Study Of Compositions And Physical Properties Of KNN-based Piezoelectric Ceramics

Piezoelectric materials are an important type of functional materials that can realize the conversion between mechanical energy and electrical energy.It is widely used in national economy,science and technology,modern national defense and other fields.Lead-based piezoelectric ceramics,represented by Pb(Zr,Ti)O3(PZT)ceramics,are widely used in piezoelectric devices because of their excellent piezoelectric properties.PZT ceramics are classified as a class of materials that should be restricted in the future because of their volatile and toxic lead elements.Therefore,exploring and developing lead-free piezoelectric ceramics to replace PZT ceramics is a very urgent and practical important issue.In recent years,(K,Na)NbO3-based ceramics(abbreviated as KNN-based ceramics)have attracted much attention due to their good piezoelectric properties and high Curie temperature.The piezoelectric properties of KNN-based ceramics could be greatly improved by doping.Different doping elements result in quite different effects on increasing the d33 values and also on the phase transition temperatures.Li+ can replace K+ and Na’ and further enhance Tc.but greatly reduce the orthogonal-tetragonal transition temperature To-T and rhombic-orthogonal transition temperature Tr-o Sb5+ can replaces Nb5+ in(K0.5Na0.5)NbO3.Adding(Bi,Na)HfO3 and other ABO3-type materials to(K,Na)(Nb,Sb)O3 can increase TR-O and reduce To-T,which makes the orthogonal phase compressed,and thus R-T phase boundary may appear near room temperature.The R-T phase boundary is similar to the morphotropic phase boundary in PZT ceramics and can improve piezoelectric properties.Based on the above research background,the compositions and physical properties of KNN-based piezoelectric ceramics were studied in this paper.The specific research contents and results are as follows:1.A series of(1-xK0.48Na0.52Nb0.96Sb0.04O3-xBi0.5Na0.5HfO3(abbreviated as(1-x)KNNS-xBNH)ceramics were prepared by two-step sintering method through conventional solid-state reaction.The piezoelectric properties,piezoelectric temperature stability,dielectrnc properties,ferroelectric properties,phase transitions,crystal structure,microstructure and domain structure were investigated.The effect of BNH doping on phase transition temperatures was elucidated.By properly adjusting BNH doping amount in(1-x)KNNS-xBNH component,excellent piezoelectric properties with high piezoelectric constant d33 of 540 pC/N and electromechanical coupling coeficient kp of 0.56 were obtained in the 0.955KNNS-0.045BNH ceramic.It is inferred that the excellent piezoelectric properties of this ceramics observed at room temperature are closely related to the O-T and R-O phase transitions as well as the hierarchical nanodomain structure.2.A basic compositional series of(1-x)K0.48Na0.52Nb0.96Sb0.04O3-xBaZrO3(abbreviated as(1-x)KNNS-xBZ)were designed,and ceramics with such compositions were prepared.Their piezoelectric properties and temperature stability were investigated.It was found that the electromechanical coupling coefficient kp ceramics is stable in the orthogonal phase temperature region,but decreases greatly in the tetragonal phase temperature region.If the thermal cycle test is done only in the orthogonal phase temperature region,the temperature stability of kp is very good,that is,kp not only does not change with the temperature but also does not change much before and after the thermal cycle experiment.Especially,the 0.97KNNS-0.03BZ compositional ceramic shows piezoelectric constant d33 of 238pC/N and kp of 0.55 at room temperature,and kp is very stable in the temperature range from-30℃ to 110℃.On the basis of the above research,a compositional series of(0.97-y)K0.48Na0.52Nb0.96Sb0.04O3-0.03BaZrO3-yBi0.5Na0.5HfO3 ceramics were prepared.and the doping effect of(Bi.Na)HfO3 was studied.The results show that the piezoelectric constant d33 increases with the increase of BNH doping,but the temperature stability of kp decreases.3.A compositional series of(1-x)(K0.5Na0.5)1-yLiy(Nb0.93Sb0.07)O3-x(Bi0.5Na0.5)HfO3(abbreviated as(1-x)KNNSLy-xBNH)ceramics were prepared.The effects of Li doping and BNH doping on phase transition temperatures and piezoelectric properties were studied.By optimizing the Li and BNH doping amounts,a 0.98KNNSL0.02-0.02BNH ceramic with piezoelectric constant d33 of 408 pC/N and electric coupling coefficient kp of 0.54 was obtained.The piezoelectric properties,temperature stability and domain structure of this ceramic were studied in detail.It was found that the piezoelectric temperature stability is closely related to the O-T phase transition.The hierarchical nanodomain structure in domain structure plays a very important role in piezoelectric properties and temprature stability.