(Na0.5K0.5)NbO3-BASED LEAD-FREE HIGH POWER PIEZOELECTRIC TRANSFORMERS

Open Access
Author:
Gurdal, Erkan Ahmet
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
February 17, 2011
Committee Members:
  • Kenji Uchino, Thesis Advisor
Keywords:
  • transformer
  • high power
  • lead-free
  • Piezoelectric
Abstract:
Lead-zirconate-titanate (PZT) has been overwhelmingly studied and it has dominated the last 50 years in piezoelectric applications. Although PZT has excellent piezoelectric properties and it has nearly completed its optimization from the industrial viewpoint, PZT and lead-based piezoelectric materials are no longer welcome in piezoelectric applications. The problem revealed when the environmental concerns rose in the last 15 years. Lead (Pb) is a known toxic material for human health and the environment. This fact was also stated by restriction of hazardous substances (RoHS) recently in 2006 by the European Union (EU) and alternative lead-free piezoelectric materials have became important. As a result, lead-free piezoelectric materials have been extensively studied in the past decade. However, high power characteristics of the lead-free materials under equilibrium conditions have not been looked into in the literature. This thesis enlightens high power characteristics of a sodium-potassium-niobate (NKN) based ceramic and its application as a candidate material for piezoelectric transformers by comparing to PZT. (Na0.5K0.5)(Nb0.97Sb0.03)O3 was prepared with 1.5% mol CuO addition. Disk-shaped and rectangular plate samples were sintered with conventional sintering methods. High power characteristics were investigated with our high power piezoelectric characterization system (HiPoCS). Distinctly different from PZTs, the disk-shaped NKN ceramics did not exhibit a decrease in mechanical quality factor (Qm) with increasing vibration velocity (up to 0.4 m/s). The rectangular plate NKN ceramics showed a different trend in mechanical quality factor (Qm). In these ceramics, Qm decreased with increasing vibration velocity. This behavior is superior to the one in PZTs because the maximum vibration velocity (vrms measured with 20°C increase of the temperature on the nodal point) obtained in the NKN ceramics was 0.8 m/s. Ring-dot piezoelectric transformers made from disk-shaped NKN ceramics, revealed power density as high as 25 W/cm3 at the matching impedance point, comparable to that obtained for the conventional PZT transformers. In conclusion, the NKN ceramics possess better high power characteristics compared to PZT, which qualify it to be an alternative lead-free piezoelectric material for piezoelectric transformers.