Four-dimensional printing refers to the manufacture of geometries that can reshape or self assemble over time, with the influence of external stimuli such as force, temperature or magnetism. It is particularly applicable to ceramics which, because of their tendency to stiffness and brittleness, are very difficult to print in complex shapes.
This has been a barrier to the structural application of ceramics, with the result that they have been largely excluded from the revolution that 3D printing has brought to the use of polymers and metals. The development from City University of Hong Kong (CityU) is to use a “ceramic ink” to print flexible forms that can be turned into a conventional ceramic material with heat treatment.
In a paper in the journal Science Advances, a team led by material scientist Prof Lu Jian describes how the ink, made from elastomeric poly (dimethylsiloxane) mixed with crystalline nanoparticles of zinc oxide, 20 to 50nm in diameter, could be printed and then deformed through stretching, origami-like folding, or by using pre-defined joints and creases into complex shapes and then heat treated to turn them into rigid ceramic bodies.
"The whole process sounds simple, but it's not," said Professor Lu. "From making the ink to developing the printing system, we tried many times and different methods. Like squeezing icing on a cake, there are a lot of factors that can affect the outcome, ranging from the type of cream and the size of the nozzle, to the speed and force of squeezing, and the temperature."
The team developed two techniques for shaping the bodies. In the first, two forms – a 3D-printed ceramic precursor and substrate – were first made with the new ink. The substrate was stretched and joints for connecting it to the precursor were printed onto it. The precursor was then placed onto the stretched substrate, which was allowed to relax, morphing the materials into the desired shape.
In the second method, a pattern was directly printed onto the stretched ceramic precursor. It was then released under computer control to morph into the final form. Both techniques can produce complexly curved and textured forms which were not accessible through previous additive manufacturing techniques.
Prof Lu believes that electronic devices will be an important application sector for this technology. Ceramics transmit electromagnetic signals much better than metals, and they are expected to play a much more important role in the manufacture of housings for products such as mobile phones when 5G networks come into use.
The resistance of ceramics to defamation and heat means that these products will also be attractive to the aerospace industry, particularly in space applications. "Since ceramic is a mechanically robust material that can tolerate high temperatures, the 4D-printed ceramic has high potential to be used as a propulsion component in the aerospace field," said Prof Lu.
The 4D printing project has taken two and a half years to reach this stage, but is not yet complete. Prof Lu now hopes to improve the mechanical properties of the material, in particular by reducing its brittleness.
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