At one-atom thick, graphene is the lightest and strongest material in the world. Harnessing its power, however, has not been easy as it’s not a simple process to manipulate the world’s toughest material. Additive manufacturing (AM) may hold the answer.
Various researchers are currently working to find ways to 3D print the “wonder material,” as it is sometimes called, in ways that maintain its desired physical properties. Most methods involve processing graphene as an aerogel, which ultimately reduces the amount of graphene in the final composition to just 20 percent or so.
At University at Buffalo,a new process has been developed that maintains the physical properties of graphene and enables the 3D printing of macroscale structures. In fact, the parts printed with the process have been dubbed by the Guinness World Records as the “least dense 3D-printed structure” yet made. ENGINEERING.com spoke with the chief researcher on the project, Chi Zhou, assistant professor of industrial and systems engineering, to learn more.
3D Printing with Graphene
To understand what makes Zhou’s method so unique, it’s important to understand why 3D printing graphene has been so difficult traditionally. Zhou explained that, until recently, it’s been an issue of bonding the material layer by layer.
“The major difficulty of printing graphene is caused by the complex graphene morphology and nontrivial material forming mechanism,” Zhou said. “It is very challenging, if not impossible, to assemble the 2D graphene sheets by heat fusing, photocuring, chemical bonding as is typically used in traditional 3D printing technologies.”
To overcome these issues, researchers have attempted to 3D print graphene aerogel through micro-extrusion techniques. “The key concept of these techniques is to develop printable graphene ink by increasing the viscosity and form it as shearing thin material. However, these approaches suffer from several drawbacks, including the necessity of fillers, possible poor bonding and limited structure complexity,” Zhou explained.
Previous research has included work by the Imperial College London and Lawrence Livermore National Laboratory (LLNL). The Imperial College London team 3D prints using graphene-oxide combined with a responsive polymer to extrude the material as a paste. LLNL has 3Dprinted graphene-oxide into a silica gel.
Freeze-Casting Graphene
However, Zhou’s group—which also included Dong Lin from Kansas State University and Qiangqiang Zhang from Lanzhou University in China—took a different approach. The team modified an open-source fused filament fabrication 3D printer to use an inkjet printhead, rather than an extrusion head, to deposit a graphene oxide and water mixture in a freezer on a cold plate at -20°C.
The low temperatures aid the object in keeping its shape and as the ice crystals grow, they squeeze the graphene sheets that make up each layer into a 3D network.Upon printing, the structure is freeze dried, causing the ice to evaporate and leaving only a graphene aerogel.
This process, known as freeze-casting, is sometimes used in ceramics. Water solidifies in an anisotropic manner and, when combined with another material, such as ceramic particles, in a slurry, it’s possible to directionally control the temperature. Ice crystals form on one side and spread across a temperature gradient on the material, redistributing the particles of the mixture in the process.
“The end result is graphene aerogel, a porous and superlight material in which the liquid part of the gel is replaced by air, allowing it to retain its shape at room temperature,” Zhou said.“The process has proven to be very effective and successful.One of the blocks along the way is the proper thermal management, which is the key to the process performance (reliability, resolution and integrity).”
Zhou pointed out that unique to his group’s technology is the ability to change the scale of the 3D-printed structure to both produce large and small objects. “Our method can 3D print freestanding multiscale graphene aerogel, where the macroscale architecture is controlled by multi-nozzle inkjet printing and the microscale morphology is controlled by freeze casting,” Zhou explained.“That means, we can print lattice structure with both tunable macropores and micropores. The graphene density we are able to achieve is 0.5 milligrams per centimeter, which is the lightest 3D-printed structure in the world so far.”
Applications of 3D Printing with Graphene
For this reason, Zhou’s team took home the Guinness World Record for “least dense 3D printed structure.” More importantly, the capabilities of the team’s process open a range of interesting and potentially groundbreaking applications.
“This technology can fabricate the graphene aerogel in a controllable and scalable manner,” Zhou said. The tailored macroarchitecture of graphene aerogel can unblock the exotic physicochemical properties (mechanical, electrical, thermal and acoustic properties) of 2D graphene sheet, and finds enormous applications, including but not limited to, separation, all-solid-state batteries, micro pressure sensors, flexible electrodes and electrochemical catalyst templates. We are currently working on topics related to energy applications of graphene aerogel.”
To get there, Zhou and his colleagues will need to expand on their existing work. Zhou said that they plan to optimize the technology in terms of “process reliability, scalability and part geometry and functional integrity.” They will also extend the technology to 3D print with other nanomaterials aside from graphene.
“This process could be an important step toward making graphene commercially viable.We already received multiple requests from industry to commercialize this technology,” Zhou concluded.