United States Department of Commerc

National Institute of Standards and Technology

NATL INST. OF STAND S TECH R.I.C.

A11103 710735

N!ST

PUBLICATIONS

Iflftf) Special Publication 826

High- Temperature Superconductivity,

Abstracts of NIST Publications,

1987-1991

Mary E. DeWeese, Editor

NIST Special Publication 826

High- Temperature Superconductivity:

Abstracts of NIST Publications,

Edited by

Mary E. DeWeese

Electromagnetic Technology Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, CO 80303

Supersedes NIST Special Publication 759

1987-1991

November 1991

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office

and Technology Washington: 1991

Special Publication 826

Natl. Inst. Stand. Technol.

Spec. Publ. 826

90 pages (Nov. 1991)

CODEN: NSPUE2

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
CONTENTS

Page

FOREWORD v

1. COMPOSITIONAL ANALYSIS 2

2. CRITICAL CURRENT AND CRITICAL TEMPERATURE 6

3. CRYSTAL STRUCTURE 23

4. ELASTIC CONSTANTS AND PHONON SPECTRA 33

5. ELECTRICAL CONTACTS 40

6. ELECTRONIC STRUCTURE 43

7. ENERGY GAP AND TUNNELING SPECTRA 53

8. JOSEPHSON EFFECT AND DEVICES 57

9. MAGNETIC MEASUREMENTS 63

10. PHASE EQUILIBRIUM 72

11. PROCESSING—BULK MATERIAL 75

12. PROCESSING—THIN FILMS 80

AUTHOR INDEX 83

iii

ABBREVIATIONS

CAC = Center for Analytical Chemistry,

National Measurement Laboratory

CBS = Center for Basic Standards,

National Measurement Laboratory

CCP = Center for Chemical Physics,

National Measurement Laboratory

CEEE = Center for Electronics and Electrical Engineering,

National Engineering Laboratory

CRR = Center for Radiation Research,

National Measurement Laboratory

EEEL = Electronics and Electrical Engineering Laboratory

(formerly CEEE)

IMSE = Institute for Materials Science and Engineering

MSEL = Materials Science and Engineering Laboratory

NML = National Measurement Laboratory

v

FOREWORD

Robert A. Kamper

The National Institute of Standards and Technology (formerly the National Bureau of Standards)

has been involved with research in superconductivity since soon after Kammerlingh Onnes's original

discovery in 191 1. Our first contribution to the literature was the Silsbee hypothesis to account for the

critical current of a type-I superconductor, published by Francis Silsbee in 1916. The discoveries of the

high-field superconductors in the late fifties and the Josephson effect in the early sixties introduced the

possibility of serious engineering with superconductors and were followed by significant expansion of the

program at NBS. A formal program to promote the application of superconductivity to physical standards

and measurement techniques was established in 1969 and has been an active contributor to the

development of superconducting electronics ever since. In the early seventies a program to define and

establish techniques to measure the engineering characteristics of practical high-field superconductors was

established. This has contributed to the development of reliable conductors and to the success of many

large-magnet projects.

The discovery of the high-temperature superconductors introduced the need to extend research from

metals to ceramics, for which NBS was also well prepared, with an established program in ceramics and

an extensive array of facilities to analyze and map the composition, structure, and electronic energy

spectrum of materials in a wide range of scales.

This collection of abstracts represents the first results of applying all these resources to hightemperature superconductors. Many of the papers have authors from several institutions. The National

Institute of Standards and Technology encourages collaborative research projects to maximize the benefit

of its resources to the U.S. research and development community. Anyone wishing to discuss the

possibility of a collaborative research project should feel free to make contact with any of the NIST

authors represented here.

vii

HIGH-TEMPERATURE SUPERCONDUCTIVITY:

ABSTRACTS OF NIST PUBLICATIONS, 1987-1991

Mary E. DeWeese, Editor

This is a collection of abstracts from 243 papers published between March

1987 and August 1991 covering various aspects of superconductivity

research. The work of nine divisions of the National Institute of Standards

and Technology (formerly the National Bureau of Standards) in both

Boulder, Colorado, and Gaithersburg, Maryland, is represented.

Key words: bismuth superconductors; critical current; critical field;

critical temperature; high Tc ; high-temperature

superconductors; low-temperature superconductors;

superconducting ceramics; superconductors; thallium

superconductors; YBCO; yttrium superconductors

1
1. COMPOSITIONAL ANALYSIS

STANDARD X-RAY DIFFRACTION POWERPATTERNS OF FIFTEEN CERAMIC PHASES, Winnie

Wong-Ng, Howard F. McMurdie, Boris Paretzkin, Camden R. Hubbard, Alan L. Dragoo, and James

M. Stewart,* Powder Diffraction, Vol. 2, No. 2, pp. 106-117 (June 1987). See pp. 191-201 for

accelerated publication with additional coauthors, Yuming Zhang* and Katherine L. Davis.

[IMSE/Ceramics Division]

*University of Maryland

See following abstract.

STANDARD X-RAY DIFFRACTION POWDER PATTERNS OF SIXTEEN CERAMIC PHASES,

Winnie Wong-Ng, Howard F. McMurdie, Boris Paretzkin, Yuming Zhang,* Katherine L. Davis, Camden

R. Hubbard, Alan L. Dragoo, and James M. Stewart,* Powder Diffraction, Vol. 2, No. 3, pp. 191-201

(September 1987). [IMSE/Ceramics Division]

*University of Maryland

The following sixteen reference patterns of boride, silicide, nitride, and oxide ceramics represent

the second group of reference patterns of ceramic phases measured at the National Bureau of

Standards under the project "High Quality Reference Patterns and Total Digital Powder Patterns

of Technologically Important Ceramic Phases." Included in the sixteen reference patterns are

reports for two high Tc superconducting oxide phases (CuLa, g Sr0 204 and BajQLjYO,) plus one

related phase (BaCuY205). In addition to these new phases, five other patterns represent phases

previously not contained in the PDF and eight represent major corrections to data in the file. The

general methods of producing these X-ray powder diffraction reference patterns are described in

this journal, Vol. 1, No. 1, p. 40 (1986).

X-RAY POWDER CHARACTERIZATION OF Ba^CujO^, W. Wong-Ng, R.S. Roth, L.J.

Swartzendruber, L.H. Bennett, C.K. Chiang, F. Beech, and C.R. Hubbard, Advanced Ceramic

Materials: Ceramic Superconductors, Vol. 2, No. 3B, pp. 565-576 (July 15, 1987). [IMSE/Ceramics

Division]

The X-ray powder diffraction technique was used in order to characterize the high Tc superconductor phase Ba^CujO^ prepared under different conditions. High quality reference

powder patterns for three compositions are presented. An attempt is made to correlate the

differences of lattice parameters with oxygen content and superconductivity.

MEASUREMENT OF SUPERCONDUCTOR STOICHIOMETRY BY PROMPT AND DELAYED

NEUTRON ACTIVATION, Richard M. Lindstrom, Transactions, American Nuclear Society, Nuclear

Activation Analysis/Applications-I, Vol. 56, pp. 231 (1988). [CAC/Inorganic Analytical Research

Division]

Prompt and delayed neutron activation analysis have been applied to the nondestructive

measurement of the major metals in small samples of YBa2Cu3 07 superconductors, starting

materials, and intermediate products. The small activation cross section and low gamma ray

2
energies of Y-90 m make the determination of yttrium imprecise in the presence of the large

quantities of Cu-64 radioactivity. The physical parameters for neutron-capture prompt

gamma-ray activation analysis (PGAA) are more attractive than delayed counting for

stoichiometry measurements. The PGAA spectra of Y, Ba, and Cu show three, four, and six

analytically useful gamma ray lines respectively, with nicely balanced intensities for the

composition range of interest. A 100 mg sample gives a Poisson precision for yttrium (the

weakest element) of 2% after only a 2 hour irradiation. Since the concentrations and mole ratios

Y/Cu and Ba/Cu may be calculated immediately after the irradiation ends, the results can be

returned within a day.

APPLICATION OF WAVELENGTH DISPERSIVE DIGITAL COMPOSITIONAL MAPPING TO

HIGH-TEMPERATURE SUPERCONDUCTORS, R.B. Marinenko, D.E. Newbury, D.S. Bright, R.L.

Myklebust, and J.E. Blendell, Microbeam Analysis— 1988, D.E. Newbury, Editor, San Francisco Press,

pp. 37-40 (1988). [CAC/Gas and Particulate Science Division, and IMSE/Ceramics Division]

The new high-Tc superconductors are advanced ceramic materials prepared from oxide powders

which are pressed and sintered. Their physical properties depend on the microstructure, which

in turn depends strongly on the processing sequence used in their production. We have used

wavelength-dispersive (WDS) x-ray compositional mapping to study the microstructure of the

Ba2YCu306+x ceramics being produced at the National Bureau of Standards so as to understand

better the processing procedures and the resulting physical properties.

MEASUREMENT OF SUPERCONDUCTOR STOICHIOMETRY BY PROMPT AND DELAYED

NEUTRON ACTIVATION, Richard M. Lindstrom, Transactions, American Nuclear Society, Nuclear

Activation Analysis/Applications-I, Vol. 56, pp. 231 (1988). [CAC/Inorganic Analytical Research

Division]

Prompt and delayed neutron activation analysis have been applied to the nondestructive

measurement of the major metals in small samples of YBa2Cu3 07 superconductors, starting

materials, and intermediate products. The small activation cross section and low gamma ray

energies of Y-90 m make the determination of yttrium imprecise in the presence of the large

quantities of Cu-64 radioactivity. The physical parameters for neutron-capture prompt

gamma-ray activation analysis (PGAA) are more attractive than delayed counting for

stoichiometry measurements. The PGAA spectra of Y, Ba, and Cu show three, four, and six

analytically useful gamma ray lines respectively, with nicely balanced intensities for the

composition range of interest. A 100 mg sample gives a Poisson precision for yttrium (the

weakest element) of 2% after only a 2 hour irradiation. Since the concentrations and mole ratios

Y/Cu and Ba/Cu may be calculated immediately after the irradiation ends, the results can be

returned within a day.

INITIAL STAGES OF DEGRADATION OF SUPERCONDUCTOR SURFACES: 02 , H2 , C02 , AND

CO CHEMISORPTION IN La2-xSrxCu04 , Richard L. Kurtz, Roger Stockbauer, Theodore E. Madey,

Donald Mueller,* Arnold Shih,* and Louis Toth,* Physical Review B, Vol. 37, No. 13, pp. 7936-7939

(May 1, 1988). [CCP/Surface Science Division]

*Naval Research Laboratory

3
The initial stages of degradation of high-Tc superconductor surfaces by interaction with

atmospheric gases have been studied using ultraviolet photoelectron spectroscopy. 02 has little

effect, while H20 adsorbs dissociatively with an initial sticking coefficient S0 — 1, and forms a

surface hydroxide. C02 adsorbs forming a carbonatelike surface species with an initial sticking

coefficient of —0.3. CO is observed to result in similar adsorbed surface species with a much

lower sticking coefficient of ~ 3 x 10"4

.

APPLICATION OF WAVELENGTH DISPERSIVE DIGITAL COMPOSITIONAL MAPPING TO

HIGH-TEMPERATURE SUPERCONDUCTORS, R.B. Marinenko, D.E. Newbury, D.S. Bright, R.L.

Myklebust, and J.E. Blendell, Microbeam Analysis— 1988, D.E. Newbury, Editor, San Francisco Press,

pp. 37-40 (1988). [CAC/Gas and Particulate Science Division, and IMSE/Ceramics Division]

The new high-Tc superconductors are advanced ceramic materials prepared from oxide powders

which are pressed and sintered. Their physical properties depend on the microstructure, which

in turn depends strongly on the processing sequence used in their production. We have used

wavelength-dispersive (WDS) x-ray compositional mapping to study the microstructure of the

Ba2YCu306+x ceramics being produced at the National Bureau of Standards so as to understand

better the processing procedures and the resulting physical properties.

STANDARD X-RAY DIFFRACTION POWDER PATTERNS OF FOURTEEN CERAMIC PHASES,

Winnie Wong-Ng, Howard F. McMurdie, Boris Paretzkin, Alan L. Dragoo, Powder Diffraction, Vol.

3, No. 2, pp. 113-121 (June 1988). [IMSE/Ceramics Division]

The following fourteen reference patterns of carbide, nitride, telluride, and oxide ceramics are

reported. Included in the fourteen reference patterns are data for three high Tc superconducting

oxide and related phases (Ba2Cu03 , CuSr02 , and Ba2Cu3Y06). The general methods of producing

these X-ray powder diffraction reference patterns are described in this journal, Vol. 1, No. 1,

p. 40 (1986).

STANDARD X-RAY DIFFRACTION POWER PATTERNS OF FIFTEEN CERAMIC PHASES, Winnie

Wong-Ng, Howard F. McMurdie, Boris Paretzkin, M.A. Kuchinski, and Alan L. Dragoo, Powder

Diffraction, Vol. 4, No. 2, pp. 106-121 (June 1989). [IMSE/Ceramics Division]

Fifteen reference patterns of oxide ceramics are reported. Included in the fifteen reference

patterns are data for nine high critical temperature superconducting oxide and related phases

Ba2Cu3Er07, Ba32Cu, 7Er0 g061 * xC02, Ba2Cu3Ho07, B^Cu3(Pt05 Y0 5)07, Ba3Dy409 , Ba3Yb409 , (Bao.6Sro.4)2Cu3Y07 , (Bao gSr0 2)2Cu3Y07 , and CuNd204

. The general methods of producing these

X-ray powder diffraction reference patterns were described previously in this journal (Vol. 1,

No. 1, pg. 40 (1986)). The symbols used in this article are defined in the PDF cards.

STANDARD X-RAY DIFFRACTION POWER PATTERNS OF FOURTEEN CERAMIC PHASES,

Winnie Wong-Ng, Howard F. McMurdie, Boris Paretzkin, M.A. Kuchinski, and Alan L. Dragoo,

Powder Diffraction, Vol. 4, No. 1, pp. 40-54 (1989). [IMSE/Ceramics Division]

4
Fourteen reference patterns of oxide ceramics are reported. Included in the fourteen reference

patterns are data for nine high critical temperature superconducting oxide related phases:

(BaCuEr205 , Ba2Cu3Eu07 , BaCuGd205, Ba2Cu3Gd07, Ba2. 4Cu1 .gLa3.609. 6, BaCu5La4013 , Ba^CujLaLjO?, BajQ^SmQ, and (Bao 4Sr0 6)2Cu3Y07

. The general methods of producing these

X-ray powder diffraction reference patterns were described previously in this journal (Vol. 1,

No. 1, pg. 40 (1986)). The symbols used in this article are defined in the PDF cards.

5
2. CRITICAL CURRENT AND CRITICAL TEMPERATURE

TRANSPORT CRITICAL-CURRENT CHARACTERISTICS OF Y1 Ba2Cu3Ox

, J.W. Ekin, A.J. Panson,*

A.I. Braginski,* M.A. Janocko,* M. Hong,t J. Kwo,t S.H. Liou,t D.W. Capone, II, $ and B.

Flandermeyer,$ Materials Research Society, Vol. EA-11, pp. 223-226 (June 1987).

[CEEE/Electromagnetic Technology Division]

*Westinghouse Research & Development Center

fAT&T Bell Laboratories, Murray Hill

fArgonne National Laboratory

Voltage versus current (V-I) characteristics were measured at magnetic fields up to 24 T at a temperature of 77 K in several YiBajCujO, samples fabricated at different laboratories. Critical

temperatures, Tc

, measured by resistivity were about 93 K. All samples showed linear V-I

characteristics at current levels much greater than the critical current, Ic . However, the slope was

significantly less than the normal resistance at Tc

. The slope increased with magnetic field and

reached the normal resistance value only at fields greater than 24 T. Values of the transport

critical-current density Jc near zero magnetic field were generally low and variable (about 1 to

200 A/cm2

). The transport Jc fell sharply when magnetic field was applied, decreasing by about an order of magnitude between 10"3 T and 1 T. This leads to an effective upper critical field for

transport critical currents in Y1 Ba2Cu3 Ox that is significantly less than the upper critical field

defined by the field at which the resistance increases to the normal-state value. The transport Jc appears to be significantly less than Jc calculated from magnetization data on similar samples.

These results are consistent with the transport critical-current in the Y1 Ba2Cu3Ox system being

dominated by a "weak-link" region between high-Jc grains.

STUDIES OF NbTi STRANDS FROM CORELESS RUTHERFORD CABLES, L.F. Goodrich, E.S.

Pittman, J.W. Ekin, and R.M. Scanlan, IEEE Transactions on Magnetics, Vol. MAG-23, No. 2, pp.

1642-1645 (March 1987). [CEEE/Electromagnetic Technology Division]

The electromechanical properties of NbTi strands extracted from coreless Rutherford cables were

studied to clarify the relative effects of strand location and field angle on current degradation that occurs in cables that have been compacted into a keystone shape. Detailed critical-current

measurements were made on two samples which were fabricated under controlled conditions.

These are prototype cables for high energy physics applications. Specific factors that are addressed are the nature, location, and amount of degradation. This information is intended to lead to methods for reducing the amount of critical-current degradation in cable manufacture.

RELATIONSHIPS BETWEEN CRITICAL CURRENT AND STRESS IN NbTi, J.W. Ekin, IEEE

Transactions on Magnetics, Vol. MAG-23, No. 2, pp. 1634-1637 (March 1987). [CEEE/Electromagnetic

Technology Division]

The effects of various types of stress on the critical current of a multifilamentary NbTi

superconductor are reported. Degradation of critical current due to axial tension applied at 4 K,

transverse compression applied at 4 K, and hairpin bending strain applied at room temperature

has been measured. The degradation from axial tension is much greater than from transverse

compression in many practical cases because the soft copper matrix limits the build-up of

6
transverse compression. The degradation from typical levels of transverse compression is only

about 4% at 8 T, for example. For axial tension, on the other hand, higher stresses can occur

that will degrade the critical current by 24%, for example, at 7 T and 2.7% strain. Both the

axial-tensile and the transverse-compressive stress effects are about 98% reversible; thus the

degradation will be seen only when the conductor is under operational stress. The results indicate

that a primary origin of the critical current degradation in NbTi is a stress-induced reversible

decrease in the upper critical field.

EVIDENCE FOR WEAK LINK AND ANISOTROPY LIMITATIONS ON THE TRANSPORT

CRITICAL CURRENT IN BULK POLYCRYSTALLINE Y^C^O,, J.W. Ekin, A.I. Braginski,* A.J.

Panson,* M.A. Janocko,* D.W. Capone, II,t

N.J. Zaluzec,t B. Flandermeyer,t O.F. de Lima,t M. Hong,$ J. Kwo,$ and S.H. Liou,$ Journal of

Applied Physics, Vol. 62, No. 12, pp. 4821-4828 (December 15, 1987). [CEEE/Electromagnetic

Technology Division]

*Westinghouse Research & Development Center

tArgonne National Laboratory

$AT&T Bell Laboratories, Murray Hill

Measurements of the transport critical-current density (Jc), magnetization Jc , and

magnetoresistance in a number of bulk sintered samples of YiBa2Cu3Ox from several different

laboratories indicate that the transport Jc is limited by weak-link regions between high Jc regions.

The weak-link Jc has a Josephson character, decreasing by two orders of magnitude as the

magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain-boundary

region in Y1 Ba2Cu3Ox shows no observable impurities or second phases to the scale of the [001]

lattice planes (~ 12 A). The effect of intrinsic conduction anisotropy is discussed. A currenttransfer model is proposed in which weak conduction along the c axis plays a role in limiting Jc at grain boundaries. Orienting the grains in the powder state during processing may result in

enhanced transport Jc in bulk conductors.

TRANSPORT CRITICAL CURRENT CHARACTERISTICS OF Y^CuA AND POSSIBILITIES

FOR ITS ENHANCEMENT, J.W. Ekin Advanced Ceramic Materials: Ceramic Superconductors,

Vol. 2, No. 3B, pp. 586-592 (July 1987). [CEEE/Electromagnetic Technology Division]

Several general processing methods for increasing the critical current density, Jc , in bulk sintered

Y1 Ba2Cu3 Ox superconductor are outlined. Data indicate that the transport Jc in bulk

polycrystalline specimens is dominated by a weak-link region between high-Jc grains and that

potentially much higher Jc may be possible. Two possible causes of such a weak-link

phenomenon are considered: low-Tc phases or impurities localized in the grain boundary region,

and anisotropy of the superconducting properties. Several methods for minimizing the weak-link

effects to increase the Jc are discussed.

JOSEPHSON JUNCTION MODEL OF CRITICAL CURRENT IN GRANULAR Y1 Ba2Cu307 ^ SUPERCONDUCTORS, Robert L. Peterson and Jack W. Ekin, Physical Review B Rapid

Communications, Vol. 37, No. 16, pp. 9848-9851 (June 1, 1988). (CEEE/Electromagnetic Technology

Division]

7
We calculate the transport critical current density in a granular superconductor in magnetic fields

below about 5 x 10"3 T. The field dependence in this region is assumed to be controlled by

intragranular or intergranular Josephson junctions. Various model calculations are fit to transport

critical current bulk Y1 Ba2Cu307

.s ceramic superconductors, whose average grain size somewhat

exceeds 10 /an. The results yield an average junction cross-sectional area (thickness x length)

of 4-6 fim2

. If the junctions are at the grain boundaries, a London penetration depth of about

150-300 nm is inferred, consistent with other estimates. We conclude that Josephson junctions

are limiting the transport critical current in these samples and that they lie at the grain

boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning

boundaries.

MAGNETIC-FIELD-MODULATED MICROWAVE-ABSORPTION DETECTIONIN A Bi-Sr-Ca-Cu-0

SUPERCONDUCTOR, J. Bohandy,t F.J. Adrian,t B.F. Kim,t K. Moorjani,t R.D. Shull, L.J.

Swartzendruber, L.H. Bennett, and J.S. Wallace, Journal of Superconductivity, Vol. 1, No. 2, pp. 191-

196 (1988). [IMSE/Metallurgy Division]

tThe Johns Hopkins University, Applied Physics Laboratory

Superconducting transitions are observed at 1 10 K, 100 K, and 72 K in a nominally BiSrCaCu^

ceramic using the novel technique of magnetic-field-modulated microwave-absorption detection.

The response of the BiSrCaCu2Ox ceramic to an external magnetic field differs markedly from

that of YBa2Cu3 0, and, in particular, -dTc/dH is much greater in the bismuth sample.

IRREGULARITY IN Nb-Ti FILAMENT AREA AND ELECTRIC FIELD VERSUS CURRENT

CHARACTERISTICS, J.W. Ekin, Cryogenics 1987, Vol. 27, pp. 603-608 (November 1987).

[CEEE/Electromagnetic Technology Division]

There is a correlation between irregularity in filament area ("sausaging") and the shape of a superconductor's electric field (E) versus current (I) relationship. The shape of the E-I

characteristic is quantified in terms of the resistive transition parameter, n, defined by E « P.

Low values of n less than about 20 correlate with a wide filament diameter distribution, while n values over 50 correspond to a distribution more than 2.5 times smaller. It is proposed that

the low-field (constant) value of n be used as an index of filament quality in evaluating different

superconductors for practical applications. A model is also suggested to explain this effect in

terms of a locally depressed filament critical current, which forces current to transfer across the

normal matrix material into neighboring filaments. The relationship between n and the statistical

distribution of filament diameters may be useful as an easy method of estimating the extent of

sausaging in practical multifilamentary Nb-Ti superconductors from measurements of n.

TRANSVERSE STRESS EFFECT ON MULTIFILAMENTARY Nb3Sn SUPERCONDUCTOR,

J.W. Ekin, Advances in Cryogenic Engineering Materials, Vol. 34, pp. 547-552 (1988).

[CEEE/Electromagnetic Technology Division]

A large reversible degradation of the critical current of multifilamentary Nb3Sn superconductors

has been observed under the application of uniaxial compressive stress applied transverse to the

conductor axis at 4 K. In bronze-process multifilamentary Nb3Sn, the onset of significant

8
degradation occurs at about 50 MPa. The intrinsic effect of transverse stress on the upper critical

field is about ten times greater than for axial stress. Although transverse stress on Nb3Sn

filaments is less than axial stress in most applications, it will need to be considered in the internal

stress design of large magnets because of the greater sensitivity of Nb3Sn to transverse stress. The effect scales with conductor thickness and this will place limits on conductor dimensions and

the spacing between distributed reinforcement in large magnets.

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS, INTERIM REPORT, JANUARY

1 986-DECEMBER 1987, L.F. Goodrich, ed., National Bureau of Standards NBSIR 88-3088, 75 p. (February 1988). [CEEE/Electromagnetic Technology Division]

A cooperative program with the Department of Energy, the National Bureau of Standards, other

national laboratories, and private industry is in progress to develop standard measurement

practices for use in large scale applications of superconductivity. This report describes research

for the period January 1986 through December 1987. It contains the results of critical current

studies on the effect of power-supply current ripple, measurements on cable strands, an

interlaboratory comparison (round robin) on a large NbTi monolithic conductor, and a Nb3Sn

round robin. Several useful current supply circuits have been developed. The reduction in

coupling losses in multifilamentary NbTi conductors has been addressed by a study of the

magnetic properties of matrix material consisting of dilute alloys of Mn in Cu. In addition,

vibrating-sample magnetometry is shown to be adaptable to the measurement of coupling losses,

in addition to hysteresis losses, in multifilamentary conductors.

CURRENT RIPPLE EFFECT ON SUPERCONDUCTIVE DC CRITICAL CURRENT

MEASUREMENT, L.F. Goodrich, S.L. Bray, and A.F. Clark, Advances in Cryogenic Engineering

Materials, Vol. 34, pp. 1019-1026 (1988). [CEEE/Electromagnetic Technology Division]

The effect of sample-current power-supply ripple on the measurement of dc critical current is reported. Measurements were made on multifilamentary NbTi superconductor. In general, ripple

in a current supply becomes more significant above 500 A because effective filtering is difficult.

The presence of current ripple reduces the measured dc critical current. Ripple also causes noise

at the input to the voltmeter used for the measurements. The quantitative effect of current ripple

was studied using a battery current supply modified to allow the creation of ripple current with

variable frequency and amplitude. Problems common to all large-conductor critical-current

measurements are discussed.

EFFECT OF TRANSVERSE COMPRESSIVE STRESS ON THE CRITICAL CURRENT AND UPPER

CRITICAL FIELD OF Nb3Sn, J.W. Ekin, Journal of Applied Physics, Vol. 62, No. 12, pp. 4829-4834

(December 15, 1987). [CEEE/Electromagnetic Technology Division]

A large reversible degradation of the critical current of multifilamentary Nb3 Sn superconductors

has been observed when uniaxial compressive stress is applied transverse to the conductor at 4 K.

In bronze-process multifilamentary Nb3Sn, the onset of significant degradation occurs at about

50 MPa. In an applied field of 10 T, the magnitude of the effect is about seven times larger for

transverse stress than for stress applied along the conductor axis. The transverse stress effect

9
increases with magnetic field and is associated with a reversible degradation of the upper critical

field. The intrinsic effect of transverse stress on the upper critical field is about 10 times greater

than for axial stress. Although axial stresses on the Nb^Sn filaments are greater than transverse

stresses in most applications, the transverse stress effect will need to be considered in the internal

design of large magnets because of the greater sensitivity of Nb3Sn to transverse stress. It is shown that the transverse stress from the Lorentz force on the conductor is proportional to conductor thickness. This will place limits on conductor dimensions and the spacing between

distributed reinforcement in large magnets. The effect may be particularly significant in cabled

conductors where large transverse stress concentrations can occur at strand crossover points.

BATTERY POWERED CURRENT SUPPLY FOR SUPERCONDUCTOR MEASUREMENTS, S.L.

Bray, L.F. Goodrich, W.P. Dube, Review of Scientific Instruments, Vol. 60, No. 2, pp. 261-264

(February 1989). [CEEE/Electromagnetic Technology Division]

To measure the critical current of superconductors, a high output current supply is required. In

addition to high current capability, the supply should be designed to reduce ground loop

problems, respond linearly to an input control signal, and minimize output noise. A current

supply with these qualifications has been constructed and tested. Although the supply was

originally designed for testing conventional superconductors at high current levels, it has also

been successfully used in measurements on the high-critical-temperature ceramic superconductors

where the maximum current output was less than 1 A. The supply can produce 1000 A output

current with a noise level of approximately 0.05 A peak-to-peak. The specifics of the current

supply's designs and performance are given.

CRYOGENIC BATHYSPHERE FOR RAPID-VARIABLE-TEMPERATURE CHARACTERIZATION

OF HIGH-TC SUPERCONDUCTORS, John Moreland, Y. Li,* R. Folsom, and T.E. Capobianco,

Review of Scientific Instruments, Vol. 59, No. 12, pp. 2535-2538 (December 1988).

[CEEE/Electromagnetic Technology Division]

*Chinese Academy of Physics, Beijing, China

A bathysphere consisting of an inverted Dewar flask for submersible operation in cryogenic fluids

is used to measure the resistance of superconductors, including high-Tc superconducting copper

oxides, as a function of temperature from 4 K to 300 K. We describe the cryostat incorporating

the bathysphere and present data on NbTi (44% Ti) and YBa2Cu307. i with respective

superconducting transition temperatures of 9.5 K and 91.5 K. There are several advantages to

the bathysphere method. The cryostat is of simple compact design easily adapted to high-field

applications where magnet bore size is a limiting factor. The sample and thermometer are thermolyzed in the dry vapor trapped at the top of the bathysphere. Temperature can be varied

rapidly from 300 K to 4 K at a rate of 1 K/min with less than 0. 1 K thermal lag between the

sample and thermometer.

