

degradation in structurally reinforced Nb^Sn conductors and details of liquidtin-inf iltrated Nb^Sn superconductors described in Sec. 2C.

SUMMARY OF MAIN RESULTS :

* The first studies of strain vs. critical-current were made on a Chevrelphase superconductor, PbMo^Sgj Chevrel phase superconductors were found to

have a large strain effect, comparable in magnitude to A-15 superconductors

like Nb^Sn.

* Electromechanical properties measurements of an experimental liquid-tininfiltrated Nb^Sn conductor showed it to have an irreversible-strain limit

twice as large as bronze-process superconductors and a significantly higher

overall critical-current density; the liquid-infiltration process thus has the

potential for development of a practical Nb^Sn conductor with both superior

critical-current density and extremely good mechanical properties.

* Experimental internal-tin Nb^Sn superconductors have a high irreversiblestrain limit (^0.9%), but a low strain-free upper critical field and conse- quently low critical-current density at fields above 12 T.

IV
Experimental jelly-roll Nb^Sn superconductors have a very low irreversiblestrain limit (0.14%) but high overall critical-current density over a wide

range of magnetic field compared with other long-length Nb^Sn superconductors.

Thermal contraction data were obtained on several new candidate materials

for superconductor sheathing. With respect to degradation of the critical

current from differential thermal contraction, it was found that high Mn

stainless steel offers no significant advantage over JBK-75 and 300 series

austenitic stainless steels as a Nb^Sn reinforcing material.

* A new diagnostic tool for probing the energy gap of practical superconductors has been developed using electron tunneling.

The participation of a number of people in this project is gratefully

acknowledged: M. Austin for performing some of the thermal expansion

measurements; J. Sherman and M. Allen for their help in machining the

apparatus; and D. Rule and V. Cima for their help in reducing much of the

data. Many helpful discussions were held with the other members of the

Superconductor and Magnetic Measurements group. Special thanks to A. F.

Clark, R. B. Goldfarb, and L. F. Goodrich for a critical reading of this

report

.

v

CONTENTS

Page

PREFACE iii

ABSTRACT ix

UNIAXIAL STRAIN-EFFECT CHARACTERIZATION OF NEW HIGH-FIELD

EXPERIMENTAL SUPERCONDUCTORS 1

ELECTROMECHANICAL AND METALLURGICAL PROPERTIES OF LIQUID- INFILTRATION

Nb-Ta/Sn MULTIFILAMENTARY SUPERCONDUCTORS

J. W. Ekin and M. Hong 3

EFFECT OF UNIAXIAL STRAIN ON THE CRITICAL CURRENT AND CRITICAL FIELD

OF CHEVREL PHASE PbMo,S 0 SUPERCONDUCTORS

D O

J. W. Ekin, T. Yamashita, and K. Hamasaki 17

EFFECT OF UNIAXIAL STRAIN ON Nb Sn WITH Mg ADDITIONS

J. W. Ekin 30

HIGH-FIELD UNIAXIAL STRAIN EFFECT CHARACTERIZATION OF CANDIDATE

Nb Sn SUPERCONDUCTORS FOR FUSION APPLICATIONS: Internal Tin,

Jelly Roll, Bronze Process

J. W. Ekin 36

CONSTRUCTION AND INITIAL TESTING OF A TRANSVERSE-STRESS-EFFECT APPARATUS

J. W. Ekin 55

THERMAL CONTRACTION OF SEVERAL CANDIDATE SHEATHING AND STRENGTHENING

MATERIALS FOR SUPERCONDUCTORS

J. W. Ekin 60

ELECTRON TUNNELING INTO SUPERCONDUCTING FILAMENTS USING MECHANICALLY

ADJUSTABLE BARRIERS

John Moreland and J. W. Ekin 78

APPENDIX A: Effect of Stainless Steel Reinforcement on the Critical

Current Versus Strain Characteristic of Multifilamentary

Nb^Sn Superconductors

J. W. Ekin, R. Flukiger, and W. Specking 94

APPENDIX B: Further Investigations of the Solid-Liquid Reaction and

High-Field Critical Current Density in LiquidInfiltrated Nb-Sn Superconductors

M. Hong, D. M. Maher, M. B. Ellington, F. Heilman,

T. H. Geballe, J. W. Ekin, and J. T. Holthuis 97

APPENDIX C: Japan Trip Report, December 5-14, 1984

J. W. Ekin 101

ELECTROMECHANICAL PROPERTIES OF SUPERCONDUCTORS FOR DOE FUSION APPLICATIONS

J. W. Ekin, J. Moreland, and J. C. Brauch

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

This is an interim report presenting data on superconductor performance

under mechanical load, which are needed for the selection of superconductors

and the mechanical design of superconducting magnets for DOE fusion energy

systems. A further aim of the reported research is to measure and understand

the electromechanical properties of promising new superconductor materials

with strong application potential at high magnetic fields. Results include

the following. The first strain vs. critical-current studies were made on a Chevrel-phase superconductor, PbMo^Sg. Chevrel-phase superconductors were

found to have a large strain effect, comparable in magnitude to A-15 superconductors like Nb„Sn. Electromechanical-property measurements of an experimental

liquid-tin-infiltrated Nb^Sn conductor showed it to have an irreversible

strain limit twice as large as bronze-process superconductors and a significantly higher overall critical-current density; the liquid-infiltration

process thus has the potential for development of practical Nb„Sn conductor

with both superior critical-current density and extremely good mechanical

properties. Electromechanical parameters were obtained on several Nb^Sn

conductors that are candidate materials for superconducting fusion magnets,

including conductors fabricated by the bronze, internal-tin, and jelly-roll

processes. Thermal contraction data are reported on several new structural

materials for superconductor sheathing and reinforcement, and a new diagnostic

tool for probing the energy gap of practical superconductors has been developed

using electron tunneling.

Key words: electromechanical; electron tunneling; energy gap; fusion; ICCS

superconductor; liquid-infiltration superconductor; Nb^Sn; PbMo^Sg; stress

effect; transverse stress effect; uniaxial stress effect

IX

UNIAXIAL STRAIN-EFFECT CHARACTERIZATION OF NEW

HIGH-FIELD EXPERIMENTAL SUPERCONDUCTORS

1. "ELECTROMECHANICAL AND METALLURGICAL PROPERTIES OF LIQUID-INFILTRATION

Nb-Ta/Sn MULTIFILAMENTARY SUPERCONDUCTORS" J. W. Ekin and M. Hong,

Appl. Phys. Letters ^5, 297 (1984).

2. "EFFECT OF UNIAXIAL STRAIN ON THE CRITICAL CURRENT AND CRITICAL FIELD

OF CHEVREL PHASE PbMo S SUPERCONDUCTORS" J. W. Ekin, T. Yamashita,

and K. Hamasaki, IEEE Trans. Magn. MAG-21 , 474 (1985).

3.

"EFFECT OF UNIAXIAL STRAIN ON Nb^n WITH Mg ADDITIONS" J. W. Ekin.

1

Appl. Phys. Letters 4^5, 297 (1984)

ELECTROMECHANICAL AND METALLURGICAL PROPERTIES OF LIQUIDINFILTRATION Nb-Ta/Sn MULTI FILAMENTARY SUPERCONDUCTORS

J. W. Ekin

National Bureau of Standards

Boulder, Colorado 80303

and

M„ Hong

AT&T Bell Laboratories

Murray Hill, New Jersey 07974

Abstract

Data are presented on the strain dependence of the critical current and

critical field of Nb-Ta/Sn superconductors fabricated by the liquid Sn infiltration process. The results show that liquid infiltrated Nb-Ta/Sn superconductors have several significant advantages over bronze-process Nb/Sn superconductors: an overall J that is 3 to 10 times higher for magnetic fields in

c the range 13 to 20 T, an irreversible (damage) strain limit twice as large,

and a elastic-strain sensitivity less than half as large at fields above

M6 T. These improved properties are attributed to several unique characteristics of the liquid infiltration process: a tough Nb-Ta matrix, fine equiaxial A15 grains and a uniform stoichiometric Sn concentration.

3
This letter reports results showing that liquid-infiltrated Nb/Sn composites have a damage strain tolerance nearly double that of commercial bronzeprocess Nb^Sn conductors and the highest overall critical-current density

measured thus far for a Nb/Sn-based superconductor at fields above 13 T.

Unlike conventional bronze-process Nb^Sn conductors, liquid-infiltrated

Nb^Sn conductors offer the possibility of simultaneously obtaining both a

desirable metallurgical microstructure of small, equiaxial A15 grains and a 1-3 uniform, stoichiometric Sn concentration across the reacted A15 region. As

a result, high critical temperatures of 17.9 K, determined inductively, with

sharp transition width (<_ 0.3 K) can be obtained along with extremely high

critical current densities J (10 5 A/cm2 at 12 T and 1 0*4 A/cm2 at 19 T, both

c 3

at 4.2 K). The addition of 2 wt.% Ta to the niobium has been found to

increase further the overall critical current to 1.6 x 10 4 A/cm2 at 4.2 K and

* * 4

20 T because of an enhancement of the bulk upper critical field, •

The question remained however of how the mechanical properties of liquidinfiltrated conductors compared with those of conventional bronze-process conductors. From the practical standpoint of conductor development, it is important to optimize not only J^, but also the strain limit t ^ rrev where the conductor

is permanently damaged. Also, the strain sensitivity of the of the conductor

is significant in setting strain limits for the mechanical design of high field

magnets.”* It is the purpose of this letter to report measurements of the

prestrain e^, E

j rrev

and the strain sensitivity of and for a liquidinfiltrated Nb-2wt . na/Sn composite and compare the parameters for this type

of conductor with those of conventional bronze-process Nb^Sn conductors.

Liquid infiltrated Nb-2wt . ZTa/Sn composite wires were fabricated using

laboratory-scale powder-metallurgy processing techniques. First, a Nb-2wt.%Ta

alloyed ingot was made by arc-melting pure components under an argon atmosphere.

4
The starting materials were Nh and Ta with 99. 9 % purity and very low oxygen

content. The arc furnace employed a water-cooled nonconsumable tungsten

electrode and a copper hearth. High-purity argon was passed over heated

zirconium to remove residual oxygen and nitrogen and through a cold trap to

condense moisture before being introduced into the furnace. The samples were

inverted on the copper hearth and remelted four times to completely alloy the

Nb and Ta. No noticeable weight loss of Nb or Ta was observed after melting.

The arc-cast ingot was heat treated in a vacuum furnace at 1250°C for 24 hours

to achieve further homogenization. The Nb-2wt.%Ta alloyed powder was then

obtained from the homogenized ingot using a hydride-dehydride technique. NbTa

powder ('''37-50 pm) was put in a cylindrical rubber mold and isostatically

compacted at 220 MPa to form a porous rod. The sintering process was conducted

at 2250°C for one hour under a vacuum of 1.3 x 10 4 Pa. After sintering, the

porous Nb-Ta rod was immersed in a liquid Sn bath for infiltration.

The infiltrated composite, consisting of a Nb-Ta matrix and a continuous

three-dimensional network of Sn, was clad with monel 400 tubing for wire

fabrication. The composite, 5 mm in diameter, was first swaged and then drawn

down to 0.23 mm in diameter or smaller. No difficulty was encountered and no

intermediate annealing was required during the wire drawing. Laboratory-scale

Nb-Ta/Sn wires more than 20 meters long were obtained. A short sample for

testing vs. strain was prepared by chemically removing the monel and heat

treating over a range of temperature from 750 C to 950 C to form the superconducting A15 phase. Copper stabilizer was later electroplated on the test

sample. Note that all the processing steps are adaptable to industrial

applications, except that the monel cladding would be replaced by a Ta diffusion barrier and copper stabilizer tube. In other tests, starting billets

13 mm in diameter have been drawn down to 0.13 mm in diameter, again with no

5
intermediate annealing required. Therefore, the scale-up of the liquid-infiltration Nb-Ta/Sn or Nb/Sn process is feasible. The overall chemical composition of the wires was (Nb-2wt.%Ta)-8wt.%Sn.

Critical temperatures (T

c

) were measured inductively at a frequency of

100 Hz in an applied field of 4 x 10 5 T. Critical currents were obtained

using an apparatus designed to simultaneously apply tensile strain, current,

and a perpendicular magnetic field to short wire samples at 4.2 K. A detailed

description is given in Reference 6. A critical-current criterion of 2 yV/cm

was used. The overall noncopper critical current density described below was

determined using the combined area of tin, superconductor and unreacted Nb,

excluding only the copper-stabilizer area.

The data were also corrected for the current carried by the stabilizer,

* which can become significant at very high fields as is approached. This

is accomplished by measuring the resistance of the conductor in the normal

state, R , either at fields above the critical field or above the critical

n strain.^ The normal current carried by the conductor is then calculated as

I c E /R , where E is the electric field criterion used to determine the n c n c critical current. I is then subtracted from the measured critical current to n obtain the true critical current of the superconductor. For this sample 1^ was determined to be 21 mA, which amounts to about 4% of I at 23 T, and

c becomes rapidly less significant at lower fields.

Figure 1 shows the strain dependence of the critical current at fields

(B * y H) from 10 to 23 T. Data were obtained by measuring the critical

o current density as a function of magnetic field under constant applied

strain. The strain was then increased, and J

c again obtained as a function of

B. The process was repeated for higher and higher strains until eventually

the superconductor was permanently damaged at the irreversible strain point

6
e . From these data the J vs. e curves of Fig. 1 were obtained. It is

irrev c seen that J passes through a maximum at a strain e of 0.132. This is quite

c in low compared to conventional bronze-process Nb^Sn conductors where is

g typically between 0.2 and 0.4%. The low value of e^ is the result of the

absence of a bronze matrix in liquid-infiltrated conductors; thermal contraction of the bronze matrix material is primarily responsible for the compressive prestrain introduced into the Nb^Sn reaction layer of conventional

bronze-process conductors.

The irreversible strain limit e

^ rrev was determined by measuring the

unloaded at 10 T only. The + symbols in Fig. 1 indicate the critical

current measured after unloading from the correspondingly labeled strain

point. These data indicate the irreversible strain limit for this conductor

is more than 1.1%. This corresponds to an intrinsic irreversible strain limit

c « e. - e of about 0.9%, which is nearly double that of bronze0 , irrev irrev m

process conductors. There e^

£ rrev *

s consistently about 0.5% regardless of

the value of e . Consequently, the bending strain limits on the mechanical

m

design of high-field magnets are also about double those of bronze process

conductors

.

The high irreversible strain limit e ^ rrevcan t> e explained by the unique

metallurgical microstructures created in the reacted Nb-Ta/Sn composite wire.

Prior to the reaction to form the superconducting A15 phase, an extremely high

density of dislocation was observed in the Nb-Ta matrix, which is due to heavy

deformation during the wire drawing process. Reaction at 750-900°C promptly

formed the A15 phase. However, the existing dislocations in the matrix would

not be annealed out since the reaction temperature was very low compared to

the melting point of the Nb-Ta alloy. This was confirmed with high-resolution

transmission electron microscopy as shown in Fig. 2. It is observed that a

7
very high density of dislocations still remains in the Nb-Ta matrix and the

dislocation-free region is the A15 phase with small, equiaxial grains. In

contrast to this, the dislocations in the Cu-Sn matrix of the bronze-process

Nb-Sn wires were completely annealed out during the heat treatment because of

the low melting point of the Cu-Sn. Therefore, after reaction the liquid

infiltrated Nb-Ta/Sn wires still maintain a strong matrix while the conventional bronze-process Nb/Sn wires possess a soft Cu-Sn matrix.

During the reaction, the Nb-Ta dominantly and swiftly diffused into the

liquid Sn because more defects were created in the liquid Sn than in the solid

Nb-Ta. However, a small amount of Sn also quickly spread over the Nb-Ta

matrix via pipe diffusion (because of the high density of dislocations) and by

grain boundary diffusion. These were observed with a scanning transmission

electron microscope coupled with an energy dispersive x-ray spectrometer.

The rapid solid (Nb-Ta) /liquid (Sn) interaction produces fine, equiaxial

A15 grains across the reacted filament. This microstructure results from the

random A15 nucleation sites in the liquid Sn region, which cannot be obtained

from solid-state diffusion that occurs in bronze-process Nb/Sn wire. The A15

filaments, between 0.2 and 1.0 ym, lie parallel to each other about 5 ym

apart. The fine A15 filaments are also in strong contrast to those in the

bronze-process wires which are typically more than 4.0 ym in diameter.

As has been discussed, a tough Nb-Ta matrix was strengthened by the high

density of dislocations along with some Sn alloying. This, coupled with small

A15 grains, fine A15 filaments and interfilament spacing contribute to the

high irreversible strain limit e Jirrev

Figure 3 shows a comparison of the peak (nearly strain-free) critical

current density at e for the liquid infiltrated conductor of this study, a m

typical conventional bronze-process conductor, and a NbSn£ powder conductor.

8
Note that a comparison of peak values eliminates the effect of varying

compressive prestress among different conductors, which can result in misleading comparison of initial (as-received) values.^ Figure 3 shows the

critical current density of the liquid infiltrated conductor to be more than

three times greater than bronze-process conductors at low fields and more than

an order of magnitude greater at high fields. This is the largest overall J

c measured for a Nb/Sn conductor over the field range from 13 T to 20 T. The

overall critical current density is about 1.8 x 10 4 A/cm2 at 20 T, which is

high enough for consideration of Nb/Sn superconducting magnets designed to be

used at or above 20 T. The extremely high overall J

£

at high fields is the

result of the combination of a unique A15 microstructure, as was discussed

earlier in this paper, and a uniform stoichiometric Sn concentration across

the A15 region. The latter phenomenon can be explained by a new Nb-Sn phase

2 3 9 diagram *

’ in which the phase spread of the A15 structure is considerably

narrower than that of the previously accepted phase diagram.^

* Figure 4 shows the bulk average upper-critical field as a function of

* strain. is the field at which the pinning force extrapolates to zero. It

was determined by fitting the measured pinning force curve, F(=J

c B) vs. B, at

a given strain level to a general function suggested by pinning theories: J^B =

A (B/B (1 - B/B _)^. The fit is made in terms of the four parameters B

C Z CZ C /

A, p, and q. Here p and q are independent of strain and had the values of 0.5

6 * and 1.4, respectively. The strain dependence of B

£

for the liquid-infiltrated

conductor is shown in Fig. 4 by open circles. The solid line represents the

A nearly universal strain dependence of B^ for bronze-process binary Nb^Sn. As

'* * seen in Fig. 4, the elastic strain dependence of B

^ for liquid-tin infiltrated

Nb^Sn with 2 wt.% Ta added is nearly identical to that of bronze-process Nb^Sn

conductors

.

