Until now, it has been difficult to apply additive manufacturing techniques to ceramic because of its high melting point. Existing 3D-printed ceramic precursors are difficult to deform, which hinders the production of ceramics with complex shapes.
A research team at the City University of Hong Kong overcame these challenges with a new type of ‘ceramic ink,’ a mixture of polymers and ceramic nanoparticles. The 3D-printed precursors printed with this new substance are soft, and able to be stretched to up to three times their initial length. They also allow origami folding, as well as the creation of more complex shapes.
The development also opens the door to 4D-printing, which adds the element of time to additive manufacturing: printed objects can re-shape or self-assemble over time with external stimuli such as mechanical force, temperature or a magnetic field.
In this research, the team made use of the elastic energy stored in the stretched precursors for shape morphing. When the stretched ceramic precursors are released, they undergo self-reshaping. After heat treatment, the precursors turn into ceramics.
"The whole process sounds simple, but it's not," said Professor Lu Jian, the mechanical engineer who led the research. "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."
It took more than two-and-a-half years for the team to overcome the limitations of the existing materials and to develop the whole 4D ceramic printing system.
In the first shaping method, a 3D-printed ceramic precursor and substrate were first printed with the novel ink.
The substrate was stretched using a biaxial stretching device, and joints for connecting the precursor were printed on it. The precursor was then placed on the stretched substrate. With the computer-programmed control of time and the release of the stretched substrate, the materials morphed into the designed shape.
In the second method, the designed pattern was directly printed on the stretched ceramic precursor. It was then released under computer-programming control and underwent the self-morphing process.
"With the versatile shape-morphing capability of the printed ceramic precursors, its application can be huge!" said Lu. One potentially promising application is for electronic devices. Ceramics are much better at transmitting electromagnetic signals than metallic materials, and will play an increasingly important role in the manufacture of electronics with the arrival of 5G networks. Consumers could design their own complex ceramic outer shells for their devices.
The innovation could also be useful in the aerospace industry, where ceramics are often used as heat-shields. "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 Lu, adding that the next step would be to reduce the material's brittleness.