GA structure and sample/structure evolution from initial compounds to final NbN superconductors. (A) GA before and after processing, with the unit cell indicated by the black cube. (B) (Top) Chemical structures of compounds and (bottom) schematic of synthesis and processing steps with photographs of the final materials. Block terpolymers (ISO) are combined with the Nb2O5 sol-gel precursors in a common solvent. Hybrid block copolymer/Nb2O5 GA structures are generated by solvent evaporation–induced self-assembly. After calcination in air, the mesoporous Nb2O5 GAs are transformed to NbN GAs in a two-step nitriding process. Scale bars in all photographs represent 1 cm. NH3, ammonia.
Building on nearly 2 decades’ worth of research, Prof. Ulrich Wiesner’s team at Cornell, says it’s the first time a superconductor, in this case niobium nitride (NbN), has self-assembled into a porous, 3D gyroidal structure (a complex cubic structure based on a surface that divides space into 2 separate volumes that are interpenetrating and contain various spirals). Pores and the superconducting material have structural dimensions of only ~10nm, which could lead to entirely novel property profiles of superconductors.
Currently, superconductivity for practical uses such as in MRI and fusion reactors is only possible at near absolute zero (-459.67F), although recent experimentation has yielded superconducting at a balmy -70 C (-94F). MRIs use superconducting magnets, but the magnets constantly have to be cooled, usually with a combination of liquid helium and nitrogen.
Wiesner’s group started by using organic block copolymers to structure direct sol-gel niobium oxide (Nb2O5) into 3-dimensional alternating gyroid networks by solvent evaporation-induced self-assembly. Simply put, the group built 2 intertwined gyroidal network structures, then removed one of them by heating in air at 450 degrees.
The team’s discovery featured a bit of “serendipity,” Wiesner said. In the first attempt to achieve superconductivity, the niobium oxide (under flowing ammonia for conversion to the nitride) was heated to a temperature of 700 degrees. After cooling the material to room temperature, it was determined that superconductivity had not been achieved. The same material was then heated to 850 degrees, cooled and tested, and superconductivity had been achieved. “We tried going directly to 850, and that didn’t work,” Wiesner said. “So we had to heat it to 700, cool it and then heat it to 850 and then it worked.”
Wiesner said the group is unable to explain why the heating, cooling and reheating works, but “it’s something we’re continuing to research,” he added. Limited previous study on mesostructured superconductors was due, in part, to a lack of suitable material for testing. The work by Wiesner’s team is a first step toward more research in this area.”We are saying to the superconducting community, ‘Hey, look guys, these organic block copolymer materials can help you generate completely new superconducting structures and composite materials, which may have completely novel properties and transition temperatures. This is worth looking into,'” Wiesner said.
http://www.news.cornell.edu/stories/2016/01/first-self-assembled-superconductor-structure-created
http://advances.sciencemag.org/content/2/1/e1501119 http://phys.org/news/2016-01-self-assembled-superconductor.html
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