The Structure And Piezoelectric Properties Of Bi_0.5Li_0.5ZrO₃ And MZrO₃（M=Ca,Sr,Ba） Co-doped KNN-based Ceramics

Piezoelectric ceramics is an indispensable and important functional material in modern society,which is widely used in many modern high-tech fieldsdue to the interconversion of electric energy and mechanical energy,such as medical treatment,electronics,military industry,etc..At present,the main piezoelectric ceramics are lead based ceramics in the commercial market,whose toxicity is extremelyharmful to both the environment and human society,so making lead-free piezoelectric ceramics to replace lead based ceramicsis the inevitable trend of development.Potassium sodium niobate based（（K,Na）NbO₃）ceramics are one of the environmentally friendly materials attracting great attention in the field oflead-free piezoelectric ceramics due to their excellent piezoelectric properties and a series of advantages,and are also a hot research topic in the field of piezoelectric ceramics at recent years.In order to improve the piezoelectric properties of（K,Na）NbO₃-based ceramics to be close to that of lead-based ceramics,it was found that by doping modification,the construction of polycrystalline phase boundary in（K,Na）NbO₃-based ceramics can effectively improve the piezoelectric properties.Therefore,phase boundary composition has become a mainstream research direction in the development of（K,Na）NbO₃ based ceramics.In this paper,the modification of（K,Na）NbO₃-based ceramics by both adding Bi_0.5Li_0.5ZrO₃ and MZrO₃（M=Ca,Sr,Ba）was studied.In the first place,the（0.98-x）K0.4Na0.6NbO₃-x(Bi_0.5Li_0.5)ZrO₃-0.02CaZrO₃ ceramics were designed and prepared by the traditional solid-state method,and the effects of Bi_0.5Li_0.5ZrO₃-doped ceramics on the crystal structure,microstructure and piezoelectric properties were studied with using the doped BiO.5Li0.5ZrO₃ content as variables in the ceramics.By the means of X-ray diffraction and temperature-dependent dielectric constants,the results showed that the change of BiO.5Li0.5ZrO₃ content can cause the rhomb ohedral-orthorhombi c phase transition temperature and the orthorhombic-tetragonal phase transition temperature of（K,Na）NbO₃ based ceramics to change towards room temperature,so that the phase structure of ceramics at room temperature changes from orthogonal phase to multi-phase coexistence to rhombohedral phase.On this basis,the best piezoelectric properties of the ceramics were obtained when the Bi_0.5Li_0.5ZrO₃ content isx=0.035,which included the piezoelectric constant d33=241 pC/N,the plane electromechanical coupling coefficient kp=0.37,and the Curie temperature TC is about 340℃.At the same time,the optimalceramic has rhombohedral-orthorhombic-tetragonal phase boundaries.However,itshowed a trend that the ceramic propertiesincreased at first and then decreased,indicating that the rhombohedral-orthorhombic-tetragonal phase co-existed structure significantly improved the ceramic properties.Then,the effect of MZrO₃（M=Ca,Sr,Ba）on（0.98-x）K0.4Na0.6NbO₃-x(Bi_0.5Li_0.5)ZrO₃-0.02MZrO₃ ceramics was systematically studied,and the differences in properties of CaZrO₃,SrZrO₃ and BaZrO₃ as well as the common properties of Bi_0.5Li_0.5ZrO₃ in this system were analyzed and discussed from the aspects of crystal structure,grain morphology and electrical properties.According to the tests of three kinds of ceramics,it was found that the doping amount of Bi_0.5Li_0.5ZrO₃is a critical value near x=0.03:when the doping amount x>0.03,the ceramics will generallyhave a large reduction in grain size and a huge weakening in piezoelectric properties.Among them,the piezoelectric properties were obtained that the ceramics doped with SrZrO₃component the piezoelectric constantd33=263 pC/N,the plane electromechanical coupling coefficientkp=0.3871,and the ceramics doped with BaZrO₃component the piezoelectric constantd33=307 pC/N,the plane electromechanical coupling coefficientkp=0.3336.Compared with the ceramics doped with CaZrO₃,the the piezoelectric constants d33 of the two kinds of ceramics are significantly increased,but the Curie temperature TC significantly decreased,which the specific values are about 259℃ and 226℃,respectively.At the same time,the ceramics doped with SrZrO₃ have special abnormal grain growth and obvious relaxation phase transformation phenomenon,which lead to the problems of low density,poor ferroelectric property and low temperature stability showing that they are difficult to apply in high temperature condition.And,although the ceramics doped with BaZrO₃ have good ferroelectric properties and good temperature stability compared with those of doped with SrZrO₃,their properties are greatly affected by the composition change of Bi_0.5Li_0.5ZrO₃content,andkp are quite much less than the other two ceramics.And at the Bi_0.5Li_0.5ZrO₃ conten tx=0.03,the ceramics all have rhombohedral-orthogonal-tetragonal phase coexistence structure,whichhavethe best piezoelectric properties in its own composition.At last,0.95K0.4Na0.6NbO₃-0.03(Bi_0.5Li_0.5)ZrO₃-0.02SryBa0.8-yCa0.2ZrO₃ ceramics were designed and prepared by combining CaZrO₃,SrZrO₃ and BaZrO₃,to improve the piezoelectric properties and temperature stability of KNN ceramics by changing the Sr/Ba ratio.The phase structure,microstructure,piezoelectric properties and other aspects of the ceramics were studied.Due to the addition of（Sr,Ba,Ca）ZrO₃,the rhombohedral-orthorhombic phase transition was adjusted to near room temperature in advance,resulting in the ceramicswith a series of phase structure changes.At the same time,when the Sr value is y=0.5,the ceramic has rhombohedral-orthorhombic-tetragonal phase boundaries and has the best performance value,that is,d33=291 pC/N,kp=0.3055.Compared with the previous three kinds of ceramics,the prepared ceramics have several different characteristics:（1）It has almost the highest piezoelectric constant d33 in this paper,but its overall the plane electromechanical coupling coefficientkp is significantly lower than other ceramics;（2）It has excellent temperature stability,even when the working temperature close to the Curie temperature TC,the reduced piezoelectric constant d33 is still higher than 50%of the maximum value;（3）The Curie temperature TC has a certain component stability,showing that TC is not affected obviouslyby Bi_0.5Li_0.5ZrO₃ content and Sr,Ba,Ca ratio in a certain range.These characteristics show that the ceramic has a very good significance for further research and development prospects in the field of the KNN-based piezoelectric ceramics.