An RE-based bulk oxide superconductor produced by the so-called “melt” method, that is, a rare-earth-based oxide superconductor where RE2BaCuO5 (211) phases are finely dispersed in a single crystal-like REBa2Cu3O7-x (here, “RE” is one or more rare earth elements including Y), has a larger magnetic flux pinning force compared with other oxide superconductors. In particular, it has a high critical current density even at a high temperature near the liquid nitrogen temperature (77K), so utilization for bulk magnets, magnetic levitation systems, current leads, and various other fields of application can be expected. To such a bulk material, fine amounts of Pt, Rh, Ce, etc. are added to thereby refine the RE2BaCuO5 phases to about 1 μm.
For example, the melt method such as the QMG (Quench and Melt Growth) method disclosed, in PLTs 1, 2, 3, etc. is a technique which once raises the temperature to a temperature region where the RE2BaCuO5 phases or RE4Ba2Cu2O10 phases and a liquid phase mainly comprised of B—Cu—O can be copresent, cools to right above the peritectic temperature where REBa2Cu3O7-x (123) is formed, and gradually cools from that temperature to cause crystal growth, controls the nucleation and crystal orientation, and obtains a large-sized bulk material comprised of single crystal grains. Further, by adding Ag to a shaped article including REBa2Cu3O7-x, RE2BaCuO5, and other materials, a bulk material in which several μm to several hundred μm of Ag particles are dispersed is obtained. A material to which Ag is added is superior in machineability with less chipping due to machining compared with a material to which it is not added. Further, near the boiling point (77K) of liquid nitrogen, in a Y-based material or Gd-based material, the critical current tends to become larger.
The seeding method using a seed crystal with a high peritectic temperature to cause crystal growth disclosed in PLT 2 uses a RE2Ba2Cu3O7-x single crystal-like sample with a higher melting point (peritectic temperature) than the RE1Ba2Cu3O7-x-based oxide superconductor which the seed crystal tries to produce. This method of production of a single crystal bulk heats the starting precursor of the RE1Ba2Cu3O7-x-based oxide superconductor to an intermediate temperature between the peritectic temperature of RE1Ba2Cu3O7-x and the peritectic temperature of RE2Ba2Cu3O7-x whereby the RE1Ba2Cu3O7-x breaks down resulting in a RE12BaCuO5 phase or RE14Ba2Cu2O20 phase and a liquid phase mainly comprised of Ba—Cu—O in a copresent state and brings one surface of the RE2Ba2Cu3O7-x crystal in contact with the precursor. After this, this is cooled to the peritectic temperature of RE1Ba2Cu3O7-x to cause the formation of RE1Ba2Cu3O7-x and then is gradually cooled near the peritectic temperature to thereby grow crystal at the same orientation as the crystal orientation of the contact surface of the RE2Ba2Cu3O7-x.
To disperse Ag in a single crystal-like bulk, Ag is usually added in about 10 to 20 mass %. NPLT 1 discloses Y-based materials in which 3, 5, 7, 10, 15, 20 mass % of Ag are added and shows the presence of any crystal growth, structures, etc. at those conditions at various temperatures. If the amount of addition of Ag, that is, the amount of addition of Ag20 to the sample weight, is 5 mass % or less, it is known that, conditional on a structure being obtained where Ag particles do not precipitate at 970° C. or less, the Ag grains change in shape to disk shapes or spherical shapes.