F. Schoenstein

Synthesis, microstructure and electromechanical properties of undoped (K0·5Na0·5)NbO3

Abstract Conventional solid state mixed oxide route using manual and ball milling is investigated for the preparation of K0·5Na0·5NbO3 (KNN) ceramics. Microstructure engineering was made using two milling methods and sintering techniques, and the crystal growth; then electromechanical properties were investigated as a function of sintering temperature, densification and grain size. The sintering conditions were set at 920°C/5 min for spark plasma sintering and 1090–1120°C/10 and 48 h for classical sintering. KNN crystal was grown using floating zone technique under nitrogen gas, where the translation and rotation speeds were fixed at 3 mm h−1 and 20 rev min−1 respectively. Piezoelectric and dielectric performances were measured and related to the microstructure. High kt (33 to 48%), kp of 18 to 48% and d33 of 127–140 pC N−1 were reached for relative densities of 84 to 96%. KNN ceramics are now available for the design of ultrasonic sensors.

Electromechanical Properties and Microstructure of Undoped K0.5Na0.5NbO3 Ceramics and KNbO3-NaNbO3 Crystals

K0.5Na0.5NbO3 (KNN) was manufactured by spark plasma sintering (SPS), which is a fast sintering method allowing to control the grain growth. Different samples of KNN are sintered with SPS at 920°C under 50 MPa for 5 minutes. High densities over than 97% are achieved. In order to make domain engineering, KNN crystals are grown by floating zone method. Stable molten zone is reached when oxygen or nitrogen gas flux is used, leading up to 50 mm length of crystals. High electromechanical coupling factor kt about 46 %, kp around 45 % and ε33S/ε0 of 253 are achieved for KNN ceramics poled at optimum electric field about 3 kV / mm. KNN crystal boule exhibits kt about 40 % against 34 % for KNN ceramic, both poled at 1 kV / mm. These results are promising to replace PZT for transducers applications.