LOTS OF POTENTIAL: This high-resolution transmission electron microscopy image depicts the phase boundary in bismuth ferrite with [left to right] areas with tetragonal, rhombohedral and tetragonal structure. Image: © SCIENCE/AAAS
Gadget makers often rely on piezoelectricity—the ability that some solids have to produce voltage when pressure is applied to them—to power tiny embedded systems, such as a BlackBerry Storm 2's touch screen or a car's airbag sensor. Whereas lead-based compounds typically have the greatest piezoelectric potential, the heavy metal has fallen out of favor as device-makers push to eliminate it from all electronics in an attempt to reduce toxic waste.
"Driven by global environmental concerns, there is currently a strong push to discover practical lead-free piezoelectrics for device engineering," a team of University of California, Berkeley, researchers posit in a study to be published Friday in Science. The researchers report finding a viable alternative in specially prepared bismuth ferrite film that has piezoelectric properties on par with lead-based compounds.
Finding alternatives to lead has not been easy. Although the European Union's (E.U.) Restriction on the Use of Certain Hazardous Substances (RoHS) went into effect in 2006—effectively banning new electronics (pdf) that contain certain levels of lead, cadmium, mercury and other toxic chemicals—lead has been difficult to phase out due its versatility. (It serves as an ingredient in solder and plastic, for example.) In fact, lead in electronics is so pervasive that the RoHS had to include a list of exemptions to the metal's ban, most notably in piezoelectronic devices (pdf).
Although researchers have known for years about bismuth ferrite's piezoelectric properties, it could not be made to produce enough voltage to be considered as a replacement for lead, says Ramamoorthy Ramesh, a professor of physics and of materials science and engineering at U.C. Berkeley who contributed to the research led by Robert Zeches, one of Ramesh's graduate researchers at Berkeley's Department of Materials Science and Engineering. "No one could control the structure of bismuth so that it could perform as well as lead and lead-based compounds as a piezoelectric substance," he adds.
By experimenting with bismuth ferrite films of different thicknesses grown on different types of substrates, Zeches was able to create a film that could generate more piezoelectric "strain"—measured in terms of charge generated—than previously possible. The key was finding a thickness-substrate combination that caused the film to form with the right mixture of tetragonal- and rhombohedral-shaped crystals. (Tetragonals are characterized by having three axes at right angles to one another, two equal in length and the third a different length; rhombohedrals resemble a cube stretched or flattened along one diagonal axis.) Piezoelectric strain is created when pressure is applied to bismuth ferrite, forcing its tetragonal crystals to change shape into rhombohedrals (and vice versa). As strain is increased more electricity is produced by the piezoelectric effect. "Robert took a big step forward by finding the right combination of bismuth ferrite crystal shapes to boost the amount of strain produced," Ramesh says.
The researchers are now trying to determine which factors—substrate type, film thickness or crystal composition and position—influence the film's piezoelectric potential the most, Zeches says. "The sweet spot is in intermediate thickness films," he adds, "which is where you get the mixed crystal formations that have the greater piezoelectric effect."
The Berkeley research is too early in its development to determine when bismuth ferrite-based materials might be incorporated into consumer electronics or how it might affect cost. Most likely, continued success with the compounds would more immediately result its use in nanoscale data storage devices.