Study Of Piezoelectric Properties,Domain Structure And 1-3 Composites Of (K,Na) NbO3-based Ceramics

Piezoelectric materials are a very important type of functional materials that can realize the mutual conversion between the mechanical signal and the electrical signal through the piezoelectric effect.Lead-based piezoelectric materials represented by lead zirconate titanate ceramics(abbreviated as PZT ceramics)are widely used to produce many important technological products such as ultrasonic transducers,resonators,filters,sensors and drivers.However,lead-based piezoelectric materials will cause the volatilization of toxic lead element during production and use process,which causes great harm to the environment and the human health.In addition,many countries have issued a series of laws to prohibit or restrict the use of lead-containing materials in the electronic equipment.Thus,it is a very important and urgent subject to investigate lead-free alternatives for replacing PZT ceramics.In recent years,(K,Na)NbO3-based ceramics(abbreviated as KNN-based ceramics)have attracted significant attention as a promising type of lead-free piezoelectric materials.Pure KNN ceramics possess high Curie temperature TC of about 415℃ and excellent piezoelectric temperature stability,but only modest piezoelectric properties with piezoelectric coefficient d33 of 125 pC/N and planar electromechanical coupling coefficient kp of 0.40.Obviously,compared to PZT ceramics,the piezoelectric properties of pure KNN ceramics are not good enough to meet the needs of most practical applications.Therefore,since 2004,researchers have used a lot of methods to improve the piezoelectric properties of KNN-based ceramics,and the method of compositional modification is considered as the most effective method.In the early stages,efforts on compositional modification of KNN-based ceramics were mainly given to the aspect of partially replacing K1+and Na1+at the A site with Li1+,and substituting Nb5+at the B site with Ta5+and/or Sb5+.In this class of compositionally modified KNN ceramics,enhanced piezoelectric properties are usually obtained due to the downward shift of orthorhombic-tetragonal(O-T)phase transition to the vicinity of room temperature.In a recent important progress,large d33 was achieved in a class of KNN-based ceramics that undergo a rhombohedral-tetragonal(R-T)phase transition near room temperature.Incorporating some ABO3-type substances like BaZrO3,(Bi,Na)ZrO3,BaHfO3 and(Bi,Na)HfO3 into(K,Na)NbO3 or(K,Na)(Nb,Sb)O3 can shift orthorhombic-tetragonal(O-T)phase transition downward and simultaneously shift the rhombohedral-orthorhombic(R-O)phase transition upward to compress the temperature region of orthorhombic phase.When the temperature region of orthorhombic phase is completely compressed,an R-T polymorphic phase transition appears near room temperature.Considering that this R-T phase transition may have a similar effect to the classical morphotropic phase boundary(MPB)in PZT ceramics,it is believed that this modified KNN ceramics can obtain a very large piezoelectric constant d33.Although this progress is an important breakthrough for the development of KNN-based ceramics,several substantial issues remain to be studied and confirmed.(1)Whether some KNN-based modified ceramics with high d33 values reported in the recent literature are actually in the R-T coexistence state at room temperature is still a very controversial academic basic problem.(2)Whether similarly large or even larger d33 can be achieved in KNN-based ceramics by a way other than constructing an R-T phase boundary is a problem worth exploring and studying.(3)How to simultaneously achieve strong piezoelectric performance and good temperature stability in KNN-based ceramics is an important problem that needs to be solved when facing applications.Unlike the nearly temperature-independent MPB of PZT ceramics,strong piezoelectric temperature dependences are usually observed in those KNN-based ceramics with high piezoelectric properties.However,the temperature stability of piezoelectric materials is an essential technical metric for the practical applications.