[Image above] Bilayer white graphene (middle layer) combined with calcium silicate creates a multifunctional ceramic with high strength and toughness, according to a Rice University lab. The material may be suitable for construction and refractory materials and applications in the nuclear industry, oil and gas, aerospace and other areas that require high-performance composites. Credit: Rouzbeh Shahsavari; Rice University

We at Ceramic Tech Today have reported on various ways scientists are researching and studying graphene in the lab.

Graphene is a 2-D one-atom-thick layer of carbon arranged in a hexagonal lattice. It’s reported to be the strongest material in the world due to its densely-packed arrangement of atoms. And it has some awesome properties, including durability, lightness, heat and electrical conductivity, and impermeability.

But could there be an even stronger and more durable material than graphene? A material that could make strong ceramic materials stronger?

Rice University assistant professor of civil and environmental engineering Rouzbeh Shahsavari—who, together with a colleague wrote about sustainable approaches to strengthening and enhancing concrete materials in last month’s ACerS Bulletin—has created a high-performance ceramic composite that is strong, durable, and resistant to heat and radiation. The composite could be used not only in the construction and refractories industries, but also in oil and gas and other industries requiring highly functioning and durable composite materials.

By layering hexagonal boron nitride (hBN)—otherwise known as white graphene—in between outer layers of tobermorite, a calcium-silicate mineral, Shahsavari essentially created an hBN sandwich. hBN is similar to graphene in that it is 2-D and is structured with linking hexagons. But instead of carbon, hBN is made up of boron and nitrogen atoms.

“This work shows the possibility of material reinforcement at the smallest possible dimension, the basal plane of ceramics,” Shahsavari explains in a Rice news release. “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.”

When tobermorite and hBN get together, the charge transfer between boron atoms and tobermorite causes buckling of the hBN sheets, which gives the composite its superpowers of high strength and toughness—two properties that typically trade off against each other in engineered materials, according to Shahsavari in the release.

Shahsavari compared the tobermorite/hBN composite with only tobermorite and found that the composite was three times as strong and 25% stiffer than tobermorite. He also discovered that the composite showed a yield strength of 25 GPa with a yield strain of up to 20% when compressed—as opposed to 10 GPa and 7% yield strain with only tobermorite.

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

An additional benefit of the discovery is that thermal and radiation tolerance also increased, which could “prevent deterioration of ceramics and increase their lifetime, thereby saving energy and maintenance costs.” 

The new discovery could prove to be useful in industries that operate under extreme conditions, such as nuclear power plants.

“The hybrid hBN/cement in nuclear industry can provide enhanced strength and toughness properties, enhanced thermal tolerance, as well as enhanced radiation tolerance, which are all needed to increase the lifetime and safety of concrete in nuclear power plants,” Shahsavari explains in an email. “Similarly, cementitious materials used in deep extreme conditions of oil and gas reservoirs and or even construction materials can have longer lifetime by properly intercalating the optimum hBN into their structure.”

He suggests the process could work with other materials beyond tobermorite, such as molybdenum disulfide, niobium diselenide, and other similar materials and ceramics.

“hBN is an interesting thin 2-D material with several exotic properties, which can effectively reinforce its surrounding matrices to impart multiple functionalities—provided that the mixture acts as a homogeneous composite.”

Shahsavari’s paper, published in ACS Applied Materials & Interfaces, is “Intercalated hexagonal boron nitride/silicates as bilayer multifunctional ceramics” (DOI: 10.1021/acsami.7b15377).

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