Wouldn't it be amazing if an airplane could adjust its wings to catch the changing air currents to improve its fuel efficiency? Well, MIT engineers report they may have found a way for structures -- and materials -- to move in this way, essentially morphing from one shape into another. It could lead to airplanes that morph on demand from one that is energy efficient to one that provides more agility. The material could also be used in boats so their hulls would change shape to be more efficient, no matter the size of the waves.
MIT's work involves a new application of a familiar device: the rechargeable battery. Papers describing the team's progress appeared earlier this year in Advanced Functional Materials and Electrochemical and Solid-State Letters.
Batteries expand and contract as they are charged and recharged. "This has generally been thought to be something detrimental to batteries. But I thought we could use this behavior to another end: the actuation, or movement, of large-scale structures," said Yet-Ming Chiang, the Kyocera Professor in the Department of Materials Science and Engineering (MSE).
Previous work on "smart materials" called piezoelectrics have already been developed and can change shape in less than the blink of an eye, but they do so on almost a microscopic level. They wouldn't be capable of moving a wing the distance necessary to affect flight.
Similarly, shape-memory alloys have characteristics useful to large-scale actuation, but they require temperature control to work. "So to make them work you've got to keep them warm and insulate them. And if you insulate them, it takes a long time to cool them down to their original shape.
In the quest for materials that would allow such morphing, engineers have recently focused on nature's approach to the problem. For example, a plant that bends toward the light, quickly furls its leaves when touched, or pushes a concrete sidewalk aloft with its roots is essentially moving fluids between cells.
Enter the solid compounds used to store electrical energy in lithium rechargeable batteries. Researchers thought they could be made to work in a similar way. The movement of ions to and from these materials during charging and recharging were thought to be analogous to the moving fluids in plants.
Researchers began testing commercially available rechargeable batteries of a prismatic form, then designed their own devices composed of graphite posts surrounded by a lithium source. The results were promising.
Among other things, they found that the batteries continued to expand and contract under tremendous stresses, a must for devices that will be changing the shape of, say, a stiff helicopter rotor that's also exposed to aerodynamic forces.
Other key advantages of the approach: The electrically activated batteries can operate at low voltages (less than five volts) as compared to the hundreds of volts required by piezoelectrics. The materials that make up the batteries are also inherently light.
The researchers have already demonstrated basic battery-based actuators that can pull and push with large force. Later this year, they hope to demonstrate the shape-morphing of a helicopter rotor blade. The morphing capability should allow for a more efficient design, ultimately making it possible for a vehicle to carry heavier loads. Team members say that other applications, including miniaturized devices for Micro-Electrical-Mechanical Systems (MEMS), may flow from these initial demonstrations.
The researchers emphasize that much work remains to be done, such as refining the design of the battery for optimal operation in a morphing vehicle.
For more information go to http://webmit.edu/newsoffice/2006/morphing.html