Electrifying cement with nano carbon black

Since its invention several millennia ago, concrete has come to play an important role in the advancement of civilization and is used in countless construction applications – from bridges to buildings. And yet, despite centuries of innovation, its function has remained above all structural.

A multi-year effort by MIT Concrete Sustainability Hub (CSHub) researchers, in collaboration with France’s National Center for Scientific Research (CNRS), has sought to change that. Their collaboration promises to make concrete more sustainable by adding new functionalities, namely electron conduction. Electronic conductivity would allow the use of concrete for a variety of new applications, ranging from self-heating to energy storage.

Their approach is based on the controlled introduction of highly conductive nanocarbon materials into the cement mix. In a paper in Physical Review Materials, they validate this approach while presenting the parameters that dictate the material’s conductivity.

Nancy Soliman, the paper’s lead author and a postdoctoral fellow at MIT CSHub, believes this research has the potential to add a whole new dimension to what is already a popular construction material.

“This is a first-order model of the conductive cement,” she explains. And it will bring [the knowledge] needed to scale up this kind [multifunctional] materials. ”

From nanoscale to state-of-the-art

In recent decades, nanocarbon materials have really taken off because of their unique combination of properties, including conductivity. Scientists and engineers have previously proposed the development of materials that can impart conductivity to cement and concrete when incorporated into them.

For this new work, Soliman wanted to ensure that the nanocarbon material they selected was affordable enough to be produced at scale. She and her colleagues chose nanocarbon black – an inexpensive carbon material with excellent conductivity. They found that their predictions about conductivity were confirmed.

“Concrete is an insulating material by nature,” says Soliman, “but when we add black nanocarbon particles, it changes from an insulator to a conductive material.”

By including nanocarbon black in just 4 percent of their blends, Soliman and her colleagues found they could reach the percolation threshold, the point at which their samples could conduct a current.

They found that this current also had an interesting result: it could generate heat. This is because of what is known as the Joule effect.

“Joule heating (or resistive heating) is caused by interactions between the moving electrons and atoms in the conductor, explains Nicolas Chanut, a co-author on the paper and a postdoc at MIT CSHub.” The accelerated electrons in the electric field exchange kinetics. energy every time they collide with an atom, generating vibrations from the atoms in the lattice, which manifests as heat and an increase in the temperature in the material. “

In their experiments, they found that even a small voltage – as low as 5 volts – the surface temperatures of their samples (about 5 cm³ in size) to 41 degrees Celsius (about 100 degrees Fahrenheit). While a standard boiler can achieve similar temperatures, it is important to consider how this material would be implemented in comparison to conventional heating strategies.

“This technology could be ideal for indoor underfloor heating,” explains Chanut. “Usually radiant heating is done indoors by circulating heated water in pipes that run under the floor. But this system can be challenging to build and maintain. When the cement itself becomes a heating element, the heating system becomes easier to install and maintain. more reliable. Moreover, the cement offers a more homogeneous heat distribution thanks to the very good distribution of the nanoparticles in the material. “

Nanocarbon cement can also have a variety of uses outdoors. Chanut and Soliman believe that nanocarbon cement, when implemented in concrete pavements, can reduce durability, durability and safety concerns. Many of those concerns stem from the use of salt for thawing.

“In North America we see a lot of snow. To remove this snow from our roads, we need to use de-icing salt, which can damage the concrete and pollute the ground water,” notes Soliman. The heavy trucks used to scatter roads are also both heavy ejectors and expensive to run.

By enabling radiant heating in sidewalks, nanocarbon cement can be used to thaw sidewalks without road salt, potentially saving millions of dollars in repair and operating costs while taking away safety and the environment. In certain applications where maintaining exceptional road conditions is paramount, such as airport runways, this technology can prove particularly beneficial.

Tangled wires

While this state-of-the-art cement offers elegant solutions to a range of problems, achieving multi-functionality posed a variety of technical challenges. For example, without a way to align the nanoparticles in a functioning circuit – known as the volumetric wiring – in the cement, their conductivity would be impossible to exploit. To ensure ideal volumetric wiring, researchers examined a property known as tortuosity.

“Tortuosity is a concept that we introduced by analogy to the field of diffusion,” explains Franz-Josef Ulm, a leader and co-author of the paper, a professor in the MIT Department of Civil and Environmental Engineering, and the faculty adviser. at CSHub. . “In the past, it described how ions flow. In this work, we use it to describe the flow of electrons through the volumetric wire.”

Ulm explains tortuosity using the example of a car driving between two points in a city. While the distance between those two points may be two miles as the crow flies, the actual distance traveled may be greater due to the circuity of the streets.

The same is true for the electrons traveling through cement. The path they must travel within the monster is always longer than the length of the monster itself. The extent to which that path is longer is the tortuosity.

Achieving the optimal tortuosity means balancing the amount and distribution of carbon. If the carbon is dispersed too much, volumetric wiring will become sparse, resulting in high tortuosity. Likewise, without enough carbon in the sample, the tortuosity will be too great to form a direct, efficient, high conductivity wiring.

Even adding large amounts of carbon can be counterproductive. At some point, the conductivity will stop improving and, in theory, will only increase costs if implemented to scale. As a result of these intricacies, they tried to optimize their mixes.

“We found that by refining the volume of carbon we can achieve a tortuous value of 2,” says Ulm. “This means that the path the electrons travel is only twice as long as the sample.”

Quantifying such traits was essential for Ulm and his colleagues. The goal of their recent paper was not only to prove that multipurpose cement was possible, but that it was also viable for mass production.

“The most important thing is that an engineer wants to pick things up, he needs a quantitative model,” Ulm explains. “Before you mix materials together, you want to be able to expect certain repeatable properties. That is exactly what this document outlines; it separates what results from boundary conditions -[extraneous] environmental conditions – from what is actually due to the fundamental mechanisms in the material. “

By isolating and quantifying these mechanisms, Soliman, Chanut and Ulm hope to provide engineers with exactly what they need to implement multipurpose cement on a larger scale. The path they have mapped out is promising – and, thanks to their work, should turn out not to be too winding.

This story has been republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site featuring news on MIT research, innovation, and education.