Lead Free BNBT Type Ceramics: A Useful Material for Sensors and Ultrasound Applications

Abstract

Ferroelectric, piezoelectric and pyroelectric materials, belong to the family called smart materials and have been extensively studied for several years, not only because they have interesting physical phenomena but also because it can be used in wide variety of applications in the most diverse fields, ranging from the automotive industry as temperature or pressure sensors, even in scientific circles as polarizers or light waveguide modulators (Moulson and Herbert, 2003). Among the smart materials most widely used are those based on combinations of lead titanate (PbTiO3) and lead zirconate (PbZrO3), leading to the formation of so-called lead zirconate-titanate (PZT), PZTs for many years has been that present the best piezoelectric and pyroelectric characteristics, however due to environmental efforts in different countries are doing, such as the Directive of the European Parliament and the Council on restriction of the use of certain hazardous substances in electrical and electronic equipment, and the restriction of hazardous substances in the USA, it is necessary to enforce research in order to find materials to replace the PZTs. There are no materials that display the excellent characteristics that PZT presents that is why in order to replace PZT system it is necessary to divide applications and develop materials which suite for each application. Several lead free materials have been studied as possible replaces for PZT; barium titanate BaTiO3 (BIT) is one of the most studied ferroelectric oxide with perovskite structure, it has been used in piezoelectric applications such as sonar due to its high electromechanical coupling factor, however BIT has low Curie temperature (Tc= 120° C), (Jaffe, 1971). Potassium niobate KNbO3 is another lead free ferroelectric material with high Tc (425° C) but to obtain dense ceramic body requires long soaking time (Jaeger & Egerton, 1962). The bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT) is an excellent candidate for use in lead-free ferroelectric ceramics because BNT is strongly ferroelectric and has a high Curie temperature (Tc = 320◦C) (Smollenky et al., 1961; Buhrer, 1962). However data on BNT properties are scarce due to the difficulty in poling process for these ceramics. Recently BNT-based solid solutions which can be poled easily have been studied (Takenaka et al., 1989, 1991; Sasaki et al., 1999; Rajeev & Dviwedi, 2005).