Now that YBCO-coated conductors have been commercialized, a number of YBCO coils have been developed. However, their basic performances have not been systematically investigated so far. Here, we demonstrate that of a YBCO double pancake coil. The critical current of an epoxy impregnated YBCO double pancake coil was substantially degraded, i.e. the normal voltage appears above 8 A, only 18% of that for the dry coil. It was inferred that degradation occurs if the cumulative radial stress developed during cool down exceeds the critical transverse stress for the YBCO-coated conductor (typically 10 MPa). Under these conditions, the conductor was debonded at the interface between the buffer layer and YBCO layers, or fractured in the YBCO layer itself, causing cracks on the YBCO layer, resulting in a significant decline of the critical current. These negative effects are suppressed if the coils are dry wound or impregnated with paraffin, as the bonding strengths between turns are negligible and therefore turns are separated if the cumulative radial stress tends to be tensile. For non-circular coils in which epoxy impregnation is inevitable, degradation due to cumulative tensile transverse stress is still the major problem.

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Degradation of the performance of a YBCO-coated conductor double pancake coil due to epoxy impregnation

Significant efforts can be found throughout the literature to optimize the current-carrying capacity of Nb3Sn superconducting wires. The achievable transport current density in wires depends on the A15 composition, morphology and strain state. The A15 sections in wires contain, due to compositional inhomogeneities resulting from solid-state diffusion A15 formation reactions, a distribution of superconducting properties. The A15 grain size can be different from wire to wire, and is also not necessarily homogeneous across the A15 regions. Strain is always present in composite wires, and the strain state changes as a result of thermal contraction differences and Lorentz forces in magnet systems. To optimize the transport properties, it is thus required to identify how composition, grain size and strain state influence the superconducting properties. This is not possible accurately in inhomogeneous and spatially complex systems such as wires. This article therefore gives an overview of the available literature on simplified, well-defined (quasi-)homogeneous laboratory samples. After more than 50 years of research on superconductivity in Nb3Sn, a significant amount of results are available, but these are scattered over a multitude of publications. Two reviews exist on the basic properties of A15 materials in general, but no specific review for Nb3Sn is available. This article is intended to provide such an overview. It starts with a basic description of the niobium–tin intermetallic. After that, it maps the influence of Sn content on the electron–phonon interaction strength and on the field–temperature phase boundary. The literature on the influence of Cu, Ti and Ta additions will then be summarized briefly. This is followed by a review of the effects of grain size and strain. The article concludes with a summary of the main results.

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A Review of the Properties of Nb3Sn and Their Variation with A15 Composition, Morphology and Strain State

Superconducting magnets have a variety of industrial, medical and research applications. This review discusses the prospects for MgB2 superconductor for practical magnet applications vis-?-vis the intermetallic low (LTS) and high temperature superconductor (HTS) cuprates. It has high TC (39?K), high JC (105?106?A?cm?2 at 4.2?K and in the self-field) and HC2 (15?20?T at 4.2?K) in wire/tape geometry, with great scope for further improvement in the coming years, making it a promising candidate for practical applications. The superconducting properties of MgB2 differ from those of LTS and HTS in many ways. Besides the unusually high TC, MgB2 has a large coherence length, low anisotropy and transparent grain boundaries. The most important difference between MgB2 and other practical superconductors is that it has two superconducting gaps originating from two different bands. Tuning the scattering rates between the two bands improves the superconducting properties and the practical applicability of MgB2. The different methods of fabrication of MgB2 conductors are described and compared. Fabrication of long length MgB2 conductors is relatively easy and less expensive as compared to HTS and the method allows the use of a variety of sheath materials with suitable barriers or reinforcement. The conductors have much better mechanical properties for practical applications. The critical issues and the challenges to be addressed for realization of MgB2 superconductors as the first choice for high field magnet applications are discussed. At present MgB2 is most suited for 20?25?K operation in fields of 1?2?T.

