Tobias Habisreuther

Magnetic processes in hysteresis motors equipped with melt-textured YBCO

Several hysteresis motors have been constructed with an output power up to 500 W at T=77 K. The rotors of these machines consist of melt-textured YBCO. In this work, the authors present detailed investigations on the magnetic processes in these rotors. Spheres were cut from melt-textured YBCO and investigated by rotating in vector-VSM. From these measurements, torque moments on the sample, hysteretic losses and the slip between external field and the sample can be calculated. The same dependence of the output power on the magnetic field for the samples as for the constructed motors can be derived. Influences of the critical current density and the domain size can be separated by changing the temperature.

High-pressure synthesis of MgB2 with addition of Ti

Abstract Magnesium diboride-based material high-pressure synthesized at 2 GPa and 800 °C for 1 h from Mg and B (taken in the stoichiometry ratio of MgB 2 ) with addition of 2–10 wt.% of Ti demonstrated the critical current density ( j c ) higher than 100 kA/cm 2 at 20 K up to 3 T and at 33 K in 0 T field. At 20 K the critical current density higher than 10 kA/cm 2 was observed up to 5 T field. In the magnetic fields up to 2 T high-pressure synthesized MgB 2 (with 10% of Ti) at 20 K has a critical current density comparable to that of Nb 3 Sn at 4.2 K. XRD patterns of magnesium diboride with Ti addition exhibited no evidence of unreacted titanium and only one compound with titanium was identified, namely, titanium dihydride TiH 2 (or more strictly TiH 1.924 ). The sample with the highest critical current density and the irreversibility field in the temperature range of 25–10 K contained some amount of rather homogeneously dispersed pure Mg and high amount of Mg–B inclusions.

Inscription of first-order sapphire Bragg gratings using 400 nm femtosecond laser radiation

The paper describes the implementation of fiber Bragg gratings inscribed by femtosecond laser pulses with a wavelength of 400 nm. The use of a Talbot interferometer for the inscription process makes multiplexing practicable. We demonstrate the functionality of a three-grating multiplexing sensor and the temperature stability up to 1200 °C for a single first-order Bragg grating.

Batch-processed melt-textured YBCO with improved quality for motor and bearing applications

Results on an established batch process preparing melt-textured YBCO of high quality and in large quantities are reported. We used a standard composition Y1.5Ba2Cu3O7?x+1?wt %?CeO2 without further doping to fabricate single domain YBCO monoliths in different sizes and shapes (cylindrical, quadratic) as well as rectangular multi-seeded YBCO monoliths. Up to 2?3?kg of melt-textured YBCO blocks were grown, reproducible in one box furnace run. Top seeding by a self-made SmBCO was improved and rationalized. Optimization of an oxygen annealing treatment led to macro-crack free YBCO monoliths. Each YBCO monolith was characterized by integral levitation force and field mapping. In a single domain, a quadratic monolith with a edge length of 38?mm, a maximum induction of 1.44?T at 77?K and a distance of 0.5?mm was frozen. The reproducibility of the batch process is guaranteed. Mean maximum induction from 1.1 to 1.2?T at 77?K per batch was reached. A trapped magnetic field of 2.5?T was achieved between two single domain monoliths in a gap of 1.5?mm at 77?K. Depending on the application, function elements with different sizes, designs and more or less complex geometry are constructed in several working steps by cutting, machining, bonding and passivation. Selected function elements were checked with field mapping at 77?K. The results of our function elements in HTSC reluctance motors with an output power of up to 200?kW using single domain material are shown. We report on a fly wheel system DYNASTORE and a system to levitate people.

The inclusions of Mg?B (MgB12?) as potential pinning centres in high-pressure?high-temperature-synthesized or sintered magnesium diboride

A systematic study of the structure and superconductive characteristics of high-pressure?high-temperature (2?GPa, 700?1000??C)-synthesized and sintered MgB2 without additions from different initial powders was performed. Among various secondary phases Mg?B inclusions with a stoichiometry close to MgB12 were identified. With an increasing amount of these inclusions the critical current density increased. So these inclusions can be feasible pinning centres in MgB2. The highest jc values in zero field were 1300?kA?cm?2 at 10?K, 780?kA?cm?2 at 20?K and 62?kA?cm?2 at 35?K and in 1?T field were 1200?kA?cm?2 at 10?K, 515?kA?cm?2 at 20?K and 0.1?kA?cm?2 at 35?K for high-pressure-synthesized magnesium diboride and the field of irreversibility at 20?K reached 8?T. The average grain sizes calculated from x-ray examinations in materials having high jc were 15?37?nm.

