Piezoelectric ceramic fibre/polymer 1-3 composites for transducer applications

Abstract

Three kinds of bulk ceramics and ceramic fibres were fabricated using the sol-gel method. Their compositions are Pb(Zn1/3Nb2/3)0.30(Zr0.52Ti0.48)0.70O3 [PZN-PZT], Pb0.85Sm0.1Ti0.98Mn0.02O3[PSmT] and Pb(Li1/4Nb3/4)0.01(Mn1/3Nb2/3)0.06Zr0.46Ti0.47O3 [PLN-PMN-PZT]. PSmT ceramic shows a large piezoelectric anisotropy (kt/kp~10), has a piezoelectric constant d33 of 51 pC/N, electromechanical coupling coefficient kt of 0.44, kp < 0.05 and Qm of ~561. PZN-PZT is a "soft" PZT ceramic, which has a d33 of ~600 pC/N, kt of 0.48, kp of 0.61 and Qm of ~65. PLN-PMN-PZT is a "hard" PZT ceramic, which has a d33 of 315 pC/N, kt of 0.50, kp of 0.59 and a high Qm of 1675. The crystalline structures of these three kinds of ceramics were determined using X-ray. Three types of ceramic fibres were fabricated by a gel-spinning technique. After proper heat treatment, they were found to have properties similar to that of the bulk ceramics. Their morphology had also been examined with scanning electron microscopy. Using epoxy as a matrix, three kinds of fibre/epoxy 1-3 composites were fabricated. These composites were fabricated into disks; and the disks were ground, polarized and then characterized. The results show that the length/width ratio of the fibres in PSmT fibre/epoxy 1-3 composites does not play an important role in the disk vibrations because PSmT has a low kp. The kt values of the PSmT 1-3 composites with ceramic volume fraction, ranging from 0.2 to 0.68, are normally larger than the kt of the bulk ceramics. In order to prevent the coupling between the lateral resonance of the individual fibre and the thickness resonance of the 1-3 composites, fine PZN-PZT ceramic fibres with a diameter of 25 撘峻 were used. The 1-3 composites disks fabricated from these fine ceramic fibres have a pure thickness resonance peak. The kt values of the PZN-PZT 1-3 composites with ceramic volume fraction ranging from 0.25 to 0.62, are normally larger than the kt, and are close to the k33 of the bulk ceramics. The PLN-PMN-PZT fibre/epoxy 1-3 composites fabricated from 50 撘峻 diameter ceramic fibres were used in low frequency transducer fabrication ( ~10 MHz). The kt values of this 1-3 composite with ceramic volume fraction, ranging from 0.25 to 0.45, are also larger than the kt, but close to the k33 of the bulk ceramics. Some concave 1-3 composite disks with non-uniform thickness were fabricated. Their kt values were found to be higher than that of the disks with a uniform thickness and with the same ceramic fibre loading. Transducers for medical imaging were assembled from the three kinds of ceramic fibre/epoxy 1-3 composites and their bandwidth, centre frequency, insertion loss, kt and beam profile were measured. The -6dB bandwidth of a PSmT fibre/epoxy focused 1-3 transducer, made from a 5 mm concave disk (with 0.68 fibre loading) and silicone rubber backing was found to be 156%. Effect of backing materials on the transducer performance was investigated. The insertion loss of the transducers with porous poly(styrene) backing was found to be -23.9 dB. A small area PZN-PZT fibre/epoxy 1-3 transducer was assembled from a 2.4 mm concave disk with 0.62 fibre loading. The bandwidth and insertion loss of the transducer was found to be 164.6% (at -6 dB) and -37.5 dB, respectively.