9
Although the elastic strain dependence of B . Is nearly the same for the

cZ

two types of conductors, the dependence of on elastic strain is very

different. This arises from two factors.** First, the peak (nearly strain

v * free) bulk upper critical field B

j

fo r the liquid infiltrated Nb-2wt . ZTa/Sn

* conductor was approximately 23.5 T compared with a B 0 of about 21 T for czm bronze-process Nb_Sn. Thus, for a given field, J of the liquid-infiltrated

j c Nb-2wt . ZTa/Sn conductor is less affected by strain because the reduced field

,

. * *

(b * B/B 0 ) of this conductor is smaller. The further B is from B the less

cz cZ

* is affected by the strain-degradation of B Secondly, the pinning force

1 A 2 F of the Nb-Ta/Sn conductor approaches zero as ( 1—b )

’ rather than (1-b) , as

in the case of binary Nb^Sn. The steeper the slope of the F vs. B character-

* 11 istic of a conductor, the more sensitive is J to changes in B Thus, for

c cz both these reasons, the elastic strain sensitivity of J

c in the Nb-2wt . ZTa/Sn

* conductor is less than that of binary Nb^Sn. Further enhancement of B

c 2m

By

additions of Ta slightly greater than 2 wt.% should produce higher values of

and further reductions in the J

c strain sensitivity. 12

In summary, Nb-Ta/Sn conductors fabricated by the liquid Sn-inf iltration

process offer a unique combination: an overall which is 3 to 10 times

higher than conventional bronze-process multif ilamentary Nb/Sn conductors, an

irreversible (damage) strain limit twice as large, and a J

c elastic strain

sensitivity comparable to other Nb^Sn conductors with ternary additions.

Valuable discussions and experimental assistance from J. T. Holthuis of

Lawrence Berkeley Laboratory are gratefully acknowledged. The authors thank

J. C. Brauch for his assistance in reducing the data. The support of part of

this work by the Naval Ship Research and Development Center and the Center for

Electronics and Electrical Engineering, National Bureau of Standards is

gratefully acknowledged.

10
References

1. M. Hong, G. W. Hull, Jr., J. T. Holthuis, W. V. Hassenzahl, and J. M.

Hong, Appl . Phvs . Lett . 42:621 (1983).

2. M. Hong, G. W. Hull, Jr., J. T. Holthuis, W. V. Hassenzahl, and J. W.

Ekin, IEEE Trans . Magn . MAG-19:912 (1983).

3. M. Hong, F. Heilman, D. H. Maher, G. W. Hull, Jr., and T. H. Geballe

(unpublished)

.

4. M. Hong, G. W. Hull, Jr., E. 0. Fuchs, and J. T. Holthuis, Materials

Lett . 2(2) (1983).

5. J. W. Ekin, Ch. 7 in: Superconductor Materials Science , S. Foner and B.

Schwartz, eds., Plenum Press, New York (1981) pp. 455-509.

6. J. W. Ekin, Cryogenics 20:611 (1980).

7. J. W. Ekin, "Strain Effects in Superconducting Compounds - An Overview

and Synthesis," Adv . Cryog . Eng . 30:823 (1984).

8. J. W. Ekin, IEEE Trans . Magn . MAG-15: 197 (1979).

9. R. A. Shiffman and D. M. Bailey, High Temperature Science 15:165 (1982).

10. J. P. Charlesworth, I. Macphail, and P. E. Madsen, J_. Mat . Sci . 5:580

(1970).

11. J. W. Ekin, H. Sekine, and K. Tachikawa, J_. Appl . Phys . 52:6252 (1981).

12. See Figure 6 in D. 0. Welch, Adv . Cryog . Eng . 30:671 (1984).

11
j

)

!

(

,
Non-Copper

J

c

(A

An

2)

Fig. 1. Effect of uniaxial tensile strain on the critical current of a liquid- it : i Nb-2wt . %Ta/Sn superconductor at magnetic fields from 10 to 23 T. The pa ram- f. • cates the strain at which the critical current is a maximum; ini i <’

Die strain limit where the superconductor is permanently damaged.

1 3
Fig.

2.

Typical

transmission

electron

micrograph

showing

fine

equiaxed

A15

grains

embedded

in

the

Nb-Ta

matrix

filled

with

a

high

density

of

dislocations.

The

mark

in

the

figure

is

0.1

pirn

long.
Fig. 3. Comparison of peak critical-current densities measured at for

several Nb/Sn based superconductors: triangle - liquid Sn infiltrated,

wt.%Ta added to Nb ; square - NbSn 0 powder process; circle - modified jelly

roll process, 1.5 at.%Ti added to“Nb; inverted triangle - 13wt%Sn bronze

process

.

15
16

Fig.

4.

Comparison

of

the

relative

effect

of

uniaxial

strain

on

the

upper

critical

field

of

liquid-infiltrated

Nb-Ta/Sn

and

commercial

bronze-process

Nb^Sn

superconductors.

The

maximum

value

of

the

upper

critical

field,

B*

is

23.5

T

for

the

Nb-Ta/Sn

conductor

compared

with

approximately

21T
IEEE Trans. Magn. MAG-21 , 474 (1985)

EFFECT OF UNIAXIAL STRAIN ON THE CRITICAL CURRENT AND

CRITICAL FIELD OF CHEVREL PHASE PbMo^S c SUPERCONDUCTORS*

o o

J. W. Ekin

National Bureau of Standards

Boulder, Colorado 80303

and

T. Yamashita and K. Hamasaki

Technical University of Nagaoka

Nagaoka, Japan

Abstract

The first measurements of the effect of uniaxial strain on the critical

current of a Chevrel phase superconductor, PbMo,S 0 , have been obtained at 4.2

D O K in magnetic fields from 2 T to 24 T. The data show there is a very

significant reversible effect of elastic strain on the critical current of

PbMo,S , comparable in magnitude to that observed in Nb„Sn. This is because

both the peak pinning force and upper critical field are very sensitive to

elastic strain. A correlation is made between the elastic strain effect,

radiation sensitivity, and phonon anharmonicity in the A15, BI, C15, and

Chevrel phase superconductors.

I ntroduction

All A15 superconductors tested thus far show a significant reversible

effect of uniaxial strain on their critical current, 1 10 critical field, 4 ’ 11

and critical temperature. 1 ^’ 1 ^ All Bl and C15 superconductors tested thus far

show no measurable strain effect. 14 ^ What about Chevrel phase superconductors? No electromechanical measurements on the critical parameters of these

superconductors have been reported. The objective of this study was to have a

first look at the effect of uniaxial strain on the critical current and

critical field of a Chevrel phase superconductor, PbMo,S_. Critical current

b 8

17
versus strain measurements were made over a wide range of magnetic field from

2 T to 24 T. This was done in order to study the effect of strain on both the

peak pinning force (which dominates the character of the critical current at

low fields) and the upper critical field (which dominates at high fields).

Sample Preparation

Tests were conducted on both wire and tape samples, each made by an

electroplating technique. 17 ’ 18 For the wire sample, a Mo wire of 0.3 mm

diameter was electroplated with lead. The plating time was adjusted to

produce a layer of lead approximately 0.5 mm thick on the surface of the Mo

wire. The wire was then sealed in an evacuated quartz tube with crushed MoS^

powder. The quartz tube was heated to about 1000°C, and held there for 6 to

14 hours. After heat treatment, it was slowly cooled to room temperature in

about 5 hours. The final sample was a wire with a Mo core of about 220 ym

diameter and a Pb-Mo-S layer about 110 ym thick. The sample was then coated

with additional lead to stabilize the superconductor for critical-current

testing.

^Contribution of NBS , not subject to copyright.

Manuscript received September 10, 1984

18
The PbMo,S 0 tape sample was made by electroplating a Mo tape, 1 . 5 mm vT ide

and 50 pm thick, with lead. The tape was then heat treated in MoS 0 vapor for

14 hours at about 1000°C. The final sample was a tape with a Mo core less

than 10 pm thick, surrounded by a PbMo,S 0 layer about 125 pm thick. The tape

D O was then coated with additional lead for stabilization.

Data were obtained using a high-field uniaxial-strain apparatus described

previously. 6 A 2 pV/cm electric-field criterion was used in determining the

critical-current values presented below. Accuracy of the critical current

measurements is about ±1%. The strain was measured to within ±0.02%.

Results

Effect of Uniaxial Strain on Critical Current

Figure 1 shows the effect of uniaxial strain e on the critical current I

of the PbMo,S 0 wire sample at magnetic fields from 2 T to 19.5 T. The strain

D o effect was reversible for strains less than 0.2%; that is, below 0.2% the

critical current recovered to its former value upon removing the uniaxial

force on the superconductor. Beyond 0.2% strain, however, the critical

current did not completely recover when the force was removed, indicating the

onset of permanent damage.

Up to this irreversible strain limit the critical current of PbMo,S 0 showed

o o a dramatic dependence on elastic strain (note that the ordinate is a logarithmic scale). This reversible strain effect is comparable in size to that seen

in Nb^Sn. Figure 2 shows the elastic strain effect in a typical multifilamentary Nb^Sn conductor strand 6 for comparison. The large percentage change in

the critical current of both types of superconductors is noted. The reversible degradation of the critical current of PbMo,S 0 at strains less than 0.2’'

u o (the irreversible strain limit) amounts to about a 20% change in I at low

fields (2 T) , 40% at high fields (18 T)

.

19
A major difference between the two types of superconductors is the scaling

of the strain effect with magnetic field. In Nb^Sn the sensitivity of the

critical current to uniaxial strain at low fields is relatively small, but

increases significantly as the field is increased. In PbMo,S , on the other

b o hand, the strain sensitivity of the critical current is relatively great at

both low and high fields. This is due to a combination of the very high upper

critical field in PbMo^Sg and the relatively high strain sensitivity of the

pinning force in PbMo,S , as described in more detail in the Discussion and

D O Summary section.

Another difference between Figs. 1 and 2 is the absence of any peak in the

J versus e curves for PbMo,S„. This results from the inverse differential

c 6 8 thermal contraction between the superconducting material and matrix in PbMo,S 0 6 o compared with Nb^Sn. In bronze-process Nb^Sn conductors, the bronze matrix

contacts more than the Nb^Sn, placing the superconductor in compression when

cooled from the reaction temperature of 700°C. Mo, however, contracts about

three times less than the PbMo,S 0 on cooling from the reaction temperature of

6 o about 1000°C. 19 In the PbMo^Sg conductors studied here, this places the

PbMOgSg portion of the conductor under axial tension (and radially into

compression, probably enhancing the shear strain effect). In the wire PbMo^Sg

sample, the effect of the Mo is relatively large because the volume ratio of

PbMo^Sg-to-Mo in this sample is only 1.8.

Consequently a tape PbMo^Sg sample was tested, because with the tape

geometry a ^50% higher volume ratio of PbMo,S -to-Mo could be obtained and the

b o amount of axial pretension reduced. Figure 3 shows the I versus e data

obtained in a PbMo-S_ tape sample with a PbMo,S n - to-Mo ratio of 2.9. The

6 8 6 8

effect of uniaxial strain on the critical current of the tape sample was

reversible up to the fracture strain of ^0.4%. The tensile prestrain in this

20
sample is smaller, near zero ( <0 . 05%) , as evidenced by the nearly zero slope

of the I versus e curves at zero applied strain. Figure 3 can be compared

c directly with the data for Nb^Sn in Fig. 2 by considering only the tensile

side of the Nb^Sn data (that is, the unshaded region in Fig. 2).

The tensile prestrain initially present in the wire PbMo^Sg sample was

determined to be about +0.2% from a correlation of the I versus e data in

c

Fig. 3 with those in Fig. 1. This prestrain value is also consistent with an

approximate analysis of the differential thermal contraction rates and volume

fractions of PbMo,S n and Mo in each conductor. Thus the data on the wire

6 8

PhMo,S 0 sample in Fig. 1 should be compared with the Nb„Sn data at strains

0 O -3 above 0.5% in Fig. 2. Also note that the initial I values for the PbMo.,S 0

wire sample are not optimum strain-free values, but are degraded by the

tensile prestrain about 10% at low fields, 15% at high fields.

Effect of Uniaxial Strain on B . c.2 The strain dependence r of the upper critical field B „ (B = y H) of PbMo,S 0 c2 o 6 8 was determined by fitting the I versus B curve at constant strain to a

general function suggested by pinning theory: J^B - K(B/B*

2m

) ^ ( 1-B/B* 0

) ^

.

The fit is made in terms of the four parameters K, p, and q. Here K is a

strain-dependent parameter proportional to the peak pinning force, and p and

q are parameters independent of strain that were measured to have the values

0.14 and 1.8 respectively. The asterisk on B* indicates a bulk average

upper-critical field.

Figure 4 shows the results obtained for B*^ as a function of strain. The

results for the wire and tape PbMo^Sg samples have been combined assuming

e =0% for the tape conductor and e =0.2% tension for the wire conductor, n m

The maximum strain-free value of B* at 4.2 K was B* = 36 T. The results

c 2 c 2m

21
shown in Fig. 4 indicate that the relative degradation of B* for PbMo,S„ is

c2 6 8

comparable or slightly greater than in Nb^Sn. For Nb^Sn under tension, d(ln

B* )/d((?e@ *^) = 1250,6 whereas for PbMo,S 0 , the preliminarv value obtained

cz bo

from Fig. 4 is d(ln B* )/d(@e@ ’^) = 1600. Thus the effect of strain on the

c z upper critical field of PbMOgSg is relatively large, comparable to, or even

slightly greater than for Nb^Sn.

Effect of Strain on Peak Pinning Force

The relative strain dependence of the peak pinning force of PbMo,S c is

6 8

contained in the parameter "K" described in the preceding section. For the

A15 superconductors, the relative strain sensitivity of the peak pinning force

is nearly identical with that of the strain sensitivity of B* . That is, c2

K(e) a B* (e). However, for PbMo,S 0 , K(e) a [B* (e)] n

, where n was measured

cZ b 8 cz

to be in the range from 2 to 3.

Discussion and Summary

At low magnetic fields (less than about half B*^) the character of the

critical current is dominated by the peak pinning force. 6 Thus, in comparing

Figs. 1-3, the strain sensitivity of PbMogSg is greater than that of NbgSn

mainly because of the very large effect of strain on the peak pinning force of

PbMo,S relative to Nb_Sn.

b 8 3 At high fields, in contrast, the behavior of the critical current is

dominated bvy B* .6 Since B* is much larger in PbMo,S 0 than in Nb„Sn, its

c2 c2 68 3

effect on the critical current of PbMo,S 0 is smaller, even though the strain

6 8

sensitivity of B*^ in PbMogSg is comparable to that of NbgSn. For example,

the effect of strain on the critical current of NbgSn at 19 T in Fig. 2 is

greater than in PbMo,S 0 because these data were obtained very close to the

o o

22
B* of Nb-Sn (21 T) . In PbMo,S , however, the maximum field where data was c_m 3 oo

obtained (24 T) is still only about 66% of B* (36 T) . Thus, because of the

cZm

high value of B* in PbMo,S n , the relative effect of strain on the critical

c2m 6 8

current of PbMo,S 0 is smaller than in Nb„Sn at fields above 3 14 T. OO J The large reversible effect of strain on the critical current of PbMo^Sg

probably accounts partially for the unexpectedly low critical-current

densities reported in several earlier studies. For example, in their comprehensive grain-size study of PbMo,S_ Seeber et al.19 noted that the observed

o o critical-current density was lower than they had expected based on a

grain-boundary pinning model. They speculated this was due to cracks and

deformation. This is probably the main reason, but our results show that the

low critical-current density may be due also, at least in part, to reversible

degradation of the critical current caused by differential thermal contraction

between PbMo,S 0 and the Mo matrix material in their samples. The volume ratio

n 8

of PbMo,S c to Mo in their wire samples was about 0.3, considerablv less than bo

the ratios of 3 and 28 for the samples measured in this study. This would

have resulted in more tensile prestrain in the PbMo^Sg layer of their samples

and an accompanying large I degradation.

In summary, these data show that uniaxial strain has a significant effect

on I and B* of PbMo,S„. The effect is reversible, up to an intrinsic

c c2 6 8

irreversible strain limit of 3 0.4% for these samples. At low magnetic fields

the strain effect on the critical current is greater than in Nb^Sn and results

from a peak pinning force which is more sensitive to strain than in Nb^Sn. At

high fields the strain effect on the critical current results from an upper

critical field which has a strain sensitivity comparable to that of Nb^Sn.

However, the effect on the critical current of PbMo,S_ is less than in Nb.,Sn

6 8 3 at fields above 3 14 T because the value of B* for PbMo,S„ is much greater c2m 6 8

than for Nb^Sn.

23
Generalizing from the fact that every A15 superconductor tested so far

exhibits a significant strain effect, 10 it is anticipated that the elastic

strain sensitivity of the critical parameters of PbMo,S c will extend to the

D O entire class of Chevrel phase superconductors. Unlike the B1 and C15

superconductors, both A15 and Chevrel phase superconductors have significant

phonon anharmonicity at liquid helium temperatures, which could account for

the extreme elastic strain sensitivity of their superconducting properties . 10

The same correlation with crystal phase also occurs for radiation damage.

The critical current of both A15 and Chevrel phase superconductors is much

more susceptible to radiation damage than the B1 and C15 superconductors.

Acknowledgments

This work was sponsored by the Department of Energy, Office of Fusion

Energy under interagency agreement #DE-AI0 1-84ER52 1 1 3 . The high magnetic

fields needed for this study were obtained using the National Magnet Laboratory magnet facilities. The assistance of J. C. Brauch in reducing the data

is gratefully acknowledged.

References

1 . 2.

3.

J. W. Ekin, Appl. Phys . Lett. 29, 216 (1976).

D. S. Easton, R. E. Schwall, Appl. Phys. Lett. 29, 319 (1976).

J. L. McDougall, Proc. of the Int'l. Cryogenic Eng. Conf . , IPC Science

4.

5.

and Technology Press, 6, 396 (1976).

J. W. Ekin, IEEE TransTMagn. MAG-15, 197 (1979).

R. Roberge, S. Foner, E. J. McNiff, Jr., B. B. Schwartz, and J . L. Fihey,

6.

7.

IEEE Trans. Magn. MAG-13, 687 (1977).

J. W. Ekin, Cryogenics 20, 611 (1980).

D. U. Gubser, T. L. Francavilla, D. G. Howe, and L. D. Jones, Appl. Phys.

8.

9.

Lett. 3J_, 230 ( 1977).

J. W. Ekin, IEEE Trans. Magn. MAG-17, 658 (1981).

J. W. Ekin and A. I. Braginski, IEEE Trans. Magn. MAG-15, 509 (1980) . 10.

11.

12.

J. W. Ekin, Adv. Cryog. Eng. 30, 823 (1984).

G. Rupp, IEEE Trans. Magn. MAG-15, 189 (1979).

T. Luhman and M. Suenaga, IEEE Trans. Magn. MAG-13, 800 (1977)

24
13 . H. Hillman, H. Kuckuck, H. Pfister, G. Rupp, E. Springer, M. Wilhelm, K.

Wohlleben, and G. Ziegler, IEEE Trans. Magn . MAG- 13 , 792 (1977). G.

Rupp, IEEE Trans. Magn. MAG- 1

3

, 1565 (1977).

