Engineers at MIT have come up with a new metamaterial, tiny star-shaped structures that actually shrink when heated, a result which could have applications in a wide range of technologies from printed circuit boards and aerospace materials to window frames and water pipes.
Save for a few lone examples such as water at certain temperatures, graphene and some quartzes, almost everything expands when heated. It’s a natural consequence of adding kinetic energy to a material that its atoms and molecules will vibrate faster, taking up more room in the process. And as we all know from the rash of potholes that appear on our roads every spring, this expansion and contraction due to heating and cooling can play havoc with our built environments, to say nothing of computer boards and other precision technologies extremely sensitive to changes in temperature.
Thus, scientists and materials engineers are continually in search of structures that behave oppositely and undergo “negative thermal expansion.” Now, researchers at the Massachusetts Institute of Technology and the University of Southern California have created a 3-D printing technique which uses projected light to print out tiny structures from liquid resin, producing star-shaped metamaterials composed of interconnected copper and polymer beams. Think, if you will, of those collapsible toy latticework balls that can shrink and expand, but imagine them half a centimetre-wide and composed of a copper and polymer (okay, don’t bother imagining, just take a look at the video below).
Put under heat up to 282 degrees Celsius, the star-shaped metamaterials contract by about 0.6 per cent in volume, which may not seem like much, but to materials specialists, the major breakthrough is in the lack of outward expansion. “The very fact that it shrinks is impressive,” says Nicholas X. Fang, an associate professor of mechanical engineering at MIT and lead author of the new study.
Researchers say that the new materials could be combined with normal, thermally expanding materials to create products and technologies that are overall resistant to expansion under heat.
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Advances in manufacturing technologies such as 3-D printing have led to countless innovations, with the growth in metamaterials being a prime example. Metamaterials are engineered composites which, often due to their unique structure and geometry, express properties not found in their component parts.
The thermal properties of metamaterials are themselves an area which is likely to become important in the quest for alternative sources of energy. Engineers at the University of Alberta in Edmonton, Alberta, have been working on using thermal metamaterials to create thermophotovoltaics which can convert thermal radiation into electricity. Where traditional solar cells convert only a fraction of solar radiation into electrical power, leaving much of the energy unused as dissipated thermal energy, the thermaphotovoltaic cell can absorb much more energy – either from a source like the sun or other heat-producers such as coal and oil-based power plants and car engines. “In the broadest terms, what we’re essentially doing is heating up an object, and then collecting the energy it gives off,” says Sean Molesky of the U of A’s Department of Electrical and Chemical Engineering. “But we’re using unique metamaterials to build these objects, which allows us to absorb efficiently, and also emit the energy at specific wavelengths that are more efficient for photovoltaic energy conversion.”
Below: Heat-induced shrinkage (Massachusetts Institute of Technology (MIT)