Lead-Free or Low-Lead Single Crystal Piezoelectric/Ferroelectric Actuators

Technology #7797

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A photograph of a composite actuator assembled from cubic single crystalsAn XRD pattern of the crystal mass, demonstrating crystallographic texturing of the polycrystalsA series of plots of electric field-induced strain vs. electric field for (a) <001>, (b) <011>, (c) <111> oriented PZN-PT single crystal plates.
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Inventors
Professor Yet-Ming Chiang
Department of Materials Science, MIT
External Link (dmse.mit.edu)
Professor Nesbitt Hagood
Department of Aeronautics and Astronautics, MIT
Sossity Sheets
Gregory Farrey
Andrey Soukhojak
Haifeng Wang
Managed By
Christopher Noble
MIT Technology Licensing Officer - Clean and Renewable Energy
Patent Protection

Piezoelectric actuators and method of making same

US Patent 6,231,779

Applications

Low-lead and lead-free high strain single crystal piezoelectrics would be ideal for implementation in a wide range of transducer and electromechanical actuator technologies.

Problem Adressed

Lead-containing actuators, including lead-based perovskites such as PbxZr1-xTiO3 (PZT), traditionally possess the highest field-induced strains and most efficient electromechanical coupling of known piezoelectric compounds, but come with environmental and safety concerns. Nontoxic substitutes of lead-based actuators with comparable performance to lead-based perovskites are currently unavailable. However, a breakthrough in the development of lead-free and low-lead single crystal actuators in the alkaline-bismuth titanate family Na1/2Bi1/2TiO3 (NBT-BT, NBT-KBT, and NBT-KBT-BT crystals) has made it possible to grow lead-free substitutes with  superior piezoelectric actuation. In addition, single crystals exhibit superior actuation capability compared to a polycrystal of the same material.

 

Technology

Single crystals were grown by spontaneous nucleation from slowly-cooled oxide melts from the alkaline-bismuth titanate family and formulated to contain the target crystal composition. Impurities were prevented by using a flux of alkaline oxide and bismuth oxide. Weight losses from crystal growth were less than 1%. Rhombohedral phase crystals exhibited low-hysteresis actuation with up to 0.25% free strain and were stable over a wide range of temperatures unlike other preexisting rhombohedral crystal actuators. Tetragonal phase compositions exhibited free strains as high as 0.85%. Altogether, these values are nearly ten times greater than that of many of the best performing polycrystalline lead perovskites and may in addition exhibit improved mechanical robustness. The above mentioned crystals' actuation capabilities are retained at uniaxial compressive stress levels (50-100 MPa) that would cause traditional lead perovskites to inactivate.

Advantages

  • Lead-free and low-lead alternatives to traditional lead-based piezoelectric compounds which bypass the traditional environmental and safety considerations of lead-based actuators
  • Higher elastic modulus, greater mechanical strength, and lower density than traditional lead-based piezoelectric compounds
  • Single crystal structures exhibit superior actuation capability compared to polycrystal structures
  • Strain energy densities are much greater than that of the best polycrystalline lead perovskites