On the Measurement of the Electromechanical Coupling Coefficient ktUsing Different Characteristic Frequencies

The thickness coupling coefficient kt has been measured in several piezoceramic disks and a piezocomposite plate, using different pairs of resonance/antiresonance characteristic frequencies. In addition, a specific computer program has been written to evaluate the approximate coupling factors from these pairs of characteristic frequencies. The exact one-dimensional expression for the electrical impedance of a thickness extensional piezoelectric resonator, extended with a complex elastic constant for the account of intrinsic mechanical losses, is used. The different relative errors are computed. In particular, it is shown that the approximations fn = fs and fm = fp, which are sometimes employed, produce a notable error for low Qm materials. The definition of the characteristic frequencies (fp, fs), (fn, fm), (fr, fa) and the expression used for the evaluation of kt are those given in ANSI/IEEE Std 176 (1987).