A. V. Troitskii

Dependences of HTS tape critical parameters on fluences under irradiation with heavy ions and high energy electrons

Experimental data on the study of modifications of two types of the YBCO(123)-based composite tape under irradiation with electrons with the energy of 23 MeV and 132Xe27+ (167 MeV), 84Kr17+ (107 MeV), and 40Ar8+ (48 MeV) ions in a wide range of irradiation doses are presented. It is shown that no changes in the HTS transition temperature and critical current occurred under electron irradiation up to the dose D ≈ 10−4 disp/a (displacements per atom). The threshold dose of irradiation with 132Xe27+ ions, at which superconductivity disappears, is determined. In comparison with the initial value, the critical current of 40Ar8+ ions is found to increase by ≈18% in the zero magnetic field at T = 77 K. This effect can be explained by the pinning centers for Abrikosov vortices induced by relatively small ion irradiation doses. High doses of heavy ions lead to partial or complete amorphization of a superconductor, which results first in a decrease in the critical current and transition temperature and then in a total disappearance of superconductivity. X-ray diffraction is used to study the changes in the crystal structure of the YBCO(123) superconductor under ion irradiation.

Application of shock waves for the improvement of current-carrying properties of YBCO(123) and Bi(2223) HTSC tapes in magnetic fields

Shock waves with a leading-edge pressure of ∼1011 Pa, which were produced in a plasma focus setup, were used to increase the critical current density in YBCO(123) and Bi(2223) HTSC tapes. It was shown that the effect of chemically inactive high-temperature high-density plasma on the HTSC tapes leads to an irreversible increase in the critical current in high magnetic fields. The improvement of the current-carrying properties of the YBCO(123) HTSC tape is confirmed also by the results of scanning Hall magnetometry at 77 K. In particular, in a field of 8 T applied perpendicular to the c axis (H ⊥ c), the increase in the critical current after shock-wave treatment is ∼60%. In the case of the Bi(2223) tape, the critical current in a zero field in the sample portion subjected to shock-wave action was found to be twice as high as that in the untreated portion (100 and 50 A, respectively). The increase in the critical current can be related to a number of possible structural transformations of the superconducting core. First of all, an increase in the density of current-carrying core, which leads to an increase in weak bonds at grain boundaries, is possible. In this case, the formation of nanosized defects, which are responsible for an increase in the force of pinning of Abrikosov vortices, is also possible.

Influence of external magnetic field on laser-induced gold nanoparticles fragmentation

Laser-assisted fragmentation is an efficient method of the nanoparticles size and morphology control. However, its exact mechanisms are still under consideration. One of the remaining problems is the plasma formation, inevitably occurring upon the high intensity laser irradiation. In this Letter, the role of the laser-induced plasma is studied via introduction of high-intensity external magnetic field (up to 7.5 T). Its presence is found to cause the plasma emission to start earlier regarding to a laser pulse, also increasing the plume luminosity. Under these conditions, the acceleration of nanoparticles fragmentation down to a few nanometers is observed. Laser-induced plasma interaction with magnetic field and consequent energy transfer from plasma to nanoparticles are discussed.

Properties of low-resistance joints between HTS tape conductors prepared by soldering

The electric resistance of contacts between superconducting tape conductors on the basis of the GdBa2Cu3O7 − x compound (2G) is investigated in an external magnetic field up to 5 T at T = 77 K. The mechanical tensile strength of both the initial conductors and the contacts is measured.

Current-carrying capability of GdBa2Cu3O7−x HTSC tapes in magnetic fields in the temperatute range of 2–100 K

The current-carrying capability of the second-generation HTSC tapes based on GdBa2Cu3O7−x (GdBCO), produced by the SuperOx Company by the pulsed laser deposition method is studied. Critical currents inmagnetic fields aremeasured by resistive andmagnetic (using a SQUID magnetometer) methods. The results obtained are compared with characteristics of an YBCO tape grown by chemical deposition (SuperPower, USA).

Radiation effects in high-temperature composite superconductors

We investigate how irradiation with high-energy ions (167-MeV 132Xe27+, 107-MeV 84Kr17+, and 48-MeV 40Ar8+) and 2.5-MeV protons affects the critical parameters of high-temperature superconductor tapes based on YBa2Cu3O7–x and GdBa2Cu3O7–x compounds. The ion ranges in the multilayer structures under study and the thermal regimes of the irradiated samples are calculated using SRIM and thermal-peak models. The calculated results make it possible to estimate the size of radiation-induced defects which serve as the pinning centers of Abrikosov vortices. The performed investigations enable us to reveal that, in the irradiated structures, an increase in the critical current, improvement in the adhesion between the superconducting layer and the substrate, and a reduction in internal stresses are observed under exposure to low levels of Ar- and Kr-ion irradiation. The critical current and the critical temperature decrease at higher fluences and, finally, the phenomenon of superconductivity disappears if the fluence continues to increase. In the case of 2.5-MeV proton irradiation, the radiation resistance of the GdBa2Cu3O7–x samples is found to be higher than that of the YBa2Cu3O7–x tapes.

The magnetic behavior and mechanical properties of low resistance joints of GdBa2Cu3O7 − δ 2G tapes

The electrical properties of contacts of second-generation (2G) GdBCO superconducting tape conductors, which are soldered by Rose’s and PbSn solders, are investigated in an external magnetic field up to 5 T at a liquid nitrogen boiling temperature. The joint resistance at T = 77 K in the case of PbSn solder is approximately half that in the case of Rose’s alloy. The rise in the contact magnetoresistance with the field is weak and independent of the orientation of the magnetic field, and it is saturated in fields on the order of 3 T for both solders. The mechanical tensile strength of initial tape conductors and contacts is measured at room temperature. The ultimate strength of contacts produced by the PbSn solder is more than twice that for the contact made by Rose’s alloy. In the latter case, the ultimate tensile strength is lower than the critical stress of superconductor degradation.

Investigation into the critical current of second-generation wire-tapes based on the GdBa2Cu3O7 (GdBCO) high-temperature superconductor fabricated by pulsed laser deposition

The results of measurements of the functional characteristics of 2G HTSC wires produced by SuperOx-Japan using pulsed laser deposition are given. Our investigations show that the HTSC tape fabricated by pulsed laser deposition is practically not worse than the HTSC tape fabricated by chemical vapor deposition (SuperPower, United States) in its current-carrying characteristics in an external magnetic field.

Modification of superconducting YBCO(123) tape at current transport at T = 77 K

The effect of long-term direct current transport through a rectilinear sample of a second-generation composite tape on the basis of a high-temperature superconductor YBCO(123)YBa2Cu3O7 − x, was studied at T = 77 K (I = 0.7Ic for 300 h and I = 1.3Ic for 100 h). If in the first case the critical current value did not change, in the second case, when the current passed through a sample in the mixed state, the critical current decreased both at zero field and in magnetic fields of up to 10 T. Scanning Hall magnetometry applied for studying the tape current-carrying ability and structural defects in samples allowed us to detect a microscopic defect arising in a sample after current overload. Experimental results were interpreted in the framework of an idea about the mixed character of the electric conductivity of YBCO(123) in the normal phase.

Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters Tc and Ic of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7Ic) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ∼30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.