Novel High Power Piezoelectrics for Transformers and Actuators
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Novel High Power Piezoelectrics for Transformers and Actuators Shashank Priya1 and Kenji Uchino International Center for Actuators and Transducers, Materials Research Institute Pennsylvania State University, University Park, PA 16802. Abstract This letter reports a Pb(Zr0.5Ti0.5)O3 (PZT) based composition, which provides high dielectric, piezoelectric and electromechanical properties. The composition corresponding to 0.2 PZN – 0.8 PZT + 0.5 wt% MnO2 provides the radial mode coupling factor (kp) of 0.58, longitudinal mode piezoelectric constant (d33) of 277 pC/N, dielectric constant (ε33/εo) of 946 and mechanical quality factor (Qm) of 1402. X-ray diffraction, dielectric and polarization-electric field measurements were done to investigate the origin for this high performance. It was found that high soft properties are obtained due to the shifting of PZT composition towards morphotropic phase boundary (MPB) and high hard properties are obtained due to the generation of dipolar defects which pin the polarization. The properties of the composition corresponding to 0.9 PZT – 0.1PMnN (Zr/Ti = 0.51/0.49) modified with 6 mol% Sr were found to be: kp = 0.53, Qm = 800, d33 = 274, ε 33 ε o = 1290 and tanδ = 1.1 %, with a TCF of –8 ppm/oC (-20 to 80oC). High piezoelectric properties with low TCF shows the suitability of this composition for ultrasonic devices used under fluctuating thermal environment. Introduction Modified Pb(Zr,Ti)O3 (PZT) based ceramics with combinatory hard and soft characteristics are required for high power applications such as piezoelectric transformer, ultrasonic motor and high power actuators. All these devices operate at resonance condition 1
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because the mechanical vibration is amplified by mechanical quality factor, Qm. At resonance, a high power condition is defined in terms of the vibration velocity.1 In this study, high power refers to the vibration velocity level when the temperature rise in the material is 20 oC. At resonance a higher vibrational energy is obtained in a material with higher mechanical quality factor, Qm and higher piezoelectric constant, d. Further high electromechanical coupling factor, k is required for higher power in transformers and larger bandwidth in actuators. A piezoelectric transformer is a two-port energy converter. On one port the electrical energy is transformed into mechanical vibrations and on the other port mechanical energy is transformed into electrical signal. At each transformation, the energy of the system reduces by a factor of k2 (k stands for electromechanical coupling factor), which implies that after the two transformations the input energy is reduced by a factor of k4. Hence, a transformer material should have a higher k. The current output at the output port (or the output power) is dependent upon the impedance of the material, which in turn is inversely proportional to the capacitance of the material. A material with higher capacitance will have lower impedance and hence it can draw
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