Fabrication and characterization of CaKFe₄As₄ round wires sintered at high pressure

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, J _c, 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 J _c 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.