Study of barium modified bismuth sodium titanate/epoxy composites and their transducer applications

Abstract

Perovskite lead-free ceramics barium modified bismuth sodium titanate, 0.94 (Bi1/2Na1/2) TiO3-0.06BaTiO3 (BNBT-6), was fabricated using a mixed-oxide method. Characteristics of this ceramics, including the microstructure, ferroelectric, dielectric and piezoelectric properties were characterized. The BNBT-6 ceramics has a large remnant polarization Pr of 30.5 uC/cm2 and electromechanical coupling coefficient kt of 0.46 and kp of 0.25, which are comparable to those of traditional lead zirconate titanate (PZT) ceramics. Therefore, it has good potential to replace PZT if lead-containing piezoceramics are to be phased out in the future. BNBT-6 ceramic/epoxy 1-3 composites with different ceramic volume fractions were fabricated by the dice-and-fill technique. Characterization of these composites was carried out and found to agree with theoretical modelling. The thickness electromechanical coupling coefficient kt of the composites can be improved to 0.5-0.6. The mode coupling theory was applied to the ceramic rods inside the 1-3 composites. When the width to thickness ratio (L/H) of the ceramic rods is close to 1.45, the 1-3 composite shows two strong resonance modes due to the coupling of the thickness and lateral mode of the rod. By using a composite with this particular aspect ratio, a dual frequency transducer with resonances at 3.8 MHz and 5.8 MHz has been fabricated and characterized. In order to fabricate higher frequency transducers, BNBT-6 fibre/epoxy 1-3 composites were fabricated. BNBT-6 fibres were developed by two methods: one is by a sol-gel route and the other is using the viscous suspension spinning process (VSSP). After sintering with a very slow heating rate and by controlling the ambient atmosphere, dense and crack-free BNBT-6 ceramic fibres were obtained. The diameters are ~80 um and 300 um for fibres produced by the sol-gel and VSSP methods, respectively. The crystalline phase and microstructure of the ceramic fibres were checked by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Some electrical properties of the ceramic fibres, such as the dielectric and ferroelectric behavior were also investigated. Most of the electrical properties were comparable to that of the bulk BNBT-6 ceramics. BNBT-6 fibre/epoxy 1-3 composite discs were fabricated with fibre volume fractions ranging from 0.1 to 0.5 and with diameter between 2 mm to 7.5 mm. Volume fraction of the composites was determined by counting the fibre area in the SEM micrographs. Ferroelectric and piezoelectric properties of the composites were studied. Polarization of the composites was found to be proportional to the fibre volume fraction and values of the coercive field were slightly higher than that of the bulk ceramics. Composites with resonance frequencies ranging from 8 MHz to 20 MHz were obtained by adjusting their thicknesses. Composites derived from the sol-gel fibres had higher coupling coefficients (kt >= 0.60) compared with the VSSP fibre composites which have a kt, of lower than 0.52. High frequency transducers were fabricated using the BNBT-6 fibre/epoxy 1-3 composites and their characteristics were measured. In comparison, transducers assembled using sol-gel fibre composites possess better performance. A 15 MHz ultrasonic transducer with ~30% sol-gel fibre loading has a broad bandwidth of 80% and an insertion loss of -34.8 dB.