Explosive, weeping ceramics pave the way for a new shape-shifting material

An international team of researchers from the University of Minnesota Twin Cities and the University of Kiel in Germany have discovered a pathway that could lead to shape-changing ceramic materials. This discovery could improve everything from medical devices to electronics.

The research is published in open access in Naturethe first multidisciplinary scientific journal in the world.

Anyone who has ever dropped a coffee cup and seen it shatter into several pieces knows that ceramic is brittle. Subject to the slightest deformation, they burst. However, ceramics are not only used for tableware and bathroom tiles, they are used in electronics because, depending on their composition, they can be semiconducting, superconducting, ferroelectric or insulating. Ceramic is also non-corrosive and used in the manufacture of a wide variety of products, including spark plugs, fiber optics, medical devices, space shuttle tiles, chemical sensors, and skis.

At the other end of the material spectrum are shape memory alloys. They are among the most deformable or remodelable materials known. Shape memory alloys rely on this tremendous deformability when functioning as medical stents, the backbone of a vibrant medical device industry in both the Twin Cities region and Germany.

The origin of this shape-changing behavior is a solid-to-solid phase transformation. Unlike the crystallization-fusion-recrystallization process, solid-crystalline solid transitions take place only in the solid state. By changing temperature (or pressure), a crystalline solid can be transformed into another crystalline solid without entering a liquid phase.

In this new research, the path to producing a reversible shape-memory ceramic was anything but simple. The researchers first tried a recipe that worked for the discovery of new shape-memory metallic materials. This involves delicately adjusting the distances between the atoms by compositional changes, so that the two phases fit together well. They implemented this recipe, but instead of improving the deformability of the ceramic, they observed that some specimens exploded when they went through the phase transformation. Others gradually crumbled into a pile of powder, a phenomenon they called “crying”.

This video shows a sample of ceramic material whose composition is tuned to have excellent compatibility between phases, but poor compatibility at grain boundaries. It explodes while going through the phase transformation. Credit: Jascha Rohmer, University of Kiel

This video shows a sample whose composition is tuned to have excellent phase compatibility, but poor grain boundary compatibility. It gradually collapses at the grain boundaries while going through the phase transformation. A phenomenon that the authors call “crying”. Video credit: Jascha Rohmer, University of Kiel

With yet another composition, they observed a reversible transformation, easily transforming back and forth between phases, much like a shape-memory material. The mathematical conditions under which the reversible transformation occurs can be widely applied and open the way to paradoxical ceramics with shape memory.

This video shows a sample of ceramic material that morphs back and forth between phases, much like a shape memory material. The mathematical conditions under which the reversible transformation occurs can be widely applied and open the way to paradoxical ceramics with shape memory. Video credit: Jascha Rohmer, University of Kiel

“We were quite surprised with our results. Shape-memory ceramics would be an entirely new type of functional material,” said Richard James, study co-author and McKnight University Professor Emeritus in the Department of Aerospace Engineering Mechanics at the University of Minnesota. “There is a great need for shape memory actuators that can operate in high temperature or corrosive environments. But what excites us most is the prospect of new ferroelectric ceramics. In these materials, phase transformation can be used to generate electricity from small temperature differences.

The German team was responsible for the experimental part and the chemical and structural investigation at the nanometric scale.

“For the explanation of our experimental finding that, contrary to expectations, ceramics are extremely incompatible and either explode or decompose, the collaboration with Richard James’ group at the University of Minnesota was very valuable,” explains Eckhard Quandt, co-author of the study and professor at the Institute of Materials Science at the University of Kiel. “The theory developed on this basis not only describes the behavior, but also shows the way to obtain the desired compatible shape memory ceramics.”

James also highlighted the importance of the collaboration between the University of Minnesota and the University of Kiel.

“Our collaboration with Eckhard Quandt’s group at the University of Kiel has been extremely productive,” added James. “As with all of these collaborations, there’s enough overlap that we communicate well, but each group brings a lot of ideas and techniques that expand our collective ability to discover.”

In addition to James and Quandt, the research team included Lorenz Kienle from the University of Kiel Andriy Lotnyk from the Leibniz Institute for Surface Engineering, and graduate students Hanlin Gu, Jascha Romer and Justin Jetter.

The researchers were supported by the U.S. National Science Foundation, a Vannevar Bush School Fellowship in “Mathematical Design of Materials” from the U.S. Department of Defense, a Multidisciplinary University Research Initiatives (MURI) grant from the Office of Naval Research, a Mercator Fellowship from the German Research Foundation and the Reinhart Koselleck Project from the German National Science Foundation.

To read the full research paper titled “Exploding and weeping ceramics”, visit Nature website.

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