Doubled J _c of Bi₂Sr₂CaCu₂O _x wires by partial melting and recrystallization heat treatment under 10 bar after vacuum degassing of the precursor powder and pre-densifying of the wires

Abstract

Final heat treatment involving partial melting and recrystallization under a high pressure of 50–100 bar is not conducive to the preparation of Bi2Sr2CaCu2O x (Bi2212) magnets; it also greatly increases the cost of research into Bi2212 wires. To solve these problems, in this work a final heat treatment under 10 bar was used to prepare Bi2212 wires with a high bulk density and high critical current density (J c). The ambient pressure of the final heat treatment in this paper is only one-fifth of that of the traditional overpressure for final heat treatment of Bi2212 wires. The J c of the wires is more than 2.4 times higher than that of Bi2212 wires with a final heat treatment at 1 bar and is consistent with that of samples having a final heat treatment at the traditional 50–100 bar. The reason why the ambient pressure of the final heat treatment can be greatly reduced in this work is: (a) vacuum degassing of the precursor powder eliminated the adsorption and interstitial impurity gases in the long Bi2212 wires, and reduced the internal gas pressure of the wires; (b) pre-treatment at 800 °C and 250 bar compressed the Bi2212 wires to their full density. Thus, during the subsequent final heat treatment, the low ambient pressure of 10 bar can overcome the difference between the residual gas pressure inside the wires and the yield strength or creep strength of the Ag sheath, and prevent expansion of the internal gas in the wires, so that the Bi2212 wires are always close to full density. The ambient pressure of the final heat treatment in this paper needs to prevent the expansion of the Bi2212 wires not compress them, so the pressure can be very low. The 10 bar low pressure final heat treatment will solve the problems of the high cost of research into Bi2212 wires and the difficult overpressure heat treatment of Bi2212 magnets.