LLNL Sheds Light on 3D Printing Next-Gen Metamaterials

U.S. government labs have become involved in 3D printing in a big way, necessarily adopting the technology for research and development purposes since the technology's inception, but also pushing the boundaries of what 3D printing is truly capable of.

While Oak Ridge National Laboratory, located on the East Coast, has developed large-scale additive manufacturing (AM) for producing such components as complete auto chassis, Lawrence Livermore National Laboratory (LLNL) is taking 3D printing down to the micro- and nanoscale, manipulating the physical properties of objects by modifying their inner geometries.

ENGINEERING.com spoke with Christopher Spadaccini, director of the Center for Engineered Materials, Manufacturing and Optimization at LLNL, to learn about some of the research that the lab is conducting in this area, as well as the cutting-edge AM technologies that make this research possible.

Micro-SLA 3D Printing for Macro Applications

While LLNL works to advance existing commercial 3D printing systems for its own research, the lab also develops technology in house. Among the techniques created by LLNL is a process called micro-stereolithography (micro-SLA).

This technology allows Spadaccini and his team to create complex micro-geometries very quickly, allowing for the 3D printing of architected materials, also known as metamaterials. Spadaccini describes the concept of developing architected materials as “creating geometry out of the right constituent material at the right length scale to give you really unique properties, like high stiffness and strength at low densities.”

By 3D printing complex lattice structures at the microscale, for instance, it's possible to design parts that are extremely lightweight, but which have enormous strength due to the way that geometries of the objects behave on the microscale. Voids allow the prints to have a reduced mass, while the tensegrity of the microstructures maintains stiffness and strength.

Micro-SLA was originally developed by LLNL partner Nicholas Fang, associate professor of Mechanical Engineering at MIT, who helped the lab to develop its own systems. LLNL now has two micro-SLA machines up and running and has two more micro-SLA machines on the way. The lab has also developed a new iteration of the system known as Large Area Projection Micro-Stereolithography (LAPuSL), for which LLNL won an R&D 100 award last year.

Spadaccini explains that to fully take advantage of the architected material properties capable with micro-SLA, the tech must be scaled up, which resulted in LLNL constructing two LAPuSL systems, with a third on the way. “If you go down to the micro- and nanoscale, you get even better material properties—things like strength become better,” Spadaccini said. “The problem becomes getting those small features in a part of any substantial size that you might actually want to use in a real application. That’s why we built the large area micro-SLA, to get small features in big parts. That will allow you to do things like make architected materials to take advantage of nanoscale properties and make them big enough to be used in say an aerospace application or automotive application or something like that.”

In other words, super strong objects on the micro scale are fine for micro-scale applications, but to develop more practical items from these metamaterials, the technology needs to be scaled up. Right now, the LAPuSL system can print objects up to six inches in size with features as fine as 10 microns. As the technology becomes larger, it will be possible to fabricate objects for applications in defense, including body armor or aerospace structures that are tough yet lightweight.

The materials for both micro-SLA and LAPuSL are not limited to polymers, though photopolymers are typically necessary for the process. Spadaccini said that LLNL has applied the process for materials like ceramics and metals. “There are multiple ways to get to other materials. One way is to take the polymer structure that you've built and you can coat it or infill the void space with another material. You can remove the polymer through chemical etching or thermal burnout. We’ve done metals and ceramics that way,” Spadaccini said.

He continued, “Another route is to load the liquid polymer with particles floating around in suspension. You fabricate a part that then has a combination of polymer and particles. You can then sinter that part in a furnace to burn out the polymer and densify the particles.”

LLNL has performed these processes with both metal particles and ceramic particles, but Spadaccini says that it's not an easy process. “You get shrinkage. You have to have a really good suspension of particles. I don't want to trivialize it by making it sound like it's routine. It's not. It's hard, but we can do it,” he said.

Direct Ink Writing

For other applications, LLNL has developed a 3D printing technique that is less dependent on polymers. In fact, direct ink writing (DIW) can use a very broad material set, according to Spadaccini, who describes the technology as a cousin to fused deposition modeling (FDM).

DIW uses a similar process, in that a material is extruded onto a substrate; however, the material is pushed out of nanometer-scale nozzles and no phase change is applied to the material. Spadaccini elaborated on the physics of the process and the materials, “Most of the time, but not all of the time, we create an ink that is shear thinning. When you apply force, it moves. In the absence of that force, it gels and holds its shape. It's very much like toothpaste coming out of a toothpaste tube.”

“Because the material set is so broad, the application space is broad,” Spadaccini explained. “One material we have a lot of experience with is silicone. After we print it and cure, it's a thermoset and it's very soft and squishy. We make cushions and pads with it. They are things that absorb the mechanical energy like vibrations, shock; you can put them between hard components as a compliant interface. They’re the kind of things you might want to use in a helmet, in a shoe or a sneaker, in a seat cushion.”

“With our partner Jim Smay from Oklahoma State, we've developed our own nozzle system that has a mixture inside of it. Inside the tip of this tiny little nozzle, we have a tiny little rotor spinning around, and we can feed different materials in at different ratios, mix them together, and push them out of the nozzle very quickly. That allows us to change the composition of material as we print on the fly. That allows us to do some very interesting things with designed dopant profiles and changes in material within a single component,” Spadaccini said.

Other unique materials that LLNL is using with DIW are graphene and glass. With the ability to 3D print graphene aerogel, LLNL envisions the ability to create better energy storage systems, sensors and nanoelectronics due to the high surface area, high electrical conductivity, mechanical stiffness and light weight of the material. The lab is also printing with glass particles, which are sintered to create optical components and glasses. Even these items can be infused with conductive materials to create lenses with built-in semiconductors.

DIW is more easily scalable than micro-SLA and can print quite quickly. LLNL has been able to build a DIW printer capable of printing 30 cm in length at a rate of 10 cm per second—all while maintaining submicron resolution. Spadaccini mentioned that the concrete extrusion systems employed by companies like WinSun Global and WASP for the construction of buildings are, in some ways, large-scale versions of DIW. He admits, however, that LLNL's DIW technology cannot build very tall or thick objects at the moment.

More to Come

In our brief interview, Spadaccini was only able to scratch the surface of the cutting-edge projects of his lab that are literally changing the fabric of the 3D printing space. In a follow-up post, we will discuss just some of the ways LLNL is tackling the issue of quality control and simulation in industrial 3D printing and, even then, there is still plenty of ground left to cover.