On the electrical resistance of cold-pressed-annealed Bi2Sr2CaCu2O8+δ superconductor under low magnetic fields

Abstract

In polycrystalline superconductors, the broadening of the resistive transition due to flux creep strongly depend on the sample preparation method. Samples prepared by conventional sintering yield large broadening which sometimes exhibit a foot and knee structure. There are several methods of improving the quality of sintered pellets. In this study we used the cold-press-anneal method. The resistive property under low magnetic field of Bi₂Sr₂CaCu₂O_8+δ prepared this way is examined. We also examined Bi₂Sr₂CaCu₂O_8+δ prepared by conventional method.
We studied three Bi₂Sr₂CaCu₂O_8+δ samples labeled A, B and C. Sample A is a conventional pellet and it was prepared by mixing the appropriate amounts of high purity Bi₂O₃, SrCO₃, CaCO₃, and CuO for 3 hours. The mixture was then reacted between 800°C and 820°C for 60 hours with two intervening reground. After regrinding the reacted powder, pellet A (13 mm in diameter, approximate weight of 0.4 grams) was made by wet uniaxial pressing in methanol at a pressure of 70 MPa followed by sintering at 860°C for at least 20 bours. Sample C is a cold-press-anneal sample. lt was prepared first by conventional method, similar to A, then afterwards it was cold-pressed (at room temperature) to a pressure of 140 MPA then annealed at 860 C. Sample B is also a cold-press-anneal pellet made in the same way as sample C except that the cold pressure was only 70 MPA and the cold-press-anneal process was repeated twice.
In samples A and B, the resistive broadening affects the part of R(T) slightly above T_c. In contrast, the broadening of R(T) in sample C is confined to temperatures below T_c. So among the three, sample C has the best quality probably because it received the higher cold pressure during the cold-press-anneal procedure.