B. P. Mikhailov

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 conditions of shock-wave effect of plasma on the structure and current-carrying capacity of multilayer high-temperature superconducting tapes

The article presents the results of the investigation of the influence of shock-wave effects on the structure and the critical currents of multilayer high-temperature superconductors—HTS tapes produced by the EAS-E HTS (VAC). Shock-wave exposure was carried out using an installation of a plasma focus (PF) type. It was experimentally found that an increase in the critical current by 20% or more was achieved in its own magnetic field and in external magnetic fields in the range of 0.5–2.5 T. The increase depended on the conditions of the shock-wave treatment (the distance from the plasma source (PF anode) and the number of shock wave pulses). In magnetic fields of more than 3 T, the effect of an increase in the critical current was not observed. Microstructural studies revealed both a compression and destruction of the individual layers of HTS in the strike zone depending on the conditions of the impact. The most severe degradation of the structure and the critical current was shown on the tape samples treated at distances of 25–30 mm from the PF anode. The critical current increased and exceeded the initial values of untreated tapes (75–85 A) at distances of 35–65 mm. The phase composition of HTS layers by XRD changed little after shock-wave treatment. Depending on the number of shock pulses and distance from the anode in the area of treatment, the tape’s thickness was reduced owing to compression or was increased owing to swelling of the tape.

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.

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.

The effect of electron irradiation on the critical temperature and magnetoresistance of multiple-strand Ag/Bi2223 tapes

The effect of 2 MeV electron irradiation at 300 K on the critical temperature (Tc) and magnetoresistance (RB) at T < Tc of Ag/Bi2223 tapes is investigated. It is shown that a slight increase in Tc and abnormal reduction of RB are observed at low irradiation doses (∼1014 cm−2). At higher doses (∼1015 cm−2) or longer storage of irradiated specimens at 300 K, a decrease in Tc and an increase in RB are observed. Variations in the critical parameters of tapes upon electron irradiation are controlled by a spatial grid of tunnel Josephson transitions at intercrystallite boundaries. It is assumed that an increase in RB is caused by accumulation of defects at the intercrystallite boundaries. The revealed anomalous decrease in RB is related to pinning of the magnetic flux at radiation-induced defects in Bi2223 microcrystals.

Change in critical parameters of Bi-2223 high temperature superconductors under action of shock waves

Stability of the critical temperature Tc of the (BiPb)2Sr2Ca2Cu3O10 + x-based multicore tape (Bi-2223) prepared by the well-known PIT (powder in a tube) technique and exposed to shock waves acting on its surface, generated by a plasma focus setup, is studied.

The effect of pulse high-density plasma on superconducting properties of Bi-2223 multilayered tape

The effect of pulse high-power nonlinear shock waves generated at the Plasma Focus installation, which was created according to the nuclear fusion program, on some characteristics of multilayered high-temperature superconducting (HTSC) Bi-2223 tape is demonstrated. The tape is produced using the technique of HTSC powder rolling in a silver coating. A more than a twofold increase in the critical current in zero magnetic fields and external magnetic fields from 0.5 to 6 T at a temperature of 77 K is shown to take place under the action of shock waves at room temperature. The volt-ampere characteristics (VACs) of HTSC tape samples exposed to the effect of shock waves are found to drastically differ from the characteristics of the initial tapes. The shock waves suppress the “gear”-type VAC character of unprocessed plasma samples, their jump amplitudes increasing with an increase in the applied magnetic field intensity.

Calculation of thermal processes upon the laser modification of HTSC ceramics aimed at increasing the critical current

A mathematical model for the heating of a surface layer of HTSC ceramics by a CW laser beam scanned at a constant rate is considered. The calculated depths of the molten region are presented for the heating of the Bi(2223) ceramics by a CW CO2 laser. The theoretical results are in reasonable agreement with the experimental data. The calculations can be used for the optimization of laser processing of superconducting ceramics aimed at increasing the critical current density.

Performance enhancement in high-T c ceramics through melting by Joule heating, infrared lamps, and laser radiation

We have analyzed experimental data on the effect of short-term melting followed by recrystallization on the microstructure and critical current density of YBa2Cu3O7 − x, Bi2Sr2CaCu2O8 − x, and Bi2Sr2Ca2Cu3O10 − x high-Tc ceramics. The ceramics were melted using different heat sources: infrared lamps, laser radiation, and electric current. A significant increase in the critical current density of Bi2Sr2Ca2Cu3O10 − x ceramics (by a factor of 40 at 20 K and by a factor of 8 at 77 K) was achieved using cw CO2 laser irradiation. Melting TiC-doped (0.1%) Bi2Sr2Ca2Cu3O10 − x ceramics with a CO2 laser, followed by annealing, insured an even larger increase in critical current density: by a factor of 35 at 77 K. We have calculated the thickness of the molten layer produced by laser heating of high-Tc ceramics.