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Sep 6, 2018 | By Thomas

In a study published in the journal Science Advances, Lawrence Livermore Nationwide Laboratory (LLNL) researchers, together with their counterparts at Harvard College report on the hierarchical 3D printing of nanoporous gold, which they believe could revolutionize the design of chemical reactors.

Nanoporous metals have generated much interest since they combine several desirable material characteristics, such as high surface area, mechanical size effects, and high conductivity. This makes them perfect candidates for applications such as electrochemical reactors, sensors and actuators.

"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 researchers combined an extrusion-based direct ink writing 3D printing process with an alloying and dealloying process to engineer the nanoporous gold into three distinct scales, from the microscale down to the nanoscale. According to researchers, the hierarchical construction “dramatically improves mass transport and response charges for each liquid and gases.” With the ability to manipulate the catalyst's surface area to generate electrochemical reactions through 3D printing, researchers said the development could have a major impact on electrochemical plants, which currently rely primarily on thermal energy.

Combining 3D printing with an alloying and dealloying process, researchers at Lawrence Livermore National Laboratory and Harvard University were able to engineer nanoporous gold into microarchitectured hierarchical structures. Credit: Ryan Chen/LLNL 

"By controlling the multiscale morphology and surface area of 3-D porous materials, you can start to manipulate the mass transport properties of these materials," said LLNL researcher Eric Duoss. "With hierarchal structures you have channels that can handle transfer of reactants and products for different reactions. It's like transportation systems, where you go from seven-lane expressways down to multiple lane highways to thoroughfares and side streets, but instead of transporting vehicles we're transporting molecules."

Achieving the finished product required several steps. LLNL researcher Cheng Zhu and former postdoc Wen Chen created inks made of gold and silver microparticles, which were then 3D printed. The 3D printed parts were put into a furnace to allow the particles to coalesce into a gold-silver alloy. Then they put the parts into a chemical bath that removed the silver in a process called "dealloying" to form porous gold within each beam or filament.

"The final part is a 3-D hierarchical gold architecture comprising the macroscale printed pores and the nanoscale pores that result from dealloying," said Chen, who is currently a professor at the University of Massachusetts-Amherst. "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."

Researchers said the method can easily be applied to other alloy materials such as magnesium, nickel and copper, providing new possibilites to apply complicated 3D printed metal parts in fields such as catalysis, batteries, supercapacitors and even carbon dioxide reduction.

3D-printed Ag-Au structures with different macroscale geometries and microscale architectures.

(A) Optical image of a single-layer array of parallel linear filaments. Optical images of multilayer high–aspect ratio (B) spiral, (C) honeycomb, (D) hollow pillar array, (E) linear simple cubic lattice, and (F) circular radial lattice structures. (A), (B), (C), and (E) shown as-printed, and (D) and (F) shown after annealing. Scale bars, 200 μm (A) and 2 mm (B to F).

Chen, who focused on printing and post-processing parts, said the key to the process was developing inks 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.

The challenge in catalysis is in combining high surface area with rapid mass transport, according to LLNL researcher Juergen Biener.

"While additive manufacturing is an ideal tool to create complex macroscale structures, it remains extremely difficult to directly introduce the nanostructures that provide the required high surface area," Biener said. "We overcame this challenge by developing a metallic ink-based approach that allowed us to introduce nanoporosity through a selective corrosion process called dealloying."

Biener said LLNL's extrusion-based approach is universal and scalable, provides tooling-free control over the macroscopic sample shape, and—most importantly—enables integration of nanoporosity in an application-specific engineered macroporous network structure.

The project is part of a feasibility study feeding into a proposed strategic initiative to create 3D electrochemical reactors in which scientists could exert greater control over catalysts and reduce transport limitations. Instead of large electrochemical plants, typically located close to oil refineries or in remote areas, modular reactor networks could be created in a series that could be easily replaceable and transportable for relocation near sources of abundant renewable energy or carbon dioxide.

"There are a whole lot of scientific and engineering challenges left, but it could have significant impact," said Chris Spadaccini, director of LLNL's Center for Engineered Materials and Manufacturing. "Scaling up should be easier with small-scale reactors because you can parallelize. You could have an array of small 3-D reactors together instead of one large vessel enabling you to control the chemical reaction process more effectively."

Researchers said they are already starting to explore other materials that might be catalysts for other reactions. Tests on samples of the parts show that their hierarchical structures facilitate mass transportation.

Their research paper, titled "Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing", is published in the journal Science Advances.

 

 

Posted in 3D Printing Application

 

 

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Amazing wrote at 9/6/2018 6:58:38 PM:

Lawrence Livermore Nationwide Laboratory.... 😂 Harvard College.... 🤣🤣🤣

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