D.C. van der Laan

Large intrinsic effect of axial strain on the critical current of high-temperature superconductors for electric power applications

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 YBa2Cu3O7−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.

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

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.

Magnetisation and transport current loss of a BSCCO/Ag tape in an external AC magnetic field carrying an AC transport current

In practical applications, BSCCO/Ag tapes are exposed to external AC magnetic field and fed with an AC transport current. The total AC loss can be separated in two contributions: first, the transport current loss influenced by an external AC magnetic field, and second, the magnetisation loss that depends on the transport current running through the conductor. In this paper the total AC loss is considered and the role of the electric and magnetic components is compared. This comparison is made with an available analytical model for the AC loss in an infinite slab and verified experimentally for a BSCCO/Ag tape conductor. For small transport currents the magnetisation loss dominates the total loss. When the current increases, a field dependent crossover occurs, after which the transport current loss also plays a role. Qualitatively the measurements can be described well in terms of the critical state model. For magnetic field parallel to the wide side of the conductor the CSM for an infinite slab describes the measurements also quantitatively.

Dependence of the critical current of YBa2CU3O7-δ coated conductors on in-plane bending

A new method to measure the effect of in-plane bending on the critical current of YBa2Cu3O7−δ coated conductors is presented. Such a bending mode can be important in transmission cables, saddleback magnets, and double-pancake windings. A linear strain distribution over the width of the conductor develops in this bending mode, where one half of the conductor is under axial compressive strain and the other half is under axial tensile strain. A reversible reduction in critical current of up to 5% is measured in 4 mm wide conductors at a critical bending radius of 0.25–0.28 m. The critical current degrades irreversibly for bending radii less than this because the strain at the edge of the conductor that is under tension irreversibly damages the conductor. The results are described by use of a model that calculates the critical current as a function of in-plane bending radius by taking into account the strain gradient over the width of the sample and the measured dependence of the critical current on axial strain. A similar approach can be used to calculate the degradation of the critical current n of other deformation modes, such as torsion, or other more complex geometries.

Critical current versus strain research at the University of Twente

At the University of Twente a U-shaped spring has been used to investigate the mechanical properties of a large variety of superconducting tapes and wires. Several mechanisms are responsible for the degradation of critical current as a function of applied strain. A change in its intrinsic parameters causes a reversible critical current dependence in Nb3Sn. The critical current reaches a maximum at a wire-dependent tensile strain level, and decreases when this tensile strain is either released or further increased. In Bi-based tapes the critical current is virtually insensitive to tensile strain up to a sample-dependent irreversible strain limit. When this limit is exceeded, the critical current decreases steeply and irreversibly. This behaviour is attributed to microstructural damage to the filaments. This cracking of the filaments is verified by a magneto-optical strain experiment. Recent experiments suggest that in MgB2 the degradation of critical current is caused by a change in intrinsic properties and damage to the microstructure. Magneto-optical imaging can be used to investigate the influence of applied strain on the microstructure of MgB2, as is done successfully with Bi-based tapes. In all these conductors the thermal precompression of the filaments plays an important role. In Nb3Sn it determines the position of the maximum and in Bi-based and MgB2 conductors it is closely related to the irreversible strain limit.

The effect of strain on grains and grain boundaries in YBa2Cu3O7−δ coated conductors*

The role of grains and grain boundaries in producing reversible strain effects on the transport current critical current density (Jc) of YBa2Cu3O7−δ (YBCO) coated conductors that are produced with metal–organic deposition (MOD) was investigated. The strain (e) dependence of Jc for full-width coated conductors is compared with that for samples in which the current transport was limited to a few or single grain boundaries by cutting narrow tracks with a laser or focused ion beam, as well as with thin films deposited on bicrystalline SrTiO3 substrates by use of pulsed-laser deposition (PLD). Our results show that the dependences of Jc on e for the grains and for the grain boundaries from the two kinds of YBCO samples can be expressed by the same function, however with a greater effective tensile strain at the grain boundaries than in the grains. The really striking result is that the grain boundary strain is 5–10 times higher for grain boundaries of in situ PLD grown bicrystals as compared to the aperiodic, meandered, nonplanar grain boundaries that develop in ex situ grown MOD-YBCO in the coated conductor of this study.

