The technological future of everything from cars and jet engines to oil rigs, along with gadgets and appliances, will depend on microscopic sensors.
The problem is such sensors are predominantly formed of silicon, which has various drawbacks. Kevin J. Hemker, Materials Scientist and Mechanical Engineer at Johns Hopkins University, has headed a group of Researchers who have, at present, reported favorable results in synthesizing an innovative material that assists in making sure that these sensors, or microelectromechanical systems (MEMS), can fulfill the requirements of the upcoming technological frontier.
“For a number of years, we’ve been trying to make MEMS out of more complex materials,” that are highly resistant to damage and superior in conducting electricity and heat, stated Hemker, the Alonzo G. Decker Chair in Mechanical Engineering at the Whiting School of Engineering. Hemker collaborated with a team of Post-doctoral Fellows, Students, Faculty, and Research Scientists at Whiting. The outcomes of their successful experiments have been published in the latest issue of the Science Advances journal.
In many of the MEMS devices, the internal structures are smaller when compared to the width of a human hair strand and made of silicon. Although these devices operate optimally under average temperature conditions, even a moderate heat of around 200 degrees can lead to loss of their strength and their capacity to conduct electronic signals. Moreover, silicon is highly brittle and can break easily.
For these reasons, while silicon has been the heart of MEMS technologies for several generations now, the material is not ideal, especially under the high heat and physical stress that future MEMS devices will have to withstand if they are to enable technologies such as the internet.
“These applications demand the development of advanced materials with greater strength, density, electrical and thermal conductivity,” that can retain their shape and can be synthesized and shaped at the microscopic scale, explained the Authors of the paper. “MEMS materials with this suite of properties are not currently available.”
The constant search for innovative materials led the Scientists to choose combinations of metal including nickel, normally used in state-of-the-art structural materials. For instance, nickel-based superalloys are used for producing jet engines. Taking into account the high requirement for dimensional stability, the research team experimented by adding the metals tungsten and molybdenum to limit the degree to which pure nickel expands under heat.
In a piece of equipment about the size of a refrigerator in a laboratory at Johns Hopkins University, the Researchers struck the alloys with ions in order to vaporize them into atoms, thus depositing them on a substrate or a surface. The result was a film that could be peeled, thus synthesizing freestanding films that had a median thickness of 29 mm, which is less when compared to that of a strand of human hair.
These discrete alloy films were found to have exceptional characteristics. Upon being pulled, the films exhibited high tensile strength (i.e. the ability to retain their shape without being deformed or broken) which was three times more than that of high-strength steel. Although a handful of materials have similar tensile strength, they do not withstand at higher temperatures or cannot be molded into MEMS components without difficulty.
We thought the alloying would help us with strength as well as thermal stability. But we didn’t know it was going to help us as much as it did.
Kevin J. Hemker, Materials Scientist and Mechanical Engineer, Johns Hopkins University
According to Hemker, the exceptional tensile strength of the material is attributable to its atomic-scale arrangement of the internal crystal structure of the alloy. The structure renders the material with high strength and also does not hinder the ability of the material to conduct electricity.
The structure, “has given our films a terrific combination, [a] balance of properties,” stated Hemker.
Apart from being mechanically and thermally stable, the films can also resist higher temperatures. The Researches in the team are preoccupied with planning the subsequent course of development, which is, molding the films into MEMS components. According to Hemker, the research team has filed a provisional patent application for the new material.
Timothy P. Weihs, Professor of Materials Science and Engineering; Jessica A. Krogstad, Gi-Dong Sim, and K. Madhav Reddy, who were Post-doctoral Fellows during various stages of the project; Research Scientist Kelvin Y. Xie, and current Graduate Student Gianna Valentino are the other Researchers involved in the study.