The technological future of everything from cars and jet engines to the gadgets, appliances, and public utilities constituting the Internet of Things will depend on microscopic sensors.
The trouble is, these sensors—also known as microelectromechanical systems (MEMS)—are mostly made of silicon, which has its limits. While these devices work well in average temperatures, 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 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, says Kevin J. Hemker, a professor in the Department of Mechanical Engineering. He led a team that recently reported success in developing such a material.
The team started by considering combinations of metal containing nickel, which is commonly used in advanced structural materials, including nickel-based superalloys used to make jet engines. Considering the need for dimensional stability, the researchers experimented with adding the metals molybdenum and tungsten in hopes of curbing the degree to which pure nickel expands in heat.
In a piece of equipment about the size of a refrigerator within a laboratory at Johns Hopkins, the team hit targets with ions to vaporize the alloys into atoms, depositing them onto a surface. This created a film that can be peeled away, thus creating freestanding films with an average thickness of 29 microns—less than the thickness of a human hair.
These freestanding alloy films exhibited extraordinary properties. When pulled, they showed a tensile strength three times greater than 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. But we didn’t know it was going to help us as much as it did,” says Hemker.
The remarkable strength of the material is due to atomic-scale patterning of the alloy’s internal crystal structure, he says. The structure strengthens the material and has the added advantage of not impeding the material’s ability to conduct electricity.
The research team, which reported its results in Science Advances, has shown that the films can withstand high temperatures and are both thermally and mechanically stable, and the researchers are busy planning the next step of development, which involves shaping the films into MEMS components. Hemker says the group has filed a provisional patent application for the alloy.