Electrochemical actuators

Posted April 3, 2006

Today's lesson is that "one person's noise is another person's data", in the words of astrophysicists. Or more relevantly here: one person's annoying side effect is another person's brilliant solution.

Dr. Yet-Ming Chiang from the materials science department gave a nice introductory talk today about his work with battery technology, in particular his development of a useful lithium phosphate high-power cell currently being commercialized by A123. Yay, MIT spin-offs!

But the more interesting part of the talk was about his new, mostly still unpublished work on morphing materials. Basically, he noticed that lithium batteries flex significantly during charging cycles, which is highly annoying but reasonable when you consider that a significant fraction of the atoms in them are moving between the anode and cathode during use: the materials have to stretch to accomodate the extra ions.

The new discovery, however, is that this flexing is strong, since you're using a decent amount of energy to stuff just one extra atom into the material when you charge a battery, and that translates into a lot of force you can exert. We're talking tons per square inch here, and strain sizes of 10% or so... this is suddenly very interesting as an actuator, since it completely blows away piezoelectrics.

Granted, you lose in actuation speed, since it's a diffusive process and doesn't want to happen much faster than you can charge a battery: minutes to hours. But if your intended application is, say, a gradual reconfiguration of an airfoil or a boat hull to adapt to current conditions and demands, this is perfectly reasonable... you want to exert load-bearing forces over macroscopic distances, and you can put up with slower speed.

Very nice, both for the immediate technology, and for the general lesson that you can change a nuisance side-effect into a patentable invention with just a shift in perspective.