Materials Research Lab, Pennsylvania State UniversityUniversity Park, Pa.

Single-crystal piezoelectric actuators

Doug Bartholomew, Samuel Greengard, Glenn Hasek, John Jesitus, Scott Leibs, Kristin Ohlson, Robert Patton, Barb Schmitz, Tim Stevens, and John Teresko contributed to this article. While studying single-crystal vibration characteristics scientists at Pennsylvania State University made a surprising discovery. The tiny crystals had piezoelectric actuation properties more than 10 times as large as any previously known material. Apply an electric field to the crystals, and they would deform to a degree never observed before, says researcher Seung-Eek Park. The effect is analogous to the ordinary solenoid found in everything from the starter mechanism of an automobile to a wide variety of industrial machinery. Apply a voltage to the coil of a solenoid, and the resulting magnetic field moves its ferrous core. The movement can be used to do work, control machinery, or open and close doors, openings, and valves. Now scale down the solenoid to a single crystal with no moving parts, but capable of strong displacement in response to an electric field. What results is far more than just a miniature solenoid. The speed with which the displacement occurs opens up a wide variety of applications where the relatively slow movement of a solenoid is useless and where the much smaller displacement of conventional piezoelectric materials is insufficient. The Defense Advanced Research Project Agency in Washington, which funds much military research, has committed $30 million for research into Single-Crystal Piezoelectric Mechanical Transduction. The goal is to harness the phenomenon in naval sonar equipment, to use the actuation effect to push out an underwater acoustic wave to detect objects by the sonic reflection that is received from them. Another property of the materials can be used in medical ultrasound equipment. A crystal can produce a sonic wave at its natural resonant frequency which can be used to "paint" a picture of an unborn child within a mother's womb. By using single-crystal piezoelectric materials in medical ultrasound equipment, a single transducer could replace several probes while providing significantly enhanced imaging. The medical ultrasound market alone is estimated to be worth at least $10 million annually. According to Thomas Shrout, the senior scientist on the project, the next step is to scale up the crystals. Although crystals of one cubic centimeter are already being produced, that's not large enough for many potential applications. Shrout hopes that crystals of a cubic inch or more will soon be possible. Although many applications benefit from greater displacement, the other side of the coin is that the new materials can produce the same results as conventional ceramic piezoelectric materials at a tenth the size. TRS Ceramics Inc., a specialty manufacturer in State College, Pa., is already gearing up to produce commercial devices based on the development. Wes Hackenberger, director of R&D at TRS, anticipates a broad range of applications because the crystals generate "an order of magnitude greater displacement" than the ceramic devices that have been used up to now. When an electric field is applied, he explains, the single-crystal devices will exhibit a displacement of 1% compared with only 0.1% for ceramics. Greater displacement means that the devices can be used for a broad range of applications that previously were possible only with mechanical amplification. Although other properties of single-crystal piezoelectric materials have been identified and studied for some time, it was at Penn State that the high-strain properties were discovered. Hackenberger is excited about the potential the technology offers for actuator applications. Where strong high-displacement movement is necessary, he says, "you can stack up a bunch of crystal plates to increase movement." TRS is scaling up the growth of these crystals and is already able to produce crystals up to a cubic centimeter in size. That, of course, is much smaller than what can be done with silicon for semiconductor applications, but the piezoelectric crystals are a far more complex material.

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