J. Kosa

Effect of higher borides and inhomogeneity of oxygen distribution on critical current density of undoped and doped magnesium diboride

The effect of doping with Ti, Ta, SiC in complex with synthesis temperature on the amount and distribution of structural inhomogeneities in MgB2 matrix of high-pressure-synthesized-materials (2 GPa) which can influence pinning: higher borides (MgB12) and oxygen-enriched Mg-B-O inclusions, was established and a mechanism of doping effect on jc increase different from the generally accepted was proposed. Near theoretically dense SiC-doped material exhibited jc= 106 A/cm2 in 1T field and Hirr =8.5 T at 20 K. The highest jc in fields above 9, 6, and 4 T at 10, 20, and 25 K, respectively, was demonstrated by materials synthesized at 2 GPa, 600 °C from Mg and B without additions (at 20 K jc= 102 A/cm2 in 10 T field). Materials synthesized from Mg and B taken up to 1:20 ratio were superconductive. The highest jc (6×104 A/cm2 at 20 K in zero field, Hirr= 5 T) and the amount of SC phase (95.3% of shielding fraction), Tc being 37 K were demonstrated by materials having near MgB12 composition of the matrix. The materials with MgB12 matrix had a doubled microhardness of that with MgB2 matrix (25±1.1 GPa and 13.08±1.07 GPa, at a load of 4.9 N, respectively).

Transformation of the DC and AC Magnetic Field With Novel Application of the YBCO HTS Ring

With the new arrangement to be presented we can transfer the magnetic energy of DC or AC magnetic fields inductively between two independent iron cores from one closed iron core to the other one. Thus, DC magnetic field can be transformed in a controlled way. This method can result in novel applications in the field of transferring both the energy of DC magnetic field and AC electrical energy. The paper presents preliminary results of experiments with showing leading idea. The successful results of experiment open new advanced applications of superconducting regardless type of superconductors.

Qualification of the Machining and Fitting Precision of YBCO Bulks and Rings Joined Together via the Examination of the Trapped Flux

The quality of the drilled YBCO bulks and rings and the precision of the fitting of the YBCO rings joined together can be deduced with the elaborated method. In the case of concerted magnetization of tightly fitted rings and bulk the measured maximum flux density is approximately the same as it was prior to machining unless the material got damaged during machining. It is also a convenient method to test the material before the application of bulks and rings.

Novel 3-Phase Self-Limiting Transformer With Magnetic Flux Applied by Perfect Closed YBCO Wire Loops

With this solution we wanted to examine whether it is possible to create a 3-phase self-limiting transformer applying a new method never used before in the way we do. Therefore we made a new, working model for testing. The primary and secondary coils are on two independent iron cores in all the three phases. We created the coupling between the primary and secondary coils using perfect closed YBCO loops with parallel turns. The advantage of this solution is that the transformer is able to break the coupling so there is no current in the secondary coil in an ideal case. We tested the results in the case of the 1-, 2- and 3-phase fault current. The paper presents the results of our experiments and introduces new, advanced applications of the flux transformer. We considered the efficiency and significance of the perfect closed loop made from YBCO wire. The first work has made by us as possibility of application of the principle of the magnetic flux constancy in the closed loop in this field.

Fault current limiter with novel arrangement of perfect YBCO loops made of HTS wire

In this work we examine a novel arrangement of a perfect closed superconducting loop made of HTS wire in a fault current limiter. This paper presents the results of our experiments with this device, which may open a new path for advanced applications by using continuous, perfectly closed superconducting YBCO loops. During the experiments we compared the fault current limitation in two arrangements. At the same time we also test the operation and application possibilities of the continuous YBCO loop developed by our own technology.

Oxygenation of bulk and thin-walled MT-YBCO under controllable oxygen pressure

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.

Novel Self-Limiting Transformer With Active Magnetic Short Circuit Using Perfect YBCO Wire Loops

In this paper we present a new arrangement implementing a novel self-limiting transformer. We have developed a 1-phase model. We used a 3-limb transformer for one phase and we surrounded 2 limbs of the three, the primary and the secondary coils, with a common perfect closed YBCO loop. In this way we could realize the active magnetic short circuit which gives a new possibility for novel applications. Its operation is based on the principle of the magnetic flux conservation in a closed loop. This is a well-known physical law but so far not applied in this way. We used perfect closed YBCO wire loops with parallel turns. The paper presents the results of our experiments and opens new advanced applications of the perfect YBCO loops made of HTS wire. We considered the efficiency and significance of the perfect closed loop made from YBCO wire.

Novel application of the perfect closed superconducting loop made of HTS wire

The paper presents the results of our experiments with novel devices, which may open a new path for advanced applications by using continuous, perfectly closed superconducting YBCO loops. With this solution we wanted to examine whether it is possible to create novel fault current limiter, 3-phase self-limiting transformer. We considered the significance and efficiency of the perfect closed loop made from YBCO wire. The used superconducting wire was produced at SuperPower, Inc. in New York, USA. Type: SF 12050.

The effect of oxygen distribution inhomogeneity and presence of higher borides on the critical current density improvement of nanostructural MgB2

MgB2-based nanostructural materials with rather high oxygen concentration (5-14 wt.%) and dispersed grains of higher borides (MgB12, MgB7) high-pressure (2 GPa or 30 MPa) synthesized (in-situ) or sintered (ex-situ) demonstrated high superconducting characteristics (critical current density, jc, up to 1.8-1.0106 A/cm2 in the self magnetic field and 103 in 8 T field at 20 K, 3-1.5105 A/cm2 in the self field at 35 K, upper critical field up to HC2 = 15 T at 22 K, field of irreversibility Hirr =13 T at 20 K). The additives (Ti, SiC) and synthesis or sintering temperature can affect the segregation of oxygen and formation of oxygen-enriched Mg-B-O inclusions in the material structure, thus reducing the amount of oxygen in the material matrix as well as the formation of higher borides grains, which affects an increase of the critical current density. The record high HC2 and Hirr have been registered for the material high-pressure (2 GPa) synthesized from Mg and B at 600 oC having 17% porosity and more than 7 wt.% of oxygen. The attained values of the critical current, AC losses and thermal conductivity make the materials promising for application for fault current limiters and electromotors. The structural and superconducting (SC) characteristics of the material with matrix close to MgB12 in stoichiometry has been studied and the SC transition Tc=37 K as well as jc= 5×104 A/cm2 at 20 K in the self field were registered, its Raman spectrum demonstrated metal-like behavior.