Authors studied the remanent magnetic flux distribution in bulk melt-textured YBa2Cu3O7-d (YBCO) crystals after their magnetization in quasi-static and pulsed magnetic fields up to 6 T. Provided the ...magnetic pulse is sharp enough and its amplitude exceeds twice the penetration magnetic field, the pulse magnetization technique becomes sensitive to sample inhomogeneities. Using this method with appropriate parameters of the magnetic pulse, authors demonstrated that the growth of YBCO crystals in the growth sectors respond to a macroscopic arrangement of weak links. 14 refs.
The suitability of Hall probe mapping for non-destructive experimental characterizations of YBa2Cu3O7-delta bulk superconductors is discussed. Firstly, the flux penetration pattern, observed by Hall ...scans of the trapped field after activation in magnetic fields below the complete penetration field in the zero-field cooled (zfc) mode, provides valuable information on the homogeneity of the material. Secondly, the magnetoscan technique is presented, which was also applied to irradiated samples. Investigations of samples from the same batch (prior to neutron irradiation) allow us to draw conclusions about the quality of the batch process.
Peculiarities of High-Pressure Oxygenation of MT-YBCO Prikhna, T.A.; Chaud, X.; Gawalek, W. ...
IEEE transactions on applied superconductivity,
06/2007, Volume:
17, Issue:
2
Journal Article, Conference Proceeding
Peer reviewed
The oxygenation under the high 16 MPa oxygen pressure allows reducing the duration of oxygenation and in the case of thin-walled MT-YBCO ceramics (with drilled holes) leads to a decrease of the ...amount of macrocracks in the structure and to an increase in critical current density in the ab-planes. An increase in the twin density enhances the critical current density in the ab-planes of the Y123 textured structure of MT-YBCO. After oxygenation under different conditions the MT-YBCO ceramics produced from a mixture of Y123 with Y211 demonstrated a critical current density in magnetic fields above 2 T at 77 K higher in the ab-planes and lower in the perpendicular c-direction than the MT-YBCO produced from Y123 and Y 2 O 3 . Mechanical properties (hardness, fracture toughness, Young modulus) have been estimated for various types of MT-YBCO oxygenated under different conditions. Both traditional and thin-walled ceramics have a higher microhardness after oxygenation under high 16 MPa oxygen pressure.
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.
Artificial welding of melt-textured YBCO blocks opens the door to the fabrication of large, complex-shaped pieces required for applications. In order to evaluate the superconducting quality of the ...welds, we have developed a Hall probe mapping system, able to record the local magnetization at 77 K under dynamic applied fields in the range of -1 to 1 T. The system was used to characterize welded samples prepared with a new Ag induced surface melting joining technique. The magnetization maps of unwelded and welded samples of various qualities are compared and discussed. The current distributions associated to the Hall maps were calculated using the Caragol software. The magnetization and current distribution maps over the joint show that good quality welds can be reached with this joining method.
For an efficient preparation of SmBCO seeds in high quantity and quality we investigated two different ways to manufacture Sm123 grains. Sm123 bulks were grown by the top-seeded melt-textured growth ...method. Using MgO seeds single-grained Sm123 samples with a diameter of 18 mm and a thickness of 10 mm were obtained. Disc-shaped, large-sized SmBCO grains were prepared along the a-b cleavage plane of the bulks. Subsequently, the material was carefully separated into a large quantity of about 2 X 2 X 2 mm3 sized seeds with well-known crystallographic orientation. The results imply the feasibility of a production of larger SmBCO bulks in a batch process. In a second approach, Sm123 bars were melt-processed by a modified Bridgman method in a vertical furnace. Long bar samples were prepared with typical sizes of 7 mm in diameter and 140 mm in length. The sintered bars were pulled up across a hot zone of a maximum temperature of 1090 deg C and a longitudinal thermal gradient of 15 K cm-1. For the translation of the samples different constant pulling rates from 0.3 to 5.5 mm h-1 were tested. The microstructure of the samples was investigated by means of polarized optical microscopy.
