Sep. 13, 2001 FAYETTEVILLE, Ark. -- University of Arkansas researchers have found a novel physical effect of systems used in ultrasound and sonar that is ten times stronger than current methods used in these techniques. This large ratio of physical change to electric effect may be used one day to create more sensitive and more portable sonar devices and medical ultrasound equipment.
Graduate student Aaron George, visiting scientist Jorge Ininguez, now at Rutgers University, and assistant professor of physics Laurent Bellaiche report their findings in the September 6 issue of the journal Nature.
"This finding means we can drastically improve the response of devices," which will help advance sonar and ultrasound used in both the military and medical fields, Bellaiche said.
The scientists used computer models to study the properties of piezoelectric compounds, crystals that change shape when encountering an electric field, or create an electric field when they change shape. Piezoelectric compounds are found in ultrasound devices, which are used by doctors to noninvasively examine fetuses and internal organs, and in sonar, used by submarine personnel for underwater navigation and detection.
Some piezoelectric systems form crystals with two different types of atoms distributed throughout. Bellaiche and his colleagues sought to find out if they could guide the atomic arrangement of such crystals by placing the two different atoms in layers rather than randomly. They wanted to see what the piezoelectric and other responses of such crystals might be.
They selected a structure with the amount of Scandium and Niobium (Sc and Nb) atoms inside each layer as the variables, and created a model that could calculate the amounts of the two atoms, which have different atomic numbers and therefore different charges.
Scandium has a charge of +3, Niobium a charge of +5. By changing the ratio of the atoms inside each layer, the researchers create strong internal electric fields in different directions, causing the crystal to change structural phases.
Some ratios created an electrical polarization in one direction, creating a rhombohedral phase, while others switched the direction of polarization and created another phase, called an orthorhombic phase, an effect not seen before. Furthermore, in between the two polarized phases, the researchers discovered a large piezoelectric response that is ten times larger than responses currently used commercially.
"It's a new fundamental structural property," Bellaiche said.
The large piezoelectric response represents the process of changing shape, and it is at this point that a small electrical pulse can produce the largest change.
The researchers performed most of their computations at a temperature of 20 K, but the same result can be found at any temperature.
"At any range you can have a structure that will give you a huge response," Bellaiche said. Changing the temperature will just mean the large effect will take place with a different ratio of the two atoms.
The atomic structures Bellaiche and his colleagues have modeled can be grown using molecular beam epitaxy, a technique available at the University of Arkansas and some other research institutions and companies.
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