In most applications for piezoelectric materials there is continual demand for improved performance: greater movement, higher temperature limits, longer lifetime, etc. Single crystals and relaxor materials are among the subjects of current research.
Single crystals of natural or man-made materials exhibit the desirable piezoelectric properties that might be offered by a polycrystalline ceramic element if all of its domains were perfectly aligned. An expanding variety of single crystals is being developed for acoustical, optical, wireless communication, and other applications. Materials used to fabricate single-crystal piezoelectric elements include lead magnesium niobate / lead titanate (PMN-PT), lead zirconate niobate / lead titanate (PZN-PT), lithium niobate (LiNbO3), lithium niobate with dopants, lithium tetraborate (Li2B4O7 ), and quartz. Barium titanate (BaTiO3 ) is a potential non-lead source of piezoelectric crystals for low temperature and room temperature applications. Single-crystal PMN-PT and PZN-PT elements exhibit ten times the strain of comparable polycrystalline lead-zirconate-titanate elements.
Applications for single-crystal materials include actuators and diagnostic and therapeutic medical devices. A useful combination of piezoelectric and electro-optic properties makes lithium niobate and doped lithium niobate crystals very useful for surface acoustic wave (SAW) devices and electro-optical applications. A SAW chip made from a lithium tetraborate crystal can be significantly smaller than its lithium niobate or quartz counterpart. Other applications for lithium tetraborate crystals include bulk acoustic wave (BAW) devices, pagers, cordless and cellular telephones, and data communication devices. Applications for quartz crystals include timing mechanisms for watches and clocks and delay lines for electrical circuits. The performance of a single-crystal element depends on the direction in which the raw crystal is cut: a cut normal to the x axis will produce maximal potential for expanding in thickness; a crystal cut normal to the y axis will have maximal potential for shear distortion.
In relaxor materials, the transition between piezoelectric behavior and loss of piezoelectric capability does not occur at a specific temperature (Curie point), but instead occurs over a temperature range (Curie range). In addition to relative insensitivity to temperature, single crystals of some relaxor formulations exhibit very high electromechanical coupling factors — values greater than 0.9, versus values of 0.7-0.8 for conventional, lead-zirconate-titanate ceramics. This combination makes relaxors very attractive materials for actuator, transducer, and other applications. Lead magnesium niobate, lead magnesium niobate / lanthanum formulations, and lead nickel niobate currently are among the most studied relaxor materials.
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