DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Annual Report

FY 79

F.R. Fickett and A.F. Clark

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

DECEMBER 1979

—QC

I 100

.U56

80-1629

ly/y

Work performed under Department of Energy contracts

PR01-79ET52052, PR02-79ET26603, PR01-79ER 10354

and Massachusvitts Institute of Technology contract POML-112876

NBSIR 80-1629

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DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Annual Report

FY 79

F.R. Fickett and A.F. Clark

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

DECEMBER 1979

Work performed under Department of Energy contracts

PR01-79ET52052. PR02-79ET26603, PR01-79ER10354

and Massachusetts Institute of Technology contract POML-112876

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

COMTENTS

Page

Abstract v

I. INTROnUCTIOM 1

II. EVALUATION OF CURRENT STATUS 5

A. Survey Results 5

B. Round Robin Testing 6

III. PREPARATION AND DISSEMINATION OF STANDARDS 8

A. Standardization of Terminology 8

B. ASTM Committee 9

C. Suggested I(< Test Specifications 11

IV. EXPERIMENTAL PROGRAM AT NBS 12

A. Critical Current Studies 13

B. AC Loss Studies 14

C. Critical Temperature Studies 20

D. Supporting Research at NBS 26

V. CONTRACTOR PROGRAMS 27

A. Airco Superconductors 27

B. Intermagnetics General Corporation 28

C. Magnetic Corporation of America 28

D. Supercon, Inc 28

VI. REFERENCES 30

i i i

APPENDICES
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DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

F. R. Fickett and A. F. Clark

Thermophysical Properties Division

National Bureau of Standards

Boulder, Colorado 80303

Practical superconductors are complex materials and the determination of

the parameters required for designing with them is a difficult task. Many

approaches are possible for determining a given parameter and the results

depend critically on which one is chosen. The goal of this program is to

arrive at a set of useful voluntary standards for measurements on modern

practical superconductors that will be acceptable to both manufacturers and

users. Agreement on a set of standard definitions for the various parameters

is also necessary. This report describes the status of the program and

includes a brief historical introduction. The need for standards in this area

is described in detail with particular attention paid to the need for consensus

among all interested parties and our techniques for achieving it. Results from

the experimental research by NBS and a review of the wire manufacturers'

programs are presented.

Key words: Critical current; critical temperature; losses; magnetic property;

standards; superconductor.

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• V A
I. INTRODUCTION

The superconductor standards program is a cooperative effort funded by

MBS and four divisions of DOE (Energy Storage, Fusion Energy, High Energy Physics, and Magnetohydrodynamics through the Francis Bitter National Magnet Laboratory). The goal of the program is to arrive at a set of voluntary standards

for modern practical superconductors that will be acceptable to both manufacturers and users. The need for such a set of standards increases as more and

more large superconducting magnet systems are designed and constructed.

The basis for the program was set several years ago at meetings called by

MBS at The ASM Conference on the Manufacture of Superconducting Materials and

the Applied Superconductivity Conference in 1976. The manufacturers, users,

and researchers present all made extensive suggestions as to how the work

should proceed. In the years that followed a small program was initiated, with

NBS funding, to make a more formal survey of the needs and desires of the

research community. From this study and several related meetings, the

following conclusions were drawn:

For all concerned, standards were both necessary and desirable.

The small size and financial position of the wire manufacturing industry

(and its competitive nature) precluded industrial development of standards

in a reasonable period of time.

The NBS Cryogenics Division (now Thermophysical 'Properties Division) was

an "unbiased third party" with the charter, the desire, and the expertise

to carry out a superconductor standards orogram of a sufficient size that

significant progress could be made in a time span of several years.

The full program was started in mid-1979.
The development and promulgation of standards can be a very sensitive

issue, for sound financial reasons. Because of this, the program relies

heavily on continuing interactions between all interested parties to assure

that, as far as possible, a consensus will be developed on any proposed

standard. To this end a portion of the funding is subcontracted to each of the

U.S. wire manufacturers to promote development of their research capability, to

provide us with needed data, and to provide a source of funding for their

participation in work associated with test development. Also, a new ASTM

subcommittee on superconductors (ASTM B1.08) was formed early in 1979 with

excellent participation from manufacturers, funding agencies, and the national

laboratories.

An outline of the approach that we are attempting to follow in the

determination of the standards is provided by the diagram of Fig. 1. The flow

is self-explanatory but note that users and producers are involved in all

aspects of the development. They provide input on current practices, perform

round robin tests, make trials of new concepts and devices, and critioue the

final product. Furthermore, their active participation in the standards

committees assures that they will have a direct hand in preparation of the

final product.

The term "standards" as used in this report may indicate any or all of

four quite different aspects of standardization:

1. Unambiguous definition of terminology,

2. Detailed description of measurement technique,

3. Development of common experimental apparatus,

4. Preparation and characterization of reference materials.

2
APPRO

STANDARD

(ASTMJE

VAL BY

IS GROUPS

:EE, etc.)

VOLUNTARY STANDARD

Figure 1. Diagram of the steps involved in production of voluntary standards.

3
The program will touch on each of these as they relate to the parameters of

most common interest in the measurement of superconductor properties. These

parameters are:

1. Critical current,

2. Critical temperature,

3. Critical field,

4. Matrix properties, such as resistivity and stabilization

5. Transient losses, due to eddy currents and hysteresis.

The work reported here concentrates on the first two critical parameters, and

more heavily on the critical current, since that measurement is the one most

commonly used ^or characterization of practical conductors. In subsequent

years, the other properties will be added to the program.

