Günter Dr. Ries

Fabrication and critical current densities of 2223-BiPbSrCaCuO silver sheated tapes

Abstract BiPbSrCaCuO silver sheated tapes were prepared with the composition Bi1.8Pb0.33Sr1.93Ca1.93Cu3O9.89 and Bi1.8Pb0.4Sr2.0Ca2.2Cu3O10.3. The core thickness ranged from 70 ωm to 20 ωm. The formation of the 2223 phase starts from the 2212 compound by a liquid phase reaction and is strongly influenced by to the presence of silver. j c∥ ( B ∥ tape) can be related to j c⊥ ( B ⊥ tape) assuming that the field component B ∥ c axis alone controls jc(B,T). We have deduced an average misorientation of the c-axes of the platelet shaped grains of about ten degrees. This is confirmed by SEM analysis. In case of the lead-rich compound more than 70% of the samples with 20 ωm in core thickness showed critical current densities between 15000 A/cm2 and 25000 A/cm2 at 77K in zero field and the maximum values measured so far were 35000 A/cm2.

Enhancement of critical current density jcm and pinning energy U in melt textured Bi2Sr2CaCu2O8+δ on Ag-tape by heavy ion irradiation

Abstract Irradiation experiments using 502 MeV 127 I ions were performed on 2212 BSCCO melt textured layers on Ag-tape to create well defined columnar amorphous tracks (diameter: 5–10 nm). These tracks should act as very effective pinning centers, especially if they are aligned in parallel to the magnetic field. Magnetization measurements resulted in an enhancement of the magnetization current density j cm within the whole temperature range of 4.2 K up to 60 K. Enhancement factors up to 10 3 were obtained in a magnetic field of 2 T at 30 K after irradiation. We investigated our samples at magnetic inductions up to 12 T; the j cm results will be compared with critical current densities j ct derived from transport measurements revealing a pronounced lower j c -enhancement due to irradiation. The evaluation of E ( j ) curves gives evidence that granularity can be neglected in unirradiated samples, on the other hand, obvious limits for the intergrain currents exist after irradiation. According to relaxation measurements of the magnetic moment before and after irradiation with different doses of 502 MeV 127 I the activation energy U could only be increased from about 30 meV to 70 meV at 10 K and 0.1 T. The small increase of pinning energies is explained within the pancake vortex model and is consistent with recent results found on Bi 2 Sr 2 CaCu 2 O 8+δ single crystals.

Ion irradiation of layered BSCCO compounds : flux line pinning and evidence for 2-D behaviour

Abstract Previous irradiation experiments with high-energy low-Z oxygen ions and fast neutrons on both polycrystalline material and single crystals of the 2212 BSCCO compound showed that the number of effective pinning centres could be increased and jc was enhanced particularly at low temperatures but the pinning energies were not significantly changed. Whereas these projectiles create only small sized and randomly distributed defects, high energy high-Z ions like 0.5 GeV iodine ions produce continuous tracks (5–10 nm in diameter) along their path in 2212 BSCCO single crystals. Assuming conventional core pinning, the activation energies should be high enough to overcome the thermal depinning problem at elevated temperatures. In contrast to these expectations, the magnetization measurements showed that measured pinning energies at low temperatures and small fields do not exceed 70 meV after irradiation. At T = 10 K the magnetization currents could be increased up to 107 A cm−2 but they decline rapidly with field. The small pinning energies can be explained theoretically if one assumes that the elementary depinning mechanism is controlled by the thermally activated depinning of individual pancake vortices located within the CuO2 double layers. The 2-D nature is the consequence of the pronounced superconducting anisotropy in the BSCCO compounds. This behaviour is also confirmed in transport current measurements jct of polycrystalline highly textured 2223 BPSCCO silver tapes irradiated with 0.65 GeV Ni ions. The jct(B,T) dependence was investigated for both the field orientations B | tape and B ⊥ tape.

Development of a 10 m long superconducting multistrand conductor for power transmission cables

A 10 m long HTS cable conductor was stranded with an industrial winding process from 2 km of Ag/Bi2223 tapes. It was installed in a vacuum cryostat and was force cooled by pressurized liquid nitrogen. DC- and AC-load tests were performed while varying the frequency and amplitude of the current. The critical current of the conductor is 5000 A. This model of a power transmission cable demonstrates very low AC losses of at measured both with an electric transport and a calorimetric method. The AC losses vary linearly with frequency, , and have a current dependence slightly lower than . The magnitude of the losses is clearly lower than predicted by the block model version of the Bean model. The model for uniform current distribution (UCD) improves the quantitative description of the losses. From these experiments we conclude that our low loss winding design of the conductor is an early stage of an economical HTS power transmission cable.