(4)How to characterize and visualize the domain structure to study the changes in domain structure with temperature is a very critical issue for the understanding of high piezoelectric performance and temperature stability in KNN-based ceramics.Domain structure and domain-wall motion often play a significant role in the dielectric properties,the piezoelectric properties and the temperature stability of the KNN-based modified ceramics.Although many studies have been conducted on the domain structure of KNN-based ceramics through transmission electron microscopy and piezoelectric force microscopy,most of these previous studies were conducted for unpoled KNN-based ceramics at room temperature.There are few reports on the domain structure and temperature-dependent domain structure of poled KNN-based ceramics.(5)How to obtain the 1-3 type(K,Na)NbO3-based ceramic/epoxy piezoelectric composites with good piezoelectric temperature stability is a problem worth exploring and studying.Most of studies on 1-3(KNa)NbO3-based ceramic/epoxy piezoelectric composites were focused on the room-temperature electromechanical coupling properties only,and the piezoelectric temperature stability of the composites was scarcely investigated.Under the above background,this thesis mainly carried out research on the piezoelectric properties,domain configurations and 1-3 composites of potassium sodium niobate based ceramics.The main research contents and results are as follows:1.The piezoelectric performance,phase transitions,and domain configurations of 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3-0.04(Bi0.50Na0.50)ZrO3(abbreviated as KNNS-0.04BNZ)ceramics were studied,and it was found that the ceramics were of orthorhombic-tetragonal phase coexistence at room temperature,while the intrinsic and the extrinsic contribution to the piezoelectric properties of the ceramics were studied.First,the dense KNNS-0.04BNZ ceramics were prepared by the two-step sintering method,and the piezoelectric constant d33 was increased from 440 pC/N previously reported in the literature to 512 pC/N.Then,according to the analysis of the dielectric constant vs.temperature curves(the R-O and O-T phase transition temperatures corresponding to the real peaks are-40℃ and 54℃,respectively,and the R-O and O-T phase transition temperatures corresponding to the imaginary peaks are-50℃ and 42℃,respectively)and the X-ray diffraction data,the author clarifys that the ceramic is of the O-T phase coexistence at room temperature.Next,the test of piezoelectric properties with temperature,the thermal cycle experiment and the thermal aging experiment were carried out to investigate the temperature stability of KNNS-0.04BNZ ceramics.The results show that the d33 value is higher than 430 pC/N in the temperature range from-30℃ to 70℃,drops from 510 pC/N to 470 pC/N after three consecutive heating and cooling cycles from-50℃ to 150℃,and maintains above 430 pC/N after the thermal aging test in the temperature range of-50℃ to 160℃.After that,the global characteristics of domain configurations in both the unpoled and poled KNNS-0.04BNZ ceramics were investigated by using an acid-etching technique.The domain pattern of poled ceramics is composed of the relatively simple configuration of long parallel stripes in diverse widths and the hierarchical nanodomain structure of 70 nm fine stripes appearing inside some the broader stripes.In addition,based on the obtained experimental results,the influence of intrinsic and extrinsic contributions in ceramics on the piezoelectric properties of KNNS-0.04BNZ ceramics was also discussed.The author considers that the R-O and O-T phase transitions near room temperature and the corresponding characteristic domain structure play a very important role in the piezoelectric properties and piezoelectric temperature stabilities of the ceramic.2.The(0.96-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3-0.04(Bi0.5Na0.5)ZrO3-xBaZrO3(abbreviated as KNNS-0.04BNZ-xBZ)ceramics were prepared,and the effect of BaZrO3 content on the piezoelectric properties,dielectric properties and crystal structure of KNNS-BNZ-xBZ ceramics was investigated,while the physical mechanism of the strong piezoelectricity of such ceramics was also discussed.A series of KNNS-0.04BNZ-xBZ ceramics were prepared by the two-step sintering method,and it was found that the component ceramics with x=0.01 had very excellent piezoelectric properties of d33=610 pC/N,kp=0.58 and k33=0.69.According to the dielectric measurement and XRD analysis,the author clarifys that the R-O,O-T and T-C phase transition temperatures of KNNS-BNZ-0.01BZ ceramics are-29℃,43℃ and 241℃,respectively.And the ceramics are of rhombohedral-orthorhombic-tetragonal phase coexistence at room temperature.The results strongly demonstrate that the outstanding piezoelectricity can be also achieved in the KNN-based ceramics with the R-O-T phase coexistence.Through the observation of