In the following sections we discuss our evaluation of the current status

of measurement practices in the U.S. and elsewhere, explain in detail the

standards development and dissemination efforts, and describe the experimental

research program being carried out in support of this effort by NBS and the

manufacturers of practical superconductors . The program was started rather

late in the year and, as a result, progress on many of the experimental

projects during FY 79 has been limited to construction and testing of

apparatus. As of this writing (12/79) all projects are taking data and the

research is progressing well.

4
II. EVALUATION OF CURRENT STATUS

One of the most important parts of the program in these early stages is to

determine in some detail what techniques and definitions are now being used for

the evaluation of practical superconductors . Furthermore, we would like to

assess the comparability of the techniques used by the various groups. The

first of these goals has been accomplished by surveys and visits and the second

is being done by a round robin test program.

A. Survey Results

After discussions with manufacturers and users we developed a comprehensive questionnaire to be used in evaluating measurement techniques. The complete questionnaire is given in Appendix A. Many of the National Laboratories

and all of the manufacturers of wire have been contacted either by phone or

personal visits and their responses obtained. In general, the techniques now

in use for critical current measurement are very simple, usually involving a

hairpin geometry for the sample that allows insertion into a solenoidal magnet.

All measurements are made at a temperature of 4K. There is no general criterion used for the voltage that defines the critical current. Methods used to

constrain the samples vary widely and, as we will see below, may have a serious

impact on the measurement. Most of the user laboratories tend to rely on the

critical current data provided by the manufacturer; few in-house tests are

made. Other parameters, such as critical temperature and critical field are

seldom measured. Parameters relating to stability, such as matrix resistivity,

self field losses and current transfer effects are not measured regularly, nor

are the interactions between these effects and the measured critical current

5
often taken into account. One should point out that these are now usually

relatively minor, but will become of increasing importance as the size, and

perhaps complexity, of the conductors increases.

We are evaluating the survey responses in hand and collecting more as the

opportunities arise. If the final collection appears to be of sufficiently

wide interest, it will be published as an NBS report.

A similar survey of measurement techniques in Japan was performed by Dr.

G. Fujii, a guest worker at NBS from the Institute of Solid State Physics,

University of Tokyo. The results have been compiled in a report [1] that will

be published by Dr. Fujii. Since much of the extensive Japanese technology

concentrates on high field materials, especially V

3 Ga, this report makes an

excellent complement to our own survey.

B. Round Robin Testing

Round robin testing is a time-honored method for evaluating the status of

measurement systems. In our program, samples of three or four practical

superconductors are sent to various laboratories for determination of their

critical current. The materials will also be measured at NBS. Comparison of

the results (early in 1980) should indicate the extent of the problem that we

are addressing in the overall program. The materials being measured in the

test are described in Table 1. We are contemplating the addition of another

mul tifil amentary Nb3 Sn wire. Measurements are being made by all of the U.S.

wire manufacturers. We may try to extend the program to some of the national

laboratories in the next year.

Discussions are being held to decide if a round robin program should be

undertaken with the critical temperature and critical field measurements. The

6
number of laboratories involved would be very small, but their measurement

techniques are quite different.

Table 1. Superconductors for NBS Round Robin Test

Sample 1

Mul tifil amentary NbTi.

Length: 1 m Cross Section: ~0.53 x 0.68 mm

Approx. 0 lOT: 115 A

0 4T: 285 A

Sample 2

Nb3 Sn Tape Conductor (Reacted).

Length: ~0.5 m

Cross Section: 2.3 x 0.2 mm

Approx. 0 lOT: 65 A

0 4T: 175 A

Sample 3 (To be provided in December)

Nb3 Sn Mul tifil amentary Conductor (Reacted).

Length: ~1 ft. or as requested

Cross Section: 0.7 mm diameter

Approx. !(. 0 12T: 50 A

0 7T: 150 A

Round robin testing is, of course, best accomplished with carefully char

acterized materials - standard reference materials. The acquisition and char

acterization of such materials is a major effort that we plan to undertake

later in the program. As a first step we have made an informal inventory of

available materials at various manufacturers. It appears that adequate NbTi

for such a program is available, but that Mb 3 Sn would present a problem

today.

7
III. PREPARATION AND DISSEMINATION OF STANDARDS

To assure the success of our program, it is essential that we interact

with all of the appropriate elements of the standards community. This process

has already been described briefly. Here we present some details of our

approach. The first order of business is to reach agreement on the terminology

for practical superconductors . The involvement of the standards groups such as

^SWi and IEEE is also necessary. The major detailed input to these groups must

come from this program and an example of how this is accomplished is provided

by our interactions on the critical current measurement techniques.

A. Standardization of Terminology

In an effort to develop a uniform terminology, an extensive compendium of

existing terminology was made. From this review a set of standard definitions

was prepared and informally criticized by more than 50 workers in the field

both in this country and abroad. The result was four papers containing

proposed definitions under the titles:

1. Fundamental states and flux phenomena [2]

2. Critical parameters [3]

3. Fabrication, stabilization and transient losses [4]

4. Josephson phenomena [5].

The first three are now in print and the fourth will be published early in

1980. Each of the papers solicits responses from the readers and, in addition,

the ASTM committee described below is now reviewing many of the terms. Ultimately, all of the definitions will be collected in a NBS document that will

serve as a guide for their application.

8
Lest the reader think that definitions are a minor problem, consider the

situation illustrated in Fiq. 2. Here is shown the effect of stress on the

critical current of a commercial high field superconductor measured at NBS [6].

