There are carbon nanotubes, fullerenes and nano-foams, but now researchers have discovered yet another new type of carbon material: colossal carbon tubes. Thousands of times bigger than their nano counterparts, these tubes have exceptional mechanical and electrical properties and could find applications from microelectric devices to bullet-proof body armour.
In the last 20 years, researchers have discovered several new forms of carbon in addition to graphite (the type of carbon found in pencils) and diamond. The colossal carbon tubes, invented by Huisheng Peng and colleagues at Los Alamos National Laboratory in the US and Fudan University in China, are 40–100 µm in diameter and are centimetres long, which makes them — unlike carbon nanotubes — visible to the naked eye. The researchers make them using a chemical vapour deposition process that involves heating a mixture of ethylene and paraffin oil at up to 850 °C in a quartz tube furnace (Physical Review Letters in press).
Scanning electron microscopy shows that the walls of the tubes, which are around a micron thick, contain rectangular pores that range in size from hundreds of nanometres to microns. High-resolution transmission electron microscopy further reveals that the walls have a layered graphite crystal structure, and the inter-layer distance, determined by X-ray diffraction, is 0.34 nm — the same as graphite.
Light and strong
The tubes are very light and have densities similar to those of carbon nano-foams, or around 10 mg per cm3. Moreover, they are strong, with a maximum tensile stress of nearly 7 GPa, which is higher than that of carbon nanotube fibres. Indeed, this makes them 30 times stronger than Kevlar and 224 times stronger than cotton, says Peng.
And that’s not all: colossal carbon tubes are ductile and can be stretched, which makes them attractive for applications requiring high toughness. They also have high electrical conductivities of around 103 siemens per centimetre at room temperature, compared with 102 siemens per centimetre for multi-walled carbon nanotube fibres. Conductivity also increases with temperature, which implies that the tubes are semiconducting.
All these good properties mean that applications could be diverse, including lightweight body armour, such as bullet-proof vests; high-strength composites that can be shaped into parts for high-performance and lightweight vehicles; microelectronic systems and machines; and super strong cloth.
The team now plans to understand how the colossal carbon tubes form and how to control their structure better. “We also hope to improve their properties and investigate practical applications,” says Peng.