Electrifying cement with black nanocarbon

Since its invention several millennia ago, concrete has become instrumental in advancing civilization, being used in countless construction applications – from bridges to buildings. And yet, despite centuries of innovation, its function has remained primarily structural.

A multi-year effort by MIT Concrete Sustainability Hub (CSHub) researchers, in collaboration with the French National Center for Scientific Research (CNRS), aimed to change this. Their collaboration promises to make concrete more sustainable by adding new features – namely, electron conductivity. Electron 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 mixture. In a paper in Physical Review Materials, they validate this approach while presenting the parameters that dictate the conductivity of the material.

Nancy Soliman, lead author of the paper and postdoc at MIT CSHub, believes that this research has the potential to add a whole new dimension to what is already a popular building material.

“This is a first-order model of conductive cement,” she explains. “It simply came to our notice then [the knowledge] necessary to encourage the expansion of these types of [multifunctional] materials. ”

From nanoscale to prior art

In the last few decades, nanocarbon materials have proliferated due to their unique combination of properties, including conductivity. Scientists and engineers have previously proposed the development of materials that can give conductivity to cement and concrete if incorporated inside.

For this new paper, Soliman wanted to make sure that the nanocarbonate material they selected was affordable enough to be produced on a scale. She and her colleagues settled on black nanocarbon – a cheap carbon material with excellent conductivity. They found that their conductivity predictions were confirmed.

“Concrete is naturally an insulating material,” says Soliman. “But when we add black nanocarbon particles, it goes from being an insulator to a conductive material.”

By incorporating black nanocarbon into only 4% of their mixtures, Soliman and her colleagues found that they could reach the percolation threshold, at which point their samples could carry a current.

They noticed that this current also had an interesting result: it could generate heat. This is due to what is known as the Joule effect.

“Joule heating (or resistive heating) is caused by the interactions between moving electrons and atoms in the conductor,” explains Nicolas Chanut, a co-author on paper and a postdoctoral fellow at MIT CSHub. energy each time they collide with an atom, inducing the vibration of the atoms in the network, which manifests itself as heat and an increase in temperature in the material. “

In their experiments, they found that even a low voltage – up to 5 volts – could increase the surface temperatures of their samples (about 5 cm).³ as size) up to 41 degrees Celsius (about 100 degrees Fahrenheit). While a standard water heater could reach comparable temperatures, it is important to consider how this material would be implemented compared to conventional heating strategies.

“This technology could be ideal for underfloor heating,” explains Chanut. “Usually, radiant interior heating is done by circulating heated water through pipes that pass under the floor. But this system can be difficult to build and maintain. When the cement itself becomes a heating element, however, the heating system becomes easier to install. and more reliable. In addition, cement provides a more homogeneous heat distribution due to the very good dispersion of nanoparticles in the material. “

Nanocarbon cement could have various applications outdoors. Chanut and Soliman believe that if implemented on concrete sidewalks, nanocarbon cement could mitigate durability, durability and safety issues. Many of these concerns stem from the use of salt for de-icing.

“In North America, we see a lot of snow. To remove this snow from our roads, it is necessary to use defrost salts, which can damage concrete and contaminate groundwater,” Soliman notes. Heavy trucks used for salting roads are also both heavy and expensive emitters to drive.

By activating radiant heating on sidewalks, nanocarbon cement could be used to defrost sidewalks without road salt, potentially saving millions of dollars in repair and operating costs while addressing safety and environmental issues. In some applications where maintaining exceptional paving conditions is paramount – such as airport runways – this technology could prove to be particularly advantageous.

Tangled wires

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

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

Ulm explains the tortuosity with the example of a car traveling between two points in a city. While the distance between these two points, while the crow flies, could be two miles, the actual distance traveled could be greater due to the circuit of the streets.

The same goes for electrons traveling through cement. The path to be taken in the sample is always longer than the length of the sample itself. The degree to which this path is longer is tortuosity.

Achieving optimal tortuosity means balancing the amount and dispersion of carbon. If the carbon is too strongly dispersed, the volumetric wiring will become sparse, leading to high tortuosity. Similarly, without enough carbon in the sample, the tortuosity will be too high to form a direct, efficient, high-conductivity wiring.

Even the addition of large amounts of carbon could prove counterproductive. At some point, conductivity will stop improving and, in theory, would only increase costs if implemented on a large scale. As a result of these complexities, they sought to optimize their mixes.

“We found that by fine-tuning the carbon volume we can achieve a tortuosity value of 2,” says Ulm. “This means that the path that electrons take is only twice the length of the sample.”

Quantifying these properties has been vital for Ulm and his colleagues. The aim of their recent work was not only to demonstrate that multifunctional cement was possible, but that it was also viable for mass production.

“The key point is that in order for an engineer to take over things, they need a quantitative model,” Ulm explains. “Before mixing materials together, you want to be able to expect certain repeatable properties. This is exactly what this paper emphasizes; separates what is due to boundary conditions -[extraneous] environmental conditions – from what is really due to the fundamental mechanisms of the material. “

By isolating and quantifying these mechanisms, Soliman, Chanut and Ulm hope to provide engineers with exactly what they need to implement multifunctional cement on a larger scale. The path they have taken is a promising one and, due to their work, it should not be too embarrassing.

This story is courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and teaching.