The separate curves indicate the critical current behavior one observes using

the indicated criterion or definition for critical current. Note that not only

the magnitude, but also the shape of the curve is affected by the choice. All

of the definitions shown have been used at one time or another. (Our studies

have led us to conclude that the use of either the electric field criterion or

the resistivity criterion will provide a maximum of information with minimum

effort.

)

B. ASTM Committee

At the instigation of NBS, an ASTM Subcommittee on superconductors

(B01.08) was formed under the Conductor committee. The organizational meeting

was held on June 12, 1979 at ASTM headquarters. It was attended by more than

25 representatives from industry, government, and the national laboratories.

A. F. Clark was appointed to the chairmanship. The Subcommittee was divided

into six task groups: Definitions; Critical current; Physical and mechanical

properties; ac losses; Critical temperature and critical field; and Stability.

Members of the first four of these were chosen and specific tasks determined.

The minutes of the meeting are included here in Appendix B. Since the

organizational meeting, membership in the subcommittee has increased and now is

near 50. Informal sessions were held at the Cryogenic Engineering Conference

in August. The critical current task group chairman appointed three teams to

prepare a first draft of an ASTM standard for the three common methods of I(.

measurement. The next full meeting of the subcommittee is planned for the

9
STRESS (10®Pa)

Fiqure 2 . The critical current of a sinale Mh 3 Sn mul tifil amentary conductor

as a function of applied stress for various critical current criteri a

.

10
spring of 1980 with major reports due from the Definitions and Critical current

task groups.

The enthusiastic response of the entire community to this subcommittee has

been most heartening. The willingness of the members to volunteer time for the

various projects is greatly appreciated.

C. Suggested 1^ Test Specifications

To assist the critical current task group of the ASTM Committee, we have

developed a comprehensive list of proposed test specifications for critical

current measurement. This document, reproduced here as Appendix C, gives: the

variables that might be specified by the user; the control parameters on the

measurement; a series of operational checks for a valid test; and test

apparatus design considerations. The list has been sent to the committee and

to the manufacturers for their comments. Clearly, to follow each of the steps

in a given test would involve an excessive amount of time. The goal is only to

provide a paper for comment, and perhaps, to give a basis for a standards

laboratory system. Following the manufacturers input, a revised list will be

presented for discussion at the spring ASTM meeting.

11
IV. EXPERIMENTAL PROGRAM AT MBS

The measurement of critical parameters of superconductors (current,

field, and temperature) as well as other phenomena of importance in

applications (ac losses, effects of stress and fatigue, etc.) require complex

apparatus that usually must be constructed by the experimenter . In such

measurements it is not uncommon for the results from different laboratories to

be quite different even though everyone agrees on the definitions of the

appropriate terms and similar apparatus is used. One solution to this problem

is the use of a very detailed and reproducible experimental technique that has

been developed from extensive experimentation. This type of development is one

of the strong thrusts of our program.

It is also often desirable to have a standard apparatus design available

for a given test. Frequently the technique and apparatus descriptions are a

single document (e.g. ASTM Standards). Care is needed in the design phase to

insure that the device is suited to all potential users and that a given measurement can be completed in a reasonable time. Our first designs in this category are for a critical current apparatus. The final design will be subjected

to a detailed evaluation by interested parties. If possible, the apparatus

will then be sent to various laboratories for field trials of the design.

Another aspect of apparatus development occurs in the situation where

widely different techniques are used to measure what is ostensibly the same

quantity. Determining the causes of disagreement among the techniques can be a

very difficult task. A specific example is provided by ac loss phenomena in

superconductors. The losses may be determined by a calorimeter, a watt

meter, or a flux integration technique. All of these measurements are diffi12
cult and agreement between any two is rare; among three it is essentially

nonexi stent.

The NBS research program is looking at each of these aspects of standards

development. Most of our work this year has been to start the development of

critical current test procedures and devices and to begin work on the critical

temperature measurement evaluations (at NBS-Gaithersburg) . Work has also been

done on the evaluation of ac loss measurement techniques, primarily with Air

Force funding, but also with some contribution from this program.

A, Critical Current Studies

Our main experimental studies this year were devoted to investigating the

effect of various aspects of the sample holder on the measured critical current. To this end the characteristics of commercial mul tifil amentary

Nb3 $n and V 3 Ga wires were measured using copper, phenolic, and fiberglass

epoxy (NEMA G-10) sample holders. For each of these materials, was measured with the sample in a hair-pin geometry. The sample was supported in two

different ways to prevent movement caused by the Lorentz force: first with the

sample freely suspended in a slot machined in the support material, and second,

with the sample frozen in place with vacuum grease, a common practice. For the

copper sample holder, no difference was found in the magnitude of between

the grease and no grease method. For the composite materials, the

depended on the fiber direction in the sample holder. For the phenolic and

G-10 holder materials with the fiber direction paral lei to the sample, no

difference in was found between the grease and no grease method. For the

phenolic sample holder with the fiber direction perpendicul ar to the sample,

however, the grease method produced a 25% degradation of at 7T (for

Nb3 $n), and 11% (for V

3 Ga) (compared with the no-grease method). The

13
complete curves are shown in Fig. 3. This relatively large decrease in

for the phenolic sample holder, with fiber direction perpendicular to the

sample and with grease around the sample, is ascribed to compressive strain

introduced by differential thermal contraction between the sample and the

sample holder material. This conclusion is supported by calculation from

measurements made of the thermal contraction of the superconducting wires and

the holder materials (Fig. 4) and measurements of the strain degradation of the

superconductor (Fig. 5). The measurements and conclusions are described in

more detail in a recent publication [7].