14. J. W. Ekin, J. R. Gavaler, and J. Greggi, Appl. Phvs . Lett. 41 , 996

(1982).

15. K. Inoue, H. Wada, T. Kuroda, and K. Tachikawa, Appl. Phys . Lett. 38, 939

(1981).

16. H. Wada, K. Inoue, K. Tachikawa, and J. W. Ekin, Appl. Phys. Lett. 40 , 844 (1982).

17. K. Hamasaki, T. Yamashita, T.Komata, K. Noto, and K. Watanabe, Adv.

Cryog. Eng. 2UD, 715 (1984).

18. K. Hamasaki, K. Hirata, T. Yamashita, T. Komata, K. Noto, and K.

Watanabe, to be published.

19. B. Seeber, C. Rossel, 0. Fischer, and W. Glaetyle, IEEE Trans. Magn.

MAG-19, 402 (1983).

25
T T T

'irrev

Fig. 1. Uniaxial-strain dependence of the critical current of a PbMo^Sg wire

superconductor. The prestrain introduced by the Mo matrix is tensile, resulting in no maximum in the I -versus-e curve. e. is the uniaxial strain

c irrev where the the superconductor is permanently damaged, indicated by the I

- versus-e curve being irreversible upon unloading. (Short-dashed curve and crosses + show the critical current after unloading.)

26
Fig. 2. Uniaxial—strain dependence of the critical current of a typical

bronze-process Nb^Sn superconductor. Data of Fig. 1 should be compared with

these data at strains above 3 0.5%. Data of Fig. 3 should be compared with

these data at strains beyond the maximum at 0.29% strain.

27
Fig. 3. Uniaxial-strain dependence of the critical current of a PbMo S tape

superconductor. The tensile prestrain introduced by the Mo matrix is small

because of the greater PbMogSg-to-Mo ratio in the sample compared with the

PbMOgSg wire sample of Fig. 1. The critical current versus strain curve was reversible up to the fracture strain.

28
Fig. 4. Uniaxial strain dependence of the upper

critical field B*^ for PbMo^Sg. The maximum (nearlystrain-free) value of the critical field, B*^ , at 4.2 K was approximately 36 T. Values of

B* /B* for both the wire and tape samples have

,c 2 c 2m, been combined m this plot assuming no prestrain

for the tape sample and a tensile prestrain of about 0.2% for the wire sample.

(Circles indicate the tape data, squares the wire data.)

29
EFFECT OF UNIAXIAL STRAIN ON Nb^n WITH Mg ADDITIONS

J. W. Ekin

This experimental conductor was made at Lawrence Berkeley Laboratory and

is described in a paper by J. W. Wi et^ al_, Appl. Phys . Lett. V5, 792 (1984).

It contained 0.35 at % Mg in the bronze matrix prior to reaction. The test

apparatus for obtaining these data and the procedure have already been described

in Cryogenics 20_, 614 (1980). A 2 yV/cm electric field criterion was used in

determining the critical current.

The results are shown in Figs. 1 and 2 and listed in the Table. The main

conclusions regarding the electromechanical properties of this conductor

include the following:

1. The conductor had a very high strain emewhere J is maximum, almost

0.8% (see Fig. 1). This resulted in a very low initial J

c value at high

fields compared with the peak J of the conductor. For example, the initial

was depressed by a factor of 50 compared with the measured peak J at 18

T. This will be important in making comparisons with other conductors.

2. The intrinsic irreversible strain limit e . for this conductor

o , irrev

(strain beyond the maximum where permanent damage occurs) was about 0.3%.

This was relatively low compared with commercial multifilamentary Nb^Sn which

has an intrinsic irreversible strain that is consistently about 0.5%. This is

probably a result of the conductor geometry and very large compressive prestrain.

A micrographic study of Kirkendall void formation in the conductor would

probably be helpful in clarifying this.

3. The reaction-layer J at 10 T (not the overall J ) was measured to cm cm

be 2.8 GA/m2

.

4. The decrease in J with field is more rapid than for Nb„Sn wires

c 3 having other additives such as Ti or Ta. The peak J vs. B characteristic

30
for this conductor (see Fie. 2) indicates that the fall-off in J with field

is very similar to binary Nb^Sn.

The results suggest that not much Mg was incorporated into the Nb^Sn

after reaction, which would account for the electromechanical properties being

essentially the same as for binary Nb^Sn (except that the prestrain is greater)

An x-ray spectrographic analysis of the Mg content in the Nb^Sn reaction layer

was consequently performed. The analysis was done by R. Sabotini at

Brookhaven National Laboratory and showed the Mg actually incorporated into

the Nb^Sn compound was less than 0.1 at %. Thus, the amount was probably too

small to significantly alter the properties of binary Nb^Sn. It is suggested

that a further test of Mg additions to Nb^Sn be made on a sample with a higher

Mg content in the Nb^Sn compound.

31
100

Fig. 1 Uniaxial strain dependence of the critical current. The cross indicates the critical current measured after unloading from the correspondingly-labeled loaded (open-symbol) data point.

32
J

(A/m

Nb3Sn+Mg LBL

S.C. Area=l. 160E~08m2

,

Fig. 2 Peak critical current density versus magnetic field measured

at £ . m

33
Table 1

e.i l t leal current of Nb^Sn with Mg additive as a function of strain at magnetic fields from 6 T to 18 T. Jc is the critical current density referred to the area of superconductor

\ax ‘*' S t ^ie stra:*- n (i- n percent) where the critical current is maximum, and E is the electric field criterion used to determine the critical current. c

Superconductor ares =*

F

< X )

O . 00

0 . 05

o _

i F

0 .

2

7

0 . 43

0.52

G0E-000 y. ** 0 . 70 Ec * ? 001 ilJ/rm

m o Ic Pie 1 d Jc Jr R I C / I C !*!

< %

)

( Arnn a 3 }

( T a 5 la) < FA/m'v 2? <6N/w A 3>

7 R q~ FR7 4 OOO 8. OFF 77 77(7 0 0000

77 . 902 5 . 000 0 . 276 31 . 360 0 . 0000

56

.

593 F 000 4 . 979 29. 272 0 . 7267

33

.

602 0 . 000 2 . 897 23

.

1 74 0 . 6932

1 7

.

685 1 0 . 000 1. 525 1 5

.

245 0 . 5737

7 46

!

1 2 . 000 0

.

643 7 . 7 1 8 0 . 422

1

2

.

293 1 4 . 000 0. 1 98 n 769 0 . 2513

0. 474 1 6 . 000 0

.

041 0 . 554 0 . 1 1 05

o t 4 Q 1 7 000 (71 <717 £7 7 1 R 0 000(7

0 (7147 1 R (7l 00 (7 0(74 (7 OFF 0 07OP

73 56

.

707 F . 000 5 . 05 1 30

.

355 0 . 7F39

35

.

036 0 . 000 7 . 020 24

.

162 0 . 7123

1 9. 1 07 1 0 000 1 . 647 1 6

.

472 0. 5 1 98

R 7F1 1 2 000 (7 77(7 R F7P 0 477F

? 664 1 4 000 0. 230 3 . 2 1 5 0 . 291 9 0 . 590 1 6 . 000 0 . 05 1 0 . 6 1 4 0 . 1 374

0 . 057 1 R . 000 0. 005 0 . 099 0 . 04 1 9

F4 5 1 . 545 0 . 000 5 _ 306 31 . 634 0 . 7 903

77 FEF R (71071 3 . 238 7F P07 0 . 7 53 7

7 1 477 1 (71 000 1 R47 1 R 4FR 0 RQ7R

9 . 91 1 1 2 . 000 0 . 654 10. 253 0 . 5507

3 . 479 1 4 . 000 0 . 295 4. 1 5 1 0 . 3 769

0

.

04 R 16 . 000 0 . 0 7 3 1 . 159 0 .

’ 9 7 3

0

.

096 1 8 . 000 0 . 008 0 . 1 4R 0 . 070F

FJ F 4 7R7 F 000 5 . 585 33

.

506 0 . 83i p 40 Q70 R (7l(7l/7l •7 F7R 7 P 7 "77 0 . Q -77

7 ~ R7F 1 171 OOP n (7IFR 7(7 FR7 0 . •7 79

1 1 R(7lF 1 7 0(71 (7| 1 (7 1 R 1 7 7 1 4 0 FF7R

FFP 1 4 000 0 FP4 F FI 4 (7 F007

1 247 1 F 000 (7 1 (71

R

1 770 0 7P07

(71 1 7F 1 R 000 (7 0 1 F £7 ? 77 0. 1 7 R R

7F FP fi">0 F 00(71 F £71 P 7R 1 1 4 0. RPFF

705

(711 4

R 000 7 RF4 7(7 R~ 1 0 POPP

77 1 (71 <7l n 329 n 77 288 0 R74P

1 -1 . 623 1 2 .000 1 . 26

1

5

. 1 27 0 . 82 7 3

F . 400 1 4 . 000 0

.

559 7 . 820 0

.

7100

135 16 000 0. 1 64 2 . 944 0

.

4959

(71 7QF 1 R 000 0 (774 (7 F 1 7 0 7P07

1 F 7F 904

659

F . 000

. 000

F E77 7P R74 0. PRR7

49 . 0 4 . 252 34 . 254 1 . 0098

7 1 F7F 1 (71 000 n 777 77 7 ? 1 1 . 0225.

34
Table 1 continued

Superconductor c 1.tE0E~008 £*>*>. 3,1 0.78 Ec = 2.00LiU/c?^

E Ec Ic F i » 1

rl J r JcB I C / I C !*!

< X ) < % )

( Amn & r & <=, ) •r - • — - • ( T«1 a 1 (RA/m 1'? 1 ( (AW /!*)"* 1

P_ F7 - _LE 1 8 .22 1 12 . 000 1 . 571 1 9

.

050 1 . 0306

g .216 ^ 4 . 000 0. 794 1 1 . 123 1 . 0099

3 . 896 1 6 . 000 0 . 336 5

.

373 0

.

906 9

i .020 1 8 . 000 0 . 068 1 . 593 0

.

7506

0 90 0.12 77 . 674 F . 000 F 71 ~ AP 70P 1 AO 1 OF. 0 PPP A . 7A0 33

.

077 1 pppp

30 . 67S 10 000 n 662 25

.

5 '

9

1 pppp

1 7 F7F 1 2 . 000 1

.

52^ 1 6

.

285 1 Pi Pi q . 1 26 1 4 .000 0

.

787 1 1 . 0 1 4 1 . 0000

4 7 qp 1 F PPP pi 77P F P7F 1 pppp

i 7FP 1 R PiP: ,7! pi 1 1 7 n 109 1 . pppp

1 .

i 7 0 . 39 8 1 .237 R . 000 5

.

270 3 1 . 57F 0. 7954

37 . 88 1 0 . 000 3 _ 266 25

.

1 25 0

.

7702

77 c? q 1 0 . 000 1

.

942 1 9

.

422 0

.

77 qc

1 1 . 123 1 7 000 0

.

959 1 1 507 0. 5293

A . 296 1 A 17! Pi Pi p 77P F 1 RF 0

.

A 70,7

0 . 1 70 1 0 PI PIP! p Pi F 0. 2*7F p 1 ~po

0 . 7? - .01 72 . 295 6 . 000 F . 7

3"' 37 . 389 0

.

g

n p n

35
HIGH-FIELD UNIAXIAL STRAIN-EFFECT CHARACTERIZATION OF

CANDIDATE Nt^Sn SUPERCONDUCTORS FOR FUSION APPLICATIONS

J. W. EKIN

The electromechanical properties of several candidate Nb^Sn superconductors

for fusion application are presented in this section. Two are under development

the internal-tin and jelly roll processes. The third is the bronze process

conductor used in the Nb^Sn magnet for the Large Coil Project.

A. Internal-Tin Nb^Sn Conductor

High-field critical-current versus strain results were obtained for two

internal-tin conductors manufactured by Intermagnetics General Corp.* One

contained 19 subelements and had 2 yin diameter Nb filaments, the other had 61

subelements and 1 ym Nb filaments.

Characteristics of these two samples are as follows. The overall diameter

of each conductor was 0.68 mm. The volume fraction of each component before

reaction was 14% Nb , 8% Sn, 23% Cu, 7% Nb diffusion barrier, and 48% Cu

stabilizer. The reaction heat treatment was made by M. Suenaga at Brookhaven

National Lab and consisted of:

375° C for 48 h

580° C for 200 h

700° C for 48 h

The apparatus used to measure the strain effect has been described in

Cryogenics 1Q_, 614 (1980). The critical current was defined by a 2 yV/cm

criterion.

*Certain commercial materials are identified to specify the experimental study

adequately. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the

material is necessarily the best available for the purpose.

36
The critical currents were corrected for the small current carried by the

stabilizing matrix, which can become relatively significant at high fields

where the critical current 1^ approaches zero. For these data the correction

was not particularly large, however. For the 61 subelement conductor it

ranged from 72 mA at a field of 18 T to 96 mA at 16 T. (The variation with

magnetic field occurs because of the magnetoresistance of the Cu stabilizing

matrix.) For the 19 subelement conductor, the matrix-current correction

ranged from 54 mA at 20 T to 64 mA at 18 T.

There was also a very large current-transfer voltage correction that had

to be applied in determining I for these data. The correction becomes

especially important at low fields where the critical currents are high. The

current-transfer voltages in this sample were much larger than those of other

samples we have tested, probably because of the relatively high matrix resistivity. The magnitude of the effect in the tests on the 19 subelement conductor was not realized at first in conducting these tests and consequently the

V-I curves were not measured to a high enough voltage to allow the correction

to be made. The correction is nonlinear and impossible to make without

measuring I over a very wide voltage range. Consequently, accurate data for

the 19 subelement sample were obtained only at high fields where the correction

is negligible. For the 61 subelement sample, however, extended voltage data

were obtained and current-transfer corrections were made accurately over the

whole field range.

Critical-current versus strain data are shown for the 61 subelement

conductor in Figs. 1, 2, and Table I. Fig. 3 shows the verv-high-f ield data

for the 19 subelement conductor. The principal results are summarized below:

1. The compressive prestrain e was measured to be 0.39% for the 61 m

subelement conductor. It did not change significantly for the 19 subelement

37
conductor where it was measured to be 0.41%. This is a relatively large

prestrain and will result in significant degradation of the J of the conductor

c at high fields if compared with other conductors without first correcting J

to the peak value. The difference between the initial and peak values of

at 16 T, for example, amounts to a 63% decrease from the peak value. At 18 T

the decrease is 94%.

2. The irreversible strain e. was measured to be 1.25% in the 19 irrev

subelement conductor. For the 61 subelement conductor e J was at least irrev

1.2%. (Only a lower limit could be determined for this particular run because

there was not time in the magnet shift to obtain data at higher strains.) It

is expected that was at least 1.25% for this conductor as well. This irrev

is a very high irreversible strain, corresponding to an intrinsic irreversible

strain e . of about 0.9%. For comparison, typical values of e . are

o, irrev o, irrev

invariably about 0.5% in bronze-process Nb^Sn conductors (see Sec. C below).

3. The elongation at fracture was measured to be 1.3% in the 19 subelement conductor.

4. was nearly identical for the two conductors at high fields (where

the data overlapped)

.

5. The peak overall was measured to be about 680 A/mm2 at 12 T, using

a 2 yV/cm criterion. This is significantly higher than for bronze-process

Nb^Sn conductors. The improvement is even greater at 10 T. However, at

fields above 12 T, is degraded from that of bronze-process Nb^Sn conductors.

6. H* was measured to be about 2 T lower than for bronze-process Nb_Sn

cz j

(without additions) . This probably accounts for the lower at fields above

12 T noted in item 5.

38
Fig. 1 Uniaxial strain dependence of the critical current. Crosses

indicate the critical current measured after unloading from the

correspondingly-labeled loaded (open-symbol) data point.

39
NON-COPPER

J

(A/

Nb3Sn Internal Sn; 61 subel.

Area=l. 890E-07m 2

, E max=0. 39%

l ^

0

OJ

8 10 12 14 16

FIELD (T)

18 20 22

Fig. 2. Peak critical-current density measured at e as a function of

magnetic field. m

40
Non-Copper

Critical

Current

Density

(A/cm 2)

Fig. 3 Uniaxial strain dependence of the critical current at const. mt

magnetic fields.

41
Table I

Hon Cu

Critical current an internal-tin Nb^Sn superconductor with 61 subelements

as a function of strain at magnetic fields from 12 T to 18 T. J

c is the

critical current density referred to the non-copper area of the conductor,

E is the strain (in percent) where the critical current is maximum,

an§

X

E is the critical-current electric-field criterion,

c area = 1 . 890E --007 Emax = 0.39 Ec = 2 . 00uU/c m Efract = 1 .220

E Eo Ic Field Jc JcB Ic / Icm

( % ) <% ) ( Amperes ) ( Tesla ) < SfVm''2 ) ( GN / m"3 )

0.04 -.35 73.704 12.000 0.390 4.686 0.7137

28.114 14,000 0,149 2.090 0.6168

7.01 1 16.000 0,038 0,602 0.4799

1 .020 18.000 0 . 006 0,104 0.3286

0 . 13 -.26 87.536 12.000 0.464 5.554 0.8474

35.059 14,000 0.186 2.604 0.7687

9.614 1 6 , 000 0,051 0.822 0.6557

1.664 18.000 0.009 0.165 0.5229

0.31 -.09 103,310 12.000 0,547 6.566 1 . 0000

45.639 1 4 . 030 0.242 3.388 1 .0000

14.713 16.000 0.078 1 .254 1 . 0000

3.248 18,000 0.018 0.316 1 .0000

0.49 0.10 97,669 12.000 0.517 6.207 0. 9454

43.540 14.000 0.231 3.232 0.9541

13.906 16.000 0.074 1.185 0.9455

3.028 18.000 0.016 0.295 0.9337

0.60 0.21 83,547 12.000 0.443 5.31 1 0.8089

35.309 14.000 0.187 2.623 0.7741

10.101 16.000 0.054 0.863 0.6886

1 .662 18.000 0.009 0.165 0.5221

0.31 -.08 102.180 1 2 . 000 0.541 6.494 0.9891

0.74 0.35 60.565 12.000 0.321 3.852 0.5866

18.157 14.000 0.097 1 .352 0.3991

3.769 16.000 0.020 0.327 0.2610

0.40 0.01 104,250 12.000 0.552 6.625 1 .0091

0.83 0.44 43.023 12.000 0.228 2.738 0.4170

10.914 14.000 0.058 0.816 0.2407

1 .803 16.000 0.010 0.161 0. 1282

0.47 0.08 1 00.380 12.000 0.532 6.380 0.9717

0.94 0.55 28.058 12.000 0.149 1 .788 0.2723

5.747 14.000 0.031 0.433 0. 1278

0.53 0.14 92.536 12.000 0.490 5.881 0.8958

1 .02 0.63 20.060 12.000 0.107 1 .280 0.1949

0.59 0.20 85.102 12.000 0.451 5.409 0.8239

42
Table I continued.

Nb35n Internal Sn;61 subel

.

