Effect of silver addition on the mechanical and field trapping properties of Gd-Ba-Cu-O bulk superconductors

The effect of Ag addition on the microstructure and the mechanical and field trapping properties of Gd-Ba-Cu-O bulk superconductor has been investigated. The single grain Gd-Ba-Cu-O bulk superconductors 32 mm in diameter were fabricated with 0-30 mass%Ag₂O additions by the melt growth method under controlled oxygen partial pressure of 1.0%. From microscopic observations, it was found that the macro-cracks in the a-b plane decreased with Ag addition. The three-point bending test showed that the average strength of Ag-free bulk was 69 MPa at room temperature, while the strength was dramatically improved to 110-115 MPa with 10-30 mass%Ag₂O additions. The trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. The trapped field of bulk Gd-Ba-Cu-O samples with 10-20 mass%Ag₂O exhibited high values of 1.8-2.0 T at 77 K. However, the trapped field of the sample with 30 mass%Ag₂O addition was lowered to 1.1 T with decreasing the critical current density. The trapped field of Ag-added sample did not deteriorate with thermal cycles. The trapped fields at lower temperatures were also measured for the Ag-free and 20 mass%Ag₂O added bulk samples without reinforcement by the metal ring or resin. The Ag-free bulk exhibited the trapped field of 3.0 T at 65 K, however, the bulk was fractured at lower temperature due to an electromagnetic force. On the other hand, the Ag-added bulk could trap the higher field of 6.7 T at 55 K, thanks to the improvement in mechanical strength.