SPMS Prof Fan Hong Jin (left) with PhD student Mr Hu Yuzhong, both holding the new piezoelectric crystal which can flex up to 40 times more than conventional ferroelectric crystals when electricity is applied.
An international team led by researchers from Nanyang Technological University, Singapore (NTU Singapore) has developed a new material that, when electricity is applied to it, flexes and bends forty times more than its competitors, opening the way to better micro machines. Conversely, bending the material generates electricity very efficiently, opening the door to energy-harvesting applications such as recharging gadgets using body movements.
The novel material is both “electrostrictive” and “piezoelectric.” The electrostrictive property means that it can change shape when an electric current is applied: when an electric field is applied, the atoms in the material shift and cause the material to deform. The piezoelectric property, on the other hand, means that the material converts pressure into electric charge. These two properties can be used in different types of applications.
The latest breakthrough was announced in a paper published in the scientific journal Nature Materials in January 2021, authored by Professor Fan Hong Jin from the School of Physical and Mathematical Sciences (SPMS), his PhD student Mr Hu Yuzhong (the first author of this paper), Professor Junling Wang from the Southern University of Science and Technology, China (formerly a faculty member at NTU’s School of Materials Science and Engineering), and other researchers.
The scientists found that applying an electric field to the new material deforms or “strains” it by up to 22 per cent, the highest strain reported in a piezoelectric material so far. This far surpasses conventional piezoelectric materials, which only deform up to 0.5 per cent. The new material is also more energy-efficient than other piezoelectric and electrostrictive materials.
Piezoelectric materials are used in guitars, loudspeakers, sensors and electric motors. For instance, a piezoelectric pick-up is a device used in an electric guitar to convert the vibrations from the strings into an electric signal, which is then processed for music recording or to be amplified through loudspeakers. A common class of piezoelectric materials are ferroelectric crystals, which were first discovered in 1920 and are easily integrated into electrical devices. However, these materials are brittle and inflexible, bending by only about 0.5 per cent, which limits their applications to certain electronic devices.
Some ferroelectrics also contain lead, which is toxic. The presence of lead in piezoelectric devices is one reason why electronic waste is challenging to recycle. Such materials are likewise unsuitable for use in flexible electrical devices that may come in contact with the skin, such as wearable biomedical devices that track heart rate. By contrast, the new material developed by Professor Fan and his team does not contain lead.
“Being more than 40 times more flexible than similar electrostrictive materials, our new ferroelectric material may be used in actuator-type devices that flex when an electric field is applied,” said Professor Fan. “With its superior piezoelectric properties, the material can also be used in mechanical devices that harvest energy when bent, which may be useful for recharging wearable devices.”
Developing a flexible ferroelectric material
To develop a flexible ferroelectric material, the researchers modified the chemical structure of a hybrid ferroelectric compound called 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 makeup of the compound by substituting some of its chlorine (Cl) atoms for bromine (Br), which has a similar size to chlorine. This weakened the chemical bonds at specific points in the structure, making the material more flexible without affecting its piezoelectric qualities.
The new material 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.
“We think we can substantially improve on this performance in future by further optimising the chemical composition,” says Professor Fan. “We believe this type of material could play a key role in technology, such as the development of wearable Internet of Things (IOT) devices.”