Nanotube Dynamos

Two scientists in India have produced a tiny voltage in a small electrical circuit by blowing gas across a mat of carbon nanotubes and doped semiconductors. This result arises from two physical effects.

First, in the Bernoulli effect, gas rushing past a surface produces pressure differences along streamlines, which in turn can produce a temperature gradient along a material sample.

Second, in the Seebeck effect, a temperature gradient (the far ends of the material being at different temperatures) can generate a voltage difference across the sample.

In the experiment of Professor Ajay.K. Sood and his graduate student Shankar Ghosh at the Indian Institute of Science (Bangalore) gas is blown over a mat of carbon nanotubes as well as doped silicon and germanium. With a small sliver of germanium as a sample, a voltage difference of 650 micro-volts was generated. The power flow amounted to 43 nano-watts.

This doesn't sound like much power, and the researchers have not yet determined whether the effect could be scaled up (a no-moving-parts carbon nanotube/doped-semiconductor generator of electricity), but one definite near-term application would be in a new type of gas flow velocity sensor for research in problems of turbulence or aerodynamics.

Compressed air was used to produce the tiny amount of electricity, but even human breath blown at the inclined sample produced a measurable result of several micro-volts. (Sood and Ghosh, Physical Review Letters, 20 August 2004; asood@physics.iisc.ernet.in, shankar@physics.iisc.ernet.in.)