BaFe2As2 (Ba-122) and (Ba0.6K0.4)Fe2As2 (K-doped Ba-122) powders were successfully synthesized from the elements using a reaction method, which incorporates a mechanochemical reaction using high-impact ball milling. Mechanically-activated, self-susta
ining reactions (MSR) were observed while milling the elements together to form these compounds. After the MSR, the Ba-122 phase had formed, the powder had an average grain size < 1 {mu}m, and the material was effectively mixed. X-ray diffraction confirmed Ba-122 was the primary phase present after milling. Heat treatment of the K-doped MSR powder at high temperature and pressure yielded dense samples with high phase purity but only granular current flow could be visualized by magneto optical imaging. In contrast, a short, low temperature, heat treatment at ambient pressure resulted in global current flow throughout the bulk sample even though the density was lower and impurity phases were more prevalent. An optimized heat treatment involving a two-step, low temperature, heat treatment of the MSR powder produced bulk material with very high critical current density above 0.1 MAcm-2 (4.2 K, 0 T).
The K- and Co-doped BaFe2As2 (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (Hc2) of well over 50 T, Hc2 anisotropy Gamma < 2, and thin film critical current densities exceeding 1 MA
cm-2 at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behavior for [001] tilt misorientations of more than about 5 degrees, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba0.6K0.4)Fe2As2 bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MAcm-2 (SF, 4.2 K), more than 10 times higher than that of any other ferropnictide wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (Gamma ~ 1-2) compared to YBa2Cu3O7-x (Gamma ~ 5).
We show that despite the low anisotropy, strong vortex pinning and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1-xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe
1-xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.
