New Herculean Materials

Aircraft wings or helicopter rotors made of materials that can change their shape in response to electric controls have long been a dream of aeronautic engineers. It’s an application that would markedly improve the performance and fuel efficiency of aircraft. But the right shape-shifting material to make it happen has proven elusive.

Now researchers at MIT have discovered a promising approach that takes advantage of the mechanism that will eventually cause your laptop battery to fail: the expansion and contraction of electrode materials in the battery. “This is a classical case of taking lemons and making lemonade,” says Yet-Ming Chiang, a materials science and engineering professor at MIT who’s working on the project. Articles describing the work will appear in upcoming issues of Advanced Functional Materials and Electrochemical and Solid State Letters.

Much of the past research on shape-morphing materials has involved piezoelectrics, materials that change shape in response to electricity. But Chiang says he long ago decided that piezoelectrics would not work for such rigorous applications as morphing helicopter rotors.

For years, researchers in the related field of batteries have faced the problem that as ions move from one electrode to another, as the battery charges, they cause the electrode material to expand, then contract again as the battery discharges. This characteristic can cause the internal structure of the battery to break apart over time; so researchers have been searching for materials that don’t suffer from the effect. But when Chiang calculated how much mechanical energy this expansion could involve, he had a moment of euphoria. And later experiments showed that batteries have the mechanical energy needed to move a load ten times as far as piezoelectrics.

The battery’s performance does come with a tradeoff, though: speed. Piezoelectrics can easily operate at several thousand cycles per second, says Chiang. But the expansion of the battery is limited by how long it takes to charge it. Depending on how much movement is required, this could take from just over minute to “a sizable fraction of an hour,” according to Steven Hall, an aeronautics and astronautics professor at MIT involved in the project. The researchers hope to improve this by decreasing the time it takes to charge a battery.

Chiang is also working to design physically stronger batteries that can take better advantage of the electrode’s mechanical energy.

But existing commercially available batteries are good enough to build a demonstration model, which the researchers hope to have ready early next year. Eventually, a series of batteries will be set into the rotor blade and used to selectively morph it.

Such shape-shifting will allow engineers to avoid a compromise that has been at the heart of helicopter design. Helicopters are built to do two very different things – hover and cruise. As a result, they do neither particularly well. Being able to change the rotor shape in flight could optimize them for these two functions. Chiang and Hall calculate that a helicopter could then operate with about one percent less fuel, a savings that could add up considerably over time. Alternatively, military helicopters could carry two additional troops or work better in high-altitude operations in the mountains.

Rotors might just be the beginning, too. Chiang and Hall point to applications on airplanes, where changing wing shapes in-flight could bring similar improvements in performance and efficiency. They might also be useful for actively rotating solar cells to track the sun, or deploying solar cells and other appendages of satellites once they reach space.