Single-crystal lead magnesium niobate-lead titanate (PMN/PT) as a broadband high power transduction material

Two experimental underwater acoustic projectors, a tonpilz array, and a cylindrical line array, were built with single crystal, lead magnesium niobate/lead titanate, a piezoelectric transduction material possessing a large electromechanical coupling factor (k33 = 0.9). The mechanical quality factor, Q(m), and the effective coupling factor, k(eff), determine the frequency band over which high power can be transmitted; k(eff) cannot be greater than the piezoelectric material value, and so a high material coupling factor is a requisite for broadband operation. Stansfield's bandwidth criteria are used to calculate the optimum Q(m) value, Q(opt) approximately 1.2 (1-k(eff)2 1/2/k(eff). The results for the tonpilz projector exhibited k(eff) = 0.730, Q(m) = 1.17 (very near optimal), and a fractional bandwidth of 0.93. For the cylindrical transducer array, k(eff) = 0.867, Q(m) = 0.91 (larger than the optimum value, 0.7), and the bandwidth was 1.16. Although the measured bandwidths were less than optimal, they were accurately predicted by the theory, despite the highly simplified nature of the Van Dyke equivalent circuit, on which the theory is based.