CRITICAL CURRENTS OF HIGH-TC SUPERCONDUCTORS: PINNING, WEAK LINKS,

CONDUCTION ANISOTROPY, AND CONTACT RESISTIVITIES, J.W. Ekin, R.L. Peterson, and

S.L. Bray, Proceedings of the 1988 Materials Research Society International Meeting on Advanced

10
Materials, Tokyo, Japan, May 30-June 3, 1988, Vol. 6, pp. 135-144 (1989. [CEEE/Electromagnetic

Technology Division]

The coherence length £ of high-Tc superconductors in the c-direction is much smaller than for

conventional superconductors and the c-axis lattice spacing is several times larger. This intrinsic

difference has significant consequences for transport properties. Because the pinning effectiveness

of defects of average size <D> is greatest for <D>/£ = 1 and decreases rapidly for

<D>/£ < 1, defects on the scale of the atomic lattice are not effective as pinning sites in

conventional superconductors (where < D > < < £), but can play an important role in high-Tc superconductors (where <D> — £, depending on crystal lographic direction). Unfortunately,

the size of the atomic lattice spacing relative to £ can also create weak-link effects at lattice

defects. From the practical standpoint of producing high transport critical-current density Jc in

high-Tc superconductors, the conventional problem of pinning-force enhancement is replaced by

weak-link minimization. The rapid decrease of Jc at low magnetic fields (< 10"2 T in YBCO) is described well by a Josephson-junction weak-link model. The transport Jc is also nearly

independent of the angle between the applied magnetic field and suggesting highly convoluted

percolation paths in these materials. Crystal lographic conduction anisotropy is a secondary

limitation on Jc , but emerges as a prime determinant of transport Jc in the absence of weak-link

effects. The effect of conduction anisotropy on transport Jc is described in terms of a currenttransfer model. A summary is given of a method to achieve very low resistivity contacts

(~ 10"10 Q-cm2

) with oxide superconductors. Low resistivity contacts have been fabricated with

both yttrium and thallium based compounds.

RESISTANCE MEASUREMENTS OF HIGH Tc SUPERCONDUCTORS USING A NOVEL

"BATHYSPHERE" CRYOSTAT, John Moreland, Y. Li,* R.M. Folsom, and T.E. Capobianco, IEEE

Transactions on Magnetics, Vol. 25, No. 2, pp. 2560-2562 (March 2, 1989). [CEEE/Electromagnetic

Technology Division]

*Chinese Academy of Physics, Beijing

We have developed a novel cryostat for variable temperature testing of high temperature

superconductors. The cryostat is a bathysphere consisting of an overturned stainless steel Dewar

suspended in liquid helium. A sample-heater-thermometer assembly is located at the top of the

encapsulated (and thermally insulated) vapor space inside of the Dewar. The sample can be

rapidly cycled from 300 K to 4 K at an average rate of 1 K/min with a thermal hysteresis of less

than 0. 1 K. Helium vapor flows through a plug in the bottom of the bathysphere so that pressure

of the vapor is roughly ambient. This provides ample heat transfer to and from the sample to maintain thermal equilibrium in the vapor space. Results for resistance versus temperature of

some high temperature superconductors in a magnetic field are presented. Also various

definitions for thermodynamic and practical Tc 's derived from transport resistivity measurements

are suggested and discussed.

Nb3Sn CRITICAL-CURRENT MEASUREMENTS USING TUBULAR FIBERGLASS-EPOXY

MANDRELS, L.F. Goodrich, S.L. Bray, and T.C. Stauffer, IEEE Transactions on Magnetics, Vol.

MAG-25, No. 2, pp. 2375-2378 (March 1989). [CEEE/Electromagnetic Technology Division]

11
A systematic study of the effect of sample mounting techniques on the superconducting criticalcurrent measurement was made in conjunction with the VAMAS (Versailles Agreement on

Advanced Materials and Standards) interlaboratory comparison (round robin) measurements. A

seemingly small change in mandrel geometry can result in a 40% change in the measured critical

current of a Nb3Sn sample at 12 T. This is a result of a change in the conductor pre-strain at 4 K

caused by variation in thermal contraction between thick- and thin-walled fiberglass-epoxy

composite (G-10) tubes. An approximate measure of the variations in thermal contraction (from

room to liquid nitrogen temperature) indicates a 0.2% greater contraction for the thick-walled

tube. This difference, combined with strain sensitivity measurements, is consistent with the

observed decrease in critical current. Previous publications on the thermal contraction of G-10

have addressed the plate geometry, but not the tube geometry. The contraction of a G-10 plate

is highly anisotropic. The radial contraction of a tube is different than the contraction of a plate,

however, because the circumferential fiberglass is put into hoop compression by the epoxy, and

the resulting contraction is a competition between the two structural components. This appears

to be the source of the variation in thermal contraction with tube wall thickness.

CURRENT CAPACITY DEGRADATION IN SUPERCONDUCTING CABLE STRANDS, L.F.

Goodrich and S.L. Bray, IEEE Transactions on Magnetics, Vol. MAG-25, No. 2, pp. 1949-1952 (March

1989). [CEEE/Electromagnetic Technology Division]

The electromagnetic properties of NbTi strands extracted from Rutherford cables were studied

to clarify the effect of mechanical deformation, caused by the cabling process, on the current

capacity of the strands. Three different cables were studied, all of which are prototypes for the

Superconducting Super Collider's dipole magnets. The extracted cable strands were instrumented

to allow measurement of the voltage across several key regions of mechanical deformation as a

function of current and the orientation of the applied magnetic field. The resulting data are presented in terms of the strand's voltage profile as well as its critical current in order to more

thoroughly characterize the conductor's electromagnetic properties. The cable strands show very

localized reductions in current capacity that are well correlated with the regions of high

mechanical deformation. For example, at a particular field orientation, the voltage across a portion of the strand that is only 3 % of the total strand length contributes 92 % of the total strand

voltage. Two applied magnetic field orientations, parallel and perpendicular to the cable's width,

have pronounced effects on the electrical properties of the strand. Both of these magnetic field

orientations will arise in applications.

CRITICAL-CURRENT MEASUREMENTS OF Nb3Sn SUPERCONDUCTORS : NBS CONTRIBUTION

TO THE VAMAS INTERLABORATORY COMPARISON, L.F. Goodrich and S.L. Bray, Cryogenics,

Vol. 29, pp. 699-709 (July 1989). [CEEE/Electromagnetic Technology Division]

Critical -current measurements on several Nb3Sn superconductors were made as part of an

interlaboratory comparison (round robin). These measurements were made in conjunction with

24 laboratories from the European economic community, Japan, and the USA as part of the

Versailles Agreement on Advanced Materials and Standards (VAMAS). The results of the NBS

measurements, including the effect of sample mounting techniques on the measured critical

current, are given. A systematic study of the effect of measurement mandrel (tubular sampleholder made from G-10 fiberglass-epoxy composite) geometry revealed that a seemingly small

12
change in that geometry can result in a 40% change in the measured critical current at a magnetic

field of 12 T. Specifically, the radial thermal contraction of the measurement mandrel depends

on its wall thickness and, thus, so does the conductor prestrain (at 4 K) and, ultimately, the

measured critical current. Techniques for reducing variation in the measured critical current are suggested.

EFFECT OF ROOM TEMPERATURE STRESS ON THE CRITICAL CURRENT OF NbTi, S.L. Bray

and J.W. Ekin, Journal of Applied Physics, Vol. 65, No. 2, pp. 684-687 (January 15, 1989).

[CEEE/Electromagnetic Technology Division]

The effect of axial tensile stress, applied at room temperature, on the critical current of NbTi

superconducting wire was measured and compared with the effect of tensile stress applied at liquid-helium temperature (~ 4 K). The results of these measurements indicate that the effect on

the critical current is independent of the temperature at which the stress is applied. Thus, the

existing 4-K data base can be used to determine Ic degradation from room-temperature fabrication

stress, cool-down stress introduced by differential contraction, as well as 4-K stress generated by

the Lorentz force when the magnet is energized. To generalize these results for arbitrary matrixto-superconductor volume ratios, the data are presented in terms of the stress on the NbTi portion

of the composite conductor. Methods for determining the stress on the NbTi from the total

composite load are presented.

OXYGEN ISOTOPE EFFECT IN THE SUPERCONDUCTING Bi-Sr-Ca-Cu-0 SYSTEM, H. KatayamaYoshida,* T. Hirooka,* A. Oyamada,* Y. Okabe,* T. Takahashi,* T. Sasaki,* A. Ochiai,* T. Suzuki,*

A.J. Mascarenhas, J.I. Pankove, T.F. Ciszek, S.K. Deb,t R.B. Goldfarb, and Yongkang Li,$ Physica

C, Vol. 156, pp. 481-484 (1988). [CEEE/Electromagnetic Technology Division]

*Tohuku University, Japan

tSolar Energy Research Institute

^Chinese Academy of Physics, Beijing

An oxygen isotope effect is observed in mixed-phase Bi-Sr-Ca-Cu-0 superconductors when 18 0

is substituted for 160. The superconducting transition temperature Tc measured by electrical

resistivity and magnetic susceptibility, is lowered by about 0.32 K for the higher-Tc (1 10 K) phase

and by about 0.34 K for the lower-rc (75 K) phase. These results suggest a measurable

contribution to the superconductivity from phonons.

AIRY PATTERN, WEAK-LINK MODELING OF CRITICAL CURRENTS IN HIGH-TC SUPERCONDUCTORS, R.L. Peterson and J.W. Ekin, Physica C, Vol. 1577, No. 2, pp. 325-333

(1989). [CEEE/Electromagnetic Technology Division]

We have measured the transport critical current density at very low magnetic fields in samples

of superconducting bulk sintered Y,Ba2Cu3Ox , Y,Ba2Cu4Ox , and Ho^ajCujO, obtained from

several sources. The results are analyzed at low fields (< 10 mT) with a statistical model which

assumes that the current is limited by Josephson weak links (SNS or SIS Josephson junctions or microbridges) whose locations are to be determined. Each weak link is assumed to be described

by an Airy current-field pattern rather than a Fraunhofer pattern. The former has a better

13
theoretical foundation and is in better agreement with the data, varying as H

3/2 upon averaging.

The fitting procedure yields the average cross sectional area of the weak links. By assuming the

link thickness to be twice the London penetration depth at 77 K, we find that the average linear

dimensions of the links are in all cases comparable to the grain dimensions. The quantitative

analysis also confirms the percolation concept, in which a subset of weakest links controls the

transport current.

MODELING OF CRITICAL CURRENTS IN GRANULAR HIGH-TC SUPERCONDUCTORS, R.L.

Peterson and J.W. Ekin, Proceedings of High Tc Superconductors: Magnetic Interactions of High Tc Superconductors, Gaithersburg, MD, October 11-13, 1988, Progress in High Temperature

Superconductivity, Vol. 17, pp. 190-195 (1989). [CEEE/Electromagnetic Technology Division]

The transport critical current density of several samples of bulk sintered high-Tc superconductors

was measured at very low magnetic fields and fitted to a model which assumes that the

impediments to current at such fields are Josephson weak links. A sample of particular interest

was Y1 Ba2Cu3 Ox made from hydroxycarbonate precursors; the final bulk sintered sample was very

fine-grained, having an average grain size of about 1.8 urn as determined by a linear intercept

analysis. The fit to the model is excellent if the average linear dimension of the weak links is chosen to be 2.0 fim. We conclude that this sample as well as others has Josephson weak links

at its grain boundaries, and that any intragrain defects which may be responsible for flux pinning

are not the primary weak links limiting the transport Jc of bulk samples at very low magnetic

fields.

CURRENT RIPPLE EFFECT ON SUPERCONDUCTIVE DC CRITICAL CURRENT

MEASUREMENTS, L.F. Goodrich and S.L. Bray, Cryogenics, Vol. 28, pp. 737-743 (1988).

[CEEE/Electromagnetic Technology Division]

The effect of current ripple or noise on d.c. critical current measurements was systematically

studied. Measurements were made on multifilamentary NbTi superconductor. A low-noise,

battery-powered current supply was required in this study in order to make the pure d.c. critical

current measurements. Also, an electronic circuit that stimulates a superconductor's general

current-voltage characteristic was developed and used as an analysis tool. In order to make

critical current measurements in which current ripple was present, the battery supply was

modified to allow the introduction of controlled amounts of a.c. ripple. In general, ripple in a

current supply becomes more significant in current supplies rated above 500 A because effective

filtering is difficult. The effect of current ripple is a reduction in the measured d.c. critical

current; however, ripple of sufficient amplitude can result in arbitrary measurement results. The

results of this work are general and quantitatively applicable to the evaluation of critical current

data and measurement systems. An unexpected benefit of this work was a more precise method

for general critical current data acquisition. Problems common to all large-conductor criticalcurrent measurements are discussed.

VAMAS INTERCOMPARISON OF CRITICAL CURRENT MEASUREMENT IN Nb3Sn WIRES, K.

Tachikawa,* K. Itoh,t H. Wada,t D. Gould,* H. Jones,§ C.R. Walters, || L.F. Goodrich, J.W. Ekin,

14
and S.L. Bray, IEEE Transactions on Magnetics, Vol. 25, No. 2, pp. 2368-2369 (March 1989).

[CEEE/Electromagnetic Technology Division]

*Tokai University, Japan

tNational Research Institute for Metals, Japan

^Commission of the European Communities

§Clarendon Laboratory, United Kingdom

I Rutherford Appleton Laboratory, United Kingdom

The VAMAS technical working party in the area of superconducting and cryogenic structural

materials has recently carried out the first world-wide intercomparison of critical current, Ic , measurement on multifilamentary Nb3Sn wires. Three sample wires were supplied from each of

EC (European Communities), Japan and USA. The total number of participant labs were 24 (EC

11, Japan 8 and USA 5). There were few restrictions for the Ic measurement at participant labs.

The standard deviations of the I

c values reported from these labs varied among test samples, and

were 6-21% of averaged Ic 's at 12 Tesla.

INTEGRITYTESTS FOR HIGH-TC AND CONVENTIONALCRITICAL-CURRENT MEASUREMENT

SYSTEMS, L.F. Goodrich and S.L. Bray, Advances in Cryogenic Engineering Materials, Vol. 36, Part

A, pp. 43-50 (1989). [CEEE/Electromagnetic Technology Division]

Critical-current measurement systems must be extremely sensitive to the small differential voltage

that is present across the test specimen as it changes from the zero resistance state to the flux-flow

resistance state. Consequently, these measurement systems are also sensitive to interfering

voltages. Specific methods for testing the sensitivity of critical-current measurement systems and

for detecting the presence of interfering voltages are discussed. These include a simple procedure

that simulates the zero resistance state and the use of an electronic circuit that simulates the fluxflow resistance state.

THERMAL CONTRACTION OF FIBERGLASS-EPOXY SAMPLE HOLDERS USED FOR Nb3Sn

CRITICAL-CURRENT MEASUREMENTS, L.F. Goodrich, S.L. Bray, and T.C. Stauffer, Advances

in Cryogenic Engineering Materials, Vol. 36A, pp. 1 17-124 (1989). [CEEE/Electromagnetic Technology

Division]

It is typical for Nb3Sn-Cu superconductor specimens to be wound into coils on tubular specimen

holders for critical-current measurements. If the thermal contraction of the holder is different

than that of the specimen, axial strain may be applied to the specimen upon cooling from room

to liquid-helium temperature. This strain can affect the measured critical current. The thermal

contraction was measured for three different Nb3Sn-Cu superconductors. Also, the thermal

contraction was measured for several different specimen holders, all of which were made from

fiberglass-epoxy composites. The specimen holder measurements were made using an electricalresistance strain-gage technique, and they were confirmed by direct mechanical measurements.

The tubes varied in diameter, wall thickness, and fabrication technique. Some of the tubes were

made directly from tube stock, an others were machined from plate stock. The results of these

measurements show that the thermal contraction of tube stock is strongly dependent on the ratio

of its wall thickness to its radius, while the contraction of tubes machined from plate stock is relatively independent of these dimensions. Critical-current measurements of Nb3Sn-Cu

15
specimens mounted on these various holders show that the presence of differential thermal

contraction between the specimen and its holder can significantly affect the measured critical

current.

S-N-S BEHAVIOR OF GRAIN BOUNDARIES IN POLYCRYSTALLINE La185Sr0 ^CuO^, A. Roshko,

J.S. Moodera,* Y.-M. Chiang,* to appear in the Proceedings of the International M^-HTSC Conference,

Stanford, California, July 1989, Physica C, Vol. 162-164, pp. 1625-1626 (1989).

[CEEE/Electromagnetic Technology Division]

*Massachusetts Institute of Technology

The transport critical current density in polycrystalline Lat g5 Sr0 15Cu04,y samples in which the

microstructure has been well characterized and with densities greater than 95% of theoretical, has

been measured in fields from 4 x 10s to 15 T, at temperatures varied from 13 K to just below

Tc

. The dependence of critical current on temperature and magnetic field in zero and low fields,

close to Tc , is consistent with S-N-S behavior, as expected from analyses of grain boundary

composition.

DOUBLE-STEP BEHAVIOR OF CRITICAL CURRENT VS. MAGNETIC FIELD IN Y-,

Bi- AND Tl-BASED BULK HIGH-7; SUPERCONDUCTORS, J.W. Ekin, T.M. Larson, A.M.

Hermann,* Z.Z. Sheng,* K. Togano,t and H. Kumakura,t Physica C 160, pp. 489-496 (1989).

[CEEE/Electromagnetic Technology Division]

University of Arkansas

tNational Research Institute for Metals, Japan

A double-step characteristic is observed at 76 K in the transport critical current as a function of

magnetic field (10

4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high-7c superconducting

materials. The low-field, step-like drop in the critical current density Jc commences at magnetic

fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant

critical current extending from about 30 to 300 mT, and then a second drop at fields between

about 0.3 and 10 T. These features occur for all three superconductor materials and are

interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and

a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current (VI) characteristic, represented by the transition parameter n (i.e., Vx/*), has a similar double-step

shape as a function of magnetic field directly corresponding to the features of the JC (B)

characteristic.

ELECTROMECHANICAL PROPERTIES OF SUPERCONDUCTORS FOR HIGH-ENERGY PHYSICS

APPLICATIONS, Part II, J.W. Ekin, L.F. Goodrich, S.L. Bray, N.F. Bergren, R.B. Goldfarb, National

Institute of Standards and Technology, NISTIR 89-3912, 154 p. (November 1989).

[CEEE/Electromagnetic Technology Division]

This report presents data on superconductor performance under mechanical load. The data are needed for setting mechanical design constraints and measuring the electromechanical

performance of NbTi superconductors for DOE high-energy physics magnet applications. The

effect of axial tensile stress, applied at room temperature, on the critical current of NbTi

16
superconductor strands has been measured. The data show a simple result that the effect on the

critical current is independent of the temperature at which the stress is applied; this allows the

existing 4 K data base to be used to determine critical current degradation from room temperature

fabrication stress, from cool-down stress introduced by differential contraction, and from 4 K

stress generated by the Lorentz force when the magnet is energized. A study of the criticalcurrent variations along NbTi strands extracted from a Rutherford cable has been made also. The

results show that the principal mechanical degradation is extremely localized at the regions where

the NbTi strand is bent around the edge of the cable. For example, only 3% of the total strand

length can contribute 92% of the total strand voltage. A further study has been made of the

effects of bending strain on the critical current of NbTi conductors. The degradation of the

critical current from bending strain is much greater at low values of electric field than at high,

suggesting that irregularity of the filament cross-sectional area introduced by bending may be the source of the Ic degradation. The bend tolerance of an NbTi conductor can be enhanced by

increasing the local copper-to-superconductor area ratio. Measurements of the permeability,

saturation magnetization, and intrinsic coercivity of several high-permeability steel alloys were

made. The overall differences of saturation magnetization and intrinsic coercivity between coldrolled steel samples were not significant.

NOVEL PROCEDURE FOR MAPPING THE Jc-Hc2-Tc SURFACE AND ITS APPLICATION TO HIGH

TEMPERATURE SUPERCONDUCTORS, John Moreland, Y.K. Li,* L.F. Goodrich, A. Roshko, and

R.H. Ono, Proceedings of the Conference on the Science and Technology of Thin-Film Superconductors,

April 30-May 4, 1990, Denver, CO, Science and Technology of Thin-Film Superconductors 2, R.D.

McConnell and R. Noufi, editors, New York: Plenum Press, pp. 429-438 (1990).

[CEEE/Electromagnetic Technology Division]

*Chinese Academy of Science, Beijing

We have used an ac lock-in method for measuring the dynamic resistance versus current (dV/dl-I) curves to determine JC (T,H). The sample current consisted of a small constant oscillating current

added to a variable dc current. The dc current was either slowly ramped for dV/dl-I

measurements or controlled keeping the dV/dl-I constant while ramping temperature or field.

In this way, it was possible to measure JC (T) at constant H. The temperature was controlled

between 4 and 300 K using a bathysphere cryostat. The bathysphere cryostat was inserted into a high field magnet for measurements at fields up to 7 T. We have measured several high

temperature superconductors including Y1 Ba2Cu307

. a thin films. In addition, we have measured

the dV/dl-I curve of a simulator with a V-I curve having the form V = 10 /*V x (1/1.4 A)13

. Upon numerical integration, our data for the simulator are consistent with those obtained by a dc

method using an analog dc nanovoltmeter to measure the V-I curves directly.

HIGH Tc SUPERCONDUCTORS AND CRITICAL CURRENT MEASUREMENT, L.F. Goodrich and

S.L. Bray, Cryogenics, Vol. 30, pp. 667-678 (August 1990). [CEEE/Electromagnetic Technology

Division]

With the introduction of high Tc superconductors, a number of problems associated with critical

current, Ic , measurement have arisen. The existing /c measurement practices were developed and

proved for low Tc superconductors. There are substantial differences between the two classes of

materials. When the Ic concept was casually extended to the high Tc conductors, measurement

17
inconsistency, ambiguity and, in some cases, invalidity followed. A discussion of the underlying

philosophy of Ic measurement is presented and a number of measurement variables that can

influence the measured Ic are discussed. Many of the problems stem from inadequate reporting

practices, and recommendations are given for improving measurement reports.

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS, FINAL REPORT, JANUARY 1988-

DECEMBER 1989, L.F. Goodrich, R.B. Goldfarb, S.L. Bray, National Institute of Standards and

Technology, NISTIR 90-3935, 148 p. (January 1990). [CEEE/Electromagnetic Technology Division]

A cooperative program with the Department of Energy, the National Institute of Standards and

Technology, other national laboratories, and private industry is in progress to develop standard

measurement practices for use in large scale applications of superconductivity. This report

describes research for the period January 1988 through December 1989. It contains the results

of critical current studies on a large conductor Reference Material (RM), the effect of powersupply current ripple, the measurements of large conductors, and an interlaboratory comparison

(round robin) of Nb3Sn wires. Short-range variations in magnetic hysteresis loss in

multifilamentary Nb3Sn were studied. The results of participation in several interlaboratory

comparisons are described.

CURRENT SUPPLY FOR HIGH-TC SUPERCONDUCTOR TESTING, S.L. Bray and L.F. Goodrich,

Measurement Science Technology, Vol. 1, pp. 491-494 (1990). [CEEE/Electromagnetic Technology

Division]

Precise and accurate measurements of the DC critical current of high-Tc superconductors often

require a current supply that has high input-to-output isolation, high isolation from ground and

low output ripple. Also, to ensure precise current control, the supply should have low current

drift and its input-output characteristic should be linear. A design for a simple and inexpensive

current supply that has these characteristics is presented. The primary power source is a 12 C

wet-cell battery and the typical operating range is from 10 mA to 10 A. The supply's current

ripple is <0.4 mA RMS over this operating range. At an output current of 5 A the current drift

is <2% h"

1 without a warm-up period. The maximum variation of the supply's output from

linearity, over its full operating range, is <5%.

TRANSPORT CRITICAL CURRENT OF ALIGNED POLYCRYSTALLINE Y^CujO^ AND

EVIDENCE FOR A NONWEAK-LINKED COMPONENT OF INTERGRANULAR CURRENT

CONDUCTION, J.W. Ekin, H.R. Hart, Jr.,* and A.R. Gaddipati,* Journal of Applied Physics, Vol.

68, No. 5, pp. 2285-2295 (September 1, 1990). [CEEE/Electromagnetic Technology Division]

*GE Corporate Research and Development

A study of grain alignment and its effect on the dc transport critical current in fine-grained bulk

Y1 Ba2Cu3 07

.a is reported in magnetic fields from 10"4 T to 26 T. Two features distinguish the

critical current density Jc of aligned bulk YiBa2Cu307. s from unaligned material. First, the

effective critical field where the intergranular Jc approaches zero is about four times higher (30 T)

for aligned samples with field parallel to the a, b planes, than it is for polycrystalline unaligned

samples (7 T). Second, the nearly field independent plateau value of Jc between 10 mT and 1 T

18
is one to two orders of magnitude higher than typical plateau values of Jc in unaligned bulksintered Y1 Ba2Cu307.5 , for field parallel to the a, b planes. A low-field (< 10 mT) weak-link

decrease in Jc with magnetic field is still observed, but it is much smaller than for unaligned

material. These data clearly demonstrate that alignment alone significantly reduces the weak-link

problem in fine-grained polycrystalline samples with low-aspect-ratio (4:1) grains (unlike meltgrown samples where there has been some ambiguity as to the relative importance of alignment

versus large grain growth). Furthermore, the results provide strong evidence that there are two

parallel components of intergranular current conduction, one consisting of weak-linked material,

the other behaving like intrinsic intragranular material that is not weak-linked. A comparison

with unaligned Y^BajC^O,^ indicates that the volume fraction of such nonweak-1 inked material

is significantly enhanced by grain alignment, but still only 0.01%-0.1% of the grain boundary

area. Field-cooled and force-free, Jc data are also presented, along with detailed measurements

of the shapes of the voltage-current characteristics.

SWITCHING IN HIGH-TC SUPERCONDUCTOR CURRENT TRANSPORT MEASUREMENTS, L.F.

Goodrich, John Moreland, and A. Roshko, Proceedings, Applied Superconductivity Conference,

September 24-28, 1990, Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27, No. 2, pp.

1194-1197 (March 1991). [CEEE/Electromagnetic Technology Division]

Switching voltages can occur in four-wire current transport measurements of sintered high-Tc superconductors. These switching voltages are irreversible shifts in the voltage-current

characteristic of the superconductor that result in multiple branches. The voltage along these

branches can be very nonlinear as a function of current and can be positive or negative in polarity

relative to the current direction. These voltages can interfere with the correct determination of

resistivity and critical current density. Experimental data on unaligned sintered Yfid^Cn^O-,^

which illustrate the complex nature of the voltages and the confusion they can create are presented. Models based on weak links and Hc „ and on other effects, are discussed as are observations on NbTi and Nb3Sn based superconductors.

SOFTWARE TECHNIQUES TO IMPROVE DATA RELIABILITY IN SUPERCONDUCTOR AND

LOW-RESISTANCE MEASUREMENTS, L.F. Goodrich and A.N. Srivastava, NIST Journal of

Research, Vol 95, No. 5, pp. 575-589 (September-October 1990). [CEEE/Electromagnetic Technology

Division]

Software techniques have been developed to take low-amplitude data in various patterns, assign a figure of merit to a set of data readings, edit data for erroneous readings (or other experimental

variations), and to alert the experimenter if the detected errors are beyond the scope of the

software. Erroneous voltage readings from digital voltmeters, intermittent electrical connections,

and an array of similar variations in data have been detected through the use of a data editor.

The fixed limit data editor removes readings that are inconsistent with the distribution of the

majority of the data readings. The frequency of erroneous readings from a particular digital

voltmeter ranges from 1 error per 100 000 readings to 1 error per 100 readings. The magnitude

of the error can be as large as 3 percent of full scale with a zero volt input to the voltmeter. It may be necessary to have multiple meters measuring voltages in the same circuit in order to generate these erroneous readings. A systematic study was performed on the occurrence of the

internally generated erroneous voltmeter readings, and it was determined that the amount that a

19
reading was in error scaled with one of a few parameters. The software techniques described

here have been used in a variety of measurements, such as resistance-versus-temperature

measurements made on cryoconductors or superconductors, and voltage-versus-current

measurements made on superconductors to determine the critical current.

CRITICAL-CURRENT DIFFRACTION PATTERNS OF GRAIN-BOUNDARY JOSEPHSON WEAK

LINKS, R.L. Peterson and J.W. Ekin, Physical Review B, Vol. 42, No. 13, pp. 8014-8018

(November 1, 1990). [CEEE/Electromagnetic Technology Division]

We discuss the diffraction patterns and other characteristics of the critical current as a function

of magnetic field in grain-boundary Josephson barriers. Diffraction patterns occur not just for

SIS junctions but for all types of Josephson links, including SNS junctions, which may be present

at grain boundaries in high-Tc superconductors. We discuss the generality of the Airy diffraction

pattern, which is expected to characterize grain-boundary barriers in bulk material more

accurately than the Fraunhofer pattern. The transport critical-current density in many bulk,

granular high-Tc superconductors has a power-law dependence on very low magnetic fields,

characteristic of averaged diffraction patterns, and cannot be fitted by an exponential magneticfield dependence, which may result from the material properties of the barriers.

