Composite Superconductors

K. Osamura

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780824791179
• eBook ISBN: 9781351459167
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This reference examines the tremendous benefits produced by the use of superconductivity, including the realization of a commercial fusion reactor for the generation of electricity. Providing a comprehensive coverage of superconductivity and magnet design - incorporating background information for beginners as well as research advances for specialists - this work: discusses the historical development of superconductivity and its engineering applications; explains the mechanical properties of the metal matrix composite; describes the important electromagnetic factors for the design of composite superconductors; analyzes the fabrication and optimization of various composite superconductors; and assesses the future development of high Tc oxide superconductors for engineering applicaitons.;This title is intended for: physicists; metallurgists; materials scientists; materials, electrical, mechanical, cryogenic and medical engineers; and graduate students in these disciplines.

1

Introduction

Fujio Irie

Kyushu University and Kyushu Electric Power Co., Inc., Fukuoka, Japan

1. BACKGROUND

Since the discovery of superconductivity by Kamerlingh Onnes in 1911, the remarkable characteristic of lossless current has aroused strong interest in the field of basic physics as well as in industry. Following that discovery, great strides have been made in physics, as evidenced by the appearance of Ginzburg–Landau (GL) and Barden–Cooper–Schrieffer (BCS) theories on the mechanism of superconductivity as well as the discovery of the Josephson effect. This constituted the first period in the development of superconductivity, which ended around 1960, with some overlap into the next period. Then the second period began, highlighted by an emphasis on application which continues up to the present.

Today, superconducting magnet technology is in the mature stage: Many large magnets for nuclear fusion experiments have been constructed, a project involving energy storage with a huge magnet system is ongoing, the superconducting magnetic resonance imaging (MRI) market is growing steadily, and an experimental magnetically levitated train is under development as a commercial transportation system. In the field of electronics, extensive studies on Josephson computers and the development of commercial superconducting quantum interference device (SQUID) systems are under way. However, before arriving to this stage it was necessary to accumulate fundamental knowledge of flux motion, flux pinning, ac loss, instability in wires and magnets, metallurgical problems, and similar concerns.

The appearance in 1987 of oxide superconductors with very high levels of critical temperature (T_c) produced great excitement in the field of basic physics as well as in practical applications, and great effort is being expended in this area. This can be taken as the beginning of the third developmental period. In the following we outline briefly how basic engineering studies of superconductivity have developed, and how, as a result, various applications have come into focus.

2. BASIC STUDIES

2.1. Early Stage

Studies of superconductivity originated with an experiment on zero resistance of mercury by Kamerlingh Onnes in 1911. The discovery of many superconducting metals, including alloys and compounds, followed. However, it took nearly 50 more years to elucidate the basic mechanism and to provide the foundation for applications.

2.2. London and Ginzburg–Landau Theories

As important as zero resistivity is the Meissner effect, i.e., the effect of zero magnetic flux density in superconductors. This effect was identified in 1933. The London equations (1935) of F. and H. London were the first theoretical attempts to describe the relation between magnetic field and superconducting current on the basis of these fundamental features of superconductivity. An important material parameter, λ_L, now known as London penetration depth, has been derived by this theory: The magnetic field can penetrate a superconductor only up to λ_L, which is on the order of 0.01 µm. However, this holds only for the type I superconductors. For type II superconductors, to which superconducting wires for magnets belong, the magnetic flux can penetrate superconductor mass in the form of quantum flux (fluxoids).

An understanding of type II superconductors had to wait until the appearance of the Ginzburg–Landau (GL) theory in 1950. In this theory the order parameter is used as a variable, the square of its absolute value being the density of superconducting electrons. A material parameter ξ, the coherence length, is defined in the theory as a width in which the density of superconducting electrons decays gradually at a boundary between the superconducting and the normal conducting states. Type I and type II superconductors are distinguished by the difference in the parameter κ = λ/ξ, where λ is a modified penetration depth that is not very different from λ_L. The structure of a single fluxoid in type II superconductors is related with λ and ξ in such a way that it consists of a flux tube of radius λ with a normal core of radius ξ at the center. The study of fluxoid motion provided the basis for the development of theories of loss, critical current, and other characteristics, which contributed greatly to progress in the development of practical superconductors, as we show later.

2.3. Bardeen–Cooper–Schrieffer Theory

GL theory is rather phenomenological; for the appearance of a microscopic theory we had to wait an additional six years for the BCS theory (Bardeen et al., 1957). The success of this theory is based on the assumption of two electrons with opposite directions for both spins and momenta. These two electrons are attracted by the intermediation of phonons and form a Cooper pair. This theory served to elucidate microscopic problems: The superconducting electrons are proved to be electrons in Cooper pairs, the coherence length in GL theory proved to be the size of the Cooper pair, and the density of superconducting electrons or the order parameter in GL theory was found to be proportional to the electron energy gap. This gap is proportional to the critical temperature T_c and to the critical frequency or upper frequency limit of the electromagnetic wave above which the superconductivity disappears. Three years following the appearance of BCS theory, a more perfect microscopic theory was presented by Gorkov. In contrast to BCS theory, it implies the case of a variation in energy gap parameter with position in a superconductor. Because of the brilliant success of these theories, by 1960 it was believed that every microscopic mechanism of superconductivity had been clarified. However, it soon became evident that more was to come.

