Mechanical strain boosts lead-free ferroelectrics

Ferroelectric materials are used in infrared cameras, medical ultrasounds, computer memory and actuators that turn electric properties into mechanical properties and vice-versa. Most of these essential materials, however, contain lead and can therefore be toxic.

Laurent Bellaiche, Distinguished Professor of physics at the University of Arkansas, said there has been a global initiative over the last 10 years to find ferroelectric materials that do not contain lead.

The atoms in a ferroelectric material can have more than one crystalline structure. Where two crystalline structures meet is called a phase boundary, and the properties that make ferroelectric materials useful are strongest at these boundaries.

Using chemical processes, scientists have manipulated the phase boundaries of lead-based ferroelectric materials to create higher performance and smaller devices. Chemically tuning the phase boundaries of lead-free ferroelectric material, however, has been challenging.

New research from a team that includes Bellaiche and fellow U of A physicists Kinnary Patel and Sergey Prosandeev found a way to enhance lead-free ferroelectrics using strain, or mechanical force, rather than a chemical process. The discovery could produce lead-free ferroelectric components, opening new possibilities for devices and sensors that could be implanted in humans.

The results were published in the journal Nature Communications.

What are ferroelectrics

Ferroelectric materials, first discovered in 1920, have a natural electrical polarisation that can be reversed by an electric field. That polarisation remains reversed even once the electric field has been removed.

The materials are dielectric, meaning they can be polarised by the application of an electric field. That makes them highly effective in capacitors.

Ferroelectrics are also piezoelectric, which means they can generate electric properties in response to mechanical energy and vice versa. This quality can be used in sonars, fire sensors, tiny speakers in a cell phone or actuators that precisely form letters in an inkjet printer.

All these properties can be enhanced by manipulating the phase boundary of ferroelectric materials.

“In a lead-based ferroelectric, such as lead zirconate titanate, one can chemically tune the compositions to land right at the phase,” Patel said.

Lead-free ferroelectrics, however, contain highly volatile alkaline metals, which can become a gas and evaporate when chemically tuned.

A new approach

The researchers instead created a thin film of the lead-free ferroelectric material sodium niobate (NaNbO3). The material is known to have a complex crystalline ground state structure at room temperature. It is also flexible. Scientists have long known that changing the temperature of sodium niobate can produce multiple phases, or different arrangement of atoms.

Instead of a chemical process or manipulating the temperature, the researchers changed the structure of the atoms in sodium niobate by strain.

They grew a thin film of sodium niobate on a substrate. The structure of the atoms in the sodium niobate contract and expand as they try to match the structure of the atoms in the substrate. The process creates strain on the sodium niobate.

“What is quite remarkable with sodium niobate is if you change the length a little bit, the phases are changing a lot,” Bellaiche said.

To the researchers’ surprise, the strain caused the sodium niobate to have three different phases at once, which optimises the useful ferroelectric properties of the material by creating more boundaries.

“What I was expecting, to be honest, is if we change the strain, it will go from one phase to another phase. But not three at the same time. This was an important discovery,” Bellaiche said.

The experiments were conducted at room temperature. The next step will be seeing if sodium niobate responds to strain in the same way at extreme temperatures ranging from –270 to 1000°C.

Image credit: iStock.com/YouraPechkin