introduction to high-temperature superconductivity selected topics

Introduction to
High-Temperature
Superconductivity


SELECTED TOPICS IN SUPERCONDUCTIVITY
Series Editor: Stuart Wolf
Naval Research Laboratory
Washington, D.C.

CASE STUDIES IN SUPERCONDUCTING MAGNETS
Yukikazu Iwasa
INTRODUCTION TO HIGH-TEMPERATURE SUPERCONDUCTIVITY
Thomas P. Sheahen

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Introduction to

High- Temperature
Superconductivity
Thomas P. Sheahen
Western Technology Incorporated
Derwood, Maryland

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Foreword

High-temperature superconductivity (HTSC) has the potential to dramatically impact many
commercial markets, including the electric power industry. Since 1987, the Electric Power

Research Institute (EPRI) has supported a program to develop HTSC applications for the
power industry. The purpose of EPRI is to manage technical research and development

programs to improve power production, distribution, and use. The institute is supported by
the voluntary contributions of some 700 electric utilities and has over 600 utility technical
experts as advisors.
One objective of EPRI’s HTSC program is to educate utility engineers and executives
on the technical issues related to HTSC materials and the supporting technologies needed
for their application. To accomplish this, Argonne National Laboratory was commissioned
to prepare a series of monthly reports that would explain the significance of recent advances
in HTSC. A component of each report was a tutorial on some aspect of the HTSC field.

Topics ranged from the various ways that thin films are deposited to the mechanisms used
to operate major cryogenic systems. The tutorials became very popular within the utility
industry. Surprisingly, the reports also became popular with scientists at universities,
corporate laboratories, and the national laboratories. Although these researchers are quite
experienced in one aspect of the technology, they are not so strong in others. It was the
diversity and thoroughness of the tutorials that made them so valuable. The authors spent
many hours with leading experts in each topic area and went through a painstaking review
process to ensure that the information in the tutorials was complete, concise, and correct.
The tutorials that were originally published by EPRI in a newsletter format have evolved
into many of the chapters of this book. Hopefully the value that we tried to provide for our
member utilities with these tutorials will also benefit the entire industry through the
publication of this book. Utility engineers and electric equipment manufacturers will benefit
from the chapters describing the theory and characteristics of the HTSC materials. Scientists
working with the materials will appreciate the chapters that discuss the engineering of the
various applications that will make use of the HTSC materials.
Because of the HTSC’s potential for a strong impact on business and society, it is
important that new and working engineers become knowledgeable in the technology. This
book will become an invaluable resource for understanding the fundamental characteristics

of the materials and how they can be used.
Donald W. Von Dollen

Electric Power Research Institute
v


Preface

High-Temperature Superconductivity (HTSC) is most certainly a multidisciplinary field.
Drawing from physics, mechanical engineering, electrical engineering, ceramics, and metallurgy, HTSC spans nearly the entire realm of materials science. No one is expert in all these

disciplines; rather, each researcher brings a special expertise that is complemented by the
skills of colleagues. Therefore, it is necessary for each to obtain a modest understanding of
these allied specialities.
This book tries to present each of those disciplines at an introductory level, with the
goal that the reader will ultimately be able to read the literature in the field. Recognizing that
there is no need to read introductory material in your own specialty, the chapters were
organized with the expectation that each reader would skip part of the book. As a consequence, some repetition occurs in places; for example, Josephson junctions are introduced
in both Chapter 5 and Chapter 13. On the expectation that most engineers will be interested
in only a few of the applications, the later chapters are designed to stand alone.

In various places, numerical values are given for certain quantities of interest. In a
fast-moving field like HTSC, it is impossible to be absolutely up-to-date with the latest

numbers. It would be missing the point to dwell on numerical values. Rather, the intent of
the book is to convey a general understanding of the accomplishments, problems, and
motivations that lead researchers to try various ways of improving the HTSC materials.
OUTLINE

The HTSC field is also quite large, and conceptually splits nicely into applications
directed toward carrying electrical power and applications directed toward electronic circuits. This book deals primarily with the former. Electronic applications, including the very
broad field of thin-film superconductors, are given very little attention. This is because the
book grew out of a series of reports prepared at Argonne National Laboratory for the Electric

Power Research Institute (EPRI), during the period of rapid development in HTSC from
1988 to 1992. EPRI’s interest in power applications drove the choices of reporting topics,
and consequently determined the scope of this book.
There are five major divisions of the text:

1. Conventional Superconductivity—This part describes the present-day playing field
on which HTSC is striving to compete.
2.

