Superconducting Materials

The following sections are included:

• Preface

• Contents

The following sections are included:

• The T_c Barrier of BCS Superconductors Till 1986
  • Development of low temperature superconductors
  • Critical temperature 23K of Nb₃Ge: Unsurmountable T_c-barrier?

• USO: Unidentified Superconducting Object

• Discovery, Confirmation and Identification of the High Temperature Superconductors

• Further Materials Developments

• Unique Electronic Structure in the HTS Cuprates

• Characteristics of HTS Superconductivity Phenomenology
  • Anisotropy factor and crystal structure
  • Anisotropy and irreversibility
  • Anisotropy and the first order vortex lattice phase transition
  • Anisotropy and the dimensional cross over in the low temperature

• Future Material Designing
  • Practical usage of Bi2212 and Bi2223 at 4K, 20K and 77K
  • Methods to improve the current carrying performance under strong magnetic fields
  • Rhenium-doped Hg1223 system
  • Heavily lead-doped Bi2212 system
  • Trends of the developments of Bi2212 and Bi2223 wires
  • Non-cuprate materials

• Weak Pinning Potential in the Under-Doped Regime

• References

Critical temperature, T_c, is the major parameter of physics of superconductivity. Its value depends on the strength of the attractive interaction λ, the energy scale ΔE (ΔE=Ω≅Ω_D for the conventional phonon mechanism, Ω_D is the Debay temperature), and the Coulomb repulsion μ* Assume, first, that μ=0…

The pairing, that is the formation of bound pairs of electrons (holes) is a key property of the superconducting state. This phenomenon is drastically affected by presence of magnetic impurities. The thing is that magnetic impurities possess a magnetic moment. This leads to an additional interaction between the impurities and the paired electrons. The two electrons (holes) forming a Cooper pair have opposite spins and, consequently, opposite magnetic moments. Therefore, each one will be acted upon differently by an impurity atom. The latter will try to flip the magnetic moment of one of the electrons, thus destroying the bound state of the pair. Experimentally, one observes a marked change in the critical temperature and the energy gap whenever magnetic impurities are introduced. Even a small impurity concentration (a few percent) can lead to a complete destruction of the superconducting state…

The following sections are included:

• INTRODUCTION

• PARAMETERS TO AFFECT CRITICAL CURRENTS
  • Carrier density
  • Coherence length
  • Depairing current (pair breaking current)

• CRITICAL CURRENTS IN TYPE I SUPERCONDUCTORS [4]

• CRITICAL CURRENTS IN TYPE I SUPERCONDUCTORS [5]

• CRITICAL CURRENTS IN PINNED SUPERCONDUCTORS
  • Elementary pinning interaction
  • Bulk pinning force
  • Case study for Y₂BaCuO₅ inclusions [7]

• SUMMARY

• REFERENCES

The role of the DOS fluctuations in the normal properties anomalies of HTS is analyzed. It is shown how, taking into account this effect, many puzzling and long debated properties of HTS materials (e.g. the steep increase of the electrical resistivity along the c axis just above T_c, the anomalous magnetoresistance, the effects of the magnetic field on the resistive transition along the c-axis, the c-axis far infrared absorption spectrum, NMR spectra around the critical temperature etc.) can be understood, leading to a simple consistent description in terms of fluctuation theory.

The lectures are based on the review article¹

Basic flux-pinning ingredients are reviewed and a selection of recent experimental results on high-temperature and conventional superconductors is presented and discussed.

Several cases of magneto-thermal transport properties in high critical temperature uperconductors are discussed, i.e. the longitudinal thermoelectric power in a magnetic field, the Nernst effect, the electrothermal conductivity and the magneto-thermal conductivity. It is shown that the order parameter type (or symmetry) can be well distinguished because of the difference in vortex features, e.g. in an s-wave or d-wave superconductor. The field dependence serves as the best recognizable signature even in polycrystalline samples. All that is highly consistent with considerations on intra- or intergrain processes, impurity effects, weak links and sample finite size.

The electrical resistivity, thermoelectric power and thermal conductivity probe the charge and heat currents caused by the electric and temperature gradients. In the following biased review, some emphasis is put on the temperature region encompassing the critical temperature, i.e. roughly extending from the percolation temperature to the onset superconductivity temperature for High T_c Superconductors. Some analysis of the specific heat data is harder to make, but it is shown that in all cases background (or so-called normal state) contributions should be well known before extracting physical parameters, explaining features and deciding upon order parameter symmetry properties and the like. In one appendix, some Hall effect feature is presented as a measure of some carrier concentration. A weak anomaly at rather high temperature can be explained in terms of a gap opening, or superconductivity onset. In a second appendix, the oxygen tracer diffusion coefficient is shown to be anomalous in YBCO at such a high temperature (ca. 2.5 T_c) as well.

