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Ceramic metamaterials form detailed microarchitectured lattices stable at high temperatures

By Eva Karatairi February 21, 2018
Ceramic metamaterials
Scanning electron micrograph (SEM) of (a) the octet-truss lattice, (b) the cuboctahedron lattice after pyrolysis, and (c) the lattices with four different sizes. (d) SEM of the high temperature micro-gear set made of silicon oxycarbide. Credit: Xiaoyu (Rayne) Zheng

Polymer-derived ceramic metamaterials, with high temperature stability, light weight, and very detailed microstructures have been fabricated with high precision by researchers at Virginia Polytechnic Institute and State University. The group of Xiaoyu (Rayne) Zheng has designed and printed complex three-dimensional (3D), silicon-based ceramic lattices that consist of microarchitectural elements with lengths between 10 μm and a few millimeters.

The team was intrigued by the limitations of the existing high-temperature bulk ceramics. These materials are heavy and difficult to process into complex microscale structures, while at the same time their inner structure is characterized by distributed cracks, voids, and inclusions, which can lead to catastrophic fracture behavior upon loading. Addressing these limitations is important, because ceramics in general have higher strength and smaller weight than metals, properties that make them better for applications like aerospace components or catalysts support.

The researchers used large-area projection micro-stereolithography (LAPµSL) to print two series of polymer-derived ceramic lattices with varying sizes and densities, but with identical 3D geometries, one based on an octet-truss unit cell and another on a cuboctahedron unit cell. The researchers chose these two geometric configurations because they are both stretch-dominated, meaning they can carry tension or compression when loaded and are more mechanically efficient than bending-dominated unit cells. LAPµSL can produce items of hundreds of millimeters in size, with highly detailed features (tens of micrometers), and it can do so quickly and with great precision.

The basic structural feature of the lattices are solid ceramic strut elements with a square cross-section. The length of the struts was kept constant, while the relative densities of the samples were tuned by modifying the aspect ratio (thickness/length) of the struts within the unit cells.

“It is challenging to maintain fidelity of the complex 3D microscale features in polymer-derived ceramics. What we found is that, through tailoring preceramic monomers, we could achieve precision manufacturing of the ceramic,’’ Zheng says.

Polymer-derived ceramics start as preceramic organosilicon monomers—in the present study, vinyl- and thiol-siloxanes. When cured with near-UV light, these monomers are capable of forming 3D polymer structures with sub-10 μm features—like the gear teeth in (d) in the Figure. The researchers had to orchestrate the parameters for the printing very carefully to obtain the final complex cellular lattice architectures.

After printing the 3D lattices, with relative densities ranging from 1% to 22 %, compression tests were performed. “We found that smaller lattices are always stronger than bigger lattices. This also leads to [a] strength benefit when reducing the density, through incorporating smaller and slender ceramic struts, because the strength-to-density ratio in these materials has become a constant (or even slightly increased) due to the size effects,” Zheng says.

“This work has extended our knowledge of ceramic lattice mechanical properties and how to manipulate them through size effects. It also is a first step towards making fine scale ceramic lattices for high temperature applications practical,” says Christopher Spadaccini, director of the Center for Engineered Materials, Manufacturing and Optimization at Lawrence Livermore National Laboratory, who was not involved in the study.

Marcus Worsley of Lawrence Livermore National Laboratory who has created 3D graphene aerogels with designed macroscopic architectures, and also not participating in the research, says, “I think the work that Prof. Zheng is doing to push the resolution limits of 3D printing in functional ceramics is exciting. It is encouraging to see the potential enhancements in mechanical properties that can be achieved with these microarchitectured structures.” Worsley thinks that this research follows a trend that should continue in terms of expanding the materials available and minimizing the feature sizes attainable with additive manufacturing technology.

“We are now working on configuring the high temperature ceramic matrix composites to achieve tailorable thermal-mechanical properties as well as studying and designing their functionalities (resistivity, dielectric properties, etc.),” Zheng says. According to Zheng, future manufacturing can achieve direct digital fabrication of high temperature ceramic components and lightweight ceramics that can survive harsh environment, as well as direct fabrication of functional dielectric device.  

Read the abstract in the Journal of Materials Research.