Rice Lab scientists have made use of white graphene with calcium-silicate to create a multifunctional ceramic that boasts of high strength and toughness.
The new solution could eventually find its way into construction and refractory materials, and applications in the nuclear industry, oil and gas, aerospace and other areas that require high-performance composites.
Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering, suggested the incorporation of ultrathin hexagonal boron nitride (hBN) sheets between layers of calcium-silicates would make an interesting bilayer crystal with multifunctional properties.
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, 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,” he said.
The form of ceramic the lab studied, known as tobermorite, tends to self-assemble in 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, slips into the spaces between the layers as the boron and oxygen atoms bind and buckles 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.
Shahsavari’s models of horizontally stacked tobermorite and tobermorite-hBN showed the composite was three times stronger and about 25 percent stiffer than the plain material. This is because, 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 percent. The composite displayed yield strength of 25 GPa and strain up to 20 percent.
Image and content: Rouzbeh Shahsavari/Rice University Lab