This shows a coating of CNTs on a ceramic material, creating a CNT ‘felt’. Image: Fabian Schuett.

Extremely lightweight, highly conductive and more stable than steel: due to their unique properties, carbon nanotubes (CNTs) would be ideal for numerous applications, from ultra-lightweight batteries to high-performance plastics to medical implants. To date, however, it has proved difficult for science and industry to transfer the extraordinary characteristics of CNTs at the nanoscale into a functional material: the CNTs either cannot be combined adequately with other materials, or, if they can be combined, they lose their beneficial properties.

Scientists from the Functional Nanomaterials working group at Kiel University (CAU) in Germany and the University of Trento in Italy have now developed an alternative method for combining the tiny tubes with other materials that allows them to retain their characteristic properties. The scientists report their work in a paper in Nature Communications.

Industry and science have been intensively researching CNTs in order to make use of their extraordinary properties. Yet much still remains just theory. "Although carbon nanotubes are flexible-like fiber strands, they are also very sensitive to changes," explained Rainer Adelung, head of the Functional Nanomaterials working group at the CAU. "With previous attempts to chemically connect them with other materials, their molecular structure also changed. This, however, made their properties deteriorate – mostly drastically."

In contrast, the approach adopted by the research team from Kiel and Trento is based on a simple wet chemical infiltration process. The CNTs are mixed with water and dripped into an extremely porous ceramic material made of zinc oxide, which absorbs the liquid like a sponge. The dripped thread-like CNTs attach themselves to the ceramic scaffolding and automatically form a stable layer, similar to a felt, so that the ceramic scaffolding becomes coated with nanotubes. This has fascinating effects, both for the scaffolding as well as for the coating of nanotubes.

On the one hand, the stability of the ceramic scaffold increases massively, allowing it to bear 100,000 times its own weight. "With the CNT coating, the ceramic material can hold around 7.5kg, and without it just 50g – as if we had fitted it with a close-fitting pullover made of carbon nanotubes, which provide mechanical support," explained first author Fabian Schütt. "The pressure on the material is absorbed by the tensile strength of the CNT felt. Compressive forces are transformed into tensile forces."

The principle behind the increased strength is the same as with bamboo buildings, in which bamboo stems are bound so tightly with a simple rope that the lightweight material can form extremely stable scaffolding, and even entire buildings. "We do the same at the nanoscale with the CNT threads, which wrap themselves around the ceramic material – only much, much smaller," said Helge Krüger, co-author of the paper.

The materials scientists were also able to demonstrate another major advantage of their process. In a second step, they dissolved the ceramic scaffolding by using a chemical etching process, leaving behind a fine 3D network of tubes, each of which consists of a layer of tiny CNT tubes. In this way, the researchers were able to increase the felt surface area, and thus create more opportunities for reactions.

"We basically pack the surface of an entire beach volleyball field into 1cm cube," explained Schütt. The huge hollow spaces inside the three-dimensional structure can then be filled with a polymer, offering a way to connect CNTs mechanically with plastics, without modifying their molecular structure – and thus their properties. "We can specifically arrange the CNTs and manufacture an electrically conductive composite material. To do so only requires a fraction of the usual quantity of CNTs, in order to achieve the same conductivity," said Schütt.

Applications for these composite materials include battery and filter technology, a filling material for conductive plastics, implants for regenerative medicine, and sensors and electronic components at the nanoscale. The good electrical conductivity of the tear-resistant material could in future also be used for flexible electronics applications, such as functional clothing or medical technology.

"Creating a plastic which, for example, stimulates bone or heart cells to grow is conceivable," said Adelung. Due to its simplicity, the scientists agree that the process could also be transferred to network structures made of other nanomaterials, further expanding the range of possible applications.

This story is adapted from material from Kiel 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.