Properties of the HTSCs—This series of chapters describes what we know about

the basic physics, chemistry, and materials science of these compounds. Because
vii


viii

PREFACE

of the complexity and interrelatedness of several different fields here, this was the
most difficult portion of the book to unify into a coherent presentation.
3. Carrying Electricity—These chapters deal specifically with those aspects of HTSC
that relate to making wire and conducting electricity. Because of the very rapid pace
of research and development in the HTSC field, and the likely success of some of
the government–industry partnerships carrying it out, this is the portion most likely

to be in need of revision soon.
4. Near-Term Applications—The known needs of the electric power industry are
featured here, in a series of chapters that each focus on one specific application of
HTSC. These could plausibly be termed the practical applications.
5. Futuristic Applications—The HTSC field has a lot of room to grow, and in these

chapters we peer over the horizon for potential future uses of HTSC. A modest
amount of speculation is in order here, and if some exceptional breakthrough occurs
tomorrow, some of these applications may move into the practical category.
Of course, for a full understanding it is best to read all five parts. However, Parts 4 and
5 can be read without having a detailed knowledge of all that went before. In general, no
single chapter in the book is so pivotal that it absolutely must be read. From the outset, I
aimed for a reader whose other demands preclude reading everything.
Thomas P. Sheahen


Acknowledgments

Every author is always indebted to his colleagues, and so it is a standard custom in the
scientific literature to say thanks for many helpful discussions. That is not enough here. The
long hours put in by many friends and professional colleagues (heavily, but certainly not
exclusively, at Argonne National Laboratory) are deserving of much greater recognition.
First of all, several chapters are co-authored with researchers who are more skilled than
I in the pertinent subject matter. My role here was often to integrate their work into the overall
presentation of the book.
Second, at the outset I certainly did not know all the various required disciplines. I had
to be tutored in the subject matter of each report to EPRI. After that, my written drafts had
to be reviewed, corrected, and critiqued both for factual accuracy and for clarity of
presentation. In assembling and updating the tutorials to make chapters for the book, I

continued to rely very heavily on the patience and generosity of many colleagues. A lot of
very fine people took time away from their own pursuits in order to help me succeed.
Foremost among my collaborators at Argonne National Laboratory was Dr. Robert F.
Giese; we worked together in preparing the series of EPRI reports for more than 4 years.
Those reports were each roughly equivalent to the size of one chapter here. Bob's contributions have been very great indeed.

From the beginning, the primary source of up-to-date information about what was taking
place in the HTSC field was High Update, featuring the “Note Bene” section written by
John Clem of Iowa State University. The guidance through the very extensive literature

provided in this way was indispensable to the completion of our reports.
Alan Wolsky supervised the EPRI project, and Bobby Dunlap and Roger Poeppel read
and critiqued each of the EPRI reports. Much of the clarity of presentation of various topics
originated in the reviews and discussions that were held with them.
Many other Argonne scientists contributed to my education in the HTSC field, and
several reviewed individual chapters, which resulted in the elimination of a number of errors
and mistaken concepts. In this regard I am particularly grateful to Howard Coffey, Steve
Dorris, George Crabtree, John Hull, Jim Jorgensen, Dick Klemm, Hagai Shaked, J.P. Singh,
and Jack Williams.
Colleagues at the National Institute of Standards and Technology deserve recognition,
both for educating me on various subjects and for critiquing portions of the manuscript.
Chapter 3 on refrigeration follows very closely the work of Ray Radebaugh; he could easily
be called a co-author. Others who provided in-depth consultation include Frank Biancaniello,
John Blendell, Steve Frieman, George Mattingly, Steve Ridder, and Bob Roth.
Stuart Wolf of the Naval Research Laboratory worked very hard to raise my level of
knowledge of the theoretical aspects of HTSC. Two British scientists (whom I have never
ix


x

ACKNOWLEDGMENTS

met) have taught me a lot: J. E. Gordon and Martin N. Wilson have written books of such
clarity that I can only cite the old slogan “Imitation is the sincerest form of flattery” to
acknowledge my debt to them. I would have fallen far behind in my knowledge of wire
development were it not for the continuous help of Alex Malozemoff and Bart Riley of

American Superconductor Corp., and of Pradeep Haldar and Lech Motowidlo of Intermagnetics General. Roger Koch of IBM straightened out my understanding of flux pinning
considerably. Xingwu Wang of Alfred University clarified conventional SMES and its

applications to the electric utility sector. Mas Suenaga of Brookhaven explained ac losses,
and Yuki Iwasa of M.I.T. helped me to understand stability in the HTSCs. Jerry Selvaggi of
Eriez Magnetics and Gene Hirschkoff of Biomagnetics Technologies each patiently explained their devices to me. Eddie Leung of Martin Marietta corrected several lapses in my
grasp of fault current limiters.
These are but a few examples of the countless sources of help—interdisciplinary
help—from which I have benefitted en route to writing this book.
Another 20 or more researchers from national laboratories, universities, and corporations have reviewed individual chapters, and have explained and clarified one point or

another. In short, this effort has received a lot of support from friends who saw the value in
it. I am very grateful to all my colleagues who have helped me to get it right. To the extent
that errors remain in the text, I personally have to take the responsibility for them.

This book would not have been completed without the strong and direct encouragement
and support of Jim Daley of the U.S. Department of Energy and Don Von Dollen of EPRI.
Their unfailing confidence made it possible to get through some very difficult aspects of the
work.
I also wish to thank all those researchers who generously gave permission for me to
reproduce their original figures, and frequently took the trouble to provide me with pristine
copies. On the subject of actually preparing the manuscript, special thanks go to Erika

Shoemaker of Argonne for guiding me through a series of word-processing hurdles, and to
Laurie Culbert for turning many sketches into excellent figures. Finally, I greatly appreciate

the generosity of Charlie Klotz of Argonne in providing me with support services during the
later stages of writing the book.

