Structural And Electrical Properties Of Conventional And Microwave Processed Lead Free KNN Based Ceramics

Abstract

Due to their excellent piezoelectric and ferroelectric properties lead oxide based ceramics, generally represented by lead zirconate titanate [PbZrxTi1-xO3]/ (PZT), are the most widely used materials for piezoelectric actuators, sensors and transducers applications. Considering lead toxicity, there is an urgent need to develop effective lead-free ferroelectric systems, which are biocompatible and environmental friendly in nature. Several classes of materials are now being reconsidered as potentially attractive alternatives to PZT based systems. The solid solution of potassium niobate and sodium niobate, (1-x)KNbO3–xNaNbO3, was found to exhibit better piezoelectric properties around the MPB at x~0.5, which separates two orthorhombic ferroelectric phases. Therefore, K0.5Na0.5NbO3 (KNN) has been recognized as one of the most promising host materials for new lead-free piezoelectrics. However, the piezoelectric properties of KNN ceramics are not comparable to PZT ceramics. Moreover, the proper densification of KNN ceramics, synthesized by conventional process is very difficult. The presence of volatile alkali elements further makes the sintering of KNN ceramics difficult. In order to solve these problems researchers have tried to make solid solution of KNN system with other systems. LiSbO3 (LS) modification in KNN based ceramics improves the piezoelectric properties as well as the sintering behavior and makes them comparable to lead based systems. The enhanced piezoelectric and ferroelectric properties in KNN-LS ceramics are due to the presence of the orthorhombic to tetragonal (TO-T) polymorphic phase transition (PPT) temperature close to room temperature. The role of PPT in KNN-LS ceramics is similar to the MPB in PZT based systems. However, the MPB in PZT based systems is nearly independent of temperature and exists over a broad temperature range. Whereas in KNN-LS based system it is dependent on temperature and maximum properties are obtained when the PPT occurs close to RT. Hence, poling temperature will have strong effect on the piezoelectric properties of KNN-LS based ceramics.
In this work, lead-free (1-x)[K0.5Na0.5NbO3]-x[LiSbO3] (x=0, 0.04, 0.05 and 0.06)/(KNN-LS) ceramics were prepared by conventional solid-state reaction (CSSR) route. For dense morphology, pure KNN ceramics were sintered at 1120oC for 4h, whereas in LS modified KNN ceramics dense morphology was obtained at 1080oC for 4h. The structural study at room temperature (RT) revealed the transformation of pure orthorhombic to tetragonal structure with the increase in LS content in (1-x)KNN-(x)LS ceramics. Temperature dependent dielectric study confirmed the increase of diffuse phase transition nature with the increase in LS content in KNN-LS ceramics. The presence of orthorhombic to tetragonal (TO-T) polymorphic phase transition temperature (PPT) ~43oC confirmed the existence of two ferroelectric (orthorhombic and tetragonal) phases in 0.95KNN-0.05LS ceramics at RT. 0.95KNN-0.05LS ceramics showed better ferroelectric and piezoelectric properties i.e., remnant polarization (Pr) ~ 18.7 μC/cm2, coercive field (Ec) ~ 11.8 kV/cm, piezoelectric coefficient (d33) ~ 215 pC/N, planar mode coupling coefficient (kp) ~ 0.415 and remnant strain ~0.07% were obtained. These properties are still lower than the PZT based ceramics. In order to further improve the piezoelectric properties the effect of Ag, Ta and V doping in 0.95KNN-0.05LS system has been investigated in detail. It was found that the substitution of Ag+ ions in place of (K0.5Na0.5)+ ions in 0.95[(K0.5Na0.5)(1-X)AgxNbO3]-0.05LiSbO3 ¬/ KNAN-LS system initially decreased the piezoelectric and ferroelectric properties but for x=0.06 improvement in the piezoelectric and ferroelectric properties were obtained in comparison to the 0.95KNN-0.05LS ceramics. The improved properties were discussed in terms of the structural changes occurred in the ceramics. Substitution of Ta+5 ions in place of Nb+5 ions in 0.95[(K0.5Na0.5)Nb(1-x)TaxO3]-0.05LiSbO3/KNNT-LS ceramics increased the ferroelectric and piezoelectric properties without affecting the crystal structure of the ceramics. The KNNT-LS ceramics with x=0.02 exhibited maximum ferroelectric and piezoelectric properties. Moreover, the piezoelectric properties were found to be nearly independent of temperature up to ~ 200oC, which is a good characteristic requirement for the ceramic to be used in high temperature piezoelectric applications. In order to improve the sintering behavior of the ceramics, V+5 has been substituted on the Nb+5 site of the 0.95[(K0.5Na0.5)Nb(1-x)VxO3]-0.05LiSbO3/KNNV-LS ceramics. The sintering temperature was drastically reduced with the increase in V+5 content, whereas the electrical properties also decreased significantly. Among all the V+5 doped ceramics, maximum piezoelectric and ferroelectric properties were obtained in case of KNNV-LS ceramics with x=0.06.
It is well known from the previous reports on different piezoelectric ceramics that microwave (MW) processing of ceramics could be an effective way to enhance the densification behavior as well the electrical properties. To further enhance the density and the electrical properties of 0.95[K0.5Na0.5NbO3]-0.05[LiSbO3], 0.95[(K0.5Na0.5)0.94Ag0.06NbO3]-0.05LiSbO3, 0.95[(K0.5Na0.5)Nb0.98Ta0.02O3]-0.05LiSbO3 , 0.95[(K0.5Na0.5)Nb0.94V0.06O3]-0.05LiSbO3 ceramics, these ceramics were synthesized by microwave processing technique. It was found that microwave processing of these ceramics not only saved the processing time but also improved the ferroelectric and piezoelectric properties in comparision to the conventionally processed ceramics. Among all the microwave processed ceramics, 0.95[(K0.5Na0.5)Nb0.98Ta0.02O3]-0.05LiSbO3 ceramic showed maximum ferroelectric and piezoelectric properties i.e., d33~ 257 pC/N, Pr ~ 30.48 μC/cm2, kp ~ 0.48 and remnant strain ~ 0.10%.