Work has also been started on adapting a theoretical model by J. W.

Ekin [8,9] describing current transfer into a superconductor to the practical

wires and soldered joints between wires. An apparatus for testing the predicted behavior is being built.

B. AC Loss Studies

Many of the applications for superconductors require transient fields or

currents. The resultant ac power loss in such conductors often influences the

design of the system. There are a large number of variables which determine

the magnitude of the effect and present theoretical models are not good enough

to accurately predict the losses for all practical superconductors.

Measurements of ac losses are necessary in basic research to further understand

the loss mechanisms and for the characterization of commercial wire. Several

techniques exist for making the measurements, but different results are often

found depending on the technique used. The purpose of these studies is to cotipare, evaluate, and calibrate the various measurement methods. As mentioned

earlier, this research is primarily funded by the Air Force, but it is also an

14
Fiqure 3 . Critical current dearadation of Nb3Sn and V3Ga wires when fixed

with grease to a phenolic sample holder with fiber direction

perpendicular to the samole.

IF
293

TEMPERATURE, K

Figure 4. Thermal expansion of phenolic, fiber-glass epoxy (NEMA G-IOCR) , multifilamentarv Nb3 $n and V3 Ga. Warp-arain denotes the warp

direction is parallel to the measured axis; normal -grain denotes the

thermal expansion is measured perpendicular to the fiberqlass cloth.

16
Fiqure F. Relative chanae of critical current, Iq, with intrinsic strain

£o* ^cm 'S the maximum value of the critical current.

Positive eq indicates tensile strain, neaative Eq indicates

compressive strain.

17
area of concern to the standards program since, ultimately, a system of standard ac loss measurement techniques will be needed.

The various methods of measuring ac losses can generally be divided into

two categories: calorimetric and electronic. The most common calorimetric

technique measures the helium boil -off rate produced by the ac losses. Though

the technique is fairly straightforward, it becomes complex when sensitivities

approaching 1 mW are required. In our program we are striving for a

sensitivity of 1 mW or better to provide as wide an overlap as possible with

electronic techniques that have sensitivities on the order of 1 mW and lower.

To this end we have set up three experimental systems: a calorimeter; a

digital oscilloscope technique; and a lock-in amplifier technique. The goal is

to measure practical superconductors with special attention to the region of

overlap and to analyze the various techniques to determine the reasons for the

observed discrepancies. Some very preliminary results are shown in Fig. 6.

The apparatus and their operation are described in detail in a recent

publication [10].

18
POWER,

mW

300

Fiqure fi. The ac loss as a function of frequency for a imjlti filamentary NbTi

superconductor. A calorimetric value at 215 Hz is not shown since

it aorees almost exactly with the diqital value.

19
C. Critical Temperature Studies

"Definition of the Critical Temperature of Practical Superconductors"

J. F. School ey and R. J. Soulen

Temperature Measurements and Standards Division

The transition from the normal (resistive) state of a metal to the superconductive state should occur over an extremely narrow temperature region

-9 -27

(theoretical estimates vary from 10 K to 10 K). Carefully prepared specimens used for temperature reference standards fall short of this ideal (efforts

at NBS yield samples with transition widths of 10-5 |<) ^^it ambiguities in

the definition of T^ of this order are not generally of practical

significance. Such is not the case, however, for the highly strained

inhomogeneous materials of technical importance where the transitions often

extend over several Kelvins.

Further complicating the ambiguity of the proper definition of T^ in the

latter circumstances is that different methods of detecting the transition

often give quite different temperature values. Measurements of the heat

capacity anomaly, of the onset of magnetic flux exclusion (the "Meissner

effect"), and of the disappearance of electrical resistance each lead to

independent and often differing representations of the shape of the transition

and of the definition of the transition temperature. In discussing the use of

a particular superconductive material for a particular application, it is

important that the manufacturer, the vendor, and the user agree upon a

transition temperature which is consistent with the other parameters of

engineering importance, such as the critical field and the critical current.

In the following report, we summarize the progress which has been made by the

20
Temperature Measurements and Standards Division during FY 79 in identifying the

superconductive transition temperatures of practical materials in a

self-consistent manner. This progress includes the construction of an

experimental test cryostat, measurement instrumentation, and several baseline

measurements on relevant superconductive materials, including samples of

industrial wires of niobium-titanium and niobium- tin alloys in copper or

copper-alloy matrices.

Instrumentation

A cryostat was fabricated for this project in order to test specimens over

a temperature range of IK to 300K. It can be inserted into the gap of a magnet

capable of generating dc fields as large as 2T (20 kG). Sufficient electrical

leads are available for the simultaneous testing of five samples.

Temperature measurements are based upon the IPTS-68 Temperature Scale

above 13K through the use of a capsule platinum resistance thermometer calibrated within the Division. Temperature reference below 13K is provided

by both doped-germanium and rhodium-iron thermometers calibrated on the EPT-76

Provisional Temperature Scale. Any given temperature measurement throughout

the test range is expected to lie within ten millikelvins of the defined temperatures on the two reference scales. We have developed a computer-based

automatic acquisition system which can incorporate the following features:

*Programnable-rate ramping temperatures throuahout the test range.

*Adjustabl e-level temperature plateau for measurement stability.

*Measurement of the low-frequency magnetic susceptibility, including

graphical analysis of the transitions.

^Measurement of the dc electrical conductivity.

