New Class of Piezoelectrics Exhibit Large Strains

In today's issue of Nature, Huaxiang Fu and Ronald Cohen of the Carnegie Institution of Washington write that a new class of materials will revolutionize acoustic applications, including medical imaging, naval sonar and hydrophones.

These piezoelectric materials were discovered recently using conventional experimental techniques, but the origin of the much stronger electromechanical coupling over previous generation materials was unknown.

Dr. Ronald Cohen is a staff member and Huaxiang Fu is a research associate at the Geophysical Laboratory of the Carnegie Institution of Washington.

Fu and Cohen developed a model and performed first-principles computations (which use only fundamental physical quantities, such as nuclear charges, instead of experimental data) that now explain the origin of the behavior of these materials.

Piezoelectrics are used to convert sound waves to electrical signals and vice versa. The newly-discovered materials (single crystals of PMN-PT and PZN-PT) have ten times the effect of previous substances. By understanding the origin of the giant observed effect, it will now be easier to improve these materials, or discover others with even larger coupling constants.

In work supported by the Office of Naval Research, Fu and Cohen simulated the simpler material barium titanate (BaTiO3) under different applied electric fields. The scientists found that very large strains can be induced by applied electric fields due to rotating the electric polarization in a single crystal.

Normally, fields are applied along the direction of polarization and produce small strain responses. However, Fu and Cohen found that applying a field obliquely to the polarization direction leads to very much larger strains. They also explained in their article how these strains develop.

In the new single crystal materials, the strains produced by a small field can reach as large as a couple percent. How this was possible was not clear until Fu and Cohen's work.

Fu and Cohen's approach of using the computer and fundamental physics to predict the behavior of materials is relatively new to materials science. Such studies can give insights not obtainable by experiment, because the theorist can examine the different causes of behavior directly, rather than inferring the origins of a behavior indirectly.

Materials theory is thus complementary to experimental studies, which usually tell us "what" but not "why." This branch of computational theory is also distinct from the phenomenological and parameterized models, in that no experimental data input is required. Theory is thus used to understand experimental observations and to make predictions that help guide experimental studies.

The model for the new materials can be understood more clearly by considering the classic ferroelectric PbTiO3, which has a large lattice strain. It is 6% longer in one direction than in the other two; it is tetragonal. The piezoelectric effect is quite small, however.

BaTiO3 has a much smaller strain (1%) and has a low temperature structure with almost no strain (called rhombohedral). If one could make rhombohedral PbTiO3, and by applying a small field make it tetragonal, one would obtain a huge piezoelectric effect.

This is akin to what is happening in the new PZN-PT and PMN-PT materials. They are rhombohedral at zero electric field, and are like "rhombohedral PbTiO3." The implication of this picture is that other materials may be discovered that have even larger strains (6% or larger, like PbTiO3) than the new PMN-PT and PZN-PT materials, with less than 2% strain (which is still huge by previous standards).

The new materials will form the active element in future generations of transducers. A transducer is used to detect or generate acoustic waves, such as those used medically in ultrasound, or in naval applications such as sonar, mine detection, and hydrophones.

The new piezoelectrics will greatly enhance the sensitivity, range and resolution of these devices so that someday, doctors will be able to look into the body with such a high resolution that some exploratory surgery will not be necessary and naval ships will be able to see farther and more clearly underwater.

(Editor's Note: More information, color figures and images are available at this website.

(Other scientists involved in similar experimental research are Tom Shrout, Eagle Park and Eric Cross, all at Penn State and Takeshi Egami at the University of Pennsylvania.

(Scientists involved in similar theoretical work include David Vanderbilt at Rutgers and Karin Rabe at Yale.)

[Contact: Ronald Cohen]

20-Jan-2000