A possible paradigm shift in piezoelectricity – ScienceDaily
Piezoelectricity is used everywhere: watches, cars, alarms, headphones, instrument microphones, electric lighters and gas burners. One of the most common examples is probably the quartz watch, where quartz, a piezoelectric material, is a prerequisite for the watch to function.
Piezoelectric materials have the special property that their shape changes when an electric voltage is applied to the material. It also works the other way around: exposing them to mechanical impact will create electrical voltage.
Piezoelectricity is often used in sensors, actuators and resonators. In small devices, they are known as MEMS (micro-electromechanical systems). Here, materials other than quartz must be used. However, these materials often contain lead in the form of lead zirconate titanate (PZT).
This can prove to be an obstacle to the diffusion of technology, for example in the biomedical field, because lead is harmful to the organism. However, the researchers assess excellent potential for using the piezoelectric effect in a wider range of diagnostic, prognostic and therapeutic technologies if the lead could be removed.
In a new scientific article in the journal Science, Professor Nini Pryds and Professor Vincenzo Esposito of DTU Energy show that it is possible to create piezoelectric effects in materials where this is not usually possible. It paves the way for the design of lead-free and much more environmentally friendly piezoelectric materials. The research was conducted with colleagues from EPFL (Ecole Polytechnique Fédérale de Lausanne), Tel Aviv University and the University of Antwerp.
More environmentally friendly materials
The work stems from the European Biowings project, coordinated by the DTU, where several European partners are studying the development of new biomedical MEMS fabricated with lead-free thin films based on non-toxic and environmentally friendly gadolinium-doped oxide materials. . It’s a big challenge, but the potential inside, for example blood cell sorting, bacterial separation and estimating hematocrit levels, is high.
“Many micro-electromechanical systems already exist, but they often contain lead-containing materials that are harmful to human implantation. The BioWings project aims to develop biocompatible materials with properties similar to common lead-containing materials that do not contain lead or other harmful materials,” says Nini Pryds, adding:
“The new development will be a fundamental step towards environmentally friendly, high-performance piezoelectric materials for use, for example, in automotive technology and medical applications,” says Nini Pryds.
As a fundamental premise, piezoelectric materials depend on crystal symmetry. Typical piezoelectric materials have a so-called non-centrosymmetric crystal lattice. This means, for example, that when the material is pressed, an electric voltage naturally occurs through the material due to the movement of positive and negative ions relative to each other. As a result, the symmetry of the crystal is broken. For more than a century, this has been a significant obstacle to the search for new piezoelectric materials because piezoelectricity can only be created with a non-centrosymmetric crystal lattice.
Possible paradigm shift
One of the surprising results of the new study is that a large piezoelectric effect can be obtained with materials that generally do not allow it, i.e. centrosymmetric materials. Induction of piezoelectricity in centrosymmetric oxides can be achieved by using alternating current (AC) and direct current (DC) simultaneously. The field causes the positive and negative ion defects in the material to move relative to each other, resulting in an electric dipole or polarization. It breaks the crystal symmetry of the material, thereby achieving piezoelectricity in centrosymmetric crystals.
According to Nini Pryds, this concept will also be possible with other materials with similar atomic defects. It can thus help pave the way for lead-free piezoelectricity, for example in actuators and sensors.
“For now, piezoelectric materials are limited to the non-centrosymmetric crystal structure. This leads to a significant limitation in the number of materials that can be used. Our new results provide a paradigm shift towards the induction of piezoelectricity in centrosymmetric crystals, thus expanding the number of possible materials used. I expect this to have a significant effect on the design of new electromechanical devices with new biocompatible materials,” says Nini Pryds.