Thomas P. Sheahen


Contents

Part I. Superconductivity

Chapter 1. Introduction and Overview
1.1. Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. High-Temperature Superconductors . . . . . . . . . . . . . . . . . . . . . .
1.3. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4. Superconducting Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5. Wire Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6. Electric Power Applications . . . . . . . . . . . . . . . . . . . . . . . . . .
1.7. Other Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.8. Future Opportunities and Challenges . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3
4
6
7
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9

10
12
13

Chapter 2. Magnetism and Currents in Superconductors
2.1. Origins of Superconductivity . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. The Meissner Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. The London Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4. Type I and Type II Superconductors . . . . . . . . . . . . . . . . . . . . . .
2.5. Penetration Depth and Coherence Length . . . . . . . . . . . . . . . . . . .
2.6. Flux Quantization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7. The Vortex State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.8. Current Flow in Superconductors . . . . . . . . . . . . . . . . . . . . . . .
2.9. The Bean Critical State Model . . . . . . . . . . . . . . . . . . . . . . . . .
2.10. Hysteresis in Superconductors . . . . . . . . . . . . . . . . . . . . . . . . .
2.11. Practical Superconducting Wire . . . . . . . . . . . . . . . . . . . . . . . .
2.12. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15
17
20
21
23
23
26
27
27
29
31

34
34

Chapter 3. Refrigeration
3.1. Thermodynamic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2. Gas Refrigerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.3. Cryogenic Refrigerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
xi


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CONTENTS

3.4. Extreme Low Temperature Refrigeration
3.5. Economies of Scale . . . . . . . . . . .
3.6. Operating Practical Refrigerators . . . .
3.7. Summary and Conclusions . . . . . . .
References . . . . . . . . . . . . . . . . . .

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49
54
55
63
64

Chapter 4. Industrial Applications
4.1. Power Quality Conditioning in Factories . . . . . . . . . . . . . . . . . . .
4.2. Magnetic Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Utility-Based SMES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4. Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

66
71
76
78
80
80

Chapter 5. Sensitive Applications
5.1. Nuclear Magnetic Resonance Imaging (MRI) . . . . . . . . . . . . . . . .
5.2. Superconducting Quantum Interference Devices . . . . . . . . . . . . . . .
5.3. Biomagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4. Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81
86
89
95
95
96

Chapter 6. Basic Concepts of Theory of Superconductivity
6.1. Lattice Vibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2. The Fermi Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3. The Density of States . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4. Pairing in Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5. The Superconducting Energy Gap . . . . . . . . . . . . . . . . . . . . . .
6.6. The Gap and Tunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.7. Consequences of the BCS Equations . . . . . . . . . . . . . . . . . . . . .
6.8. Experimental Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .
6.9. Analysis of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.10. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

98
99
101
103
105
107
110
111
113

114
115

Chapter 7. The New Superconductors
7.1. Why It Was “Impossible” . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.2. The Discoveries of 1986–1987 . . . . . . . . . . . . . . . . . . . . . . . . 119
7.3. Hype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.4. Real Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.5. Government’s Role . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
7.6. Development of an Industry . . . . . . . . . . . . . . . . . . . . . . . . . . 130
7.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133


CONTENTS

xiii

Part II. High-Temperature Superconductivity (HTSC) Basic Properties

Chapter 8. Structure
8.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2. HTSC Crystal Structures . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3. Twinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4. Thallium, Mercury, and Bismuth Compounds . . . . . . . . . . . . . . . .
8.5. Layered Structures and Anisotropy . . . . . . . . . . . . . . . . . . . . .
8.6. Other Oxide Superconductors . . . . . . . . . . . . . . . . . . . . . . . .
8.7. Summary and Forecast . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

137
139
144
146
149
152
155
156

Chapter 9. Phase Equilibrium
9.1. Introduction to Phase Diagrams . . . . . . . . . . . . . . . . . . . . . . .
159
9.2. Two-Component Phase Diagrams . . . . . . . . . . . . . . . . . . . . . . 163
9.3. Three-Component (Ternary) Phase Diagrams . . . . . . . . . . . . . . . . 170
9.4. Phase Diagram for YBCO . . . . . . . . . . . . . . . . . . . . . . . . . . 175
9.5. Four-Component Phase Diagrams . . . . . . . . . . . . . . . . . . . . . . 181
9.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
184
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
185
Chapter 10. Effects of Doping
10.1. Structural Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.2. Valence Electrons and Charge Balance . . . . . . . . . . . . . . . . . . .
10.3. Holes vs. Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4. Magnetism and Superconductivity . . . . . . . . . . . . . . . . . . . . .
10.5. Substitution on the “A” and “B” Sites . . . . . . . . . . . . . . . . . . .
10.6. Flux Pinning by Vacancies . . . . . . . . . . . . . . . . . . . . . . . . .
10.7. Experimental Difficulties . . . . . . . . . . . . . . . . . . . . . . . . . .
10.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