ENHANCED FLUX CREEP IN Nb-Ti SUPERCONDUCTORS AFTER AN INCREASE IN

TEMPERATURE, R.W. Cross and R.B. Goldfarb, Applied Physics Letters, Vol. 58, No. 4, pp. 415-416

(January 28, 1991). [CEEE/Electromagnetic Technology Division]

The magnetic fields of Superconducting Super Collider (SSC) dipole magnets change with time

when the magnets are operated at constant current. The decay of the field is though to be a consequence of flux creep in the Nb-Ti filaments in the superconducting cables. However,

measured magnetic relaxation of small samples of SSC cable as a function of time is unlike the

large decays that are observed in the fields of the actual magnets. We have made relaxation

measurements on sample SSC conductors at 3.5 and 4.0 K after field cycling. The decay at both

temperatures was 2.8% in 50 min. However, the relaxation measured after a temperature

increase from 3.5 to 4.0 K was 4.8% in 50 min. A likely reason for the greater magnetization

decay is that, after an increase in temperature, the Nb-Ti is in a supercritical state, with shielding

currents flowing at a density greater than the new critical current density. This causes enhanced

flux creep. We suggest that a small temperature rise during the operation of SSC magnets may

contribute to the unexpectedly large magnetic field decay.

EFFECT OF MECHANICAL DEFORMATION ON Nb-Ti FILAMENT PROXIMITY-EFFECT

COUPLING AT THE EDGES OF SSC CABLES, T.W. Petersen and R.B. Goldfarb, Proceedings,

Applied Superconductivity Conference, September 24-28, 1990, Snowmass, Colorado, in IEEE

Transactions on Magnetics, Vol. 27, No. 2, pp. 1809-1810 (March 1991). [CEEE/Electromagnetic

Technology Division]

20
Magnetization as a function of transverse magnetic field and time was measured for short strands

extracted from the centers and edges of five Nb-Ti Rutherford cables designed for use in

Superconducting Super Collider dipole magnets. The multifilamentary strands all had 6-/-im

diameter filaments. Edge samples, which had severe mechanical deformation, showed small

magnetic coupling losses at low fields, compared to no coupling losses for undeformed center

strands. Sharp strand bends at cable edges decreases the interfilament spacing to the order of the

coherence length in the normal matrix material which increases the effective filament diameter

and hysteresis loss at low fields. Microscopic studies of the cables' cross sections confirmed

smaller interfilament separations in these samples. Flux creep measurements, represented by the

time dependence of magnetization, showed little difference between edge and center samples.

This indicates that the proximity-coupled matrix in edge samples is not a significant source of flux

creep.

CURRENT RECORD IN SUPERCONDUCTORS, J.W. Ekin, K. Salama,* and V. Selvamanickam,*

Nature, Vol. 350, p. 26 (March 7, 1991). [CEEE/Electromagnetic Technology Division]

*University of Houston, Texas

Critical currents in excess of 200 A at 30 T have been measured at 4.2 K in melt grown

YBa2Cu307

. Critical currents of about 10 A were obtained at 30 T at 77 K. The results

demonstrate the possibility of obtaining high supercurrents at both liquid helium and liquid

nitrogen temperatures for applications at magnetic fields of 30 T or more.

WEAK-LINK-FREE BEHAVIOUR OF HIGH-ANGLE YBa^uA^ GRAIN BOUNDARIES IN HIGH

MAGNETIC FIELDS, S.E. Babcock,* X.Y. Cai,* D.L. Kaiser, and D.C. Larbalestier, Nature, Vol.

347, No. 6289, pp. 167-169 (September 13, 1990). [IMSE/Metallurgy Division]

*University of Wisconsin

A crucial issue in both the physics and the application of high-temperature superconductors is the

low transport critical current density (JJ of polycrystalline materials. Much thinking about this

issue has been defined by the thin-film YBa2Cu307^ bicrystal experiments of Dimos et al., which

clearly showed greatly reduced values of Jrt at high-angle grain boundaries (mis-orientation angle ^ 10°). The reproduciblity of this result for a wide range of misorientation relationships led

them to conclude that all high-angle grain boundaries act intrinsically as Josephson junctions. By

contrast, we describe here two high-angle grain boundaries with superconducting properties that

clearly lack the weak-link signatures characteristic of Josephson junctions. Our bulk-scale

bicrystal results provide direct evidence that not all high-angle grain boundaries are alike in their

superconducting properties, and that at least some high-angle boundaries can carry significant

supercurrents at 77 K in high magnetic fields.

21
CRITICAL CURRENT BEHAVIOR OF Ag-COATED YB^CuA.,, R.H. Ono, J.A. Beall, T. Harvey,

C. Reintsema, M. Johansson, M. Cromar, L.F. Goodrich, J. Moreland, A. Roshko, and T.C. Stauffer,

IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 1471-1474 (March 1991). [EEEL/Electromagnetic

Technology Division]

We have studied the behavior of high-quality YBa2Cu307

.x (YBCO) thin films with Ag overlayers. We chose to study Ag in detail because of its widespread use as contact metalization and

our earlier studies of proximity effects in YBCO. The details of transport critical current

measurements are presented. The Ag coatings can reduce normal state resistance while not

degrading the critical current density, Jc.

22
3. CRYSTAL STRUCTURE

X-RAY STUDIES OF HELIUM-QUENCHED BajYC^O^, Winnie Wong-Ng and Lawrence P. Cook,

Advanced Ceramic Materials: Ceramic Superconductors, Vol. 2, No. 3B, pp. 624-631 (July 15, 1987).

[IMSE/Ceramics Division]

A series of 1 1 samples of Ba2YCu307.x (x = 0 to 1) was prepared from single phase orthorhombic

material by annealing at temperatures from 400 to 1000°C, followed by rapid quenching. All

quenchings were performed using a liquid nitrogen-cooled copper cold well through which helium

gas was passed. Lattice parameters were obtained for these materials by X-ray powder diffraction

methods to obtain dependence of cell volumes and crystal symmetry on annealing temperature.

CRYSTAL STRUCTURE OF THE HIGH-TEMPERATURE SUPERCONDUCTOR La! 85 Sr0 15Cu04 ABOVE AND BELOW Tc , R.J. Cava,* A. Santoro, D.W. Johnson, Jr.,* and W.W. Rhodes,* Physical

Review B, Vol. 35, No. 13, pp. 6716-6720 (May 1, 1987). [IMSE/Reactor Radiation Division]

*AT&T Bell Laboratories, Murray Hill

The crystal structure of La, g5Sr0 15Cu04 has been determined at 300, 60, and 10 K by neutrondiffraction powder profile analysis. The structure is of the tetragonal K2NiF4 type at ambient

temperature, but undergoes an orthorhombic distortion near 200 K which buckles the copperoxygen planes. The crystal structure of the orthorhombic phase is not significantly different

above and below Tc. The copper-oxygen coordination polyhedron changes shape in a subtle

manner at the tetragonal-to-orthorhombic phase transition, but is unchanged at Tc

.

NEUTRON STUDY OF THE CRYSTAL STRUCTURE AND VACANCY DISTRIBUTION IN THE

SUPERCONDUCTOR Ba^C^O^, F. Beech, S. Miraglia, A. Santoro, and R.S. Roth, Physical Review

B, Vol. 35, No. 16, pp. 8778-8781 (June 1, 1987). [IMSE/Reactor Radiation Division]

"

Two samples of the high-temperature superconductor Ba2YCu30c>.J with 6 = 2.0 and 2.2 have

been studied at room temperature and at 10 K, with the neutron powder diffraction method and

profile analysis. The structure of the compound is orthorhombic. The oxygen atoms are located

on four sets of sites and not on five sets as reported earlier in x-ray-diffraction studies. Of the

two sets of copper atoms, one is surrounded by four oxygen atoms at distances 1.9299(4) and

1.9607(4) A. This arrangement forms a two-dimensional framework of Cu-0 atoms that extends

indefinitely along the a and b axis. A fifth oxygen atom is located at 2.295(3) A from the copper

atom, giving a pyramidal configuration. In the other set, the Cu atoms are also surrounded by

four oxygen atoms, with distances 1.9429(1) and 1.846(2) A. In this case, however, the oxygen

atoms form rectangles connected by vertices, and resulting in chains along the b axis. In the

compound with 5 = 2.0 all oxygen sites are fully occupied. When 8 = 2.2 there are oxygen

vacancies, but these are confined to one set of positions only, specifically to the oxygen atoms

of the chains, located on the b axis. No detectable change of the structure has been observed

between room temperature and low temperature.

23
THE STRUCTURE AND PROPERTIES OF BaaYCuA, A. Santoro, S. Miraglia, and F. Beech, S.A.

Sunshine,* D.W. Murphy,* L.F. Schneemeyer,* and J. V. Waszczak,* Materials Research Bulletin, Vol.

22, pp. 1007-1113 (1987). [IMSE/Reactor Radiation Division]

*AT&T Bell Laboratories, Murray Hill

The structure of Ba2YCu306 has been determined by neutron diffraction powder profile analysis.

The cell is tetragonal P4/mmm with a = 3.8570(1) A and c = 11.8194(3) A. The cations are

in a perovskite type arrangement, with Ba and Y ordered on the A site to give a cell tripled along

c. The oxygens occupy only 2/3 of the perovskite anion sites and are ordered such that 1/3 of

the Cu is two-fold coordinated and 2/3 is five-fold coordinated. This configuration can be

derived from that of the superconductor Ba^CuA by removing the oxygen atoms along the b

axis at the (0,1/2,0) position. A pressed pellet exhibited semiconductivity with a band gap of

0.21 eV from 125-300 K. The compound is described as Ba^Cu^Cu^O^

X-RAY POWDER STUDY OF 2BaO:CuO, W. Wong-Ng, K.L. Davis, and R.S. Roth, Journal of the

American Ceramic Society, Vol. 71, No. 2, pp. C-64 to C-67 (1987). [IMSE/Ceramics Division]

A compound of composition 2BaO:CuO was synthesized during the phase equilibria study of the

BaO-Y203-CuOx system. Phase characterization has been carried out by using X-ray powder

diffraction. The crystal symmetry was found to be the same as that of Ca2Cu03 and Sr2Cu03

.

It is orthorhombic with space group Immm and lattice parameters a = 12.9655(14),

b = 4.1007(3), c = 3.9069(5) A, V = 207.72(3) A. The experimental pattern shows good

agreement, in general, with intensity values calculated by assuming Ba2Cu03 to have a structure

similar to that of Sr2Cu03 and Ca2Cu03 . Intensity discrepancy for the hOO reflections might be

due to preferred orientation.

NEUTRON POWDER DIFFRACTION STUDY OF ORTHORHOMBIC Ba2YCu3 065 , S. Miraglia, F.

Beech, A. Santoro, D. Tran Qui, S.A. Sunshine,* and D.W. Murphy,* Materials Research Bulletin,

Research Bulletin, Vol. 22, pp. 1733-1740 (December 1987). [IMSE/Reactor Radiation Division]

*AT&T Bell Laboratories, Murray Hill

The structure of orthorhombic Ba^CuAs has been refined with neutron diffraction profile

analysis. The space group of the compound is Pmmm, and the lattice parameters are a = 3.8468(1), b = 3.8747(1), and c = 11.7466(5) A. The oxygen sites at (0,0,z), l/2,0,z),

and (0,l/2,z) are fully occupied, while the sites at (0,1/2,0) are 50% full and the sites at (1/2,0,0) are vacant. A comparison of bond distances in this sample and in other compositions in the

system Ba^CuA (6.0 < 6 < 7.0) shows that the environment of the Ba atoms and of the Cu

atoms at (0,0,0) and (0,0,z) change significantly with the amount of oxygen in the unit cell.

THE X-RAY POWDER DIFFRACTION PROFILE STUDIES ON YBa2Cu3O70 AND YBa^uA 8 , Y.

Zhang,* W. Wong-Ng, B. Morosin,t C.R. Hubbard, J.M. Stewart,* and S.W. Freiman, Physica C, Vol.

152, pp. 130-132 (1988). [IMSE/Ceramics Division]

*University of Maryland

tSandia National Laboratories

24
Two barium yttrium cuprate compositions having different oxygen stoichiometries, YBa2Cu3O70

and YBa2Cu3068 , were prepared from a single phase material under different processing

conditions. A comparison of the X-ray powder diffraction (XRD) patterns of these two samples

showed a significant difference in diffraction line breadth. X-ray diffraction line profile analysis

has been employed to study the profile differences.

POSSIBLE EVIDENCE FOR SUPERCONDUCTING LAYERS IN SINGLE CRYSTAL Y^CuA-x

BY FIELD ION MICROSCOPY, A.J. Melmed, R.D. Shull, C.K. Chiang, and H.A. Fowler, Science,

Vol. 239, pp. 176-178 (January 8, 1988). [IMSE/Ceramics Division and Metallurgy Division]

The high-transition-temperature superconducting ceramic material YBa2Cu3 07. x (0 < x < 0.5)

has been examined by field ion microscopy. Specimens from nominally superconducting and

nonsuperconducting samples (determined by magnetic susceptibility measurements) were studied

by field ion microscopy and significant differences were found. Preferential imaging of atomic

or molecular layers, due to preferential field evaporation, field ionization, or both, was found in

the superconducting phase below the transition temperature and is interpreted as possible evidence

for the occurrence of relatively highly conducting layers in the YBa2Cu307

. x unit cell

perpendicular to the orthorhombic c-axis. Similar results were obtained for YbBa^UjO^.

SIGNIFICANCE OF PLANE VERSUS CHAIN SITES IN HIGH-TEMPERATURE OXIDE SUPERCONDUCTORS, Gang Xiao,* M.Z. Cieplak,* D. Musser,* A. Gavrin,* F.H. Streitz,* C.L. Chien,*

J.J. Rhyne, and J.A. Gotaas, Nature, Vol. 332, No. 6161, pp. 238-240 (March 17, 1988).

[IMSE/Reactor Radiation Division]

*Johns Hopkins University

One of the outstanding questions concerning the high-temperature superconductor YB^CuA is the relative importance of the Cu02 planes (Cu(2) site) and the CuO chains (Cu(l) site). Many

theories have stressed the importance of the 3d holes at the Cu sites, which provide an antiferromagnetic correlated background necessary for the coupling of the superconducting electrons. We

have substituted Zn(3dw

4s2

) and Ga(3J104j,24/?

1 ) in the Cu(2) and Cu(l) sites, respectively, while

maintaining the oxygen stoichiometry at 7. In the valence states of 2+ and 3+ respectively, the

3d bands of both elements are completely full, without the complication of a magnetic moment.

Here we discuss the structural changes that accompany Zn and Ga substitution, as deduced from

X-ray and neutron diffraction, and correlate these changes with the superconducting properties.

We show that the integrity of the planes is much more important than that of the chains in

sustaining high-Tc superconductivity.

STRUCTURAL PHASE TRANSITIONS OF Ba2RCu306+x SUPERCONDUCTORS, W. Wong-Ng, L.P.

Cook, C.K. Chiang, L.J. Swartzendruber, and L.H. Bennett, Adv. Cer. Materials, pp. 27-42 (May

1988). [IMSE/Ceramics and Metallurgy Divisions]

In order to gain more insight into the effect of oxygen stoichiometry on superconductivity and

the correlation between the size of the rare-earth elements and superconductivity, the phase

transitions between the orthorhombic and tetragonal structures of several high Tc superconductors

25
Ba2RCu306+x (where R = Sm, Y, Gd and Er; x

of oxygen content achieved by quenching in air. = 0 to 1) have been investigated as a function

Various measurements including x-ray diffraction, thermogravimetric analysis, and determination

of Meissner effects were carried out in order to correlate the nature of the phase transition with

crystallographic data, superconductivity and annealing temperature.

Phase transformations from orthorhombic Ba2RCu307 to tetragonal Ba2RCu306 are apparently

second order and may involve two orthorhombic regions which correspond to two Tc plateaus,

region (A), Tc of 92 K with cell parameters a < b « c/3 and region (B), Tc of from 52 to 60 K

with a < b < c/3. The transformations from orthorhombic to tetragonal for all four compounds

takes place in a temperature range of 625 to 770°C. The temperature range of the region B

plateaus also varies with different R. Both structural phase transition temperatures and

characteristics of the Tc plateaus appear to follow a trend anticipated from the dependence of the

ionic radius on the f electron count.

BISMUTH VALENCE ORDER-DISORDER STUDY IN BaBi03 BY POWDER NEUTRON

DIFFRACTION, C. Chaillout, A. Santoro, J.P. Remeika,* A.S. Cooper,* G.P. Espinosa,* and M.

Marezio,t Solid State Communications, Vol. 65, No. 11, pp. 1363-1369 (1988). [IMSE/Reactor

Radiation Division]

*AT&T Bell Laboratories, Murray Hill

tLaboratoire de Cristallographie du CNRS

Two structural arrangements have been shown to exist for the perovskite-like compound

BaB&jB&j by powder neutron diffraction data. The first is characterized by a partial (75%)

order of the Bi3+ and Bi5+ cations on the two crystallographically independent sites, while in the

second arrangement the two cations are almost 100% disordered. The structure of eleven

differently-prepared BaBi03 samples showed that the deciding factor for obtaining one or the

other arrangement is the temperature at which the sample is prepared or subsequently heattreated. On the contrary, the cooling rate and the atmosphere (air or oxygen) do not seem to be

important parameters. DTA measurements indicated that BaBi03 undergoes an additional phase

transition at 860°C on heating and 801 °C on cooling. This transition probably corresponds to

the disproportionation of the Bi into Bi3+ and Bi5+ cations. This conjecture explains why the

heat-treatment at 800°C yields compounds containing partial cation ordering. Since the symmetry

is cubic above and below the 801 °C transition, the 75% ordered BaBi03 stable between 801 °C

and 480°C would contain a trigonal dynamical distortion which would be the precursor of the

static trigonal distortion which takes place at 480°C. In every compound the partial ordering is always 75% . This could be interpreted as an indication that an additional ordering of Au3Cu type

exists on the two Bi sublattices.

NEUTRON AND ELECTRON DIFFRACTION STUDY OF YBa2Cu22Cul.77 Fe 23 0713 , P. Bordet,* J.L.

Hodeau,* P. Strobel,* M. Marezio,* and A. Santoro, Solid State Communications, Vol. 66, No. 4, pp.

435-439 (1988). [IMSE/Reactor Radiation Division]

*Laboratoire de Cristallographie du CNRS

26
The compound of formula YB2Cu2 7Fe0 307:13 has been analyzed by neutron and electron

diffraction techniques. The material is tetragonal with lattice parameters a = b = 3.8674(1),

c = 11.6687(2) A and space group P4/mmm. The Fe cations substitute only the Cu cations

located on the basal plane of the structure and can adopt three different types of coordination

(tetrahedral, pyrimidal and octahedral) depending upon the content and distribution of the extra

oxygen atoms on the plane. Calculations of the effective valence of iron cations seem to indicate

that Fe3+is present in tetrahedral coordination and Fe4+

in pyramidal and octahedral coordination,

while values of Cu2 2+ and Cu2 47+ were found for the copper cations located at (OOz) and (000),

respectively. _The electron diffraction experiments show diffuse scattering planes parallel to

(110)* and (1 10)*. Crosses of strong intensity are visible at reciprocal nodes located between

the reciprocal lattice layers. This diffuse scattering is interpreted in terms of linear clusters of

iron cations extending along the [1 10] and [110] directions having a width of a few cations. The

clusters are separated by domains of orthorhombic YBa2Cu306+x having the same orientation or

rotated of 90° one with respect to the other.

SUPERCONDUCTIVITY IN LANTHANUM BARIUM CUPRATE PEROVSKITES, S.A. Sunshine,*

L.F. Schneemeyer,* J.V. Waszczak,* D.W. Murphy,* S. Miraglia, A. Santoro, and F. Beech, Journal

of Crystal Growth, Vol. 85, pp. 632-638 (1987). [IMSE/Reactor Radiation Division]

*AT&T Bell Laboratories, Murray Hill

Samples in the system Ba^La^CujO,^ (0.0 < x < 0.5; 0 < 6 < 0.25) have been prepared

and characterized by X-ray powder diffraction, magnetic susceptibility, electrical resistivity, and

for x = 0.5 powder neutron diffraction. This neutron diffraction study on Baj 5La, 5 Cu3072S

indicates that the structure is similar to the 92 K superconductor B^YCujO, with two exceptions.

First, the "Ba site" contains 75% Ba and 25% La and second, the symmetry is tetragonal. At

x = 0.5 the material is a semiconductor, but as x decreases metallic behavior and

superconductivity are observed. Compositions with x < 0.2 contain traces of BaCu02, but

exhibit TT* « 90 K and R = O near 80 K. For x = 0.2-0.3, TT* « 60 K and R = O near 50 K are observed. For x < 0.1 orthorhombic symmetry is observed. These results provide

further evidence that oxygen stoichiometry and details of the oxygen ordering are important to

superconductivity.

ANTIFERROMAGNETIC ORDER IN YBa^u^CoA+y, P.F. Miceli,* J.M. Tarascon,* L.H. Greene,*

P. Barboux,* M. Giroud,* D.A. Neumann, J.J. Rhyne, L.F. Schneemeyer,t and J.V. Waszczak,t

Physical Review B, Vol. 38, No. 13, pp. 9209-9212 (November 1, 1988). [IMSE/Reactor Radiation

Division]

*Bell Communications Research

tAT&T Bell Laboratories, Murray Hill

We present a neutron scattering study of oxygen-deficient YBa^u^Co^+y where, with

increasing Co concentration, long-range antiferromagnetic order develops on the chain sites. For

x = 0.2, two magnetic transitions are found: Below TN2 = 211 K, there is an ordered moment

on both Cu sites, while for TN2 < T < TN1 = 415 K only a "plane" moment is present. For

x = 0.8, an ordered moment appears on both Cu sites with a single transition at 435 K.

Evidence for magnetic ordering was also found by magnetic susceptibility measurements. These

27
results imply that Co substitution enhances antiferromagnetic coupling between the "plane" and

"chain" sites.

CRYSTAL CHEMISTRY OF SUPERCONDUCTORS: A GUIDE TO THE TAILORING OF NEW

COMPOUNDS, A. Santoro, F. Beech, M. Marezio,* and R.J. Cava,* Physica C, Vol. 156, pp. 693-700

(1988). [IMSE/Reactor Radiation Division]

AT&T Bell Laboratories, Murray Hill

The crystal structures of the known superconducting copper oxides can be described in terms of

two basic structural types. The series La2Can.

1 Cun 02ll+ 2, Cn302(Ba>Sr)2Can.

1 CunO2n+ 4 and

TlBaaCan.jCUnOjn+a can be viewed as made of alternating slices having the rock salt and

perovskite structure. The compounds Ba2YCu4Og and Ba4Y2Cu7014+x, on the other hand,

comprise of perovskite blocks alternating with blocks in which a crystallographic shear is present.

The effect of this shear is that of forming double chains of edge sharing squares with oxygen

atoms at the corners and copper atoms at the center. The superconductor Ba2YCu307 can be

described in terms of both structural types and may be considered as an intermediate type between

the other two. The basic building blocks of these superconducting materials can be further

broken down into constituent nets (or meshes). This description allows one to envisage new

structures built from these meshes containing the key structural elements present in the currently

known superconductors. As such, the structural schemes used in this description may be used

as a guide in the preparation of new materials with interesting electronic properties.

NEUTRON POWDER DIFFRACTION STUDY OF Pb2Sr2YCu3 08 , THE PROTOTYPE OF A NEW

FAMILY OF SUPERCONDUCTORS, R.J. Cava,* M. Marezio,* J.J. Krajewski,* W.F. Peck, Jr.,* A.

Santoro, and F. Beech, Physica C, Vol. 157, pp. 272-278 (1989). [IMSE/Reactor Radiation Division]

AT&T Bell Laboratories, Murray Hill

The structure of Pb2Sr2YCu3Og has been analyzed with powder neutron diffraction techniques and

profile analysis. The results of this study confirm the general structural features determined by

X-ray single-crystal methods. The material has pseudo tetragonal symmetry, but is orthorhombic,

space group Cmmm with lattice parameters a = 5.3933(2), b = 5.4311(2), and

c = 15.7334(6) A. The orthorhombic distortion is caused by the disordering in the a lane of the

oxygen atoms of the PbO layers over the general position of the space group, with x = 0.275(5)

and y = 0.402(5) A. The structure of this compound can be derived from mat of YBa2Cu3 06 by

substituting the Cu layers with blocks of ...PbO.Cu.PbO... . Of the two Cu atoms in the

asymmetric unit, one has two fold coordination while the other has five fold pyramidal

coordination with the apex elongated along the c-axis.

MAGNETIC TRANSITIONS IN THE SYSTEM YBa2Cu28Co02O6+y , P.F. Miceli,* J.M. Tarascon,* P.

Barboux,* L.H. Greene,* B.G. Bagley,* G.W. Hull,* M. Giroud,* J.J. Rhyne, and D.A. Neumann,

Physical Review B, Vol. 39, p. 12375 (1989). [IMSE/Reactor Radiation Division]

*Bell Communications Research

We have studied the oxygen dependence of the two magnetic transitions (antiferromagnetic

ordering of chains and planes) in YB^C^ gCOo.jOe+y using neutron scattering. It is found that

28
both transition temperatures increase with decreasing oxygen concentration. At y « 0.37 ( = y0) the two transition temperatures are equal, so that chains and planes order at a single transition

temperature for y <

y0

. For y = 1 the compound is superconducting at 60 K. Therefore, this

system qualitatively exhibits the magnetic and superconducting properties of pure YBa2Cu3 06+y

while providing an important insight on the oxygen dependence of chain site magnetic ordering.

A discussion is presented which also includes results on Ni and Al substitutions.

STRUCTURE AND MAGNETIC PROPERTIES OF NONSUPERCONDUCTING DOPED Co AND

Fe-Bi2Sr2Cu,.xMxOy

, J.M. Tarascon,* P.F. Miceli,* P. Barboux,* D.M. Hwang,* G.W. Hull,* M.

Giroud,* L.H. Greene,* Yvon Le Page,t W.R. McKinnon,t E. Tselepis,t G. Pleizier,t M. Eibschutz,$

D.A. Neumann, and J.J. Rhyne, Physical Review B, Vol. 39, p. 11587 (1989). [IMSE/Reactor

Radiation Division]

*Bell Communications Research

tNational Research Council, Canada

$AT&T Bell Laboratories, Murray Hill

The structure and magnetic properties of the Bi2Sr2Cui.xMxOy (M = Co and Fe) materials were

studied. The limits of solid solution formation are at x = 0.5 for the Fe system and x = 1 for

the Co system. Crystals of the new Bi2Sr2CoOy phase were grown and the structure established

by x-ray crystallography. The subcell is the same as that of the 10 K superconductor,

Bi2Sr2CuOy

, but the superstructure is different, as it exhibits a commensurate modulation of

periodicity 4 instead of 5. Extra oxygen is accommodated in the Bi layers, as in Bi2Sr3Fe20y

, and

the structure of the Bi-0 layers can be described as 50% rocksalt-type and 50% oxygen-deficient

perovskite for x = 1, but with disorder at the oxygen positions. The formal valence of Co in

this compound is about 2.5 + 0.2 as deduced from structural and chemical measurements,

whereas Fe adopts the oxidation state +3 as deduced by Mossbauer measurements. Bi2Sr2CuOy

is an antiferromagnetic insulator with the spins lying within the Co02 sheets and the

antiferromagnetic ordering temperature (TN) is sensitive to processing conditions and composition

changes. The high anisotropy of the susceptibility suggests that Bi2Sr2CuOy may be an Ising or xy antiferromagnet.

CRYSTALLOGRAPHY, Anthony Santoro, Chapter 4 in High Tc Superconductivity, New York:

Springer-Verlag, pp. 84-121 (1990). [IMSE/Reactor Radiation Division]

The structures of the superconducting compounds belonging to the systems BaPb,.xBix03 , La2.xMxCu04_y, Ba2YCu3Ox , Ba2.xLa, +xCu3Ox , and Bi2CaSr2Cu2Ox are described in detail in this

chapter, and their relationship with the structure of perovskite is illustrated from a structural point

of view. The role of neutron diffraction to locate light atom in the presence of heavy metals is underlined, and a brief description of the Rietveld method is given. Compounds with

crystal lographic shear, such as BajYC^Og and Ba4Y2Cu7 0i4+x, are briefly described at the end

of the chapter.

X-RAY DIFFRACTION STUDY OF A THERMOMECHANICALLY DETWINNED SINGLE

CRYSTAL OF YBa2Cu306+x, Winnie Wong-Ng, Frank W. Gayle, Debra L. Kaiser, Steven F. Watkins,*

29
and Frank R. Fronczek,* Physical Review B, Vol. 41, No. 7, pp. 4220-4223 (March 1, 1990).

[IMSE/Ceramics Division]

*Louisiana State University

A twin-free single crystal of YBa2Cu3 06+x (YBC) has been studied at room temperature and

115 K by x-ray diffraction analysis. The crystal was grown from a Y-Ba-Cu-0 melt in a gold

crucible, leading to a 2% substitution of Au for Cu. Twins were removed from the as-grown

crystal by a novel thermomechanical process, resulting in an orthorhombic (Pmmm), singledomain crystal. Oxygen positions and occupancies in the Cu-0 basal plane have been refined,

showing that the 0(5) site is completely vacant, and the 0(4) atoms are offset from the

crystallographic mirror plane positions by 0. 15 A in a zig-zag fashion. Gold, which is a common

impurity in crystals grown by the present technique, was found to occupy Cu(l) sites only. Weak

superlattice reflections suggest a possible three-dimensional ordering of O and/or Au. To our knowledge, this paper reports the first high resolution x-ray study of an intentionally detwinned,

superconducting YBC single crystal.

STRUCTURAL PHASE TRANSFORMATION STUDIES OF THE HIGH Tc SUPERCONDUCTING

MATERIALS Ba2RCu3 06+x, IN AIR,, W. Wong-Ng, L.P. Cook, C.K. Chiang, M.D. Vaudin, D.L.