*Measurement of the heat capacity.

21
Measurements to Date

1. Niobium-Tin. We have examined two types of samples of this material.

One was prepared in our own facilities by the vapor deposition of

high-purity tin on a one-mm niobium wire of 99.9+% purity. It is

identified as NBS #80, and its susceptibility-temperature curve is

shown in Fig. 7. The second type of sample is from a copper-clad

0.7 mm diameter high- field magnet wire prepared by imbedding some

2800 Nb filaments in a bronze matrix and subsequently reacting the

wire for 240 hours at 750°C. The sample was measured in its annealed

state. Its susceptibility-temperature curve is shown in Fig. 8, with

the data of NBS #80 superimposed for comparison. These results must

be regarded as very preliminary, but they indicate that the bulk of

the superconductive transition in the commercial material is

substantially broader and occurs at a lower temperature than that

which is characteristic of unstressed Nb3$n.

We expect to make quantitative correlations between the transitions in the two

types of samples in terms of internal material stresses and proximity effect

calculations.

2. Niobium and Niobium-Titanium. We have examined one sample of

commercially-prepared niobium-titanium superconductive wire. This

sample is a wire of rectangular cross-section 0.53 mm by 0.68 mm

containing nearly 200 filaments of ^^b^gTig^^ imbedded in a copper

matrix. The susceptibility-temperature curve for this sample is

shown in Fig. 9. In view of the proximity of its transition

temperature to that of pure niobium, we show also in Fig. 9 both

susceptibility and heat capacity data for sample OR-1, a 3-mm

22
NboSn

NBS/^80

(sjjun AjBJiiqjv) Aiinqiideosns

23

Fiqure

7.

Siisceptibi

1

i

ty

transition

of

Hb3

Sn

preparoH

bv

vapor

Opposition

of
T„

of

Nb,Sn

(siiun AjBJiiqjv) Aunqqdaosns

24

Figure

8.

Susceptibility

transition

of

commercial

Nb3$n

mul

tifil

amentary

wire

compared

to

the

reference

sample

of

Fig.

7.
(Sjjun /llBJUqjV) AjlOBdBQ |B8H

(sjiun AjBJiiqjv) Ajjiiqiideosns

25

Figure

9.

Susceptibility

transition

of

commercial

MbTi

mul

tifil

amentary

wire.
diameter wire prepared from electron-beam zone- refined niobium. The

strong difference in transition width between the two types of

samples is not surprising, considering their very different

compositions and states of anneal. We anticipate the testing of the

commercial materials in much the same fashion, however, in order to

establish the most meaningful definitions of the commercial material

parameters.

D. Supporting Research at NBS

Since this is our first report on this program, we list here other

research programs in the group that are related to this work. It is an

important feature of the laboratory that we have people engaged in experimental

work covering a wide range of properties at low temperatures. They provide us

with a very valuable source of data and consultation on many aspects of

superconductors and related materials. The projects, funded by a variety of

agencies are:

Stress and fatigue effects in superconducting wires

Stress and fatigue effects in potted superconducting coils

Training of superconducting coils

Electrical, thermal and mechanical properties of stabilizing metals

New high field superconductors

In-si tu processing of superconductors

Thermal expansion

Elastic properties

Thermal conductivity

Magnetic properties

The reader who is interested in more details of any of these proarams is

encouraged to call one of the authors.

26
V. CONTRACTOR PROGRAMS

Four industrial contractors, all manufacturers of superconducting wire,

are working on the program (see Appendix D). To date most of their

contribution has been in helping to staff the ASTM committee, collaborating

with NBS in the formulation of the test program, and construction of test

apparatus. Their experimental work has just started and, thus, most of the

description here will be of their proposed programs. Some data has been

received, but will be kept for a later report. All of the tests described are

to be made on commercial wires, usually from the manufacturer' s own stock. In

addition, each contractor is participating in the round robin tests described

in Section II 'and will critique the NBS-designed test fixtures.

A. Airco Superconductors

A report is in preparation comparing the critical current measured in

strands with that found when the strands are cabled.

The experimental program will determine: a) the effect of power supply

ripple on the I(- measurement; b) how different methods of constraint affect

the measured !(.; c) how I^ varies with direction of an applied magnetic

field with respect to the current; d) the effect of wire diameter on the current transfer length; and e) the effect on I(. of a variation of the magnetic

field strength over the sample length. In each case the goal is to provide

data allowing a judgement to be made of the measurement precision to be expected from a given standard test jig.

27
B. Intermagnetics General Corporation

This project is investigating the effect of self field on the critical

current of relatively large {>5 kA) superconductors. Measurements are being

made to evaluate the effect of twist pitch and matrix resistivity as well as

terminal configuration. They are developing a test fixture specifically to

allow accurate determination of the residual resistance ratio of the

stabilizing metal in the composite superconductor. A first model has been

constructed and a number of tests made.

C. Magnetic Corporation of America

A comprehensive series of short sample tests are being made on 25-30 NbTi

wires. The wires cover a range of copper/superconductor ratios and several

have high resistivity cupro-nickel matrices. The effect of the criterion

chosen for defining is also being studied in these tests. A number of the

samples have been measured in three different types of sample holders

(straight, hairpin, and noninductively wound spool) to determine if the sample

configuration has an effect on the result. In addition, studies of current

transfer through joints in superconducting wires are being made on about twenty

samples. The samples are chosen such that the effect of varying wire

diameters, filament twist pitch, matrix material, and matrix-to-superconductor

ratio can all be evaluated. The emphasis of these measurements is on

relatively large conductors (Iq > lOOOA)

.

