Chapter 3 Development of KNN-Based Piezoelectric Materials Shashaank Gupta, Deepam Maurya, Yongke Yan, and Shashank Priya 3.1 Introduction Piezoelectric materials are technologically important because of their application in various kinds of devices including ultrasonic medical imaging, ultrasonic nonde- structive testing, speakers, resonators, gas igniters, gyroscope, pressure sensors etc [1–3]. Piezoelectrics are finding applications in new emerging areas as well such as micromotors, energy harvesting devices, magnetoelectric sensors, and high power transformers [4–6]. Owing to their excellent piezoelectric and ferroelectric properties, PbZr x Ti (1x) O 3 (PZT) and other lead-based materials such as Pb(Mg 1/ 3 Nb 2/3 )O 3 (PMN), Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 (PMN-PT) and Pb(Zn 1/3 Nb 2/3 ) O 3 –PbTiO 3 (PZN-PT) have been dominating the defense and civilian applications in the past few decades [7, 8]. But with increasing concern about environment, it is the need of the hour to develop new eco-friendly piezoelectric materials with properties comparable to lead-based piezoelectrics. Extensive research has been conducted on the development of lead-free piezoelectric materials with high piezo- electric coefficient and electromechanical coupling factor. Out of the various possible choices, most widely investigated lead-free systems are K x Na (1x) NbO 3 (KNN), Na 0.5 Bi 0.5 TiO 3 (NBT) and BaTiO 3 (BT) based materials. In recent past, there have been a number of articles published reviewing the current status of research focused on these lead-free piezoelectric compositions [9–16]. Potassium sodium niobate is considered as a leading lead-free candidate among these options with high Curie temperature and good piezoelectric properties [17, 18]. Crystal structure of KNN is studied by many groups and lacks consistency [3, 19–21]. The composition near x ¼ 0.5 is of greater interest because of superior piezoelectric and ferroelectric properties. Superior properties of this composition are often attributed to the presence of polymorpic phase boundary (PPB) between two orthorhombic S. Gupta • D. Maurya • Y. Yan • S. Priya (*) CEHMS, Virginia Tech, Blacksburg, VA 24061, USA e-mail: spriya@vt.edu S. Priya and S. Nahm (eds.), Lead-Free Piezoelectrics, DOI 10.1007/978-1-4419-9598-8_3, # Springer Science+Business Media, LLC 2012 89