We fabricated (Ba,K)Fe 2 As 2 superconducting wires and tapes using the powder-in-tube method and hot isostatic pressing (HIP). HIP wires and tapes showed a high value of transport critical current density (J c ) exceeding 100 kAcm −2 at T=4.2 K and the self-field. Transport J c in the HIP wire reached 38 kAcm −2 in a high magnetic field of 100 kOe. This value is almost twice larger than the previous highest value of J c among round wires using iron-based superconductors. Enhancement of J c in the wires and tapes was caused by improvement of the drawing process, which caused degradation of the core, formation of microcracks, weak links between grains, and random orientation of grains. Details of the effect of the improved fabrication processes on the J c are discussed. Keywords: superconducting wires and tapes, (Ba,K)Fe 2 As 2 , critical current density, powder-intube (PIT), hot isostatic pressing (HIP)

Abstract(Ba,K)Fe 2 As 2 superconducting wires and tapes are fabricated by using hot isostatic pressing (HIP) technique, and their superconducting properties are studied. In the HIP round wire, transport critical current density (J c ) at 4.2 K has achieved record-high value of 175 kA/cm 2 at zero field, and exceeds 20 kA/cm 2 even at 100 kOe. Improvement of polycrystalline powder synthesis may play a key role for the enhancement of J c . In the HIP tape, even larger transport J c of 380 kA/cm 2 is realized at zero field. Based on magnetization and magneto-optical measurements, possible further enhancement of J c is discussed.

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We report the fabrication of (Sr,Na)Fe 2 As 2 superconducting tapes by the powder-in-tube technique and their characteristics, including the transport critical current density J c at 4.2 K up to 140 kOe, the magnetic J c estimated from magnetic hysteresis curves, magneto-optical (MO) images, and scanning electron microscopy images. In a tape sintered at 875 °C for 1 h, the transport J c reaches 26 kA/cm 2 at 4.2 K and 100 kOe for a field perpendicular to the tape surface. When the field is parallel to the tape surface, the magnetic J c exceeds the practical level of 100 kA/cm 2 at 4.2 K below 25 kOe. Analysis of the MO images reveals clear current discontinuity lines in the core, indicating that the current flows homogeneously and the connections between grains are strong in the core.

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