Team incorporates stretched CNTs into carbon composites

Researchers from North Carolina State University (NCSU) say they have developed new methods for stretching carbon nanotubes (CNTs) and using them to create carbon composites.

By stretching the CNT material before incorporating it into a composite for use in finished products, the researchers straighten the CNTs in the material, which significantly improves its tensile strength and enhances the stiffness of the composite material and its electrical and thermal conductivity.

Researchers used a rotating spool to create ribbon-like composite materials that have a high percentage of carbon nanotubes, for use in products ranging from aircraft to bicycles.

Carbon-fibre composites are currently used to build aircraft and other products where strong, lightweight materials are desirable. However, researchers have long thought that if these composites could be made with CNTs they could be as strong, but 10 times lighter.

According to NCSU, creating a strong CNT composite requires four features: first it needs long CNTs, which are more effective at carrying loads; then, the CNTs need to be aligned in rows; third, the CNTs in the material are held together by a polymer or resin and there must be a high ratio of CNTs to polymer in the finished composite material; and finally, CNTs need to be as straight as possible so that the material bears weight evenly.

For decades, researchers have been unable to achieve these goals but now a research team, led by Dr Yuntian Zhu, a professor of materials science and engineering at NCSU, has developed a solution.

‘The new technique begins with a CNT array that looks like a forest of CNTs growing up out of a flat substrate,’ Zhu said in a statement. ‘By grabbing the CNTs at one end of the array, we are able to pull them over onto their sides — and all of the other CNTs in the array topple in the same direction.’

The aligned CNTs are then wound onto a rotating spool and sprayed with a polymer solution to bind the CNTs together. This creates a ribbon-like composite material that has a high percentage of CNTs by volume, which can be used to make CNT composite structures for use in finished products such as aircraft and bicycles.

This, however, does not address the need to straighten the CNTs. To achieve this, Zhu and his team stretch the CNTs as the nanotubes are being pulled onto the rotating spool.

This process is said to improve the tensile strength of the CNT composite ‘ribbon’ by approximately 90 per cent (to an average of 3.5 gigapascals) and stiffness by more than 100 per cent.

By straightening the CNTs, the researchers were also able to almost triple the CNT composite’s thermal conductivity to 40W per metre per Kelvin. Electrical conductivity was increased by 50 per cent to 1,230 siemens per metre.

The paper on stretching the CNTs to straighten them, Ultrastrong, Stiff and Multifunctional Films Assembled from Superaligned Carbon Nanotubes, is published online in the inaugural issue of the journal Materials Research Letters.