D. Supercon Inc.

This project is studyina the variation of critical current found in a production run of NbTi superconductor . The variation oT I^ along the final wire

in both long and short segments is beina determined. Similar measurements on

28
samples from a number of other production runs will be made to give an indication of the lot-to-lot variation in wires processed by the same method. The

second part of the study is an investigation of parameters effecting the

stability of the wires, especially the effect of filament uniformity. To this

end the concepts of recovery current and minimum propagation current are being

reviewed and practical measurement practices developed.

29
VI . REFERENCES

[1] G. Fujii, "Present Practices in Japan for the Measurement and Definition

of Various Superconducting Parameters" (To be published).

[2] R. L. Powell and A. F. Clark, "Definitions of Terms for Practical Superconductors 1. Fundamental States and Flux Phenomena", Cryogenics 17,

697-701 (1977).

[3] R. L. Powell and A. F. Clark, "Definitions of Terms for Practical Superconductors 2. Critical Parameters", Cryogenics 1^, 137-141 (1978).

[4] D. T. Read, J. W. Ekin, R. L. Powell, and A. F. Clark, "Definitions of

Terms for Practical Superconductors 3. Fabrication, Stabilization, and

Transient Losses", Cryogenics J^, 327-332 (1979).

[5] F. R. Fickett, S. B. Kaplan, R. L. Powell, R. Radebaugh, and A. F. Clark,

"Definitions of Terms for Practical Superconductors 4. Josephson

Phenomena", Cryogenics (To be published).

[6] A. F. Clark and J. W. Ekin, "Defining Critical Current", IEEE Trans. Mag.

MAG-13 , 38-40 (1977).

[7] G. Fujii, J. W. Ekin, R. Radebaugh, and A. F. Clark, "Effect of Thermal

Contraction of Sample Holder Material on Critical Current", Advances in

Cryogenic Engineering ^ (To be published).

[8] J. W. Ekin, "Critical Transfer in Mul ti fil amentary Superconductors.

I. Theory", J. Appl . Phys. 3406-3409 (1978).

[9] J. W. Ekin, A. F. Clark and J. C. Ho, "Current Transfer in Mul tifil amentary Superconductors. II. Experimental Results", J. Appl. Phys.

3410-3412 (1978).

[10] R. Radebaugh, G. Fujii, D. T. Read, and A. F. Clark, "A Standards Program

for AC Losses in Superconductors", Proc. XVth International Congress on

Refrigeration (To be published).

30
APPENDIX A

SUPERCONDUCTING STANDARDS PROGRAM

Questions to Users

The purpose of this questionnaire is to aid in the development of standard

definitions and measurement practices for superconducting parameters.

It will also be used in the writing of a document which discusses current

measurement practices.

Is it okay for NBS to publish results of this questionnaire?

Specifications

1. What superconducting parameters are used in coil design?

2. How are specifications derived from design data?

3. What parameters are in specifications - get copy of specs, if

possible (a) does p refer to area of superconducting part or

ove'rall conductor?

4. Do you perform measurements to check specs?

(a) How many measurements?

(b) How much time and money is spent on checking specs?

(c) How often do you find material does not meet specs?

5. Who is supplier of superconductor?

Measurement Techniques

I measurements

c

1. What materials do you test, i.e. NbTi and/or Nb^Sn?

2. What range of conductor sizes are tested?

(a) What is largest size conductor you can test?

3. At what temperature are tests done, 4.2K or other?

4. Are only individual strands of large conductors tested?

(a) if so, is Ic of overall conductor calculated as the sum of individual strands?

5. Method of Measurement

(a) hair pin, coil geometry, or straight sections

(b) length of sample

(c) length between voltage taps

(d) holder material - GIO, phenolic, or other

(e) holding method, grooves, grease, solder, thread, etc.

(f) drawing or sketch of holder

A-1
6. Instrument Capability

(a) dia. of magnet bore

(b) max. magnetic field

(c) was magnet calibrated?

(d) maximum current for sample

(e) voltage sensitivity, i.e. how many uV/cm? - noise level?

what kind of amplifier is used?

7. What other parameters are measured?

(a)

^c2’ ^cl

(c) AC losses

(d) physical mechanical properties

(e) stability

A-2
APPENDIX B

MINUTES

TECHNICAL MEETING

ASTM SUBCOMMITTEE B0I.08 on SUPERCONDUCTORS

Held June 12, 1979

ASTM Headquarters, Philadelphia, PA

In the afternoon session, chairman A1 Clark discussed the role of NBS

and pointed out that it is not a regulatory agency but a research base

which can be used as input to ASTM. Next the various tasks and priorities which need to be considered were discussed at some length. The

tentative task group structure set up in the morning meeting was reviewed

further. However, after further discussion it was agreed upon by the

group that the expertise of the members might be better utilized in

specific subject areas, each dealing with all the categories in the

scope, except for standard definitions. On MOTION, duly seconded, the

group voted to accept the task group structure shown in the table

below. Since the v/ork of Task group #1 (Definitions) covers all subject

areas, the other task groups are to refer any recommendations regarding

Final Task Group Structure

definitions in their subject area to Task group #1. Most of those

present volunteered for service on the various task groups. As of this

meeting the membership of each task group is as follows:

Task group #1 (Definitions): Steve J. St. Lorant - Chairman

Alan F. Clark

Roger Boom

Michael. J. Superczynski , Jr.

