Richard D. James from the University of Minnesota works as a Mercator Fellow on shape memory materials at Kiel University
Often stents are used for the treatment of circulatory disturbances of the heart. These ultrathin metallic tubes keep the vessels open and the blood flowing. In many cases, they are made out of shape memory alloys that can return to a “memorized” original state after deformation. This way, a stent inserted into a tiny plastic tube at the end of guidewire can expand to the shape of a stent when deployed in a coronary artery. These superelastic materials used in numerous medical and industrial applications require a sufficient lifetime.
As a Mercator Fellow of the DFG Richard D. James from the University of Minnesota (right) will investigate shape memory materials at Kiel University together with Professor Eckhard Quandt.
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Ceramics are considered by most researchers to be far too brittle to serve as shape memory materials. With the support of a high-risk Reinhart Koselleck-Project of the German Research Foundation (DFG) Professor Eckhard Quandt, from Kiel University and Professor Richard D. James from the University of Minnesota believe that this conventional wisdom may be unjustified. Now Quandt has won a Mercator Fellowship of the DFG for James to expand on this work. The internationally renowned expert on shape memory materials will work on the project for three years. Together they want to drive further the development of new materials for other applications in medicine, industry and energy.
Triggered by temperature and stress shape memory materials can change between different states. Thereby they pass several crystalline phase transformations. Since these are reversible, these materials can return to former states. "I want to understand the full set of physical principles that govern reversibility and how these phases can fit together in many ways without stressed transition layers", outlines Professor Richard D. James from the Department of Aerospace Engineering Mechanics at the University of Minnesota. James and Quandt call this characteristic of some phases "Supercompatibility". They have studied it with the focus on shape memory materials for some years and now they want to intensify their cooperation.
"In our current project, we want to figure out the range of factors that influence the lifetime of shape memory materials and how we could optimize them. This way we could develop new materials that allow new applications in medicine and industry", Quandt, Professor for Inorganic Functional Materials, explains. In his Reinhart Koselleck-Project on Crystallographically Compatible Ceramic Shape Memory Materials he is working on shape memory materials made of ceramics that could be especially advantageous: Unlike metal materials, they could also be used at high temperatures, for examples as an actuator in engines. However, their phase transformations are not reversible enough so far, so more research is necessary.
Another main topic of the research cooperation of James and Quandt on shape memory materials is the phenomenon hysteresis: when you cool a material it transforms at one temperature and when you heat it back up again it transforms at a higher temperature. It has been known for a while that the hysteresis can be nearly eliminated if you satisfy a certain condition on the transformation matrix. Quandt and James conjecture that this works even better when supercompatibility is fulfilled. During the term of the Mercator fellowship they plan to examine theories like that.
A much-noticed paper in the leading journal Science was the starting point for closer collaborations between James and Quandt. Here Quandt reports on an ultralow-fatigue shape memory alloy film system based on TiNiCu that allows at least 10 million transformation cycles. His working group was successful in developing a process to develop well-defined pure polycrystalline materials, that can also be used for shape memory materials. “With high-quality materials you can study each factors’ impact on its fatigue much more precisely”, Quandt explains. The possibility to produce high-quality materials by thin film technology has already spawned a successful spin-off company.
Mercator Fellow James, one of the international leading theorists for shape memory materials, is now collaborating with the project. During the last years some joint publications have already been published, also in cooperation with the Research Training Group 2154 “Materials for Brain” at Kiel University. Both scientists are optimistic, they can take this research area one major step forward.