188
190
191
191
194
198
199
201
202

Chapter 11. Mechanical Properties
11.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2. Microscopic Perspective . . . . . . . . . . . . . . . . . . . . . . . . . .
11.3. Fracture Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.4. Measurement Methods . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.5. Mechanical Properties of HTSCs . . . . . . . . . . . . . . . . . . . . . .
11.6. Novel Ways to Improve Strength . . . . . . . . . . . . . . . . . . . . . .
11.7. Comparison to Fiber Optics . . . . . . . . . . . . . . . . . . . . . . . . .
11.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

203
205
207
212
214
219
220
221

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Chapter 12. Theory of HTSCs
12.1. The Normal-State Fermi Surface . . . . . . . . . . . . . . .
12.2. Macroscopic Theories . . . . . . . . . . . . . . . . . . . . .
12.3. Interacting Electrons . . . . . . . . . . . . . . . . . . . . . .
12.4. The Density of States in HTSCs . . . . . . . . . . . . . . . .
12.5. A Two-Band, Two-Gap Theory . . . . . . . . . . . . . . . .
12.6. Comparison with Data . . . . . . . . . . . . . . . . . . . . .
12.7. Universal Curves . . . . . . . . . . . . . . . . . . . . . . . .
12.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 13. Weak Links
13.1. Josephson Junctions . . . . . . . . . . . . . . . . . . . . . . . . . . .

13.2. SQUIDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3. Grain Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.4. Experimental Observations . . . . . . . . . . . . . . . . . . . . . . .
13.5. Optimizing Current Across Grain Boundaries . . . . . . . . . . . . . .
13.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Part III. Carrying Electricity

Chapter 14. Flux Pinning
14.1. The Irreversibility Line . . . . . . . . . . . . . . . . . . . . . . .
14.2. Basic Concepts of Flux Pinning . . . . . . . . . . . . . . . . . .
14.3. Thermal Activation . . . . . . . . . . . . . . . . . . . . . . . . .
14.4. Irreversibility and Flux Creep . . . . . . . . . . . . . . . . . . .

14.5. Flux Lattice Melting . . . . . . . . . . . . . . . . . . . . . . . .
14.6. Vortex Glass Model . . . . . . . . . . . . . . . . . . . . . . . .
14.7. Anisotropy Effects . . . . . . . . . . . . . . . . . . . . . . . . .
14.8. Creating Strong Pinning Sites . . . . . . . . . . . . . . . . . . .
14.9. Implications for Conducting Current . . . . . . . . . . . . . . .
14.10. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .263
. . . . .265
. . . . . 268
. . . . 270
. . . . . 273
. . . . . 275
. . . . . 279
. . . . 282
. . . . 283
. . . . . 287
. . . . . 288

Chapter 15. Processing Methods
15.1. Kinetics and Thermodynamics . . . . . . . . . . . . . . . . . . . . . .
15.2. Measurement of Processed Materials . . . . . . . . . . . . . . . . . .
15.3. Real Time Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . .
15.4. BSCCO: The Two-Powder Process . . . . . . . . . . . . . . . . . . .

.
.
.
.

292
296
302
. 303


CONTENTS

15.5. Melt Processing in YBCO . . . . . . . . . . . . . . . . . . . . . . . . .
15.6. Volatility and Thallium Compounds . . . . . . . . . . . . . . . . . . . .
15.7. Postprocessing: Irradiation . . . . . . . . . . . . . . . . . . . . . . . . .
15.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xv

305
310
314
315
316

Chapter 16. Wire
Thomas P. Sheahen and Alan M. Wolsky
16.1. The Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317
16.2. YBCO: Early Attempts . . . . . . . . . . . . . . . . . . . . . . . . . . . 318
16.3. Powder-in-Tube Method . . . . . . . . . . . . . . . . . . . . . . . . . . 321
16.4. Direct Tape Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326
16.5. Monofilament Wires . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

16.6. Multifilament Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
338
16.7. Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
341
16.8. Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344
16.9. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
346
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

Chapter 17. Protecting Against Damage
Thomas P. Sheahen and Robert F. Giese

17.1. Physics vs. Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2. Measurement of Specific Heat . . . . . . . . . . . . . . . . . . . . . . .
17.3. Specific Heat of Superconductors . . . . . . . . . . . . . . . . . . . . .
17.4. Specific Heat and Stability . . . . . . . . . . . . . . . . . . . . . . . . .
17.5. Quenching and Flux Jumping . . . . . . . . . . . . . . . . . . . . . . . .
17.6. Composite Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.7. Quench Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.8. Types of Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.9. Experimental Verification of the Model . . . . . . . . . . . . . . . . . .
17.10. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

349
351
353
357
358
360

363
366
368
371
372

Chapter 18. AC Losses
18.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
373
18.2. AC Loss Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
18.3. Designing Against AC Losses . . . . . . . . . . . . . . . . . . . . . . . 378
18.4. HTSC Theory of AC Losses . . . . . . . . . . . . . . . . . . . . . . . . 381
18.5. Measuring AC Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
18.6. Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
18.7. Theory/Experiment Comparison . . . . . . . . . . . . . . . . . . . . . . 391
18.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392
References
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393


xvi

CONTENTS

Part IV. Electric Power Applications of HTSC

Chapter 19. Transmission Lines
John S. Engelhardt, Donald Von Dollen,
Ralph Samm, and Thomas P. Sheahen
19.1. Underground Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397

