Rolling a small steel cylinder across an array of carbon nanotubes (CNTs) is a quick and easy way of preparing “buckypaper” — a thin material that is an excellent conductor of both heat and electricity. Invented by physicists at Tsinghua University, China, the new technique could be used to make materials that boost the performance of high energy density supercapacitors or remove heat from computer chips.

The team's production method really is as simple as it sounds. They begin with arrays of millions of CNTs that have been grown on a silicon substrate using a well-established technique. The arrays, which resemble a forest with all the CNTs aligned perpendicular to the silicon surface, are about 10 cm in diameter and the CNTs are about 100 µm tall.

Flattened forest

To make a piece of buckypaper, Changhong Liu and colleagues place a very thin microporous membrane on top of a CNT array and then push a steel cylinder slowly across the sample — which knocks all the CNTs over in the same direction and flattens them between the membrane and silicon substrate. Next, the membrane and buckypaper is peeled off the silicon substrate and the membrane is removed by washing the sample with ethanol — leaving just the buckypaper (Nanotechnology 19 075609).

The team claim that their method is a significant improvement over previous attempts at making buckypaper that involved filtering a liquid suspension of CNTs in a high magnetic field. Paper made in this way often has poor mechanical, thermal and electrical properties because it is difficult to ensure that the process results in a uniformly-thick material in which all the CNTs point in the same direction.

Swanning about

Liu and his colleagues report that their dry technique produces a strong and flexible film and demonstrate the claim by folding their CNT material into an origami swan.

The team also put their material to more practical use by using it to make supercapacitors — devices that can store up to 1000 times more electrical energy than standard capacitors and are often used when a large but brief surge of energy is required, such as driving the starter motor of a large engine. Supercapacitors are also being used in some prototype fuel-cell and hybrid cars to improve acceleration.

Superior supercapacitors

Buckypaper shows great promise for use as capacitor electrodes because it has a rough surface with a very large surface area — and the capacitance of a device is proportional to the surface area of its electrodes. "From our comparison measurements, our buckypaper-based supercapacitors were at least twice as good as the commercially purchased carbon fiber-based capacitors," said Liu.

The team also found that the buckypaper was a very good conductor of heat, having a thermal conductivity of about 330 W/(m K). This is the highest value of any known CNT film and about the same as copper. “The material's high in-plane thermal conductivity means that it can be used to transmit heat from confined areas,” said Liu. “For example, our tightly aligned buckypaper could be used to solve thermal management problems in microelectronic packaging.”