Bi2223超伝導相とSr-V-O酸化物の界面反応

Abstract

We have developed Ag-sheathed multifilamentary Bi2223 ((BiPb)2Sr2Ca2Cu3Ox) tapes with Sr6V2O11(SVO) barriers. The superconducting properties of the critical current density, Jc and a. c. losses of the tapes strongly depend on the purity of SVO, especially on the residue of SrCO3 in it. In order to examine the behavior of an SVO barrier in detail, we prepared pellet samples with seven layers, which were stacked by barrier and Bi2223 layers one after the other, and examined the reaction between them. The five kinds of barrier layers, such as SrCO3, Sr6V2O11(6-2SVO), 5 and 10 mass%V2O5-added 6-2SVO, and Sr3V2O8 (3-2SVO), were used. The pellet samples were heat-treated at 1113 K for 72-360 ks in air. In the barriers of SrCO3 and 6-2SVO, including the residue of SrCO3, the large Bi2212 (Bi2Sr2Ca1Cu2Ox) grains are grown perpendicular to the interface to form the Bi2212 layers with a thickness of about 100 μm by the reaction between the barrier and Bi oxide. The majority of the Bi-oxide layers are composed of Bi2223 fine grains parallel to the interface. The thickness of the formed Bi2212 layers decreases when the content of SrCO3 in the barriers is decreased. In the reaction, SrCO3 reacts with the Bi oxides, and the elements of Pb and Cu diffuse and distribute homogeneously in the barrier layers. The residue of SrCO3 in 6-2SVO decreases obviously when the additional content of V2O5 is increased. In the 6-2SVO barriers with 5 and 10 mass% added-V2O5, only the very fine Pb oxide grains of (Sr, Ca)2PbO4 are precipitated in a row along the interface. No precipitation occurs for the 3-2SVO barrier. The 6-2SVO is stable, and the 3-2SVO is transformed into the 6-2SVO phase during the heat treatment. We can conclude that, for the use of 6-2SVO as a barrier, it is very important to remove the residue of SrCO3 in the 6-2SVO to prevent the formation of Bi2212 layers and, for that purpose, a small addition of V2O5 is effective.