19.2. Capacity Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
19.3. Superconducting Transmission Systems . . . . . . . . . . . . . . . . . . 403
19.4. HTSC Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . 407
19.5. Near-Term Applications for HTSC Cable Systems . . . . . . . . . . . . . 410
19.6. Long-Range Possibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
19.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

Chapter 20. Levitation
John R. Hull and Thomas P. Sheahen
20.1. The Meissner Effect . . . . . . . . . . . . . . . . . . . . . . . . .
20.2. The “Force Banana” . . . . . . . . . . . . . . . . . . . . . . . . .
20.3. Forces on Moving Magnets . . . . . . . . . . . . . . . . . . . . .
20.4. Magnetic Levitation Vehicles . . . . . . . . . . . . . . . . . . . .
20.5. Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.6. Flywheel Energy Storage . . . . . . . . . . . . . . . . . . . . . .
20.7. Outlook and Summary . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.
.
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.
.
.
.
.

. . . 415
. . . 418

. . 419
. . . 421
. . . 425
. . . 429
. . . 430
. . . 431

Chapter 21. Superconducting Magnetic Energy Storage
Susan M. Schoenung and Thomas P. Sheahen
21.1. Economic Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.2. Big vs. Small SMES . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.3. HTSC SMES Calculations . . . . . . . . . . . . . . . . . . . . . . . . .
21.4. Unique Features of HTSC SMES . . . . . . . . . . . . . . . . . . . . .
21.5. Refrigeration System and Energy Efficiency . . . . . . . . . . . . . . .
21.6. Cost of Major Components . . . . . . . . . . . . . . . . . . . . . . .
21.7. Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

433
. 435
. 436
. 439
. 441
443
. 445
. 446
. 446

Chapter 22. Electric Motors

Howard E. Jordan, Rich F. Schiferl, and Thomas P. Sheahen
22.1. Conventional Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449
22.2. SuperconductingMotors . . . . . . . . . . . . . . . . . . . . . . . . . .450


CONTENTS

xvii

22.3. Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.4. Motor Design Principles . . . . . . . . . . . . . . . . . . . . . . . . . .
22.5. Specific Design: 10,000 hp Motor . . . . . . . . . . . . . . . . . . . . .
22.6. Cryogenics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.7. Actual Motor Construction . . . . . . . . . . . . . . . . . . . . . . . . .
22.8. Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22.9. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 451
453
455
457
459
461
462
463

Chapter 23. Fault Current Limiters
Robert F. Giese, Magne Runde, and Thomas P. Sheahen
23.1. Fault Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23.2. Utility Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.3. Superconducting Fault Current Limiters . . . . . . . . . . . . . . . . . .
23.4. Stability and Switching . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.5. Considerations for In-Line SCFCLs . . . . . . . . . . . . . . . . . . . .
23.6. Cost Competition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23.7. Other Switching Applications . . . . . . . . . . . . . . . . . . . . . . .
23.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

465
466
469
474
477
479
479
480
481

V. Future Possibilities

Chapter 24. New Refrigerators
24.1. Liquid Hydrogen . . .
24.2. Cold Gaseous Helium .
24.3. Liquid Neon Cryostat .
24.4. Magnetic Refrigeration
24.5. Summary . . . . . . .
References . . . . . . . . . .

. . . .

. . . .
. . . .
. . . .
. . . .
. . . .

. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .

485
487
491
492
496
497

Chapter 25. Applications to Measurement and Process Control
25.1. Principles of Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . .
499
25.2. HTSC SQUIDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
25.3. Applications of HTSC SQUIDs . . . . . . . . . . . . . . . . . . . . . . 506
25.4. Magnetic Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508
25.5. Digital Circuit Applications . . . . . . . . . . . . . . . . . . . . . . . . . 509
25.6. Competing Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
25.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512

xviii

CONTENTS

Chapter 26. High Magnetic Fields
26.1. Energy Density and Magnetic Pressure . . . . . . . . . . . . . . .
26.2. High Fields Using BSCCO . . . . . . . . . . . . . . . . . . . . .
26.3. Applications to Research Facilities . . . . . . . . . . . . . . . . .
26.4. Manufacturing Processes . . . . . . . . . . . . . . . . . . . . . . .
26.5. Magnetic Separation . . . . . . . . . . . . . . . . . . . . . . . . .
26.6. Future Applications . . . . . . . . . . . . . . . . . . . . . . . . .
26.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.
.
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.

.
.
.
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.

.
.
.

.
.
.
.
.
.
.
.

. 515
. 517
. 518
. 522
. 523
. 526
. 530
. 531

Chapter 27. Organic Superconductors
27.1. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533
27.2. Contemporary Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . 534
27.3. Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535
27.4. Structural Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538
27.5. Future Expectations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539
27.6. Carbon-60 Superconducting Compounds . . . . . . . . . . . . . . . . . . 539
27.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541
Chapter 28. Aerospace Applications
28.1. NASA’s Perspective . . . . . . . . . . . . . . . . . . . .
28.2. Near-Term Applications . . . . . . . . . . . . . . . . . .
28.3. Applications of High Magnetic Fields . . . . . . . . . . .
28.4. Future Expectations . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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. . . . .
. . . . .
. . . .
. . . . .
. . . . .

