Particles of mechanically sheared flash boron nitride, as seen through a scanning electron microscope. The arrow shows the direction of shear force applied to the material. Image: Tour Group/Rice University.
Particles of mechanically sheared flash boron nitride, as seen through a scanning electron microscope. The arrow shows the direction of shear force applied to the material. Image: Tour Group/Rice University.

Scientists at Rice University who ‘flash’ materials to synthesize substances like graphene have turned their attention to boron nitride, highly valued for its thermal and chemical stability.

The process by the Rice lab of chemist James Tour exposes a precursor to rapid heating and cooling to produce two-dimensional materials, in this case pure boron nitride and boron carbon nitride. Both have until now been hard to create in bulk, and nearly impossible to produce in easily soluble form.

In a paper in Advanced Materials, Tour and his team detail how flash Joule heating, a technique introduced by the Tour lab in 2020, can be tuned to prepare purified, microscopic flakes of boron nitride with varying degrees of carbon. Experiments with the material showed that boron nitride flakes can be used as part of a powerful anticorrosive coating.

“Boron nitride is a highly sought 2D material,” Tour said. “To be 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 looks like graphene but with alternating boron and nitrogen atoms instead of carbon. Boron nitride is soft, so it’s often used as a lubricant and as an additive to cosmetics, and is also found in ceramics and metal compounds to improve their ability to handle high heat.

Rice chemical engineer Michael Wong recently reported that boron nitride is also an effective catalyst in helping to destroy PFAS, a dangerous ‘forever chemical’ found in the environment and in humans.

Flash Joule heating involves stuffing source materials between two electrodes in a tube and sending a quick jolt of electricity through them. For producing graphene, the materials can be just about anything containing carbon, including food waste and used plastic car parts. The process has also successfully isolated rare-earth elements from coal fly ash and other feedstocks.

In experiments led by Rice graduate student Weiyin Chen, the lab fed ammonia borane (BH3NH3) into the flash chamber 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 extraneous elements and again with 150 volts to complete the process, with a total flashing time of less than a second.

Microscope images showed that the resulting flakes are turbostratic – that is, misaligned like badly stacked plates – with weakened interactions between them. That makes the flakes easy to separate.

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

The flashed compound proved more than 92% better at protecting the 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.

According to Chen, the conductivity of the precursor can be adjusted not only by adding carbon but also by adding iron or tungsten. He said that the lab also sees potential for flashing 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 features. We’ve demonstrated flashing metastable phase metal carbides and transition metal dichalcogenides, and this part is worth more research.”

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.