The technological future of everything from cars and jet engines to oil rigs, as well as the gadgets, appliances and public utilities that comprise the internet of things, will depend on microscopic sensors.

The trouble is these sensors are mostly made of silicon, which has its limits. Now, a team of materials scientists led by Kevin Hemker at Johns Hopkins University has succeeded in developing a new material that promises to help ensure that these sensors, also known as microelectromechanical systems (MEMS), can continue to meet the demands of the next technological frontier.

"For a number of years, we've been trying to make MEMS out of more complex materials" that are more resistant to damage and better at conducting heat and electricity, explained Hemker, chair in mechanical engineering at the Whiting School of Engineering. Hemker worked with a group of students, research scientists, post-doctoral fellows and faculty at Whiting. The results of their successful experiments are reported in a paper in Science Advances.

Most MEMS devices have internal structures smaller than the width of a strand of human hair and shaped out of silicon. These devices work well in average temperatures, but even modest amounts of heat – a couple of hundred degrees – causes them to lose their strength and their ability to conduct electronic signals. Silicon is also very brittle and prone to breaking.

For these reasons, while silicon has been at 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 be utilized in technologies such as the internet of things.

"These applications demand the development of advanced materials with greater strength, density, electrical and thermal conductivity" that hold their shape and can be made and shaped at the microscopic scale, the authors wrote in the paper. "MEMS materials with this suite of properties are not currently available."

The pursuit of new materials led the researchers to investigate nickel-containing metal alloys, which are commonly used as advanced structural materials: nickel-based superalloys, for example, are used to make jet engines. Considering the need for dimensional stability, the researchers experimented with adding molybdenum and tungsten to nickel in hopes of curbing the degree to which pure nickel expands in heat.

"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 Hemker, Johns Hopkins University

In a piece of equipment about the size of a refrigerator in a laboratory at Johns Hopkins, the team hit targets with ions to vaporize the alloys into atoms, depositing them onto a surface or substrate. This created a film that can be peeled away, thus creating freestanding films with an average thickness of 29µm – less than the thickness of a human hair.

These freestanding alloy films exhibited extraordinary properties. When pulled, they showed a tensile strength – meaning the ability to maintain shape without deforming or breaking – three times greater than high-strength steel. While a few materials have similar strengths, they either do not hold up under high temperatures or cannot be easily shaped into MEMS components.

"We thought the alloying would help us with strength as well as thermal stability," said Hemker. "But we didn't know it was going to help us as much as it did."

He said the remarkable strength of the material is due to atomic-scale patterning of the alloy's internal crystal structure, which strengthens the material and has the added advantage of not impeding the material's ability to conduct electricity. This structure "has given our films a terrific combination, [a] balance of properties," Hemker said.

The films can withstand high temperatures and are both thermally and mechanically stable. Team members are now busy planning the next step in development, which involves shaping the films into MEMS components. Hemker said the group has also filed a provisional patent application for the alloy.

This story is adapted from material from Johns Hopkins University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.