No abstract

No abstract

No abstract

Thick ͑800 m͒ polyvinylidene fluoride/trifluoroethylene ͑P͑VDF-TrFE͒͒ copolymer films for transducer applications are poled under applied voltage at elevated temperatures. By using different heat treatments, poling temperatures, and poling time, we are able to prepare a uniformly poled film with a single resonance peak at 1.3 MHz, or a nonuniformly poled film with two resonances ͑1.3 and 2.6 MHz͒, or a film with bimorph structure with a single resonance at 2.6 MHz. The nonuniform polarization which arises from charge injection from the cathode is checked by the pressure wave propagation method. The polarization mechanisms in these thick films are expected to be similar to those previously reported for thin films. The results obtained in this work may lead to practical applications because they suggest a means for controlling transducer frequency by poling.

Ultrasonic attenuation and phase velocity in a solid insulating material are determined in a broad frequency band by studying the propagation of a pressure pulse in a sample of this material submitted to an electric field of known value. During the propagation of the pulse in the sample, which is placed between short-circuited electrodes, a current is generated in the external circuit. This signal gives directly the time dependence of the pressure pulses entering and exiting the sample, from which the frequency-dependent attenuation and phase velocity are deduced by Fourier analysis. The pressure pulse is generated by the impact of a laser pulse on an absorbing surface adjacent to one face of the sample. New results obtained in polyethylene and silicone samples are presented. The proposed method presents the following advantages: It avoids the use of a transducer, reduces the impedance matching requirements, and provides information in a broad frequency band by a single measurement.