Voltage-current characteristic in Bi2Sr2CaCu2O8 from transport and magnetization measurements

The authors investigated the voltage-current (V-I) relation in polycrystalline melt processed Bi2Sr2CaCu2O8 over eleven orders of magnitude. From direct resistive measurements as well as analysis of the decay of magnetization with time they found evidence that the V-I curves in both regimes are part of a common V-I relation following a power law E alpha jP. From transport V-I curves they calculated the irreversible magnetization to be expected after a decay time of 120 s and found good agreement with the measured magnetization including reversibility above an irreversibility field B*.

Modelling the interactions between magnets and granular high-Tc superconductor material with a finite-element method

A computational model for the interaction of magnets with HTS material of arbitrary grain size and critical current density is presented. The two-dimensional solution technique is based on a finite-element method and the superconductor is assumed to obey a power-law voltage-current characteristic. The currents inside the grains are generated by the periodic movement of a magnet stack consisting of permanent magnets and iron shims which gives an exciting field with a strong gradient. The considered application of this is a so-called high magnetic gradient bearing. The motivation for this work was to investigate the dependence of the levitation force and loss on the grain size and the intragrain jc.

Comparison of flux pinning enhancement in fast neutron irradiated Bi-2212 single crystals and polycrystalline melt samples

Abstract Single crystals and melt samples of the Bi-2212 compound were irradiated with fast neutrons up to doses of 1.1 · 10 18 n/cm −2 . The magnetization currents and the irreversibility field of the single crystals showed an increase of more than one order of magnitude. The activation energies of the single crystals were enhanced almost to a level characteristic for the melt samples. Irradiation of the polycrystalline melt samples led to a moderate increase in magnetization currents only at low temperatures while the pinning energies remained almost unchanged. The different irradiation induced changes of the magnetic properties are influenced by the defects present in single crystals and polycrystalline materials before irradiation.

Pinning model derived from resistive measurements on melt processed Bi2Sr2CaCu2O8

Abstract On melt processed samples of the 86 K superconductor Bi 2 Sr 2 CaCu 2 O 8 we have performed resistive measurements in the low field limit B ⩽0.13 T and 40 K⩽ T ⩽77 K. The voltage drop is found to rise exponentially with current E ∝exp j / j 0 , which is interpreted in terms of thermal activation of pinned flux lines. An activation energy U 0 ( T )⩽90 meV is derived from the transition width j 0 ( T ) and is related to a plausible core pinning interaction of flux lines with normal conducting precipitates. This reproduces the measured j c ( B, T ) values in the whole regime investigated. We conclude that pinning centers must have a minimum size in order to control flux creep. Finally we demonstrate that conventional summation of the single site pinning forces cannot account for the observed macroscopic depinning current density.

Comparison of energy storage in flywheels and SMES

Abstract Generally valid expressions and scaling rules are derived, which relate the stored magnetic energy in a SMES and flywheel to the required minimum amount of superconducting, structural and stabilizing material. Results are widely independent of details like magnet type and dimensions, the only specific design variable is a magnetic coil length h . It is shown how the experimentally observed design limit j 2 eff E 0 ⩽2×10 23 J A 2 /m 4 follows from force arguments.

AC losses in a flexible 10 m long conductor model for a HTS power transmission cable

Abstract To demonstrate the possibility of manufacturing a HTS power transmission cable with low AC losses, we fabricated a 10 m long cable conductor and a cryosystem. The conductor was wound in a four-layer design out of 2 km Ag/Bi-2223 tapes. We determined a critical current of 5000 A. Loss measurements were performed with an electric method which detects the voltage drop along the conductor with a lock-in technique and a calorimetric method which measures the temperature rise along the conductor. Both methods yield the same low loss values of only 0.8 W/m at 77 K and 2000 A rms /50 Hz. This is due to the low loss winding scheme we used which assures an equal current distribution in all four layers through transformatoric coupling. We applied the uniform current distribution model and added the nonlinear V – I curve to describe quantitatively the obtained results.