After nearly two decades’ worth of research, a multidisciplinary team at Cornell has created a self-assembled, three-dimensional gyroidal superconductor.
Ulrich Wiesner, a materials science and engineering professor who led the group, says it’s the first time a superconductor, in this case niobium nitride (NbN), has self-assembled into a porous, 3-D gyroidal structure. The gyroid is a complex cubic structure based on a surface that divides space into two separate volumes that are interpenetrating and contain various spirals. Both the pores and the superconducting material have structural dimensions of only around 10 nanometers, a size that could lead to entirely novel property profiles for superconductors.
Superconductivity for practical uses, such as in magnetic resonance imaging (MRI) scanners and fusion reactors, is currently only possible at near absolute zero (-459.67ºF), although recent experimentation has yielded superconducting at temperatures nearing -94ºF. “There’s this effort in research to get superconducting at higher temperatures, so that you don’t have to cool anymore,” Wiesner says. “That would revolutionize everything. There’s a huge impetus to get that.”
Wiesner and his coauthor Sol Gruner have been dreaming for over two decades about making a gyroidal superconductor in order to explore how this would affect the superconducting properties. The difficulty was in figuring out a way to synthesize the material, and their breakthrough was the decision to use NbN as a superconductor.
When superconducting, electrons flow without resistance but the resultant energy is still an expensive proposition. 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-D alternating gyroid networks by solvent evaporation-induced self-assembly. Simply put, the group built two intertwined gyroidal network structures and then removed one of them by heating in air at 450 degrees.
The team’s discovery featured a bit of “serendipity,” Wiesner says. 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. But superconductivity was achieved later, after the same material was heated to 850 degrees, cooled and then tested. “We tried going directly to 850 and that didn’t work,” Wiesner says. “So we had to heat it to 700, cool it and then heat it to 850 and then it worked—only then.” Wiesner says the group is unable to explain why the heating, cooling and reheating works, but “it’s something we’re continuing to research.”
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 says.
The group’s findings are detailed in a paper published in Science Advances.