Electro-mechanical Coupling Factor
The electro-mechanical coupling factor, k measures the electro-mechanical energy conversion efficiency. It is expressed by the simple relationship
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(31.3) |
The superscripts, E and T signify the values of S and ε under constant electric field and constant stress condition, respectively.
Frequency Constant
When an unconstrained piezoelectric material is subjected to an alternating current, the material shows resonating behaviour at certain frequencies. For a disc piezoelectric element of diameter Dp , and thickness t , the resonating frequencies corresponding to the radial and axial modes ( fr and fa , respectively) are expressed in terms of the two frequency constants Nd and Nt as
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(31.4) |
The frequency constants are important for applications of piezoelectric material as a mechanical resonator. Typically, such resonances occur at driving frequencies beyond 20 MHz; and hence are of less importance in vibration control. High frequency applications of piezoelectric resonators include piezoelectric ultrasonic motors and piezoelectric probes. The other three coefficients ( d, g and k ), however, play crucial roles in vibration control applications.
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