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.
The evolution of the superconducting properties of round wires of (Ba,K)Fe 2 As 2 fabricated by the powder-in-tube (PIT) method is systematically studied. After establishing the method to obtain the largest transport critical current density (J c ) in round wires using the hot isostatic press technique, we investigated how the transition temperature (T c ), J c , and microstructures change at each step of the wire fabrication. Unexpectedly, we find that superconducting properties of the wire core are significantly damaged by the drawing process. Systematic measurements of J c and T c of the core superconductor after each drawing and sintering process clarified the evolution of degradation by the drawing process and recovery by heat treatment. Keywords: superconducting wires, hot isostatic pressing (HIP), (Ba,K)Fe 2 As 2 , powder-in-tube (PIT), critical current density, effects of drawing and sintering
We report the fabrication of small (Ba,A)Fe 2 As 2 (A: Na, K) coils using 10 m-class long round wires, fabricated by powder-in-tube (PIT) method. Coils are sintered using hot-isostatic-press (HIP) technique after glass-fiber insulations are installed. Critical current (I c ) of the whole coil using (Ba,Na)Fe 2 As 2 and (Ba,K)Fe 2 As 2 are 60 A and 66 A under the self-field, and the generated magnetic fields at the center of the coil reach 2.6 kOe and 2.5 kOe, respectively. Furthermore, the largest transport critical current density (J c ) and I c in (Ba,Na)Fe 2 As 2 wires picked up from the coil reach 54 kAcm −2 and 51.8 A at T = 4.2 K under a magnetic field of 100 kOe, respectively. This value exceeds transport J c of all previous iron-based superconducting round wires. Texturing of grains in the core of the wire due to the improvement of the wire drawing process plays a key role for the enhancement of J c .
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