On the electrical resistance of cold-pressed-annealed Bi2Sr2CaCu2O8+δ superconductor under low magnetic fields
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 Bi2Sr2CaCu2O8+δ prepared this way is examined. We also examined Bi2Sr2CaCu2O8+δ prepared by conventional method.
We studied three Bi2Sr2CaCu2O8+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 Tc. In contrast, the broadening of R(T) in sample C is confined to temperatures below Tc. So among the three, sample C has the best quality probably because it received the higher cold pressure during the cold-press-anneal procedure.