Flexible crystal enables efficient, bendable electronics

Researchers from Nanyang Technological University, Singapore (NTU Singapore) have developed a ferroelectric material that is both electrostrictive and piezoelectric. Its electrostrictive properties mean it can change shape when an electric current is applied, while its piezoelectric properties mean the material can convert pressure into electric charges. Their work has been described in the journal Nature Materials.

Ferroelectric crystals were first discovered in 1920 and have been used to make piezoelectrics for over 70 years, as they are easily integrated into electrical devices. However, they are brittle and inflexible, bending only 0.5%, which largely limits their application in electronic devices such as actuators (parts that convert an electric control signal into mechanical motion; for example, a valve that opens and closes).

Some ferroelectrics also contain lead, which is toxic, and its presence in piezoelectric devices is one of the reasons why electronic waste is challenging to recycle. Traditional ferroelectrics such as perovskite oxides are also unsuitable for flexible electrical devices that are in contact with the skin, such as wearable biomedical devices that track heart rate.

To develop a flexible ferroelectric material, the NTU researchers modified the chemical structure of a hybrid ferroelectric compound C₆H₅N(CH₃)₃CdCl₃, or PCCF in short, which can potentially bend up to a hundred times more than traditional ferroelectrics. To increase the material’s range of movement further, the scientists modified the chemical make-up of the compound by substituting some of its chlorine (Cl) atoms for bromine (Br), which has a similar size to chlorine, to weaken the chemical bonds at specific points in the structure. This made the material more flexible without affecting its piezoelectric qualities.

The scientists found that when an electric field is applied, the hybrid material can be strained up to 22% — the highest strain reported in a piezoelectric material so far. This far surpasses conventional piezoelectric materials that only deform up to 0.5% when a current is passed through them. The new material is also more energy efficient than other piezoelectric and electrostrictive materials and is easy to manufacture, requiring only solution-based processing in which the crystal forms as the liquid evaporates — unlike typical ferroelectric crystals that require the use of high-powered lasers and energy to form.

“Being more than 40 times more flexible than similar electrostrictive materials, the new ferroelectric material may be used in highly efficient devices such as actuators and sensors that flex when an electric field is applied,” said NTU Professor Fan Hong Jin. “With its superior piezoelectric properties, the material can also be used in mechanical devices that harvest energy when bent, which will be useful to recharge wearable devices.

“We think we can substantially improve on this performance in future by further optimising the chemical composition, and we believe this type of material could play a key role in the development of wearable devices for the Internet of Things (IoT), one of the key technologies enabling the Fourth Industrial Revolution.”

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