Kaiser, F. Beech, L.J. Swartzendruber, L.H. Bennett, and E.R. Fuller, Jr., Journal of Material Research,

Vol. 3, No. 5, pp. 832-839 (September/October 1988). [IMSE/Ceramics Division]

To understand the crystal chemistry and to gain more insight into the effect of oxygen

stoichiometry on superconductivity, we have investigated the phase transformations between the

orthorhombic and tetragonal structures of several high Tc superconductors, Ba2RCu306+x, where

R = Sm, Gd, Y, Ho and Er, and x = 0 to 1. Various techniques including X-ray diffraction,

thermogravimetric analysis, neutron scattering, transmission electron microscopy, and

determination of Meissner effect have been used to study the nature of the phase transition.

AC magnetic susceptibility measurements suggest that the transformation from the oxygen-rich

orthorhombic phase to the oxygen-deficient tetragonal phase involves two orthorhombic phases,

designated here as O(A) and 0(B). A present there is no clear evidence of where the phase

boundary between 0(A) and 0(B) is. Electron diffraction studies performed on an yttrium

material quenched from 670 °C show both a splitting of the strong diffraction spots as a result of

twinning and also extra, weak, diffuse reflections indicating that the a-axis cell dimension is doubled. Neutron diffraction results show the absence of long-range ordering of oxygen; the

electron diffraction results correspond to short-range oxygen ordering.

The structural phase transition temperatures, oxygen stoichiometry and characteristics of the Tc plateaus appear to follow a trend anticipated from the dependence of the ionic radius on the

number of f electrons as R progresses across the lanthanide series. Rare earth elements with a smaller ionic radius stabilize the orthorhombic phase to higher temperatures and lower oxygen

content. Also, the superconducting properties, viz., the Tc values, are less sensitive to the

oxygen content for materials with smaller ionic radius.

30
CRYSTAL STRUCTURE, ATOMIC ORDERING AND CHARGE LOCALIZATION IN

Pb^Y^C^CiijOg+i (x = 0, 6 = 1.47), M. Marezio,*t A. Santoro, J.J. Caponi,t E.A. Hewat,$ R.J.

Cava,* and F. Beech, Physica C, Vol. 169, pp. 401-412 (1990). [IMSE/Reactor Radiation Division]

*AT&T Bell Laboratories, Murray Hill

fLaboratoire de Cristallographie, France

$LETI-CENG, France

Neutron, X-ray and electron diffraction measurements have been carried out on

Pb2Sr2Y1 .xCaxCu308+i samples. The oxygen incorporated in the structure during the oxidation is located on the (Cu) planes sandwiched between the two (PbO) layers. A theoretical composition

of 6 = 2 is possible, although in practice only a stoichiometry corresponding to 5 = 1 .9 has been

achieved so far. The extra oxygen present for 6 > 0 forms ordered structures in which the Cu

cations of the (CuOa) planes have square planar coordination in Pb2Sr2YCu309 , either square

planar, pyramidal, and octahedral, or only pyramidal coordination in Pb2Sr2YCu3 09 5 , and

exclusively octahedral coordination in Pb2Sr2YCu3 Oi0 . In the range of composition 0 < 8 < 1,

mixtures of two phases are obtained, one with 6 = 0 stoichiometry and the other with 6=1, and

whose relative quantities depend on the total amount of oxygen incorporated by the sample. The

positive charges induced in Pb2Sr2YCu3 Og+a by oxygen incorporation oxidize the Cu1+ cations to

2 + and some of the Pb2+ cations to 4+ . An order between Pb2+ and Pb4+is established and this

localization hinders the charge transfer to the conducting (Cu02) planes and, for this reason, no

superconductivity is present in oxidized samples. The cation valences are estimated from the co- ordination numbers and from the bond length-bond strength relationship.

Pb2Sr2YCu3Og becomes superconducting at ~ 80 K when some of the trivalent Y cations are replaced by divalent Ca. In this case the extra positive charges oxidize the Cu2+ cations in the

Cu02 planes instead of Pb2+

to 4+ and Cu1+ to 2 + , as does the incorporation of oxygen. This

different behavior can be explained as a concentration effect which changes the

oxidation/reduction potentials. When heat treated at 500°C in 02, Pb2Sr2YCu3Og behaves

similarly to the undoped compound. The oxygen uptake suppresses the superconducting transition

which is re-established by heat treating the sample at the same temperature in N2

.

INCORPORATION OF GOLD INTO YBa^uA: STRUCTURE AND Tc ENHANCEMENT, Marta

Z. Cieplak,*t Gang Xiao,* C.L. Chien,* A. Bakhshai,* D. Artymowicz,* W. Bryden,* J.K. Stalick,

and J.J. Rhyne, Physical Review B, Vol. 42, No. 10, pp. 6200-6208 (October 1, 1990). [IMSE/Reactor

Radiation Division]

*The Johns Hopkins University

tAlso with Polish Academy of Sciences

The structural and superconducting properties of gold-doped YBa^^Oy compounds have been

studied with x-ray and neutron diffraction, optical microscopy, weight-loss analysis,

magnetization, and resistivity measurements. The solubility of Au in YBa2Cu307 is close to 10

at. %, below which all samples are thermally stable up to the processing temperature (950°).

Metallic Au grains precipitate out when the Au content exceeds 10 at. %. Analysis of the x-ray

and neutron-scattering intensities shows that Au substitutes for the Cu(l) chain site exclusively

and has a probable valence state of Au3+

. The main effect of Au doping is a substantial uniaxial

lattice expansion, while the orthorhombic structure is preserved. We present a detailed set of

structural data including the Cu-0 bond lengths and interplanar distances in a Au-doped sample.

31
Unlike all other known dopants on Cu sites, the Au dopant is not detrimental to the

superconducting properties. On the contrary, Tc is slightly enhanced over that of YBa2Cu307

. Such an enhancement, observed for the first time for a Cu site dopant, is intrinsic and

reproducible. We discuss the possible causes of the Tc increase and the features of Au dopant

useful in the investigation of high-Tc superconductivity.

NEUTRON DIFFRACTION STUDY OF THE "BROWN PHASE" BaNd2Cu05 , Judith K. Stalick and

Winnie Wong-Ng, Materials Letters, Vol. 9, No. 10, pp. 401-404 (June 1990). [MSEL/Ceramics

Division]

A neutron Rietveld refinement study of the "brown phase" BaNd2Cu05 shows that this material

is isostructural with the La analog, but adopts a totally different structure from that of the "green

phase" BaR2Cu05 formed for R = Y and most lanthanide elements. BaNd2Cu05 is tetragonal,

space group P4/mbm(126), with Z = 2 and refined cell parameters a = 6.7015(1) and

c = 5.8211(1) A. The BaNd2Cu05 framework is built from edge- and face-sharing BaO10 and

NdOg polyhedra, while in the green phase structure the smaller R

3+ ion is surrounded by seven oxygen atoms. The square-planar Cu04 groups are isolated in the structure.

32
4. ELASTIC CONSTANTS AND PHONON SPECTRA

THE BULK MODULUS AND YOUNG'S MODULUS OF THE SUPERCONDUCTOR Ba^YO,, S.

Block, G.J. Piermarini, R.G. Munro, and W. Wong-Ng, Advanced Ceramic Materials: Ceramic

Superconductors, Vol. 2, No. 3B, p. 601 (July 15, 1987). [IMSE/Ceramics Division]

The isothermal equation of state of the high temperature superconducting ceramic material

Ba^CugOy has been determined by measurements in a diamond anvil pressure cell using an energy dispersive x-ray diffraction method. The orthorhombic unit cell lattice parameters

(a = 3.8856 A, b = 11.6804 A, c = 3.8185 A) were found to have compressions of (2.0%,

2.3%, 1.1%) respectively over the pressure range from one atmosphere to 10.6 GPa at room

temperature. Subsequent equation of state analysis of the approximately linear compression of

the volume determined that the isothermal bulk modulus was 196 ± 17 GPa. Young's modulus

was estimated to be 235 ± 20 GPa assuming that the Poisson's ratio for BajYCuA was 0.3

which is typical of many ceramics.

PHONON DENSITY OF STATES OF SUPERCONDUCTING YBa^uA AND THE NONSUPERCONDUCTING ANALOG YBa2Cu3 06, J.J. Rhyne, D.A. Neumann, J. A. Gotaas, F. Beech, L. Toth,*

S. Lawrence,* S. Wolf,* M. Osofsky,* and D.U. Gubser,* Physical Review B, Vol. 36, No. 4, pp.

2294-2297 (August 1, 1987). [IMSE/Reactor Radiation Division]

*Naval Research Laboratory

Neutron scattering has been used to study the vibrational density of states and the atomic structure

of the high-temperature superconductor YBajCuA and the analogous nonsuperconducting

compound YBajCu-A The density of states of the superconductor shows a strong double peak

at about 20 meV and a second major maximum near 70 meV with additional less-intense features

present at intermediate energies. The YBa2Cu306 material shows a similar energy spectrum above

45 meV; however, below this energy there are significant differences associated with the O

vacancies in the linear b-axis chain site.

ELASTIC CONSTANTS AND DEBYE TEMPERATURE OF POLYCRYSTALLINE Y,Ba2Cu3 07. x , H.M. Ledbetter, M.W. Austin, S.A. Kim, and Ming Lei, Journal of Materials Research, Vol. 2, No. 6,

pp. 786-789 (November-December 1987). [IMSE/Fracture and Deformation Division]

Using ultrasonic methods, the quasi-isotropic elastic stiffness of void-containing Y,Ba2Cu3 07

_ x were determined. By a composite-material model, these were corrected to the void-free state. From these, the Debye characteristic temperature was calculated. All the elastic stiffnesses fall well below those of polycrystalline BaTi03

, an approximate crystal-structural building block of

Y,Ba2Cu3 07

. x

. The low apparent stiffness may result from oxygen vacancies, which soften

interionic forces. Also, it may result from microcracks, which reduce elastic stiffness without

lowering mass density.

SHEAR-MODULUS CHANGE BELOW Tc IN YB^CuA.,, H.M. Ledbetter, M.W. Austin, S.A. Kim,

T. Datta,* C.E. Violet,t Journal of Materials Research, Vol. 2, No. 6, pp. 790-792 (NovemberDecember 1987). [IMSE/Fracture and Deformation Division]

33
*University of South Carolina

tLawrence Livermore National Laboratory

The ultrasonic transverse-wave velocity in the high-Tc metal-oxide superconductor YBa2Cu307.x between 275 and 4 K was measured. Above Tc the velocity shows normal behavior: during

cooling, it displays a monotonic increase with decreasing slope. During cooling below Tc the

shear modulus departs from normal behavior, increasing as AG/G = b[l - (T/Tc)

n

], where n = 3.

These results depart dramatically from those expected for a simple second-order normalsuperconducting phase transition.

POISSON-RATIO ANOMALIES IN THE Y.Ba^O^ SUPERCONDUCTOR, H.M. Ledbetter, S.A.

Kim, T. Datta,* J. Estrada,* and C.E. Violet,t Physical Review Letters, October 1987. [IMSE/Fracture

and Deformation Division]

*University of South Carolina

tLawrence Livermore National Laboratory

When cooled from 295 to 4 K, polycrystalline Y,Ba2Cu307.x shows anomalous Poisson-ratio

behavior. Instead of an expected slow and steady decrease, with a minimum at zero temperature,

the superconductor shows two additional minima and maxima, the latter centered at 190 K and

70 K. Magnetic-susceptibility transition begins at 80 K. Thus, the lower maximum relates to the superconducting phase transition. Because the Poisson ratio reveals interatomic-bonding

changes, we ascribe the upper maximum to small electronic changes, possibly precursors to the

superconducting transition.

LOW-TEMPERATURE ELASTIC CONSTANTS OF A Y1 Ba2Cu3 07. x SUPERCONDUCTOR, H.M.

Ledbetter, S.A. Kim, M.W. Austin, T. Datta,* J. Estrada,* and C.E. Violet,f Physical Review B,

October 1987. [IMSE/Fracture and Deformation Division]

*University of South Carolina

tLawrence Livermore National Laboratory

Using ultrasonic methods, from 295 to 4 K, we determined the elastic constants of the high-Tc superconductor YjBa2Cu307. x

. We report the Poisson ratio and three elastic-stiffness moduli:

bulk, shear, Young. During cooling, the stiffnesses increase smoothly and monotonically until

Tc , where they increase abruptly. Above Tc , the shear modulus increases normally; but the bulk

modulus shows a slightly abnormal increase. Especially, the Poisson ratio behaves abnormally

versus temperature.

REENTRANT SOFTENING IN PEROVSKITE SUPERCONDUCTORS, T. Datta,* C. Almasan,* J. Estrada,* C.E. Violet,t and H.M. Ledbetter, Physical Review Letters, Vol. 37, No. 13, pp. 7502-7505

(May 1, 1988). [IMSE/Fracture and Deformation Division]

*University of South Carolina

tLawrence Livermore National Laboratory

We suggest a model—reentrant elastic softening—that achieves three useful results. First, and

principally, it reconciles existing sound-velocity-elastic-constant measurements with

34
thermodynamics. Second, it leads to Debye characteristic temperatures that agree with those

from specific-heat and phonon density-of-states determinations. Third, it links elastic-constanttemperature behavior in Y-Ba-Cu-0 and La-Sr-Cu-O. The model predicts a superconducting-state

elastic stiffness lower than the normal state.

MEASUREMENT OF SUPERCONDUCTOR STOICHIOMETRY BY PROMPT AND DELAYED

NEUTRON ACTIVATION, Richard M. Lindstrom, Transactions, American Nuclear Society, Nuclear

Activation Analysis/Applications-I, Vol. 56, pp. 231 (1988). [CAC/Inorganic Analytical Research

Division]

Prompt and delayed neutron activation analysis have been applied to the nondestructive

measurement of the major metals in small samples of YBajCuaO, superconductors, starting

materials, and intermediate products. The small activation cross section and low gamma ray

energies of Y-90 m make the determination of yttrium imprecise in the presence of the large

quantities of Cu-64 radioactivity. The physical parameters for neutron-capture prompt

gamma-ray activation analysis (PGAA) are more attractive than delayed counting for

stoichiometry measurements. The PGAA spectra of Y, Ba, and Cu show three, four, and six

analytically useful gamma ray lines respectively, with nicely balanced intensities for the

composition range of interest. A 100 mg sample gives a Poisson precision for yttrium (the

weakest element) of 2% after only a 2 hour irradiation. Since the concentrations and mole ratios

Y/Cu and Ba/Cu may be calculated immediately after the irradiation ends, the results can be

returned within a day.

APPLICATION OF WAVELENGTH DISPERSIVE DIGITAL COMPOSITIONAL MAPPING TO

HIGH-TEMPERATURE SUPERCONDUCTORS, R.B. Marinenko, D.E. Newbury, D.S. Bright, R.L.

Myklebust, and J.E. Blendell, Microbeam Analysis 1988, D.E. Newbury, Editor, San Francisco

Press, pp. 37-40 (1988). [CAC/Gas and Particulate Science Division, and IMSE/Ceramics Division]

The new high-Tc superconductors are advanced ceramic materials prepared from oxide powders

which are pressed and sintered. Their physical properties depend on the microstructure, which

in turn depends strongly on the processing sequence used in their production. We have used

wavelength-dispersive (WDS) x-ray compositional mapping to study the microstructure of the

Ba2YCu306+x ceramics being produced at the National Bureau of Standards so as to understand

better the processing procedures and the resulting physical properties.

ELASTIC PROPERTIES OF METAL-OXIDE SUPERCONDUCTORS, Hassel Ledbetter, Journal of

Metals, Vol. 40, No. 1, pp. 24-30 (January 1988). [IMSE/ Fracture and Deformation Division]

This article reviews the elastic properties of the new perovskite-related superconductors,

especially YiBa2Cu307

. x

. Considered are such related matters as atomic size, crystal structure,

microstructure, twins, and microscopic mechanism. Also examined are related physical

properties such as Debye characteristic temperature, thermal expansivity, specific heat, ultrasonic

attenuation, and phonon density-of-states.

35
REENTRANT SOFTENING IN COPPER-OXIDE SUPERCONDUCTORS, C.E. Violet,* T. Datta,t

H.M. Ledbetter, C. Almasan,t and J. Estrada,! in High Temperature Superconductors, Materials

Research Society Symposium Series, Vol. 99, pp. 375-378 (1988). [IMSE/Fracture and Deformation

Division]

*Lawrence Livermore National Laboratory

fUniversity of South Carolina

We suggest a model—reentrant elastic softening—that achieves three useful results. First, and

principally, it reconciles existing sound-velocity -elastic-constant measurements with

thermodynamics. Second, it leads to Debye characteristic temperatures that agree with those

from specific heat and phonon density-of-states. Third, it links elastic-constant— temperature

behavior in Y-Ba-Cu-0 and La-Ba-Cu-O. The model predicts a lower superconducting-state

elastic stiffness.

ELASTIC CONSTANTS OF POLYCRYSTALLINE YjBaAbO^, H.M. Ledbetter, S.A. Kim, and

D.W. Capone,* in High-Temperature Superconductors II, Materials Research Society Symposium Series,

pp. 293-296 (1988). [IMSE/Fracture and Deformation Division]

*Materials Science Division, Argonne National Laboratory

We report the elastic constants of a polycrystalline Y^ajCujOy., superconductor between 295 and

4 K. Roughly speaking, the elastic constants behave regularly. However, small departures from

regular behavior provide much interest. We draw the following conclusions from the

measurements: (1) Elastic constants show irregularities below and above Tc . During cooling,

all the elastic constants show an irregularity near 200 K. (2) Within measurement error (about

1 part in 1000), none of the elastic stiffnesses shows an abrupt change at Tc , 91 K. (3) Below

Tc , all the elastic constants show regular behavior, except v (Poisson ratio). (4) During cooling,

between 160 and 70 K, the material behaves like it undergoes a sluggish phase transformation.

(5) The large (4 percent) decrease in Poisson ratio is unexpected. It indicates large interatomicforce changes. (6) The unusual flatness of v(T) near 295 K suggests unusual material -property

changes above ambient temperatures.

HYSTERETIC PHASE TRANSITION IN Y1 Ba2Cu307.x SUPERCONDUCTORS, H.M. Ledbetter and

S.A. Kim, Physical Review B, Vol. 38, No. 16, pp. 11857-11860 (December 1, 1988). [IMSE/Fracture

and Deformation Division]

We studied ultrasonic-wave velocities, both longitudinal and shear, in Y1 Ba2Cu3 07.x , between 5

and 295 K, during both cooling and warming. Both waves, especially the longitudinal, show

thermal hysteresis. The results suggest a hysteretic phase change that occurs between 160 and

170 K during cooling and between 170 and 260 K during warming. This phase-change

hypothesis explains anomalies in several physical properties. The phase change agrees with

thermodynamic-instability predictions. We confirmed the hysteresis in Ho-Ba-Cu-O, where it is smaller man in Y-Ba-Cu-O, and in Eu-Ba-Cu-O, where it is larger. In a companion perovskite,

BaTi03 , we observed zero hysteresis. At Tc , 91 K, sound velocities show no measurable change

in either magnitude or slope. This continuity disputes the current popular view that, contrary to thermodynamics, elastic stiffness increases upon cooling through Tc into the superconducting

36
state. We believe that stiffening results from the usual thermal effects after a phase

transformation from a stiffer phase.

ELASTIC CONSTANTS OF THE POLYCRYSTALLINE Bi-Pb-Sr-Ca-Cu-0 SUPERCONDUCTOR,

H.M. Ledbetter, S.A. Kim, R.B. Goldfarb, and K. Togano,* Physical Review B, Vol. 39, No. 13, pp.

9689-9692 (May 1, 1989). [IMSE/Fracture and Deformation Division and CEEE/Electromagnetic

Technology Division]

*National Research Institute for Metals, Japan

For polycrystalline (Bi-Pb)2Sr2Ca2Cu301o, we report the 5 - 295-K elastic constants: shear

modulus (G), Young modulus (E), bulk modulus (fl), and Poisson ratio (v). Both G and E show

nearly normal temperature variation. During cooling, B softens between 215 and 65 K. The

Poisson ratio decreases abruptly at 215 K and decreases almost five percent in the 295 - 5-K

region. In its temperature dependence, this compound resembles YBa2Cu3 07 . However, it shows

much lower elastic stiffness. Corrected to the void-free state, we calculate a Debye temperature

of 312 K, versus 436 K for YBa^u-A. The Poisson ratio, 0.20, agrees well with that of

YBa2Cu307 , 0.21 . We use Kresin's model to interrelate Debye temperature, critical temperature,

and electron-phonon interaction parameter for four classes of copper-oxide superconductors.

MADELUNG POTENTIALS AND VALENCES IN THE Y.Ba^uA SUPERCONDUCTOR, H.

Ledbetter and M. Lei,* Physica C, Vol. 166, pp. 483-485 (1990). [IMSE/Fracture and Deformation

Division]

institute for Metals Research, PRC

Using Ewald's method, we calculated the ion-site potentials and Madelung energy of YiBa2Cu307 (orthorhombic, Pmmm, No. 47). We considered the effects of copper-ion and oxygen-ion

valences. Among seven suggested copper-oxygen ion-charge configurations, only two gave a low

electrostatic bonding energy to agree with oxygen vacancies on 01 sites. Only one configuration

gives a realistic bulk modulus. We perturbed this configuration by equalizing the Cul-Cu2

valences and introducing a hole into the Cu02 plane at the oxygen sites. This configuration also

agrees well with observation.

GRUNEISEN PARAMETER OF Y,Ba2Cu307 , Hassel Ledbetter, Physica C, Vol. 159, pp. 488-490

(1989). [IMSE/Fracture and Deformation Division]

Contrary to reports that the Griineisen parameter of Y1 Ba2Cu307 is approximately 3.0, I argue

that this parameter is approximately 1.5, a value consistent with metal oxides. I offer three

arguments. One depends on a lower bulk modulus (B), near 101 GPa, than found in highpressure X-ray diffraction studies, which yield bulk-modulus values up to 200 GPa. The second

depends on an ionic-crystal -model calculation of the Griineisen parameter. The third depends on

the Anderson-Griineisen parameter determined by measuring dB/dT.

37
IS Y^CiiaOy STIFF OR SOFT?, H. Ledbetter and M. Lei,* Journal of Materials Research, Vol. 5,

No. 2, pp. 1-4 (February 1990). [IMSE/Fracture and Deformation Division]

institute for Metals Research, PRC

Using several measured and calculated physical properties, we argue that the high-Tc metal-oxide

superconductor YjBajCujOy is elastically soft compared with BaTi03 or SrTi03. We conclude

that the bulk modulus equals approximately 107 GPa, despite several high-pressure-x-ray

diffraction studies that report values up to approximately 200 GPa. Part of the argument uses an ionic-crystal-model calculation of the bulk modulus.

ELASTIC CONSTANTS, DEBYE TEMPERATURES, AND ELECTRON-PHONON PARAMETERS

OF SUPERCONDUCTING CUPRATES AND RELATED OXIDES, Hassel Ledbetter, Ming Lei,* and

Sudook Kim, Phase Transitions, Vol. 23, pp. 61-70 (1990). [IMSE/Fracture and Deformation Division]

institute for Metals Research, PRC

Using both measurements and modeling, we studied the cohesive and related properties of several

oxides. Superconducting oxides include La185Sr015CuO4 , Y^^Cu^, and (Bi-Pb)2Sr2Ca2Cu3O10 . Related nonsuperconducting oxides include SrTi03 , BaTi03 , and LajCuO,,. For these materials,

we give the complete quasiisotropic elastic constants corrected to the void-free state. From elastic

constants and atomic volume, we calculated Debye characteristic temperatures, 6D. Using

Kresin's model, valid for all values of the electron-phonon parameter, X, we estimated X from

Tc and 0D. For the superconducting cuprates, X ranges from 0.92 (La—0) to 7.96 (Tl —0).

Except perhaps for Tl —0, these parameters fall within a range predicted by theory. For all the

above materials, we show the 295-5-K variation of 0D. We support our elastic-constant

measurements with Born-model calculations of the bulk modulus.

SPECIFIC HEAT OF THE HIGH-TC SUPERCONDUCTOR (Bi,

.

66Pbo.34)Ca2Sr2Cu301 o, R.A. Fisher,*

S. Kim,* Y. Wu,* N.E. Phillips,* H.M. Ledbetter, and K. Togano,t Physica C, Vol. 162-164, pp.

502-503 (1989). [IMSE/Fracture and Deformation Division]

*Lawrence Berkeley Laboratory

tNational Research Institute for Metals, Japan

The specific heat (C) was measured for H=0 and 7T in the ranges 0.4 to 20K and 65 to 125K.

The coefficient of the low-temperature linear term in C was 0+0.5 mJ/K2»mole. On initial cooling, an anomaly in C was observed at Tc , but there were dramatic temperature hysteresis

effects.

38
LOW-TEMPERATURE ELASTIC CONSTANTS OF POLYCRYSTALLINE LajCiA AND

La, g5Sr0 , 5Cu04, Hassel Ledbetter, S.A. Kim, C.E. Violet,* and J.D. Thompson,! Physica C, Vol.

162-164, pp. 460-461 (1989). [IMSE/Fracture and Deformation Division]

*Lawrence Livermore National Laboratory

tLos Alamos National Laboratory

Using ultrasonic methods, we measured the 295--4-K elastic constants of superconductive

La,.g5Sr0 , 5Cu04 and nonsuperconductive LajCuO^ These materials show two elastic-constant

similarities: nearly the same ambient-temperature elastic constants and an elastic-stiffness

minimum in the 20--40-K region. Their principal difference is that La2Cu04 softens 3% during

cooling, while La, ^Sr,, , 5Cu04 softens 30%. This reversible wide-temperature-range softening

resembles a magnetic phase transition. Perhaps it relates to the large drop in spin-excitation

intensity found by Shirane and coworkers using inelastic neutron scattering.

39
5. ELECTRICAL CONTACTS

METHOD FOR MAKING LOW RESISTIVITY CONTACTS TO HIGH-TC SUPERCONDUCTORS,

J.W. Ekin, A.J. Panson,* and B.A. Blankenship,* Applied Physics Letters, Vol. 52, No. 4, pp. 331-333

(January 25, 1988). [CEEE/Electromagnetic Technology Division]

*Westinghouse Research & Development Center

A method for making low-resistivity contacts to high Tc superconductors has been developed,

which has achieved contact surface resistivities less than 10 fiQ cm2 at 76 K and does not require

sample heating above ~ 150°C. This is an upper limit for the contact resistivity obtained at high

current densities up to lOMO3 A/cm2 across the contact interface. At lower measuring current

densities the contact resistivities were lower and the voltage-current curve was nonlinear, having

a superconducting transition character. On cooling from 295 to 76 K, the contact resistivity

decreased several times, in contrast to indium solder contacts where the resistivity increased on

cooling. The contacts showed consistently low resistivity and little degradation when exposed

to dry air over a four-month period and when repeatedly cycled between room temperature and

76 K. The contacts are formed by sputter depositing a layer of a noble metal—silver and gold

were used—on a clean superconductor surface to protect the surface and serve as a contact pad.

External connections to the contact pads have been made using both solder and wire-bonding

techniques.

HIGH-TC SUPERCONDUCTOR/NOBLE-METAL CONTACTS WITH SURFACE RESISTIVITIES IN

THE 10 10 0 CM2 RANGE, J.W. Ekin, T.M. Larson, N.F. Bergren, A.J. Nelson,* A.B. Swartzlander,*

L.L. Kazmerski,* A.J. Panson,t and B.A. Blankenship, t Applied Physics Letters, Vol. 52, No. 21, pp.

1819-1821 (May 23, 1988). [CEEE/Electromagnetic Technology Division]

*Solar Energy Research Institute

fWestinghouse Research and Development Center

Contact surface resistivities (product of contact resistance and area) in the 1010 0 cm2 range have

been obtained for both silver and gold contacts to high Tc superconductors. This is a reduction

by about eight orders of magnitude from the contact resistivity of indium solder connections. The

contact resistivity is low enough to be considered for both on-chip and package interconnect

applications. The contacts were formed by sputter depositing either silver or gold at low

temperatures (< 100°C) on a clean surface of Y,Ba2Cu307.J (YBCO) and later annealing the

contacts in oxygen. Annealing temperature characteristics show that for bulk-sintered YBCO

samples there is a sharp decrease in contact resistivity after annealing silver/YBCO contacts for

1 h above ~600°C. Oxygen annealing for longer times (8 h) did not reduce the contact

resistivity of silver contacts as much as annealing for 1 h. Auger microprobe analysis shows that

indium/YBCO contacts contain a significant concentration of oxygen in the indium layer adjacent

to the YBCO interface. Silver and gold contacts, on the other hand, contain almost no oxygen

and have favorable interfacial chemistry with low oxygen affinity. Silver also acts as a "switchable" passivation buffer, allowing oxygen to penetrate to the YBCO interface at elevated

temperatures, but protecting the YBCO surface at room temperature.

40
EFFECT OF OXYGEN ANNEALING ON LOW-RESISTIVITY CONTACTS FOR HIGH-TC SUPERCONDUCTORS, J.W. Ekin, A.J. Panson,* and B.A. Blankenship,* High Temperature

Superconductors, M.B. Brodsky, H.L. Tuller, R.C. Dynes, and K. Kitazawa, eds., Proceedings,

Materials Research Society, Vol. 99, High Temperature Superconductors, Pittsburgh, PA, pp. 283-286

(1988). [CEEE/Electromagnetic Technology Division]

*Westinghouse Research and Development Center

A method for making low resistivity contacts to high-Tc superconductors has been developed,

consisting of depositing noble metal contact pads (silver or gold) on a clean superconductor

surface at low temperatures (< 150°C). After annealing the silver contact pads in oxygen at intermediate temperatures (<500°C) for one hour, contact resistivities less than 2 x 10"8 Q-cm2

at 76 K are obtained, about six orders of magnitude less than for indium-solder contacts. Before

annealing, the contact resistivities are still very low, in the 10"6 to 10"5 fi-cm2 range at 76 K,

which would be useful when contacts with low fabrication temperatures are required. The

voltage-current characteristics of the contacts are strongly nonlinear after annealing, having a superconducting transition character. This is ascribed to the critical current of the

superconducting material being exceeded at the contact interface. External connections to the

contact pads have been made using both solder and thermosonic wire-bonding techniques.