Bruce P. Strauss

Roger J. Soulen

B-1
Bruce P. Strauss - co-chairman

Jack W. Ekin - co-chairman

Ronald M. Scanlon

William A. Fietz

Phillip A. Sanger

Robert Schwall

Robert N. Randall

Henry Riemersma

Phillip A. Sanger - co-chairman

Bruce Zeitlin - co-chairman

Jack W. Ekin

Fred N. Mazandarany

Robert W. Randall

Robert H. Remsbottom

Masaki Suenaga

Meyer Garber - Chairman

William Fietz (Stuart Shen)

Ray Radebaugh

Henry Riemersma (Richard Wagner)

Joe D. Thompson

(Michael Walker)

Task group #5 and #6: Not appointed yet.

Names in parentheses indicate those not present but volunteered by

others. Each task group met separately for about an hour to decide on the proper course of action for their own groups. Reports from each

task group were presented to the entire subcommittee. These reports are

as follows:

Task group #1 (Definitions)

This group will start with the three sets of definitions published so far by the National Bureau of Standards (Cryogenics 2Z> 697 (1977),

Cryogenics 18, 137 (1978), and Cryogenics 19, 327 (1979)). Irrelevant

definitions will be deleted and others the group feels are important

will be added. Chairmen of the other task groups were asked to submit

to Dr. St. Lorant any definition changes or additions which relate to

their specific task group. Both intellectually satisfying definitions

and practical definitions will be considered. A preliminary set of

definitions will then be ready by the next meeting of the subcommittee.

Task group #2 (Critical current)

1. All tests would be defined in terms of measurable variables such as current, voltage, magnetic field, and physical distance between the

voltage taps.

2. The task group decided to write recommended tests of the following

type:

Task group #2 (Critical Current):

Task group #3 (Phys. Mech. prop):

Task group #4 (ac losses):

a. Hairpin

B-2
b. Coiled sample (either non-inductively wound or inductively

wound)

c. Straight, short sample - simple solenoidal test magnet

d. Straight sample - split-pair test magnet

The range of these tests would be less than 600 amps so it could be

defined with low noise power supplies. Future work would include

tests for higher-transport-current samples.

3. At the ICMC conference in August, each member of the task group

will submit a listing of suggestions, practices, and tolerances for

specifying test methods in each of the above areas. The suggestions

will be distributed among the members for preparation of firstdraft test procedures.

Task group #3 (Phys. Mech. Prop.)

We divided into three groups and leaders assigned as follows:

Group I. Physical Properties

1. Component Percentages - Phil Sanger, Airco

2. Physical Dimensions and Workmanship - Bob Remsbottom,

University of Wisconsin

3. Twist Pitch - Bruce Zeitlin, IGC

4. Configuration Bob Randall, Supercbn

Group II. Electrical Properties

Resistivity or Resistance Ratio - Phil Sanger

Group III. Mechanical Properties

yield strength

ultimate strength

thermal contraction

modul us stress/strain characteristics

residual stresses

fatigue properties (thermal and mechanical cycling)

Leaders: Fred Mazandarany, GE; Mas Suenaga, BNL. It was also

suggested that John Zbasnik, LLL and Dewey Easton, ORNL be asked to serve on Group III.

We agreed that the objective for our next meeting is to have (a) a definition, (b) suggest a test method for measuring, (c) research the

existing ASTM for present specifications (thanks to B. Remsbottom), and

(d) suggest tolerances.

Our next meeting would then be used to critique and possibly assign a second member to specifically review individual tasks.

B-3
At the end of our June 12 meeting, it became apparent to one of us (BAZ)

that Group III Mechanical Properties might better be served by a separate

Task Group because of the lack of basic test data and procedures. This

will be discussed at the next meeting.

In the longer range, we hope that once the specifications are defined

for the wire, similar specifications will be defined for the more complex conductors such as braids, cables, etc.

Task group #4 (ac losses)

The group spent most of the time trying to organize the area of ac losses. The preliminary organization divides ac loss measurements into

three categories: (I) type of sample, (II) type of excitation, (III)

type of measurement.

Each of those categories is broken down as follows:

I. Type of Sample

A. small or short sample

B. long cables

C. magnets

D. ac motors, generators, transformers

E. rf devices (linac)

II. Type of Excitation

A. quasi-dc

B. 20Hz - 20kHz

C. pulses and ramps

D. ripple on dc

III. Type of Measurement

A. magnetization

B. wattmeters (digital or analog multipliers, phasemeters)

C. ac bridges

D. lock-in voltmeter

E. calorimeter (boil-off or adiabatic)

Future work will be to do more detail work on the outline. The first

priority will be in small conductors at low frequencies. In many cases

all measurements methods would be acceptable but in some cases certain

methods may be preferable. We plan to study this further.

The next meeting of the ASTM B01.08 Subcommittee on Superconductors will

be held at 4:30 pm on August 23, 1979 during the time of the CEC/ICMC

meeting in Madison, Wisconsin. Most of the task groups will be meeting

on Monday, August 20.

The meeting adjourned at about 3:30 pm.

B-4

Respectfully submitted,

Ray Radebaugh

Secretary, B01.08
APPENDIX C

NBS CRITICAL CURRENT TEST SPECIFICATION SUGGESTIONS

A.

B.