Observations of quantized vortices or flux lines or fluxons in superconducting thin specimens can be carried out by means of transmission electron microscopes equipped with a field emission gun. Since from the electron optical point of view the fluxon can be considered as a pure phase object, special phase contrast techniques should be employed in order to generate contrast in the images.

The main ideas underlying the model developed for describing the interaction between the fluxon and the electron beam are here presented and applied to the interpretation of the main features of images obtained from the various experimental set-ups.

The following sections are included:

• Introduction

• Superconducting Bismuthates
  • Barium based systems
  • Strontium based systems
  • Conclusion

• High T_c Cuprates
  • Hg-based superconductors
  • Carbonate substitutions
    • Substitutions in Hg-based superconductors
    • Oxycarbonates

• Conclusion

• References

The following sections are included:

• Structural design of high temperature superconductors

• Synthesis of HgBa₂Ca_n−1Cu_nO_2n+2+δ

• Crystal structures of the HgBa₂Ca_n−1Cu_nO_2n+2+δ phases

• Superconductivity properties

• Superconductivity under high pressure

• Acknowledgments

• References

The following sections are included:

• INTRODUCTION

• OXYGEN-CONTROLLED MELT GROWTH (OCMG) PROCESS
  • The OCMG process for LRE-Ba-Cu-O
  • Effect of p(O₂) on T_c

• J_c-B PROPERTIES IN OCMG-PROCESSED LRE-Ba-Cu-O BULKS

• GRAIN ENLARGEMENT

• SUMMARY

• REFERENCES

The following sections are included:

• Introduction

• Powder synthesis methods

• Kinetic evaluation of powder synthesis

• Forming

• Sintering

• Conclusions

• References

The following sections are included:

• Introduction

• Chemical and physical properties of Bi-based superconductor

• Powder preparation [23]

• Oxygen nonstoichiometry and thermal stability of the 2223 phase

• Hot pressing of Bi-Pb-Sr-Ca-Cu-O superconductors
  • Densification behavior during hot pressing
  • Relationship between microstructure and superconducting properties [35]

• Sinter forging of Bi-2223 superconductors [36,37]

• Mechanical properties of Bi-2223 superconductors [38]

• Summary

• References

YBa₂Cu₃O_x, Bi₂Sr₂CaCu₂O_x, and (Bi,Pb)₂Sr₂Ca₂Cu₃O_x are difficult to densify by simple heat treatment. Strategies to densify these compounds by sintering, and their limitations and relations to diffusion data, are discussed.

The following sections are included:

• Introduction

• Anisotropic critical current density in single-crystalline samples

• Dependence of Grain-Boundary Critical Current Densities on the misorientation between the adjacent grains

• Strategy for increasing J_c of polycrystalline superconductors

• Experimental examples for increasing J_c
  • (Bi, Pb)₂Sr₂Ca₂Cu₃O_y (Bi-2223)
  • YBa₂Cu₃O_7−x (YBCO)

• Summary

• References

Bending strength, fracture toughness, and elastic modulus data have been acquired for YBa₂Cu₃O_x, Bi₂Sr₂CaCu₂O_x, (Bi,Pb)₂Sr₂Ca₂Cu₃O_x, and TIBa₂Ca₂Cu₃O_x bulk specimens. These data strongly suggest that the maximum possible tensile strain without fracture of bulk compacts, tapes, or wires is =0.2%. In Ag-sheathed bulk conductors, residual stresses have been measured and shown to be beneficial. In these composite conductors, fractures produced by large strains can be partially accommodated by current shunting through the Ag.

The methods for depositing thin films using vacuum processes are generally divided in two main classes: Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD), such as sputtering, laser ablation, evaporation, molecular beam epitaxy (MBE). This article will focus on PVD techniques used to prepare High Temperature Superconducting (HTS) thin films, illustrating the main characteristics of sputtering, thermal and electron beam evaporation, and in particular of MBE. Moreover, we will only deal with some representative and typical results, aiming at an illustration of the most often encountered problems and solutions.

