Advanced Ceramics: Piezoceramics
Piezoelectricity is the creation of an electric charge (or voltage) by applying pressure. Discovered by Pierre and Jacques Curie in 1880, this phenomenon occurs in many non-metallic crystals with non-centrosymetric crystal structures and some degree of ionic bonding. Upon the application of stress, a polarization charge (P) per unit area is created which equals ds, where s is an applied stress and d is the piezoelectric modulus. The constant, d, which defines piezoelectric behavior, is a third rank tensor.1
Thus, piezoelectric behavior is highly dependent on the direction(s) along which a crystal is stressed. For example, in a quartz crystal, stressing along the [100] direction will cause polarization (a voltage), while stressing along the [001] will not.
While some single crystal ceramics, such as quartz, are still used in piezoelectric devices, the emergence of polycrystalline barium titanates (BaTiO3) in the 1940s, and lead zirconate titanates (PbZrTiO3), known as PZT, in the 1950s, led to most modern applications of piezoceramics. Both materials (and modifications of them) have high values of d, and thus can create a relatively large voltage for a given stress-called a direct piezoelectric effect. Alternatively, the materials produce a "large" strain if an electric field is applied-called a converse piezoelectric effect.
For polycrystalline piezoceramics to work, the electrically charged dipoles within the entire piezoelectric component must be aligned by placing the piezoceramic within a high electric field-a process known as poling.
The ability of piezoceramics to almost instantaneously convert electrical currant to mechanical displacement and vice versa makes them useful transducers. The efficiency of conversion between mechanical and electrical energy (or the converse) is measured by a parameter called the coupling coefficient. PZT based materials are widely used because of their high coupling coefficients.
Applications
Sonar. The use of piezoceramics (converse mode) to generate a pulsed underwater acoustic wave and then receive a reflected echo (direct mode)-sonar-was the first practical application of piezoelectrics. By the end of World War I, French engineers could measure the depth of submerged objects using quartz transducers. Contemporary sonars generally have a pulse emitter of PZT based material optimized for generating strong pulses. The "receiver" consists of arrays of hydrophones with ceramic compositions optimized to produce the maximum electrical signal from a minimal echo. Modern hydrophone transducers are composite structures with piezoceramic rods mounted in a polymer. This enhances the output voltage per unit pressure. Key to these systems are very sophisticated electronics that can separate physical and instrumental noise from the actual returned signal.Medical ultrasound. Many of us have first seen our unborn children or grandchildren thanks to medical ultrasound technology. This application is somewhat related to sonar, in that both use pulse echo methods to send and receive signals. However, in this short-range application, transducer arrays can be designed to form images.
Micropositioning and micro motors. The converse piezoelectric effect produces solid state motion. Piezoceramics are used as actuators in many micropositioning applications, such as fiber optic alignment, mirror tilting for optical communication and imaging, wafer mask alignment in the semiconductor industry, and hydraulic servo valves. In exceptional cases, positioning can be controlled to a nanometer or less.
Piezoelectric transformers. If a two-layer transducer is designed so that a low voltage applied to the first layer produces a strain, a larger strain is imposed on the second layer. This causes the second layer to generate a higher voltage than applied to the first layer. Thus we have a step-up transformer. When operated at the appropriate frequencies, voltage increases from 3 V to 1500 V are attainable. Such transformers (inverters) are used to power cold-cathode fluorescent lamps for notebook computer displays.
Active noise and vibration damping. Noise and vibration are produced by waves pulsing through the air or structural materials. Creating an "anti-wave" of the same frequency spectrum, but opposite amplitude, will cancel or at least decrease the objectionable noise or vibration. Piezoceramic transducers coupled with sophisticated signal detection and generation electronics are used for such damping applications.
Markets
A recent report by the National Science Foundation cited above estimates the U.S. market for these materials at ~$1.5 billion. Other estimates of the U.S. market range down to ~$400 million. The global market is estimated at over $11 billion. Clearly, piezoceramics represent a large and growing segment of the global and U.S. advanced ceramic market.For Further Reading
1. For an in-depth description of the mathematics of piezoelectricity, consult chapter VII in Physical Properties of Crystals, by J. F. Nye, Oxford, 1964.2. A good introduction to piezoelectric ceramics can be found in Modern Ceramic Engineering , 2nd Ed., Chapter 6, by D. W. Richerson, Marcel Dekker,1992.
3. A good discussion of piezoelectricity and sonar can be found in Electronic Materials, Chapter 4, eds. N. Braithwaite and G. Weaver, Butterworths, 1990.
4. NSF Workshop Report: Fundamental Research Needs in Ceramics, April 1999,
(NSF Grant # DMR-9714807), Workshop Chairs: Y-M Chaing and K. Jakus), available at www-unix.ecs.umass.edu/~jakus/nsf/.
5. To see a dynamic simulation of a piezoceramic motor, see www.edoceramic.com/PEMot.htm.