2.4. Josephson Effect

The next big step in basic theory came in 1962 with the discovery of the Josephson effect (Josephson, 1962). This effect occurs in the junction where two superconducting electrodes are separated by a very thin insulating layer or a very small weak link. The effect implies that a superconducting current tunneling through the junction produces a phase difference between the superconducting electrons of the two electrodes and that the time derivative of the phase difference generates a voltage difference between the electrodes. From this effect two very interesting and useful characteristics can be derived: the critical current changes periodically with the magnetic flux in the junction and the V–I curve of the tunneling current has voltage steps whose width is proportional to the frequency of an electromagnetic wave applied to the junction. Practical applications of the Josephson effect, such as very high speed switching elements in computers, very high sensitivity magnetometers, and very high precision voltage standards, are an outgrowth of these characteristics.

2.5. Studies of Nonmetal Superconductors

Although the second period is characterized by increased growth in applications starting around 1960, basic studies continued during this period, especially on organic and oxide superconductors. Superconductivity in organic materials was suggested theoretically by Little (1964), and experimental results for organic materials were obtained in 1980. However, the T_c values obtained were less than 10 K, which is much lower than that expected by theory, and the mechanism was proved to be other than the exiton mechanism suggested theoretically. The first study of oxide superconductors appeared in 1975. This was preceded by a study of Ba–Pb–Bi–O (Sleight et al., 1975) that reported a T_c value of 12 K, although this work attracted little attention. Another 11 years elapsed until the discovery of the very high T_c oxides.

2.6. Nonideal Type II Superconductors

Studies supplying direct bases for superconductor application began early in the second period and continued to support the development of various applications. For practical materials, termed nonideal type II superconductors, the important phenomena are the critical current and the voltage generated by a current exceeding the critical current. These were explained by Kim et al. (1963) as follows. A fluxoid located at a point of current density J receives a driving force Jφ₀ (φ₀-flux quantum) which is counterbalanced by a force from pinning centers. This force has a threshold value called a pinning force, J_cφ₀. When J is larger than J_c, the driving force exceeds the pinning force, which leads to fluxoid movement and a voltage drop. Obviously, a high J_c value is required for practical applications of superconductors. At the same time, however, high-J_c-value material has a high loss density when the applied field changes. In this case a movement of fluxoids is generated by the introduction or removal of fluxoids from the surface of the sample due to the change in applied field. A loss is generated by movement of the fluxoid passing over a point of pinning. This loss is called as a pinning loss and the point of pinning is called the pinning center. The loss is also known as a hysteresis loss because the change of applied field in this case causes hysteresis in a magnetization of the sample.

Where the field variation is very slow, the current density in every point of the sample is thought to be kept just equal to J_c. This assumption is known as the critical-state model. The concept for this model was introduced by London (1962), Bean (1962), and Kim et al. (1962). The simple functions which they used for the pinning force are called the Bean–London model and the Kim model. Other models have been presented that provide quantitative descriptions of practical materials, in which the model having a general form is that presented by Irie and Yamafuji (1967).

2.7. Studies of Pinning

Soon after the aforementioned studies of pinning phenomena and of their metallurgical meaning were begun, certain principles were set forth. The pinning force of a pinning center experienced by one straight fluxoid is called the elementary pinning force. The total pinning force due to pinning centers in a unit volume, P_v, was originally thought to be a simple summation of elementary pinning forces in this volume. However, because of the elastic nature of a fluxoid lattice, the foregoing rule was found not to hold generally. It was pointed out by Yamafuji and Irie (1967) that the elastic property of the fluxoid lattice should be taken into account in this problem, and as a result, P_v was found to be proportional to the square of the elementary pinning force and inversely proportional to the elastic constant of the lattice. The former is now known as the simple summation rule and the latter as the dynamic definition of pinning force. The “summation problem” has now been studied in detail, and the P_v values measured experimentally were shown in most cases to lie between those calculated by the two formulas noted above.

3. PRACTICAL SUPERCONDUCTORS

3.1. Development of Wires

The first half of the 1960s was also a time of great development in high-field materials for superconducting wires. The foundation of practical materials was established at the same time. In 1961 for the first time, wires of Nb–Zr were manufactured and wound into a small magnet that produced 4 T. Such wires were later produced commercially. However, they soon gave way to Nb–Ti wires, which are more ductile than Nb–Zr and easier to handle. Although the value of critical current required has become much higher, Nb–Ti is still the representative material for magnet use.

Another group of superconductors are the A15 compounds. Among these, Nb₃Sn was found by Kunzler et al. (1961) to have the highest critical field of materials known at that time. They made wires by drawing and heat treatment of a Nb tube filled with Nb and Sn powders. A magnet wound by this wire, which produced 7 T, was the first truly high-field magnet to be developed. However, because of the brittle nature of Nb₃Sn, the magnet was very unstable. Several years later Nb₃Sn tapes with better mechanical characteristics were developed. These consist of Nb₃Sn deposited onto stainless steel tapes in the gas phase. Magnets wound by this tape produced fields greater than 10 T. They were supplied commercially and used for experiments in physics. However, due to the large demagnetization factor, tape conductors are intrinsically unstable and tape-wound magnets have now disappeared.

The idea of multifil...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• 1 Introduction
• 2 Superconducting Properties
• 3 Mechanical Properties of Composite Superconductors
• 4 Factors for Design of Composite Superconductors
• 5 Fabrication Methods
• 6 Engineering Applications
• Index