Ferroelectric composites for pyro-and piezoelectric applications
Composites made by dispersing a ferroelectric ceramic powder in a polymer host have been fabricated and evaluated for their pyroelectric and piezoelectric properties. Lead zirconate titanate (PZT5A), calcium modified lead titanate (PTCa) and lanthanum doped lead zirconate titanate (PLZT) have been used as the electroactive dispersoid while the polymer polyvinylidene fluoride PVDF and its copolymer with trifluorethylene P(VDF/TrFE), have been used as hosts. It has been found that the electro-active properties were highest in the composite using PTCa powder obtained through a quenching process dispersed in P(VDF/TrFE). The pyroelectric coefficient p, in this composite was 1301.1C/m 2K, while its Figure Of Merit pie, where E is the permittivity, had a value around 2 pE/m2K the latter value being comparable to that of the pure ceramic. The piezoelectric constant d33 in the same composite had a value of 48 pC/N, the thickness mode electromechanical coupling factor kt was 0.24 and the acoustic impedance was equal to 16 MRayl. These piezo-properties are not so attractive for an ultrasonic transducer because PZT5A has better characteristics for an emitter while P(VDF/TrFE) on its own performs better as a receiver. In this work a measurement technique has been developed which has been applied successfully to the measurement of the polarization hysteresis in composites. A value of 0.15 C/m 2 for the remnant polarization has been found for the composite mentioned above, while its coercive field was 18 MV/m. A model, so-called mixed connectivity cube, has also been described which attempts to predict the properties of the composites based on the properties of the constituent materials with an emphasis on their connectivity.