Electrical properties of CuO doped-KNN ceramics and 1-3 piezocomposites

Background, Motivation and Objective Lead free piezoelectric materials have been attracting increasing attention in recent years as new materials to prevent serious problems caused by the high toxicity of lead. Among lead-free metarials, potassium sodium niobate [(K0.5Na0.5)NbO3 -(KNN)] is considered to be a promising candidate due to its comparable piezoelectric, ferroelectric properties to PZT and its high Curie temperature. The densification of KNN is problematic but it can be enhanced and one of them is addition of a sintering aid. There are no studies on lead-free piezoelectric materials in the fiber form which can find possible use in piezocomposite transducer applications. In this study,KNN was produced in two different forms; as bulk ceramics and as fibers. 1-3 composite ceramics were also prepared from fibers. Statement of Contribution/Methods KNN powders were prepared by the conventional solid state reaction method from oxide and carbonate powders. After the calcination of KNN, copper oxide (CuO) was added to the KNN powder as a sintering aid in three different ratiosThe green KNN and KNN-CuO pellets were sintered at various temperatures ranging from 1070°C to 1110°C. The samples were poled by applying a DC electric field of 5 kV/mm for 30 minutes at 80ºC. The KNN fibers with different CuO molar ratios were drawn using a novel alginate gellation technique. Fibers were dried 24 h at the room temperature and sintered at 1090°C-4 hour. Results Increasing CuO ratio was found to cause drastic changes on the microstructure. Addition of 0.5mol% CuO caused a densification of KNN whereas addition of 1.5mol% CuO led to further densification, formation of a liquid phase between the grains and grain growth. A similar trend was also observed in the microstructures of the KNN fibers. Well formed pure and CuO-added KNN fibers were produced with diameters from 150μm to 300μm. Dielectric constant and loss of 0.5mol% CuO added KNN were found to be 320 and %1.4, respectively at 100 KHz. 2Pr was determined to be ~32 μC/cm2 and the coercive field (Ec) was measured as 10 kV/cm. The planar mode coupling coefficient (kp) and mechanical quality factor (Qm) of the KNN sample containing 0.5mol% CuO were calculated from admittance-frequency measurements taken from disc shaped thin pellets using the resonance method. The kp and Qm were 0.29 and 398, respectively. Piezoelectric coefficient d33 of this sample was measured as 120 pC/N. These measurements were also repeated for various CuO additions. The CuO added KNN fibers were embedded in to an epoxy matrix and 1-3 piezocomposites were prepared with various fiber volume ratios. The dielectric and piezoelectric properties of these piezocomposites with respect to fiber content were also investigated. Discussion and Conclusions The CuO addition was found to be a viable sintering aid to obtain dense KNN ceramics. This is a first report in the literature on fabrication of lead-free piezoelectric ceramic fibers and 1-3 piezocomposites.