Counterintuitive metamaterial can lead to the creation of heat-resistant electronic circuits

Virtually all solid materials, from rubber and glass to granite and steel, expand when heated. Only in very rare cases, certain materials go against the system and shrink when heated. For example, cold water is compressed if it is heated from 0 to 4 degrees Celsius, before it begins to expand.

Engineers at MIT and the University of Southern California introduced a new addition of strange materials to this class. A team led by Nicholas X. Fang, an associate professor of engineering mechanics at MIT, has created star-shaped structures consisting of connected crossbars or farms. These sugar cube-sized structures quickly contract, being heated to 282 degrees Celsius.


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Farms are made up of ordinary materials that expand when heated. Fang and his colleagues guessed that if they were joined in a special way, they would be able to pull the structure inward, causing it to shrink like a Goberman sphere toy.

Researchers believe that their creation belongs to "metamaterials" - composite materials, the configuration of which has strange and often counterintuitive properties, usually not found in nature.

In some cases, it may not be useful to compress these structures themselves, but to resist their expansion when heated. Such materials can be used, for example, in the manufacture of computer chips, deformed during prolonged heating.

“Printed circuit boards can heat up when the CPU is running, and heating can affect their performance,” says Fang. “Therefore, [in designing] it is necessary to take this property of their property very carefully into account.”

Ingredients for printing

In the mid-90s, scientists proposed theoretically possible structures whose structure could give them the property of “negative thermal expansion” (negative thermal expansion, NTE). To do this, it was necessary to create three-dimensional lattice structures of two materials with different coefficients of expansion when heated. When the entire structure is heated, one material should expand faster and tighten the other inwards, with the result that the overall size of the structure decreases.

“The theoretical papers talked about how such structures could violate the usual limitations of thermal expansion,” says Fang. “But at the time, [scientists] were limited to the technology of creating things. And here we saw a great opportunity for micromanufacturing, demonstrating this concept. ”

In the laboratory, Fang developed a 3D printing technology called microstereolithography, which uses light for layer-by-layer printing of very small structures in a liquid resin.

“Now we can use the microstereolithography system to create thermomechanical metamaterials that will make the impossible possible earlier,” says Spadaccini, director of the Center for the Development and Production of Materials. “They have thermomechanical properties that are unattainable for ordinary materials.”

“We can make an inkjet printer for printing and curing different ingredients in the same way,” says Fang.

Inspired by the theoretical platform, Fang and his colleagues printed small three-dimensional star-shaped structures from interconnected crossbars. Each of them is made of either a solid and slowly expanding material containing copper, or of an elastic rapidly expanding polymer. The inner crossbars are elastic, and the outer ones are hard.

“By correctly positioning the components of the grid, we will ensure that even with the expansion of each crossbar, they tighten the entire grid inside,” says Fang.

“We work with temperature offset,” says Wang. “These materials have different thermal expansion coefficients, therefore, as the temperature rises, they interact with each other and pull inwards, so that the total volume of the structure decreases.”

Experiment space

The researchers tested their composite structures by placing them in a glass flask and slowly raising its temperature, from room temperature to 282 degrees. It was found that at first the structure retains its shape, and then gradually bends inwards and contracts.

“It shrinks by 0.6%,” says Fang. This does not seem to be such a great achievement, but Fang adds that "the fact of compression is impressive." For most practical applications, according to Fang, designers would prefer structures that simply do not expand when heated.

In addition to their experiments, the researchers created a computer model for calculating the interaction of connected crossbars, the distance between them, and the degree of expansion. Compression of the structure is controlled by two main parameters - the size of the individual crossbars and their relative stiffness, directly related to the rate of thermal expansion.

“We have developed a method of adjustment, having in the computer model separate components with different stiffness and speed of expansion, and we can make a specific crossbar or part of the structure deviate or expand as we need,” says Fang. - There is space for experiments with other materials, for example, with carbon nanotubes, which are lighter and stronger. You can achieve interesting results by experimenting in a laboratory with different structures. "