BNL 11-28-84 In=var

area * 1 . 890E -007 Emax = 0 .39 Ec = 2,00uU/cm Efract = 1 .220

E Eo Ic Field Jc JcB Ic/Icm

( % ) ( % ) ( Anperes ) (Tesla) ( GPi/n^Z ) ( GN/n A 3 )

1 .09 0.70 13.867 12.000 0.073 0.880 0.1342

0.63 0.24 76.261 12.000 0.404 4.842 0.7382

1.17 0.78 8.590 12.000 0.045 0.545 0.0831

0.69 0.30 67.564 12.000 0.357 4.290 0.6540

43
B . Jelly-Roll Nb^Sn with 1.1 at% Ti additions

High-field I^-strain results were also obtained for a jelly-roll, internaltin conductor #CRE1031 containing about 1.1 at% Ti in the Nb^Sn compound. The

wire was manufactured by Teledyne Wah Chang Corp.* and was heat treated for

100 h at 680° C using the "cold tails" technique to contain the tin. The wire

diameter was 0.71 mm, and in figuring the noncopper critical current densities,

an area of 2.217 x 10 ^ m

2 was used. The noncopper area of the sample was

nominally 56%. The noncopper area consisted of 10% volume fraction vanadium

diffusion barrier, 38.4% Nb, and 51.6% Sn-Cu matrix. The Sn-Cu matrix was

38.9 wt% tin with respect to Cu.

The apparatus used to measure the strain effect was the one described

earlier in Cryogenics 2(), 614 (1980). The critical current was defined by a

2 yV/cm criterion. The critical currents were also corrected for the small

current carried by the stabilizing matrix, which can become significant at

high fields. For these conductors the correction was small, however, amounting

to 75 mA. The current-transfer voltage correction was negligible even though

the separation between current and voltage taps was only 5 mm.

I versus strain data are shown at magnetic fields ranging from 15 T to

19.6 T in Figs. 4 and 5 and Table II. The principal results are summarized

below:

1. The compressive prestrain was measured to be 0.22%. This is a

relatively small prestrain. Values of typically range from 0.2% to 0.4%

for commercial bronze-process Nb^Sn conductors (see Sec. C) .

2. The irreversible strain e. was measured to be 0.36%. This is a xrrev very low irreversible strain, corresponding to an intrinsic irreversible

strain e . of about 0.14%. For comparison, typical values of e o,irrev o,irrev

are invariably about 0.5% in bronze-process Nb^Sn conductors (see Sec. C) .

44
Thus the handling limits of this conductor in its present form will be restrictive compared to other Nb^Sn conductors.

3. The peak overall was measured to be about 718 A/mm2 at 15 T, using

a 2 yV/cm criterion. This is a relatively high overall for Nb^Sn conductors.

45
Fig. 4 Uniaxial strain dependence of the critical current. Crosses

indicate the critical current measured after unloading from the

correspondingly labeled loaded (open-symbol) data point.

46
NON-COPPER

J

(A/

Wah Chang Internal Sn JR #CRE1031

Area=2. 2 1 7E-07m2

. E max=0. 221

FIELD (T)

Fig. 5. Peak critical-current density measured at e as a function of

magnetic field.

47
Table II. Critical current of an internal tin jelly-roll Nb3Sn conductor (//CRE1031) as a function of strain at magnetic fields from 15 T to 19.6 T. J c is the

critical current density referred to the non-copper area of the conductor,

^max

'*‘ S P ea^ stra in » and E^ is the critical-current electric-field criterion.

Non Cu area = 2.217E-007 Em ax = 0.22 Ec = 4.00uU/cm

E Eo I c Field J c JcB I c / 1 c m

% ) ( X ) ( Amperes ) (Tesla ) < GA/m"2 ) <GN/rT3 )

0.0-3 -.20 1 92.780 14.000 0 . 870 12.174 0 . 0000

153.160 1 5.000 0.691 1 0 . 36 0 0.9620

115.830 1

6

. 000 0.522 8.359 0.9245

61.405 18.000 0.277 4.985 0.8743

41 .295 15.000 0.186 3.539 0.6339

31 . I 60 19.600 0.141 2.757 0.8034

0. 10 -.14 157.230 15.000 0.709 10.638 0.9876

123.770 16.000 0.558 6.932 0.9830

93.305 17.000 0.421 7.155 0.9728

67.215 18.000 0 . 303 5.457 0.9570

45.430 19.000 8.205 3.893 0.9174

35.045 19.600 0. 158 3.093 0. 9056

n nn W . 4- ^ -.01 159.210 15.000 0.718 10.772 1 .0000

125.270 16.000 0.565 9.041 1 . 0000

95.915 1 7 . 000 0.433 7.355 1 .3000

70.235 18.000 0.317 5. '702 1 .0000

49.520 19.000 0.223 4.244 1 . 0000

38 . 570 19.600 0. 174 3.410 1 . 0030

0.3E 0. 13 149.060 15.000 0 , 672 10.085 0 . 9363

1 14.350 IE. 000 0.516 8.253 0.9128

86.265 17.000 0.369 6.616 0.8996

63.005 18.000 0.284 5.115 0.8971

44.630 19.000 0.201 3.825 0.9013

34.645 19.600 0. 156 3.063 0.8982

0. 15 -.09 159.730 15.000 0.720 10.807 1 .0333

0.50 0.27 1 03 . 750 15.000 0.468 7.020 0.6517

77.925 16.000 0.351 5.624 0.6221

57.065 17.000 0.257 4.377 0.5952

39.125 1 8 . 000 0.176 3.177 0.5571

25.365 19.000 0.114 2. 174 0.5122

18.865 19.600 0.085 1 . E68 0.4891

0.21 -.02 120. 1 10 15.000 0.542 8.127 0.7544

0.59 0.36 69.415 15.000 0.313 4.697 0.4360

51 .235 16.000 0.231 3.698 0.4090

34.430 17.000 0.155 2.640 0.3590

22.800 18.000 0. 103 1 .851 0.3245

14.405 1 9.000 0.0E5 1 .235 0.2909

10.428 19.500 0.047 0.917 0 . 0000

48
Table II continued.

Uah Chang Internal Sn JR #CRE 1 03

1

area * 2 . 2 1 7E-007 Emax = 0.22 Ec = 4.00u ,J/'cm

E Eo Ic Field Jc JcB Ic/Icm

< S ) ( X ) ( Amperes ) (Tesla ) <GA/m A 2

)

(GN/rT3>

0.25 0.02 9 1 .125 15.000 0.41 1 6.165 0.5724

0.E8 0.45 32.045 15.000 0.145 2.168 0.2013

25. 145 16.000 0.113 1.815 0.2007

19.135 17.000 0.086 1 .467 0.1995

12.07S 16.000 0.054 0.981 0.1720

7.710 19.000 0.035 0.661 0.1557

5.445 19.500 0.025 0.479 0 . 0000

0.30 0.07 38.750 15.000 0.175 2.622 0.2*34

49
C . Bronze Process Nb^Sn

As a reference, we also present here data on the effect of axial tensile

strain measured in a bronze-process Nb^Sn conductor obtained earlier. This

conductor was manufactured by Oxford Superconductors* (//336-1-X) and used as

the strand material in the Westinghouse* LCP conductor. It was reacted for

240 hours at 750°C.

The data were obtained using the high-field magnet facilities at the

Francis Bitter National Magnet Laboratory and an apparatus designed and built

at NBS (used in the above two studies) . The J -e results for the Oxford

c Superconductor strand are shown in Figs. 6 and 7, and tabulated in Table III.

The main results are as follows:

1. The compressive prestrain was measured to be 0.3%. This is typical

for bronze-process Nb^Sn conductors, where the prestrain is usually in the

range 0.2% to 0.4%.

2. The irreversible strain limit e. was measured to be about 0.8%, irrev

corresponding r 0 to an intrinsic irreversible strain limit e . of 0.5%. The

o, irrev

value of e is invariably about 0.5% in commercial bronze-process Nb„Sn

o, irrev 3

conductors

.

50
Fig. 6 Critical-current density versus axial-tensile strain for the ’

1 conductor strand at constant magnetic fields from 7 T to 20 T.

51
NON-COPPER

J

(A/m

AIRCO 336-1-X 10-2-79 (AM)

Area= 1 . 260E"07m2

. E max=0. 29%

8 10 12 14 16 18 20 22

FIELD (T)

. Peak critical-current

magnetic field.

density measured at

52

e as mFig. 7 a function of
Table ill Critical current of an individual strand from the LCT ICCS conductor

as a function of strain at magnetic fields from 7 T to 19 T. J c is

the critical current density referred to the non-copper area of the

conductor, E^x is the strain (in percent) where the critical current

is maximum, and E

c is the electric field criterion used to determine

the critical current. _

AIRCO 336-- 1 — X Non Cu area = 1 . 350E-007 Emax = 0. 287 Ec = 2. OOuV/cm

E Ed Ic Field Jc JcB Ic/Icm

<*/.) (7.) (Amperes) (Tesla) (GA/m~2) (GN/m~3)

0.03 -.26 151. 100 7.000 1.119 7.835 0.922

123.300 8.000 0.913 7.307 0.915

82.000 10.000 0.607 6.074 0.898

51.300 12.000 0.380 4.560 0. 859

28. BOO 14.000 0. 213 2.987 0.802

12.800 16.000 0.095 1.517 0.699

4. 150 18.000 0.031 0. 553 0.572

1.970 19.000 0.015 0.277 0.500

0. IB -. 10 159.500 7. 000 1. 1B2 8.271 0.973

131.800 8.000 0.976 7.810 0.978

88.800 10.000 0.658 6.578 0.973

57.900 12.000 0.429 5. 147 0.970

34.400 14.000 0.255 3.567 0.958

17. 150 16.000 0. 127 2.033 0.937

6 . 560 18.000 0.049 0.875 0.905

3.470 1 9 . 000 0.026 0.488 0.881

0.29 0. 00 163.900 7.000 1.214 8.499 1 . 000

134.800 8.000 0.999 7.988 1.000

91.300 10.000 0.676 6.763 1 . 000

59.700 12.000 0.442 5.307 1 . 000

35.900 1 4 . 000 0.266 3.723 1 . 000

18.300 16.000 0. 136 2. 169 1 . 000

7.250 18.000 0.054 0.967 1 . 000

3.940 19.000 0.029 0.555 1.000

0.42 0. 13 162.000 7.000 1.200 8.400 0. 988

132.000 8 . 000 0.978 7.822 0.979

88. 800 10.000 0.658 6.578 0.973

57.500 12.000 0.426 5.111 0.963

34 . 200 14.000 0.253 3.547 0.953

16.800 16.000 0. 124 1.991 0.918

5.930 18.000 0.044 0.791 0.818

2.950 19.000 0.022 0.415 0. 749

0.57 0. 28 152.800 7.000 1. 132 7.923 0.932

123.300 8.000 0.913 7.307 0.915

81.300 10.000 0.602 6.022 0.890

50.600 12.000 0.375 4.498 0. 84B

27.500 14.000 0.204 2.852 0.766

1 1 . 300 16.000 0.084 1.339 0.617

2.850 18.000 0.021 0. 380 0.393

1.170 19.000 0.009 0. 165 0. 297

0.39 0. 10 134.000 8.000 0. 993 7.941 0.994

53
Table III (cont)

AIRCO 336--1-X

Non Cu area = 1 . 350E-007 Emax = 0. 287 Ec = 2 . OOuV/cm

E Ed Ic Field Jc JcB Ic/ Icm

(“/.) (7.) (Amperes) (Tesla) ( GA/m''2) (GN/m~3)

0.71 0.42 140.500 7.000 1.041 7.285 0.857

112.000 8.000 0.830 6.637 0.831

71.000 10.000 0.526 5.259 0.778

41.800 12.000 0.310 3.716 0.700

19.700 14.000 0. 146 2.043 0.549

6.360 16.000 0.047 0.754 0.348

1. 180 18.000 0.009 0. 157 0. 163

0.330 19.000 0.002 0.046 0.084

0.48 0. 19 124.000 8.000 0.919 7.348 0.920

0. 83 0.54 127.500 7.000 0.944 6.611 0.778

100.300 8.000 0.743 5.944 0.744

61 . 400 1 0 . 000 0.455 4.548 0.673

33.400 12.000 0.247 2.969 0.559

13.400 14.000 0.099 1.390 0.373

3.370 16.000 0.025 0.399 0. 184

0.425 18.000 0.003 0.057 0.059

0. 55 0.27 115.800 8.000 0.858 6.862 0.859

54
CONSTRUCTION AND INITIAL TESTING OF A TRANSVERSE-STRESS-EFFECT APPARATUS

J. W. Ekin and J. C. Brauch

An apparatus was designed and built to simultaneously apply transverse

pressure, current, and a perpendicular magnetic field to superconductor

strands at 4 K. The construction was started under a small grant (1/3 staff

year) from the MIT Plasma Fusion Center. The apparatus was modified, construction completed, and the apparatus used to conduct initial tests on a singlestrand Nb^Sn superconductor in the first year of the present DOE/OFE contract.

In its present configuration the apparatus is capable of applying up to

600 A to a single strand superconductor in a transverse field of 10 T in poolboiling liquid helium. The magnetic field is produced by a 10 T split-pair

mult if ilamentary-Nb^Sn magnet with 1-inch-square radial-access ports. Field

accuracy is ±0.2% and homogeneity over the sample test section is on the order

of ±0.2%.

An illustration of the apparatus is shown in Fig. 1 and photographs are

shown in Figs. 2 and 3. Materials in the load frame were selected for low

magnetic susceptibility. The sample is compressed between two stainless steel

anvils. The edges of the anvils are rounded where the sample enters between

the two anvil heads so as to reduce stress concentration at that point.

The ends of the sample are soldered to two copper blocks which introduce

current into the superconductor. A thin sheet of fiberglass-epoxy composite

is used to insulate the copper blocks and also to support the Lorentz force

experienced by the conductor during testing. Compressive pressure is applied

to the sample by a 13.3 kN (3000 lb) servohydraulic actuator capable of either

load, stroke, or strain control.

55
An initial test was conducted on the Nb^Sn conductor used as strand

material in the Westinghouse LCP coil. The apparatus performed well for the

first test. A preliminary threshold value for reversible transverse-stress

degradation was measured. Additional samples, however, should be measured

with several apparatus modifications before a threshold value can be reported

which we could certify for engineering design purposes. Such tests are in

progress

.

The apparatus is extremely versatile and present plans under the DOE

contract call for testing a variety of individual candidate strands for fusion

magnets, as well as smaller ICCS conductors and larger monolithic superconductors.

56
4-

Upper Stainless Steel

Compression Fixture

Lower Stainless Steel

Compression Fixture

t i

t

Superconductor

Test Strand

Copper Current Block

Figure 1. Apparatus for measuring the effect of transverse pressure

on the critical current of superconductor strands.

57
58

Figure

2.

Overview

of

transverse

compression

apparatus,

showing

magnet-insert

portion

at

left,

main

force-reaction

cylinder

in

center,

and

head

plate

at

right.
Figure 3. Head plate showing mechanical linkage to

actuator and vapor-cooled sample current

servo-hydraulic

leads

.

59
THERMAL CONTRACTION OF SEVERAL CANDIDATE SHEATHING AND STRENGTHENING MATERIALS

FOR SUPERCONDUCTORS

J. W. Ekin

Most structural materials being considered for reinforcing or sheathing

Nb^Sn conductors significantly degrade the critical current of the

1-4

superconductor. In the case of austenitic stainless steel sheathing

materials, such as JBK-75 for example, the critical current is degraded

between 25% and 70% at 12 T for typical superconductor packing fractions; see

4 1 Fig. 1. (The degradation is greater at higher magnetic fields. ) This

occurs because of a large mismatch in thermal contraction between Nb^Sn and

the sheathing material. If other candidate structural materials could be

identified with a thermal contraction coefficient closely matching that of

Nb^Sn, the improvement in overall current density would be considerable.

Consequently, thermal expansion measurements were made on several structural

materials to quantitatively evaluate their suitability for superconductor

sheathing and reinforcement applications.

Incoloy 903 and Incoloy 905* (4 to 400 K)

The first tests were made on Incoloy 903 and Incoloy 905 over a low

temperature range from 4 K to 400 K. Samples of Incoloy 903 and Incoloy 905

were cut from bulk pieces of each alloy. The samples were in the shape of

cylinders measuring approximately 1 cm in diameter and 3 cm in length.

* Certain commercial materials are identified to specify the experimental

study adequately. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the

material is necessarily the best available for the purpose.

60
Temperature was measured using a type E thermocouple. Tables 1 and 2 show the

thermal expansion data as a function of temperature. A 6-data point smoothing

routine was used to obtain thermal-expansion values at even-temperature values

every 10 K. Figures 2 and 3 show the smoothed data.

From these data, the total thermal contraction was determined from 4 K to

700°C (the approximate reaction temperature for Nb^Sn) , assuming an

extrapolation from 400°C to 700°C based on Reference 5. This extrapolation

shows a significant change in slope at approximately 672 K. This is ascribed

to a magnetic transformation since the alloy exhibits Curie temperature

behavior. The same extrapolation was used for both Incoloy 903 and 905,

although the data of Ref. 5 were obtained for Incoloy 903 only. The results

are tabulated in Table 3 and plotted in Fig. 4. Similar data for Nb^Sn on

cooling from 700°C are also tabulated in Table 3 and Fig. 4. Based on the

comparison between these two materials shown in Fig. 4, the strain that would

be introduced into Nb^Sn from differential thermal contraction with Incoloy

alloy sheathing material has a maximum value of 0.2%. (It may be less than

that, depending on the tightness of the mechanical bond between the sheathing

material and the superconductor, and the relative amount of each. 2 ’ 3

)

Thus, the prestrain arising from using Incoloy sheathing material in ICCS

should be significantly smaller than that from the stainless-steel sheath

material used in the LCP ICCS. This is shown in Table 3 and Fig. 4 where the

thermal contraction of stainless steel in cooling from 700°C is given for

comparison.

In Fig. 4 the thermal contraction of a free-standing Nb^Sn/Cu-bronze LCP

strand is also presented, determined from a measurement of versus uniaxial

strain for this conductor (described in section 2 of this report)

.

61
A sheathing material such as Incoloy with a thermal contraction coefficient

closely matching that of Nb^Sn could also reduce the compressive prestrain

introduced into the Nb^Sn from the bronze and copper matrix in a free-standing

strand. Some reduction in prestrain would be an advantage since the effect of

0.3% compressive prestrain on the critical current is still relatively

significant (11% at 10 T, for example); see Fig. 1 of Sec. 2C. However a

small compressive prestrain of about 0.2% is recommended since this provides a

buffer for additional tensile strain introduced by the Lorentz force on

energizing the magnet. An exact strain balance between differential thermal

contraction, winding strain, and Lorentz force strain can be calculated in the

critical regions of the magnet on a case-by-case basis.

