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RESEARCHERS at Massachusetts Institute of Technology (MIT), US, have discovered an entirely new way of generating electricity in carbon nanotubes using heat.

A team led by Michael Strano, professor of chemical engineering, found the new phenomenon in multi-walled carbon nanotubes.

The pulse of heat is created by an explosive, cyclotrimethylene trinitramine, better known as RDX, which surrounded the carbon nanotubes in a 7 nm annular shell. This is ignited at one end of the nanotube using a laser or a high-voltage spark. A thermal wave at a temperature of 3000K travels down the nanotube 10,000 times faster than the normal spread of the chemical reaction. The immense heat feeds back to the explosive coating, perpetuating a wave which is guided down the nanotube. The heat carries electrons with it, generating a current.

Thermopower waves are known and have been studied but Strano was the first to predict that they could be guided by nanotubes and carry an electric current. He says that the team was “really surprised by the size of the resulting voltage peak” in initial experiments, which was far higher than predicted in thermoelectric calculations.

“There's something else happening here. We call it electron entrainment since part of the current appears to scale with wave velocity,” says Strano.

He believes that the thermal wave picks up and carries charge carriers, such as electrons and electron holes in much the same way as floating debris is picked up and transported by ocean waves.

Further developments to the technique have resulted in a current of 7 ?kW/?kg, which is about 100 times greater than the current produced by an equivalent-sized lithium ion battery.

Strano says that the discovery opens up a complete new field of energy research, and that possible applications could include very small electronic devices such as environmental sensors or treatment devices that could be injected into the body. Work will now focus on improving the efficiency of the system, as most energy is currently given off as heat and light. The team will also look at using different types of fuel or coating for the carbon nanotubes in an attempt to create alternating current.

The research was published in Nature Materials (doi: 10.1038/nmat2714).