Researchers from the University of Twente, The Netherlands, established a huge step forward in high-resolution metal printing, or 3D printing pure metals. Laser light was used to melt copper and gold into micrometre-sized droplets that were subsequently positioned onto a substrate.
A 3D structure was created by repeating this process, stacking thousands of drops to form micro-pillars with a height of 2 millimetres and a diameter of 5 micrometres. In addition, the researchers printed vertical electrodes in a cavity, as well as lines of copper. In effect, virtually any shape can be printed by smartly choosing the location of the drop impact.
Because metals melt at a high temperature, controlled deposition of metal droplets is particularly challenging. Thermally robust nozzles are required to process liquid metals, but these are hardly available. For small structures in particular (from 100 nanometres to 10 micrometres) there was no good solution for this problem.
By using high laser energy the impact velocity of the metal droplets was increased. When these fast droplets impact onto the substrate, they deform into a disk shape and solidify in that form. The disk shape is essential for a sturdy 3D print: it allows the researchers to firmly stack the droplets on top of each other. In previous attempts, physicists used low laser energies. This allowed them to print smaller drops, but the drops stayed spherical, which meant that a stack of solidified droplets was less stable.
In their article in Advanced Materials, the researchers explain which speed is required to achieve the desired drop shape. They had previously predicted this speed for different laser energies and materials, which means the results can be readily translated to other metals as well.