A thermopower-wave generator produces up to 0.2 to 0.3 volts and 0.1 to 0.2 amperes of electrical current. The current is generated as a pulse, typically several milliseconds long. We can also increase the capacity and current by arranging nanotubes in parallel or increase the voltage by putting a number of nanotubes in series.
The current appears to scale up or down with wave velocity. We initially assumed that the true mechanism here wasn’t wave velocity but rather wave-front temperature, itself the effect of a stronger thermal reaction that swept more electrical carriers along the tube. But after testing several fuels that reacted at different speeds on a nanotube, we confirmed that wave velocity is in fact the more important factor. We are still trying to explain why this is the case in order to understand more about how thermal waves couple with electrons and electron holes. One of our future goals is to find equations that help us calculate the relationship between the temperature, the voltage, and the wave‑front behavior. So far, we’ve seen that the fuel reaction needs to generate localized temperatures around a thousand degrees Celsius in order to start a reaction that’s fast enough to kick off the thermal wave.
While that might seem like an absurdly high temperature for use in any practical application, the high heat is contained within an area smaller than a cell. It’s so highly localized and insulated by the nanotubes that we think the high temperature would be safe in almost any device, even ones inside the human body.
Early in our research, we began to see ways that we could modify and control carbon nanotubes, hoping to demonstrate their usefulness in future systems. Many electrical applications require only the sorts of power pulses that thermopower wave generators provide.
Because the length of the nanotube determines the duration of a reaction and how many charge carriers are entrained by the system, it therefore determines the energy and duration of the power pulse. By changing the length and choosing a fuel that supplies the right energy, we can effectively set the system’s propagation velocity.
During our experiments, we made thermopower wave generators that had many different dimensions and properties and tested them in different conditions. Interestingly, the smallest generators produced the largest power densities.
Thermopower wave generators are also remarkable in other ways. For instance, before the nanotubes are ignited, the chemical energy can be stored in the fuel coating indefinitely. Batteries can leak or erode over time, but carbon nanotubes stay completely intact until lit, as well as after a reaction. They also have simple designs, which can take advantage of standard industrial micro- and nanofabrication technologies, and they have potential as nanogenerators. And they’re rechargeable—reapplying the fuel is all it takes to launch another thermopower wave. Carbon nanotubes are sturdy enough to remain intact after the reaction, even though reaction temperatures exceed 1000 °C around the nanotube and the devices operate in open air.
Encouraging as some of these results have been, there is much we need to learn before we can turn carbon nanotube generators into a commercially viable power source. For instance, the 200 to 300 millivolts our systems have been able to put out so far isn’t enough for most applications. We hope to find out whether different materials or mixtures of materials could produce more voltage. We’d also like to try liquid and gaseous fuels, which could work together with microfluidic systems.
On another front, we recently discovered that changing the conductivity of carbon nanotubes, by doping or other means, alters the propagation velocity of the thermal waves along the tube. And by changing certain properties of the fuel it should, in theory, be possible to produce both alternating and direct current with a single reaction; we’ve already seen that the rate of a reaction can decrease and increase as it travels.
Right now, we’re trying to better understand the physics of thermopower waves. Already there are multiple angles to explore when it comes to taming these exotic waves and, ultimately, finding out if they’re the wave of the future.
About the Authors
Michael S. Strano and Kourosh Kalantar-zadeh share an interest in extremely small systems. While on sabbatical from the Royal Melbourne Institute of Technology, in Australia, Kalantar-zadeh joined Strano’s nanotechnology research group at MIT. The team was working on measuring the acceleration of a chemical reaction along a nanotube when they made the serendipitous discovery that the reaction generated power. Now the two researchers are using their combined expertise in chemistry and nanomaterials to explore this phenomenon.