Study On Electrical Properties Of KNN Based Lead-free Dielectrics With Wide Temperature Range

PbZrxTi(1-x)O3(PZT) based compositions have been the dominant piezo-ceramic in thefield of actuators and transducers for a long time because of its excellent piezoelectricproperties. One of the limitations of PZT is its toxic nature which posing a strong threat to ourenvironment; the other one is the thermal degradation of its electromechanical propertiesabove150oC. This directive gave a boost to the search for lead-free candidates which hadgood thermal stability. Sodium potassium niobate K0.5Na0.5NbO3(KNN) having orthorhombiccrystal structure at room temperature (RT) is a possible alternative for high temperatureapplications which having Curie point (Tc) close to420oC. However, orthorhombic totetragonal transition at~200oC deteriorates the thermal stability of piezoelectric and dielectricproperties between RT and Tc. In addition, the piezo-response of KNN is much inferior to thatof PZT. In last decade, doping of different A-site (Ca, Ba, Li, et al.) and B-site(Ta, Ti, et al.)elements has been reported to reduce the orthorhombic-tetragonal transition temperature (To-t)close to RT, resulting in an improved room temperature piezoelectric and dielectric response.By further lowering the To-t, it is possible to achieve a broad existence of tetragonal regionfrom RT to Tc, which could impart good thermal stability of the piezoelectric and dielectricproperties. Very few studies have addressed the thermal stability of electrical properties inKNN based ceramic system.Hence in this thesis we report the thermal stability of KNN based compositionsespecially in dielectric properties for we aim to find attractive properties for application ashigh-temperature dielectrics. Following works are carried out:(1) Lead-free dielectrics of (1-x)K0.5Na0.5NbO3-xSrVO3(KNN-xSV, x=0.02、0.04、0.08and0.16) and (1-x)K0.45Na0.49Li0.09NbO3-xBiAlO3(KNLN-xBA, x=0.0、0.02、0.05、0.08and0.10) were synthesized and its temperature dependent dielectric properties wereinvestigated systemically.(2) Single-phase (K0.5Na0.5)1-3xEuxNbO3(KNENO, x=0.005，0.01，0.03and0.05) and(K0.5Na0.5)1-xEuxNb1-xTixO3(KNENTO, x=0.005，0.01，0.03and0.05) ceramics wereprepared to study the effects of Eu and (or) Ti ions substitution on their phase structureand high temperature dielectric.(3)0.996(0.95K0.5Na0.5NbO3-0.05LiSbO3)-0.004BiFeO3doped with LiF (KNNLSBF/xLiF,x=0.0,1.0,1.5and2.0) piezo-ceramics were synthesized to study the effects of LiFadditive on the piezoelectric, dielectric and ferroelectric properties.