High-Mn Steel Alloys (77 K to 1073 K)

Two high-Mn steel alloys were also tested. Because of some thermal

contraction data reported for 25Mn-5Cr-lNi steel by Yoshimura a_1 . , it had

been hoped that high-Mn steel alloys would have a low thermal contraction more

closely matching Nb^Sn. Yoshimura e_t a^L. obtained data at low temperatures

from 77 K to 293 K. In this regime they reported a thermal contraction

coefficient about 50% of that of 300-series austenitic stainless steels.

To test the potential of high-Mn steel alloys as a superconductor

sheathing material, thermal contraction measurements were obtained on two

compositions at both low and high temperatures from 77 K to 800° C. The first

sample was 25Mn+5Cr+lNi steel and was the same alloy tested by Yoshimura e_t

al . The second was 25Mn-14Cr-5Ni stainless steel and was chosen because it

also had a high-Mn content, but in addition, it had sufficient Cr to make it

rust resistant. The composition of each alloy is given in Table 4.

62
The measurements were carried out at Energy Materials Testing Laboratory

(EMTL) under contract to NBS using a fused silica dilatometer in general

accord with ASTM-E-228. Both heating and cooling data were obtained and no

significant differences were observed between the two types of measurements.

Overall accuracy based on measurements against NBS SRM-739 was estimated by

EMTL to be ± 0.002% absolute. The coefficient of thermal contraction coefficients for each alloy are compared with those typical of JBK-75 and 300-series

stainless steels in Table 5. The results for each alloy are shown in Figs. 5

and 6. The data are tabulated in Table 6.

The following conclusions can be drawn from these data:

1. Below room temperature, the coefficient of thermal contraction of the

high-Mn steels is about 60% of that of the 300-series austenitic stainless

steels

.

2. Above room temperature, however, the thermal contraction coefficient

of the high-Mn steels is significantly greater than for the 300-series austenitic stainless steels.

3. The combination yields a total thermal contraction from 700° C to 4 K

that is just about the same for both the high-Mn and 300-series austenitic

stainless steels.

These data indicate that the high-Mn steels will be no better than JBK-75

and 300-series austenitic stainless steels as sheath materials, with respect

to degradation of the critical current of Nb^Sn arising from differential

thermal contraction. This is shown in Fig. 4 and Table 3 where the thermal

contraction on cooling from 700° C is given for free-standing Nb^Sn and

compared with several other materials.

63
Conclusion

In summary, the data indicate the mismatch in thermal expansion between

Nb^Sn and Incoloy 903 or Incoloy 905 (amounting to 0.2%) is significantly less

than that (0.9%) for conventional stainless-steel sheathing materials [Fe +

(16-19)Cr + (6-14)Ni + X] or high-Mn steel alloys [Fe + (23-25)Mn + (5- 14)Cr

+ (l-5)Ni + X]. The use of Incoloy should result in significant increases in

the critical current of sheathed and reinforced superconductors. The increase

in the critical current of ICCS conductors, for example, can be greater than

50% at 12 T, depending on the conductor packing fraction (see Fig. 1). The

increase will he more at higher fields.

It is suggested that further screening and testing of alternative Incoloy

alloys be done. The high-temperature extrapolation of the data for Incoloy

905 were based on measurements of Incoloy 903 and should be measured directly.

High-temperature thermal contraction and mechanical properties of Incoloy 905

and other modifications should be measured.

ACKNOWLEDGMENT

The skill and care of M. Austin in making the low-temperature thermal

expansion measurements of Incoloy 903 and 905 are gratefully acknowledged.

64
REFERENCES

1. "Strain scaling law for flux pinning in practical superconductors. Part

1: Basic relationship and application to Nb^Sn conductors," J. W. Ekin,

Cryogenics 2JD, 611 (1980).

2. "Mechanical properties of high-current multifilamentary Nb^Sn conductors,"

R. M. Scanlan, R. W. Hoard, D. N. Cornish, and J. P. Zbasnik, p. 221,

Filamentary A15 Superconductors , ed. by M. Suenaga and A. F. Clark, Plenum

Press, New York (1980)

.

3. "Effect of stainless steel reinforcement on the critical current versus

strain characteristic of multifilamentary Nb^Sn superconductors," J. W.

Ekin, R. Flukiger, and W. Specking, J. Appl. Phys. 54_, 2869 (1983).

4. "Effects of Incoloy 903 and tantalum conduits on critical current in Nb^Sn

cable- in-conduit conductors," M. M. Steeves, M. 0. Hoenig, and C. J.

Cyders, Adv. Cy. Eng. _27_ (1982).

5. "Determination of structural engineering properties of Incoloy 903 and

CTX-1 alloys," p. 121, George C. Marshall Space Flight Center, Alabama

35812, National Aeronautics and Space Administration, Contract NAS8-30929,

Paul E. R.uff, Battelle, Columbus Laboratories, 505 King Ave., Columbus,

Ohio 43201.

6. "Microstructures, low temperature toughness and thermal expansion

coefficients of high manpanese-chromium austenitic steels," H. Yoshimura,

N. Yamada, H. Yada, H. Honma, and T. Ito, Trans. ISIJ 16, 98 (1976).

65
>x

Helium Fraction (%)

Fig. 1. Noncopper critical current density at 12 T and 4.2 K as

a function of helium fraction for Incoloy 903, tantalum

and stainless steel conduits. Single wire data (no conduit) are shown for reference. Volume fractions of 1-903, Ta and

steel were 0.68, 0.48 and 0.32 respectively. (From Ref. 4)

66
INCOLOY 903

Z00rJ5B -

100

5(0

0 r

-50

-IBB

”150 r

-200

=

1

if. 50

' i i i I i i ' i i i i i I i i > i i i < i » I i i i i 1 1 i i i I i i » » i i i i i I i i i ' i i i i i I i i i i i i i i i I 100 150 Z00 S50 900 350 400

TEMRERRTURE K

Figure 2. Thermal expansion versus temperature for

Incoloy 903 interpolated to even temperature values.

67
INCDLOY 905

Z00

150

100

IT

<CD 50 -

0\

DJ

1 -l -50

-100

-150

-200 li_L 0

i i i i i ^i A i ^ i i i i i i i I i i i i i i i i i 1 i i i i i i i i i I i i i i i i i i i I i i i i i i i i i I i : i i i i i i i I i i i I A A 50 100 150 200 250 300 350 400

TEMPERATURE K

Figure 3. Thermal expansion versus temperature for

Incoloy 905 interpolated to even temperature values.

68
THERMAL

EXPANSION

(%)

Fig- 4. Comparison of the thermal expansion of various materials

with Nb^Sn over the temperature range from 700 C to 4 K.

69
THERMAL

EXPANSION

(%)

TEMPERATURE (K)

Fig. 5 Thermal expansion versus temperature for Fe-25Mn-5Cr-lNi steel.

70
THERMAL

EXPANSION

(%)

Fig. 6 Thermal expansion versus temperature for Fe-25Mn-14Cr- 5\' i stainless steel.

71
903PLT

Table 1. Thermal expansion data for Incoloy 903 from 20 K to

400 K interpolated to even temperature values.

TEMPERATURE K <L-Lo>/Lo <10

20 -193. 157

30 -193.583

40 -194. 107

50 -191.537

60 -167.417

70 -183.991

60 -178.91

90 -174.401

100 -166.774

110 -155.792

120 -147.507

130 -140.506

140 -132.556

150 -124.37

160 -115.997

170 -107.513

160 -99.0061

190 -90.2696

200 -81.307

210 -72.5205

220 -63.665

230 -54.8301

240 -45.9367

250 -36.964

260 -27.6313

270 -19.7223

260 -11.9646

290 -2.96064

300 6.30729

310 12.6236

320 20.2361

330 29.2727

340 38. 1345

350 46.9056

360 55.6306

370 64.3733

360 72.8874

72
905PH

Table 2. Thermal expansion data for Incoloy 905 from 20 K to

400 K interpolated to even temperature values.

TEMPERATURE K <L-Lo>/Lo <10

20 -207.982

30 -206.09

40 -201.066

90 -196.351

€0 -192.246

70 -183.461

80 -176.033

90 -171.393

100 -165.949

110 -159.503

120 -152. 149

130 -144. 191

140 -135.944

150 -127.472

160 -118.744

170 -109.846

160 -100.604

190 -91.5626

200 -82.3797

210 -73.3016

220 -64.4067

230 -56.847

240 -47.7631

250 -39. 1919

260 -30.5982

270 -22.0411

260 -13.4566

290 -3. 10587

300 5.69815

310 13.2291

320 21.8436

330 30.5793

340 39.3465

350 48.8927

360 56.9727

370 65.8374

360 74.662

73
Table 3. Total thermal contraction of various materials on cooling from 700°C

to 4.2 K.

Material AL/L (973 K *— <

Nb

3

Sn 0.74% t

Incoloy 903 0.94% *+

Incoloy 905 0.95% *+

Stainless Steel [Fe+( 16-19)Cr+(6-14)Ni+ZX ] 1.62% t

High Mn Steel [25Mn-5Cr-lNi] 1.62% *

High Mn Stainless steel [23Mn-14Cr-5Ni] 1.64% *

Nb^Sn/Cu-bronze (LCT strand) 1.03% +t

Nb 0.68% t

Cu 1.63% +

* Measured

tt Calculated from J (e) data

c t From following references:

1) Handbook on Materials for Superconducting Magnets (Battelle,

Columbus, OH 1975).

2) Paul E. Ruff; Battelle, Columbus, OH; Determination of Structural

Engineering Properties of Incoloy 903 and CTX-1 Alloys, pl21; George C.

Marshall Space Flight Center, Alabama 35812, National Aeronautics and

Space Administration, Contract NA58-30929.

3) Y. S. Touloukian, R. K. Kirby, R. E. Taylor, and P. D. Desai,

Thermophysical Properties of Matter (IFI/Plenum, New York, 1975), Vol.

12 .

74
Table 4. Composition of the high-Mn steels

Element

Mn

Cr

Ni

Si

N

A1

C

Nb

P

S

25Mn-5Cr-lNi

25.6

4.67

0.97

0.86

0.040

0.009

0.22

0.043

0.024

0.001

23Mn-14Cr-5Ni

23.3

13.7

5.3

0.302

0. 15

0.035

0.034

0.0025

0.0007

75
Table 5. Comparison of thermal contraction coefficients for 300-series and

high-Mn steel alloys.

Material

a(77-293 K)

(10

b /K)

a(293-973 K)

(10

b

/K)

AL/L(4-973 K)

(%)

Fe+( 16-19)Cr+(6-14)Ni+EXi

(304L,316, 321 , etc.

)

12.9 18.6 1.626

Fe-25Mn-5Cr-lNi

(Nippon Steel)

7.3 21.2 1.622

Fe- 2 3Mn- 1 4Cr- 5N

(Nippon Kokan)

8.2 21.2 1.645

76
• le 6. Thermal Expansion Values for High-Mn and 300-Series Steel Alloyj

(Composition A)* (Composition B)* (Compositic

T T AL/Lo AL/Lo AL/Lo

(K) (C) (%) (%) (%)

4 -269 (-0.18) (-0.20) (-0.36)

77 -196 -0. 158 -0.178 -0.279

100 -173 -0.146 -0. 167 -0.254

150 -123 -0. 115 -0. 135 -0.195

200 - 73 -0.079 -0. 100 -0.130

250 - 23 -0.041 -0.056 -0.061

293 20 0.000 0.000 0.000

400 127 0. 140 0. 170 0.166

500 227 0.315 0.353 0.336

600 327 0.522 0.553 0.517

700 427 0.749 0.773 0.708

800 527 0.994 1.009 0.907

900 627 1.249 1.254 1.114

973 700 1.439 1.440 1.266

1000 727 1.507 1.505 1.322

1073 800 1.702 1.692 1.558

Composition A (Fe + 25Mn + 5Cr + INi + £Xi) High-Mn steel

Composition B (Fe + 23.3Mn + 13.7Cr + 5Ni + ZXi) High-Mn stainless steel

Composition C (Fe + (16-19)Cr + (6-14)Ni + ZXi) : 300-series stainless steel

alloys

Measured

t From Y. S. Touloukian, R. K. Kirby, R. E. Taylor, and P. D. Desai,

"Thermophysical Properties of Matter (IFI/Plenum, New York, 1975), Vol.

12 .

77
ELECTRON TUNNELING INTO SUPERCONDUCTING FILAMENTS USING MECHANICALLY

ADJUSTABLE BARRIERS

John Moreland and J. W. Ekin

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

Abstract

A new type of squeezable electron tunneling (SET) junction has been

developed for tunneling into superconducting filaments. Stable, mechanically

adjustable tunneling barriers between the native surfaces of sputtered Nb

films and 30 ym diameter Nb filaments were established in liquid helium at 4

K. The current versus voltage characteristics of these SET junctions were

used to determine the superconducting energy gap at the surface of the filaments. Since the filaments were etched from commercial superconducting magnet

wire, this type of tunnel junction shows proiaise as A diagnostic probe of

superconducting materials for high-field magnets.

PACS numbers: 74.50+r, 74.70.Ps, 73.40Gk, 73.40.Rw

Contribution of National Bureau of Standards not subject to copyright.

78
Electron tunneling spectroscopy is an excellent probe of the superconducting state. It is the preferred method for determining the superconducting

energy gap and the nature of the electron-phonon interaction since the conductance of a superconducting tunnel junction as a function of applied bias is

directly related to the density of states of thermal excitations in a supercon1 2

ductor as a function of energy.

*

Tunnel junctions stable enough for electron tunneling spectroscopy have

traditionally relied on the formation of an insulating oxide barrier between

3 4 conducting surfaces. * Native oxides grown under controlled conditions have

been the foundation for tunneling experiments on superconductors^’^ until

recent developments that utilized artificial barriers including silicon oxide^

8 9

and aluminum oxide ’ insulators, and squeezable electron tunneling (SET)

. 10,11 junctions

The latter technique has also been adapted to bulk materials including Si

(ref. 12), (TMTSF)^ C£0^ (ref. 13) and HgCdTe (ref. 14) crystals. Though tunneling contacts to bulk materials have been made using some of the other methods

mentioned above, mechanically adjustable junctions have additional versatility

because a nondestructive tunneling contact can be translated relative to the

surface of a sample. The electron tunneling microscope developed by Binnig

and Rohrer^, for example, works on this principle, substantially increasing

the flexibility of tunneling measurements. Very recently Elrod, de Lozanne

and Quate have shown that vacuum tunneling microscopes can be adapted to

cryogenic experiments on superconductors.^

We present here a SET junction that takes advantage of the filamentary

nature of superconductors drawn to small diameters in a reaction or stabilization matrix. Experimental current versus voltage (I-V) curves for SET junctions

consisting of Nb filaments with Nb thin-film counter electrodes are discussed

79
along with data obtained for thin-film Nb-Nb junctions at 4 K. To our knowledge these are the first successful mechanically adjustable tunneling barriers

between native Nb surfaces and the first SET junctions with a filament electrode.

Figure 1 shows the top view of a miniaturized SET junction used to

determine the superconducting energy gap of the thin-film Nb counter electrodes. It consisted of electrode strips supported by flexible substrates

that were separated by thin-film spacer strips. Alternating electrode and

spacer strips were deposited on polished 50.8 mm (2 inch) diameter, 0.4

mm-thick Si wafers that were then scribed and broken into 6.4 x 12.7 mm (0.25

x 0.50 inch) chips. To obtain the geometry shown in Fig. 1, two of the chips

were assembled at right angles with opposing electrodes to form the junction.

A strip pattern was used to simplify the alignment of the opaque substrates.

A layer of photoresist that protected the films from shards and dust that

accumulated during scribing was removed with an acetone spin-wash just prior

to junction assembly.

Superconducting counter electrodes were used because they gave rise to

distinct structure in the I-V curves that provided a quality check for the

tunneling barrier even if the sample electrodes were normal. A material with

T^ > 4 K was experimentally convenient. Preliminary measurements of Nb

filament-Pb film junctions revealed I-V characteristics identical to those

obtained for Pb film-Pb film junctions. This indicated that the soft Pb had

transferred to the Nb filaments. Sonnenfeld has observed the same phenomenon

14

for Pb film-HgCdTe crystal SET junctions. Sputtered Nb films were our next

choice. Despite being more difficult to deposit, they showed no signs of

rubbing off, and had larger T^’s and larger energy gaps than those obtained

for Pb films.

80
All films were prepared using standard microelectronic photolithographic

processing (based on Shipley 1350 photoresist^) in a clean room supplemented

by laminar flow work stations. Evaporated SiO spacers were delineated using a -4

lift off process. Evaporations took place at a pressure of 2 x 10 Pa and a

rate of 1 nm/s. The target voltage was about 475 V. The Nb films were then

etched in a reactive 0

^ - CF^ plasma.

Previously, dust contamination was minimized by acetone vapor degreasing

precleaned substrates prior to evaporations.^ We found that using Si wafers

polished and precleaned by the manufacturer in conjunction with clean room

processing without vapor degreasing also yielded reliable SET junctions. At

first we were apprehensive about using contact masks to expose the photoresist

apparently the photoresist was tough enough to protect both the substrate and

films deposited in preceding layers.

An iron electromagnetic squeezer scaled down from the original version^

was used to compress the junctions to tunneling distances. The squeezer was

mounted on the end of a dip stick for rapid experiments in a liquid helium

storage Dewar. A copper chamber on top of the Dewar allowed the squeezer and

junction assembly to warm in a dry helium atmosphere. This kept water from

condensing on the apparatus and facilitated rapid (ss 10 min.) thermal cycling.

The electrodes and spacers were 0.2 ym and 1.0 ym thick, respectively, so that

the electrode surfaces were nominally 1.6 ym apart before squeezing. The

squeezing force was applied along concentric circles 1 mm and 6 mm in diameter

on either side of the junction. Under these conditions squeezing forces of

about 0.25 N were necessary to make junction contact. This was in agreement

with the force calculated for the given loading geometry of the Si substrates

according to thin plate deflection formulas.^

81
Differential dl/dV (conductance) versus voltage curves were measured with

a tunneling spectrometer that utilized a feedback circuit to keep a constant

18 modulating voltage (250 pV, 100 Hz) across the junction. Voltage was swept

at a constant rate of 100 pV/s. All measurements were done at 4 K.

Figure 2 shows an experimental I-V curve typical of those obtained for Nb

film-film SET junctions immersed in liquid helium at 4.0 K. Notice the

distinct current inflections that, theoretically, should be separated by a

voltage AV *= ^A^^ /e for a tunnel junction with identical superconducting

electrodes having an energy gap of A^^ (e is the electron charge)."* The

junctions could be reversibly adjusted in that the I-V curves could be continuously changed from an open contact, to tunneling behavior, and finally to a

short. Once a short was established, tunneling behavior could be obtained

again by decreasing the squeezing force.