543
544
547
553
553

Appendix A. Measurement of Critical Current . . . . . . . . . . . . . . . . . . . . 555
A.1. Magnetization Measurement of
. . . . . . . . . . . . . . . . . . . . .
555
A.2. Transport Measurement of . . . . . . . . . . . . . . . . . . . . . . . . . 557
A.3. Contact Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559
A.4. Progress Toward Standards . . . . . . . . . . . . . . . . . . . . . . . . . . 559
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561
Appendix B. Magnetic Measurements Upon Warming or Cooling . . . . . . . . . .

563

Donn Forbes and John R. Clem
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568
Glossary

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 571


I
SUPERCONDUCTIVITY


1
Introduction and Overview

The field of superconductivity, once a mere laboratory curiosity, has moved into the realm
of applied science in recent years. Even more applications may become possible because of

the discovery of ceramic superconductors, which operate at comparatively “high” temperatures.
1.1.

SUPERCONDUCTORS

What is a superconductor? For most materials, which are normal conductors, whenever
electrical current flows, there is some resistance to the motion of electrons through the
material. It is necessary to apply a voltage to keep the current going, to replace the energy
dissipated by the resistance. Ordinary copper wire in a house is a good conductor, with only
a little resistance; the filament in a light bulb has a high resistance, and generates so much
heat that light is given off. Electronics is based on components in which the resistance
changes under control of an input voltage; these components are made of semiconductors.
A superconductor, in contrast, is a material with no resistance at all.
A lot of metals, but not all, show modest electrical resistance at ordinary room
temperatures, but turn into superconductors when refrigerated very near to absolute zero.

The first metal discovered to be a superconductor was mercury,1 soon after the invention (in
1908) of a cryogenic refrigerator that could attain the temperature at which helium becomes
a liquid: 4.2 K = –452°F. In the subsequent 60 years, many more superconductors were found
at these very low temperatures. By the 1960s, certain alloys of niobium were made that
became superconductors at 10–23 K. It was generally believed on theoretical grounds that
there would be no superconductors above 30 K.
Since a superconductor has no resistance, it carries current indefinitely without requiring
voltage or an expenditure for electricity. Once the current is started, it continues for
“geological” time durations, provided that the superconductor is kept cold. For many years,
the requirement of refrigeration to extremely low temperatures had the effect of confining
superconductivity to the realm of research laboratories. The cost of running a superconducting persistent current loop is simply the cost of refrigeration, which in most cases means
the cost of purchasing liquid helium—about $7 per liter.
Electromagnets are the most important application of current loops, but it is expensive
to run a large electromagnet built out of ordinary wire like copper. By the 1970s, it became
cost effective (in some cases) to pay the price for refrigerating a superconductor instead of
paying the utility for electricity lost through resistance. In this way an industry evolved, in
3


4

CHAPTER 1

which large superconducting magnets were used in certain applications. One familiar use
was in hospitals, where Magnetic Resonance Imaging (MRI) has become a standard
diagnostic tool for scanning the body to see what is wrong inside. The cost of running such

a device is far less than “exploratory surgery.”
1.2.

HIGH-TEMPERATURE SUPERCONDUCTORS

There would be a lot more practical uses for superconductivity if it weren’t for the very
high cost of liquid helium coolant. Any gas will liquefy at sufficiently low temperatures; for

example, oxygen becomes liquid at 90 K and nitrogen at 77 K. It is far less costly to liquefy
these gases than to liquefy helium. Liquid nitrogen sells for about six cents per liter (in
truckload quantities); moreover, it has a much greater cooling capacity than liquid helium.
For any application in which liquid nitrogen can replace liquid helium, the refrigeration cost
will be about 1000 times less.

There are several ceramics, based on copper oxide, which remain superconducting near
100 K. For example, the compound yttrium barium copper oxide (YBCO) has been found
to be superconducting up to 92 K. This may not seem like a “high” temperature to most
people, but to the engineers figuring the cost of refrigerants, it is high enough: liquid nitrogen

is sufficient to cool YBCO into its superconducting range. Additional important ceramic
superconductors include BSCCO (bismuth strontium calcium copper oxide) and TBCCO

(thallium barium calcium copper oxide); and HBCCO (mercury barium calcium copper
oxide). The latter has the highest critical temperature of superconductivity, Tc = 133 K =
–220°F. Table 1.1 presents the chemical formulas and Tc values for each of these compounds.
The ceramic superconductors of greatest interest are very anisotropic compounds; that
is, their properties are quite different in different crystalline directions. For that reason,

researchers take considerable pains to obtain good grain alignment within any finite-sized
sample. Figure 1.1 is a drawing of the molecular structure of YBCO. The structure is
essentially that of a sandwich, with planes of copper oxide in the center, and that is where

the superconducting current flows. The compounds BSCCO and TBCCO are even more

pronounced in their anisotropy; indeed, very little current can flow perpendicular to the
copper oxide planes in those lattices.
The role of the elements other than copper and oxygen is secondary. In YBCO, yttrium
is only a spacer and a contributor of charge carriers; indeed, nearly any of the rare earth


