Tech

A new generation of lead-free piezoelectric materials could lead to greener actuators, sensors and even ultra-dense data storage. The new materials, which generate an electric field when squeezed, copy the crystal structure of conventional piezoelectric materials but use bismuth instead of lead.

Since the EU banned lead (see Uncovering the hazards in our electronic gadgets) from electrical devices in 2006, scientists and manufacturers have been scrambling to find alternatives to key materials that depend on the toxic metal for their function. In particular, piezoelectrics – materials that can convert mechanical stress into an electric field and vice versa – have proved challenging to replace. Now a new design, based on clever crystal geometry, could soon deliver comparable performance with less environmental impact.

The best-performing piezoelectric materials are currently made from lead zirconium titanate (PZT). This has a crystal structure consisting of atoms arranged in the shapes of squares and diamonds. An electric field causes the diamonds to flip into square shapes, changing the structure of the crystal and altering its overall length by up to 10 per cent.

Robert Zeches, Ramamoorthy Ramesh and colleagues at University of California, Berkeley have developed a way to reproduce this behaviour using layers of non-toxic bismuth ferrite, which naturally forms into a square crystal structure. They start with an electrically charged grid of squares that are slightly smaller than those usually formed by bismuth ferrite. They then deposit the bismuth ferrite onto the grid using an ultraviolet laser. As the film builds up, some of the squares maintain their shape, while others get skewed as they attempt to form the natural, larger crystal structure.

The result is a mixture of box and diamond shapes, which behave in similar way to those in PZT. In the presence of the electric field, the new material can change its length by around 1.5 per cent – not as much as PZT, but enough for many practical purposes. The team says these could include making ultra-dense data storage in which a bit is stored in a region of the crystal that is extended. The data would be written and read using an atomic force microscope tip.

"One of the drivers of this research is trying to find ferroelectric materials that perform on a par with lead-based compounds but that don't have lead in them," says Zeches.

"We want something that could be a viable replacement for what's in the field."

The team still needs to improve the material's piezoelectric response, says Muhtar Arhart, a materials researcher at the Carnegie/Doe Alliance Center in Washington DC, not involved with the research. "If it doesn't have lead but still has huge piezo properties, that would be very nice. Lead has all kinds of hazardous properties."

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