Nanoscale 3D printing is a small-scale technique that many researchers are working to further develop, and 3D printing on the nanoscale with metal is one of the major challenges. But researchers at Caltech have developed a technique that makes it possible to 3D print complex nanoscale metal structures. Once their process is scaled up, it could be used for applications like creating 3D logic circuits on computer chips, building tiny medical implants, and engineering ultra-lightweight aircraft components.
Materials scientist Julia Greer, who runs the Greer Group and is a Professor of Materials Science, Mechanics, and Medical Engineering in Caltech’s Division of Engineering and Applied Science, is also an expert when it comes to 3D printing extremely small architectures. Her group 3D prints structures using all sorts of materials, like organic compounds and ceramics, and they’ve built 3D lattices with beams that are only nanometers across.However, metals have proven difficult to 3D print when making structures that are half the width of a human hair, which is what the group specializes in.
Two-photon lithography is a 3D printing technique that can produce nanoscale features smaller than one-hundredth the width of a human hair. 3D printing at the nanoscale involves this process, where a high-precision laser zaps the liquid in certain locations of the material with two photons. But while this can harden liquid polymers into solids, it has not proven very effective when it comes to fusing metal together.
“Metals don’t respond to light in the same way as the polymer resins that we use to manufacture structures at the nanoscale. There’s a chemical reaction that gets triggered when light interacts with a polymer that enables it to harden and then form into a particular shape. In a metal, this process is fundamentally impossible,” Greer explained.
But Andrey Vyatskikh, one of Greer’s graduate students, got creative, and developed a resin using organic ligands, which are molecules that bond to metal. The resin mostly consists of polymer but carries along 3D printable metal. By synthesizing these organic scaffolds that contain metal ions, he was able to 3D print metallic structures that are much smaller than previously possible.
The research team described the new technique in a study, titled “Additive Manufacturing of 3D Nano-Architected Metals,” that was just published in the journal Nature Communications; co-authors of the paper include Vyatskikh, Stéphane Delalande of the Centre Technique de Vélizy in France, Caltech Resnick Sustainability Institute Postdoctoral Scholar Akira Kudo, Xuan Zhang of Tsinghua University in China, mechanical engineering graduate student Carlos Portela, and Greer. The Department of Defense funded their research.
For the experiment detailed in the paper, Vyatskikh created a liquid that strongly resembles cough syrup by bonding together nickel and organic molecules. Then, the team designed a structure and created it using a two-photon lithography process: the laser can create stronger chemical bonds between the molecules, which allows them to harden into building blocks. The nickel is incorporated into the structure, since the molecules are bonded to the nickel atoms.
The abstract reads, “Most existing methods for additive manufacturing (AM) of metals are inherently limited to ~20–50 μm resolution, which makes them untenable for generating complex 3D-printed metallic structures with smaller features. We developed a lithography-based process to create complex 3D nano-architected metals with ~100 nm resolution. We first synthesize hybrid organic–inorganic materials that contain Ni clusters to produce a metal-rich photoresist, then use two-photon lithography to sculpt 3D polymer scaffolds, and pyrolyze them to volatilize the organics, which produces a >90 wt% Ni-containing architecture. We demonstrate nanolattices with octet geometries, 2 μm unit cells and 300–400-nm diameter beams made of 20-nm grained nanocrystalline, nanoporous Ni. Nanomechanical experiments reveal their specific strength to be 2.1–7.2 MPa g^(−1) cm^3, which is comparable to lattice architectures fabricated using existing metal AM processes.”
The structure was placed into an oven that heated it up slowly to 1,000°C in a vacuum chamber, which is far below the melting point of nickel but hot enough to vaporize the organic materials, leaving behind only the metal parts of the structure. The metal particles were also fused together, and while this unique heating process, called pyrolysis, vaporized a large amount of the material in the structure, its dimensions shrank by 80%. But its proportions and 3D printed shape remained.
Lead author Vyatskikh explained, “That final shrinkage is a big part of why we’re able to get structures to be so small. In the structure we built for the paper, the diameter of the metal beams in the printed part is roughly 1/1000th the size of the tip of a sewing needle.”
The structure featured in the paper has some minor impurities, and some voids left by the vaporized organic materials, so obviously the technique still needs a little work. In addition, Greer said the process needs to be scaled up in order to produce more material, and she and Vyatskikh are investigating if they can use the process to 3D print other materials, like semiconductors, ceramics, and piezoelectric materials, and even materials that are often used in industry but hard to make in small 3D shapes, like titanium and tungsten.
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