INTRODUCTION AND OVERVIEW

5

elements (holmium, erbium, dysprosium, etc.) can be substituted for yttrium without
changing the transition temperature Tc significantly. Often the formula is written as
(RE)1Ba2Cu3O7, to emphasize the interchangability of other rare earths (RE) with yttrium.
The bismuth compounds exhibit the interesting property of being micaceous; that is,
they are like mica. The crystal lattice shears easily along the bismuth oxide planes, and this
allows BSCCO to be deformed and shaped with less difficulty than the other ceramic
superconductors. This advantage has led researchers to invest more effort in making wire
out of BSCCO: lengths of over one kilometer have been made so far.
Unfortunately, the new high-temperature superconductors have two major drawbacks:
they are very brittle (like most ceramics), and they do not carry enough current to be very
useful. One problem is that of brittleness. Ceramics are by nature brittle, and so is copper
oxide. The idea of making wire out of ceramics would be a subject of derision, were it not
for the example set by fiber optics. It is true that if one makes a strand of sufficiently tiny
diameter, then a cable made from such strands can have a bending radius of a few centimeters
without over-straining the individual strands. For the high temperature superconducting
materials, the engineering task of overcoming brittleness is proving more difficult than it
was for fiber optics.
A more important drawback is that the magnetic properties of these materials are
substantially different from conventional metallic superconductors. The workhorse material
of low temperature superconducting magnets, niobium-titanium (NbTi), allows lines of

magnetic flux to penetrate in such a way that these lines tend to stay put: the phenomenon
is known as flux pinning. By contrast, the exceptional crystalline structure of the copper
oxide superconductors causes the magnetic flux lines to fragment (they become shaped like
sausages), and hence they move around readily, thus dissipating energy and defeating the
advantage of superconductivity. In one of those perverse conspiracies of nature, the crystal-


6

CHAPTER 1

line properties that offer the best chance to circumvent the brittleness problem are the very
same properties that tend to degrade flux pinning.

1.3. HISTORY
Before continuing with what HTSCs may lead to, it is appropriate to look back and see
what they have come from. The history of high-temperature superconductivity as a field
distinct from ordinary superconductivity is very brief. It began in late 1986 when news spread

that J. George Bednorz and Karl Müller of the IBM research laboratory in Zurich, Switzerland, had reported2 the observation of superconductivity in lanthanum copper oxides doped
with barium or strontium at temperatures up to 38 K. This caused tremendous excitement
because 38 K was above the ceiling of 30 K for superconductivity that had been theoretically
predicted almost 20 years earlier (and which had become an unquestioned belief among
scientists and engineers interested in superconductivity).
Once the barrier was broken, hundreds of scientists rushed to try various chemical
compounds to see which one would give the highest
In March 1987, the American
Physical Society meeting included a session dealing with new discoveries in superconductivity. That session, which lasted all night, had over 1000 people trying to squeeze in the

doors of the meeting room, and would later be remembered as “the Woodstock of physics.”

At that point, the compound yttrium barium copper oxide (YiBa2Cu3O7, or just YBCO for
short) took center stage,3 because of it’s high value of
Subsequently, attention was focused on copper oxides, and before long the compound
bismuth lead strontium calcium copper oxide was found4 with Tc= 105 K. That was followed
5
by the discovery in 1988 of thallium barium calcium copper oxide, with Tc = 125 K. Almost

five years elapsed before the mercury compounds6 boosted the Tc record to 133 K. Under
extremely high pressure,7 Tc can be pushed over 150 K.

As soon as one superconductor had reached a temperature above 77 K, the era of
high-temperature superconductivity had arrived. Some observers believed that roomtemperature superconductors were just around the corner waiting to be discovered. A number
of exuberant articles appeared in the popular press extolling the many ways our lives would
change. Others realized the stunning advantage associated with having superconductors near
100 K and turned their attention to studying and improving the properties of the compounds
already discovered.
Moreover, all previous (low-temperature) superconductors require expensive ($7 per
liter) liquid helium to cool them to around 4 K. Also, substantial skill and training is required
to transfer liquid helium from one container to another without freezing the apparatus.
Consequently, only rarely has conventional superconductivity emerged from the physics lab.
In the meantime, anyone can pour liquid nitrogen, so a major obstacle to using superconductors in practical applications vanishes if they can operate above 77 K.
These features of superconductivity, well known in 1987, have provided the driving
force to sustain superconductivity research ever since. The payoff has been so great that
many researchers have devoted major resources to pursuing practical applications. Of course,
the path toward high-temperature superconductivity has never been all roses, and the
research community has had to sustain itself through several early disappointments. The
bubble generated by the popular press didn’t exactly burst, but deflated around 1990. The
early exuberance was replaced by the sober realization that there are many serious obstacles
to overcome in physics, materials science, and mechanical and electrical engineering before

INTRODUCTION AND OVERVIEW

7

these new superconductors find widespread practical application. Serious research managers
do not expect to see any large-scale applications until the twenty-first century. Some early
applications to delicate sensors and electronic devices are beginning to appear in the

mid-1990s.
1.4.