SILVER SCREEN CONTACTS TO SINTERED YBaCuOx POWDER FOR RAPID

SUPERCONDUCTOR CHARACTERIZATION, John Moreland and L.F. Goodrich, Proceedings of the

Applied Superconductivity Conference, IEEE Transactions on Magnetics, Vol. MAG-25, No. 2, pp.

2056-2059 (March 1989). [CEEE/Electromagnetic Technology Division]

We have developed a new method for making current contacts and voltage taps to YBaCuOx

sintered pellets for rapid superconductor characterization. Ag wire screens are interleaved

between calcined powder sections and then fired at 930°C to form a composite pellet for

resistivity and critical current measurements. The Ag diffuses into the powder during the

sintering process forming a proximity contact that is permeable to oxygen. Contact surface

resistivities (area-resistance product) range from 1 to 10 jt-cm2 at 77 K for the Ag-powder

interface. In this configuration, current can be uniformly injected into the ends of the pellet

through the bonded Ag screen electrodes. Also, Ag screen voltage contacts that span a cross

section of the pellet may provide an ideal geometry for detecting voltage drops along the pellet,

minimizing current transfer effects.

OHMIC CONTACTS TO HIGH-TC SUPERCONDUCTORS, J.W. Ekin, Proceedings of the SPIE

October 1989 Conference, Processing of Films for High Tc Superconducting Electronics, Vol. 1 187, pp.

359-364 (1990). [CEEE/Electromagnetic Technology Division]

This note summarizes and gives references describing the details of a method for reducing high

Tc contact surface resistivities, pc , to the 10" 10 fl cm2 range pc = RA, where R is the contact

resistance and A is the contact area). The reduction was obtained using both gold and silver

contacts, and represents a decrease in contact resistivity by over eight orders of magnitude from

that obtained using indium solder connections. We have obtained most of the results so far for

YBajCujO^ (YBCO) but preliminary data for the Bi and Tl based compounds will also be

summarized.

41
PROSPECTS FOR HIGH TEMPERATURE SUPERCONDUCTOR-SEMICONDUCTOR CONTACTS,

John Moreland, J.W. Ekin, and T. Larson, Proceedings of the ASM International Third Electronic

Materials and Processing Congress, August 20-23, 1990, San Francisco, CA, pp. 195-201 (1991).

[CEEE/Electromagnetic Technology Division]

Development of high temperature superconductor-semiconductor contacts with very low contact

resistivities is required for microelectronic applications such as circuit interconnects or superconductor-semiconductor-superconductor (S/Semi/S) proximity effect devices. Several thinfilm deposition methods have been developed for forming very low resistance contacts to high

temperature superconductors. For example, we have measured contact resistivities as low as

10"10 Q-cm2 for annealed Ag films on sintered YBajQ^O^ samples. Such a contact could

potentially be part of a superconductor-semiconductor contact that relies on an intervening normal

metal, spanning superconductor and semiconductor interfaces. Presently we are surveying

materials commonly used to make ohmic contacts to Si and GaAs for their compatibility with

high-temperature superconductors. Prime material characteristics necessary for potential contact

structures are (1) low chemical reactivity of the intervening materials with high-temperature

superconductors, (2) limited interdiffusion of multilayer constituents, and (3) stable, low contact

resistivity, ohmic I-V characteristics.

PROSPECTS FOR HIGH TEMPERATURE SUPERCONDUCTOR-SEMICONDUCTOR CONTACTS,

John Moreland, J.W. Ekin, and T. Larson, New Technology and Electronic Packaging, B.R. Livesay

and N.D. Nagarkan, editors, Materials Park, Ohio: ASM International, pp. 195-201 (1990).

[CEEE/Electromagnetic Technology Division]

Development of high temperature superconductor-semiconductor contacts with very low contact

resistivities is required for microelectronic applications such as circuit interconnects or superconductor-semiconductor-superconductor (S/Semi/S) proximity effect devices. Several thinfilm deposition methods have been developed for forming very low resistance contacts to high

temperature superconductors. For example, we have measured contact resistivities as low as

10"10 Q-cm2 for annealed Ag films on sintered YBa2Cu307.J samples. Such a contact could

potentially be part of a superconductor-semiconductor contact that relies on an intervening normal

metal, spanning superconductor and semiconductor interfaces. Presently we are surveying

materials commonly used to make ohmic contacts to Si and GaAs for their compatibility with

high-temperature superconductors. Prime material characteristics necessary for potential contact

structures are (1) low chemical reactivity of the intervening materials with high-temperature

superconductors, (2) limited interdiffusion of multilayer constituents, and (3) stable, low-contact

resistivity, ohmic I-V characteristics.

42
6. ELECTRONIC STRUCTURE

RESONANT PHOTOEMISSION STUDY OF SUPERCONDUCTING Y-Ba-Cu-O, Richard L. Kurtz,

Roger L. Stockbauer, Donald Mueller,* Arnold Shin,* Louis E. Toth,* Michael Osofsky,* and Stuart

A. Wolf,* Physical Review B Rapid Communications, Vol. 35, No. 16, pp. 8818-8820 (June 1, 1987).

[CCP/Surface Science Division]

*Naval Research Laboratory

Ultraviolet photoelectron spectra of 93-K superconducting compound, YBa^UjOy, have been

obtained using photon energies ranging from 60 to 106 eV. Resonant photoemission is used to

identify the chemical origin of the features in the valence-band electronic structure.

SOFT X-RAY ABSORPTION AND EMISSION SPECTRA AND THE ELECTRONIC STRUCTURE

OF THE Ba2YCu3 07

. x SUPERCONDUCTOR, K.-L. Tsang,* C.H. Zhang,* T.A. Callcott,* L.R.

Canfield, D.L. Ederer, J.E. Blendell, C.W. Clark, N. Wassdahl,t J.E. Rubensson,t G. Bray,t N.

Mortensson,t J. Nordgren,f R. Nyholm,$ and S. Cramm,§ Physical Review B, Vol. 37, No. 4, pp.

2293-2296, (February 1, 1988). [CRR/Radiation Physics Division, IMSE/Ceramics Division]

*University of Tennessee

tUniversity of Uppsala, Sweden

$University of Lund, Sweden

§Universitat Hamburg, West Germany

We present T-beam-excited soft-x-ray emission spectra and total-photoelectron-yield spectra in

the 20-600 eV photon energy range for Ba2YCu307. x. In soft-x-ray emission, the N45 structure

of Ba, the M4 5 spectrum of Y, and the K spectrum of O provide a direct measure of the /Hype

partial density of states (p-PDOS) localized on the respective atomic sites. In each case the pPDOS is very small at the Fermi energy eF

. The maximum in the O K spectrum is 2.6 + 0.5 eV

below cF. This result provides direct confirmation that the shoulder previously found at eF

- 2.3 eV in photoelectron emission spectra is associated with O p orbitals. No changes are observed between spectra taken above and below Tc

.

SYNCHROTRON RADIATION STUDIES OF THE ELECTRONIC STRUCTURES OF HIGH-TC SUPERCONDUCTORS, Richard L. Kurtz, AIP Proceedings No. 165, American Vacuum Society Series

3: Thin Film Processing and Characterization of High-Tc Superconductors, J.M.E. Harper, R.J. Colton,

and L.C. Feldman, eds., p. 222 (1988). [CBS/Surface Science Division]

Experimental measurements of the electronic structure of the high-Tc superconductors

La2_xSrxCu04 and YBajC^O^ are reviewed and discussed in the context of determining the

strengths of the interactions within the valence bands. It is concluded that the Cu 3d - O 2p

valence bands are highly hybridized and there are strong electron-electron correlations. It is shown that the Cu3+ configuration (3^) is energetically unfavorable and the Cu in these systems

is best thought of as having an approximate valence of +2. The influence of surface

stoichiometry on the electronic structure is discussed and preliminary molecular adsorption studies are reported.

43
TOTAL DIELECTRIC FUNCTION: ALGEBRAIC SIGN, ELECTRON-LATTICE RESPONSE, AND

SUPERCONDUCTIVITY, Philip B. Allen,* Marvin L. Cohen,t and David R. Penn, Physical Review

B, Vol. 38, No. 4, pp. 2513-2525 (August 1, 1988). [CRR/Radiation Physics Division]

*Naval Research Laboratory and Department of Physics, State University of New York, Stony Brook

fDepartment of Physics, University of California, Berkeley

The interaction between two test charges in a solid can be described in terms of a total dielectric

function that includes electronic and lattice polarization. Crystal stability requires the eigenvalues

of e"

1 to be ^1. Some implications for superconductivity are discussed. A total dielectric

function for the electron-lattice system is derived in the mean field approximation and its inverse

is explicitly constructed. The low-lying poles of e

l give the correct phonon frequencies as determined by the usual dynamical matrix.

A PHOTOEMISSION STUDY OF HIGH Tc OXIDES, Donald Mueller,* Arnold Shih,* Louis E. Toth,*

Michael Osofsky,* Stuart A. Wolf,* Richard L. Kurtz, and Roger L. Stockbauer, Physical Review B,

Rapid Communications, Vol. 37, pp. 7936 (1988). [CCP/Surface Science Division]

*Naval Research Laboratory

A number of ceramic compounds including YBa2Cu307 have been reported to superconduct at temperatures in excess of 35 K. A critical feature in the processing of these compounds required

to obtain good superconductive properties appears to be a final annealing stage in an oxygen

containing atmosphere to increase the oxygen content of the materials. Without the oxygen

anneal step, the superconducting transition temperature drops and the drop in the temperature

versus resistance curve becomes less defined.

To aid in the development of a better understanding of the electronic structure in this class of

materials, we have examined changes in the electronic density of states for YBajC^O* with

oxygen content via ultraviolet photoelectron spectroscopy. Samples with x = 6.95, 6.5 and 6.05

were examined.

X-RAY POWDER DIFFRACTION CHARACTERIZATION OF BaR2Cu05 (R= YTTRIUM AND THE

LANTHANIDES) AND RELATED COMPOUNDS, W. Wong-Ng, M.A. Kuchinski, H.F. McMurdie,

and B. Paretzkin, Powder Diffraction, Vol. 4, No. 1 (March 1989). [IMSE/Ceramics Division]

A series of BaO:RxOy :CuO materials, where R=Y, La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er,

Tm, Yb, and Lu, has been prepared and characterized by X-ray powder diffraction methods. All

BaR2Cu05 phases, commonly referred to as the "green phases," are orthorhombic with space

group PBcn and are isostructural with each other. These single phase materials could be prepared

with most lanthanides, except for La, Ce, Pr, Nd, and Tb. Possible reasons for the exceptions

are discussed. Both La and Nd tend to form a brown solid solution of Ba^R^xCu^O,^ with a tetragonal space group of P4/mbm. The major phases found in the Ce, Pr, and Tb

compositions are the perovskite-related structures BaR03 , and in the Pr case, Ba2PrCu3 06+x as well. The cell parameters of the green phase materials increase progressively from the Lu

compound to the Sm compound: 'a' ranges from 7.0506(6) to 7.2754(4) A, 'b' from 12.0534(8)

to 12.4029(7) A, V from 5.6099(5) to 5.7602(3) A, and the cell volume from 467.75(6) to 519.78(4) A

3

. A correlation of the crystallographic data with the size of the R elements is given.

44
RESONANT PHOTOEMISSION AND CHEMISORPTION STUDIES OF Tl-Ba-Ca-Cu-O, Roger L.

Stockbauer, Steven W. Robey, Richard L. Kurtz, D. Mueller,* A. Shin,* A.K. Singh,* L. Toth,* and

M. Osofsky,* AIP Proceedings No. 182, Hi Tc Superconducting Thin Films, Devices, and

Characterization, G. Margaritondo, R. Joynt, and M. Onellion, eds., p. 276 (1989). [NML/Center for

Chemical Physics]

*Naval Research Laboratory, Washington, DC

Resonant photoemission has been used to study the electronic states and electron-electron

interactions in a bulk sample of Tl-Ba-Ca-Cu-0 high temperature superconductor. The electronic

structure, i.e., broad peaks in the valence band at 3.2 and 5.5 eV and satellites at 10 and 13 eV

binding energy, is similar to that observed for the 1-2-3 materials. This indicates that the

electronic states and interactions are similar in these different classes of superconductor.

The surface reactivity has been probed with photoemission using controlled exposures of

atmospheric gases. Both H20 and C02 react with a sticking probability of near 0.2, forming

hydroxide and carbonate species, respectively. 02 is non-reacting while CO reacts only slightly.

RESONANT EXCITATION OF AN OXYGEN VALENCE SATELLITE IN PHOTOEMISSION FROM

HIGH-TC SUPERCONDUCTORS, Richard L. Kurtz, Steven W. Robey, Roger L. Stockbauer, D.

Mueller,* A. Shih,* and L. Toth,* Physics Review B, Vol. 39, No. 7, pp. 4768-4771 (March 1, 1989).

[NML/Center for Chemical Physics]

*Naval Research Laboratory, Washington, DC.

A detailed analysis of the intensities of valence band photoelectron features of superconducting

YBa2Cu307_x and semiconducting La, 85Sr0 05CuO4 has revealed a resonance in the peak located at a binding energy of =9.5 eV for photon energies spanning the onset of O 2s excitations. This

demonstrated conclusively that the feature is associated with oxygen excitations. The origin of

the satellite is described and its disappearance on superconducting surfaces is explained.

ELECTRONIC STRUCTURE OF HIGH-Te SUPERCONDUCTORS STUDIED USING

PHOTOELECTRON SPECTROSCOPY, Richard L. Kurtz, Steven W. Robey, Roger L. Stockbauer, D.

Mueller,* A. Shih,* L. Toth,* A.K. Singh,* and M. Osofsky,* Vacuum, Vol. 39, Nos. 7 & 8, pp. 611-

615 (1989). [NML/Center for Chemical Physics]

*Naval Research Laboratories, Washington, DC.

Fundamental information about the structure of the valence band and the chemical valence states of the various constituents of the La-Sr-Cu, Y-Ba-Cu, Bi-Sr-Ca-Cu, and Tl-Ca-Ba-Cu oxides have

been obtained using photoelectron spectroscopy. These results show that the one-electron theories

do not adequately describe the electronic structure of these superconductors. The atomic origins

of the features observed in the valence bands have been investigated by studying photoemission

resonances and changes in excitation cross-sections with photon energy. Results to date suggest

that these materials have varying densities of states at the Fermi level, valence bands composed

of O 2p and Cu 3d states, and display no significant changes in the band structure associated with

the superconducting behavior when the temperature is lowered below Tc. In addition, the

complex surface chemistry of these oxides makes it essential to study the surface stoichiometry

45
and the interaction of simple molecules. 02 and CO are found to interact only weakly with the

surfaces of the materials studied to date, while H20 and C02 react strongly, forming hydroxides

and carbonates.

ELECTRONIC CORRELATIONS AND SATELLITES IN SUPERCONDUCTING OXIDES, K.J.

Chang,* Marvin L. Cohen,f and David R. Penn, Physical Review B, Vol. 38, No. 13, pp. 8691-8696

(1988). [CRR/Radiation Physics Division]

*Xerox Corporation

tDepartment of Physics, University of California

The satellite observed at binding energy of 12-13 eV below the Fermi level in the high-Tc oxide

superconductors La2.x SrxCu04 and YB^Cu^O^ is viewed as originating from a state with two

holes bound at the same Cu site. As in the case of Ni metal, the satellite is caused by an intra-iishell shakeup process into a Cu 3<f final state and its intensity is enhanced at resonance because

of a super-Koster-Kronig transition. Based on the f-matrix approach for the hole self-energy, we

study the effect of electronic correlations on the one-electron band structure. We examine the

position of the satellite and find a large Coulomb interaction energy of ~5 eV at the Cu site

when the experimental satellite position is duplicated by the theory. Since this energy is comparable with the 3d bandwidth, the two-hole bound state is a high-energy excitation. This

indicates that in the ground state a creation of two holes at the Cu site is unlikely and thus holes

are formed at the O sites when Sr is substituted for La in La2Cu04 and when the oxidation is increased in YBa2Cu3 07^.

OXYGEN PARTIAL DENSITY OF STATES CHANGE IN THE YBajCujO, COMPOUNDS FOR

x « 6, 6.5, 7 MEASURED BY SOFT X-RAY EMISSION, C.H. Zhang,* T.A. Callcott,* K.-L.

Tsang,* D.L. Ederer, J.E. Blendell, C.W. Clark, T. Scimeca,t and Y.-W. Liu,$ Physical Review B,

Vol. 39, No. 7, pp. 4796-4799 (March 1, 1989). [CRR/Radiation Physics Division, IMSE/Ceramics

Division]

University of Tennessee

{"University of Hawaii, Honolulu

^Institute of High Energy Physics, Beijing, China

We present oxygen K soft x-ray emission spectra for the YB^C^O, compounds with x nominally

equal to 6, 6.5, and 7 and compare them with x-ray emission spectra determined from recent

band-structure calculations. The K emission spectrum of O provides a measure of the /Hype

local partial density of states (p-LPDOS) at the oxygen sites. As x decreases from 7-6 to about

6.5, a chemical shift of the entire spectrum to lower energy indicates that screening is modified

for all oxygen sites. The integrated intensity of the spectra is nearly unchanged by oxygen

removal, indicating an increase in /7-LPDOS per oxygen site. These results and changes in the

spectral shape suggest that itinerant electron density near the O atoms is reduced and bound

electron density is increased as oxygen is removed.

46
ELECTRONIC DENSITY OF STATES OF HIGH TEMPERATURE SUPERCONDUCTORS FROM

SOFT X-RAY SPECTROSCOPY, T.A. Callcott,* K.-L. Tsang,* C.H. Zhang,* D.L. Ederer, C.W.

Clark, N. Wassdahl,t J.E. Rubensson,t G. Bray,t N. Mortensson,t J. Nordgren,f R. Nyholm,$ and

S. Cramm,§ Extended Abstracts, High-Temperature Superconductors II, April 5-9, 1988, Reno, Nevada

(1988). [CRR/Radiation Physics Division, IMSE/Ceramics Division]

*University of Tennessee

tUniversity of Uppsala, Sweden

^University of Lund, Sweden

§Universitat Hamburg, West Germany

We present electron and photon beam excited soft x-ray emission spectra for the YBa2Cu307

. x superconductor. The soft x-ray emission spectra determine the p-type partial density of states (pPDOS) localized on the Ba, Y, and O atomic sites. The p-PDOS are very small at the Fermi

energy eF, and exhibit peaks at 3.5 to 4 eV below cF

. K emission spectra of O confirm the

interpretation that the structure observed in previous photoemission measurements is associated

with the O 2p orbitals. The copper L^ emission spectrum, which samples d-PDOS, and the O

K emission spectrum have been modeled theoretically and are in excellent agreement with the soft

x-ray emission spectrum. Differences between the model and the experiment are attributed to correlation effects.

SOFT X-RAY ABSORPTION AND EMISSION SPECTRA OF THE YBa^O^ SUPERCONDUCTOR, K.-L. Tsang,* C.H. Zhang,* T.A. Callcott,* L.R. Canfield, D.L. Ederer, J.E. Blendell,

C.W. Clark, Drexel High Tc Conference Proceedings (July 1987). [CRR/Radiation Physics Division,

IMSE/Ceramics Division]

*University of Tennessee

We present total photoelectron yield spectra and electron-beam excited soft x-ray emission spectra

for the YBa2Cu307

. x superconductor. The 2 + valencies of Cu and Ba in the compound are confirmed by total photoelectron yield measurements. Emission spectra determine the p-type

partial density of states (p-PDOS) localized on the Ba, Y, and O atomic sites. The p-PDOS are very small at the Fermi energy ep , and exhibit peaks at 3.5 to 4 eV below eF

. K emission spectra

of O confirm the interpretation that the structure observed in previous photoemission

measurements is associated with O p orbitals.

SOFT X-RAY ABSORPTION AND EMISSION SPECTRA AND THE ELECTRONIC STRUCTURE

OF SOME EXOTIC MATERIALS, D.L. Ederer, L.R. Canfield, T.A. Callcott,* K.-L. Tsang,* C.H.

Zhang,* E.T. Arakawa,t SPIE, Vol. 911 X-Ray and VUV Interaction Data Bases, Calculations, and

Measurements, pp. 75-79 (1988). [CRR/Radiation Physics Division]

*University of Tennessee

tOak Ridge National Laboratory

The technique of soft x-ray fluorescence spectroscopy (SXE) is complementary to that of

photoemission spectroscopy (PES). SXE probes the local partial density of states (PDOS), selects

dipole allowed symmetries, and is not necessarily surface sensitive. PES on the other hand,

averages over the DOS and can be used to measure the dispersion of the energy bands. PES is also very surface sensitive. We present measurements on the high Tc superconductors, the

47
quasicrystalline phase of AIMn, and the LiAl intermetallic alloy. These measurements provide

insight for theoretical modeling. In the case of the high Tc compound and the intermetallic

compound the measurements are in good agreement with the theory. However, for the

quasicrystals the measurements provide new insights to challenge theory.

SOFT X-RAY ABSORPTION AND EMISSION SPECTRA AND THE ELECTRONIC STRUCTURE

OF THE Ba2YCu3 Ov. x SUPERCONDUCTOR, K.-L. Tsang,* C.H. Zhang,* T.A. Callcott,* L.R.

Canfield, D.L. Ederer, J.E. Blendell, C.W. Clark, N. Wassdahl,t J.E. Rubensson,t G. Bray,t N.

Mortensson,t J. Nordgren,t R. Nyholm,$ and S. Cramm,§ Journal de Physique, Colloque C9,

supplement au n° 12, Tome 48, C9-1 193 - C9-1 197 (d&embre 1987). [CRR/Radiation Physics Division,

IMSE/Ceramics Division]

*University of Tennessee

tUniversity of Uppsala, Sweden

^University of Lund, Sweden

§Universitat Hamburg, West Germany

We present e-beam excited soft x-ray emission spectra and total photoelectron yield spectra in the

20-600 eV photon energy range for the Ba2YCu307. x superconductor. We confirm the 2 +

valency of Cu in the compound by total yield measurements. In soft-x-ray emission, the NiS

spectrum of Ba, the M4>5 spectrum of Y, and the K spectrum of O provide measures of the /Hype

partial density of states (p-PDOS) localized on the respective atomic sites. In each case the pPDOS is very small at the Fermi energy with the first peak in the p-PDOS lying 3.5 to 4 eV

below the Fermi energy. The K spectra of O confirm the interpretation that the structures

observed in the photoemission measurements are associated with the 0 2p orbitals. Finally no

changes are observed between spectra taken above and below Tc

.

SOFT X-RAY EMISSION SPECTRA AND THE BONDING OF ALUMINUM, T.A. Callcott,* K.-L.

Tsang,* C.H. Zhang,* D.L. Ederer, and E.T. Arakawa,t Journal de Physique, Colloque C9, supplement

au n°12, Tome 48, C9-1053 - C9-1057 (decembre 1987). [CRR/Radiation Physics Division]

*University of Tennessee

tOak Ridge National Laboratory

The 1^3 soft x-ray emission (SXE) spectra of Al in metallic Al, Al-Mn alloys, dilute Al-Mg

alloys, the intermetallic compounds LiAl and Ni3Al, the semiconducting alloy (AI-Ga)As and

insulating A1203 are presented. The spectra provide a measure of the s-like partial density of

states (PDOS) localized at the Al atoms and show prominent qualitative features that may be

identified with each of the major types of bonding in solids, i.e., metallic, covalent and ionic.

The spectra of metallic Al and of the Al-Mn alloys have parabolic densities of states characteristic

of nearly free electron PDOS's. Al in Mg alloys have metallic PDOS's with a strong

enhancement of the low energy region of the spectrum which is associated with screening of the

extra charge on the Al ion core.

48
In LiAl, the Al has tetragonal local symmetry, the spectrum is similar to that of silicon and the

bonding is a combination of covalent and metallic. In AlxGa,.xAs alloys, the Al has tetragonal

local symmetry, but the spectrum is strongly modified from the characteristic covalent form by

the presence of a strong ionic component in the bonding. Finally, the spectrum of A1203 shows

features characteristic of localize electronic orbitals and ionic bonding.

ELECTRONIC STRUCTURE OF THE ICOSAHEDRAL AND OTHER PHASES OF ALUMINUMMANGANESE ALLOYS STUDIED BY SOFT-X-RAY EMISSION SPECTROSCOPY, D.L. Ederer,

R. Schaefer, K.-L. Tsang,* C.H. Zhang,* T.A. Callcott,* and E.T. Arakawa,t Physical Review B, Vol.

37, No. 15, pp. 8594-8597 (May 15, 1988). [CRR/Radiation Physics Division]

*University of Tennessee

fOak Ridge National Laboratory

Aluminum Lj 3 soft-x-ray emission spectra have been observed for several aluminum-manganese

alloys exhibiting icosahedral, decagonal, and the normal crystalline symmetries. The spectra of

all the alloys are qualitatively similar and are slightly narrower ( — 0.2 eV) than the width

observed for metallic aluminum. Compared to the sample with crystalline symmetry, the

quasicrystals show no chemical shift and a decrease in the partial density of states of s and d

symmetry at the Fermi energy, contradicting the predictions of several calculations.

WIGNER-SEITZ LOCAL-ENVIRONMENT STUDY OF THE HIGH-TC SUPERCONDUCTORS, M.

Melamud, L.H. Bennett, and R.E. Watson,* Physical Review B, Vol. 38, No. 7, pp. 4624-4631

(September 1, 1988). [IMSE/Ceramics Division]

*Brookhaven National Laboratory

The near-neighbor environments and the bonding of atoms in the high-Tc superconductors are

studied using a Wigner-Steitz-cell construction. Assuming metallic radii for the atoms, it is shown that the Ba, Y, and La atoms have large coordination numbers, implying a threedimensional bonding scheme. The La-Cu-0 type ( = 40 K) and the Y-Ba-Cu-0 type ( = 90 K)

superconductors both display the same bonding characteristics.

MOSSBAUER STUDY OF THE EFFECT OF OXYGEN STOICHIOMETRY ON THE HIGH Tc SUPERCONDUCTOR Y1 Ba2(Cu0. OTFeo.a3)307. x

, L.J. Swartzendruber, L.H. Bennett, M.Z. Harford,* and

Mark Rubinstein,* Journal of Superconductivity, Vol. 1, No. 2, pp. 219-230 (1988). [IMSE/Ceramics

Division and Metallurgy Division]

*Naval Research Laboratory

The Mossbauer effect of oxygen-depleted YBa2Cu307

. x and PrBa^^O^ doped with 3% 57Fe has

been investigated at room temperature. A manifold of quadrupole-split spectra has been found,

whose parameters are in general agreement with those found by other workers. In addition, the

Mossbauer spectra show that a fraction of the Fe sites develop magnetic order at room

temperature when the oxygen content is reduced. It is demonstrated that the observed

asymmetries in the Mossbauer spectra can be the result of a preferential alignment of the platelike

crystallites that arises during the normal sample preparation process. The tendency to bond with

49
the oxygen atoms is presumed to be responsible for the discreteness of the Mossbauer spectra as a function of oxygen depletion.

MOSSBAUER HYPERFINE FIELDS IN RBa2(Cu0. 97Fe0.o3)307.x SUPERCONDUCTORS, Mark

Rubinstein,* M.Z. Harford,* L.J. Swartzendruber, and L.H. Bennett, Journal de Physique, Colloque C8,

Supplement au n* 12; Tome 49 (decembre 1988). [IMSE/Ceramics Division and Metallurgy Division]

*Naval Research Laboratory

Room temperature

57Fe Mossbauer spectra of RBa^Cuo ^Feoc^CVx [R= Y, Pr, Er] were obtained

from samples with varying jc. A magnetically split hyperfine field spectrum was observed for the

most oxygen-deficient Y sample, for all the Pr samples, and for none of the Er samples.

MAGNETIC PROPERTIES OF SOME HIGH Tc SUPERCONDUCTING COMPOUNDS, P.F. Miceli,*

J.M. Tarascon,* B.G. Bagley,* L.H. Greene,* P. Barboux,* G.W. Hull,* M. Giroud,* J.J. Rhyne, and

D.A. Neumann, in High Temperature Superconductors, Vol. 20, Tenth Winter Meeting on Low

Temperature Physics, World Scientific Publishing, p. 89 (1989). [IMSE/Reactor Radiation Division]

*Bell Communications Research

The magnetic properties of two high temperature superconducting compounds having Co

substitutions for Cu are studied by neutron scattering. The "123" system can exhibit two

antiferromagnetic transitions or superconductivity depending upon the composition. In the "n= 1

phase of the Bi based system, the Co substitution displays antiferromagnetic ordering with a small

ferromagnetic canting of the spins.

THEORETICAL MODELS FOR HIGH-TEMPERATURE SUPERCONDUCTIVITY, R.C. Casella, II Nuovo Cimento, Vol. 10 D, N. 12, pp. 1439-1448 (Dicembre 1988). [IMSE/Reactor Radiation Division]

A semi-phenomenological analysis is given of the effects of certain band structure features on the

gap ratios 2A/kBTc for high-Tc superconductors, including multigap systems. In addition to

phonons other intermediate bosons (IB) mediating the superconducting interaction are considered.

Interesting results emerge when the IB energy exceeds the widths of possible narrow peaks in the

density of states associated with sub-bands presumably belonging to substructures such as stacked

Cu-0 planes. Comparison with experiment is made. In particular, data obtained by Warren et

al. via nuclear-spin relaxation on BajYC^O^ can be interpreted within the present framework

in terms of a model having an IB of energy >1 eV, which exceeds the predicted width

(^0.3 eV) of a peak in the density of states containing the normal-state Fermi level. This

suggests that the IB is not a phonon.

