We fabricated CaKFe4As4 round wires through a powder-in-tube method followed by hot isostatic pressing up to 175 MPa. The transport critical current density, Jc, at 4.2 K almost reached the level for practical applications (100 kA cm−2) under self-field and reached 7.6 kA cm−2 at 100 kOe. Magneto-optical images confirmed that the grains were well-connected at low temperatures. Although high-pressure sintering at high temperatures enhanced the Jc and density of the core of the wires, X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy analyses indicated that impurity phases were present in both the polycrystalline powder and the core of the wire.
(Nd,Eu,Gd)Ba2Cu3Ox (NEG123) bulk shows high irreversibility fields over 14 T at 77.3 K. In order to study the origin of the high irreversibility field, transport measurements for NEG123 bulk were performed as functions of temperature, high magnetic fields (up to 27 T) and field angle. We found that c-axis correlated disorder exists for samples with 3 and 40 mol% NEG211, although their irreversibility fields are different. The enhancements of the irreversibility fields at 77.3 K due to the c-axis correlated disorder are almost the same (about 1 T) for both samples and are in agreement with that of Y123 bulk with a twin boundary. The partially entangled vortex liquid and the Bose glass phases, which are induced by the c-axis correlated disorder, play an important role in the improvement of the irreversibility field. However, in order to explain the much higher irreversibility field, we should take account not only of the c-axis correlated disorder but also of the other pinning centres such as the nanolamella.
MgB2 tapes were fabricated by the in situ powder-in-tube method using C2H4O3-treated B powder as a source material. It is found that the Jc values of MgB2 tapes are significantly improved by treating B powder with C2H4O3. At 4.2 K, the Jc of the C2H4O3-treated sample sintered at 800 °C reaches 5 × 103 A cm − 2 at 12 T. The C2H4O3-treated B improves not only the upper critical field through enhancing intraband scattering but also the flux pinning by increasing grain boundaries and C substitution effects. A larger amount of intragranular structural defects and grain boundaries were found in the C2H4O3-treated MgB2 matrix. Benefiting from the good crystallinity, the C2H4O3-treated tape sintered at 800 °C shows a bigger active area fraction and a higher Jc than the sample sintered at 700 °C. These results suggest that treating B powder with C2H4O3 is an effective way to enhance the critical current density of MgB2 superconductors.
To increase critical current density (JC) in magnetic fields (B) parallel to the c-axis, 2-dimensional artificial pinning centers (APCs) are introduced into YBa2Cu3O7-δ (Y123) films. Multilayer films alternately stacked a-axis oriented Y123 and Pr123 (Y123/Pr123 multilayer films) have been grown on SrLaGaO4 substrates by pulsed laser deposition. Magnetic field angular dependences of the JC showed peaks not only in B // a-axis but also in B // c-axis of the Y123/Pr123 multilayer films. This result indicates that the Y123/Pr123 multilayer films have c-axis-correlated pinning centers. Consequently, it is suggested that the Pr123 layers act as 2-dimentional APCs.
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