In 2008 a new family of high-temperature superconductors that contain Fe and As was discovered. There has been a flurry of work on these and related pnictide superconductors (containing N, P or As) since then and the critical temperature (for operation) has been raised to 55 K. These materials are particularly interesting to the physics community because they have an intriguing combination of magnetic and superconducting properties, and they are of significance to the applied superconductivity community because they appear to have superconducting properties that may make them suitable for practical applications. However, practical applications require high-quality bulk material with clean, well-connected grain boundaries that allow supercurrent to move from grain to grain across the material. The bulk pnictide superconductors made today suffer from severely limiting current transport across grain boundaries due to multiphase mixtures that typically contain a Fe-As phase that covers the grain boundaries.
TECHNICAL DETAILS: The intellectual merit of this project lies in investigating phase relations and reaction pathways to understand how to process pnictide superconductors to make high-quality bulk material that has clean, well-connected grain boundaries. Knowing what phases should be present in a sample (phase relations) and what phases are actually present (reaction pathways) guides the development of new heat treatments to make bulk samples. The resulting high-quality samples can be used to evaluate key bulk properties such as the critical current density in polycrystalline samples. This work investigates the Ba-Fe-As- Co system, which contains the Ba(Fe1-xCox)2As2 superconductor. The broader impact of this research is to involve students and teachers in research on pnictide superconductors and expose lay audiences to the wonders of superconductivity and the world of superconductor processing. Specifically, Hellstrom continues to recruit summer students from underrepresented groups and to work with K-12 teachers during the summer. He also gives talks on superconductivity at the Rotary Club and other local civic organizations.
This research project has had significant successes: it has produced a simple synthesis technique to make high-purity samples of the high-temperature superconductor BaFe2As2 (Ba-122) (the element As is in column 15 in the periodic table, which are called the pnictogens, so Ba-122 is a pnictide superconductor), it has fabricated round Ba-122 wire that yielded the surprising positive result that the supercurrent transport across grain boundaries is much higher than the low value expected from bicrystal experiments, and it has demonstrated that the critical current density in magnetic field is within a factor of 10 of the level that is generally accepted as being adequate for practical applications. Many superconducting materials have been discovered over the years, but few of them have the combination of intrinsic properties that make them viable for potential applications, such as high-field magnets for NMRs (nuclear magnetic resonance) that are crucial to study the structure of proteins. Ba-122 is one of these few, and our research focused on understanding whether it is possible to make bulk Ba-122 that can ultimately be used for practical high-current applications. When we started, the existing synthesis techniques yielded Ba-122 mixed with significant amounts of unwanted nonsuperconducting phases that blocked supercurrent and masked the intrinsic transport properties of polycrystalline Ba-122. To solve the problem, we developed a simple synthesis technique, which is a mechanochemical self-sustained reaction (MSR), that eliminated almost all of the unwanted phases, in particular the Fe-As phases, which can coat grain boundaries and block current transport across grain boundaries. After we were able to make high-purity samples reproducibly, we switched our focus to measuring the superconducting properties of bulk Ba-122. An absolutely key property for bulk superconductors is how the supercurrent transports across grain boundaries. Before we started this research, we had done thin-film studies of current transport across idealized Ba-122 grain boundaries made by growing Ba-122 thin films on bicrystal substrates. The bicrystals had specific orientations, and the current dropped off with increasing misorientation angle across the grain boundary. This suggested that to have high critical current density we would have to texture Ba-122 conductors to create low-angle grain boundaries, which is what has to be done with the YBa2Cu3O7-δ and Bi2Sr2Ca2Cu3O10 high-temperature superconductors. But, instead we got a very pleasant surprise – Ba-122 does not need to be textured. We fabricated round wires of Ba-122 using our high-purity MSR powder and measured the current transport properties of bulk Ba-122. The polycrystalline Ba-122 in the round wires is untextured, and the wire geometry is ideal to measure the transport properties of Ba-122. An added advantage is that magnet designers and magnet builders prefer round wire over flat tapes. Surprisingly, the round wires had much higher critical current density (0.01 MA/cm2 at 4.2 K, 12 T) than we expected from the bicrystal thin-film studies. This high value demonstrated the welcome, positive result that real grain boundaries in bulk Ba-122 can behave differently than the idealized grain boundaries in bicrystal thin-film studies. But even though the critical current density is higher than expected, it is still lower than in thin-film single crystals, which clearly shows that even though the grain boundaries are much better than we initially expected, they are still limiting the critical current density in bulk Ba-122. We began studies to understand how grain boundaries limit the critical current density with the ultimate goal of developing methods to improve supercurrent transport across Ba-122 grain boundaries. These studies have been done by graduate and undergraduate students, and have provided summer research experiences for undergraduates through the NSF-REU (Research Experience for Undergraduates) program. Our results have been disseminated through oral and poster presentations at conferences and in journal articles, including two articles published in Nature Materials.
