A star in the world of ceramic engineering

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Image: Ling Li with starfish skeleton and 3D printed scale model. Photo by Alex Parish of Virginia Tech.
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Credits: Virginia Tech

Compared to metal and polymer-based materials, ceramics can withstand high temperatures and corrosive environments, but their brittle nature makes them vulnerable to breakage. This behavior can cause problems for innovators trying to create lightweight porous versions of these materials and explains why ceramic foam is not normally used as a structural component.

Video, starfish skeleton and ceramics: https: //video.vt.edu/media/Ling%20Li%3A%20Starfish%20skeletons%20and%20ceramics/1_5d5abu01

Faced with the daunting task of developing lightweight, high-strength ceramic materials, mechanical engineering assistant Ling Li sought design inspiration from a surprising collaborator, the hump-shaped starfish of the tropical Indo-Pacific. .. By investigating the complex and highly ordered mineralized skeletal system of this rare marine species, Li and his team have properties that could lead to the development of a whole new class of high-performance lightweight ceramic composites. I found an unexpected combination of. Science magazine A recent cover story featured their discoveries.

Being porous makes it lighter

Industries such as the automotive and aerospace manufacturing industries are keenly interested in designing powerful and lightweight materials that combine the economics and strength of better fuel efficiency. The industry finds this balance difficult because stronger materials generally have higher densities and are therefore heavier.

Through millions of years of evolution, nature has come up with creative ways to solve this problem. That is to use a porous material. The introduction of internal porosity can create very lightweight and mechanically efficient materials.

Some examples of porous materials exist in nature. These include the human skeleton system, plant stems, and honeybee urticaria. If you place these natural materials under a microscope, you will immediately see that they are filled with small gaps and chambers. Natural growth forms these porous biological structures very efficiently, which often results in unexpectedly complex internal shapes.

In the laboratory of biological and biologically inspired materials, Li and his team aim to develop new material design principles to address the mechanical weaknesses of ceramic foam and building materials. We are investigating lightweight ceramic structures.

“Our overall goal is to learn from nature, get inspiration and develop new porous materials,” says Li. “Nature offers many excellent material lessons for designing porous materials with both strength and damage tolerance.”

Earlier, the team discovered that the unique chamber-based bioceramic construction of the instep (the internal skeleton of the squid) allows for control of buoyancy while at the same time being strong, stiff and resistant to destruction. This project and other similar projects motivated the team to explore additional applications for natural porous design on a microscale.

Starfish Skeleton: Naturally Designed Ceramic Lattice

In this work, Lee and his team turned to the skeleton of a humped starfish. Widely distributed throughout the Indo-Pacific region, this type of dry skeleton is often used for home decor. These starfish are characterized by conical protrusions that rise from the dorsal surface, discouraging predators.

While observing samples of these starfish skeletons at the Nanoscale characterization and Manufacturing Laboratory (NCFL), Li and Ph.D. students Ting Yang (co-lead author of the paper and now at the Massachusetts Institute of Technology) Doctor of Philosophy) made intriguing observations. On the microscale, the starfish skeleton showed a very regularly arranged lattice structure. A branch that is quite different from the previously studied porous structure of the instep and sea urchin spines. In fact, the unique skeletal tissue of this starfish shows the highest structural regularity ever reported from this group of invertebrates. Such a regular grid structure shows a striking similarity to the space frame truss structure commonly used in modern human construction projects.

The team wondered how this natural ceramic lattice material achieved mechanical protection because the starfish skeleton is made of calcite, which is a crystalline form of calcium carbonate (chalk). Any child who is accustomed to playing outside knows that sidewalk chalk is very fragile and fragile. However, the starfish body shows high strength and flexibility. Clarifying the underlying principles of this structure may help solve the challenge of making stronger porous ceramics.

What the team found was unexpected. Like other starfish species, the hump-shaped star skeleton is made up of many millimeter-sized skeletal elements called ossicles. These ossicles connect with soft tissues and allow the animal to move flexibly. Li and his team found that each ossicle is composed of branches connected via nodes with veins similar to the structure of the Eiffel Tower, and is composed of a mathematically uniform microlattice structure. I found. More interestingly, the team discovered that the uniform structure of the microlattice is essentially a single crystal structure at the atomic level due to the arrangement of its atoms.

“This unique material is like a periodic grid carved from a single crystal of calcite,” Li said. “This near-perfect microlattice has never been reported in nature or manufactured synthetically. Most very regular grid materials combine materials with small crystals to create composites. Made by doing, but this is new. It has grown as a single piece. “

This structure allows starfish to strategically strengthen their skeleton and enhance protection in specific directions. In addition, animals can thicken branches along selected directions and specific areas, and the human body has the ability to change the local shape of porous bone to adapt to physical activity. It seems that the mechanical performance can be improved in a similar way. In starfish, researchers also found areas where the structure appeared to change the regular grid pattern of its design. This is a function that suppresses the expansion of cracks when the microlattice is destroyed.

Patricia Dove, a biomineralization expert, a prominent university professor, and a CP Mile Science professor at Virginia Tech’s Faculty of Earth Sciences, said this biological discovery was in the field of biologically inspired innovation. He said it could have a big impact.

“Starfish and other echinoderms living in a highly predatory seafloor environment reveal a world of material innovation that is essential for survival,” Dub said. “Biology directs the formation of notable skeletons such as starfish, using only seawater and some organic constituents. This new study of the properties of the underlying mechanical engineering is a new material design. It has great potential as a frontier. ”

What’s next?

Knowing the architecture of natural ultrastructure meant a big step forward, but Li and his team asked more questions. Was there a key to how creatures could grow their skeleton? Could it shed some light on how to reproduce them?

Li and his collaborators used 3D printing to model and generate large versions of these complex lattice structures for both research and educational purposes. This is an approach that helps you understand the complexity of these unique geometries. The 3D printing model created by Li’s team was certainly visually exciting, but the technology needed to bring a new, more powerful ceramic microarchitecture to market is still in the future. Currently, 3D printers produce structures at the micrometer level, but ceramic printing requires firing of the final product, which can result in a large number of uncontrolled small pores and cracks. These imperfections make the structure very fragile. Li hopes that continued progress in the field of 3D printing and a further understanding of the formation mechanisms of biological structures such as starfish skeletons will ultimately provide a solution.

“Nature can assemble mineral precursors to form complex structures at room temperature and ambient pressure,” says Li. “That’s something that modern human technology can’t achieve today. Virginia Tech has a keen research interest in the mineral structures found in nature, and one day this exciting research direction was inspired by a wide range of organisms. We hope that it will lead to development, stronger and lighter materials. “

Other authors of this paper include Virginia Tech graduate students Hongshun Chen, Zhifei Deng, Liuni Chen, postdoctoral fellow Zian Jia, Harvard University James C. Weaver, and Bowdoin College Emily Peterman.

Additional Information

This work was funded by the National Science Foundation, the Air Force Science Research Bureau, and the Institute for Applied Sciences for Critical Technology, Virginia Tech.

Journal cover:

article: Hump starfish damage tolerance, dual scale, single crystal microlattice, Kobu starfish.. Chemistry (2021). Doi: 10.1126 / science.abj9472


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