The superconducting energy gap was estimated from the position of the

peaks in the dl/dV spectrum and found to be A r . n = 1.4 meV independent of the

i Ilm

junction resistance measured at 10 mV bias for R(10 mV) > 10 kfl. This value

19

for the energy gap was close to the previous measurements by Broom et a_l

g using native oxide and by Wolf, Zasadsinski, Osmun and Arnold using aluminum

oxide barriers. Both of these previous measurements gave about 1.5 meV at a

lower temperature of 1.4 K. Assuming Tc K 9 K, Aq ssl.5 meV, and a BCS energy

gap temperature dependence (see, for example, ref. 5), this implies A = 1.43

meV at 4 K. Thus the SET-junction results agreed with the earlier

measurements using oxide barriers to within our experimental precision (~ 0.1

meV) .

The section of a SET junction that consisted of a superconducting filament held against the substrate of a thin-film counter electrode is shown

in figure 3. The filament was held by a friction fit between tapered brass

mandrels that press fit into tapered brass sleeves. A Be-Cu spring compressed

82
between the fiberglass-epoxy holder and one of the brass sleeves kept the

filament in tension. This configuration provided rugged cryogenic electrical

contacts to the filament. A pointed thumb screw reacted against the squeezing

force of the electromagnet and allowed the holder to pivot so that the counter

electrode substrate was flush against the plunger. Since the filament rested

on the evaporated spacers, the counter electrode-filament spacing was

determined by the amount of bow in the substrate supporting the electrode and

the spacers. The amount of substrate bow was adjusted by applying forces as

shown in the figure. Using the Nb film counter electrodes described above

with different filaments from the same wire led to a variable series of

junctions ranging from those that were shorted without any squeezing to those

that required an unusually large squeezing force. This may have been due to

inherent surface bowing of the Si chips in the fiberglass-epoxy holder, dust

particles (all experiments occurred outside the clean room), or variations in

the average filament diameter. Fortunately, the high compliance

electromagnetic squeezer system allowed for large variations in the initial

filament-film separation.

The Nb filaments (~ 30 ym in diameter) are obtained by etching away the

Cu-Sn bronze matrix of an unreacted Nb^Sn magnet wire using a 50/50 solution

of nitric acid and water. Their surface topology is illustrated in figure 4

with electron micrographs of the filament-film SET junction and various

magnifications of a single filament. Tunneling current (s) most likely flow

through the small bump(s) (<1 ym in size) apparent along the surface of the

filament (Fig. 4d)

Included in figure 2 is an experimental I-V curve measured for a Nb

filament-Nb film junction immersed in liquid helium at 4.0 K. Like the

film-film junctions, the filament-film junctions were reversibly adjustable.

Since the counter electrodes used for filament measurements were calibrated in

83
the previous film-film SET junction experiment, the energy gap of the filament

could be determined from the relation"*

iV - < 24 filament

+ 2fi fllm> /e

where AV was the voltage separating the gap edges determined from the peaks of

the dl/dV data.

Again AV was independent of the junction resistance for R(10 mV) > 10 kfi.

Given A^^ *1.4 meV, we infer that “1*0 meV. A lower superconducting energy gap obtained for the filaments may not be surprising since they

were fabricated in a Cu-Sn bronze matrix. X-ray microanalysis showed a

minimum surface contamination of 10 wt.% Sn and 3 wt.% Cu.

Tunneling data could be repeated after establishing a short in the

barrier. This implied that these SET junction contacts were nondestructive in

the sense that the energy gap of the materials remained the same before and

after contact.

In conclusion, SET junctions provide a viable method for tunneling to

bulk superconducting materials. The following comments summarize some of our

results for superconducting filaments.

2

1) We obtained small area (< 1 ym ) low leakage tunneling barriers

between native Nb surfaces. They had continuously adjustable

resistances. The adjustability was probably due to a combination of

changes in the barrier area and thickness, possibly facilitated by

the elasticity of the substrate and the Nb near contact points, or

the presence of He between the Nb surfaces.

2) Tunneling gaps (AV) determined from differential dl/dV data were

independent of junction resistance for R(10 mV) > 10 kft. Together,

84
tunneling gaps measured for Nb film-film and filament-film junctions

Rave 6 A^, *= 1.4 meV and A,.., = 1.0 meV. The difference was film filament

not surprising since the surfaces of the filaments contained Cu and

Sn from the bronze matrix of the magnet wire (tunneling electrons

probe only the surface of a superconductor to a depth of about one

coherence length)

3) The junctions were stable against moderate room vibrations. They

could be rapidly cycled (~ 10 min.) to cryogenic temperatures in a

dry helium atmosphere so that counter electrodes could be calibrated

and reused in later experiments.

We look forward to extending this technique to NbTi and Nb^Sn filaments.

Some particularly intriguing prospects include studying the effects of various

chemical treatments on the properties of a superconducting surface and then

depth profiling the energy gap by successively etching and probing the surface

with a SET junction.

Acknowledgments

The authors wish to thank T. Capobianco, A. Clark, F. Fickett,

R. Goldfarb, L. Goodrich, P. Hansma, R. Harris, K. Kilhstrom, F. Lloyd,

D. Moreland, D. Moses, D. Rudman, and A. West, for their advice and helpful

expedition of this project. This work was partially sponsored by the Department of Energy, Office of Fusion Energy, under interagency agreement

IDE-AI01-84ER521 13. Additional support was provided to one of us (J.M.) as a

National Research Council Postdoctoral Associate.

85
References

1. D. J. Scalapino, J. R. Schrieffer, and J. W. Wilkins, Phys, Rev. 148 , 263

(1966).

2. W. L. McMillan and J. M. Rowell, Superconductivity, Vol. 1, edited by R.

D. Parks, 561 (Dekker, New York, 1969).

3. Tunneling Spectroscopy , edited by P. K. Hansma, (Plenum, New York, 1982).

4. R. V. Coleman, R. C. Morris, and J. E. Christopher, Methods of

Experimental Physics v. 11 Solid State Physics , edited by R. V. Coleman,

123 (Academic, New York, 1974).

5. I. Giaever, Tunneling Phenomena in Solids , edited by E. Burstein and S.

Lundqvist, 255 (Plenum, New York, 1969).

6. E. L. Wolf, Rep. Prog. Phys. 4J_, 1439 (1978).

7. D. A. Rudman and M. R. Beasely, Appl. Phys. Lett. _3j6, 1010 (1980).

8. E. L. Wolf, J. Zasadzinski, J. W. Osmun, and G. B. Arnold, J. Low Temp.

Phys . _40, 1980)

9. J. S. Moodera, R. Meservey, and P. M. Tedrow, Appl. Phys. Lett. 4_1_, 488

(1982).

10. J. Moreland, S. Alexander, M. Cox, R. Sonnenfeld, and P. K. Hansma, Appl.

Phys. Lett. 43, 387 (1983).

11. J. Moreland and P. K. Hansma, Rev. Sci. Instrum. 5^5, 399 (1984).

12. J. Moreland, J. Drucker, J. P. Kotthaus, A. Adams, R. Kvass, and P. K.

Hansma, Appl. Phys. Lett. 4_5, 104 (1984).

13. J. Moreland, Ph.D. Thesis, University of California, Santa Barbara

(1984)

14. R. Sonnenfeld, J. Moreland, A. Adams, R. Kvass, and P. K. Hansma, to be

published in J. Appl. Phys.

86
15. G. Binnig and H. Rohrer, Surf. Sci. 126 , 236 (1983).

16. S. A. Elrod, A. L. de Lozanne, and C. F. Quate, Appl. Phys. Lett. 45 ,

1240 (1984).

17. Certain commercial materials are identified in this paper to specify the

experimental study adequately. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the material identified is necessarily the best

available for the purpose.

18. A. F. Hebard and P. W. Shumate, Rev. Sci. Instrum. ^5, 529 (1974).

19. R. F. Broom, S. I. Raider, A. Oosenbrug, R. E. Drake, and W. Walter, IEEE

Electron Dev. ED-27

,

1998 (1980).

87

WIRE SOLDERED

TO CLAMP -

BRASS

CLAMPS

DEPOSITED

SPACER

STRIPS

FLEXIBLE

SUBSTRATES

DEPOSITED

ELECTRODE

STRIPS

1 mm

Figure 1. Top view of a miniaturized SET junction showing

intersecting deposited spacer and electrode strips

Though opaque Si chips were used, transparent

substrates are shown for clarity. Brass screw

clamps are used to make electrical contact to the

electrodes

89
(vu) iN^aano

90

Figure

2.

Typical

I-V

curves

of

Nb

film-Nb

film,

and

Nb

film-Nb

filament

SET

junctions.

The

squeezer

apparatus

was

immersed

in

liquid

helium

at

4

K.
I

o

<

ID

0C

IDO

cco

LL

91

Figure

3.

Section

of

the

filament

holder

for

film-filament

tunneling

experiments.

The

squeezing

force

from

an

electromagnet

bows

the

counter

electrode

substrate

against

a

filament.

The

t

ilament

is

tensioned

against

the

substrate

by

the

spring

loaded

mandrel

holder.
4-1 cd

CL)

d

d o

bO •rl 4-J d 4-1 d B O 0) d B d d d

c/d •r-

i—i CO •H x> H 4-4 a W

CD) 4-4 T3 O

0) 4-1

4-1

d CD

a CD

d

d

b o

<Dd •H • d i—i 4-J 0) d •H d N d 14-4 i—i •H 1 d CD d B nd d rd d d

•H

d •H S <4-1 o /^N S

5-4 4-J XI d.

4-4

U

0) i

i d3 4-4 • 01 14-4 CD v I rd d x) a o CO

4-) 4-1

d cx

CD

d 4-1

B

-d o d

4-J 4-J cu d i

! 0) CO CD i

i b c

i—

d 0) d d i

i d 0) B

•H -d 4-J CD

4-4 ao

d

d

d ^~N -D o o

la

d o d 4-4 d . o CD

14-4 bO

d o co d o

<D •r-4

H

cd CX cd 4-J -d CD d

ex •U o •H d d d d 0) a 4-1 bO -d CD O 4-1 4-1 d d / N

CJ bO a) cj •H d B e *rH di—

• d o •H d o 4d 4-1 0) d cn 4-J

4-J Jd 0) u 0) a B

<u

dd i—

•H *H

w £ d d)

OJd3

bO

•H

Pm

92
APPENDICES

A. "Effect of Stainless Steel Reinforcement on the Critical Current Versus

Strain Characteristic of Multifilamentary Nb„Sn Superconductors"

J. W. Ekin, R. Flukiger, and W. Specking, published in J. Appl. Phys . 54,

2869 (1983).

B. "Further Investigations of the Solid-liquid Reaction and High-field

Critical Current Density in Liquid-infiltrated Nb-Sn Superconductors"

M. Hong, D. M. Maher, M. B. Ellington, F. Heilman, T. H. Geballe, J. W.

Ekin, and J. T. Holthuis, IEEE Trans. Magn. MAG-21 , 771 (1985).

C. Trip report covering visits made December 5-14, 1984 to superconductor

research laboratories at Kyushu University, (Fukuoka, Japan); Japanese

National Railway "MAGLEV" Test Site, (Miyazaki, Japan); Japanese Atomic

Energy Research Institute (Tokai, Japan); Tohoku University (Sendai,

Japan) ; and the DOE Japan-US Workshop on High-Field Superconducting

Materials for Fusion (Tsukuba, Japan).

93
APPENDIX A

Effect of stainless steel reinforcement on the critical current versus strain

characteristic of multifilamentary Nb3Sn superconductors

J. W. Ekin

Electromagnetic Technology Division, National Bureau ofStandards, Boulder, Colorado 80303

R. Flukiger and W. Specking

Kernforschungszentrum Karlsruhe, Institut fiir Technische Physik, 7500 Karlsruhe, Postfach 3640, Federal Republic of Germany

(Received 25 August 1982; accepted for publication 1 1 January 1983)

A series of multifilamentary Nb3 Sn superconductors were fabricated containing from 0% to 52

vol % stainless steel reinforcement strands as an integral part of the conductor. Critical current

versus strain measurements are reported which show that the stainless steel introduces a relatively

large compressive prestrain, em , into the superconductor, ranging from em ~0. 3 % for no stainless

steel reinforcement to em = 0.84% for 52 vol % stainless steel. Accompanying this high

compressive prestrain in the reinforced conductors is a large degradation of the conductor’s

critical current, 7C

. The Ic degradation relative to the strain-free value has been measured, for example, to be more than 50% at 13 T for em = 0.64%. Several methods are discussed for reducing the large Ic degradation in A15 superconductors containing internal reinforcement.

PACS numbers: 74.70.Ps, 74.70.Lp

In the design of large superconducting magnet systems,

a great amount of structural reinforcement is necessary to contain the magnetic forces generated when the magnet is energized. For example, it has been calculated that a fully stabilized Nb3 Sn conductor in a 12 T fusion magnet would

consist of nearly 50% stainless steel, while the Nb3 Sn itself would occupy only about 10% of the volume.' To be effec- tive in providing adequate support for the superconductor, it

is essential to have this large amount of reinforcing material

distributed throughout the winding. A superconductor fa- bricated with stainless steel as an integral part of the wire

would provide direct and uniform support of the supercon- ductor as well as simplify magnet construction, particularly

if the reinforcement were prefabricated with compound su- perconductors in their unreacted (ductile) state. In such a composite, however, the reinforcing material will have an

effect on the mechanical prestrain state of the superconducting material after reaction heat treatment. This is of particular interest in the A15 superconductors where the critical current is affected strongly by the presence of strain. 2,3

In an earlier experiment, a large compressive prestrain was reported in a loosely cabled Nb3 Sn conductor slipped

inside a stainless steel tube. 4 In the test, the ends of a short length of the conductor were compacted in order to “pin”

the superconducting strands to the stainless steel jacket. Pre- sumably because of this end compaction, it was found that

significant prestrain was introduced into the Nb3 Sn by ther- mal contraction of the outer stainless steel tube. The effect should be even greater in conductors where the superconductor and stainless steel are continuously bonded along

their entire length.

This paper reports the first results on the prestrain ef- fect ofstainless steel reinforcement internally bonded to multifilamentary Nb3 Sn strands. The test conductor was fabri- cated as a monolith with stainless steel reinforcement

bonded to the Nb3 Sn along its entire length through a solid copper interface. The results indicate that at 1 3 T the critical

current of the conductor is severely degraded relative to the same conductor without internal steel reinforcement.

Several reinforced Nb3 Sn superconductors were fabri- cated by combining small (0.2 mm diam) unreacted strands (Nb + bronze) with fine stainless steel strands (0. 1 mm diam)

into a copper matrix, see Fig. 1. The superconducting

strands were multifilamentary bronze-process Nb3 Sn consisting of 9072 niobium filaments embedded in a Cu-13

wt. % Sn bronze matrix. The stainless steel strands were German Mat. No. 1.4455 which is similar to AISI 316LN. A

compositional breakdown ofboth types of strands is given in Table I. Several methods were tried for combining the stainless

steel with unreacted superconducting composite strands. The most successful consisted of the following procedure.

The stainless steel and superconducting strands were first annealled in an inert atmosphere for 30 min at 1000 and

550 °C, respectively, to remove work hardening. They were

then loosely cabled together, lightly etched with nitric acid and electroplated with a ~ 0.1 -mm layer of copper. The soft copper sheath served to protect the superconducting strands

as the cable was swaged (12% reduction by area) into a solid

unit. The conductor was then lightly etched again with nitric

tor with internal stainless steel strand reinforcement.

2869 J. Appl. Phys. 54 (5), May 1983 0021 -8979/83/052869-03$02.40

94

© 1 983 American Institute of Physics 2869
TABLFI Compostion of strands

Superconductor strands Stainless steel strands

Strand diam 0.2 mm C 0.03% No filaments 9072 Cr 20.5%

Filament diameter 1 . 1 gm Fe 55.3%

Bronze |Cu-l3 wt %Sn|/Nb = 2.5:1 Mn 5.0%

Reaction temp 700 'C Mo 3.0%

Reaction time 20 h N 0.17%

Ni 16.0%

acid and finally electroplated with an additional layer of copper for stability Nb,Sn was then formed by reacting the

entire composite (superconductor + stainless steel) at 700 °C

for 20 h in an inert atmosphere of argon. Samples were fabri- cated with stainless steel fractions as high as 52% (where the copper stabilizer volume has been omitted from the stainless-steel volume fraction determination).

The critical current of several of the samples was measured as a function of uniaxial strain applied at 4.2 K using an apparatus described in Ref. 2. The measurements were earned out in a magnetic field perpendicular to the wire axis, produced by a 13 T solenoidal magnet. A critical-current criterion of 2 n V/cm was used. Figure 2 shows the strain dependence of the critical current measured for a single (uncombined) strand of superconductor. The cntical current has been normalized by its maximum (strain-free) value Jcm in order to show the relative degradation. Also, the results have been plotted as a function

of intrinsic strain defined as e0=e — em . Here em is the strain

at which the critical current is maximum and corresponds to the magnitude of compressive prestrain introduced into the superconducting material by thermal contraction of the sur- rounding matrix material. 5 This normalization has been

made to remove the effect of the compressive prestrain in comparing the data, e is the strain applied to the conductor

as a whole, where e0is the intrinsic strain actually exper- ienced by the superconducting filaments within the conductor. Negative values of the intrinsic strain 60 indicate the

FIG. 2. Relative critical current degradation Ie /Icm as a function of intrinsic strain e0 in a multifilamentary Nb3 Sn conductor containing no stainless

steel reinforcement. e„ is the compressive prestrain introduced into the su- perconductor by thermal contraction of the matrix material, e0=e — em .

2870 J. Appl. Phys., Vol. 54, No. 5, May 1 983

FIG. 3. Relative critical current degradation /c //cm as a function of intrinsic strain e0, in a multifilamentary Nb3 Sn conductor containing 33 vol %

stainless steel reinforcement. Note the large increase in em compared with

Fig. 2 and the accompanying degradation of the initial critical current val- ue.

superconducting material is under compression, while positive values indicate tension. Figure 3 shows similar data obtained on a conductor

containing 33 vol % stainless steel reinforcement. Note that the Ic /Icm vs e0 curve is exactly the same as in Fig. 2, excepi

the magnitude of the compressive prestrain em present in the reinforced conductor (0.64%) is more than double that for the unreinforced superconducting strand. The curve in Fig.

3 is simply an extension of the curve in Fig. 2 to larger compressive strain values. The shapes of these curves also agree within 5% with those previously measured on other unrein- forced commercial Nb3 Sn conductors2 and is characteristic

for all Nb3 Sn. Results obtained on another stainless-steel reinforced

superconductor containing 52 vol % stainless steel were si- milar to those shown in Fig. 3, except the amount of compressive prestrain present in the superconductor was signifi- cantly greater (0.83%). Thus the magnitude of em is a strong

function of the volume fraction of reinforcing stainless steel present.

