White graphene makes ceramic stronger and tougher

Bilayer white graphene (middle layer) combined with calcium silicate (top and bottom layers) creates a multifunctional ceramic with high strength and toughness, according to a scientist at Rice University. Image: Rouzbeh Shahsavari/Rice University.
Bilayer white graphene (middle layer) combined with calcium silicate (top and bottom layers) creates a multifunctional ceramic with high strength and toughness, according to a scientist at Rice University. Image: Rouzbeh Shahsavari/Rice University.

A little hexagonal boron nitride (hBN) in ceramics could give them outstanding properties, according to a scientist at Rice University.

In a paper in ACS Applied Materials and Interfaces, Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering, has suggested that incorporating ultrathin hBN sheets between layers of calcium silicates would make an interesting bilayer crystal with multifunctional properties. The resultant ceramic would not only be only tough and durable but resistant to heat and radiation.

By Shahsavari's calculations, calcium silicates with inserted layers of two-dimensional hBN could be hardened enough to serve as shielding in nuclear applications like power plants. The ceramic could also prove suitable for use in construction and refractory materials, and have applications in the oil and gas and aerospace industries, as well as other areas that require high-performance composites.

Two-dimensional hBN is nicknamed white graphene and looks like graphene from above, with linked hexagons forming an ultrathin plane. But hBN differs from graphene as it consists of alternating boron and nitrogen atoms, rather than carbon atoms.

"This work shows the possibility of material reinforcement at the smallest possible dimension, the basal plane of ceramics," Shahsavari said. "This results in a bilayer crystal where hBN is an integral part of the system as opposed to conventional reinforcing fillers that are loosely connected to the host material.

"Our high-level study shows energetic stability and significant property enhancement owing to the covalent bonding, charge transfer and orbital mixing between hBN and calcium silicates."

The form of ceramic the lab studied, known as tobermorite, tends to self-assemble as layers of calcium and oxygen held together by silicate chains as it dries into hardened cement. Shahsavari's molecular-scale study showed that hBN mixes well with tobermorite, with the hBN slipping into the spaces between the calcium silicate layers as the boron and oxygen atoms bind, buckling the flat hBN sheets.

This accordion-like buckling is due to the chemical affinity and charge transfer between the boron atoms and tobermorite that stabilizes the composite and gives it high strength and toughness, properties that usually trade off against each other in engineered materials, Shahsavari said. This appears to be the result of a two-phase mechanism that takes place when the hBN layers are subjected to strain or stress.

Shahsavari's models of horizontally stacked tobermorite and tobermorite-hBN showed that the composite was three times stronger and about 25% stiffer than the plain material. This computational analysis also showed why: while the silicate chains in tobermorite failed when forced to rotate along their axes, the hBN sheets relieved the stress by first unbuckling and then stiffening.

When compressed, plain tobermorite displayed a low yield strength (or elastic modulus) of about 10 gigapascals (GPa) with a yield strain (the point at which a material deforms) of 7%. The composite displayed yield strength of 25GPa and a strain of up to 20%.

"A major drawback of ceramics is that they are brittle and shatter upon high stress or strain," Shahsavari said. "Our strategy overcomes this limitation, providing enhanced ductility and toughness while improving strength properties.

"As a bonus, the thermal and radiation tolerance of the system also increases, rendering multifunctional properties. These features are all important to prevent deterioration of ceramics and increase their lifetime, thereby saving energy and maintenance costs."

When the material was tested from other angles, differences between the pure tobermorite and the composite were less pronounced, but on average hBN significantly improved the material's properties.

"Compared with one-dimensional fillers such as conventional fibers or carbon nanotubes, 2D materials like hBN are two-sided, so they have twice the surface area per unit mass," Shahsavari said. "This is perfect for reinforcement and adhesion to the surrounding matrix."

He said other 2D materials like molybdenum disulfide, niobium diselenide and layered double hydroxide may also be suitable for the bottom-up design of high-performance ceramics and other multifunctional composite materials.

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.