How Do Piezoelectrics Work?

Piezoelectrics, found almost everywhere in modern life, are materials that are able to change mechanical stress to electricity and back again. One can find them in sonars, medical ultrasound, loud speakers, computer hard drives, and in many more places. However popular piezolectrics may be as a technology, very few people truly understand their way of working. At the Simon Fraser University at Canada, and the National Institute of Standards, researchers are working on understanding one of the main classes of these materials. These are the relaxors, behaving distinctly different from the regular materials, and exhibiting the largest effect among piezoelectrics. Most surprisingly, their discovery comes in the shape of a butterfly.

The team was examining two of the most popular piezoelectric compounds, the relaxor PMN and the ferroelectric PZT. These look very similar when viewed through a microscope, with both exhibiting crystalline structure comprising cube-shaped unit cells. These are the basic building blocks all crystals use, and they contain one lead atom and three oxygen atoms. The team found the essential difference only at the center of the cells. While the PZT had one similarly charged zirconium or titanium atom occupying the center randomly, the PMN had differently charged niobium or manganese atoms in the center. With the differently charged atoms, PMN produced strong electric fields varying from one unit cell to the other. They observed this behavior exclusively in PMN and in other relaxors, but not in PZT.

According to Peter Gehring of the NIST Center for Neutron Research, although ferroelectric PZT and PMN-based relaxors have been around for decades, the difficulty in identifying the origin of their behavioral difference was due to the inability in growing sufficiently large single crystals of PZT. For a long time, the researchers had no fundamental explanation for the reason relaxors exhibited greater piezoelectric effect, which could help guide efforts in optimizing this technologically valuable property.

Then scientists from Simon Fraser University discovered a way to grow crystals of PZT that were large enough to enable a comparison of the PZT and PMN crystals. The scientists used neutron beams and they revealed new details about the location of atoms within the unit cells. The scientists found the atoms in the PMN cells were not in their expected positions, whereas in the PZT cells had them in more or less rightly expected positions. According to Gehring, this accounted for the essentials of relaxor behavior.

Te scientists observed that neutron beams scattering off PMN crystals formed the shape of a butterfly. The characteristic blurred image revealed the nanoscale structure withing the PMN and in all other relaxor materials. However, when they studied PZT materials with the same method, they did not observe the butterfly shape. This led them to conclude relaxors offer a characteristic signature in the shape of this butterfly-shaped scattering.

The team conducted additional tests on both PMN and PZT crystals. These tests revealed for the first time that compared to PZT, PMN-based relaxors were over 100 percent more sensitive to mechanical stimulation. The team hopes these findings will help in better optimization of piezoelectric behavior in general.