Oct 24, 2016 | By Benedict

An additive manufacturing research group has developed a 3D printing material that shrinks, rather than expands, when heated. The lightweight thermal material, made up of microlattice structures, can be “tuned” to shrink in three Cartesian directions over a large range of temperatures.

Most solid materials expand when heated; only a few shrink. However, materials with “negative thermal expansion,” those that shrink when heated, can have various important uses, such as in water pipes, in spacecraft equipment, and for securing parts that move out of alignment under varying heat loads, including microchips and high precision optical mounts. A group of scientists from Lawrence Livermore National Laboratory (LLNL) and several U.S. universities has managed to 3D print lightweight metamaterials that can be "tuned" to shrink over a large range of temperatures. Their research has been published in Physical Review Letters.

The secret to the researchers' new shrinking 3D printed metamaterial is its bi-material microlattice structure, 3D printed from polymer and a polymer/copper composite material, that can flex inward. This inward flexion causes the structure to contract when exposed to heat. Crucially, this highly tunable contraction occurs in three Cartesian directions, and the researchers believe that their 3D printed material is the first to do this. The thermal expansion can also be either zero or positive, depending on how the geometry and topology of the structure are engineered.

The research group designed a series of microlattice structures made of pyramid-like shapes that would contract when heated. For the microlattice structures to bend in on themselves and thus contract, their internal struts are made from a material with greater thermal expansion. As these struts expand, an external frame made of the less expansive material forces the struts to fold into the structure’s center.

“This is a new version of a printing method we have developed and used in the past,” said principal investigator Chris Spadaccini, director of LLNL's Center for Engineered Materials and Manufacturing. “We used it to create a thermomechanical metamaterial that may enable applications not possible before. It has thermomechanical properties not achievable in conventional bulk materials.”

Using a projection microstereolithography 3D printing technique, a team at MIT was able to expand an existing system in order to fabricate multimaterials. Under the leadership of Associate Professor of Mechanical Engineering Nicholas Fang, the MIT scientists had to ensure that there was no contamination between materials by washing out the residue after every printed layer.

The researchers believe that the 3D printed material’s ability to passively adjust to local temperature changes could make it suitable for a variety of mechanical applications, in which thermal mismatches have traditionally been tackled with active control or heating and cooling. In devices where parts tend to move out of alignment under heat, the 3D printed material could be used to counteract that misalignment.

According to Qiming Wang, the paper's lead author, the microstructured 3D printed metamaterial could be used in dental fillings, which tend to move or crack when a person eats something hot, while it could also be used to fill small gaps in bridges or buildings that are normally left open to account for thermal expansion, or in precision devices such as atomic-force microscopes.

“The interesting thing is it's made of two different materials, beams and void space,” explained Jonathan Hopkins, an assistant professor of mechanical aerospace and engineering at UCLA. “When you heat it, as long as one of the beams expands more than the others, then the connecting points between each unit cell pulls inward and makes the overall lattice pull inward. It's an immediate thermal contraction, which is the unique thing about it.”

The study, which involved researchers from LLNL, the University of Southern California, MIT, and the University of California, Los Angeles, is part of a five-year Defense Advanced Research Projects Agency Defense Sciences Office program to research materials with controlled microstructure architecture. According to Wang, the next stage of the project is to fabricate zero thermal expansion materials that could solve similar problems.

 

 

Posted in 3D Printing Materials

 

 

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