In 3D printing — also known as additive manufacturing — an object is built layer-by-layer, allowing for the creation of structures that would be impossible to manufacture by conventional subtractive methods such as etching or milling.
Metals have been difficult to print, especially when trying to create structures with dimensions smaller than about 50 microns, or about half the width of a human hair. A new process creates complex nanoscale metal structures using 3D printing.
The way 3D printing works at the nanoscale is that a high-precision laser zaps the liquid in specific locations of the material with just two photons, or particles of light. This provides enough energy to harden liquid polymers into solids, but not enough to fuse metal. Metals don't respond to light in the same way as the polymer resins used to manufacture structures at the nanoscale. There is a chemical reaction that is triggered when light interacts with a polymer that enables it to harden and form into a particular shape. In a metal, this process is fundamentally impossible.
In the new process, organic ligands — molecules that bond to metal — are used to create a resin containing mostly polymer, but that carries along with it metal that can be printed, like a scaffold. In experiments, nickel and organic molecules were bonded together to create a liquid that looks like cough syrup. A structure was designed using computer software, and then was built 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, a 3D structure was printed that was initially a blend of metal ions and nonmetal, organic molecules.
The structure was then placed into an oven that slowly heated it to 1000 °C (around 1800 °F) in a vacuum chamber. That temperature is well below the melting point of nickel (1455 °C or about 2650 °F), but is hot enough to vaporize the organic materials in the structure, leaving only the metal. The heating process, known as pyrolysis, also fused the metal particles together. In addition, because the process vaporized a significant amount of the structure's material, its dimensions shrank by 80 percent, but it maintained its shape and proportions.
The structure includes some voids left behind by the vaporized organic materials as well as some minor impurities. Also, if the technique is to be of use to industry, it will need to be scaled up to produce much more material. Although they started with nickel, the researchers are interested in expanding to other metals that are commonly used in industry, but are challenging or impossible to fabricate in small 3D shapes, such as tungsten and titanium. The researchers are also looking to use this process to 3D-print other materials, both common and exotic, such as ceramics, semiconductors, and piezoelectric materials.