Investigation of microstructure of textured YBCO with addition of nanopowder SnO2

Abstract By using top seeding melt-textured growth (TSMTG) process, large single domain YBa2Cu3O7−δ/Y2BaCuO5 (YBCO) samples were prepared with addition of nanometric SnO2 and normal CeO2. Polarized light microscope studies at each step of the TSMTG process showed that before YBa2Cu3O7−δ (123) melting, the Sn-based reacted products were uniformly distributed in micrometer scale; after solidification, the trapped Y2BaCuO5 (211) and Sn-based inclusions were more homogeneously distributed and with mean sizes of about one micrometer. Composition images of scanning electronic microscope for the textured samples showed that, on the whole, the Sn-based inclusions were smaller than the 211. They must be on average below one micrometer in size. Although evidence was found that very fine Sn-based inclusions were uniformly trapped in 123 matrix, the fraction of them was small in comparison to that of 211. No significant enhancement of superconducting critical current density (Jc) was evidenced for the nanometric SnO2-doped samples at a low magnetic field ( B T ). However, values of Jc were decreased at intermediate magnetic field ( 1 T ).

Preparation and magnetic properties of YBa2Cu3O7−x single crystals containing Y2BaCuO5 and barium titanate inclusions☆

Abstract Large single crystals of YBCO are grown by melt textured growth in ceramic pellets. Starting powders of Y 1.5–1.8 Ba 2 Cu 3 O x with the addition of 5 wt% BaTiO 3 are used for the pellets. The peritectically grown single crystals are mechanically extracted from the melt textured blocks. The microstructure is characterized by optical microscopy, scanning electron microscopy and electron microprobe analysis. SEM micrographs show a high density of crystal defects like Y 2 BaCuO 5 and YBaTiO inclusions. Between the a – b planes extended planar defects are found. This growth morphology is typical of melt textured YBCO. The anisotropic magnetic properties are measured with a vibrating sample magnetometer. Magnetization loops for the external field orientation parallel to the crystal c -axis indicate critical current densities larger than 1 × 10 4 A cm −2 up to 2 T at 77 K. Typically the magnetization loops show an extended high field peak. It was found that the high field peaks are correlated with the oxygenation state of the sample. The temperature shift of the peak field is different from that of the irreversibility field. If applying the Bean or Gyorgy model to estimate the critical current densities it is important to consider sample shape effects.

Optimization of levitation forces [in superconducting magnetic bearings]

In this contribution, the authors present a systematic approach to the calculation of levitation forces in plane-parallel (infinitely extended in one direction) arrangements of permanent magnets and superconductors. Starting from an extremely idealized geometry with a very simple expression for the levitation force, they go step by step to more realistic arrangements and present the corrections in the force equations. In particular, magnet configurations with an increased field gradient which allow a higher stiffness are investigated. Finally, numerical calculations are presented which show the dependance of the levitation force on the size and the critical current density of the superconductor and the effect of magnet-iron combinations instead of magnets without iron. The calculations show that the stiffness can be increased by using magnets which consist of several sections with alternating polarity. However, this improvement can only be used for reduced bearing gaps and increased critical current density. The use of magnet-iron combinations has only little advantage.

Limitations for the trapped field in large grain YBCO superconductors

The actual limitations for the trapped field in YBa2Cu3O7?? (YBCO) monoliths are discussed. The influence of the sample geometry and of the critical current density on the trapped field is investigated by numerical calculations. The field dependence of the critical current density strongly influences the trapped field. A nonlinear relationship between the sample size, the critical current density and the resulting trapped field is derived. The maximum achievable trapped field in YBCO at 77?K is found to be around 2.5?T. This limit is obtained for reasonable geometries and high but realistic critical current densities. Such high fields have not been reached experimentally so far, due to non-optimized flux pinning and material inhomogeneities. These inhomogeneities can be directly assessed by the magnetoscan technique, and their influence is discussed. Significant differences between the a-?and the c-growth sectors were found. Limitations due to cracks and non-superconducting inclusions (e.g.?211 particles) are estimated and found to be candidates for variations of Jc on a millimetre length scale, as observed in experiments.

Increased efficiency of batch-processed melt-textured YBCO

Results on an established batch process preparing melt-textured YBCO in high quality and quantity will be reported. We used a standard composition Y1.5Ba2Cu3O7−X+1 wt% CeO2 without further doping to fabricate single-domain YBCO monoliths in different sizes and shapes (cylindrical, quadratic) as well as rectangular multi-seeded YBCO monoliths. Up to 2–3 kg melt-textured YBCO blocks are grown reproducibly in one box furnace run. Top seeding by self-made SmBCO was improved and rationalized. Optimization of oxygen annealing treatment leads to macro-crack free YBCO monoliths. Each YBCO monolith was characterized by integral levitation force and field mapping. In a single-domain quadratic monolith with an edge length of 38 mm a maximum induction of 1.44 T at 77 K and a distance of 0.5 mm was frozen. The reproducibility of the batch process is guaranteed. Mean maximum induction from 1.1 to 1.2 T at 77 K per batch was reached. A trapped magnetic field of 2.5 T was achieved between two single-domain monoliths in a gap of 1.5 mm at 77 K. Depending on the application function, elements with different sizes, designs and more or less complex geometry are constructed in several working steps by cutting, machining, bonding and passivation. Selected function elements were checked with field mapping at 77 K. Results of our function elements in HTSC reluctance motors using single-domain material are shown. We will report on a fly-wheel system and a system to levitate persons.