Densification Effects on Critical-Current Dependence on Longitudinal Strain in Bi2Sr2CaCu2O8+x Wires

There is undoubted new interest in applying Bi₂Sr₂CaCu₂O_8+x (Bi-2212) superconducting round wires for ultra-high fields above 20 T. We investigated effects of filament densification in a high-quality Bi-2212 wire made with fine Engi-Mat powder by comparing critical-current I_c dependence on longitudinal strain ϵ among samples reacted under 5- and 50-bar overpressure during the partial-melt process. The Walters spring apparatus we used for transport I_c(ϵ) measurements had a high sensitivity that helps detect the first onset of irreversible damage, as well as determine other characteristic strains that describe the transition of I_c(ϵ) from weak to steeper dependences. Results show a noticeable improvement of all the tensile characteristic strains as a result of densification. For example, average value of the irreversible strain limit ϵ_irr increased from 0.3% for 5-bar samples to 0.4% for 50-bar specimens. Moreover, the response of densified samples to strain was significantly more homogeneous along the sample length, independent of the measurement mode used. These improvements, which happened in conjunction with an increase of I_c by a factor 2.5 at 16 T and 4.2 K, are plausibly due to the elimination of pores that can concentrate stress in strained, nondensified samples where they act as crack initiators. Sensitivity of densified samples to longitudinal compression, on the other hand, increased for strain values beyond −0.28% . In this case, we believe that the continuum of Bi-2212 material in densified samples facilitates the propagation of strain-induced buckling defects as compared to nondensified samples where pores may slow this propagation and keep damage more localized. The benefits of densification, nevertheless, largely outweigh this increase of sensitivity at high compression as I_c remained significantly higher than that of nondensified samples. These results reveal very positive effects of filament densification on the conductor's strain properties that add to the significant boost of I_c it induces.