On the floor of 101 Olin Hall lays a long, ribbon-like sheet of metal, curling onto itself in a pile, shards peeling off. Above it looms a large machine that consists of, among other parts, a heating system, a crucible, and a metal casting wheel. In technical terms, it’s a modified planar-flow casting machine.
Two undergraduate researchers, Stwart Pena ‘17 and Oliver Lake ‘17, explain that the machine melts and superheats a solid block of metal, which they then release onto the spinning wheel using applied gas pressure. The wheel spins at speeds of around 10 meters per second (22 miles per hour) and is cold enough to solidify the metal. After three to five seconds of running, the result is the stretch of thin metal—known as glassy metal.
Why Glassy Metals?
Pena and Lake work under Paul H. Steen, Chemical and Biomolecular Engineering. Steen’s expertise is in examining the stability of interfaces, where two different states of matter meet.
In the case of the metal sheet, he and his students look at the metal-gas interface where molten metal comes in contact with ambient gas and the metal wheel. The goal is to understand the casting process in order to develop a way to make high-quality, consistent glassy metal ribbons at high production speeds in a manner that industry can later employ.
Why? Glassy metals are important because they enable efficient energy transfer. They’re used in transformer cores, like the ones seen on power poles, and can also be found in everyday items such as security tags on clothing and solid-state inductor devices. The material has unique energy properties thanks to its amorphous structure. In other words, the atoms inside the metal are disordered; they don’t partner up to form a crystalline structure as in conventional metals.
The lab members study the casting process by using the planar-casting machine and a proxy alloy that has the same production challenges that face glassy metal formation. The metal ribbon’s quality and thickness are determined by the crucible nozzle, the size of the gap between the nozzle and the wheel, the amount of pressure, and the speed of the wheel. Everything needs to be stable and consistent for a high-quality product. Instabilities often arise, however, from the liquid metal-gas interface.
“It’s got to go from liquid to solid and in that transition, there’s a little region where there’s a meniscus, where the surface tension holds the molten metal in,” Steen explains. “That’s subject to vibrations, natural oscillations, air entering it, which can lead to defects.”
In a project funded by the National Science Foundation, Steen and his researchers are characterizing the metal ribbon structure using x-ray diffraction and transmission electron microscopy and have come up with a mathematical model of the metal-on-metal contacting event.
Drops, Bubbles, and Bridges
Beyond the metal-gas interface, Steen’s lab also explores liquid-gas and liquid-solid interfaces, specifically in the form of drops, bubbles, and bridges.
Most recently, former graduate student Joshua Bostwick discovered solutions to a Schrödinger-like equation for the behavior of drops on a solid surface—important for such applications as atomizing droplets (for example, medical delivery, misting, spray paint) and understanding blood splatter.
The goal is to understand the casting process in order to develop a way to make high-quality, consistent glassy metal ribbons at high production speeds in a manner that industry can later employ.
“Up to now people have found ad hoc ways of thinking about droplet oscillations,” Steen says. “We found a way to put it on a real firm basis, much like the Schrödinger equation put the historically empirical chemical periodic table on a firm basis.”
In a controlled setting, Steen’s PhD student ChunTi Chang (now a postdoctorate at TU Dortmund University) oscillated tiny, one-millimeter diameter drops at different speeds using a shaker table. He then captured the drops’ natural shapes and frequencies and showed how they fit into a periodic-like table for droplets, which informs any work related to droplet motions.
One example of droplet motions is the work of graduate student Ashley Macner (now a scientist at Exxon-Mobil) on condensation and heat transfer. In collaboration with Susan Daniel, Chemical and Biomolecular Engineering, Macner looked at how condensing droplets on a surface gather, coalesce, and move. She developed a way to utilize condensation to transfer heat in low-gravity environments, such as space.
Macner, a former NASA Space Technology Research Grants Program researcher, found that heat transfer is dependent on the size and number of drops on a surface. Clearing larger drops from a surface as quickly as possible makes room for smaller drops, which in turn removes more heat.
“By understanding the process better, we can design technological solutions for the problem of getting heat out of devices or habitats where we don’t have gravity,” says Steen. Instead of depending on gravity to pull droplets off a surface, Macner chemically treated the surface to sweep droplets away to a surrounding gutter where they collected.
Steen says that his path to chemical engineering has been a huge amount of fun. Like his less-than-direct journey (Steen had early dreams of becoming a writer), his current work makes room for learning from informed experimentation.
Take the flawed, peeling, metal ribbon in 101 Olin. Pena and Lake made it by turning up the wheel speed by 50 percent to see what would happen. Upon seeing it, Steen says, “the thing is, we try to make mistakes. It’s a learning process.”