domain structure in poled KNNS-0.04BNZ-0.01BZ ceramics,it is found that the domain structure inside the crystal grains is composed of a number of lamellar domain stacks and the hierarchical nanodomain structure of fine stripes appearing inside part of lamellar domains.The author believes that the strong piezoelectricity observed in KNNS-BNZ-0.01BZ ceramics is due to the R-O-T phase coexistence and the corresponding characteristic domain structure.3.The 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3-0.03BaZrO3-0.01(Bi0.50Na0.50)ZrO3(abbreviated as KNNS-0.03BZ-0.01BNZ)ceramics were prepared,and the piezoelectric performance,dielectric properties,ferroelectric properties,crystal structure and microstructure of KNNS-0.03BZ-0.01BNZ ceramics were studied.Based the unique modification effects of BaZrO3 on the R-O phase transition and the physical phenomenon of "piezoelectric properties in some KNN-based ceramics are very stable in orthorhombic phase" previously discovered by the author’s group,the author designed the ceramics with a chemical composition of KNNS-0.03BZ-0.01BNZ.It is found that the ceramics exhibit good piezoelectric performance of d33=300 pC/N,kp=0.56 and k33=0.69 at room temperature.More importantly,d33,kp and k33 show weak piezoelectric temperature dependences in the common usage temperature range between-30℃ and 100℃,and thier change rates are less than ±10%,±5%and ±3.5%,respectively.In addition,the author also tried to understand the high piezoelectric performance and weak piezoelectric temperature dependence of KNNS-0.03BZ-0.01BNZ ceramics in the common usage temperature range from the aspects of crystal structure(orthorhombic phase),microstructure and diffuse phase transition(R-O and O-T).4.A feasible temperature-variable acid-etching method technique was developed to study the domain configurations of KNNS-0.04BNZ ceramics at different temperatures,and it was found that both acid-etching rate and domain configurations changed significantly with temperature,while several fundamental issues related to the possible domain configurations and the acid-etching were discussed on the simple mathematical basis.An improved acid-etching method was used to study the domain configurations of poled KNNS-0.04BNZ ceramics at-60℃,25℃ and 80℃,respectively.The study shows that the domain configurations of KNNS-0.04BNZ ceramics change significantly with temperature.Hierarchical nanodomain structure of fine stripes appearing inside some the broader stripes is observed at 25℃,which should be the consequence of orthorhombic-tetragonal phase coexistence.In contrast,although there is a small amount of hierarchical domain structure at-60℃,most of the domain patterns are mainly composed of simple long parallel stripes.The intersectional angle of nearly 66° is observed between adjacent sets of parallel stripes in the domain patterns of cuboid-shaped grains,which indicates that the poled KNNS-BNZ ceramics at-60℃ are still at least partly in orthorhombic phase.The etched domain patterns at 80℃ are typically composed of parallel long stripes,mostly being several hundred nanometers wide and showing quite straighter edges.The results show that the newly developed method is not only effective but also reliable for conveniently investigating the domain structure at different temperatures.According to the change of the domain structure with temperature,the author believes that the temperature-dependent structure is closely related to the high piezoelectric properties and temperature stability of KNNS-0.04BNZ ceramics.5.The author chose the previously fabricated KNNS-0.03BZ-0.01BNZ ceramics with good temperature stability to prepare a 1-3 KE piezoelectric composite with an improved dice-fill method and examined both its room-temperature electromechanical coupling properties and piezoelectric temperature stability.Compared to the KNNS-0.03BZ-0.01BNZ ceramic,the developed composites possess much larger electromechanical coupling coefficient kt of 0.54-0.67 and piezoelectric voltage coefficient g33 of 64～91×10-3 Vm/N.In addition,these composites also possess other favorable properties such as low values of acoustic impedance Z of 6.9～13.4 Mrayl,dielectric coefficient ε’ of 130～400 and mechanical quality factor Qm of 1.3～4.More importantly,kt is very stable,and the change rate of kt is less than±5%in the common usage temperature range from-30℃ to 100 ℃.The author believes that the 1-3 KE piezoelectric composites have great potential value in the application of high-frequency ultrasonic transducers.