Lead zirconate titanate/poly(vinylidene fluoride-trifluoroethylene) 1-3 composites for ultrasonic transducer applications

A new procedure for preparing lead zirconate titanate (PZT)/poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 1-3 composites with both phases piezoelectrically active is described, Sintered PZT rods are inserted into a prepoled copolymer matrix, and the composite is repoled under a lower ele ctric field. Using this new procedure, the dipoles in the two phases are al igned in either the same or opposite directions. Composite disks, of 12.7-c m diameter and 0.33- to 0.60-mm thicknesses, have been fabricated with PZT rods of 0.8 or 1 mm diameter distributed in a square pattern with 3 mm cent er-to-center separation, The ceramic volume contents of the composite disks are 3.6 and 5.6%, respectively, The resonance characteristics of the compo site disks consist of the resonance modes of the two constituent phases, bu t they are dominated by the coupled longitudinal thickness mode (H-mode) of the PZT rods, The coupled radial mode (L-mode) resonance of the PZT rods i s significant only for thin disks. The observed resonance frequencies of th e H- and L-modes agree well with the values calculated from the coupling th eory. The thickness mode resonance of the copolymer matrix (T-mode) is pres ent but hardly observable in thick disks. The composite disks have been fab ricated into transducers with air-backing and with no front face matching l ayer, and their performance characteristics have been evaluated in water. T he transmitting and receiving voltage responses of a PZT/P(VDF-TrFE) compos ite transducer are better than those of a PZT/epoxy composite transducer, T he transmitting and receiving voltage responses are improved when the PZT r ods and copolymer matrix are poled in opposite directions, especially when the resonance frequencies of the H- and T-modes are approximately equal. Wh en the phases are poled in the same direction and the resonance peaks assoc iated with the Hand T-modes just overlap, the bandwidth is improved. Using 0.33-mm thick composite disks, a transducer can be produced with three oper ating frequencies by poling the constituent phases in the same direction, o r with two operating frequencies at equal efficiency by poling the constitu ent phases in opposite directions. The PZT/P(VDF-TrFE) 1-3 composite transd ucer, especially the one with multiple operating frequencies, should be ver y promising in the applications of medical ultrasonic imaging.