Credit: © 2006 AAAS

A rectifier in an electrical circuit restricts current flow to one direction, and is often used to turn alternating current into direct current. The analogue for heat flow is a thermal rectifier, a device that could be used to direct heat out of nanoscale circuits and aid our understanding of thermal flow in biomolecules.

A group at the University of California, Berkeley has now shown how to make miniature thermal rectifiers with carbon and boron nitride nanotubes. Alex Zettl and colleagues1 coated the nanotubes with organo-platinum molecules, forming a gradient in the thickness of the molecular layer along the length of the tube. The tube was placed across heater and sensor pads and the thermal conductivity was measured in each direction along the length of the tube.

The rectification effect is actually fairly small — 2% for carbon and 7% for boron nitride nanotubes. However, the real stir from these results may be in mathematical physics. Since the direction of the rectification cannot be simply explained — heat flows more easily from the end with extra mass — the authors suggest that exotic nonlinear effects are likely at play.