Mechanical properties and densification mechanism of powder-in-tube BaxK1-xFe2As2 superconductors

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.

Transverse distribution of critical current density in Ag-sheathed Ba1-xKxFe2As2 tapes

The Ag-sheathed Ba1-xKxFe2As2 (Ba-122) monofilamentary tapes were prepared by the ex-situ powder-in-tube (PIT) method. The variation of the microstructure and superconducting properties with the thickness of the superconducting core in the cross-section of sintered tapes is studied. At the same time, the reason is studied in comparison with the unsintered tapes. The research results show that the magnetic Jc of the iron-based superconducting tapes increases continuously with distance from the core-sheath interface, which is the complete opposite of the Bi-based superconductor. The magnetic Jc of central layers for final Ba-122 tapes is about 33% higher than the Jc of the whole tape at 4.2 K and 7 T. We have found that the center of the superconducting core shows higher hardness and better texture. In addition, it is also found that there is a reaction layer at the Ag-superconductor interface. These reasons may result in the reduction of the critical current density near the interface in the tapes. Moreover, we also found the presence of a reaction layer in the hot-pressed (HP) high-performance samples. However, no unevenness was found in the unsintered samples. Therefore, the superconductivity of Ba-122 tapes will be better by reducing the reaction layer and eliminating inhomogeneity at the core-sheath interface of the sintered tapes.

Effect of Ce Content on Properties of Al-Ce-Based Composites by Powder-in-Tube Method

Al-Ce based alloys have gained recent interest and have proven to have excellent strength without heat treatment and high thermal stability. Challenges with the production of Al-Ce samples from elemental powders arise due to the elemental material before alloying being susceptible to rapid oxidation. The methodology for making superconductive wire, powder-in-tube, was used as a consolidate Al and Ce elemental powder, and Al-8 wt % Ce-10 wt % Mg composite powder into bulk nanostructured material. Powder samples are fabricated in an inert controlled atmosphere, then sealed in a tube to avoid oxidation of powders. Therefore, most of the powder is used without much loss. We used 316 stainless-steel tubes as a sheathing material. For Al-xCe wt % (x = 8 to 14) samples of elemental powder, liquid phase sintering was used and for Al-Ce-Mg powder solid-state sintering. Characterization of the bulk consolidated material after sintering, and before and after heat treatment, was made using optical and Scanning Electron Microscope imaging, Energy Dispersive Spectroscopy, Microhardness and Rockwell Hardness test. We demonstrated that microstructure stability in Al-Ce-based specimens can be retained after thermomechanical processing. Densification was achieved and oxidation of powder was avoided in most samples. In addition, we found that Fe and Ni in the sheathing material react with Al in the process, and Ce concentration modifies the reactivity the sheath.

In-Situ Manufacturing of SiC-Doped MgB2 Used for Superconducting Wire

Magnesium diboride (MgB2) is a highly potential superconducting material, in substitution of Nb3Sn, which has a critical temperature of ~ 39 K. This synthesis and manufacturing of MgB2 wire were conducted by in-situ powder in tube (PIT). The method doped with silicon carbide (SiC) was aimed to study the effect of phase formation on carbon substitution and morphological characteristics with the motivation to improve superconductivity properties. Magnesium, boron, and SiC powders were synthesized and functionally processed with stainless Steel 304 tube. Heat treatment was conducted at 750℃, 800℃, and 850℃ for 2 hours followed by furnace cooling. Characterization was carried out by x-ray diffractometer (XRD), scanning electron microscopy (SEM), and cryogenic magnet testing. The results showed that 1% SiC optimally increased the zero critical temperature of MgB2 ~ 37.18 K along with the sintering at 750℃ for 2 hours.