According to a study published in Nature Communications the process, once scaled up, could be used in a wide variety of applications, from building tiny medical implants and 3D logic circuits on computer chips to engineering ultra-lightweight aircraft components. It also opens the door to the creation of a new class of materials with unusual properties that are based on their internal structure.
Nanoscale 3D printing uses a high-precision laser to strike the liquid in specific locations of the material with two photons. Whilst this provides enough energy to harden liquid polymers into solids, it doesn’t provide enough to fuse metal.
"Metals don't respond to light in the same way as the polymer resins that we use to manufacture structures at the nanoscale," said Caltech materials scientist Professor Julia Greer. "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's graduate student Andrey Vyatskikh came up with a solution that uses organic ligands - molecules that bond to metal - to create a resin containing mostly polymer, but which carries along with it metal that can be printed, like a scaffold.
The team bonded together nickel and organic molecules to create a liquid, designed a structure using computer software, and then built this structure by zapping the liquid with a two-photon laser.
The laser creates stronger chemical bonds between the organic molecules, hardening them into building blocks for the structure. Since those molecules are also bonded to the nickel atoms, the nickel becomes incorporated into the structure. In this way, the team was able to print a 3D structure that was initially a blend of metal ions and non-metal, organic molecules.
This structure was then put into an oven that slowly heated it up to 1,000 degrees Celsius, which is below the melting point of nickel (1,455 degrees Celsius,) but hot enough to vaporise the organic materials in the structure, leaving only the metal. This pyrolysis process also fused the metal particles together.
In addition, because the process vaporised a significant amount of the structure's material, its dimensions shrank by 80 per cent, but it maintained its shape and proportions.
"That final shrinkage is a big part of why we're able to get structures to be so small," said Vyatskikh. "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 group is now refining the performance of the technique and looking at how it could be called up for industrial applications. It’s also looking at using a range of other materials, including tungsten, titanium, ceramics, semiconductors, and even piezoelectric materials.
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