Evidence that the reversible strain effect on critical current density and flux pinning in Bi2Sr2Ca2Cu3Ox tapes is caused entirely by the pressure dependence of the critical temperature

It is well known that the critical temperature of cuprate- and iron-based high-temperature superconductors changes with pressure. YBa2Cu3O7 − δ coated conductors, as well as Bi2Sr2CaCu2Ox and Bi2Sr2Ca2Cu3Ox tapes and wires, show a clear reversible effect of strain on their current-carrying capability, but no clear understanding about the origin of this effect has been obtained. For the first time, we present evidence that the pressure dependence of the critical temperature is entirely responsible for a reversible change in critical current and magnetic flux pinning in Bi2Sr2Ca2Cu3Ox tapes with strain.

Strain effects in high temperature superconductors investigated with magneto-optical imaging

In order to determine the influence of intermediate deformation steps on the mechanical behavior of Bi-based tapes, the effect of longitudinal applied strain is investigated by means of magneto-optical imaging. The strain is applied in a helium flow-cryostat. Cracks appear soon after the critical current in Bi-based tapes is degraded. All filaments form multiple cracks that grow into tape-wide cracks, running from one filament to the next. The crack location is not caused by stress concentrations in the matrix, but by the mechanically weak colony boundaries. Because of the absence of intermediate rolling steps in the production of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ tapes, a different crack structure is observed compared to Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes. The relation between the critical current and the formation of cracks is studied. The degradation in critical current before the critical strain is reached may be caused by microcracks that remain undetected by magneto-optical imaging. The influence of strain on the microstructure of YBa/sub 2/Cu/sub 3/O/sub x/ coated conductors is also investigated with magneto-optical imaging. The formation of cracks is believed to be determined by the nickel substrate and related to the Ni-grain size.

Anisotropic in-plane reversible strain effect in Y0.5Gd0.5Ba2Cu3O7 − δ coated conductors

Recent experiments have shown that reversible effects of strain on the critical current density and flux pinning strength in the high-temperature superconductor Bi2Sr2Ca2Cu3Ox can be explained entirely by the pressure dependence of its critical temperature. Such a correlation is less simple for RE–Ba2Cu3O7 − δ (RE = rare earth) superconductors, in part because the in-plane pressure dependence of its critical temperature is highly anisotropic. Here, we make a qualitative correlation between the uniaxial pressure dependence of the critical temperature and the reversible strain effect on the critical current of RE–Ba2Cu3O7 − δ coated conductors by taking the crystallography and texture of the superconducting film into account. The strain sensitivity of the critical current is highest when strain is oriented along the [100] and [010] directions of the superconducting film, whereas the critical current becomes almost independent of strain when strain is oriented along the [110] direction. The results confirm the important role of the anisotropic pressure dependence of the critical temperature on the reversible strain behavior of RE–Ba2Cu3O7 − δ. The reversible strain effect in RE–Ba2Cu3O7 − δ is expected to decrease the performance of the conductor in many applications, such as high-field magnets, but the effect may be only minor in coated conductor cables, where strain is generally not aligned with the tape axis.

Temperature and magnetic field dependence of the critical current of Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape conductors

In order to improve the understanding of the dominant mechanisms that limit the critical current in high temperature superconductors, the dependence of the critical current on magnetic field and temperature of a Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape has been investigated in detail. The critical current is measured in magnetic fields up to 8 T, at temperatures ranging from 4.2 K to 70 K. The results are compared with existing models that describe the current path as two parallel systems, one depending on weak links and the other on flux pinning. The critical current at low magnetic fields is reduced drastically by the self-field of the superconductor. At intermediate magnetic fields, the field dependence of the critical current is mainly dominated by weak links, while at higher fields it is dominated by the strong-links current path, and depends on flux pinning. To clarify the models used to describe the measurements, the temperature dependence of the parameters used in the models is studied. The temperature dependence of the parameters used to describe the weak-links current path points out that the weak links are formed by remnant Bi/sub 2/Sr/sub 2/Ca/sub 1/Cu/sub 2/O/sub x/ phase at the grain boundaries.