SUPERCONDUCTING MAGNETS

A leading use of superconductors is to produce high magnetic fields. Magnetic fields
exceeding 10 T have been produced in a handful of laboratories, but have never been

employed either in health care (MRI scans, for example) or in industry. The potential
applications for higher magnetic fields are just beyond the horizon, and therefore subject to
speculation. The idea of using very high magnetic fields (> 30 T) to separate industrial
chemicals, thus retrieving value from a waste stream and reducing pollution, is a very
attractive concept. However, such mundane considerations as the structural integrity of the
supporting framework must be brought into the engineering design, because high magnetic
fields exert very great forces, and no one has yet built a large-scale magnet of such magnitude.
Optimistic recognition of possibilities needs to be tempered with cautious engineering

pragmatism about what can actually be accomplished at a low cost. If the price of an entire
magnetic system is too high, no one will buy the device and the application will not come
into widespread use.
Meanwhile, interest has increased in applications of low-temperature superconductors;

and the possibility of using the ceramic copper oxide superconductors at low (4 K) or
intermediate (20–30 K) temperatures is worth considering. Conventional low-temperature
superconductors are often used in magnets running at 4 K, but they lose their superconductivity in high magnetic fields, typically above 6 T (= 60,000 gauss); although niobium tin

(Nb3Sn) will remain superconducting even out to 10 or 15 T. The ceramic superconductors
do much better. Bismuth strontium calcium copper oxide (BSCCO) carries adequate current
and remains superconducting well above 20 T, at 20 K. Therefore, the best way to obtain

very high magnetic fields is to use the ceramic superconductors at low temperatures.
Of course, in order to wind a coil to produce a magnetic field, the first prerequisite is to
make long lengths of wire from the copper oxide superconductors; thus, the application to
high magnetic fields awaits the development of a reliable wire-manufacturing technique.
There is no guarantee of ultimate success here, which is why ceramic superconductivity

remains a research field.
1.5.

WIRE MAKING

The critical current density

(current per cross-sectional area, A/cm2) is the major

electrical parameter of a superconductor’s performance. Therefore, the main focus in HTSC
research today is on trying to make wire with high
There are four distinct categories of

obstacles to be overcome:
•
•

•
•

Large currents in magnetic fields
Fabricating uniform long lengths of wire
Mechanical properties
Joining and contact techniques


8

CHAPTER 1

Each of these obstacles contains subcategories by which R&D activities can be classified.
Here we touch on only the first two.
1.5.1.

Large Currents in Magnetic Fields

Ceramic oxide wires present two problems that were not encountered in the earlier
development of low-temperature, intermetallic wires. The first is due to the granular nature
of these materials. Very large currents can flow within grains, but grain boundaries impede
the current flow between grains. It is necessary to achieve very good alignment between
adjacent grains in order to circumvent this problem. Methods have been developed both to
align grains and to provide “clean” grain boundaries, but these processing methods still need
improvement.
The second problem occurs when current is passed through HTSC wires (even when
grains are aligned and grain boundaries are clear) but the operating temperature exceeds a
certain value. This temperature may be as low as 30 K for some materials and as high as 90
K for others.8 It is known from development of LTSC wires that high transport current

required pinning of the magnetic flux lines that penetrate the material.9 Lorentz forces,
proportional to both current and magnetic field strength, will move the flux lines unless the
flux lines are sufficiently pinned. Flux line movement causes losses (which may exceed that
of copper resistance) even in the presence of superconductivity.

1.5.2.

Fabricating Uniform Long Lengths

For the HTSCs, none of the ordinary standard methods of making wire have proved
successful. It is not easy to make wire from the ceramic superconductors. To circumvent the
problem of brittleness, it is customary to sheath the ceramic material with some ductile metal
(usually silver) that is readily handled in wire fabrication equipment. Figure 1.2 is an
illustration of one typical process. The raw ingredients are oxide powders of the key elements
(in this illustration, BSCCO is being made). These are treated at high temperatures to make
a powder of the superconducting compound. It is often helpful to substitute different
elements.
is known (by its subscripts) as 2223, where partial
substitution of lead for bismuth is understood.

The powder is next packed into a tube of typically a half-meter length and an 8-mm
diameter (see Figure 1.2). The wire-making process of drawing, rolling, or swaging follows,

leaving a final shape well below 1 mm in diameter but very long. To restore the ceramic core
to the superconducting state, it is necessary to heat treat it further, at perhaps 800–900°C.
Finally, the wire must be annealed in oxygen very slowly (typically 100 hours) in order to
allow oxygen atoms to slowly recover their proper positions in the crystal lattice. Without
this step, only a small percentage of the material would be superconducting, and the wire
would not be useful for carrying current.
Sumitomo Electric Corp. in Japan was the first company to make over 100 m lengths

of wire. Subsequently, companies in Europe and the United States also made lengths over
100 m, and now the competition is intense. Questions of manufacturability, bending radius,
and insulation are being explored, demonstrating that companies consider wire-making to
be more than just a research venture.
There are many different varieties of processing techniques, the details of which are
proprietary within each organization. Sumitomo also made the first multifilament strands by
10
a repeated rolling and annealing process, packing as many as 1,296 fibers into a wire. By
11
1993, American Superconductor Corp. used a metal precursor process to surpass that