October 17, 1979

User Specified Variables and Suggested Tolerances

1. Critical current I c

a. reproducible precision of + 2%

b. accuracy of + 5% for independent measurement

2. Applied magnetic field, H

a. uniform to + 2% between voltage contacts

b. field ripple less than + 2% peak to peak

c. field accuracy +2%

3. Angle between current and magnetic field

a. orientation constant to +2° between voltage contacts

4., Ambient temperature, T

a. constant temperature, accurate to j^2%

5. Electric field criterion, E , for defining I

a. accuracy +10%

Control Parameters

1. Voltage current characteristics

a. I , defined by E^., reproducible precision of j^2% for

at least 3 runs at constant field

b. V-I reversible up to 120% times the voltage at E^;., to

give I^ with precision of +^2% c. current ripple less than 5% r.m.s.

d. r.m.s. noise voltage, a factor of 5 less than voltage

at Eq

e. current transfer voltage less than voltage at Ep by a factor of 2

2. Strain

a. bend radius R and wire diameter d, such that d/2R

< 0.05% for Nb 3 Sn and < 1% for NbTi, this may require

pre-reaction forming of wire to test configuration

b. no stress risers between current contacts

c. in the case of the sample fastened to the sample holder

with adhesive or grease, the differential thermal con- traction between sample and support low enough to give

strain < 0.02% for Nb 3 $n and < 0.5% for NbTi

d. in the case of a grooved sample holder, the groove

C-1
geometry will be such that the strain due to the Lorentz

force is < n.0?% for Nh3Sn and < 0.5% for NbTi

e. sample will have the same initial stress state as user product, i.e. same initial stress from diffusion

barriers, cladding and insulation

f. voltage taps will be strain free

C. Operational Checks for Valid Test

1. Quench protect circuit test

a. quench sample 5 times and not change by more than +2%

2. Lorentz force and trapped flux test

a. cycling magnetic field doesn't change by more than +2%

3. Current ramp rate test

a. Iq independent of ramp rate, to +2%, for chosen ramp

rate and half that ramp rate

4. Test for contact and current transfer heating affecting

a. measure sample power at thermal quench, for a factor of

5 range of I^, will be the same to +20%

5. Contact and current transfer heating test

a. measure power across current contacts, this will be an operational number that will indicate heating problems

D. Test Apparatus Design Concerns

1. Current transfer length dependence

a. magnetic field gradient

b. magnetic field orientation

c. composite constituents (i.e. matrix resistivity, diffusion barriers, cladding)

2. Current solder joints

a. low joint power

3. Sample protection from

a. strain in handling and mounting

b. strain upon cool down

c. large powers upon quench

C-2
E. Options

1. Test configuration (provided strain criterion is met)

a. cylinder

b. hairpin

c. straight

2. Sample holder

a. matched thermal contraction of sample and holder when

adhesive or grease is used

b. grooved holder to prevent sample strain and movement

from Lorentz force

C-3
APPENDIX D

STAFF AND CONTACTS

A significant amount of the work on this program is done under

subcontract to NBS. We list here both the NBS staff and the technical contacts

for the various contractors.

National Bureau of Standards, Thermophysical Properties Division, Boulder, CO

80303. Ph. (303) 499-1000, FTS 323+EXT.

F. R. Fickett (PI) x3785

A. F. Clark (PI) x3253

L. F. Goodrich x3143

J. W. Ekin X4448

R. Radebaugh X3710

National Bureau of Standards, Temperature Measurements and Standards Division,

Washington, D.C. 20234. Ph . (212) 921-1000, FTS 921+EXT.

J. F. School ey x3315

R. J. Soulen, Jr. x2018

Airco Superconductors, 600 Milik Street, Carteret, NJ 07008. Ph. (201)

541-1300.

P. Sanger

C. Rosen

Intermagnetics General Corporation, P.O. Box 566, Guilderland, NY 12084. Ph.

(518) 456-5456.

R. Schwall

R. McCown

Magnetic Corporation of America, 179 Bear Hill Road, Waltham, MA 02154.

Ph. (617) 890-4242.

H. Seqal

B. Strauss

Supercon Inc., 9 Erie Drive, Natick, MA 01760. Ph. (617) 655-0500.

W. Larson

K. Stohlman

D-1
NBS>114A (REV. 9-78)

U.S. DEPT. OF COMM. 2.GoVt Access! BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 80-1629

Recipteni's Access^ioft No.

4. TITLE AND SUBTITLE 5. Publication Date December 1979

Development of Standards for .Superco nductor*;

7. AUTHOR(S)

F. R. Fickett and A. F. Clark

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreet. City, ZIP)

Supported in part by:

Department of Energy

Massachusetts Institute of Technology

13. Type of Report & Period Covered Annual Report

FY 79

15. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200^word or less factual aununary of most significant information. If document includes a significant bibliography or

literature survey, mention it here,)

Practical superconductors are complex materials and the determination of

the parameters required for designing with them is a difficult task. Many

approaches are possible for determining a given parameter and the results

depend critically on which one is chosen. The goal of this program is to

arrive at a set of useful voluntary standards for measurements on modern

practical superconductors that will be acceptable to both manufacturers and

users. Agreement on a set of standard definitions for the various parameters

is also necessary. This report describes the status of the program and

includes a brief historical introduction. The need for standards in this area

is described in detail with particular attention paid to the need for consensus

among all interested parties and our techniques for achieving it. Results from

the experimental research by NBS and a review of the wire manufacturers'

programs are presented.

17. KEY WORDS ("six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Critical current, critical temperature, losses, magnetic property, standards,

superconductor.

18. AVAILABILITY [X] Unlimited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003- FS Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

UNCLASSIFIED 44

20. SECURITY CLASS 22. Price (THIS PAGE)

$6.00

UNCLASSIFIED

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