March 6-11, 2016
Potassium sodium niobate (KNN) is one of the most promising lead-free systems. However, undoped KNN has poor sinterability and by conventional sintering results in inadequate densification and, consequently, in inferior physical and electrical properties. The highly volatile alkaline sodium and potassium do not permit the increase of the sintering temperature somehow required to densify the niobium based ceramics. This conflict can be solved if the mass transport during the sintering step is enhanced, since the temperature and time needed for consolidation must be reduced in order to avoid volatilization. Among the methods reported for activation of the mass transport during the sintering process, the application of an electrical current through the sample during heating represents a promising technique for rapid densification of ceramics at relatively low temperatures . The most novel and increasingly used among Field-Assisted Sintering Techniques (FAST) is Spark Plasma Sintering (SPS). SPS has been tried before to densify KNN ceramics. Dense SPS KNN ceramics with densities around 96 % presented enhanced dielectric properties (εr = 557) [2-3]. In what concerns the electromechanical performance of piezo and ferroelectrics, their dependence on the ferroelectric domain structure and crystallographic orientation is also well known. Because the microstructure of the ceramics determines average structural orientation and domain configuration, in this work an alternative FAST is used, i.e. Spark Plasma Texturing (SPT). SPS and SPT are quite similar, however in SPT, the sample is allowed to deform freely during the pressure stage . Here we report the enhanced properties of KNN ceramics sintered by SPT. By using SPT, we were able to achieve denser ceramics (100 % densification) with a reduced average grain size (Ḡ = 1.24 µm). This ceramics present a moderate dielectric response (εr = 238), enhanced piezoelectric response (d33 = 125 pC/N) and unique ferroelectric response (Pr = 38 µC/cm2 and Ec =14 kV/cm) (Figure 1). The comparison between SPT and SPS is established and the mechanisms discussed.
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