DEPENDENCE OF Tc ON THE NUMBER OF Cu02 PLANES PER CLUSTER IN INTERPLANERBOSON-EXCHANGE MODELS OF HIGH-TC SUPERCONDUCTIVITY, R.C. Casella, Solid State

Communications, Vol. 70, No. 1, pp. 75-77 (1989). [IMSE/Reactor Radiation Division]

50
The critical temperature Tc(n) of systems, such as the Ti and Bi compounds, containing n Cu02 planes per cluster is discussed theoretically within semi-phenomenological models which are based

upon the exchange of (unspecified) high-energy bosons (co > > cophoooo ) between different Cu02 planes. Applied to representative data for the Tl2 compounds, the models predict rapid saturation

of Tc(n) at relatively low n, with the possibility of a diminution beyond n = 3. Within the

theoretical framework these qualitative features appear to be quite general for this class of

materials.

SOME NORMAL STATE PROPERTIES OF HIGH Tc COMPOUNDS, Mark Rubinstein,* M.Z.

Harford,* T.K. Chaki,* L.J. Swartzendruber, and L.H. Bennett, Proceedings of High Tc Superconductors: Magnetic Interactions, Gaithersburg, MD, October 11-13, 1988, Progress in High

Temperature Superconductivity, Vol. 17, pp. 292-302 (1989). [IMSE/Metallurgy Division]

*Naval Research Laboratory

The 57Fe Mossbauer Effect of oxygen-depleted YBCO has been investigated. A magnetically split

6-line hyperfine field spectrum was observed, indicating that Fe atoms which substitute for Cu(2)

atoms participate in antiferromagnetism of YBCO. In addition, the temperature dependence of

the resistivity of BSCCO and YBCO was investigated. Both compounds show a large increase

in resistivity as the oxygen content is depleted from the samples with increased temperature.

PROCEEDINGS OF HIGH-TC SUPERCONDUCTORS: MAGNETIC INTERACTIONS, L.H. Bennett,

Y. Flom,* G.C. Vezzoli,t Editors, Gaithersburg, Maryland, October 11-13, 1988, Progress in High

Temperature Superconductivity, Vol. 17, 404 p., New Jersey: World Scientific Publishing (1989).

[IMSE/Metallurgy Division]

*NASA-Goddard Space Flight Center

tU.S. Army Laboratory Command

This is the collected proceedings of a three-day workshop, sponsored by the National Institute of

Standards and Technology and the Goddard Space Flight Center, which was held at NIST in

Gaithersburg, Maryland, on October 11-13, 1988. The mission of the conference was to bring

together invited experts, which included both world class solid-state theorists and

experimentalists, to assess the role of magnetic phenomena as related to the great discovery of

superconductivity near and above liquid-nitrogen temperature, as well as to provide a forum for

interaction between communities representing fundamental research and application of HTSC

materials.

ELECTRON-EXCITED 4d-*4f Ba RESONANCES IN YBajC^O^: SELECTION RULES AT

INTERMEDIATE ELECTRON ENERGY, C.W. Clark, J.A.D. Matthew,* M.G. Ramsey,t and F.P.

Netzer,t Physical Review A, Vol. 40, No. 9, pp. 4902-4906 (November 1, 1989). [CRR/Radiation

Physics Division]

*University of York, England

tUniversitat Innsbruck, Austria

51
Observations of electron-excited 4</

104/-*W94/

1 giant resonances of Ba in the high-Tc superconductor YBajC^O^ are compared with previous work on the corresponding La transition

in various environments. Breakdown of dipole selection rules is observed even at electron

primary energies of a few keV, but for low-angle scattering it is possible to rationalize the results

in terms of symmetry-related selection and intensity rules appropriate to intermediate coupling.

This interpretation is supported by atomic Hartree-Fock calculations of the final-state multiplets.

52
7. ENERGY GAP AND TUNNELING SPECTRA

ELECTRON TUNNELING MEASUREMENT OF THE ENERGY GAP IN A La-Sr-Cu-0

SUPERCONDUCTOR, J. Moreland, A.F. Clark, H.C. Ku,* and R.N. Shelton,* Cryogenics, Vol. 27,

pp. 227-228 (1987). [CEEE/Electromagnetic Technology Division]

*Iowa State University

We have used the break junction technique to determine the energy gap of lanthanum-strontiumcopper-oxide, one of the new high critical temperature superconductors. The current-voltage

characteristics demonstrated a variety of tunneling behaviors. The best characteristic indicating

quasiparticle tunneling between superconducting electrodes implied an energy gap of

7.0 + 0.1 meV. Derivatives of other characteristics showed weak structure indicating possible

energy gaps up to 9 meV.

TUNNELING SPECTROSCOPY OF A La-Sr-Cu-0 BREAK JUNCTION: EVIDENCE FOR STRONGCOUPLING SUPERCONDUCTIVITY, John Moreland, A.F. Clark, L.F. Goodrich, H.C. Ku,* and

R.N. Shelton,* Physical Review B, Vol. 35, No. 16, pp. 8711-8713 (June 1, 1987).

[CEEE/Electromagnetic Technology Division]

*Iowa State University

Detailed structure in the quasiparticle tunneling has been observed in La-Sr-Cu-0 superconductive

tunneling junctions using the break-junction technique. Variability in the energy gap and

associated structure in the current-voltage curves are observed indicating significant

inhomogeneity in the superconducting properties. Large energy gaps (7.0 meV) and deep

structure in the conductance derivative are evidence for a strong-coupling mechanism.

BREAK-JUNCTION TUNNELING MEASUREMENTS OF THE HIGH-TC SUPERCONDUCTOR

Y^Cu^, J. Moreland, J.W. Ekin, L.F. Goodrich, T.E. Capobianco, A.F. Clark, J. Kwo,* M.

Hong,* and S.H. Liou,* Physical Review B, Vol. 35, No. 16, pp. 8856-8857 (June 1, 1987).

[CEEE/Electromagnetic Technology Division]

*AT&T Bell Laboratories, Murray Hill

Current-voltage tunneling characteristics in a high-critical-temperature superconducting material

containing predominantly YiBajCu-jOc^ have been measured using the break-junction technique.

Sharp gap structure was observed, with the largest superconductive energy gap measured to be

A = 19.5 + 1 meV, assuming a superconductor-insulator-superconductor junction. This energy

gap corresponds to 2A/kBTc = 4.8 at T = 4 K, for a critical temperature of 93 K (midpoint of

the resistive transition).

ELECTRON TUNNELING MEASUREMENTS IN LaSrCuO AND YBaCuO, John Moreland, J.W.

Ekin, L.F. Goodrich, T.E. Capobianco, and A.F. Clark, Materials Research Society, Vol. EA-11, pp.

73-75 (June 1987). [CEEE/Electromagnetic Technology Division]

The break junction technique whereby vacuum tunneling occurs within the fracture of a bulk

sample is used to study the LaSrCuO and YBaCuO perovskite superconductors. Structure in the

53
current versus voltage characteristics is reminiscent of previous quasiparticle curves obtained for

BCS superconducting materials. Some curves have anomalous qualities, including large dips in

the junction conductance with increasing voltage just above a well defined tunneling gap edge,

linearly increasing junction conductance with applied bias, along with features occurring near

voltage intervals with the 1, 3, 5 pattern.

ELECTRON TUNNELING MEASUREMENTS OF HIGH Tc COMPOUNDS USING BREAK

JUNCTIONS, John Moreland, L.F. Goodrich, J.W. Ekin, T.E. Capobianco, and A.F. Clark,

Proceedings of the 18th International Conference on Low Temperature Physics, Kyoto, Japan, August

20-26, 1987, Japanese Journal of Applied Physics, Vol. 26, Supplement No. 26-3, pp. 999-1000 (1987).

[CEEE/Electromagnetic Technology Division]

We report on the break junction technique and its application to the high Tc superconductors

LaSrCuO and YBaCuO. In this technique, bulk samples are fractured and the freshly fractured

surfaces adjusted to form a tunneling junction with vacuum or liquid helium as the insulating

barrier. Precise mechanical adjustment permits the study of electron tunneling phenomena

between pieces of a bulk superconductor. The current voltage characteristics of these break

junctions are variable indicating sample inhomogeneity. However, some junction settings result

in the more familiar quasiparticle signatures in the current voltage characteristics. Low leakage

junctions indicate the presence of a sharp superconductive energy gap as well as large variations

in junction conductance above the gap edge in both materials.

ANOMALOUS BEHAVIOR OF TUNNELING CONTACTS IN SUPERCONDUCTING PEROVSKITE

STRUCTURES, J. Moreland, L.F. Goodrich, J.W. Ekin, T.E. Capobianco, and A.F. Clark Advances

in Cryogenic Engineering Materials, Vol. 34, pp. 625-632 (1988). [CEEE/Electromagnetic Technology

Division]

Our break junction results for electron tunneling spectroscopy of the perovskite superconductors

La-Sr-Cu-0 and Y-Ba-Cu-0 are similar to those obtained using thin film, scanning tunneling

microscopy, and point contact methods. Energy gap structures are sometimes observed in the

measured current voltage characteristics. More often, however, the characteristics are anomalous

when compared to previous tunneling studies of BCS superconductors. The anomalies include

linearly increasing conductance with voltage, large deviations injunction conductance above the

gap edge, and junction diode action. We discuss some possible explanations for these

observations.

RECENT TUNNELING MEASUREMENTS OF 90 K SUPERCONDUCTORS AT NBS, John

Moreland, J. A. Beall, R.H. Ono, and A.F. Clark, Materials Research Society, Vol. EA14, pp. 351-353

(December 1987). [CEEE/Electromagnetic Technology Division]

Several tunneling measurements on oxide superconductors have been made at NBS in the last

year. These include break junction tunneling measurements of the energy gap, break junction

superconducting point contacts, and the operation of a break junction point contact rf SQUID

above 77 K. Until recently, these tunneling experiments have been limited to bulk samples cut

from sintered pellets and a few small single crystals. We present here further results on thin

54
films of YBajCujOx (YBCO) using squeezable electron tunneling (SET) junctions. In contrast

to the break junction tunneling experiments on bulk samples, where quite often tunneling spectra are without energy gap features, the spectra for thin-film SET junctions are rich with structure.

SINGLE CRYSTAL HoBa^uA BREAK JUNCTIONS, John Moreland, A.F. Clark, M.A. Damento,*

and K.A. Gschneider, Jr.,* Proceedings of the International Conference on High Temperature

Superconductors - Materials and Mechanisms of Superconductivity, Interlaken, Switzerland, February

29 - March 4, 1988, pp. 1383-1384 (1988). [CEEE/Electromagnetic Technology Division]

*Ames Laboratory, Iowa State University

Tunneling spectra of HoBa2Cu3Ox single crystals using the break junction method show energy

gap features. These features are variable from junction to junction possibly due to an anisotropic

gap function. The I-V curves show the peculiar square law dependence of the current on voltage seen in many tunneling measurements of polycrystalline samples of 90 K superconductors. This

may be an indication of an inherent "granularity" built into the superconducting matrix of a single

crystal.

EVIDENCE FOR THE SUPERCONDUCTING PROXIMITY EFFECT IN JUNCTIONS BETWEEN

THE SURFACES OF YBa2Cu30x THIN FILMS, J. Moreland, R.H. Ono, J.A. Beall, M. Madden, and

A.J. Nelson,* Applied Physics Letters, Vol. 54, No. 15, pp. 1477-1479 (April 10, 1989).

[CEEE/Electromagnetic Technology Division]

*Solar Energy Research Institute

We have used the squeezable electron tunneling (SET) junction technique for testing the electrical

properties of the surfaces of YBa2Cu3Ox (YBCO) thin-film electrodes. As deposited and

annealed, the surfaces of the electrodes were insulating at 4 K. Several methods were used to

improve the electrical properties of the electrodes' surfaces including rapid thermal annealing,

oxygen sputter etching, and thin Ag coating treatments. The greatest improvement occurred after a deposition of a 5 nm Ag coating and subsequent rapid thermal anneal of one set of YBCO

films. Under these conditions it was possible to make a superconducting Josephson point contact

between the surfaces of the electrodes. We think that the Ag acts as a normal-metal proximity

layer effectively shunting the degraded electrodes' surfaces.

BREAK JUNCTIONS I, John Moreland, L.F. Goodrich, J.W. Ekin, T.E. Capobianco, and A.F. Clark,

National Bureau of Standards NBSIR 88-3090, 76 p. (May 1988). [CEEE/Electromagnetic Technology

Division]

Measurements of the tunneling current-voltage characteristics of break junctions in conventional

superconductors can be used to determine their superconducting energy gap as a function of

energy. These results agree with junctions in some exotic superconductors, on the other hand,

have anomalous current-voltage characteristics compared to BCS predictions. Energy gaps and

the Josephson effect measured for the new high Tc materials YBaCuO (Tc = 93 K) and LaSrCuO

(Tc = 36K) indicate that the samples are inhomogeneous with varying gap functions depending

on the location of the tunneling contact within the break junction fracture. Break junction data

for these materials are within the strong coupling limits of BCS theory.

55
BREAK JUNCTION MEASUREMENT OF THE TUNNELING GAP OF A THALLIUM-BASED

HIGH-TEMPERATURE SUPERCONDUCTOR CRYSTAL, J. Moreland, D.S. Ginley,* E.L.

Venturini,* and B. Morosin,* Applied Physics Letters., Vol. 55, No. 14, pp. 1463-1465 (October 2,

1989). [CEEE/Electromagnetic Technology Division]

*Sandia National Laboratories

We have used the break junction method to measure the tunneling gap of a thallium-based hightemperature superconductor crystal in liquid helium at 4 K. The crystal was predominantly

T^CaBajC^Oy and had a Meissner onset at 105 K. Tunneling data showed a symmetric gap

about zero bias between two well defined conductance peaks in the conductance-versus-voltage

curve. The gap is consistent with a BCS energy gap (A) of 30 meV assuming a superconductorinsulator-superconductor electrode configuration. In addition, a supercurrent could be detected

when the break junction was operated in a point contact mode at temperatures as high as 95 K.

BREAK JUNCTION TUNNELING SPECTROSCOPY OF SINGLE-CRYSTAL BISMUTH-BASED

HIGH-TEMPERATURE SUPERCONDUCTORS, John Moreland, C.K. Chiang, L.J. Swartzendruber,

Advances in Cryogenic Engineering (Materials), Vol. 36, Part A, pp. 619-625 (1990).

[CEEE/Electromagnetic Technology Division and IMSE/Ceramics and Metallurgy Divisions]

We have measured the tunneling spectra of some high temperature superconducting crystal break

junctions at 4 K. The samples were thin plates of Bi2SrCa2Cu20g compound. The tunneling

spectra (conductance versus voltage) were not typical of BCS superconductor tunneling electrodes.

The spectra of higher-resistance break-junction settings (R > 1 MQ) show a tunneling gap on top

of a linearly increasing conductance background signal. "Harmonic" dip features in the spectra

of lower resistance break junction settings (R < 1 MQ) indicated tunneling between multiple

particles in the vicinity of the primary (highest resistance) contact of the junction. The dips

occurred at about the same current but shifted in voltage when the resistance of the break junction

was continuously adjusted to new settings.

HIGH CRITICAL TEMPERATURE SUPERCONDUCTOR TUNNELING SPECTROSCOPY USING

SQUEEZABLE ELECTRON TUNNELING JUNCTIONS, Thomas Walsh, John Moreland, R.H. Ono,

J. A. Beall, M. Cromar, T. Harvey, C. Reintsema,* and T.S. Kalkur,* IEEE Transactions on Magnetics,

Vol. 27, No. 2, pp. 840-843 (March 1991). [EEEL/Electromagnetic Technology Division]

*University of Colorado

We have performed tunneling spectroscopy measurements on squeezable electron tunneling

junctions using Bi-Sr-Ca-Cu-O, Y-Ba-Cu-O, and Nb electrodes in a variety of combinations. A

zero-bias conductance peak has been seen repeatedly in the current-voltage [I(V)] and the

conductance-voltage [G(V)] characteristics. We present a model to explain this conductance peak

in terms of quasi-particle tunneling, phase diffusion and a supercurrent. Two additional structures

have been seen repeatedly in I(V) and G(V). One of these structures has the characteristics of

an energy gap feature. The other structure, which can mimic the gap feature, is explained in

terms of the switching to the voltage state of a grain boundary junction that is in series with the

SET junction. The dependence of these features upon temperature and the force applied to the

junction is examined.

56
8. JOSEPHSON EFFECT AND DEVICES

JOSEPHSON EFFECT ABOVE 77 K IN A YBaCuO BREAK JUNCTION, John Moreland, L.F.

Goodrich, J.W. Ekin, T.E. Capobianco, A.F. Clark, A.I. Braginski,* and A.J. Panson,* Applied Physics

Letters, Vol. 51, No. 7, pp. 540-541 (August 17, 1987). [CEEE/Electromagnetic Technology Division]

*Westinghouse Research & Development Center

We have observed the Josephson effect in a YBaCuO break junction. Critical currents as high

as 10 mA were measured at 4 K for break junctions with a point contact within the fracture of

a sample. The junction was susceptible to microwave radiation showing Shapiro steps with the

ratio of V/f of 2.04 + 0.05 /xV/GHz compared to the pair tunneling value of

h/2e = 2.068 /xV/GHz. These steps were clearly visible in the current-voltage characteristics at temperatures up to 85 + 5 K.

A JOSEPHSON ARRAY VOLTAGE STANDARD AT 10 V, Frances L. Lloyd, C.A. Hamilton, J. A.

Beall, Diane Go, R.H. Ono, and R.E. Harris, IEEE Electron Device Letters, Vol. EDL-8, No. 10, pp.

449-450 (October 1987). [CEEE/Electromagnetic Technology Division]

The technology of Josephson voltage standards has been extended to an array of 14 184 junctions

which is capable of generating over 150 000 quantized voltage levels spanning the range from

-12 to +12 V. This makes possible the direct calibration of 10-V Zener reference standards

without the use of a voltage divider.

OPERATION OF A Y-Ba2-Cu-Ox RF SQUID AT 81K, J.E. Zimmerman, J.A. Beall, M.W. Cromar,

and R.H. Ono, Applied Physics Letters, Vol. 51, No. 8, pp. 617-618 (August 24, 1987).

[CEEE/Electromagnetic Technology Division]

An rf superconducting quantum interference device (SQUID) has been made from bulk Y-Ba-CuO. The device displays quantum interference effects and operates with useful signal levels up to

81 K. The SQUID is formed from a ring of Y-Ba-Cu-0 which is broken in the cryogenic

environment and then recontacted. Estimates of the SQUID noise performance are given.

EQUIVALENT FLUX NOISE IN YBa^uA RF SQUID, J.E. Zimmerman, J.A. Beall, M.W. Cromar,

and R.H. Ono, Proceedings of the 18th International Conference on Low Temperature Physics, Kyoto,

Japan, August 20-26, 1987, Japanese Journal of Applied Physics, Vol. 26, Supplement 26-3, pp. 2125-

2126 (1987). [CEEE/Electromagnetic Technology Division]

We have measured the noise in flux-locked rf SQUIDs made of bulk YBajCujO, both in a He

cooled cryostat and in liquid nitrogen (LNj). Our best results at 75 K show a spectral density

of the equivalent flux noise equal to 4.5 x lO'VcA'Hz. There is considerable variation in the

performance of SQUIDs made from nominally similar material.

57
SWITCHING NOISE IN YBa^uA " MACROBRIDGES"

, R.H. Ono, J.A. Beall, M.W. Cromar, P.M.

Mankiewich,* R.W. Howard,* and W. Skocpol,t Proceedings of the 1988 Applied Superconductivity

Conference, San Francisco, CA (August, 1988), IEEE Transactions on Magnetics, Vol. MAG-25, No.

2, pp. 976-979 (1989). [CEEE/EIectromagnetic Technology Division]

AT&T Bell Laboratories, Holmdel, NJ.

tBoston University, Boston, MA.

We have observed intermittent switching in the voltage-current characteristics (VIC) of thin film

bridges of YBajCuA,. At a fixed bias point there are multiple metastable voltage states with

lifetimes which depend on the bias current and applied magnetic field. The microbridges are made of thin (<500 nm), polycrystalline films of YBajCujO* which are patterned by liftoff into

structures with dimensions ranging from less than 1 fim to 100 /*m. Details of the fabrication

process and the measurements are presented. The results are discussed in the context of

fluctuations in the effective resistance of the bridge due to motion of trapped flux.

YBa2Cu307VINSULATOR MULTI-LAYERS FOR CROSSOVER FABRICATION, J.A. Beall, M.W.

Cromar, T.E. Harvey, M.E. Johansson, R.H. Ono, CD. Reintsema, D.A. Rudman, S.E. Asher,* A.J.

Nelson,* and A.B. Swartzlander,* Proceedings, Applied Superconductivity Conference, September 24-28,

1990, Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 1596-1599 (March

1991). [CEEE/EIectromagnetic Technology Division]

*Solar Energy Research Institute

The development of thin-film dielectrics compatible with the epitaxial growth of YBa2Cu307^ (YBCO) is crucial to the fabrication of multilayer device and circuit structures. We have

investigated the YBCO/SrTi03 (STO) system by fabricating YBCO/STO bilayers and simple

YBCO/STO/YBCO crossover structures. The thin films were deposited in situ by pulsed laser

deposition and analyzed using x-ray diffraction and scanning electron microscopy. The film

interfaces were characterized by secondary ion mass spectrometry (SIMS) depth profiling. We

have developed photolithographic and wet-etching processes for patterning the crossovers which

are compatible with these materials. The crossover structures were characterized by resistance

and insulator pinhole density as well as the superconducting properties of the patterned top and

bottom YBCO electrodes (critical temperature, Tc , and critical current density, Jc). Using SrTi03 as the insulating layer, we have made crossovers with good isolation between layers (> 100 MO)

and high Jc even in the top electrode (Jc (76 K) > 10s A/cm2

).

JOSEPHSON COUNTING ANALOG-TO-DIGITAL CONVERTER, D.L. Miller,* J.X. Przybysz,* J. Kang,* CA. Hamilton, D.M. Burnell,t Proceedings, Applied Superconductivity Conference, September

24-28, 1990, Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 2761-2764

(March 1991). [CEEE/EIectromagnetic Technology Division]

*Westinghouse, Pittsburgh

tWestinghouse, Baltimore

A superconductive analog-to-digital converter which uses a dc SQUID as a quantizer and a flipflop counter as a digitizer has been designed, fabricated, and tested. The circuit was fabricated

using a ten-level niobium process. Tests at 4.2 K demonstrated (1) counting to the full twelve-bit

accuracy of the design, (2) monotonic A/D conversion with linearity to less than 1 LSB over the

58
more than nine-bit range of conversion, (3) read-on-the-fly operation with counter overflow, and

(4) counter operation with a gate current bus.

FAR-INFRARED KINETIC-INDUCTANCE DETECTORS, E.N. Grossman, D.G. McDonald, and J.E.

Sauvageau, Proceedings, Applied Superconductivity Conference, September 24-28, 1990, Snowmass,

Colorado, in IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 2677-2680 (March 1991).

[CEEE/Electromagnetic Technology Division]

Extremely sensitive far-infrared detectors suitable for both direct detection and heterodyne

applications are possible based upon micrometer-sized thin films with thickness less than a superconducting penetration depth. The penetration depth of such a film, and therefore its inductance, varies with temperature and with quasiparticle population (described by an effective

temperature T*), resulting in both bolometric and non-equilibrium "photoinductive" responses.

Incident radiation is coupled into the small-area kinetic inductor by a lithographic antenna, and

the resulting inductance changes are amplified and converted to a voltage signal by an integrated

microstrip DC SQUID. The device is sensitive because, unlike junction-based devices with large

capacitive reactances, the kinetic inductor is naturally well matched to the antenna impedance at the far-IR frequency (y > 2A/h), and to the pre-amplifier (SQUID) impedance at microwave or video frequencies (y < < 2Alh). The best kinetic inductor materials are those with low

electronic mean free path, large penetration depth, and high critical current density. Thus,

common magnet alloys such as NbTi are the natural choice for liquid-He temperature operation.

A detailed analysis predicts a (phonon-limited) NEP of 4 x 10"17 WA/Hz for a bolometer with an iridium kinetic inductor operated at 0. 1 K. A heterodyne noise temperature of 2250 K (singlesideband) at 3 THz, with 200 MHz bandwidth, is predicted for a Nb-Ti mixer operated at 4 K.

SUPERCONDUCTING KINETIC INDUCTANCE RADIOMETER, J.E. Sauvageau, D.G. McDonald,

and E.N. Grossman, Proceedings, Applied Superconductivity Conference, September 24-28, 1990,

Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 2757-2760 (March 1991).

[CEEE/Electromagnetic Technology Division]

We are developing a bolometer based on a differential thermometer that senses temperature

changes through changes in the kinetic inductance of a superconducting thin film. The

temperature transducer is an inductance bridge patterned as an integrated circuit on a 1 cm2 Si

substrate. Two inductors from opposite arms of the bridge are patterned on a 2 mm2 thermally

isolated Si island which is supported by a 9 /*m thick Si:B membrane. The bridge is excited with

audio frequency current and the bridge imbalance is detected with a commercial DC SQUID

amplifier. The bridge is balanced by applying power to the thermally isolated island. This

thermometer is the sensor for a prototype radiometer that will provide an absolute measure of IR

power. The radiometer, which is designed for a NEP of about 10 " WZ/Hz, is intended to measure the spectrally dispersed power of a 300 K black body. This absolute radiometer is being

developed for use at the Low Background Infrared (LBIR) facility at NIST, Gaithersburg. The

noise floor of the temperature transducer for the radiometer has been measured to be 0.7 pW for a 100 s integration time. This is approximately 150 times lower noise than that of the

commercial absolute radiometer currently used at the LBIR facility in Gaithersburg.

59
A SERIES ARRAY OF DC SQUIDS, Richard P. Welty* and John M. Martinis, Proceedings, Applied

Superconductivity Conference, September 24-28, 1990, Snowmass, Colorado, in IEEE Transactions on

Magnetics, Vol. 27, No. 2, pp. 2924-2926 (March 1991). [CEEE/Electromagnetic Technology Division]

*University of Colorado

We have fabricated a series array of 100 dc SQUIDs using trilayer Nb-A10x-Nb junctions. The

SQUIDs are modulated with a common flux bias line and produce an output voltage swing of

several millivolts across the array. The large output voltage will allow direct connection to room

temperature electronics without the transformer coupling and resulting frequency limitations

commonly associated with dc SQUID amplifiers. We have measured a bandwidth of dc to at

least 175 Mhz for a 100-SQUID array. The series array will be used as the output stage for a

multistage integrated SQUID amplifier.

TRAPPED VORTICES IN A SUPERCONDUCTING MICROBRIDGE, George S. Park,* Charles E.

Cunningham,* Bias Cabrera,* and Martin E. Huber, Proceedings, Applied Superconductivity Conference,

September 24-28, 1990, Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27, No. 2, pp.

3021-3024 (March 1991). [CEEE/Electromagnetic Technology Division]

*Stanford University

Laser light pulsed onto a Nb microbridge drives it momentarily normal and changes the quantum

flux state of a superconducting inductive loop. The flux state is measured by a SQUID coupled

to the loop. With a Nd:YAG laser, vortices are never trapped in the microbridge; with a diode

laser, vortices are sometimes trapped. The spatial distribution of the trapped flux was studied.

The effect of the optical pulse fall time on the frequency of flux trapping was found to be

unimportant from 200 ns to 8 ms. Noise spectrum analysis indicates that the laser diode is 5 to

25 times noisier than the Nd:YAG laser at the characteristic frequency of the loop. This noise

is believed to be responsible for flux trapping in the microbridge.

CORRELATION OF FLUX STATES GENERATED BY OPTICAL SWITCHING OF A

SUPERCONDUCTING CIRCUIT, Charles E. Cunningham,* George S. Park,* Bias Cabrera,* and

Martin E. Huber, Physica B, Vols. 165 and 166, pp. 113-114 (1990). [CEEE/Electromagnetic

Technology Division]

*Stanford University

We pulse a superconducting microbridge with light, changing the quantum flux state of a superconducting circuit. Long sequences of pulses are used to measure the degree of correlation

between successive flux states. In a series of runs, the pulse length was changed over six decades

from 6 ns to 10 ms. The correlations fit a simple Fokker-Planck conditional probability model.

EXCESS LOW-FREQUENCY FLUX NOISE IN dc SQUIDS INCORPORATING Nb/AI OXIDE/Nb

JOSEPHSON JUNCTIONS, Martin E. Huber and Michael W. Cromar, Physica B, Vols. 165 and 166,

pp. 77-78 (1990). [CEEE/Electromagnetic Technology Division]

60
We have fabricated thin-film dc SQUIDs (Superconducting QUantum Interference Devices)

incorporating Nb/Al-Oxide/Nb tunnel junctions. The spectral density of the voltage noise, Sv, of stripline SQUIDs is characterized between 1 Hz and 2000 Hz. In this frequency range, Sv is proportional to the square of the responsivity (dV/d$) over a significant range of bias conditions

with an unusual frequency dependence. In a 7 pH SQUID, the spectral density of the flux noise,

S#, at 1 Hz is less than 10 " %

2/Hz, where $„ = h/2e. The observed noise does not appear to

be environmental; also, it is independent of the value of the junction shunt resistance and whether

the stripline material is a Pbln alloy or Nb. Subject to the constraint of a constant product of the

junction area, critical current density, and SQUID self-inductance in the SQUIDs studied, S#

is inversely proportional to the junction area.

