Professor Wen Chen of the UMass Mechanical and Industrial Engineering Department is on a team of scientists from Lawrence Livermore National Laboratory (LLNL), UMass Amherst, and Harvard University working on a process for hierarchical 3-D printing of gold that could revolutionize the manufacture of electrochemical reactors, sensors, and actuators. Chen, a former postdoctoral research scientist at LLNL, helped develop an alloying and de-alloying process that was the key to the findings.
Chen, who focused on printing and post-processing parts, said the basis for the whole process was developing inks, made of gold and silver microparticles, with well-suited flow behavior, allowing them to form continuous filaments under pressure and to solidify upon exiting the printer’s micro-nozzle to retain their filamentary shape.
According to an LLNL press report, nanoporous metals are superior catalysts for chemical reactions due to their large surface area and high electrical conductivity, making them perfect candidates for applications such as electrochemical reactors, sensors, and actuators. In a study published recently in the journal Science Advances, LLNL researchers and their counterparts at Harvard University reported on the hierarchical 3D printing of nanoporous gold, a proof of concept that researchers say could revolutionize the design of chemical reactors.
The LLNL explained that achieving the finished product required several steps. First, LLNL researcher Cheng Zhu and former LLNL postdoc Chen created inks made of gold and silver microparticles. After printing, the 3D parts were put into a furnace to allow the particles to coalesce into a gold-silver alloy. Then the two researchers put the parts into a chemical bath that removed the silver (a process called “de-alloying”) to form porous gold within each beam or filament.
“The final part is a 3D hierarchical gold architecture comprising the macroscale printed pores and the nanoscale pores that result from de-alloying,” said Chen. “Such hierarchical 3D architectures allow us to digitally control the morphology of the macropores, which allowed us to realize the desired rapid mass transport behavior.”
In the LLNL press report, Zhu and Chen said the team’s method is a model that could easily extend to other alloy materials, such as magnesium, nickel, and copper, offering a powerful toolbox to fabricate complex 3D metal architectures with unprecedented functionalities in fields such as catalysis, batteries, supercapacitors, and even carbon dioxide reduction.
“If you consider traditional machining processes, it’s time-consuming and you waste a lot of materials — also, you don’t have the capability to create complex structures,” said LLNL postdoctoral researcher Zhen Qi, a co-author on the paper. “By using 3D printing we can realize macroporous structures with application-specific flow patterns. By creating hierarchical structures, we provide pathways for fast mass transport to take full advantage of the large surface area of nanoporous materials. It’s also a way to save materials, especially precious metals.”
The LLNL press report also explained that, combining 3D printing through extrusion-based direct ink writing and an alloying and de-alloying process, researchers were able to engineer the nanoporous gold into three distinct scales, from the microscale down to the nanoscale. The researchers reported that the hierarchical structure “dramatically improves mass transport and reaction rates for both liquid and gases.” (September 2018)