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We investigate the use of 2nd-generation High-Temperature Superconductors (2G-HTSs) in the rotors of electrical motors and generators. For these devices the conductor must be wound into robust impregnated coils, which are operated in vacuum at temperatures around 30 K, in strong magnetic fields of about 2T. Differences in thermal contraction between the coil former, conductor constituents, impregnation resin, bandage and heat-sink materials (assembled at room temperature) cause mechanical stresses at operating temperature. Rotating-machine operation adds Lorentz forces and challenging centripetal accelerations up to thousands of g. Second generation-HTS conductors withstand large tensile stresses in axial direction and compression in normal direction. However, shear stresses, axial compression, and tension normal to the conductor can cause degradation in superconducting properties. Such stresses can be mitigated by correct choice of materials, coil lay-out and manufacturing process. A certain stress level will remain, which the conductor must withstand. We have manufactured many impregnated round and race-track coils, using different 2G-HTS conductors, and tested them at temperatures from 25 K to 77 K. Degradation of the superconductor in early coils was traced to the mentioned differences in thermal contraction, and was completely avoided in coils produced later. We will discuss appropriate coil-winding techniques to assure robust and reliable superconductor performance.

HTS magnetic bearings are inherently stable without an active feedback system. They provide low frictional losses, no wear and allow operation at high rotational speed without lubrication. So they are very promising for use in motors, generators and turbines. We designed and constructed an HTS radial bearing for use with a 400 kW HTS motor. It consists of alternating axially magnetized permanent magnet rings on the rotor and a segmented YBCO stator. Stator cooling is performed by liquid nitrogen, the temperature of the stator can be adjusted by varying the pressure in the cryogenic vessel. At 68 K maximum radial forces of more than 3.7 kN were found. These results range within the highest radial bearing capacities reported worldwide. The encouraging results lead us to develop a large heavy load HTS radial bearing. Currently a high magnetic gradient HTS bearing for a 4 MVA synchronous HTS generator is under construction.

Optimized design of synchronous machines can be achieved by use of HTS tape conductors. The introduction of an iron-free air-core stator winding and replacement of the rotor’s copper windings by Bi-2223 tapes allows to develop very compact HTS machines with less than half the weight and volume, higher efficiency and excellent operational behavior compared to conventional devices. In consequence these rotating machines with HTS rotors become very attractive for ship drives, power generation and industrial applications. A 400 kW synchronous HTS machine was designed, manufactured and tested at Siemens. Main goal was to demonstrate the feasibility of basic concepts. Development of a 4 MVA synchronous HTS generator is currently under way.

The interaction between high temperature superconductors (HTS) and permanent magnets allows to realize an inherently stable contactless bearing without active feedback system. It has no wear, very low friction and allows operation at high rotational speed. Its thermal inertia prevents a sudden breakdown. High magnetic gradient bearings with a special magnet configuration work both radially and axially. We have designed and constructed a test set-up with two HTS journal type bearings. Investigation of characteristic bearing features was performed in quasi-static experiments with different YBCO stators. Stiffness of the bearings and the dependence of losses per force–displacement cycle on the amplitude were characterized. Losses per force–displacement cycle were investigated in dynamic measurements with frequencies between 0.01 and 40 Hz and resulted in nearly frequency independent behaviour.

Since the discovery of superconductivity above liquid nitrogen temperature in copper oxide based compounds many experiments have been performed on the one hand to study their mixed state properties and on the other hand to check their technological applicability. Due to their well defined cylindrical shape latent particle tracks (also known as columnar defects) are a powerful tool to investigate the mixed state in high temperature superconductors (HTSC) as they provide optimized pinning centers. Therefore in this paper we will present experiments performed with HTSC samples (epitaxial thin films on SrTiO 3 substrates, single crystals and technical tapes) containing columnar defects. We will outline the influence of these linear defects on the resistive phase transition and on transport I–V curves of YBa 2 Cu 3 O 7−δ (123) thin films. All HTSC known up to now exhibit a pronounced structure-related anisotropy. Hence we present magnetic and transport measurements concerning the angular dependence of the critical current in various irradiated HTSC compounds. Furthermore experiments with technical tapes of (BiPb) 2 Sr 2 Ca 2 Cu 3 O 10 (Bi2223) will be shown giving an overview of problems concerning technological applications. Finally in irradiated and nonirradiated Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) single crystals the flux line lattice is visualized via the high resolution Bitter decoration technique.

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.

We investigated the voltage-current (V-I) characteristics in polycrystalline, melt textured and epitaxially grown Bi-2212 superconductors, in Bi-2223 melt textured Ag tapes and in Y-123 films. In all samples the V-I curves are well described by a power law behaviour V ∝ I P over many orders of magnitude in the voltage V. With increasing magnetic field and temperature the exponent p decreases with 1/B from a maximum at p=10–20 down to the ohmic limit p=1. In a double logarithmic plot the V-I lines extrapolate to a common hinge point. These results are interpreted within the frame of thermally activated flux creep. From the measurements on thin films and bulk of Bi 2 Sr 2 CaCu 2 O x we derive an expression for an effective activation energy U eff (B,T,j)=60 meV/B (1-T/T c ) 5/2 ln j o /j. We show that a number of experimental observations are in accordance with this explicit result. In particular (a) exponential decay of the critical current density with field, (b) nonlogarithmic flux creep and (c) the irreversibility field B irr (T) emerge as consequences of the specific functional dependence of U eff .

A comparative study of transport and magnetic properties was made for epitaxial YBa2Cu3O7−δ thin films. Good correspondence was found for the current-voltage characteristics determined by transport and magnetic measurements. This fact gives direct evidence for the validity of the Bean model in the case of thin films. Different types of E(J) relations were analysed. The best agreement was found for the current-voltage characteristics with the vortex activation energy dependence U(J) = UO(JO/J)μ. The temperature dependences of the parameter μ are obtained for different magnetic fields.

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.