OBSERVATION OF QUANTUM SUSCEPTANCEIN SUPERCONDUCTING TUNNELJUNCTIONS,

QingHu,* C.A. Mears,* P.L. Richards,* and F.L. Lloyd, Physica B, Vols. 165 and 166, pp. 1587-1588

(1990). [CEEE/Electromagnetic Technology Division]

*Lawrence Berkeley Laboratory

We have made the first direct measurement of the quantum susceptance which arises from the

reactive part of quasiparticle tunneling in a superconductor-insulator-superconductor (SIS)

junction. The resonant frequency of a resonator which contains an SIS junction depends on the

dc bias due to the bias dependence of the quantum susceptance. The observed 19% shift of the

resonant frequency, from 73 GHz to 87 GHz, is in excellent agreement with Werthamer-Tucker

theory.

OBSERVATION OF NONDISPERSIVE QUASIPARTICLE TUNNEL CURRENTS IN

SUPERCONDUCTING TUNNEL JUNCTIONS, Qing Hu,* C.A. Mears,* P.L. Richards,* and F.L.

Lloyd, Physical Review Letters, Vol. 64, pp. 2945-2948 (June 11, 1990). [CEEE/Electromagnetic

Technology Division]

*Lawrence Berkeley Laboratory

We have made the first direct measurement of the quantum susceptance which arises from the

nondisipative part of the quasiparticle tunneling in a superconductor-insulator-superconductor

(SIS) junction. The resonant frequency of a resonator which contains an SIS junction depends

on the dc bias due to the bias dependence of the quantum susceptance. The observed 19% shift

of the resonant frequency, from 73 to 87 GHz, is in excellent agreement with the WerthamerTucker theory. This quantum susceptance should exist in other devices such as quantum-well

structures.

QUANTUM SUSCEPTANCE AND ITS EFFECTS ON THE HIGH-FREQUENCY RESPONSE OF

SUPERCONDUCTING TUNNEL JUNCTIONS, Qing Hu,* C.A. Mears,* P.L. Richards,* and F.L.

Lloyd, Physical Review B, Vol. 42, No. 16, pp. 10,250-10,263 (1990). [CEEE/Electromagnetic

Technology Division]

*Lawrence Berkeley Laboratory

61
We have made the first direct measurement of the quantum susceptance that arises from the

nondissipative part of quasiparticle tunneling in a superconductor-insulator-superconductor tunnel

junction. The junction is coupled to an antenna and a superconducting microstrip stub to form

a resonator; the resonant frequency is determined from the response of the junction to broadband

radiation from a Fourier-transform spectrometer. A 19% shift of the resonant frequency, from

73 to 87 GHz, is observed, which arises from the change of the quantum susceptance of the

junction with dc bias voltage. This shift is in excellent agreement with calculations based on the

Werthamer-Tucker theory, which includes the quantum susceptance. We also demonstrate that

it is essential to include the quantum susceptance in our theoretical computation to explain the

photon-assisted -tunnel ing steps, which have negative dynamic conductance. Such steps are observed when the junction is pumped at slightly below the resonant frequency of the capacitor

and the stub. The quantum susceptance should exist in all tunnel devices whose nonlinear I-V

characteristics are due to elastic tunneling.

A PHASE SENSITIVE SIS MIXER TO CIRCUMVENT THE QUANTUM LIMIT, Mark F. Bocko,*

Zhao-nan Zhang,* Stephen Martinet,* and Michael W. Cromar, Proceedings, Applied Superconductivity

Conference, September 24-28, 1990, Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27,

No. 2, pp. 3391-3394 (March 1991). [CEEE/Electromagnetic Technology Division]

*University of Rochester, New York

A conventional SIS mixer, which is pumped by a single frequency local oscillator (LO) functions

as a phase-insensitive linear amplifier and may achieve a noise level near the "quantum limit" of

one-half photon added noise per unit bandwidth. A phase sensitive linear amplifier may have less

noise than this "quantum limit." We are attempting to beat the quantum noise limit with an SIS

mixer by employing a two-LO technique which makes the mixer's gain dependent upon the phase

of the incoming signal. We have experimentally demonstrated the phase sensitive gain of our two-LO mixer; the gain variation from maximum to minimum is more than 20 dB. We also

present theoretical predictions of the noise of the two-LO mixer.

62
9. MAGNETIC MEASUREMENTS

AC SUSCEPTIBILITY MEASUREMENTS NEAR THE CRITICAL TEMPERATURE OF A Y-Ba-Cu-0

SUPERCONDUCTOR, R.B. Goldfarb, A.F. Clark, A.J. Panson,* and A.I. Braginski,* Materials

Research Society, Vol. EA-1 1, pp. 261-263 (June 1987). [CEEE/Electromagnetic Technology Division]

*Westinghouse Research & Development Center

The loss component of complex susceptibility of a Y-Ba-Cu-0 superconductor near its critical

temperature is strongly dependent on ac field amplitude but virtually independent of frequency.

This implies that magnetic hysteresis is the major loss mechanism in these materials. The

temperature at which the loss first becomes positive upon warming corresponds to an equivalence

between the amplitude of the ac field and the lower critical field of the superconductor.

EVIDENCE FOR TWO SUPERCONDUCTING COMPONENTS IN OXYGEN-ANNEALED SINGLEPHASE Y-Ba-Cu-O, R.B. Goldfarb, A.F. Clark, A.I. Braginski,* and A.J. Panson,* Cryogenics, Vol.

27, p. 475-480 (September 1987). [CEEE/Electromagnetic Technology Division]

*Westinghouse Research & Development Center

The complex susceptibility of a sintered Y-Ba-Cu-0 superconductor is strongly dependent on a.c.

field amplitude, h. Very small values of h must be used for the real part of susceptibility, x\

to reach a value corresponding to bulk diamagnetism just below the critical temperature, Tc. The

imaginary part, x", represents hysteresis loss in the sample. Thus, x" versus temperature

becomes positive when h exceeds the lower critical field, Hcl , of the superconductor.

Annealing the material in oxygen gives rise to two distinct components, a relatively high-Tc , high

Hcl superconductor (denoted as "G" or "good") and a relatively low-Tc , low Hcl superconductor

(denoted as "B" or "bad"). Curves of susceptibility versus increasing temperature reflect the dual

nature of the annealed sample: x' has an inflection point at Tc of the B component and

approaches zero at Tc of the G component, while x

" has a peak at each Tc

. Both critical

temperatures decrease linearly with increasing h, though at very different rates. Hc, of the G

component is considerably greater than Hc, of the B component. The lower critical fields are

linearly decreasing functions of temperature.

Two models might explain the susceptibility data. In the grain model, the G component consists

of superconducting grains and the B component is either intergranular material, unfavourably

orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G

component is in the interior of the sample and the B component is at the sample's surface. This

condition could arise if there was oxygen depletion at the surface subsequent to total enrichment

during annealing.

MAGNETIC SUSCEPTIBILITY OF SINTERED AND POWDERED Y-Ba-Cu-O, D.-X. Chen,* R.B.

Goldfarb, J. Nogues,* and K.V. Rao,* Journal of Applied Physics, Vol. 63, No. 3, pp. 980-983

(February 1, 1988). [CEEE/Electromagnetic Technology Division]

*Royal Institute of Technology, Sweden

63
The real and imaginary parts of ac susceptibility of a sintered Y1 Ba2Cu307.a superconductor were

measured before and after powdering. The temperature-dependent susceptibility may be separated

into two contributions, one sensitive and the other relatively insensitive to the magnitude of the

measuring field. The former is partially suppressed by coarsely crushing the sample. It is completely suppressed after finely powdering, whereupon the susceptibility curves become

insensitive to the magnitude of the measuring field. Several models apparently consistent with

the results are discussed.

MAGNETIC EVALUATION OF Cu-Mn MATRIX MATERIAL FOR FINE FILAMENT Nb-Ti

SUPERCONDUCTORS, R.B. Goldfarb, D.L. Ried, T.S. Kreilick,* and E. Gregory,* IEEE Transactions

on Magnetics, MAG-25, p. 1953 (1989). [CEEE/ Electromagnetic Technology Division and

IMSE/Fracture and Deformation Division]

*Supercon, Inc.

Copper-manganese alloys have been proposed as matrix material for the reduction of coupling

losses in fine-filament Nb-Ti superconductor wires. Magnetization and susceptibility

measurements show that adverse magnetic effects arising from the spin-glass properties of this

matrix are minimal for concentrations of Mn up to at least 4 percent.

BEAN MODEL EXTENDED TO MAGNETIZATION JUMPS, R.L. Peterson, Physics Letters A, Vol.

131, No. 2, pp. 131-134 (August 8, 1988). [CEEE/Electromagnetic Technology Division]

I extend the phenomenological Bean model of magnetization in hard superconductors to include

the trains of magnetization jumps seen at low temperature in moderate-to-high magnetic fields.

As in the original Bean model, no particular mechanisms for flux pinning are invoked. The

extended model correctly accounts for the general dependence of the size of the magnetization

jumps on sample size and critical current density. The data together with the model show that

the shielding fields are approximately equal after each jump.

SUPERCONDUCTING PROPERTIES OF MELT-CAST Bi-Sr-Ca-Cu-0 (ABSTRACT), R.B. Goldfarb,

T.F. Ciszek,* and CD. Evans,* Journal of Applied Physics, Vol. 64, No. 10, pp. 5914 (15 November

1988). [CEEE/Electromagnetic Technology Division]

*Solar Energy Research Institute

Sintered solid copper-oxide superconductors generally have an intergranular coupling component

that is evident in curves of ac susceptibility versus temperature: a broad peak in the imaginary

(loss) part, x", and a corresponding inflection in the real part, x'- PolycrystallineBi-Sr-Ca-Cu-0

samples have been prepared from the melt. They are very different from typical sintered

materials. Some have a single superconducting phase, while others have two superconducting

phases. Most of the superconducting platelets are partially surrounded by nonsuperconducting

material. An unusual feature of the susceptibility curves is that they show almost no coupling

losses. This might be explained if the superconducting grains were isolated from each other, as

is the case in susceptibility measurements of powdered superconductors. However, the

characteristics of the melt-cast samples differ from those of powdered samples in three ways.

(1) The resistivity goes to zero at low temperatures, which indicates some connectivity between

64
crystallites. (2) x' approaches -1 at low temperature, indicating substantial bulk shielding. (3) x'

decreases in magnitude with increasing field at low temperature, an effect not seen in powdered

samples. We therefore believe that there is substantial superconductor grain-to-grain coupling, even though the susceptibility curves show little evidence of the usual intergranular coupling

losses. We conclude that the coupling component in these melt-cast samples is virtually lossless.

KIM MODEL FOR MAGNETIZATION OF TYPE-II SUPERCONDUCTORS, D.-X. Chen and R.B.

Goldfarb, Journal of Applied Physics, Vol. 66, No. 6, pp. 2489-2500 (September 1989).

[CEEE/Electromagnetic Technology Division]

We have calculated the initial magnetization curves and complete hysteresis loops for hard type-II

superconductors. The critical-current density, Jc , is assumed to be a function of the internal

magnetic field, H;, according to Kim's model, Jc (Hj) = k/(H0 + |H; |), where k and H0 are

constants. Our analytic solution is for an infinite specimen with finite rectangular cross section,

2a x 2b (a < b), in which a uniform field, H, is applied parallel to the infinite axis. The

calculated curves are functions of b/a, a dimensionless parameter p = (2ka),/4 /H0 and the

maximum applied field. The full -penetration field is Hp = H0[(l +p2

)

,,4 -l]. The equations for an infinite cylindrical specimen of radius a are the same as those for a specimen with square cross

section, a = b. In the limit p < < 1 and a = b, our results reduce to those of the Bean model

(Jc independent of for cylindrical geometry. Similarly, in the limit p < < 1 and b -» oo, the

results are the same as those for a slab in the Bean model. For H > 1.5 Hp, or H > 0 when

p < < 1, the width of the hysteresis loop AM may be used to deduce Jc as a function of H:

JC (H) = AM(H)/[a(l-fl/3b)].

FLUX CREEP AND ACTIVATION ENERGIES AT THE GRAIN BOUNDARIES OF Y-Ba-Cu-0

SUPERCONDUCTORS, M. Nikolo and R.B. Goldfarb, Physical Review B, Vol. 39, p. 6615 (1989).

[CEEE/Electromagnetic Technology Division]

We measured the ac susceptibility of sintered Y^ajCujO^ pellets as a function of temperature

and ac magnetic field amplitude and frequency. The imaginary part of the susceptibility, x",

exhibits two peaks. A narrow peak is located at the critical temperature of the grains. A broad

peak at lower temperature is attributed to hysteresis losses at the grain boundaries. There is a small shift in this coupling peak to higher temperature as the frequency increases from 10 to 1000

Hz. We explain the shift in terms of Anderson flux creep on a time scale of milliseconds. The

shift depends on the amplitude of the measuring field. The activation energy for flux creep

ranges from 11.9 ± 1.0 eV in the zero-field limit [0.8 A'm'1 (0.01 Oe)] to 1.2 ± 0.3 eV at 800 A • m"1 (10 Oe). We extrapolate our data to find the value for an intergrain decoupling field

of 1-2 kA'm"1 (13-25 Oe), above which flux creep presumably becomes flux flow at the grain

boundaries.

MAGNETIZATION OF IMPERFECT SUPERCONDUCTING GRAINS, R.L. Peterson, Physical

Review B, Vol. 40, No. 4 (August 1, 1989). [CEEE/Electromagnetic Technology Division]

A critical-state theory of the magnetization of superconducting grains containing

nonsuperconducting regions is presented which shows that the thickness of the sheath of

65
supercurrents around these regions can be more important than the grain dimension in

determining the magnetization. This may explain some apparently conflicting results on the

magnetization of high-Tc powders of different sizes.

MAGNETIC FIELD DEPENDENCE OF THE SUPERCONDUCTIVITY IN Bi-Sr-Ca-Cu-0

SUPERCONDUCTORS, F.J. Adrian,* J. Bohandy,* B.F. Kim,* K. Moorjani,* J.S. Wallace, R.D.

Shull, L.J. Swartzendruber, and L.H. Bennett, Physica C, Vol. 156, pp. 184-188 (1988).

[IMSE/Ceramics Division and Metallurgy Division]

*The Johns Hopkins University, Applied Physics Laboratory

The changes in superconducting behavior of Bi-Sr-Ca-Cu-0 and Y-Ba-Cu-0 high temperature

superconductors upon the application of an applied field of 0.5 T are explored, using fieldmodulated-microwave absorption. Significant differences in the behavior are noted and discussed.

CONSTRICTED DIAMAGNETIC HYSTERESIS LOOPS OBSERVED FOR THE HIGH Tc SUPERCONDUCTORS, U. Atzmony, R.D. Shull, C.K. Chiang, L.J. Swartzendruber, L.H. Bennett,

and R.E. Watson,* Journal of Applied Physics, Vol. 63, No. 8, pp. 4179-4181 (April 15, 1988).

[IMSE/Ceramics Division and Metallurgy Division]

*Brookhaven National Laboratory

Magnetic hysteresis loops, ac susceptibility, and resistivity measurements have been made on a BajYCujOy^-type high-Tc superconductor. The shape of the hysteresis loops well below Tc are reminiscent of constricted hysteresis loops observed in certain ferromagnetic materials which are usually associated with magnetic aftereffects. Similar dynamic effects, with a time constant on

the order of 10 s at 40 K, are shown to be present in the superconducting material. This dynamic

magnetic viscosity effect is in addition to the flux creep observed for longer time periods.

MAGNETIC PROPERTIES OF A CHEMICALLY SYNTHESIZED Bi(Pb)SrCaCuO

SUPERCONDUCTOR, D.R. Lundy, J.J. Ritter, L.J. Swartzendruber, R.D. Shull, and L.H. Bennett,

Proceedings of High Tc Superconductors: Magnetic Interactions, Gaithersburg, MD, October 11-13,

1988, Progress in High Temperature Superconductivity, Vol. 17, pp. 263-268 (1989. [IMSE/Ceramics

Division and Metallurgy Division]

It has been reported that the presence of lead serves to increase the fraction of high temperature

phase in the Bi-Sr-Ca-Cu-0 system prepared by a solid state reaction. To test if this is also the case for material prepared by a chemical method, a lead containing bismuth superconductor

(Bi1.5Pbo.5Sr! 5 Ca! ^CujOJ was chemically synthesized and its magnetic properties measured. The

material obtained contained a large fraction ( = 50%) of a phase with a superconducting onset

temperature near 110 K. Inspection of the AC susceptibility with a small applied transverse

magnetic field indicated the presence of additional superconducting phases with lower onset

temperatures. Flux depinning was found to occur at relatively low fields.

66
MAGNETIC LEVITATION BY ATTRACTION IN SOME HIGH Tc SUPERCONDUCTORS,

R.D. Shull, L.J. Swartzendruber, C.K. Chiang, M.K. Wu,* P.N. Peters,t and C.Y. Huang,$

Proceedings of High Tc Superconductors: Magnetic Interactions, Gaithersburg, MD, October 11-13,

1988, Progress in High Temperature Superconductivity, Vol. 17, pp. 328-339 (1989). [IMSE/Ceramics

Division and Metallurgy Division]

Columbia University

tNASA Marshall Space Flight Center

:£ Lockheed

Superconducting composites of AgO and YBa2Cu307.x are found to possess the unusual property

at 77 K of being able to levitate by "attraction" (with the composite being suspended in space

"below" a permanent magnet). At the same temperature, however, these composites are still able

to levitate a magnet above it by means of the better known method of repulsion. Magnetization

measurements as a function of field for several composites of AgO and YBaaCujO,., are presented

to explain the origin and composition dependence of the novel "attractive" force. These data

support the previous conclusion that the ability of the material to be suspended below a magnet

is dependent upon the degree of magnetic flux trapping in the superconductor and that the ability

to obtain a stable equilibrium position depends on the shape of the hysteresis loop.

MEASUREMENT OF MAGNETIC PROPERTIES IN A MELT CAST Bi-Ca-Sr-Cu-0

SUPERCONDUCTOR, L.J. Swartzendruber, L.H. Bennett, and C.F. Gallo,* Proceedings of High Tc Superconductors: Magnetic Interactions, Gaithersburg, MD, October 11-13, 1988, Progress in High

Temperature Superconductivity, Vol. 17, pp. 303-308 (1989). [IMSE/Ceramics Division and Metallurgy

Division]

*Superconix, Inc.

Magnetic measurements were made to examine the superconducting properties of a crystalline

chunk, consisting of many small crystals, prepared by casting from the oxide melt with a starting

composition of BisCajS^C^O*. AC susceptibility revealed an onset temperature of 82 K and a

transition width of ~ 10 K. Hysteresis loops at 70 K showed a very small hysteresis, indicating a low density of effective flux pinning sites at this temperature, similar to the behavior observed

for some superconducting samples of the Bi-Ca-Sr-Cu-0 system prepared by other techniques.

However, the magnetization at 10 K showed no hysteresis for fields greater than «3.5 kOe, a much smaller value than we have previously observed for other samples of this material

regardless of preparation method, or for any other material with a Tc greater than 70 K. This

critical field for flux depinning, H^,, varies with temperature as H^ = 4980(l-t)25

, where t is T/Tc

.

MAGNETIC MEASUREMENTS ON POLYMER-HIGH Tc SUPERCONDUCTORCOMPOSITES, A.S.

De Reggi, C.K. Chiang, L. Swartzendruber, and G.T. Davis, Proceedings of High Tc Superconductors:

Magnetic Interactions, Gaithersburg, MD, October 11-13, 1988, Progress in High Temperature

Superconductivity, Vol. 17, pp. 263-268 (1989). [IMSE/Metallurgy Division]

Composite materials made by dispersing powdered mixed-oxide superconductors in suitable

polymers may become important new materials in superconducting applications where contact

from one superconducting grain to another is not essential to the function, such as magnetic

67
shielding. The principal advantage of the composite over the undiluted superconductor so that

it can be processed and fabricated like a polymer using existing polymer processing equipment

and methods. Another advantage of the composite is that the polymer, if properly chosen, can

protect the surface of the powder grains from exposure to environmental chemicals which might

affect the oxides and degrade its superconducting properties.

In this study, composite materials of the 1-2-3 mixed oxide superconductor YjBajCuaO;.,

(BYCO), withx < 0. 1 and critical temperature Tc ~ 92 K, and the vinylidene fluoride polymer,

PVDF, with a glass transition temperature =233 K, were prepared with volume concentrations

ranging from 5 to 25% superconductor. Magnetic data were obtained with an ac susceptometer

and a vibrating sample magnetometer. Permeability data in the dilute superconductor range and

compared to the predictions of a model analogous to the Lorentz model for dielectrics.

MAGNETIZATION OF ANISOTROPIC SUPERCONDUCTING GRAINS, R.L. Peterson, Journal of

Applied Physics, Vol. 67, pp. 6930-6933 (1990). [CEEE/Electromagnetic Technology Division]

A critical-state calculation of the magnetization of hard type-II superconducting grains having

anisotropic critical current densities is given. The grains are assumed to present rectangular cross

sections to an applied magnetic field. The analysis shows how the critical current densities should

be inferred from magnetization measurements for various grain dimensions. For grains in the

form of platelets, the hysteresis changes with grain size. However for very elongated grains with

anisotropic critical currents, such as may be found in the high-temperature superconductors, the

magnetic hysteresis is insensitive to the lengths of the grains, and hence to powdering.

AC LOSS MEASUREMENTS OF TWO MULTIFILMENTARY NbTi COMPOSITE STRANDS, E.W.

Collings,* K.R. Marken, Jr.,* M.D. Sumption,* Ronald B. Goldfarb, and Robert J. Loughran, Advances

in Cryogenic Engineering (Materials), Vol. 36, R.P. Reed and F.R. Fickett, eds., New York: Plenum

Press, pp. 169-176 (1990). [CEEE/Electromagnetic Technology Division]

*Battelle Memorial Institute

As part of an interlaboratory comparative testing program conducted in support of the Versailles

Agreement on Advanced Materials and Standards (VAMAS), transverse-field DC hysteresis loss

measurements were made at liquid-helium temperatures at fields of up to 3 T (30 kG) on two

samples of multifilamentary NbTi composite. The strands differed widely in filament number,

were comparable in filament diameter, and one of them was provided with a Cu-Ni barrier

between the filaments. The results have been analyzed, and magnetically deduced critical current

density values obtained (for comparison with directly measured transport data) using various

standard techniques. Based on these studies, a figure-of-merit for AC loss is recommended. The

Cu-matrix strand, with its interfilamentary spacing of less than 1 jtm, exhibited pronounced

proximity-effect-induced coupling losses; this was not observed in the mixed-matrix strand which

possessed not only a Cu-Ni barrier but also an interfilamentary spacing of typically 4 /an.

68
MAGNETIC CHARACTERISTICS AND MEASUREMENTS OF FILAMENTARY Nb-Ti WIRE FOR

THE SUPERCONDUCTING SUPER COLLIDER, R.B. Goldfarb and R.L. Spomer, International

Cryogenic Materials Conference, Los Angeles, California, July 24-28, 1989, Paper FX-02, Advances in

Cryogenic Engineering (Materials), Vol. 36, pp. 215-222 (1990). [CEEE/Electromagnetic Technology

Division]

In synchrotron accelerator applications, such as the Superconducting Super Collider (SSC),

superconducting magnets are cycled in magnetic field. Desirable properties of the magnets

include field uniformity, field stability with time, small residual field, and fairly small energy

losses upon cycling. This paper discusses potential sources of problems in achieving these goals,

describes important magnetic characteristics to be considered, and reviews measurement

techniques for magnetic evaluation of candidate SSC wires. Instrumentation that might be

practical for use in a wire-fabrication environment is described. We report on magnetic

measurements of prototype SSC wires and cables and speculate on causes for instability in

multipole fields of dipole magnets constructed with such cables.

MAGNETORESISTANCE OF MULTIFILAMENT A1/A1-ALLOY CONDUCTORS, C.A. Thompson

and F.R. Fickett, Advances in Cryogenic Engineering (Materials), Vol. 36, Part A, pp. 663-669 (1990).

[CEEE/Electromagnetic Technology Division]

Previously we have shown that composite monofilament conductors consisting of very pure

aluminum confined in an Al-Fe-Ce alloy sheath show an anomalously high magnetoresistance

compared to pure aluminum. Some monofilament conductors showed values of AR/R in excess of 50 at 4 K in fields of 10 T, whereas pure aluminum values are usually an order of magnitude

smaller. Concerns that similar anomalous behavior might occur in multifilament wires of the

same materials prompted this study. Multifilamentary conductors with pure aluminum filaments

contained in an Al-Fe-Ce matrix have been investigated.

ANOMALOUS MAGNETORESISTANCE IN Al/Al -ALLOY COMPOSITE CONDUCTORS, F.R.

Fickett and C.A. Thompson, Advances in Cryogenic Engineering (Materials), Vol. 36, Part A, pp. 671-

678 (1990). [CEEE/Electromagnetic Technology Division]

The transverse magnetoresistance of several composite conductors containing a large single

filament of pure aluminum in a matrix of Al-Fe-Ce has been measured at 4 K to fields of 10 T.

The magnetoresistance (AR/R) of the composite is very large, rising to 55 in the "worst" case. Previous measurements on pure polycrystalline aluminum have always shown a rapidly saturating

behavior with a very small linear component; AR/R rarely exceeds a value of 5-6. In addition,

the magnetoresistance of the composite samples shows a structure as the field is rotated around

the current axis.

TUNNELING STABILIZED MAGNETIC FORCE MICROSCOPY: PROSPECTS FOR LOW

TEMPERATURE APPLICATIONS TO SUPERCONDUCTORS, John Moreland and Paul Rice,

Proceedings, Applied Superconductivity Conference, September 24-28, 1990, Snowmass, Colorado, in

IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 1 198-1201 (March 1991). [CEEE/Electromagnetic

Technology Division]

69
We have recently demonstrated an imaging technique referred to as tunneling stabilized magnetic

force microscopy or TSMFM. TSMFM is performed using a scanning tunneling microscopy

(STM) with a flexible, magnetic, tunneling tip in place of the usual rigid tunneling tip. TSMFM

images are therefore combinations of topography and the magnetic forces between the tip and the

sample. Room temperature TSMFM images of magnetic bit tracks on a hard disk have 100 nm

resolution and are comparable to Bitter patterns made using a ferrofluid. We have built a low

temperature STM with the hope of getting TSMFM images of the flux lattice in superconductors.

Preliminary TSMFM images of a YBa^uA (YBCO) film (Tc = 88 K) in a 50 mT field show

that relatively large magnetic forces are acting on the flexible tip while scanning at 48 K.

DIRECT EVIDENCE FOR AN EFFECT OF TWIN BOUNDARIES ON FLUX PINNING IN SINGLECRYSTAL YBajCujO^, L.J. Swartzendruber, A. Roitburd, D.L. Kaiser, F.W. Gayle, and L.H.

Bennett, Physical Review Letters, Vol. 64, No. 4, pp. 483-486 (January 22, 1990). [IMSE/Metallurgy

Division]

Magnetization measurements are reported on a nearly cubic single crystal of YBa2Cu306. x which

displays predominantly one variant of twin boundary. With this crystal, we were able to compare

the effects of different vortex and twin-boundary orientations on magnetic behavior. The results

demonstrate that there is a measurable effect due to direct pinning of vortices by twin boundaries

for applied fields perpendicular to the c axis. We also show that the large anisotropy in flux

pinning for applied fields parallel and perpendicular to the c axis is not due to pinning by twin

boundaries.

TEMPERATURE DEPENDENCE OF VORTEX-TWIN-BOUNDARY INTERACTION IN YBaaCuA,,,

A. Roitburd, L.J. Swartzendruber, D.L. Kaiser, F.W. Gayle, and L.H. Bennett, Physical Review Letters,

Vol. 64, No. 24, p. 2962 (June 11, 1990). [IMSE/Metallurgy Division]

Two recent Letters have reported studies of flux-vortex pinning by twin boundaries in single

crystals of the high-temperature superconductor YBasC^O^ (with x near 1) containing

predominantly one twin-boundary (TB) orientation. Although both investigations report a measurable effect of TB's on flux pinning, they present opposite conclusions concerning the

nature of the vortex-TB interactions. One investigation showed by magnetization measurements

that flux-vortex pinning at 10 K was greater when the applied magnetic field (H) was

perpendicular to the TBs (compared with parallel or at 45° to the TBs). In contrast, the other

investigation studied resistivity near the superconducting transition temperature ( — 93 K) and

demonstrated that pinning was greatest with the field parallel to the twin planes (compared to the

full range of H-TB orientations). We present evidence here that both results are correct, with

the apparent contradiction arising entirely from the different temperatures employed in the two

investigations.

HALL PROBE MAGNETOMETER FOR SSC MAGNET CABLES: EFFECT OF TRANSPORT

CURRENT ON MAGNETIZATION AND FLUX CREEP, R.W. Cross and R.B. Goldfarb, Proceedings,

Applied Superconductivity Conference, September 24-28, 1990, Snowmass, Colorado, in IEEE

Transactions on Magnetics, Vol. 27, No. 2, pp. 1796-1798 (March 1991). [CEEE/Electromagnetic

Technology Division]

70
We constructed a Hall probe magnetometer to measure the magnetization hysteresis loops of

Superconducting Super Collider magnet cables. The instrument uses two Hall-effect field sensors

to measure the applied field H and the magnetic induction B. Magnetization M is calculated from

the difference of the two quantities. The Hall probes are centered coaxially in the bore of a superconducting solenoid with the B probe against the sample's broad surface. An alternative

probe arrangement, in which M is measured directly, aligns the sample probe parallel to the field.

We measured M as a function of H and field cycle rate both with and without a dc transport

current. Flux creep as a function of current was measured from the dependence of ac loss on the

cycling rate and from the decay of magnetization with time. Transport currents up to 20% of the

critical current have minimal effect on magnetization and flux creep.

