From left, Dr Benjamin M. Gallant, Dr Dominik J. Kubicki and Dr Shrestha Banerjee in front of a solid-state NMR instrument in the Molecular Sciences Building at the University of Birmingham.
A UK research team has unveiled a soft, lead-free piezoelectric material that converts motion into electricity. Its efficiency is comparable to commercial lead-based ceramics, while being easier to process and far less toxic.
The hybrid material, based on bismuth iodide, is described in a new paper in the Journal of the American Chemical Society. Researchers at the University of Oxford, the University of Birmingham and the University of Bristol developed the material, which generates an electric charge when it is pressed, bent or otherwise deformed. It can also move when an electric field is applied.
Piezoelectrics sit inside autofocus systems, inkjet print heads, ultrasound transducers, car airbag sensors and an array of wearable and IoT devices that scavenge energy from motion. The global piezoelectric materials market is estimated at more than $35 billion and is growing, driven by automotive sensing, medical devices, industrial robotics and consumer electronics.
The problem is that the workhorse material in this space, lead zirconate titanate (PZT), is roughly 60% lead by composition. That creates tension with tightening environmental and worker-safety regulations, especially as piezo devices proliferate. The new material contains no lead and can be made at room temperature rather than the roughly 1,000 C° processing temperatures often used for PZT ceramics.
“With performance comparable to commercial piezoelectrics but made from non-toxic bismuth, this discovery is a new pathway toward environmentally responsible technologies that can power sensors, medical implants and flexible electronics of the future,” said Dr. Dominik Kubicki of the University of Birmingham in a release.
Hybrid halobismuthate with a twist
A crystal of the newly discovered piezoelectric material viewed under a microscope.
The material is an organic inorganic “halobismuthate” built around bismuth iodide. It is soft and mechanically flexible, yet still delivers a strong piezoelectric response.
Piezoelectric research has often faced a trade-off. Rigid ceramics such as potassium sodium niobate (KNN) offer high efficiency but are brittle and difficult to process, while soft polymers such as PVDF are flexible but generate relatively weak electrical charges. The new halobismuthate helps bridge that gap, offering the mechanical compliance needed for next generation wearables while approaching the energy conversion performance typically associated with rigid, high temperature ceramics.
Lead author Dr. Esther Y. H. Hung of the University of Oxford’s Department of Physics said the key was engineering a very specific kind of structural instability in the crystal.
By tuning how the organic and inorganic building blocks interact through halogen bonding, the team created a phase that is close to a symmetry-breaking transition. That balance between order and disorder appears to boost the piezoelectric effect.
“By fine-tuning the interactions between the organic and inorganic components, we were able to create a delicate structural instability that breaks symmetry in just the right way,” Hung said. “This interplay between order and disorder is what gives the material its exceptional piezoelectric response. It is a different approach to piezoelectricity than in traditional materials such as lead zirconate titanate, and that is what has led to these big improvements.”
Probing structure from crystal to atoms
Understanding how and why the material works required structural probes across multiple length scales.
rom left, Dr Benjamin M. Gallant, Dr Esther Y.H. Hung and Dr Harry C. Sansom conducting single crystal X-ray diffraction measurements on the new piezoelectric material using synchrotron radiation at the Diamond Light Source, Oxfordshire, UK.
Researchers at the University of Birmingham used single-crystal X-ray diffraction to map the atomic arrangement and track how it changes as the structure distorts. Solid-state nuclear magnetic resonance (NMR) measurements then provided complementary insight into the local chemical environment and dynamics.
“As an early career researcher, it is exciting to participate in research with the power to transform our society. Almost every device we use in our daily lives contains piezoelectrics,” said Dr. Benjamin Gallant of Birmingham, who led the NMR work.
The group also used synchrotron X-ray diffraction at the Diamond Light Source in Oxfordshire to study the crystals under high-intensity beams, allowing more precise structural models of the new phase.
That combination of techniques let the team link specific bonding motifs and structural instabilities to the measured piezoelectric response. The same design principles could carry over to other organic inorganic materials that mix molecular building blocks with inorganic frameworks.
Because the new material is processed at room temperature from solution, it could be compatible with flexible substrates and roll-to-roll manufacturing, which are attractive for wearables and large-area sensor sheets. It may also simplify integration with temperature-sensitive electronics or polymers.
The work was jointly supervised by Professor Henry Snaith (Oxford), Dr. Harry Sansom (Bristol) and Dr. Dominik Kubicki (Birmingham), bringing together expertise in new materials, crystal design and atomic level characterization.