The following sections are included:

• INTRODUCTION

• BASIC OF BULK APPLICATIONS

• PASSIVE DEVICES
  • Superconducting Bearing
  • Load Transport System
  • Hysteresis Motors
  • A Combination with Superconducting Magnet

• ACTIVE DEVICES
  • Trapped Field Magnets
  • Superconducting Motor
  • Second Generation Maglev
  • Magnetic Separation

• SUMMARY AND PROSPECTS

• REFERENCES

Superconducting Tunnel Junctions (STJs) are amongst the more energy sensitive cryogenic detectors. They are amazingly versatile, being able to measure the energy released both by ionizing and non-ionizing particle/radiation. As a result, the application of STJ detectors are wide ranging, from the detection of X-ray in radioastronomy and the measurement of single optical photon counting to the industrial X-ray fluorescence analysis and biopolymer mass spectrometry.

We present some of the more accepted and recognized experimental results on the microwave surface resistance in high-T_c superconductors in an external magnetic field. We briefly describe typical experiments employing resonant cavities, and how the surface resistance is extracted from measured quantities. We present experimental results on thin films and granular superconductors, and we describe the typical absorption phenomena that arise in such materials. Granularity-induced dissipation, related to Josephson coupling between superconducting grains is analyzed first. We then show how microwave experiments can be particularly suitable in the determination of the parameters of the vortex motion, in particular the fluxon viscosity, in films as well as in granular materials.

The following sections are included:

• Introduction

• Superconductors at Microwave Frequencies
  • DISPERSION RELATION
  • SURFACE IMPEDANCE
  • ATTENUATION

• Devices
  • INTRODUCTION
  • DELAY LINES
  • INTEGRATED ANTENNA AND MIXER
  • RESONATORS
  • OSCILLATORS
  • FILTERS
  • FUTURE DEVELOPMENTS

• TI-Based HTS Thin Films
  • INTRODUCTION
  • CHEMISTRY OF THE THALLIUM-BASED HTS PHASES
  • THIN FILM DEPOSITION TECHNIQUES
  • SUBSTRATES AND BUFFER LAYERS
  • PROCESSING, PROPERTIES AND MICROSTRUCTURE

• CONCLUSIONS

• Acknowledgements

• References

The vanishing of the electrical resistance is a key property of superconducting materials when cooled at low temperatures below their critical values. The replacement of normal conducting metals, as copper, by superconductors can result in considerable improvements in the equipments of the power sector with respect to their efficiency, size and performances, allowing a significant energy saving and a subsequent reduction of polluting emissions at the generation sites. The discovery of high temperature superconductors with T_c values above 77 K hold out the promise of superconducting technologies with lower capital and operating cost, if compared to devices based on low T_c superconductors.

The following sections are included:

• Introduction

• High Temperature Superconducting Cuprates at Glance

• Preparation of HTS Composite Tapes: the Powder-in-tube Method
  • PIT' initial stage: Precursor powder and sheathing material
  • PIT' middle stage: cold-working deformation process
  • PIT' final stage: sintering heat treatment and subsequent thermo-mechanical steps
  • PIT' modifications: multifilamentary and concentric tapes
    • Multifilamentary
    • Concentric

• Preparation of HTS Multilayered Conductors: The Accordion-Folding Method

• Summary

• References

Several low critical-current applications of high-T_c superconductors (HTS) are possible, some of which already have industrial potential. Three of them are presented, which make use of YBCO or BiSCCO extended thick (> 50 µm) films sprayed on metallic substrates by means of the HVOF thermal spray technique. The more extensively studied, so far, is magnetic shielding for cancellation of the magnetic field inside enclosures. With a single layer, attenuation factors of the external magnetic field greater than 10⁴ are obtained for static magnetic fields, much better than using multiple layers of conventional high-permeability materials. The maximum shielded field of ≈ 1•10⁻⁴ T can easily be improved to ≈ 0.1 T by using a secondary shield made of low-cost ferromagnetic materials. A second application, under development, is thermal shielding, taking advantage of the fact that below T_c a superconducting surface reflects most of die thermal radiation. Heat transfer can be reduced to levels comparable with those obtained with the best multilayer insulation techniques. A third application is for magnet current leads, with currents < ≈100 A. In this case, the powder-in-tube technique or the use of bulk materials is exceedingly costly and the current density provided by these techniques needlessly high. The use of low-conductivity metal strips coated with superconducting thick films allows one tabetter compromise performances and costs. A review of the state-of-the-art is provided and some of the latest results are reported.