71
10. PHASE EQUILIBRIUM

PHASE EQUILIBRIA AND CRYSTAL CHEMISTRY IN THE SYSTEM Ba-Y-Cu-O, R.S. Roth, K.L.

Davis, and J.R. Dennis, Advanced Ceramic Materials: Ceramic Superconductors, Vol. 2, No. 3B, pp.

303-312 (July 15, 1987). [IMSE/Ceramics Division]

Preliminary phase equilibria diagrams are constructed for the "binary" systems BaO-V2Y203 , BaO-CuOx, V2Y203-CuOx

, and the "ternary" system BaO-V2Y203-CuOx

. Some melting data are used to outline an approximate primary field for the phase responsible for high temperature

superconductivity in this system, BajYCujO^. A discussion is given of the crystal chemistry of

the phase(s) near the composition Ba3YCu2 Ox

.

PHASE EQUILIBRIA IN THE SYSTEM Ba-Y-Cu-0-C02 IN AIR, R.S. Roth, C.J. Rawn, F. Beech,

J.D. Whitler, J.O. Anderson,* Ceramic Superconductors II Research Update, Man F. Yan, Editor,

Waterville, Ohio: The American Ceramics Society, Inc., pp. 13-26 (1988). [IMSE/Ceramics Division

and Reactor Radiation Division]

*NBS American Ceramics Society Research Association

Phase equilibria studies in the Ba-Y-Cu-0 system have continued, to determine the

thermodynamic relations and the primary phase field of Ba2YCu306+x . The high Cu-0 containing

liquid quenches as another phase with a characteristic x-ray diffraction pattern dependent on the

temperature of the heat treatment. The compound previously described as 3BaO: l/2Y203 :2CuO

which is a tetragonal pervoskite type phase with a = 2a' and c = 2a' is now shown to exist as a solid solution region between, but not including, the 3:1:2, 4:1:2 and 5:1:3 compositions. A

series of completely single phase specimens can be prepared at 950°C with the formula

Baj 2Y0 g Cu2. x 06 4_x (0.1 < x 0.4). This phase does not form at all when prepared with Ba02 in a pure oxygen atmosphere and it is concluded that the compound is chemically and structurally an oxycarbonate. The superconducting compound Ba^Cu-jO^ was found to melt in air through

a four phase field with BaY2Cu05 , BaCu02 and liquid gradually increasing in amounts from about

975° to the incongruent melting point at 1002° + 2°C. As four phases cannot coexist in

equilibrium, through a temperature range, in the system BaO:Y203 :CuOx a fourth component,

C02

, is required to explain the experimental melting data.

PHASE EQUILIBRIA IN PORTIONS OF THE SYSTEM SrO-CaO-Bi20-3-CuO, PART 1 - THE

SYSTEM SrO-CaO-CuO, R.S. Roth, C.J. Rawn, J.J. Ritter, and B. Burton, Journal of the American

Ceramic Society, Vol. 72, No. 8, pp. 1545-1549 (1989). [IMSE/Ceramics Division]

A new phase, probably CaCu02

, is stable only below =740°C, and it has an orthorhombic

average subcell: Cmca, a = 10.588(1), b = 2.8122(4), c = 6.3245(6) x 0.1 nm. Very weak

superstructure reflections indicate two distinct supercells: an orthorhombic cell with the b axis

multiplied by 10, and a monoclinic cell, C2/c or equivalent, with amon = 10.946 = a, bmon = 6.345 = cmon = 2.8100 x 12 = 33.72 x 0.1 nm = 12c, and j8 = 105.5°. At 950°C the

ternary SrO-CaO-CuO system has extensive regions of solid solution: complete solution is observed for the 2:1 series (Sr,Ca)2Cu03

; the 1:1 series (Sr,Ca)Cu02 accepts Ca up to = Sr0 ^Cao 7502; and the 14:24 series "Sr14Cu24041 " extends to at least "Sr7Ca7Cu24041 ". A new

ternary phase is reported, and it has a very narrow range of homogeneity (Ca1 . xSrx)Cu02 with

72
x = 0.15. The composition (Cao.g4Sro. 16)Cu02 has a primitive tetragonal unit cell with

a = 3.867(2) and c = 3.2195(2) x 0.1 nm. Another solid solution series is stable only above = 1000°C. It extends from the SrO-CuO binary into the ternary and is probably (Sr,.xCax )2Cu03

. The composition (Sr, 0Ca! 0)CuO3 is orthorhombic Immm with a = 3.306, b = 12.319,

c = 3.807 x 0.1 nm.

PHASE EQUILIBRIA AND CRYSTAL CHEMISTRY IN THE QUATERNARY SYSTEM Ba-Sr-Y-CuO IN AIR, R.S. Roth, C.J. Rawn, J.D. Whitler, C.K. Chiang, and W. Wong-Ng, Journal of the

American Ceramic Society, Vol. 72, No. 3, pp. 395-399 (1989). [IMSE/Ceramics Division].

Studies in the Sr-Y-Cu-o and Ba-Sr-Y-Cu-0 systems have revealed that Sr will substitute for Ba

in (Ba,Sr)2YCu3 06+x up to about 60 percent. There are no ternary compounds in the Sr-Y-Cu-0

system equivalent to the three ternary phases in the Ba system. A new binary phase,

"Sr14 Cu24041 ", CuO « 63.158 mole % was found, which forms a solid solution with Y203 to a

Sr: Y ratio of approximately 2:1. This phase can also incorporate considerable amounts of Ba and

Ca as well as many other large ions.

CRYSTAL CHEMISTRY AND PHASE EQUILIBRIA STUDIES OF THE BaO-l/2 R203-CuO SYSTEMS

III: X-RAY POWDER CHARACTERIZATION AND DIFFRACTION PATTERNS OF Ba3R3Cu6014+x, R = LANTHANIDES, W. Wong-Ng, C.K. Chiang, B. Paretzkin, and E.R. Fuller, Jr., Powder

Diffraction, Vol. 5, No. 1, pp. 26-32 (March 1990). [IMSE/Ceramics Division]

The superconductor related phases Ba3R3Cu6014+x, (or Ba2_zR1+zCu307+x , with z = 0.5), where

R = Pr, Nd, Sm, and Eu, have been prepared and characterized by X-ray powder diffraction, ac magnetic susceptibility measurement, resistivity measurement and thermogravimetric analysis

(TGA). Attempts to make this compound with R = Gd, Dy, Y, Er, and Lu were not successful.

This compound does not appear to form for rare-earth elements, R, with an ionic size smaller

than Eu. The oxygen content of these materials was estimated by TGA. The Ba3R3Cu60,4+x

compounds which were sintered at 950°C and annealed in oxygen at 550°C were found to be

non-superconducting above 10 K. Previously reported results for the R=La compound have

indicated that it was superconducting with a transition temperature of 15 K. The oxidationreduction behavior of these materials is similar to that of the superconductor phases Ba2RCu306+x .

These isostructural compounds are tetragonal with a space group of P4/mmm. The cell

parameters range from a = 3.8612(4), c = 11.5624(14) A, V = 172.38(4) A

3 in the Eu

compound to a = 3.8893(3), c = 11.6370(11) A, and V = 176.03(3) A

3 in the Pr compound.

The standard diffraction patterns of these materials are given.

CRYSTAL CHEMISTRY AND PHASE EQUILIBRIA OF THE BaO-R2 03CuO SYSTEMS, Winnie

Wong-Ng, Boris Paretzkin, and Edwin R. Fuller, Jr., Advances in X-Ray Analysis, Vol. 33, pp. 453-465

(1990). [IMSE/Ceramics Division]

Two important factors, the progressively decreasing size of the lanthanides, which is known as the lanthanide contraction, as well as the stability of different oxidation states of these elements

influence the prediction of compound formation in the Ba-R-Cu-0 systems. A systematic

73
investigation of these lanthanide systems and comparison with the Y system has revealed a

correlation of the effect of the above factors, in particular the size factor, on the trend of phase

formation, solid solution formation and phase compatibility diagrams of the Ba-R-Cu-0 systems.

For example, it has been found that the smaller the size of R

3+ or the greater the mismatch

between Ba2+ and R

3+ in the solid solution formation. This differing extent of solid solution

formation influences the ternary phase relationships.

74
11. PROCESSING—BULK MATERIAL

LOW TEMPERATURE THERMAL PROCESSING OF BajYCuA., SUPERCONDUCTING

CERAMICS, C.K. Chiang, L.P. Cook, S.S. Chang, J.E. Blendell, and R.S. Roth, Advanced Ceramic

Materials: Ceramic Superconductors, Vol. 2, No. 3B, pp. 530-538 (July 15, 1987). [IMSE/Ceramics

Division and Polymer Division]

Thermal processing of superconducting ceramics BajYC^O^, in the temperature range between

50°C and 750°C was studied. The oxygen content of the ceramic was reversible under thermal

treatment. The temperature for optimizing oxygen content was discussed.

THERMAL ANALYSIS OF BajYC^O^ AT 700-1000°C IN AIR, L.P. Cook, C.K. Chiang, W. WongNg, and J. Blendell, Advanced Ceramic Materials: Ceramic Superconductors, Vol. 2, No. 3B, pp. 656-

661 (July 15, 1987). [IMSE/Ceramics Division]

Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of Ba2YCu3 07. x using

conventional platinum cells yields a series of well-defined reversible thermal events between 850

and 1000°C. At least one of these is interpreted as being due to the products of platinum

reaction. However, changes in slope are also observed on TGA curves taken using non-reactive

MgO cells. The other thermal events may therefore represent minor phase changes in

Ba2YCu307.x in response to variations in temperature/stoichiometry.

PROCESSING - PROPERTY RELATIONS FOR Ba^CuA., HIGH Tc SUPERCONDUCTORS, J.E.

Blendell, C.K. Chiang, D.C. Cranmer, S.W. Freiman, E.R. Fuller, Jr., E.D. Krasicka, Ward L.

Johnson, H.M. Ledbetter, L.H. Bennett, L.J. Swartzendruber, R.B. Marinenko, R.L. Myklebust, D.S.

Bright, and D.E. Newbury, Advanced Ceramic Materials: Ceramic Superconductors, Vol. 2, No. 3B,

pp. 512-529 (1987). [IMSE/Ceramics, Fracture & Deformation, and Metallurgy Divisions, NML/Gas

& Articulate Science Division]

Ceramic materials exhibit strong relationships between their physical properties, their ceramic

microstructures, and the processing sequence that produced these microstructures and properties.

This interrelation between processing, microstructures, and properties is particularly true for the

new high Tc superconducting ceramic oxides of the type Ba2YCu307. x

. To characterize some of

these relations, a number of properties have been measured for these superconducting ceramics,

and related to the microstructures and processing sequence. The Ba2YCu307. x ceramics were

prepared by powder processing techniques, followed by dry pressing and sintering. Sintering was

conducted in both air and flowing oxygen at various temperatures, followed by annealing at various temperatures and environments. Superconducting properties, such as the transition

temperature and the width of the transition, were measured by both electrical conductivity and

AC magnetic susceptibility; both showing a strong sensitivity to annealing temperature and

annealing atmosphere. The microstructure and density was also strongly dependent on processing

conditions. In this regard compositional mapping proved to be an important technique for

quantifying the microstructural properties. Structural properties, which will be important for the

reliable use of the materials, were characterized through the fracture strength, the fracture

toughness, and the elastic moduli.

75
RELATIONSHIP OF ELECTRICAL, MAGNETIC, AND MECHANICAL PROPERTIES TO

PROCESSING IN HIGH Tc SUPERCONDUCTORS, J.E. Blendell, C.K. Chiang, D.C. Cranmer, S.W.

Freiman, E.R. Fuller, Jr., E.D. Krasicka, Ward L. Johnson, H.M. Ledbetter, L.H. Bennett, L.J.

Swartzendruber, R.B. Marinenko, R.L. Myklebust, D.S. Bright, and D.E. Newbury, American Chemical

Society Symposium Series 351, "Chemistry of High Temperature Superconductors," pp. 240-260

(September 1987). [IMSE/Ceramics Division, Fracture and Deformation Division, and Metallurgy

Division; and CAC/Gas and Particulate Science Division]

The interrelation between processing, microstructure, and properties is an important factor in

understanding the behavior of ceramic materials. This type of understanding will be particularly

important in the development of new high Tc superconducting ceramic oxides of the type

Ba2YCu307.x

. As an initial effort in understanding these relations, a number of properties have

been measured for these superconducting ceramics and related to their microstructure and

processing sequence. The Ba2YCu307.x ceramics were prepared by powder processing techniques,

followed by dry pressing and sintering in both air and flowing oxygen at various temperatures.

The sintered bodies were annealed at various temperatures and environments. Superconducting

properties, such as the transition temperature and the width of the transition, were measured by

both electrical conductivity and AC magnetic susceptibility; both of these properties show a strong

sensitivity to annealing temperature and atmosphere. The microstructure and density were also

strongly dependent on processing conditions. In this regard, compositional mapping proved to be an important technique for quantifying microstructural variations. Mechanical properties, such

as elastic modulus, hardness, and fracture toughness, which will be important for the reliable use of these materials in large scale structures, were also determined.

DENSIFICATION, SUSCEPTIBILITY AND MICROSTRUCTURE OF Ba,YCu306+x, John E. Blendell

and Laura C. Stearns, Annual Meeting of the Basic Science Division of the American Ceramic Society,

November 1-4, 1987, Orlando, Florida; Ceramic Transactions, Vol. 1, Part B, Ceramic Powder Science,

pp. 1146-1155 (1988). [IMSE/Ceramics Division]

The densification of Ba2YCu3 06+x for different sintering conditions has been measured. The

effects of compaction pressure, atmosphere and temperature have been investigated.

Measurements of the AC magnetic susceptibility have been used to determine the superconducting

properties. Dense samples were found to have less superconducting material as compared to

porous samples. Compositional mapping of the samples showed variations in composition in the

samples, corresponding to Cu-rich regions and Y-poor regions. Sinter-forging was done in order

to align the grains.

LOW TEMPERATURE APPROACHES TO SUPERCONDUCTIVE MATERIALS: A CHALLENGE

IN CHEMICAL SYNTHESIS, Joseph J. Ritter, Ceramics Powder Science II, American Ceramic Society,

Westerville, OH, pp. 79-84 (May 1988). [IMSE/Ceramics Division]

The synthesis of Ba-Y-Cu oxide ceramic powders is an essential factor in the development of

superconductive devices. While the major effort in the superconductor research is supported with

powders synthesized by conventional high temperature techniques, the problem of generating

precursors to these materials by low temperature routes presents a significant challenge. Results

from four different chemical approaches ranging from the precipitation of Y-Ba-Cu

76
hydroxycarbonates to reactions between Y-Ba alkoxides with Cu(OH)2 are given. All of these

systems progress through an intermediate triphasic mixture of BaC03 , CuO and Y203 before

being converted to YBa2Cu3 06+x between 800 and 950°. Superconductive behavior is measured

in each case after annealing the specimens in Oj at —600°. The benefits of ideal and practical

low temperature processing are discussed.

SYNTHESIS AND MAGNETIC PROPERTIES OF THE Bi-Sr-Ca-Cu OXIDE 80 and 110 K

SUPERCONDUCTORS, J.S. Wallace, J.J. Ritter, E.R. Fuller, Jr., L.H. Bennett, R.D. Shull, and L.J.

Swartzendruber, Physical Review B, Vol. 39, No. 4, pp. 2333-2335 (February 1, 1989).

[IMSE/Ceramics Division and Metallurgy Division]

We report on the observation of both the 80- and 110-K superconducting transitions in a nominally BiSrCaCu2Ox material produced by a new chemical synthesis procedure as well as by

solid state reaction. We also discuss a method for the confirmation of the presence of

superconductivity when only a small amount is present.

COMPOSITIONAL CONTROL OF THE MICROSTRUCTURE OF Ba2YCu306+x, John E. Blendell,

Carol A. Handwerker, Mark D. Vaudin, and Edwin R. Fuller, Jr., Journal of Crystal Growth, Vol. 89,

pp. 93-100 (1988). [IMSE/Ceramics Division]

Although Ba2YCu3 06+x exhibits superconductivity at high temperatures, the critical current density

(Jc) in bulk polycrystalline materials, prepared by sintering, is several orders of magnitude below

useful values. One possible source of low Jc in polycrystalline materials is the presence of

resistive grain boundary junctions resulting from second phases or impurity segregation.

Chemical composition plays a major role in determining the phases present and hence the grain

boundary characteristics. Preliminary composition measurements of Ba2YCu306+x include the

ratio of major elements, impurity concentrations at the trace level, compositional mapping of

major elements by electron probe microanalyses, and observations of sintered microstructures.

Results on liquid phase formation and carbon contamination introduced during powder processing

and from exposure to atmospheric C02 indicate that a great degree of compositional control is required for any meaningful characterization of the Ba-Y-Cu-0 system.

A STUDY OF THE HYDROXYCARBONATE PRECURSOR ROUTE TO THE YBa^uA.x HIGH

Tc SUPERCONDUCTOR, Joseph J. Ritter and James F. Kelly, High Temperature Superconducting

Compounds: Processing & Related Properties, S.H. Whang and A. DasGupta, Editors, The Minerals,

Metals & Materials Society, pp. 303-310 (1989). [IMSE/Ceramics Division]

The precipitation of Ba, Y and Cu as precursors to the YBa2Cu307. x high Tc superconductor is an attractive route to this material. However, studies in our laboratory indicate that powder

homogeneity is strongly dependent upon ionic concentrations and system pH. Moreover, we have

assessed the composition of the precipitated moieties. These data have been incorporated into

controlling the precipitation process in order to optimize particle size and powder homogeneity.

Further processing of these powders has yielded polycrystalline solids with sharp transitions near 95K and high transport current densities.

77
CRYSTAL CHEMISTRY AND PHASE EQUILIBRIA STUDIES OF THE BaO-R203-CuO SYSTEMS.

IV. CRYSTAL CHEMISTRY AND SUBSOLIDUS PHASE RELATIONSHIP STUDIES OF THE CuORICH REGION OF THE TERNARY DIAGRAMS, R= LANTHANIDES, Winnie Wong-Ng, Boris

Paretzkin, and Edwin R. Fuller, Jr., Journal of Solid State Chemistry, Vol. 85, pp. 117-132 (1990).

[IMSE/Ceramics Division]

In the BaO-V£R203-CuO systems, where R = lanthanides and yttrium, general trends of phase

formation, solid solution formation and phase relationships are correlated with the ionic size of

R. In air at 950°C the phase relationships in the CuO-rich region of these ternary diagrams

progressively change from the La system through the Nd, Sm, Eu, Gd, Y, Ho systems to the Er

system. First, the La system has the greatest number of ternary compounds. Second, the

superconductor material, Ba2RCu306+x, exhibits a solid solution of Ba2.zR1+zCu306+x for the first

half of the lanthanide elements with a range of formation which varies with the ionic size of R.

Third, a trend is observed regarding the tie-line connections between BajRCuOs, CuO, and the

phases Ba2.zR1+zCu306+x, and the binary phase R2Cu04 , or R2Cu205

. For the first half of the

lanthanides, except for La, a compatibility join is found to connect R2Cu04 and the tetragonal end

member of the Ba2.zR1+zCu306+x phase. In systems where R has a smaller ionic size, R = Eu

and beyond, the tie-line connection switches to join the BajRCuOs phase and the CuO phase. For

R = Dy and beyond, the binary phase R2Cu04 is replaced by the binary phase R2Cu205

.

POSSIBLE "PROXIMITY MATRIX" ROUTE TO HIGH CURRENT CONDUCTORS, John Moreland,

Y.K. Li,* J.W. Ekin, and L.F. Goodrich, Advances in Cryogenic Engineering (Materials), Vol. 36, Part

A, pp. 413-421 (1990). [CEEE/Electromagnetic Technology Division]

*Chinese Academy of Science, Beijing, China

The conductance of point contacts between the surfaces of superconducting YBa2Cu307.4 thin films

is very low. This is probably due to a native insulating surface layer. The conductance of these

point contacts can be markedly increased by vacuum depositing and subsequently annealing a thin

layer of Ag into the surface of the films. We believe that what might be described as a normalmetal superconducting "proximity matrix," is formed at the surface of the Ag coated YB^CujO^

films. In this paper, we describe our efforts to adapt this method to YBa2Cu3 07_5 powder. In

particular, we have developed a procedure for vacuum deposition of very thin Ag coatings onto

the surface of YBa2Cu3Ox powder grains. The Ag-treated powder is then pelletized, sintered,

annealed, and cut to form small conducting bars for electrical transport testing.

ORIGIN OF GRAIN BOUNDARY WEAK LINKS IN BaPblxBix03 SUPERCONDUCTOR, T.

Takagai,*t Y.-M. Chiang,* and A. Roshko, Journal of Applied Physics, Vol. 68, No. 11, pp. 5750-5758

(1990). [CEEE/Electromagnetic Technology Division]

*Massachusetts Institute of Technology

tPermanent address: DENKA, Tokyo Japan

Although BaPbo.75Bio.aO3 (BPB) has a comparatively large superconducting coherence length of

~7 nm and no reported anisotropy in its superconducting parameters, polycrystalline BPB

exhibits the same rapid decrease in transport critical current density (JJ with low applied field

(< ~50 Oe) that is characteristic of grain boundary weak links in cuprate superconductors (e.g.,

La2.xSrxCu04 , YBa2Cu307_x). We have studied the effects of processing thermal history on the

formation and morphology of grain boundary phases, and on the composition of BPB boundaries

78
with and without second phase, in order to understand the origin of these weak links. Scanning

transmission electron microscopy and Auger electron spectroscopy results show the presence of

a Pb-Bi-Ba-0 phase that is wetting and liquid above ~570°C, but which retracts to three-grain

junctions upon slow cooling or annealing at lower temperatures. However, weak-link behavior

persists in materials with retracted secondary phase, as well as in hot isostatically pressed samples

that never exceed the secondary phase melting temperature. It is found that the grain boundaries

remain Bi- and Pb-rich even after the retraction of secondary phases; samples that never exceed

the melting temperature of the secondary phase show absence of segregation at some but not all grain boundaries. The composition of the grain boundaries as well as Jrt vs temperature

measurements indicate that the boundaries act as SIS tunnel junctions.

MOLECULAR CHEMISTRY AND THE SYNTHESIS OF PRECURSORS TO ELECTRONIC

CERAMIC MATERIALS, J.J. Ritter, J.F. Kelly, D.E. Newbury, and D.B. Minor, NIST Special

Publication 804, Chemistry of Electronic Ceramic Materials, Proceedings of the International Conference,

August 17-22, 1990, Jackson, Wyoming, pp. 123-136 (January 1991). [MSEL/Ceramics Division and

CAM]

The use of molecular chemical reactions to generate precursors to ceramic materials has been the

subject of considerable study for more than three decades. This approach, in many cases, offers

distinct advantages over conventional high-temperature syntheses particularly in the areas of

improved homogeneity, purity, particle size regulation and amenability to intelligent processing

control. Generally, within the broad spectrum of chemical schemes available for ceramic

precursor synthesis, the molecular processes are poorly understood. Moreover, the reactions are

often interpreted either through "chemical intuition" or by analogy to similar chemistries, rather

than from measurements on the relevant systems. An additional complicating factor arises from

our lack of knowledge about the mechanisms operating during the conversion of precursors to

ceramic phases.

This presentation elaborates on several chemical routes which have been used to synthesize

ceramic precursors. The discussion includes systems where control over the molecular

architecture may lead to ceramic phases at low temperatures. Other systems, which essentially

deliver random, intimately mixed assemblies of the elements, are also examined. The various

approaches are illustrated with examples from research programs at NIST which have focused

on the chemical synthesis of precursors to barium polytitanates, metal carbides and borides,

electrically conductive oxides and nitrides, and the YBCO high-temperature superconductor.

THERMOMECHANICAL DETWINNING OF SUPERCONDUCTING YBa^UjO^ SINGLE

CRYSTALS, Debra L. Kaiser, Frank W. Gayle, Robert S. Roth, and Lydon J. Swartzendruber, Journal

of Materials Research, Vol. 4, No. 4, pp. 745-747 (July/August 1989). [IMSE/Metallurgy Division]

A method for the complete removal of twins from single crystals of superconducting YBa2Cu307. x

is described. The process depends on ferroelastic behavior found to exist in the phase, and

should be generally applicable to the layered perovskite-type phases containing accommodation

twins resulting from a tetragonal-to-orthorhombic transformation on cooling. The twin-free,

superconducting single crystals will enable investigation of a-b anisotropy of properties as well

as crystal structure determination without complication by the presence of microtwins.

79
12. PROCESSING—THIN FILMS

SUPERCONDUCTIVITY IN BULK AND THIN FILMS OF La185Sr015CuPw AND BajYC^O^, K.

Moorjani,* J. Bohandy,* F.J. Adrian,* B.F. Kim,* R.D. Shull, C.K. Chiang, L.J. Swartzendruber, and

L.H. Bennett, Physical Review B, Vol. 36, No. 7, pp. 4036-4038 (September 1, 1987). [IMSE/Metallurgy Division and Ceramics Division]

*Johns Hopkins University

A laser-ablation technique was used to deposit thin films from the bulk oxides La, ^Stq ^CuO^

and Ba^CujO^, whose superconductivity properties were investigated by dc resistivity, complex

ac susceptibility, and microwave response. The latter technique was employed to establish that

the thin films have superconducting regions with properties similar to the bulk materials.

MICRO-RAMAN SPECTROSCOPY OF HIGH-TC SUPERCONDUCTORS IN THE Y-Ba-Cu-0

SYSTEM, E.S. Etz, Winnie Wong-Ng, J.E. Blendell, and C.K. Chiang, Microbeam Analysis— 1988,

pp. 187-192 (August 1988). [CAC/Gas and Particulate Science Division and IMSE/Ceramics Division]

Preliminary results are reported on the (unpolarized) room-temperature micro-Raman spectra of

several types of samples and predominant phases in the Y-Ba-Cu-0 ceramic system. These

investigations provide microstructural characterization of these materials with a spatial resolution

equal to that obtained from electron microprobe microanalysis for elemental composition.

Experimental detail is given of the measurement parameters employed in the Raman microprobe

study of these materials. The samples examined are polycrystalline ceramic powders, sintered

ceramic pellets, and a specimen of single-crystal material. Of interest are the determination of

the component phases, mixed stoichiometrics, and crystal structures. The micro-Raman spectra are interpreted on the basis of the reported vibrational data (Raman and infrared) for materials

in this system of both tetragonal and orthorhombic symmetry. Conclusions are drawn relative

to the heterogeneity of these polyphase materials.

PROPERTIES OF YBa^u-A^ THIN FILMS GROWN ON OFF-AXIS-CUT MgO SUBSTRATES, S.E.

Russek, B. Jeanneret, D.A. Rudman, and J.W. Ekin, Proceedings, Applied Superconductivity

Conference, September 24-28, 1990, Snowmass, Colorado, in IEEE Transactions on Magnetics, Vol. 27,

No. 2, pp. 931-934 (March 1989). [CEEE/Electromagnetic Technology Division]

A series of YBajC^O^ films has been reactively sputtered on off-axis-cut MgO substrates. All

the films were oriented with the c axis normal to the substrate regardless of substrate orientation,

indicating that growth dynamics is a major factor influencing film orientation on non-latticematched substrates. As the substrate orientation is moved off the [100] direction the films

showed a decrease in transition temperature and showed properties indicative of an increased

density of weak links. The films grown on high-angle substrates showed better properties than

the films grown on low-angle substrates. Films grown on (1 10) MgO were as good as films

grown on (100) MgO.

80
MORPHOLOGY OF SILVER ON YBs^cUaO^ THIN FILMS, A. Roshko, R.H. Ono, J. A. Beall, J. Moreland, A.J. Nelson,* and S.E. Asher,* IEEE Transactions on Magnetics, Vol. 27, No. 2, pp. 1616-

1618 (March 1991). [EEEL/EIectromagnetic Technology Division]

*Solar Energy Research Institute

The morphology of silver layers deposited and annealed on laser ablated YBajCiLjO^ films has

been examined. Silver was found to dewet the YBa2Cu3 07.3 (001) surface on annealing in oxygen

and nitrogen. This dewetting behavior may be kinetically inhibited by using either thick silver

layers or short anneals and low temperatures. Silver layers 70 nm to 2.5 fim thick were studied

on films deposited on SrTi03 , LaA103 , and MgO substrates. Anneal times were varied from

6.5 min to 15 hrs at temperatures from 200 to 700 °C.

81

AUTHOR INDEX

Adrian, F.J. 8,66,80 Cieplak, M.Z. 25,31

Allen, P.B. 44 Ciszek, T.F. 13,64

Almasan, C. 36 Clark, A.F. 9,53-55,57,63

Anderson, J.O. 72 Clark, C.W. 43,46-48,51

Arakawa, E.T. 47-49 Cohen, M.L. 44,46

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Asher, S.E. 58,81 Cook, L.P. 23,25,30,75

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* U.S. G.P.O. : 1991-31 1-891 :40061 85

BL-1 14A U.S. DEPARTMENT OF COMMERCE

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RIRI IDftRAPHIP n ATA QHFFT

1. PUBUCATION OR REPORT NUMBER

NIST/SP-826

2. PERFORMING ORGANIZATION REPORT NUMBER

B91-0289

3. PUBLICATION DATE

November 1991

4. TITLE AND SUBTITLE

High-Temperature Superconductivity: Abstracts of NIST Publications, 1987-1991

S. AUTHOR(S)

Mary E. DeWeese (Editor)

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

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both Boulder, Colorado, and Gaithersburg , Maryland, is represented.

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bismuth superconductors; critical current; critical field; critical temperature; high Tc

; hightemperature superconductors; low-temperature superconductors; superconducting ceramics;

superconductors; thallium superconductors; YBCO; yttrium superconductors

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