Mechanical properties and densification mechanism of powder-in-tube Ba _x K_1−x Fe₂As₂ superconductors

Abstract

BaxK1-xFe2As2 (BaK-122) iron-based superconductors (IBSs) have been considered to be promising for high-field applications. The transport J c performance of BaK-122 wires and tapes is continuously enhanced by introducing advanced fabricating methods. The mass density of BaK-122 superconducting core in wires and tapes is important to the transport J c performance and related to the mechanical behavior during preparation. In this work, the mechanical property parameters including Poisson's ratio-density, yield strength-density, and elastic modulus-density of BaK-122 IBS powder were examined via uniaxial compression experiments. The density-dependent mechanical constitutive of BaK-122 was obtained for the first time. The relationship function between density and Vickers hardness of BaK-122 was established as HV0.05=0.0249ρ5.332 based on the numerical simulation of hardness testing, and a method for characterizing the BaK-122 core density was developed. It had been found the sheath materials and preparation method have great influences on the stress state of the BaK-122 core, and then affect the density. The densification mechanism and corresponding improvement method were revealed to provide guidance for preparing high-density BaK-122 wires and tapes. Finally, the generalized relationship between density and the superconducting transport J c was established according to lots of experimental data from multiple BaK-122 samples, which has confirmed the positive correlation of ρcore and J c. We comparatively discussed the various cold-work and heat-treatment processes used in our team for preparing the BaK-122 wires and tapes, and the critical factors affecting the transport performance were summarized.

Significant enhancement of transport Jc in Cu/Ag-sheathed (Ba,K)Fe2As2 superconducting tapes by pre-composite technique

“…The hardness values of the tape fabricated using the pre-annealing method by Li et al [32] reaches 112.70, which is also lower than that of the pre-composite tape. Previous study has concluded that hardness and density are positively correlated in Ba-122 polycrystalline superconducting materials [28]. The increase in hardness of the pre-composite tape indicates the increase in density, which promotes the improvement of transport performance.…”

“…The increase in hardness of the pre-composite tape indicates the increase in density, which promotes the improvement of transport performance. The hardness of Ba-122 tape is highly dependent on the characteristic of the sheath material, and generally the greater the thickness and mechanical strength of the sheath, the greater the increase in hardness for the superconducting core [28]. The pre-composite method is equivalent to making the initial thickness of the sheath higher, which helps to improve the strength of the mechanical process and leads to an increase in the density of the superconducting core to some extent.…”

“…Apart from that, because of the low mechanical strength, the irreversible strain of Ag-sheathed 122-type IBS tapes is too low to withstand high electromagnetic forces [25], making it less practical. Researchers studied the use of iron [26], copper [27], stainless steel [28], Monel alloy [29] and other sheath materials to fabricate Ba-122 wires and tapes. Among these materials, it was found that Cu, as a material with similar electrical and thermal conductivity to Ag, low price, high residual resistivity ratio and good mechanical properties, is able to effectively maintain the transport stability of superconducting tapes under the condition of magnetic flux jumping [30].…”

Interface effects on the current transport properties of multi-layered (Ba, K)Fe2As2 superconducting wires

“…25 According to the research on the relationship between the Vickers hardness and mass density, a hardness value of 200 corresponds to about 90% theoretical density of the Ba-122 sample. 50 In this work, hardness tests were conducted for the local areas in the superconducting filaments on the well-polished transverse section of the Ba-122 tape-in-tube wires. The obtained hardness data are summarized in Fig.…”