CONVENTIONAL and FLASH SINTERING of STOICHIOMETRIC, ALKALINE or NIOBIUM EXCESS K0.5Na0.5NbO3 (KNN) CERAMICS
Gülcan Çorapcıoğlu
Materials Science and Engineering, PhD Dissertation, 2016
Thesis Jury
Prof. Dr Mehmet Ali Gülgün (Thesis Advisor), Assoc. Prof. Dr. Cleva Ow-Yang, Assoc. Prof. Dr. Burc Mısırlıoğlu, Prof. Dr. Miran Ceh, Assoc. Prof. Dr. Ebru Mensur Alkoy
Date &Time: May 10th, 2016 – 10:00 AM
Place: SUNUM, G-111
Keywords : Lead-free piezoelectric, sodium potassium niobate ceramics, sintering, flash sintering, scanning transmission electron microscopy (STEM-EDX)
Abstract
The most well known piezoelectric material is solid solution of PbZrO3-PbTiO3 (Pb(Zr1-xTix)O3 or PZT) and it derivatives. However, the large amount of lead contained (60%) in PZT materials became a serious problem, due to the environmental concerns and government regulations against hazardous substances.
Sodium Potassium Niobate ceramics abbreviated as KNN (K1-xNaxNbO3) is one of the high temperature candidates among the lead-free piezoelectric ceramics. It is a solid solution between ferroelectric KNbO3 and anti-ferroelectric NaNbO3. It has a perovskite crystal structure.
The obstacles in front of KNN in replacing PZT in most applications are related to the poor sintering ability of KNN. The difficulties in the sintering of KNN were associated with the narrow range of sintering temperatures, volatilization of alkali elements, and formation of secondary phases.
In the present work, conventional and flash sintering of KNN with varying alkaline and niobium compositions were investigated. Conventional sintering behavior was investigated by dilatometry. Results showed that excess alkaline composition causes a two-step sintering behavior whereas stoichiometric and niobium excess compositions have aone-step sintering behavior. Transient liquid phase sintering was proposed as an active sintering mechanism for alkaline excess KNN.
Flash sintering was applied to KNN. Flash sintering is a two electrode method initiated at the University of Colorado. The abrupt nature of the sintering process in flash sintering distinguishes it from other Field-Assisted-Sintering Techniques (FAST) which require high pressure, high current and high operational costs. A theoretical density of 94% was achieved in 30 s. under 250V/cm electric-field at 990°C. Sintering time was reduced a few hours and sintering temperature wasreduced 100-150°C compared to conventional sintering. Detailed microstructural and chemical investigations of the sample showed that there was inhomogeneous Na, K distribution and it resembles a core-shell structure where K was more in the shell and Na was more in the core region. STEM-EDX analysis showed that this Na and K variation was between and most importantly within grains. The inhomogeneous distribution of Na and K was correlated with the doubling of the unit cell within the grain along 002 direction. Compositional equilibrium is achieved after a heat treatment at 1000°C for 4 h. The compositional variations appeared to have been linked to grain boundary melting during flash and consequent recrystallization as the sample cooled.