Metal process

Chemists adapt instant process to make more valuable nanomaterials

Rice University scientists who “flash” materials to synthesize substances like graphene have turned to boron nitride, highly valued for its thermal and chemical stability.

Chemist James Tour’s Rice Lab process exposes a rapidly heating and cooling precursor to produce two-dimensional materials, in this case pure boron nitride and carbon boron nitride. Both have so far been difficult to create in bulk and nearly impossible to produce in an easily soluble form.

The lab report Advanced materials details how Joule flash heating, a technique introduced by the Tour lab in 2020, can be tuned to prepare microscopic purified flakes of boron nitride with varying degrees of carbon.

Experiments with the material have shown that boron nitride flakes can be used as part of a powerful anti-corrosion coating.

“Boron nitride is a highly sought-after 2D material,” Tour said. “Being able to make it in bulk, and now with mixed amounts of carbon, makes it even more versatile.”

At the nanoscale, boron nitride comes in several forms, including a hexagonal configuration that resembles graphene but with alternating boron and nitrogen atoms instead of carbon. Boron nitride is soft, so it is often used as a lubricant and as an additive for cosmetics, and is also found in ceramics and metal compounds to improve their ability to withstand high heat.

Rice chemical engineer Michael Wong recently reported that boron nitride is an effective catalyst to help destroy PFAS, a dangerous “eternal chemical” found in the environment and in humans.

Flash Joule heating involves placing source materials between two electrodes in a tube and sending a rapid electrical discharge through them. For graphene, the materials can be just about anything with carbon in it, food scraps, and used plastic car parts to be just two examples. The process was also successful in isolating rare earth elements from coal fly ash and other raw materials.

In experiments conducted by Rice graduate student Weiyin Chen, the lab introduced ammonia borane (BH3NH3) to the flash chamber along with varying amounts of carbon black, depending on the desired product. The sample was then flashed twice, first with 200 volts to degas the sample of foreign elements and again with 150 volts to complete the process, with a total flash time of less than one second.

Microscopic images showed that the flakes are turbostratic, i.e. misaligned like poorly stacked plates, with weakened interactions between them. This makes the flakes easy to separate.

They are also easily soluble, which led to the anticorrosion experiments. The lab mixed flash boron nitride with polyvinyl alcohol (PVA), painted the compound onto a copper film, and exposed the surface to electrochemical oxidation in a bath of sulfuric acid.

The flashed compound was found to be over 92% better at protecting copper than PVA alone or a similar compound with commercial hexagonal boron nitride. Microscopic images showed that the compound created “tortuous diffusion pathways for corrosive electrolytes” to reach the copper, and also prevented metal ions from migrating.

Chen said the conductivity of the precursor can be adjusted not only by adding carbon but also with iron or tungsten.

He said the lab sees potential to flash additional materials. “Precursors that have been used in other methods, such as hydrothermal and chemical vapor deposition, can be tried in our flash method to see if we can prepare more products with metastable characteristics,” Chen said. “We have demonstrated flashing metastable phase metal carbides and transition metal dichalcogenides, and this part deserves more research.”

The study’s co-authors are Rice alumni John Tianci Li, Wala Algozeeb, Paul Advincula, Emily McHugh and Duy Xuan Luong, graduate students Chang Ge, Zhe Yuan, Jinhang Chen, Kexin Ling, Chi Hun Choi, Kevin Wyss and Zhe Wang, research scientist Guanhui Gao and Yimo Han, assistant professor of materials science and nano-engineering. Tour is the TT and WF Chao Professor of Chemistry as well as a professor of computer science and materials science and nanoengineering at Rice.

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Material provided by rice university. Original written by Mike Williams. Note: Content may be edited for style and length.