The irreversible strain, eirrev , 6 where the conductor was

permanently damaged was also measured to be quite high

for these materials. Values of eirrev of up to 1 .87% were measured in the samples with the highest amount of stainless

steel reinforcement. However, this high value of e, rrcv is due

to the exceptionally large values of em in these samples. In terms of the intrinsic tensile strain experienced by the super- conductor, permanent fracture consistently occurred at

<r0 ~l%.

These results can be explained in terms of the differential thermal contraction between the Nb,Sn superconductor

and the stainless steel reinforcing strands. When cooled from

the reaction heat treatment temperature of 700 °C to 4 2 K

stainless steel contracts about 1.64%, 78 whereas Nb.Sn contracts only 0.75%,8 see Table II. The difference in thermal

contraction for the two materials, ~ 0.9%, is the maximum

amount of precompression that could be introduced into the Nb3 Sn by thermal contraction of the stainless steel The ac- tual compressive prestrain, em , depends on the relative

Ekin, FlOkiger, and Specking 2870

95
TABLE II. Relative thermal contraction for the indicated temperature

ranges.

700 "C to 4.2 K 20 °C to 4.2 K

Stainless steel 1.64% 0.30%

NbjSn 0.75% 0.15%

strengths and volume fractions of the two materials. In a typical nonreinforced multifilamentary Nb3 Sn

conductor, the thermal contraction of bronze (1.68%)8 is similar to that of stainless steel, but bronze alone yields at high temperatures during cooldown from ~700°C, result- ing in an em of typically only 0.2%-0.4% (the copper stabilizer has an even lower yield strength than bronze and, to

first order, its thermal contraction effect has been neglected

here).In stainless steel reinforced conductors, however, the

stainless steel strands have a much higher yield strength than

bronze. Because the steel yields very little during cooldown,

the superconductor is subjected to a much greater compressive prestrain. In the example shown in Fig. 3, 33% stainless

steel reinforcement resulted in em = 0.64%. As described in the previous section, 52% stainless steel reinforcement re- sulted in em — 0.83%. The latter conductor has an em near

the maximum (0.9%) that would be expected, based on the

difference in thermal contraction rates for stainless steel ver- sus Nb3 Sn in Table II. So, to summarize, the stainless steel reinforcement results in a significantly larger compressive

prestrain than in conventional bronze-matrix superconductors, not because its thermal contraction is any greater than

bronze (or copper), but because of its higher yield strength,

particularly at high temperatures.

The very high compressive prestrain introduced by the

stainless steel presents a significant mechanical design problem. The conductor cannot be left in this state of high pre- compression or more than 50% of the current carrying capibility of the conductor is lost, as shown in Fig. 3. On the hand

it is very hard to apply the uniaxial strain design criterion9 in

this case and simply design the magnet so the combination of

fabrication strain and magnetic forces compensate the ther- mal contraction effect. A tensile applied strain of 0.6%-

0.9% is too much compensation to safely introduce without

the possibility of a major mechanical failure in the conductor

or magnet structure. At least four possibilities exist for handling the problem

of large compressive prestrain in reinforced conductors.

(1)

Partially compensate the compression by prestraining the conductor a moderate amount during magnet fabri- cation. Only about 0.2%-0.3% strain can be compensated

by this technique, however, because of the practical tensile limits of the structural materials in the conductor. The re- maining compressive strain in the superconducting material

will still result in a reduced current-carrying capacity.

(2) Join the reinforcing material to the superconductor

at room temperature after reaction. In this manner, only an

additional 0. 15% compressive prestrain is introduced by the

stainless steel in cooling to 4.2 K (see Table II). The potential

for damage to the superconductor during joining and handling can be extreme in this case, however.

(3) Use structural reinforcement with a smaller thermal

contraction coefficient more nearly matching that of the su- perconductor.

(4) Use a superconducting material with a lower strain

sensitivity such as Nb-Ta/Sn (Ref. 10) or Nb-Hf/Sn-Ga. 1

For example, Nb-Hf/Cu-Sn-Ga conductors have been fabri- cated and tested in which the strain degradation of the criti- cal current has been measured to be about half that for commercial Nb3 Sn at 13 T." In this case the large compressive

prestrain introduced by the structural reinforcement does

not result in such a large critical current degradation, and

the added precompression acts as an extra safety margin in conductor handling during magnet fabrication. These materials are only in the development stage, but their decreased

strain sensitivity would be particularly advantageous in this situation. This work was performed while J. W. Ekin was a visit- ing scientist at the Kemforschungszentrum Karlsruhe

(KfK) from June to August 1981. Support was provided by

KfK and partially by the Naval Ship Research and Development Center, Maryland. Helpful discussions with C. Schmidt, W. Maurer, F. Arendt, P. Komarek, W. Heinz, P. Turowski, A. Nyilas, W. Schauer, N. Tonk, and A. F. Clark

during the course of this work are gratefully acknowledged.

'W. Maurer and F. Arendt (private communication).

2 J. W. Ekin, Cryogenics 20, 61 1 (1980).

3 J. W. Ekin, IEEE Trans. Magn. MAG-17, 658 (1981).

4 R. M. Scanlan, R. W. Hoard, D. N. Cornish, and J. P. Zbasnik, in Fila- mentary A l 5 Superconductors, edited by M. Suenaga and A. F. Clark (Ple- num, New York, 1980), p. 221.

5 G. Rupp, IEEE Trans. Magn. MAG-13, 792 (1977).

6

J. W. Ekin, IEEE Trans. Magn. MAG-15, 197 (1979).

7 R. J. Corruccini and J. Gniewek, Thermal Expansion of Technical Solids

at Low Temperatures, NBS Monograph 29 (National Bureau of Standards, Boulder, Colorado, 1961).

8 Y. S. Touloukian, R. K. Kirby, R. E. Taylor, and P. D. Desai, Thermophysical Properties ofMatter New York, 1975), Vol. 12, p. 1 138, 601, 790.

9

J. W. Ekin, Adv. Cryog. Eng. 24, 306 (1978).

I0 R. Fliikiger, in Filamentary A 15 Superconductors, edited by M. Suenaga and A. F. Clark (Plenum, New York, 1980), p. 299.

"J. W. Ekin, H. Sekine, and K. Tachikawa, J. Appl. Phys. 52, 6252 (1981).

2871 J. Appl. Phys., Vol. 54, No. 5, May 1983 96

Ekin, Flukiger, and Specking 2871
APPENDIX B

IELE TRANSACTIONS ON MAGNETICS. VOL. MAG-21, NO. 2, MARCH 1985 771 FURTHER INVESTIGATIONS OF THE SOLID-LIQUID REACTION

AND HIGH-FIELD CRITICAL CURRENT DENSITY IN

LIQUID-INFILTRATED Nb-Sn SUPERCONDUCTORS

M. Hong. D. M. Maher, and M. B. Ellington

AT&T Bell Laboratories, Murray Hill, NJ 07074

F. Heilman and T. H. Geballe* Department of Applied Physics, Stanford University, CA 94305

J. W. Ekin

National Bureau of Standards, Boulder, CO 80303

J. T. Holthuis Lawrence Berkeley Laboratory, Berkeley, CA 94720

ABSTRACT

Superior superconducting properties, such as high Jc's and Tc’s, have been obtained from reacted liquid-infiltrated Nb-Sn composite

wires. These excellent properties are attributed to the chemistry and structure of the material, which is prepared by a unique solid (Nb) - liquid (Sn) reaction. From heat capacity measurements,

sharp bulk superconducting transitions of the A15 phase occur at 17.2-18 K and the weight fraction of A15 in the composite wire is —23%. Analytical electron microscopy techniques have shown that: the microstructure of these conductors consists of alternating large- grain and small-grain filaments; these two types of filaments correspond to BCC Nb(Sn) and cubic A15 Nbj^jSn^* phases,

respectively; the A15 filaments (<0.5 «un) are chemically

homogeneous in terms of measured X-ray intensity ratios to within ±7%, which implies that x — 1.5; and the A15 grains are

essentially free of extended lattice disorder down to a resolution of —0.34 nm. Recent work in which Nb is alloyed with Ta has shown

that these superconducting properties can be improved upon; e.g. high overall Jc

’s of — 1.8xl 0

4 A7cmJ at 20 T and 4.2 K have been

measured. Also, the liquid-infiltrated Nb(Ta)-Sn composites have a damage strain tolerance nearly double that of commercial bronzeprocessed Nb-Sn conductors.

L INTRODUCTION

High critical current densities (Jj) in superconducting wires operating at fields of 12 T and above are always needed, especially

for applications in magnetic fusion reactors. Considerable effort has

been made to achieve this objective. Reacted liquid-infiltrated NbSn multifilamentary composite wires carry the highest Jc measured

thus far among Nb-Sn based superconductors at fields above

12 T.

1-2 Unlike the “in-situ" method, the bronze-process technique, and their variants in which A15 is formed through a solid-state diffusion mechanism, the A15 phase formation in the liquidinfiltrated wire results from a rapid solid Nb-liquid Sn reaction. The difference between these two reaction mechanisms was

discussed previously.

3

The present work was undertaken in order to extend our understanding of this solid-liquid reaction by using both

heat capacity measurements and analytical electron microscopy

(AEM). AEM techniques included selected-area diffraction, high-resolution imaging, scanning transmission electron microscopy

(STEM), and energy-dispersive x-ray spectroscopy (EDXS). The

heat capacity measurements showed that sharp bulk superconducting transitions of the A15 phase occur at 17.2-18 K. The AEM studies showed that the A15 filaments are chemically

homogeneous to within ±1.5at%Sn.

Finally, when a small amount of Ta (— 1 at%) is alloyed with the Nb, we found an enhanced B*2 , high overall Jc

’s at high fields, and

excellent mechanical properties.

Manuscript received September 10, 1984.

D. EXPERIMENT

1. MATERIAL PREPARATION

The liquid-infiltration technique is a well established technique

in powder metallurgy. The experimental procedure used to fabricate the laboratory-scale wire was described earlier.

1 The wire was in the form of a multifilamentary composite containing Nb [or Nb(Ta)l (-5.0 nm) and Sn (<0.5 »im). The Nb [or Nb(Ta)] and Sn filaments are intimately tangled with each other. The diameter

of the tested composite wire was 0.22 mm or smaller. The overall chemical composition was approximately Nb-8at%Sn (or Nblat%Ta-8at%Sn). The chemical composition can be controlled by

the starting Nb [or Nb(Ta)] powder size, the compacting pressure, and the sintering condition. The A15 phase was then formed by a reaction at 750* C or above.

For the Nb(Ta)-Sn wire, first a Nb-lat%Ta alloyed ingot was made by arc-melting pure components under a higb-purity argon

atmosphere. The starting materials were Nb and Ta of 99.9%

purity and very low oxygen content. The arc furnace was equipped

with a water-cooled nonconsumable tungsten electrode and a copper

hearth. The arc-cast ingot was homogenized at 1250"C for 24 hours. The Nb(Ta) powder was then obtained from the homogenized ingot using a hydride-dehydride technique.

2. SUPERCONDUCTING TRANSITION MEASUREMENTS

Normal-superconducting transition temperatures were measured

using resistive (electrical), inductive (magnetic), and calorimetric methods. Inductive critical temperatures were measured at a frequency of 100 Hz and an applied field of 4xlO-5T. The samples

used in the inductive Tc measurements were in a wire form They

were either cut into small pieces or wrapped helically so that several turns were contained within a 3 mm diameter. The sample chosen for heat capacity measurements was cut from

the same wire that was used for the inductive Tc and the critical current Jc measurement, as well as the AEM studies Heat capacity was measured from 1.8 to 30 K using a method developed for small sample calorimetry and described elsewhere.

7

3. ANALYTICAL ELECTRON MICROSCOPY

For AEM studies, longitudinal sections (—3 mm in length) were

obtained by mechanical polishing and ion milling The results reported here are for a reaction treatment of 96 hours at 750'C

EDXS was carried out in the scanning transition mode using a 5 nm probe, and area selection was done from annular dark-6eld images (only reacted zones which extended into the vacua were

analyzed) and considerable care was taken to ensure that individual grains were oriented into a weakly diffracting condition Nei peak

intensities (FWHM) for the Nb and Sn K-alpba lines (k.,) at 16.6 keV and 25.3 keV, respectively, were ralioed This ratio ^Nt/^Sn^k. W4S taken as a figure of merit in assessing bomogeneit)

of reacted A15 filaments

0018-9464/85/0300-0771S01 .00©198J IEEE

97
772

4. CRITICAL CURRENT MEASUREMENTS

Critical currents were obtained using an apparatus designed to simultaneously apply tensile (train, current, and a perpendicular

magnetic field to short wire samples at 4.2 K.* A critical-current

criterion of 2 mV

/

cm was used. The overall non-copper critical current density described below was determined using the combined

area of A1S and BCC phases, excluding only the copper-stabilizer area.

ID. RESULTS AND DISCUSSION

1. NORMAL-SUPERCONDUCTING TRANSITIONS

For the Nb-Sn wires, inductive Tc onsets of 17.9 K with a very sharp transition width of 0.2 K were obtained at the very initial stage of a 950 X reaction. This indicates that the solid-liquid reaction is rapid. Prolonged heat treatments up to 200 hours at 950 'C did not change either the Tc onsets or the transition width. The reaction rate is slower with heat treatments at 750"C. A

slightly lower Tc onset of 17.6 K was observed and a somewhat

wider transition width of 3.1 K occurred at the initial stage of this heat treatment. Nevertheless, aging for 24 hours increased the Tc to 17.9 K and narrowed the transition width. This may be due to an improvement of the ordering parameter within the A15 phase.

For the Nb(Ta)-Sn wires, inductive Tc onsets of 18.1-18.2 K

along with a very sharp transition width of <0.3 K were observed

with reaction at 800-950 ‘C. The measured Tc onsets are (lightly higher than those of the reacted binary Nb-Sn composite wires. Heat capacity data for a sample which was heat treated at 950 ’C for 3 hours and weighed 7.6 mg are shown in Fig. 1. The yaxis units are mJ/g-atom-K2 where 1 g-atom of Nb-8at% Sn weighs

94.96 grams. The superconducting transitions of the BCC and A15

phases occur at 6. 7-7.0 K and 17.7-18 K, respectively. The

detailed analysis of the data has been given elsewhere.9 The

amount of A15 in the sample was estimated to be 1.76 mg or 23 wt%. For the area of high transitions, the discontinuity between

17.7 K and 18 K represents 1.2 mg which is approximately 70% of the presumed A15 phase in the sample. Extrapolating the data to an ideally sharp transition, as shown by the solid line in Fig. 1 , gives

1.7 mg of A 15, the same value as found above, but with a Tc from

17.2 to 18.0 K. This transition is still quite sharp and, using Tc versus at %Sn data, 10 indicates less than a 0.5at% variation in the Sn concentration within the A15 phase.

Fig. 1. Heat capacity data for a liquid-infiltrated Nb-at%Sn wire

reacted at 950 ‘C for 3 hours.

By contrast, the discontinuity at 7 K only represents 1.7 mg or 30% of the presumed BCC phase. The transition temperature of the remaining BCC phase is smeared out below 6.7 K. This wide

transition width indicates a variation in the amount of Sn dissolved

in the Nb. This variation of Sn concentration in the Nb was also found earlier using STEM/EDXS.11

This sharp superconducting transition of the A15 phase, which

forms by reacting liquid-infiltrated "Nb-Sp composite wire and is observed by heat capacity measurements, is in strong contrast to the

transitions which are observed for in-situ and bronze-processed NbSn conductors. Heat capacity measurements exhibited small and

very broad superconducting transitions for these two conductors. 12,13

It should be borne in mind that the A15 phase in the latter conductors forms through a solid-state diffusion reaction while the A15 formation in the liquid-infiltrated conductors is due to a rapid

solid-liquid reaction. It is clear that the solid-liquid reaction gives a uniform composition across the A15 while significant compositional

variations result from the solid-state diffusion reaction. Data in Fig. 1 indicate that the heat capacity technique is, indeed, a proper method to measure the exact amount of A1 5 in the

wire. This is the first time that a 23 wt% of A15 phase has been

detected in a reacted liquid-infiltrated Nb-Sn wire. Since an overall Jc of nearly 2xl04 A/cm2 at 20 T and 4.2 K has been achieved in this wire,2 the true critical current density carried by the A1 5 region

may be near 105 A/cm2 at 20T and 4.2 K. It is also suggested that

this heat capacity measurement may be used to study other multifilamentary A15 superconducting wires when the information on the amount of A15 is needed.

2. ANALYTICAL ELECTRON MICROSCOPY

The microstructure of a liquid -infiltrated Nb-Sn composite which was reacted for 96 hours at 750 X has been characterized by

conventional transmission electron microscopy4 and high-resolution electron microscopy.5 The basic microstructure of the composite

consists of alternating large-grain and small-grain filaments (sec

Fig. 2). The small-grain filaments are typically 0.3 to 0.5 mid in width and several tens of micrometers in length. Selected-area

diffraction patterns (spatial resolution —1 nm diameter) which were

obtained from areas similar to that shown in Fig. 2 could be indexed

self-consistently in terms of a BCC Nb phase and a cubic A15

phase G.e. Nt^^xSoj^x)- If the Nb lines were taken as the reference for obtaining the camera constant of the microscope, then each pattern up to 14 d-spacings was consistent with those expected from the A15 phase Nb75Sn2j. From the point of view of extended lattice defects the two

filaments were quite different. The BCC Nb filaments contained a

very high density of dislocation arrays, mostly comprised of small

dislocation loops (see right-hand filament in Fig. 2), whereas the A15 filaments consisted of a heterogeneous population of nearly

perfect grains ranging in size from —10 nm to —0.2 Min. A

significantly high percentage of the grain boundaries exhibited image contrast behavior typical of low-angle boundaries, and no

evidence for inclusions was observed either within grains or at grain boundaries. Moreover no dislocation contrast was observed within

the grains. The degree of perfection was further confirmed from

high-resolution images, which were obtained,, from individual grains and across adjacent grains. An example is shown in Fig. 3, where

the (110) lattice spacings are resolved and no contrast features that are above the point-to-point resolution limit of the microscope (0.34 nm) can be attributed to lattice disorder.

Energy-dispersive X-ray spectroscopy was used to assess the chemical homogeneity of these reacted A15 filaments. The

measured intensity ratio (In^/IsdIk.’ wl> ere I is the net integrated

intensity from the K^, lines of Nb and Sn, was taken as a figure of merit and is denoted here as x Spectra were obtained only from

thin, isolated filaments (<100 nm thick) which protruded into the vacuum. Typically three spectra were recorded “across” a filament and the number of sets recorded “along” the filament varied

98
773

Fig 2. Bright-field image illustrating the filamentary nature of

reacted liquid-infiltrated Nb-Sn. The A15 phase ( —-0.35 nm wide)

is bounded on both sides by the BCC phase.

