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Behaviour of ‘slush like’ smart materials explained

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University of Pennsylvania researchers who carried out a detailed study into the unique behaviour of relaxor ferroelectrics found that they have “striking similarities” to water.

Piezoelectrity is the interchange of mechanical and electrical energy which occurs when an electric field is applied to a material. This causes the dipoles within it to reorient, and the material to change shape, causing vibrations. Piezoelectric materials are particularly useful in ultrasound technology, and are essential in the manufacture of sensors, transducers and other electrical components.

A better understanding of the properties of piezoelectric materials could lead to considerable improvement in the performance of electric devices. Relaxor ferroelectrics – a type of smart material – are especially effective piezoelectric materials, although it has never quite been understood why.

In order to enhance limited understanding of the material and its properties, a team of researchers led by Professor Andrew Rappe, a professor of chemistry at the University of Pennsylvania, set about forming the most comprehensive model of how these materials work.

“This allows us to observe behaviours that take a long time to happen or only happen deep inside a material, and this gives us unique perspectives on complicated behaviours,” said Professor Rappe.

Previously, it was assumed that dipoles in the material act independently of each other, making it easy for them to respond to external stimuli such as electric fields, However, the researchers have now shown that at cooler temperatures the dipoles form regions of alignment which result in them being separated by a ‘wall’. These walls lead to the material’s highly piezoelectric properties.

This mirrors the behaviour of water at cooler temperatures, when dipoles become more strongly aligned. The researchers describe the smart material as “slush like” in a Nature paper reporting their findings.

“It’s exciting to be able to build up a model from individual electrons up to millions of atoms at finite temperature and observe complex properties,” said Professor Rappe.

“And it’s exciting that observing those complex properties gives us new productive directions where we can enhance materials that will efficient convert energy for useful devices to help people.”

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