https://iopscience.iop.org/article/10.1088/0953-2048/20/1/R01/pdf

Prospects for MgB2 superconductors for magnet application

Spectroscopy, and “NMR on a Chip” for Chemistry, Biochemistry, and Industry Sergey S. Zalesskiy,† Ernesto Danieli,‡ Bernhard Blümich,*,‡ and Valentine P. Ananikov*,†,§ †Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991, Russia ‡Institut für Technische Chemie und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, D-52074 Aachen, Germany Department of Chemistry, Saint Petersburg State University, Stary Petergof, 198504, Russia

Miniaturization of NMR systems: desktop spectrometers, microcoil spectroscopy, and "NMR on a chip" for chemistry, biochemistry, and industry.

The electromechanical properties of YBa2Cu3O7−δ coated conductors under high axial compressive strain are measured; they show no irreversible degradation in critical current up to −2% strain. The high degree of elasticity of the ceramic layers in these conductors is beneficial when used in high-field applications, but has not been fully exploited. The results presented here lead to the introduction of a new method of producing YBa2Cu3O7−δ coated conductor cabling for use in low ac-loss and high-field magnet applications, where coated conductors are wound around a former with a relatively small diameter. This concept allows for full transposition of the conductors, a high cable critical current, low inductance, and a relatively high engineering current density. The feasibility of the concept is demonstrated by constructing several prototype cables and by comparing the cable critical current to that of a straight sample under axial compression.

https://iopscience.iop.org/article/10.1088/0953-2048/22/6/065013/pdf

YBa2Cu3O7―δ coated conductor cabling for low ac-loss and high-field magnet applications

The strain dependence of the critical current was studied for YBCO and DyBCO coated conductors with different buffer layers on Hastelloy substrates. A maximum of I/sub c/ was observed for both the YBCO and DyBCO tapes, however the sign of the strain at the I/sub c/ peak was opposite for the two superconductors. A reversible variation of I/sub c/ with applied strain was found and the reversible strain limit was observed to depend on the buffer layer. For the IBAD-CeO/sub 2//YSZ buffered YBCO tapes, I/sub c/ recovers reversibly when the applied strain is reduced starting from 0.30%. For those with an ISD-MgO buffer layer the irreversible degradation starts at a strain less than 0.22%. The reason for this difference is discussed based on microscopic observations. Quenching occurred during V-I measurements after the applied strain exceeded 0.30%, which is close to the yield strain of the composite tape.

Reversible strain dependence of critical current in 100 a class coated conductors

The influence of mechanical strain on the critical current (I/sub c/) is investigated for (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/(Bi-2223)/ Ag-0.2wt%Mg-0.3wt%Sb superconducting tapes at 77 K. The tensile axial strain along tape length is successfully induced to the sample by using a U-shape holder. Continuous change of the axial strain can be obtained by changing the distance between both ends of the holder. The U-shape holders made of Ti, SUS304, or brass are used to examine the effect of thermal strain due to the contraction caused by the cooling. For example, the results for the samples glued to Ti or brass holder are as follows. Strain dependence of normalized I/sub c/ (I/sub c//I/sub c/(at as-cooled state)) is affected by holder material. A steep decrease of I/sub c/ occurs when we apply 0.1% and 0.3% strain for Ti and brass holder, respectively. The different thermal expansion (-0.15% for Ti and -0.37% for brass from 300 K to 77 K) explains it. All the results for normalized I/sub c/ vs. strain relation fall on a master curve with taking into account of the effect of the thermal strain. Thermal expansion of the tape from 300 K to 77 K is measured to be -0.35% by using a strain gage. The critical strain ( c) where a steep decrease of I/sub c/ occurs is evaluated to be 0.27% from self-contracted state of the tape.

