Scientists have identified a chemical route to an innovative insulating nanomaterial that could lead to large-scale industrial production for a variety of applications – including in spacesuits and military vehicles. The nanomaterial – a thousand times thinner than a human hair, stronger than steel and non-flammable – could block the radiation from astronauts and, for example, support the armor of military vehicles.
Collaborative researchers at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have proposed a step-by-step chemical route to the precursors of this nanomaterial, called boron nitride nanotubes (BNNT), for large-scale production.
The breakthrough brings plasma physics and quantum chemistry together and is part of the expansion of research at the PPPL. “This is pioneering work that is taking the laboratory in new directions,” said PPPL physicist Igor Kaganovich, lead researcher on the BNNT project and co-author of the article describing the results in the journal Nanotechnology.
Staff identified key steps in the chemical pathway in the formation of molecular nitrogen and small clusters of boron that can chemically react with each other when the temperature created by a plasma jet cools, said lead author Yuri Barsukov of the Peter the Great St. Petersburg Polytechnic University. He developed the chemical reaction pathways by performing quantum chemical simulations with the help of Omesh Dwivedi, a PPPL intern at Drexel University, and Sierra Jubin, a Princeton graduate student in plasma physics.
The interdisciplinary team consisted of Alexander Khrabry, a former PPPL researcher now at Lawrence Livermore National Laboratory, who developed a thermodynamic code for this research, and PPPL physicist Stephane Ethier, who helped the students put together the software and set up the simulations .
The results solved the mystery of how molecular nitrogen, which has the second strongest chemical bond among diatomic or double-atom molecules, can nonetheless break apart through reactions with boron to form different boron nitride molecules, Kaganovich said. “We spent a lot of time thinking about how to make boron – nitride compounds made from a mixture of boron and nitrogen,” he said. “We found that, in contrast to much larger boron droplets, small clusters of boron interact easily with nitrogen molecules. That is why we needed a quantum chemist to carry out the detailed quantum chemical calculations with us. “
BNNTs have properties similar to carbon nanotubes, which are produced by the ton and can be found in everything from sporting goods and sportswear to dental implants and electrodes. But the greater difficulty in making BNNTs has limited their uses and availability.
Evidence of a chemical pathway to form BNNT precursors could facilitate BNNT production. The process of BNNT synthesis begins when scientists use a 10,000 degree plasma jet to convert boron and nitrogen gases into a plasma made up of free electrons and atomic nuclei or ions embedded in a background gas. This shows how the process works:
• The jet vaporizes the boron, while the molecular nitrogen remains largely intact;
• The boron condenses into droplets when the plasma cools down;
• The droplets form small clusters when the temperature drops to a few thousand degrees;
• The critical next step is the reaction of nitrogen with small clusters of boron molecules to form boron-nitrogen chains;
• The chains become longer due to collisions and fold into precursors of boron nitride nanotubes.
“During high-temperature synthesis, the density of small boron clusters is low,” says Barsukov. “This is the main obstacle to mass production.”
The results have opened a new chapter in BNNT nanomaterial synthesis. “After two years of work we found the way,” said Kaganovich. “When boron condenses, it forms large clusters that nitrogen does not react with. But the process starts with small clusters that nitrogen reacts with, and there is still a percentage of small clusters as the droplets get bigger, ”he said.
“The nice thing about this work,” he added, “is that we were able to go through all of these processes together in an interdisciplinary group because we had experts in plasma and fluid mechanics and quantum chemistry. Now we have to compare the possible BNNT results of our model ”with experiments. That will be the next level of modeling. “
Support for this research comes from the DOE Office of Science.