The possibility to saturate MT-YBCO with oxygen up to about 7 oxygen atoms per one unit cell of YBa2Cu3O7-δ at 800 °C under controllable oxygen pressure (from 0.5 kPa to 16 MPa) has been established. ...On condition that the material heating to high temperatures occurs in the nitrogen atmosphere with gradual substitution of it for oxygen and further increase of the oxygen pressure, the saturation of MT-YBCO by oxygen takes place practically without the macrocrack formation and results in the decrease of microcrack density parallel to ab-planes, that leads to the increase of critical current density and mechanical characteristics. It has been shown that in the case of a higher twin density, the material demonstrates higher critical currents, jc. Bulk and thin-walled MT-YBCO oxygenated at high controllable pressure and 800 °C contained a large amount of twins (4-11 and 20-22 μm-1, respectively) and were practically free from dislocations and stacking faults, but demonstrated very high values of jc at 77 K: jc higher than 10 kA/cm2 was observed in the fields up to 5T in ab-planes and up to 2 T in the direction of the c-axis. Specially detwinned MT-YBCO containing a large amount of dislocations (up to 1012 cm-2) demonstrated critical currents lower by about one order of magnitude, which gives us ground to conclude that the presence of twins is extremely important for attaining high critical currents in MT-YBCO. The high-pressure oxygenation allows one to essentially reduce the duration of the oxygenation process as compared to that under atmospheric pressure.
ORMOCER coated Fiber-Bragg-Gratings (FBGs) were investigated at cryogenic temperatures. Below the Bragg wavelength of uncoated FBG is nearly independent on temperature. ORMOCER coated FBG are ...temperature dependent over the whole temperature range investigated from 10 to 300 K. For 50-300 K, the ORMOCER coating contributes to an additional linear temperature shift of the Bragg wavelength of 2.4 pm/K. Below 40 K the temperature dependence decreases to 1.0 pm/K. ORMOCER coated FBGs can be used as sensor at cryogenic temperatures.
The developed method of high-pressure synthesis (HPS) allows producing nanostructural highly dense material based on MGB2, which possesses the highest superconducting and mechanical characteristics ...among the known world analogues, in the form of blocks that are suitable for application in SC electromotors and pumps. Additions of Zr can increase critical current density (jc) of synthesized at 2 GPa and 750-800 °C MGB2 in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen forming the ZrH2. The formation of ZrB2 phase at higher synthesis temperatures (about 950 °C) in HPS MGB2 does not result in the jc increase. Some increase in jc of HPS MGB2 at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH2 impurity phase from appearing and hydrogen from being introduced into the material structure. Besides, the presence of additions in HPS MGB2 promotes the formation of a larger amount of Mg-B (most likely MGB2) inclusions in the Mg-B-O material matrix that in turn leads to the increase of jc of the material in magnetic fields.
The oxygenation of MT-YBCO under isostatic oxygen pressure (up to 16 MPa) at 900-800 deg C allowed reduced process time, lower macrocracking, and reduced microcracks. Additionally higher critical ...currents, trapped fields and mechanical characteristics can be attained. At 77 K thin-walled MT-YBCO had a jc in the ab plane of 85 kA/cm2 at 0 T and higher than 10 kA/cm2 in fields up to 5 T and the irreversibility field was 9.8 T. In the c-direction jc was 34 kA/cm2 in 0 T and higher than 2.5 kA/cm2 in a 10 T field. At 4.9 N-load the micohardness, Hv, was 8.7+/-0.3 GPa in the ab-plane and 7.6+/-0.3 GPa in the c-direction. The fracture toughness, K1C, was 2.5+/-0.1 MPa*m0.5 (ab-plane) and 2.8+/-0.24 MPa*m0.5 (c-direction). The samples with a higher twin density demonstrated a higher jc, especially in applied magnetic field. The twin density correlates with the sizes and distribution of Y211 grains in Y123. The thin-walled ceramics that demonstrated the highest jc contained about 22 twins in 1 mum and were practically free from dislocations and stacking faults. The maximal trapped field of the block of thin-walled ceramic oxygenated at 900-800 deg C and 16 MPa was doubled as compared to that oxygenated at low temperature under ambient pressure.
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