Strain effect in Bi-based oxide/Ag superconducting tapes

We investigated the intrinsic strain effect on critical current, Ic, and tensile fracture behaviour in the RE-123 coated conductors (RE =  Y, Dy or Sm) with Hastelloy C-276 substrates. It is found that the SmBCO coated conductors exhibit an intrinsic strain effect on Ic which is similar to that in the DyBCO ones. Ic decreases monotonically with increasing applied tensile strain but it can recover reversibly up to the strain of 0.32% when the strain is relieved. Quenching occurred for the coated conductors with Hastelloy C-276 substrate in the narrow strain region of 0.27–0.35% regardless of the buffer layers and superconducting materials. In order to reveal the reason for such quenching, we measured mechanical properties of the Hastelloy C-276 tapes alone and coated conductors at 77 K. The as-received and as-polished Hastelloy tapes show continuous yielding, which results from homogeneous plastic deformation. On the other hand, the Hastelloy tape annealed at 963 K corresponding to the deposition condition for the superconducting layer exhibits discontinuous yielding. In the annealed Hastelloy tapes, occurrence of Luders bands is observed, indicating that plastic deformation progresses inhomogeneously. Discontinuous yielding is also confirmed in the coated conductor. The yield strains of the coated conductors are almost identical to the strains at quenching. Characteristic fracture behaviour is observed, that transverse crack arrays are initiated from the tape edge and they propagate into the central part. The crack arrays orient to a certain angle to the loading axis, which is similar to that of the Luders bands. The similarities in the initiation site and the angle to the loading axis between the Luders bands and the crack arrays and close values of quenching strains to the yielding one strongly suggest that quenching is attributable to the discontinuous yielding in the Hastelloy substrate.

https://iopscience.iop.org/article/10.1088/0953-2048/18/12/020/pdf

Tensile fracture behaviour of RE-123 coated conductors induced by discontinuous yielding in Hastelloy C-276 substrate

The uniaxial strain dependence of the critical current was examined for YBCO coated conductors with IBAD-CeO2/YSZ or ISD-MgO buffer layers on Hastelloy substrates. Ic increased with increasing applied strain, reached a maximum and decreased for higher strain values. The reversible strain region of Ic variation was observed. The reversible strain limit depends on the buffer layers. For the CeO2/YSZ buffered tape, Ic recovered the initial value even after the applied strain reached 0.30%. On the other hand, an irreversible degradation of Ic was observed at the strain less than 0.22% for the MgO buffered tape. The overall relationship between the Ic normalized by the peak value and the intrinsic strain in YBCO obeyed a unified scaling function. Quenching occurred at the strain close to the yield strain of the Hastelloy substrate.

https://iopscience.iop.org/article/10.1088/0953-2048/18/3/027/pdf

Intrinsic strain effect on critical current and its reversibility for YBCO coated conductors with different buffer layers

As a milestone in the 1-GHz NMR magnet project that is being carried out at the Tsukuba Magnet Laboratory (TML), a 920-MHz high-resolution NMR magnet was successfully manufactured. It is made of 15%Sn-bronze-processed (Nb, Ti)/sub 3/Sn, Ta-reinforced (Nb, Ti)/sub 3/Sn, and NbTi conductors. The room-temperature bore of the cryostat is 54 mm in diameter. All the coils are cooled with pressurized superfluid helium at 1.55 K. The magnet was moved to a new building of the TML in December 2001. A persistent operation at 920 MHz started in April 2002. The field homogeneity after correcting with superconducting shim coils was less than 0.1 ppm in a sample volume. Field decay decreased to below 1.3 Hz/h at the beginning of July. NMR measurements using this magnet started in July.

Kikuo Itoh

Papers

Reversible strain dependence of critical current in 100 a class coated conductors

Strain effect in Bi-based oxide/Ag superconducting tapes

Tensile fracture behaviour of RE-123 coated conductors induced by discontinuous yielding in Hastelloy C-276 substrate

Intrinsic strain effect on critical current and its reversibility for YBCO coated conductors with different buffer layers

Operation of a 920-MHz high-resolution NMR magnet at TML