Study of lead-free piezoelectric ceramics for transducer applications

Abstract

Bi0.5Na0.35K0.1Li0.05TiO3 (abbreviated as BNKLT) is a relatively new lead-free perovskite-type ceramic with good piezoelectric properties. In this work, we aimed at improving its piezoelectric properties by the texturing technique, and exploring the feasibility of using it in ultrasonic transducer applications. A novel approach has been established to prepare large plate-like Bi0.5Na0.5TiO3 (BNT) templates for texturing the BNKLT ceramic. Bismuth layer-structured ferroelectric Bi4.5Na0.5Ti4O15 (BNiT) particles were first synthesized by a molten-salt process at 1100oC for 4 h, which were then transformed to perovskite BNT templates by a topochemical reaction at 950oC for 2 h. As the topochemical reaction involved only the changes in chemical composition and structure, the plate-like morphology of the BNiT particles were retained, giving large plate-like BNT templates with a diameter of 5 um and a high degree of grain orientation. The large BNT templates were used to prepare textured BNKLT ceramics by a Reactive Template Grain Growth (RTGG) method. BNT templates (20 wt%) were first mixed with BNKLT calcined powders to form a slurry, which was then tape cast into a thin green sheet. After stacking a number of the green sheets, the samples were reactively sintered at 1085oC for 12 h. Our results revealed that the BNT templates were effectively aligned by the tap casting process. Because of the similar structures, the large BNiT templates grew at the expense of the BNKLT grains, producing large plate-like grains (10 um) embedded in the matrix with small grains (2 um). As inherited from the high degree of grain orientation, the textured ceramic contained a large fraction of grains aligned with their a-axis normal to the sample surface, giving a large Lotgering factor of 0.6. Moreover, the piezoelectric properties were improved, giving a large piezoelectric coefficient d33 (300 pC/N) and large electromechanical coefficients kt and kp (0.56 and 0.39, respectively). However, the depolarization temperature Td of the ceramic is not very high (~ 130oC). The textured ceramic has been fabricated into a single-element pulse-echo-type ultrasonic transducer. With the use of an epoxy/tungsten powder composite (94 wt% tungsten) as the backing layer, the transducer exhibited a short ringdown time and a broad bandwidth (~ 44%). Although the insertion loss is not very small (-37 dB), the transducer provided a good axial resolution. So, it should be a promising candidate for medical applications.