For the first time scientists have created a superconductor, in this case niobium nitride (NbN), that self-assembles into a porous, 3D 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. Pores and the superconducting material have structural dimensions of only around 10 nanometers, which could lead to entirely novel property profiles of superconductors.
Superconductivity for practical uses, such as in magnetic resonance imaging (MRI) scanners and fusion reactors, is only possible at near absolute zero (-459.67 degrees below zero), although recent experimentation has yielded superconducting at a comparatively balmy 94 degrees below zero.
“There’s this effort in research to get superconducting at higher temperatures, so that you don’t have to cool anymore,” says Ulrich Wiesner, an engineering professor at Cornell University who led the group. “That would revolutionize everything. There’s a huge impetus to get that.”
Superconductivity, in which electrons flow without resistance and the resultant energy-sapping heat, 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 and Sol Gruner, a physics professor, had been dreaming for more than 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.
The breakthrough was the decision to use NbN as the superconductor. This was born from a conversation between Wiesner and Cornell physicist James Sethna, a coauthor on the paper published in Science Advances. Wiesner recalled asking Sethna what he thought of the possibility of a gyroidal superconductor, and what material should be used.
Sethna, who was writing a paper on superconductors at the time, felt that NbN would be the best option.
Heat, cool, and then heat again
Wiesner’s group started by using organic block copolymers to structure direct sol-gel niobium oxide (Nb2O5) into 3D alternating gyroid networks by solvent evaporation-induced self-assembly. Simply put, the group built two 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 says.
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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, superconductivity was not achieved. The same material was then heated to 850 degrees, cooled and tested, and the scientists confirmed that they had achieved superconductivity.
“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.”
New structures are now possible
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.
“Now that we have these periodically nanostructured and porous materials, we can start to ask questions about structure property relationships,” he says. “Or we can fill the pores with a second material, that may be magnetic or a semiconductor, and then study the properties of these new superconducting composites with very large interfacial areas.”
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This latest effort is groundbreaking in terms of bringing together the organic and inorganic science communities, Wiesner adds.
“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 National Science Foundation and the US Department of Energy supported the research.
Source: Cornell University