Fig. 3. A high-resolution image from an individual grain within an A15 filament where only the <110> Bragg beams

(d no - 0.374 nm) and zero-order beam were included in the aperture and the image was taken near the Scherzer defocus.

depending on the extent to which the filament protruded into the vacuum. Results from 60 spectra (i.e. 5 filaments) gave X “ 5.5 ± 0.4 which is a variation of ±7%. If the following

assumptions are made: i) CNb + CSn — 100, where C is in at %; and

ii) CNb/Cs„ — KNbSn x. where K is the k ratio;

5 then the observed

variation in x can be expressed as a variation in composition for any

value of CNb. For CNb in the range 72 to 76at% the result is ±1.5at%. Clearly this degree of homogeneity is consistent with the sharp superconducting transitions which are observed in the heat

capacity data at 17.2 to 18 K.

3. Jc (t,H) OF Nb(Ta)-Sn WIRES

Figure 4 shows the strain dependence of the critical current of

the Nb(Ta)-Sn wires at fields from 10 to 23 T. Jc passes through a maximum at a strain ^ of 0.13%. The parameter indicates the

strain at which the critical current is a maximum. This is quite low compared with conventional bronze-processed conductors where t,,, is typically between 0.2 and 0.4%. The low value of is the result of

the absence of a bronze matrix in liquid-infiltrated conductors; thermal contraction of the bronze-matrix material is primarily

responsible for the compressive prestrain introduced in the reacted A15 regions of conventional bronze-processed conductors. From Fig. 4, the irreversible strain limit for this conductor

is more than 1.1%. Note that the indicates where the superconducting composite wire is permanently damaged. This

corresponds to an intrinsic irreversible strain limit “ tuTtv ~ *ir of about 0.9%, which is nearly double that for bronze-processed conductors where is consistently about 0.5% regardless of the value of Correspondingly, the bending strain limits on the mechanical design of high-field magnets are also about double those of bronze process conductors.

A comparison of peak Jc values eliminates the effect of varying

compressive prestrain among different conductors. Figure 4 shows

that the critical current density of the liquid-infiltrated conductor is more than three times higher than that of bronze-processed conductors at low fields and more than an order of magnitude

higher at high fields. The overall critical current density is about 1.8xl04 A/cm2 at 20 T, which is high enough to consider the

feasibility of Nb-Sn superconducting magnets which will be used at or above 20 T. The extremely high overall Jc at high fields is the

result of the combination of a unique A15 microstructure and a uniform stoichiometric Sn concentration across the A15 region, as thown in the AEM analyses and the heat capacity -measurement of the present work. Detailed discussion on the mechanical properties of this wire was given elsewhere. 14

In this work the Ta concentration of lat% may not be optimal From recent work of Schwall et a!.,

15 a higher concentration of Ta.

e.g. 3-5at%, should produce better properties Research along this line is in progress.

IV. SUMMARY

We have used heat capacity measurement and AEM techniques

to study the solid (Nb) - liquid (Sn) reaction in multifilamentary

liquid-infiltrated Nb-Sn superconducting wires A uniform

stoichiometric chemical composition of Nb-25at%Sn was found at the reacted A15 regions from both the heat capacity and the AEM

studies. Furthermore, the beat capacity data indicate that the weight fraction of the A15 in the reacted wire is —23‘t

Correspondingly the volume fraction is —21$ This value is slightly lower than the A15 volume fraction in bronze-processed Nb-Sn

wire, which is usually around 30%

99
774

Fig. 4. Overall critical current density (J^ of liquid-infiltrated (Nb-1 at%Ta)-8at%Sn wire as functions of magnetic fields and

uniaxial strain at 4.2 K.

The rapid solid-liquid reaction, in which the Nb predominantly

diffuses into the liquid Sn, produces A15 grains with an equiaxial

ahape. Moreover, from the lattice-imaging studies, these A15 grains are essentially defect-free down to a resolution of 0.34 nm.

ACKNOWLEDGEMENTS

We thank S. Nakahara of AT&T Bell Laboratories for valuable

discussions. The Jc (t,H) data were obtained at the Francis Bitter National Magnet Laboratory which is supported by the National Science Foundation. The work performed at Stanford University was supported by Air Force Office of Scientific Research under

contract F 49620-83-C-0014. The work pefformed at the National Bureau of Standards was supported by the Naval Ship Research and Development Center and the Department of Energy, Office of Fusion Energy under interagency agreement #DE-AI01-

84ER521 13.

• Also with Bell Communication Research, Inc., Murray Hill, New Jersey 07974

REFERENCES

[1] M. Hong, G. W. Hull, Jr., J. T. Holthuis, W. V. Hassenzahl, and J. M. Hong, Appl. Phys. Lett. 42, 621 (1983).

[2] M. Hong, G. W. Hull, Jr., E. O. Fuchs, and J. T. Holthuis, Mater. Lett. 2, 165 (1983).

[3] M. Hong, G. W. Hull, Jr., J. T. Holthuis, W. V. Hassenzahl, and J. W. Ekin, IEEE Trans. Magn. MAG-19, 912 (1983).

[4] M. H. Lovetto and R. E. Smailman, “Defect Analysis in Electron Microscopy,” Science Paperbacks, Chapman and

Hall, London (1975).

15] J. C. H. Spence, “Experimental High Resolution Electron Microscopy,” Clarendon Press, Oxford (1981).

[6] N. J. Zaluzec, “Introduction to Analytical Electron Microscopy,” Eds., J. J. Hren, J. I. Goldstein, and D. C. Joy, Plenum Press, New York, pp. 121-167 (1979).

[7] S. R. Early, F. Heilman, J. Marshall, and T. H. Geballe, Physica 107B, 327 (1981). We have also found that lat%Ta alloyed with Nb has further enhanced the superconducting properties such as Jc

’s and the mechanical strain tolerance. These improvements are believed due

to the increase of the bulk upper critical fields. Further

optimization in terms of the Ta concentration is now in progress. Another important aspect of the research on the liquid-infiltrated Nb-Sn wire is the feasibility for an industrial scale-up. We have

discussed this point in the Appendix.

V. APPENDIX

SCALE UP

Previously, we were able to fabricate the Nb-Sn liquid-infiltrated wire of 0.13 mm diameter from a starting composite rod of 5 mm

diameter. In order to test the feasibility of the industrial scale up of

this process, in this work, a starting billet 13 mm in diameter was drawn down to a wire of 0.13 mm in diameter. An area reduction of 104 has been achieved. No intermediate annealing was applied

during the wire-drawing process. The overall Jc

’s carried by the wire with the heavy area reduction is similar to what we have

previously reported. Currently, research along the line is two-fold: a

larger starting billet of 25 mm in diameter and a rebundling

technique. The results will be reported in the near future.

[8]

J. W. Ekin, Cryogenics 20, 61 1 (1980).

19] M. Hong, F. Heilman, T. H. Geballe, and G. W. Hull, Jr., to be published in Appl. Phys. Lett.

[10] H. Devantay, J. L. Jorda, M. Decroux, J. Muller, and R.

Flukiger, J. Mater. Sci. 16, 2145 (1981).

[11] D. M. Maher, M. Hong, M. B. Ellington, D. L. Stubbs, T. H. Geballe, and G. W. HuU, Jr., Bull. Am. Phys. Soc. 28, 261 (1983).

[12] H. L. Gegel, D. C. Yeh, J. T. Pajor, and J. C. Ho, IEEE

Trans. Magn. MAG-13, 788 (1977).

[13] S. S. Eucker, J. D. Verhoeven, and D. K. Finnemore,

J. Appl. Pbys. 54, 1471 (1983).

[14] J. W. Ekin and M. Hong, Appl. Phys. Lett. 45, 297 (1984).

[15] R. E. Schwall, G. M. Ozeryansky, S. Foner, and E. J. McNiff, Jr., J. Appl. Phys. 56, 814 (1984).

100
APPENDIX C

Summary Foreign Trip Report

December 4-14, 1984

Fukuoka, Japan

Miyazaki, Japan

Tsukuba, Japan

Tokai, Japan

Sendai, Japan

by

John W. Ekin

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Boulder, Colorado 80303

Submitted to the Office of Energy Research

Department of Energy

Washington, D.C. 20545

January 15, 1985

Report No. 500

101
January 15, 1984

SUMMARY FOREIGN TRIP REPORT

From: John W. Ekin

Physicist

Superconductors and Magnetic Materials Group, 724.05

Locations Visited: Fukuoka, Japan 5-6 December 1984

7-8 December 1984

9-11 December 1984

Miyazaki, Japan

Tsukuba, Japan

Tokai, Japan 12 December 1984

Sendai, Japan 13-14 December 1984

Purpose of Trip :

1) To obtain information on new superconductor materials being developed in

Japan needed for several NBS, NBS/DoE, research programs on superconductors

2) To reach agreement on an exchange of superconductor samples for electromechanical testing and an exchange of reference materials for development

of international standards for critical-current testing.

3) To present an invited talk on electromechanical properties of high-field

superconductors at the Third U.S. /Japan Workshop on High-Field Superconducting Materials for Fusion.

Trip Summary :

KYUSHU UNIVERSITY (Fukuoka, Japan)

1) The Faculty of Engineering is a very active group with a strong emphasis

on theory of superconductors. I obtained much information pertinent to our own electromechanical research effort here.

2) Research areas include: Flux pinning and basic electromagnetic properties (Profs. Matsushita and Yamafuji), electromagnetic properties of

wires and conductors (Profs. Sumiyoshi and Yamafuji), stability and

safety of wires and conductors (Profs. Funaki and Irie), stability and

safety of magnets (Profs. Takeo and Irie), and Josephson junctions

(Profs. Yoshida and Irie).

3) Recent topics include: Flux pinning in Chevrel superconductors (Profs.

Matsushita and Yamafuji), longitudinal field effects on different pin

types (Profs. Matsushita and Yamafuji), powder metallurgy wire (Prof.

Sumiyoshi), powder metallurgy wire for ac use (Prof. Sumiyoshi), ac loss

of wires and cables (Prof. Sumiyoshi), transmission of normal parts

(Prof. Funaki), Josephson ac oscillator (Prof. Irie), and magnet fabrication research (Prof. Irie).

102
4) Prof. Irie, who is quite famous for his theories on pinning, will be

retiring at the end of next year. Kyushu University intends to hold a world

conference on superconductor pinning in his honor. We discussed some details of the conference which will probably be held in November of 1985.

5) Prof. Matsushita predicts that there will be a different power dependence of

the pinning term in the strain-scaling law for irradiated superconductors.

We discussed a collaborative experiment to test this and are now pursuing

the preliminary phases.

6) Prof. Matsushita thinks that powder processed wire will ultimately be the

best conductor for pulsed magnets because of the very fine filament size

(and hence low hysteretic losses) that can be achieved. Twenty percent Kb

can be introduced into the conductor before f ilament-to-f ilanent contact is

achieved, compared to only ten percent Nb for in-situ superconductors.

JAPANESE NATIONAL RAILWAY "MAGLEV" TEST SITE (Miyazaki, Japan)

1) They have achieved speeds of 500 km/hr in an unmanned test vehicle and 300

km/hr in a manned test vehicle.

2) Currently, tests are being conducted in three different areas: stress and

vibration, damping, and aerodynamic shapes.

3) They are also developing on-board liquid-helium refrigerators.

4) One of the major problems for the utilization of this technology in Japan is

the mountainous terrain. The cost of tunneling would make it prohibitively

expensive. Consequently there are no near-term plans to build commercial

MAGLEV tracks in Japan. A superconducting MAGLEV link from Tokyo to Narita

airport had been planned but recently has been ruled out. They think there

are possibilities for commercialization in the U.S. and Germany.

JAPAN-U.S. WORKSHOP ON RIGH-FIELD SUPERCONDUCTING MATERIALS FOR FUSION

(Tsukuba, Japan)

Eight scientists were invited from the U.S. and about 40 research scientists

attended from Japanese universities, government laboratories, and industry.

During the last four years there has been a rapid growth in both basic

and applied superconductor research programs in Japan. Basic superconductor

research is now conducted at many research laboratories in Japan, and

Japanese industry has built up the engineering know-how and manufacturing

processing for a large variety of NbTi, Nb^Sn and other materials. The

Japanese have demonstrated that large Nb^Sn coils can be fabricated commercially. In short, superconductor research for fusion has been targeted for

103
major support by the Japanese government, and their programs row appear to

have reached, and in some area, passed ours in both scope and depth.

Agreement was reached at the workshop to broaden the exchange in superconductor research. Emphasis at first will be given to exchanging laboratory

reference sample materials for testing by both countries and for joint

development of standard testing procedures and standard reference materials.

Arrangements were made for collaborative studies of the electromechanical

properties of a number of new experimental conductors being developed in

Japan.

Specific highlights include:

1) In the area of critical current measurements, I emphasized that the

current practice of intercomparing commercial superconductors in

terms of their prestrain critical current density can be very

misleading. Comparisons should be made in terms of the strain-free

critical current, especially at high fields (10 tesla and above).

Other problems emphasized at the conference included exactly those

problems we are working on: critical-current characterizations in

terms of the power law dependence of the voltage-current curve,

field-angle dependences, current-transfer effects, and sausaging

effects

2) Radiation — two radiation limits were discussed which will affect

the choice of materials for fusion applications:

a) The waiting time after reactor shut down before maintenance can begin. Prof. T. Okada presented some data on superconductor

performance in this regard. The elements Sn, Ta, Hf, Mo, Ga

,

Cr have the longest waiting time and should be avoided! Ti and

V were the best performers. Conclusion — Nb^Sn and V Ga

present serious design problems in fusion applications! On the

other hand, Nb^Al and NbTi are good choices. NbN should also

be good, but no data were available. Another conclusion is

that Ta, Hf, and Ga should be avoided as additives to Nb^Sn in

favor of Ti.

b) The operating time before significant radiation damage occurs

to the superconductor. Prof. T. Okada and Dr. E. Dalder

presented some preliminary data from the literature on radiation degradation of critical current density. Nb^Sn was among

the worst conductors. No data on the B1 or C15 superconductors

were presented. This appears to me to be an area where we have

insufficient information on which to base any decisions.

The first problem is fundamental and may well dictate which superconductors are ultimately selected for fusion magnets. The second

of these two problems is more amenable to solving by providing

enough radiation shielding for the superconductor. The problem is

to know how much is enough.

104
3) The irreversible strain damage limit can be increased sixty percent

by adding Ti to the bronze matrix of Nb^Sn conductors. It can be

doubled in commercial Nb^Sn conductors by a hot-isostatic-pressure

(HIP) treatment.

4) In-situ V^Ga and V^Ga-tape conductors are being developed commer- cially in Japan because they offer the possibility to achieve

significantly higher magnetic fields than Nb^Sn conductors, even Nb^Sn with additives.

5) Arrangements were made for collaboration on a study of electromechanical properties at high fields of a number of new experimental

conductors being developed in Japan. The following organizations

agreed to send superconductor specimens for strain effect testing by

NBS

Tohoku University T. Anayama Standard Nb^Sn

NRIM* Y. Tanaka, K. Tachikawa Nb

3

Ge

NRIM* K. Togano, K. Tachikawa Nb

3 (Al,Ge)

Kobe Steel T. Horiuchi HIP of Ti added

Nb

3

Sn

Hitachi-NRIM K. Kamata, K. Tachikawa Ti-in-bronze

conductor

Univ. of Nagaoka K. Hamasaki, T. Yamashita PbMo^Sg with

minimum Mo

Furukawa Y. Tanaka In-situ V

T 3

Ga

Toshiba S. Murase Internal Sn

Nb

3

Sn

Vacuum Metallurgical Co. T. Noguchi In-situ \

7 3 Ga

*NRIM is the Japanese National Research Institute for Metals

JAPANESE ATOMIC ENERGY RESEARCH INSTITUTE (Tokai, Japan)

Privately, S. Shimamoto commented to me that Japan has considerable

concern about the possibility of insulation failure in wind-and-react magnet

construction, which would result in coil melt-down in the shorted region.

This is the reason for their strong preference for react-and-wind construction

of magnets for fusion. It permits much more reliable coil insulation since

the insulation does not have to be subjected to a high-temperature heat

treatment

Shimamoto also commented that for superconductor coils larger thar. Large

Coil Task (LCT) magnets, he likes the Internal Cable in Conduit Superconductor

(react and wind), not because of stability reasons, but for the distributed

reinforcement feature to handle the very large forces.

105
TOHOKU UNIVERSITY (Sendai, Japan)

Since my visit last March they now have their 23-tesla hybrid magnet

operating. A 15-tesla copper poly-helix magnet has also been in regular

operation. The 30-tesla hybrid magnet is still under construction.

A. Nagata is very much interested in setting up to measure uniaxial

strain effects on the critical current of superconductors using their highfield magnet facilities.

Benefits

Information obtained from these visits and the workshop will provide

direction to our work on electromechanical properties of superconductors for

fusion-magnet development.

The agreement reached to exchange superconductor standard reference

materials is necessary to the establishment of international standards in this

new field.

The collaborative agreements for electromechanical property testing were made on experimental Japanese conductors which are not being fabricated in the

U.S., or represent a modification of U.S. made superconductors. An evaluation

of their performance will help U.S. programs by guiding our own efforts toward

those superconductors having the best performance or promise of performance,

eliminating wasted U.S. effort on development of lower performance conductor

types and providing the needed data base to develop advanced high-field

superconductors in the U.S.

106
NBS-I I4A

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR-86/3Q44

2. Performing Organ. Report No. 3. Publ ication Date

March 1986

4 . TITLE AND SUBTITLE

1 Li t I KOMLCHANICAL properties of superconductors for doe fusion applications

5. AUTHOR(S)

J. W. Ekin, J. Moreland, and J. C. Brauch

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

Department of Energy

Office of Fusion Energy

Washington, DC 20545

10. SUPPLEMENTARY NOTES

] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) This is an interim report presenting data on superconductor performance under

mechanical load, which are needed for the selection of superconductors and the mechanical design of superconducting magnets for DOE fusion energy systems. A further aim of

the reported research is to measure and understand the electromechanical properties of

promising new superconductor materials with strong application potential at high magnetic fields. Results include the following. The first strain vs. critical-current

studies were made on a Chevrel-phase superconductor, PbMo S . Chevrel-phase superconductors were found to have a large strain effect, comparable in magnitude to A-15

superconductors like Nb Sn. Electromechanical-property measurements of an experimental

liquid-tin- inf iltrated Nb Sn conductor showed it to have an irreversible strain limit

twice as large as bronze-process superconductors and a significantly higher overall

critical-current density; the liquid-infiltration process thus has the potential for

development of a practical Nb Sn conductors with both superior critical-current density

and extremely good mechanical properties. Electromechanical parameters were obtained

on several Nb Sn conductors that are candidate materials for superconducting fusion

magnets, including conductors fabricted by the bronze, internal-tin, and jelly-roll

processes. Thermal contraction data are reported on several new structural materials

for superconductor sheathing and reinforcement, and a new diagnostic tool for probing

the energy gap of practical superconductors has been developed using electron tunneling

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon i > electromechanical; electron tunneling; energy gap, fusion; ICCS superconductor; liquidinfiltration superconductoi ; Nb^Sn; PbMo^Sg;

uniaxial stress effect

stress effect; transverse stress effect;

13.

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