Like turning coal to diamond, adding pressure to an electrical material enhances its properties. Now, University of Illinois at Urbana-Champaign researchers have devised a method of making ferroelectric thin films with twice the strain, resulting in exceptional performance.
Led by Lane Martin, a professor of materials science and engineering, the group published its results in the journal Advanced Materials.
Ferroelectric materials, metal oxides with special polarization properties, are used in a number of advanced electronics applications. When electricity is applied, they can switch their polarization, or the direction of their internal electric field, which makes them useful in devices such as computer memories and actuators. Ferroelectric materials are especially useful in aerospace applications because they are less susceptible to radiation than traditional semiconductors.
Strain in these materials can alter their properties and improve their performance. A lot of research in ferroelectric materials has focused on making strained thin films with alternating layers only a few nanometers thick of materials with slightly different crystal structures.
“It turns out that if you put pressure on certain types of materials, the properties completely change,” Martin said. “In our case we administer pressure by straining or stretching thin versions of these materials like one would stretch plastic wrap to fit on a bowl. You can induce things that don’t exist at ambient conditions; you can make phases and properties that don’t exist.”
The films are made of lead zirconate titanate (commonly called PZT). The relative amounts of zirconium (Zr) and titanium (Ti) in the films determine the shape of the crystals. Traditionally, films of PZT have been made up of a single composition, grown on a substrate with a slightly different crystal structure to cause strain in the PZT. However, too much strain causes the PZT to revert to its original crystal structure. This limits researchers’ ability to change the properties of these materials for better device performance.
The Illinois researchers overcame this limitation by gradually shifting the concentrations of Zr and Ti as they grew the thin films, incrementally changing the crystal structure. From layer to layer, the structures are very similar, yet the composition of the PZT at the top and bottom of the film is very different, transitioning from a PZT composition with 80% Zr to 80% Ti. This gradual change, instead of the usual layered approach, results in little localized strain but large overall strain.
“We have taken a material with similar mechanical properties to a dinner plate, the same kind of hardness, and effectively figured out a way to stretch that plate without breaking it,” Martin said. “With our method, we’ve been able to extend our ability to strain these materials. We go to the nanoscale so we can pull on these films and dramatically change the shape, and that affects the properties.”