PART I CRYOSTAT DESIGN AND MATERIALS SELECTION x Contents 1.5 Examples of measurement cryostats and cooling methods-high transport current (e 1 A) 30 1.5.1 Immersion test apparatus 30 1.5.2 Variable-temperature high-current measurement cryostats 32 1.5.3 Measurements near the superfluid-transition temperature 32 Lambda-point refrigerator 33 Saturated-liquid-container refrigerator 34 1.5.4 Variable-angle cryostats for measurements in a magnetic field 36 1.6 Addenda: safety and cryogen handling 37 1.6.1 Safety: how we can go wrang Cryogenic problems Less common cryogenic problems Vacuum foibles Unhealthy materials 1.6.2 Transferring cryogenic liquids Liquid nitrogen Liquid helium Procedure for transferring liquid helium Helium-transfer problems 1.7 References 1.7.1 Further reading 45 1.7.2 Chapter references 46 54 2.3.2 Low pressure (free-molecule case) 55 2.4 Radiative heat transfer 55 2.4.1 Superinsulation/multilayer insulation 57 2.5 Heat conduction across liquid/solid Interfaces 59 2.5.1 Liquid-helium/solid Interfaces 59 2.5.2 Liquid-nitrogen/solid Interfaces 61 2.6 Heat conduction across solid/solid Interfaces 62 2.6.1 Solder joints 64 2.6.2 Varnish and glue joints 64 Contents xi 2.6.3 Pressed contacts and heat switches 65 2.6.4 To grease, or not to grease? 66 2.7 Heat conduction across solid/gas Interfaces 2.8 Other heat sources 69 2.8.1 Joule heating 2.8.2 Thermoacoustic oscillations 2.8.3 Superfluid-helium creep 71 2.8.4 Adsorption and desorption of exchange gas 71 2.9 Examples of heat-transfer calculation 2.9.1 Case 1: simple dipper probe immersed in liquid helium 2.9.2 Case 2: dipper probe operated in variable-temperature mode in a superconducting magnet 2.9.3 Case 3: variable-temperature sample chamber 81 2.10 References 2.10.1 Further reading 2.10.2 Material property information an the internet 2.10.3 Chapter references xii Contents Low-melting-temperature solders 107 Soldering aluminum-a tough case 108 3.3.5 Sticky stuff 108 3.4 Construction example for a basic dipper probe 109 3.5 Sizing of parts for mechanical strength 113 3.5.1 Yield strength 113 3.5.2 Euler buckling criterion 3.5.3 Deflection of beams and plates 3.5.4 Pressure and vacuum loading 3.6 Mechanical motion at cryogenic temperature xviii Contents 8.5 Example calculations of minimum contact area 8.5.1 NI3-11 at 4 K 341 Contacts immersed in liquid helium 341 Contacts in helium gas or vacuum 342 8.5.2 Nb3Sn at 4 K: resistive-matrix contribution 343 8.5.3 High-T, superconductors at 77 K 344 Contacts in nitrogen gas or vacuum 346 8.6 Spreading-resistance effect in thin contact pads and example calculations 346 8.6.1 YBCO-coated-conductor contacts 347 8.6.2 Thin-film contacts 348 8.7 References 349 8.7.1 Further reading 349 8.7.2 Chapter references 350 PART III SUPERCONDUCTOR CRITICAL-CURRENT MEASUREMENTS AND DATA ANALYSIS 351 9 Critical-Current Measurements 353 9.1 Introduction 353 9,1.1 Transport method vs. contactless methods of measuring critical current 354 9.1.2 Defining critical-current density 355 9.1.3 The overall picture: dependence o...

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An Nb-Sn filament mounted on a flexible glass beam can be broken to form an electron tunneling junction between the fracture elements. Breaking the filament in liquid helium prevents oxidation of the freshly exposed fracture surfaces. A sharp superconducting energy gap in the I-V characteristics measured at 4 K indicates the formation of a high-quality tunneling barrier between the fracture elements. The resistance of the junction can be continuously adjusted by varying the surface bending strain of the beam. An estimated 0.1 nm change in the barrier thickness produces about an order of magnitude change in the resistance over the range from 105 to 108 Ω. The exponential character of this dependence shows that the tunnel junction is freely adjustable without intimate contact of the junction elements. ‘‘Break’’ junctions made in this way offer a new class of tunneling experiments on freshly exposed surfaces of a fractured sample without the oxide barrier previously required for junction stability. Such experiments provide a simple technique for tunneling to new materials and may eliminate complications that can be encountered during interpretation of data obtained using oxide barriers.

A remarkably large reversible reduction in the critical current of "second generation" high-temperature superconductors for electric power applications has been measured with a new technique over a wide range of mechanical strain. The effect amounts to a 40% reduction in critical current at 1% compressive strain in self-magnetic field, and is symmetric for compressive and tensile strains. The intrinsic effect is measured in highly aligned multigranular YBa 2 Cu 3 O 7−d coated conductors made by different processes, including superconductors with nanoscale pinning centers. This effect and its magnitude are expected to have a significant impact on power applications and provide a useful new parameter for probing the fundamental nature of current transport in high-temperature superconductors.

A large, magnetic-field-dependent, reversible reduction in critical current density with axial strain in YBa 2 Cu 3 O 7−δ coated conductors at 75.9 K has been measured. This effect may have important implications for the performance of YBa 2 Cu 3 O 7−δ coated conductors in applications where the conductor experiences large stresses in the presence of a magnetic field. Previous studies have been performed only under tensile strain and could provide only a limited understanding of the in-field strain effect. We now have constructed a device for measuring the critical current density as a function of axial compressive and tensile strain and applied magnetic field as well as magnetic field angle, in order to determine the magnitude of this effect and to create a better understanding of its origin. The reversible reduction in critical current density with strain becomes larger with increasing magnetic field at all field angles. At 76 K the critical current density is reduced by about 30% at −0.5% strain when a magnetic field of 5 T is applied parallel to the c-axis of the conductor or 8 T is applied in the ab-plane, compared to a reduction of only 13% in self-field. Differences in the strain response of the critical current density at various magnetic field angles indicate that the pinning mechanisms in YBa 2 Cu 3 O 7−δ coated conductors are uniquely affected by strain.